A method for improved work-function measurements in-situ to X-ray photoelectron spectroscopy (XPS) is presented. The method is based on the combination of two techniques involving irradiation each, however with opposite irradiation artifacts. Results can thus be processed to provide work-function values of high reliability. An estimate of the (systematic) error in each measuring technique is obtained as well. Thus, application to poorly conducting specimens and insulators is enabled, with which the extent and reliability of XPS-derived information can be improved significantly, starting from the standard chemical analysis and ending in novel XPS-based electrical characterizations.
Saxena S. K., Bera S., Bendikov T., Pecht I., Sheves M. & Cahen D. (2025) Advanced Functional Materials. e28948
The integration of functional proteins into solid-state electronic devices remains a central challenge in molecular bioelectronics due to the fragile nature of protein structures and their complex charge-transport behavior. Here, we present a robust crosswire evaporated top-contact device based on bacteriorhodopsin (bR) single-bilayers (SBL), configured as Au/Cys/bR(SBL)/eC/Au (simplified as Au/bR/eC). The evaporated carbon (eC) top electrode forms a conformal, non-invasive contact that suppresses filament formation and ensures electrical integrity across the crosswire intersecting area (≈200 µm<sup>2</sup>). Structural and spectroscopic analyses confirm that the solid-state bR films retain the native absorption spectrum and exhibit functional photocycle activity after electrode deposition, indicating that their native conformation is not significantly altered. Remarkably, electron transport (ETp) through the ≈9 nm bR-SBL junctions is temperature-independent within 300 K10 K, excluding thermally activated hopping, while the length is incompatible with coherent tunneling. Under green illumination, the junctions exhibit a reversible, photo-induced current enhancement (J<sub>green</sub>/J<sub>dark</sub> ≈ 2), ascribed to light-driven conformational changes rather than direct photoexcitation. The Au/bR/eC architecture thus establishes a thermally non-activated, conformationally mediated transport mechanism via a stable, cryo-compatible solid-state protein junction. This work provides a scalable platform for integrating light-responsive biomolecules into future bio-optoelectronic and neuromorphic devices.
Khaykelson D., Diab G. A., Cohen S. R., Kashti T., Bendikov T., Pinkas I., Teixeira I. F., Tarakina N. V., Houben L. & Rybtchinski B. (2025) Nano Letters. 25, 49, p. 17230-17236
Structurally heterogeneous materials present major challenges for characterization due to their complex nanoscale order. Sodium poly(heptazine imide) (NaPHI), a layered carbon nitride photocatalyst, exemplifies this complexity, with its precise structure remaining unresolved. Here, we uncover new structural insights into NaPHI using energy-filtered four-dimensional scanning transmission electron microscopy combined with machine-learning-based diffraction image segmentation, supported by transmission electron microscopy, atomic force microscopy, X-ray diffraction, and Raman spectroscopy. At the mesoscale, NaPHI flakes display bent morphologies, while nanodiffraction patterns reveal features characteristic of stacking disorder. Based on these insights, we modeled a NaPHI-layered structure incorporating out-of-plane undulations (waves) with amplitudes of ∼0.5 Å and wavelengths of 23 nm. This model reproduces the observed line features in nanodiffraction patterns and agrees with powder X-ray diffraction data, thereby bridging local and bulk structural information. The introduced approach uses data-driven machine learning to identify statistically significant features, offering a robust framework for structural analysis of semi-crystalline materials.
Maity A., Leskes M. & Bendikov T. (2025) ACS Applied Energy Materials. 8, 21, p. 15781-15787, 15781-1578
X-ray Photoelectron Spectroscopy (XPS), a surface-sensitive technique with atomic-scale sensitivity, provides unique insights into the elemental composition, as well as the chemical and electronic states of elements within a material. Given its ability to analyze surfaces with high precision, XPS has become a prominent tool in battery research. Despite its numerous advantages and high reliability, XPS has certain limitations caused by experimental requirements. Factors such as X-ray radiation, high vacuum conditions, and necessity for charge neutralization can potentially alter the sample composition during analysis, leading to misinterpretations of artificial reaction products as genuine battery reactions processes. This concern is particularly critical in the analysis of nonaqueous battery systems containing liquid electrolytes, where volatile components are highly sensitive to vacuum conditions and X-ray exposure. In this study, we systematically investigate the effects of XPS conditions on Lithium bis(Trifluoromethane)Sulfonimide (LiTFSI), a widely used component in nonaqueous battery electrolytes. We demonstrate how prolonged vacuum exposure, application of a low-energy electron flood gun (eFG) and extended X-ray irradiation contribute to the continuous degradation of LiTFSI. Additionally, we propose partial solutions to mitigate these decomposition processes, thereby improving the reliability of XPS measurements in battery research.
Fereiro J. A., Tomita M., Bendikov T., Bera S., Pecht I., Sheves M., Cahen D. & Ishii H. (2025) Small Methods. 9, 4, 2401204
Proteins are attractive as functional components in molecular junctions. However, controlling the electronic charge transport via proteins, held between two electrodes, requires information on their frontier orbital energy level alignment relative to the electrodes Fermi level (E<sub>F</sub>), which normally requires studies of UV Photoemission Spectroscopy (UPS) with HeI excitation. Such excitation is problematic for proteins, which can denature under standard measuring conditions. Here high-sensitivity soft UV photoemission spectroscopy (HS-UPS) combined with Constant Final State Yield Spectroscopy (CFS-YS) is used to get this information for electrode/protein contacts. Monolayers of the redox protein Azurin, (Az) and its Apo-form on Au substrates, have HOMO onset energies, obtained from CFS-YS, differ by ≈0.2 eV, showing the crucial role of the Cu redox centre in the electron transport process. It is found that combined HS-UPS/CFS-YS measurements agree with the Photoelectron Yield Spectroscopy (PYS), showing potential of the HS-UPS + CFS-YS as a powerful tool to characterize and map the energetics of a protein-electrode interfaces, which will aid optimizing design of devices with targeted electronic properties, as well as for novel applications.
Malik N., Shimon L. J., Houben L., Kossoy A., Pinkas I., Kaplan-Ashiri I., Bendikov T., Lahav M. & van der Boom M. E. (2025) Chemistry - A European Journal. 31, 7, e202403577
The branched metal-organic frameworks (MOFs) are the first superstructures of this kind, and the growth mechanism may explain crystal shapes of other materials. The mechanism of the formation of fascinating structures having a hedrite, sheaf or spherulite appearance are detailed. The branching can be controlled, resulting in crystals that either exhibit multiple generations of branching or a single generation. These structures might result from an increasing number of defects on fast-grown rods. As the basal facets become less reactive, material is added to the prism facets, leading to secondary nucleation and triangular branches. These triangular structures are connected to the rod surface, growing longer than the central rod. Electron diffraction analyses show that the sheafs are polycrystalline structures with their fantails consisting of single-crystalline nanorods deviating gradually from each other in their orientation. The crystallographic structure consists of channels with opposite handedness. The accessibility of the nanochannels and the porosity of the superstructures are demonstrated by chromophore diffusion into the channels. The confinement and alignment of the chromophores inside the channels resulted in polarized-light dependent coloration of the crystals; the polycrystallinity generated areas having different optical properties.
Ami S. D. B., Javitt L. F., Siddiqui S. A., Cohen H., Ehre D., Dubey K. D., Lahav M. & Lubomirsky I. (2025) Journal of the American Chemical Society. 147, 49, p. 45119-45131
Here, we report on the chemistry of electrofreezing of water near -0.5 °C as induced by aluminum and magnesium electrodes. We suggest that the processes are triggered by the action of hydrated Al3+ and Mg2+ ions or their hydrated hydroxides, as created by electrolysis and the electric field. The uniqueness of octahedral hydrated Al3+ and Mg2+ ions lies in their high polarizing power, which acidifies the coordinated water that interacts with the bulk water to create hexagonal ice-like architectures. The aluminum electrodes induce icing when used as an anode (+50 V), while magnesium electrodes induce icing either as an anode or cathode at the same voltage, where the formation of the hydroxide ions is plausible. Furthermore, molecular dynamic simulations suggest that these species induce in their surroundings, under the influence of the electric field, "ice-like" assemblies that trigger the icing process. Those simulations suggest that the order in which the species lower the entropy of the system is Al(OH)3 > Mg(OH)2 > Al3+ > Mg2+.
Miali M. E., Shimanovich U. & Cohen H. (2025) Physical Chemistry Chemical Physics. 27, 37, p. 19591-19629
X-ray photoelectron spectroscopy (XPS) is a popular analytical technique in materials sciences owing to its versatile coverage of broad energy ranges and the reliability of its quantitative compositional analysis. Hence, tailoring XPS capabilities to the research frontiers of biological systems and nature-inspired materials can potentially be of great value. However, the application of XPS in bio/organic systems encounters critical inherent challenges, specifically amplified by the rich nuances that are at the heart of biological functions. The present mini review describes some of these difficulties, showing that by combining electrical-sensing capabilities in situ with standard XPS chemical analysis, diverse and effective solutions can be achieved. A related method, termed chemically resolved electrical measurements (CREM), is described, and case study examples are provided, ranging from self-assembled monolayers of small molecules to relatively large supramolecular sugars and proteins. A detailed discussion is dedicated to specimen stability issues, charge capturing and hot-charge transport functionalities, for which the CREM approach provides particularly attractive capabilities and a template for advanced characterization strategies.
Baer D. R., Camci M. T., Cant D. J., Chambers S. A., Cohen H., Aydogan Gokturk P., Morgan D. J., Shchukarev A., Sherwood P. M., Suzer S., Tougaard S. & Watts J. F. (2025) Journal of Vacuum Science and Technology A. 43, 4, 040801
Because of the importance of surfaces and interfaces in many scientific and technological areas, the use of x-ray photoelectron spectroscopy (XPS) has been growing exponentially. Although XPS is being used to obtain useful information about the surface composition of samples, much more information about materials and their properties can be extracted from XPS data than commonly obtained. This paper describes some of the areas where alternative analysis methods or experimental design can obtain information about the near-surface region of a sample, often information not available in other ways. Experienced XPS analysts are familiar with many of these methods, but they may not be known to new or casual XPS users, and sometimes, they have not been used because of an inappropriately assumed complexity. The information available includes optical, electronic, and electrical properties; nanostructure; expanded chemical information; and enhanced analysis of biological materials and solid/liquid interfaces. Many of these analyses can be conducted on standard laboratory XPS systems, with either no or relatively minor system alterations. Topics discussed include (1) considerations beyond the \u201ctraditional\u201d uniform surface layer composition calculation, (2) using the Auger parameter to determine a sample property, (3) use of the D parameter to identify sp<sup>2</sup> and sp<sup>3</sup> carbon information, (4) information from the XPS valence band, (5) using cryocooling to expand range of samples that can be analyzed and minimize damage, and (6) using electrical potential effects on XPS signals to extract chemically resolved electrical measurements including band alignment and electrical property information.
Cohen H. (2025) Surface and Interface Analysis. 57, 7, p. 528-536
An inherent difficulty in X-ray photoelectron spectroscopy (XPS) regards the frequent emergence of spectral shifts due to probe-induced surface charging. Various energy-scale correction methods were already proposed, yet it is generally agreed that no fundamental answer to this problem has been demonstrated so far. Here, a method is proposed that can, in principle, address this issue and lead to a transformative improvement in XPS analyses. The method is based on extrapolation to the \u201czero exposure state\u201d of the sample, based on a reference measurable at nearly zero charging conditions. Temporal changes in the surface potential can thus be monitored at high accuracy and subsequently be implemented to get the desired energy-scale calibration. This methodology demonstrates remarkable improvements in reliability and applicability of both the chemical and electrical XPS-based analyses.
Bera S., Mishuk E., Li P., Das S., Keshet S., Garusi S., Tunik L., Edri E., Selzer Y., Pecht I., Vilan A., Sheves M. & Cahen D. (2025) Small. 21, 49, e06560
The design, fabrication, and application of robust metal/protein/metal junctions are presented with ultrathin (≈20 nm) protein films demonstrating long-term stability in ambient conditions and preserving their electron transport behavior also at ≈10 K. These junctions establish a reliable platform with a permanent contact configuration, where the confined protein layer retains its functional activity after metal contact evaporation on the protein. A bottom-up micropore device (MpD) fabrication strategy is used and optimized to ensure reproducibility. The sub-nanometer roughness of the bottom electrode is preserved within the micropore, enabling uniform protein layer deposition and film formation. In the MpD structures, protein layers are integrated between Au-covered substrates and an e-beam evaporated Pd contacts. Depositing multi-layered protein films allows for defining film widths, as tested by the atomic force microscopy (AFM)-based scratching technique. The films are composed of human serum albumin (HSA) and bacteriorhodopsin (bR). Pd's preferred 2D growth minimized metal penetration and short circuits. Impedance phase response analysis shows that ≈60% of the junctions are functional ones, demonstrating the effectiveness of the fabrication approach. These protein-based MpD junctions provide a basis for future stable platforms for electron transport studies of bio- and other soft materials.
Bera S., Vilan A., Das S., Pecht I., Ehre D., Sheves M. & Cahen D. (2025) Advanced Materials. 38, 2, e07654
The solid-state protein junctions have shown efficient electron transport over a few tens of nanometer lengthscale. This work demonstrates, how the contact resistance ((Formula presented.)) of a solid-state protein junctions, treated as a contact-limited process, which can be extracted quantitatively from the measured junction resistance (R<sub>P</sub>) by using the extrapolated zero-length resistance and series resistance (R<sub>S</sub>). Alternating current (impedance spectroscopy) and direct current measurements are used to examine charge transport in junctions of human serum albumin (HSA) and bacteriorhodopsin (bR) films with varying thicknesses. Three contact configurations, SiAu, Aueutectic gallium indium (EGaIn), and, in a micropore device (MpD), AuPd, are compared. While SiAu and AuEGaIn junctions exhibit substantial (Formula presented.) that are ascribed to interfacial oxides and electrostatic proteinelectrode interactions, MpD effectively eliminates (Formula presented.), enabling measuring the intrinsic electron transport across HSA and bR films. The exponential length-dependence of R<sub>P</sub> shows a transport decay constant (β) that varies with interfacial conditions, underscoring the role of contact engineering. By minimizing (Formula presented.), exceptionally low β values (≈0.71.1 nm<sup>−1</sup>) are found, proving that, indeed, proteins can have outstanding charge transport efficiencies.
Wang Y., Feng Y., Qi P., Qu K., Zhang Y., Huang B., Xie Z., Vilan A. & Guo C. (2025) Small. 21, 26, 2500346
Peptides have been demonstrated as promising candidates for constructing bioelectronic devices. The charge transport through peptides can be effectively modulated via intrinsic composition, structure, and, most notably, interfacial engineering. However, achieving specific electrical properties, such as solid-state rectification, remains a challenge in peptide-based bioelectronics. To fill this gap, a solid-state heterojunction is designed based on the uniform and densely-packed bilayers composed of peptide and quantum dots (QDs) self-assembled monolayer. The presence of a QDs monolayer markedly regulates the rectification and static dielectric constant (ɛ<sub>r</sub>) of the peptide-based heterojunctions. The solid-state rectification ratio of peptide junctions can be raised up to ≈10<sup>3</sup>, and the ɛ<sub>r</sub> increases by a factor of 3. Both the rectification ratio and ɛ<sub>r</sub> of peptide/QDs heterojunctions increase with enhanced non-covalent interactions between peptide side chains and QDs. The interfacial interaction modulates the coupling across the interface, influencing the energy level alignment at the peptide/QDs interface. This leads to distinct charge transport pathways under opposite bias polarities, thus achieving the regulation of rectification. These findings suggest that strengthening interfacial interactions can increase both rectification and ɛ<sub>r</sub>. This deepens the understanding of interface-regulated molecular charge transport and offers a theoretical framework for optimizing the biomolecular solid-state rectification.
Danieli Y., Houben L., Rechav K., Brontvein O., Kaplan-Ashiri I., Pinkas I., Vilan A. & Joselevich E. (2025) ACS Applied Materials and Interfaces. 17, 19, p. 28336-28349
Transition metal dichalcogenides, notably MoS<sub>2</sub>, have garnered substantial attention owing to their excellent optical and electrical properties. While various methods have been employed to grow MoS<sub>2</sub>, resulting in nanostructures with diverse dimensionalities, controlling the lattice orientation and synthesizing aligned nanostructures beyond 2D remain a formidable challenge. In this study, we report the epitaxial growth of aligned MoS<sub>2</sub> nanofin-nanoribbon hybrids, each consisting of a horizontal nanoribbon with a vertical lamellar structure (\u201cfin\u201d) in its center. Structural analysis reveals epitaxial relations that induce the growth into three isomorphic orientations following the 3-fold symmetry of the C-plane sapphire substrate. The nanofin-nanoribbon hybrid was integrated into a fin-channel photodetector with response times on the scale of tens of μs and high photocurrent. Furthermore, the nanofin-nanoribbon hybrids are incorporated into n-type \u201cfin-FET\u201d transistors, showing on-off ratios on the order of ∼10<sup>3</sup> at room temperature. The performance of these devices is discussed in terms of the efficient fabrication process, devoid of postgrowth steps, and the unique dimensionality of the device, which realizes a high optical path in the fin-shaped channel. This work demonstrates the integration of MoS<sub>2</sub> into efficient fin-channel electronic and optoelectronic devices, laying the foundation for large-scale integration of TMDs into devices with nonstandard channel configurations.
Steinberg Y., Sebti E., Moroz I. B., Zohar A., Jardón-Álvarez D., Bendikov T., Maity A., Carmieli R., Clément R. J. & Leskes M. (2024) Journal of the American Chemical Society. 146, 35, p. 24476-24492
Sodium ion batteries (SIB) are among the most promising devices for large scale energy storage. Their stable and long-term performance depends on the formation of the solid electrolyte interphase (SEI), a nanosized, heterogeneous and disordered layer, formed due to degradation of the electrolyte at the anode surface. The chemical and structural properties of the SEI control the charge transfer process at the electrode-electrolyte interface, thus, there is great interest in determining these properties for understanding, and ultimately controlling, SEI functionality. However, the study of the SEI is notoriously challenging due to its heterogeneous nature and minute quantity. In this work, we present a powerful approach for probing the SEI based on solid state NMR spectroscopy with increased sensitivity from dynamic nuclear polarization (DNP). Utilizing exogenous (organic radicals) and endogenous (paramagnetic metal ion dopants) DNP sources, we obtain not only a detailed compositional map of the SEI but also, for the first time for the native SEI, determine the spatial distribution of its constituent phases. Using this approach, we perform a thorough investigation of the SEI formed on Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> used as a SIB anode. We identify a compositional gradient, from organic phases at the electrolyte interface to inorganic phases toward the anode surface. We find that the use of fluoroethylene carbonate as an electrolyte additive leads to performance degradation which can be attributed to formation of a thicker SEI, rich in NaF and carbonates. We expect that this methodology can be extended to examine other titanate anodes and new electrolyte compositions, offering a unique tool for SEI investigations to enable the development of effective and long-lasting SIBs.
Kaslasi H., Rakita Y., Kaplan-Ashiri I., Hodes G. & Bendikov T. (2024) Crystal Growth and Design. 24, 15, p. 6421-6430
Interest in halide perovskites (HaPs) is motivated by the combination of superior optoelectronic properties, ease of synthesis, and a surprisingly low density of electrically active defects. HaPs possess high chemical sensitivity, especially those having an organic cation at their A position (AMX<sub>3</sub>). X-ray photoelectron spectroscopy (XPS) is a surface technique with sensitivity that goes down to a single atomic layer and provides unique information that relates the elemental composition with the chemical and electronic states of the elements in the material. Our study focuses on XPS imaging in combination with selected small-area spectra and uses aged (3 years old) solution-grown single crystals of mixed A-cation Cs<sub>x</sub>MA<sub>1-x</sub>PbBr<sub>3</sub> (MA = CH<sub>3</sub>NH<sub>3</sub><sup>+</sup>) HaPs as a candidate for investigating intracrystal heterogeneity. With XPS, we followed the variations in chemical composition by measuring crystals at different regions down to 50 μm diameter of the samples. By comparing the surface of the crystals with their cross-section, we found significant changes in the Cs<sup>+</sup> and Br<sup>-</sup> concentrations, which increase toward the interior of the crystal. Contrarily, concentrations of carbon and nitrogen predominate on the top surface and especially at crystal edges, which form a partial covering of the crystals beyond the visible crystal boundaries, something that is not seen by electron microscopy analysis and shows the advantage of the XPS for measurements of light elements. Besides demonstrating the utility of the XPS technique, this compositional heterogeneity within the Cs<sub>x</sub>MA<sub>1-x</sub>PbBr<sub>3</sub> crystals reveals novel insights into the complex chemical nature of what may be seen as uniform single crystals and brings crucial information for their understanding.
Snarski L., Biran I., Bendikov T., Pinkas I., Iron M. A., Kaplan-Ashiri I., Weissman H. & Rybtchinski B. (2024) Advanced Functional Materials. 34, 7, 2309742
While individual single-wall carbon nanotubes (SWCNTs) have remarkable strength and electrical conductivity, SWCNT networks fabricated from dispersions have inferior properties due to nanotube bundling, limiting the potential applications of SWCNT materials. Herein, a common dye molecule (purpurin) is used to exfoliate SWCNTs via noncovalent functionalization and to fabricate SWCNT materials by a simple solution-based process. The advantageous noncovalent interactions result in efficient exfoliation and metallic SWCNT enrichment, affording SWCNT materials with high mechanical robustness and electrical conductivity. This method is used to prepare mechanically robust SWCNT films and flexible transparent conductive electrodes.Purpurin, a common dye molecule, is utilized to efficiently exfoliate single-wall carbon nanotubes (SWCNTs) in an aqueous solution. The advantageous noncovalent interactions result in metallic SWCNT enrichment, affording the fabrication of flexible, robust, and highly electrical conductive SWCNT networks. Transparent conductive electrodes based on those networks show excellent optoelectronic performance suitable for application in touch screens and LEDs.image
Crans K. D., Cohen H., Nehoray A. A., Oron D., Kazes M. & Brutchey R. L. (2024) Nano Letters. 24, 50, p. 16015-16021
Additive engineering of lead halide perovskites has been a successful strategy for reducing a variety of deleterious defect types. Ionic liquids (ILs) are a unique group of such additives that have been used to passivate halide vacancies in both bulk lead halide perovskites and their colloidal nanocrystal analogues. Herein, we expand the types of defects that can be addressed through IL treatments in CsPbBr<sub>3</sub> nanocrystals with a novel phosphonium tribromide IL that heals metallic lead surface defects through redox chemistry. This new type of surface treatment leads to a significant increase in PLQY and outperforms equivalent treatments with non-redox-active bromide ILs. Such redox-active ligands widen the scope of defect types that can be addressed in semiconductor nanocrystals.
Javitt L. F., Cohen H., Weissbuch I., Ehre D., Lahav M. & Lubomirsky I. (2024) Journal of Physical Chemistry C. 128, 37, p. 15443-15450
In search of different chemical mechanisms for electro-freezing of supercooled water (SCW), we describe here the differences between the inert electrodes of Co and Ni and their ability to be converted into ice makers. The formation of crystalline domains by anodic electrochemical treatment of cobalt in 10 μM KOH solution triggers ice nucleation of supercooled water at ∼−3 °C without the application of voltage. X-ray diffraction and XPS demonstrate that these crystalline domains are composed of oxyhydroxide molecules. As predicted by their Moiré fringes, crystals of hexagonal ice grow epitaxially on top of these domains along their [002] direction. In contrast, Ni anodes under similar electrochemical treatment do not form crystalline domains of the oxyhydroxides and therefore do not yield ice-nucleating surfaces.
Freidzon D., Wachtel E., Cohen H., Houben L., Kossoy A., Brontvein O., Varenik M., Frenkel A. I., Ehre D. & Lubomirsky I. (2024) Solid State Ionics. 411, 116572
Coupling between an electrochemical reaction and a functional material property has been termed electro-chemo-X, or EC-X, where X can refer to mechanical, optical, magnetic or thermal properties. Recently, our group has demonstrated a two-terminal electro-chemo-mechanical (ECM) membrane actuator operating under ambient conditions and containing a Ce<sub>0.8</sub>Gd<sub>0.2</sub>O<sub>1.9</sub> solid electrolyte layer sandwiched between two Gd-doped ceria/TiO<sub>x</sub> nanocomposite thin films. Reducing one nanocomposite film while oxidizing the other was observed to produce reversible volume change thereby driving membrane actuator operation. Here, we use the same electrolyte and nanocomposite layer pair (the upper one as the ion reservoir and the lower, as the active layer) to further explore the EC-X effect. We demonstrate the suitability of the nanocomposite for a three-terminal, thin film-based resistivity switch. We find that application of ±6 V ( Ce<sup>+4</sup> is similarly effective in leading to increased nanocomposite conductivity, while reduction produces the opposite effect. With the expectation that the response time can be significantly shortened, the proposed resistivity switch may be suitable for future applications such as sensors, neuromorphic computing or spintronics.
Rosenberg A., Solomonov A., Cohen H., Eliaz D., Kellersztein I., Brookstein O., Kozell A., Wang L., Wagner H. D., Daraio C. & Shimanovich U. (2024) ACS Applied Materials and Interfaces. 16, 7, p. 9210-9223
Biology resolves design requirements toward functional materials by creating nanostructured composites, where individual components are combined to maximize the macroscale material performance. A major challenge in utilizing such design principles is the trade-off between the preservation of individual component properties and emerging composite functionalities. Here, polysaccharide pectin and silk fibroin were investigated in their composite form with pectin as a thermal-responsive ion conductor and fibroin with exceptional mechanical strength. We show that segregative phase separation occurs upon mixing, and within a limited compositional range, domains ∼50 nm in size are formed and distributed homogeneously so that decent matrix collective properties are established. The composite is characterized by slight conformational changes in the silk domains, sequestering the hydrogen-bonded β-sheets as well as the emergence of randomized pectin orientations. However, most dominant in the composites properties is the introduction of dense domain interfaces, leading to increased hydration, surface hydrophilicity, and increased strain of the composite material. Using controlled surface charging in X-ray photoelectron spectroscopy, we further demonstrate Ca ions (Ca<sup>2+</sup>) diffusion in the pectin domains, with which the fingerprints of interactions at domain interfaces are revealed. Both the thermal response and the electrical conductance were found to be strongly dependent on the degree of composite hydration. Our results provide a fundamental understanding of the role of interfacial interactions and their potential applications in the design of material properties, polysaccharide-protein composites in particular.
Cohen A., Li J., Cohen H., Kaplan-Ashiri I., Khodorov S., Wachtel E. J., Lubomirsky I., Frenkel A. I. & Ehre D. (2024) ACS Applied Electronic Materials. 6, 2, p. 853-861
The local environments of Sc and Y in predominantly ⟨002⟩ textured, Al<sub>1-x</sub>Do<sub>x</sub>N (Do = Sc, x = 0.25, 0.30 or Y, x = 0.25) sputtered thin films with wurtzite symmetry were investigated using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x = 0.25, both Sc<sup>3+</sup> and Y<sup>3+</sup> ions are able to substitute for Al<sup>3+</sup>, thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modeling the scandium local environment, extended XAFS (EXAFS) analysis suggests that when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 mol % Sc) in wurtzite (Al, Sc)N. XPS spectra provide support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.
Kundrat V., Cohen H., Kossoy A., Bonani W., Houben L., Zalesak J., Wu B., Sofer Z., Popa K. & Tenne R. (2024) Small. 20, 14, 2307684
Uranium is a high-value energy element, yet also poses an appreciable environmental burden. The demand for a straightforward, low energy, and environmentally friendly method for encapsulating uranium species can be beneficial for long-term storage of spent uranium fuel and a host of other applications. Leveraging on the low melting point (60 °C) of uranyl nitrate hexahydrate and nanocapillary effect, a uranium compound is entrapped in the hollow core of WS<sub>2</sub> nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO<sub>2</sub> nanoparticles anchor within the WS<sub>2</sub> nanotube lumen are obtained through this procedure. Such methodology can find utilization in the processing of spent nuclear fuel or other highly active radionuclides as well as a fuel for deep space missions. Moreover, the low melting temperatures of different heavy metal-nitrate hydrates, pave the way for their encapsulation within the hollow core of the WS<sub>2</sub> nanotubes, as demonstrated herein.
Shein-Lumbroso O., Gerry M., Shastry A., Vilan A., Segal D. & Tal O. (2024) Nano Letters. 24, 6, p. 1981-1987
Electronic flicker noise is recognized as the most abundant noise in electronic conductors, either as an unwanted contribution or as a source of information on electron transport mechanisms and material properties. This noise is typically observed when a voltage difference is applied across a conductor or current is flowing through it. Here, we identify an unknown type of electronic flicker noise that is found when a temperature difference is applied across a nanoscale conductor in the absence of a net charge current or voltage bias. The revealed delta-T flicker noise is demonstrated in molecular junctions and characterized using quantum transport theory. This noise is expected to arise in nanoscale electronic conductors subjected to unintentional temperature gradients, where it can be a performance-limiting factor. On the positive side, delta-T flicker noise can detect temperature differences across a large variety of nanoscale conductors, down to atomic-scale junctions with no special setup requirements.
Bera S., Fereiro J. A., Saxena S. K., Chryssikos D., Majhi K., Bendikov T., Sepunaru L., Ehre D., Tornow M., Pecht I., Vilan A., Sheves M. & Cahen D. (2023) Journal of the American Chemical Society. 145, 45, p. 24820-24835
A key conundrum of biomolecular electronics is efficient electron transport (ETp) through solid-state junctions up to 10 nm, often without temperature activation. Such behavior challenges known charge transport mechanisms, especially via nonconjugated molecules such as proteins. Single-step, coherent quantum-mechanical tunneling proposed for ETp across small protein, 2-3 nm wide junctions, but it is problematic for larger proteins. Here we exploit the ability of bacteriorhodopsin (bR), a well-studied, 4-5 nm long membrane protein, to assemble into well-defined single and multiple bilayers, from ∼9 to 60 nm thick, to investigate ETp limits as a function of junction width. To ensure sufficient signal/noise, we use large area (∼10<sup>-3</sup> cm<sup>2</sup>) Au-protein-Si junctions. Photoemission spectra indicate a wide energy separation between electrode Fermi and the nearest protein-energy levels, as expected for a polymer of mostly saturated components. Junction currents decreased exponentially with increasing junction width, with uniquely low length-decay constants (0.05-0.5 nm<sup>-1</sup>). Remarkably, even for the widest junctions, currents are nearly temperature-independent, completely so below 160 K. While, among other things, the lack of temperature-dependence excludes, hopping as a plausible mechanism, coherent quantum-mechanical tunneling over 60 nm is physically implausible. The results may be understood if ETp is limited by injection into one of the contacts, followed by more efficient charge propagation across the protein. Still, the electrostatics of the protein films further limit the number of charge carriers injected into the protein film. How electron transport across dozens of nanometers of protein layers is more efficient than injection defines a riddle, requiring further study.
Hamo Y., Neudert A., Bendikov T., Lahav M. & van der Boom M. E. (2023) Journal of the American Chemical Society. 145, 32, p. 18075-18083
We demonstrate here the assembly of a nanolayer of electrochromic iron complexes on the top of composite layers of cobalt and ruthenium complexes. Depending on the ratio of the latter two complexes, we can tailor materials that show different electron transport pathways, redox activities, and color transitions. No redox activity of the top layer, consisting of iron complexes, is observable when the relative amount of the ruthenium complexes is low in the underlying composite layer because of the insulating properties of the isostructural cobalt complexes. Increasing the amount of ruthenium complexes opens an electron transport channel, resulting in charge storage in both the cobalt and iron complexes. The trapped charges can be chemically released by redox-active ferrocyanide complexes at the film-water interface.
Gercci Y., Yosef-Tal N., Bendikov T., Dodiuk H., Kenig S. & Tenne R. (2023) Polymers. 15, 14, 3061
The effect of semiconducting tungsten disulfide (WS<sub>2</sub>) nanoparticles (NPs), functionalized by either methacryloxy, glycidyl, vinyl, or amino silanes, has been studied in photocuring of acrylate and epoxy resins (the latter photocured according to a cationic mechanism). The curing time, degree of curing (DC), thermal effects, and mechanical properties of the radiation-cured resins were investigated. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses confirmed that a silane coating was formed (14 nm) on the NPs surface having a thickness of 14 nm. Fourier transition infrared (FTIR) was used to determine the DC of the nanocomposite resin. The curing time of the epoxy resin, at 345385 nm wavelength, was 10 to 20 s, while for acrylate, the curing time was 7.5 min, reaching 92% DC in epoxy and 84% in acrylate. The glass transition temperature (Tg) of the photocured acrylates in the presence of WS<sub>2</sub> NPs increased. In contrast to the acrylate, the epoxy displayed no significant variations of the Tg. It was found that the silane surface treatments enhanced the DC. Significant increases in impact resistance and enhancement in shear adhesion strength were observed when the NPs were treated with vinyl silane. A previous study has shown that the addition of WS<sub>2</sub> NPs at a concentration of 0.5 wt.% is the optimal loading for improving the resins mechanical properties. This study supports these earlier findings not only for the unmodified NPs but also for those functionalized with silane moieties. This study opens new vistas for the photocuring of resins and polymers in general when incorporating WS<sub>2</sub> NPs.
Yahalom N., Snarski L., Maity A., Bendikov T., Leskes M., Weissman H. & Rybtchinski B. (2023) ACS Applied Energy Materials. 6, 9, p. 4511-4519
Lithium-sulfur (Li-S) batteries (LSBs) have high energy densities and employ inexpensive materials. However, the poor sulfur conductivity and rapid capacity fading hamper their applications. We developed a free-standing composite cathode based on multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs), whose fabrication follows a solution-based, scalable method. The two CNT types create a synergic effect: SWCNTs result in high conductivity, high surface area, and mechanical strength/flexibility; MWCNTs larger pores ensure facile ionic diffusion and trapping of lithium polysulfides. The composite cathode exhibits a peak discharge capacity of 1221 mAh/g, maintaining 876 mAh/g after 100 cycles at a 0.1C rate.
Fereiro J. A., Bendikov T., Herrmann A., Pecht I., Sheves M. & Cahen D. (2023) Journal of Physical Chemistry Letters. 14, 12, p. 2973-2982
We demonstrate that the direction of current rectification via one of natures most efficient light-harvesting systems, the photosystem 1 complex (PS1), can be controlled by its orientation on Au substrates. Molecular self-assembly of the PS1 complex using four different linkers with distinct functional head groups that interact by electrostatic and hydrogen bonds with different surface parts of the entire protein PS1 complex was used to tailor the PS1 orientation. We observe an orientation-dependent rectification in the currentvoltage characteristics for linker/PS1 molecule junctions. Results of an earlier study using a surface two-site PS1 mutant complex having its orientation set by covalent binding to the Au substrate supports our conclusion. Currentvoltagetemperature measurements on the linker/PS1 complex indicate off-resonant tunneling as the main electron transport mechanism. Our ultraviolet photoemission spectroscopy results highlight the importance of the protein orientation for the energy level alignment and provide insight into the charge transport mechanism via the PS1 transport chain.
Kumar S., Damle V. H., Bendikov T., Itzhak A., Elbaum M., Rechav K., Houben L., Tischler Y. & Cahen D. (2023) ACS Applied Materials and Interfaces. 15, 19, p. 23908-23921
Two-dimensional (2D) halide perovskites, HaPs, can provide chemical stability to three-dimensional (3D) HaP surfaces, protecting them from exposure to ambient species and from reacting with contacting layers. Both actions occur with 2D HaPs, with the general stoichiometry R<sub>2</sub>PbI<sub>4</sub> (R: long or bulky organic amine) covering the 3D ones. Adding such covering films can also boost power conversion efficiencies of photovoltaic cells by passivating surface/interface trap states. For maximum benefit, we need conformal ultrathin and phase-pure (n = 1) 2D layers to enable efficient tunneling of photogenerated charge carriers through the 2D film barrier. Conformal coverage of ultrathin (
Modak A., Velayudham P., Bendikov T., Mohan R. & Schechter A. (2023) Catalysis Science and Technology. 13, 4, p. 1180-1195
Developing an active, durable and cost-effective air cathode catalyst is critical for commercializing proton exchange membrane fuel cells (PEMFCs). CeO<sub>2</sub> has been reported as a scavenger of harmful peroxide by-products, yet it is less efficient in O<sub>2</sub> electrocatalysis due to its insufficient electronic conductivity. In this work, a novel approach is proposed to mitigate this limitation. We prepared CeO<sub>2</sub> nanoparticles incorporated into a porous organic polymer (POP) followed by pyrolysis with KOH to form a mixed phase of ceria and CeC<sub>x</sub> incorporated in a conducting nitrogen-doped carbon matrix (CeC<sub>x</sub>/N-doped carbon), exhibiting high porosity (1320 m<sup>2</sup> g<sup>−1</sup>) and specific capacitance (37.7 F cm<sup>−2</sup>). Pt nanoparticles impregnated onto CeC<sub>x</sub>/N-doped carbon revealed preferential binding and nucleation with the support offering enhanced oxygen reduction reaction (ORR) activity in acid and alkaline electrolytes. Structural characterization by XRD, TEM, and XPS discloses a successful synthesis of a composite hybrid catalyst comprising Pt on CeC<sub>x</sub> and N-doped activated carbon, designated as Pt/KC@CeO<sub>2</sub>. The ORR activity in 0.5 M H<sub>2</sub>SO<sub>4</sub> shows a higher onset (0.84 V/RHE) and limiting current density, −3.5 mA cm<sup>−2</sup>, comparable with that of commercial Pt/C. Importantly, Pt/KC@CeO<sub>2</sub> exhibits a low peroxide yield (
Haber S., Solomatin N., Shapira A., Bendikov T., Brontvein O., Ein-Eli Y. & Leskes M. (2023) Journal of Power Sources. 560, 232693
Surface degradation is a major limitation in the utilization of high energy cathodes. Cathode coatings provide a promising route for achieving interfacial stability. In this case, there is interest in developing coatings which will provide efficient ion transport across the cathode-electrolyte-interface (CEI). Aluminum based coatings, particularly metal fluorides, have shown great promise for stabilizing the high voltage cathode, LiNi0.5Mn1.5O4. Despite the clear practical advantages of coatings, the chemical basis for their improved performance remains ambiguous. Here we present an in-depth investigation of a nominal AlF3 coating on LiNi0.5Mn1.5O4 based on solid state NMR spectroscopy to determine the composition and function of the Al/F coating layer. NMR results, supported by X-ray photoelectron spectroscopy and electron microscopy, reveal that the actual composition of the deposited layer is an amorphous AlOF phase. Isotope exchange is used to follow the spontaneous exchange of lithium ions across the CEI, revealing improved transport in coated cathodes. Finally, NMR experiments provide evidence for insertion of lithium ions in the Al/F coating following electrochemical cycling. The results suggest that lithium insertion into a nominally lithium-less coating plays a role in its improved performance, highlighting lithium content as an important factor in designing beneficial coatings for higher energy cathodes.
Schwarz A., Alon-Yehezkel H., Levi A., Yadav R. K., Majhi K., Tzuriel Y., Hoang L., Bailey C. S., Brumme T., Mannix A. J., Cohen H., Yalon E., Heine T., Pop E., Cheshnovsky O. & Naveh D. (2023) npj 2D Materials and Applications. 7, 1, 59
Recent research on two-dimensional (2D) transition metal dichalcogenides (TMDCs) has led to remarkable discoveries of fundamental phenomena and to device applications with technological potential. Large-scale TMDCs grown by chemical vapor deposition (CVD) are now available at continuously improving quality, but native defects and natural degradation in these materials still present significant challenges. Spectral hysteresis in gate-biased photoluminescence (PL) measurements of WSe<sub>2</sub> further revealed long-term trapping issues of charge carriers in intrinsic defect states. To address these issues, we apply here a two-step treatment with organic molecules, demonstrating the \u201chealing\u201d of native defects in CVD-grown WSe<sub>2</sub> and WS<sub>2</sub> by substituting atomic sulfur into chalcogen vacancies. We uncover that the adsorption of thiols provides only partial defect passivation, even for high adsorption quality, and that thiol adsorption is fundamentally limited in eliminating charge traps. However, as soon as the molecular backbone is trimmed and atomic sulfur is released to the crystal, both bonds of the sulfur are recruited to passivate the divalent defect and the semiconductor quality improves drastically. Time-dependent X-ray photoelectron spectroscopy (XPS) is applied here together with other methods for the characterization of defects, their healing, leading energies and occupation. First-principles calculations support a unified picture of the electronic passivation of sulfur-healed WSe<sub>2</sub> and WS<sub>2</sub>. This work provides a simple and efficient method for improving the quality of 2D semiconductors and has the potential to impact device performance even after natural degradation.
Major G. H., Pinder J. W., Austin D. E., Baer D. R., Castle S. L., Čechal J., Clark B. M., Cohen H., Counsell J., Herrera-Gomez A., Govindan P., Kim S. H., Morgan D. J., Opila R. L., Powell C. J., Průša S., Roberts A., Rocca M., Shirahata N., Šikola T., Smith E. F., So R. C., Stovall J. E., Strunk J., Teplyakov A., Terry J., Weber S. G. & Linford M. R. (2023) Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films. 41, 3, 038501
Due to significant advances in instrumentation, many previously specialized techniques have become "routine"in user facilities. However, detailed knowledge held by experts has often not been relayed to general users, so they often rely on entry-level information, basic principles, and comparison with literature results for data analysis. As a result, major errors in the data analysis of multiple surface and material analysis techniques, including in x-ray photoelectron spectroscopy (XPS), have been appearing in the scientific literature. Representative examples of serious errors in XPS data analysis are shown in this work. We suggest that surface and material analysis, and perhaps even science in general, are in a state of "pre-crisis."We use two (logistic) models from population biology to suggest that bad analyses self-correct if they remain below a critical number. However, beyond a threshold, the literature can become useless because of the perpetuation of faulty analyses and concomitant loss of its self-correcting ability. XPS is used by scientists in many communities because of the power of the technique and high-quality instrumentation that is commercially available. Those who make new surfaces and materials face unique challenges because of the large number of surface and material analytical techniques that are often needed to characterize their materials. Graduate students and post-docs are often provided with only minimal instruction on using surface and material characterization methods. High fees for instruments may affect both the quality and the quantity of the data people collect. The Prisoner's Dilemma is a model from game theory that describes situations with reward structures that encourage uncooperative behavior and lead to suboptimal outcomes. However, the outcomes of Prisoner's Dilemma are not inevitable - their consequences change if their reward structures change. The current system does not appear to incentivize detailed learning of surface and material characterization techniques and careful material characterization. Prisoner's dilemmas appear to lead to other undesirable consequences in science. The concerns raised in this work suggest that many manuscripts are incompletely reviewed at present. The different stakeholders in this problem, including authors, research advisers, subject matter experts, reviewers, scientists who notice examples of faulty data analysis, editors, journals and publishers, funding agencies, scientific societies, leaders at universities and research centers, and instrument vendors, can improve the current situation. This work provides specific recommendations for each of these stakeholders. For example, we believe that authors are primarily responsible for the correctness of their work, not reviewers or editors; we question the wisdom of listing the names of the editor and reviewers on a paper; we are grateful for the significant contributions that have been made by subject matter experts to produce standards and tutorial information; the high cost of instrument time at some institutions may limit student access and result in suboptimal analyses; staff scientists often need to be better recognized for their intellectual contributions to studies; publishers may wish to allow selective reviewing of specific sections of papers related to material characterization; the reviewing at some open access journals may be inadequate; while it had its shortcomings, the pre-open access model of publishing incentivized the production and publication of high-quality work; audits of the products (scientific papers) of funding agencies may be necessary; collaboration needs to be encouraged to a greater extent at some institutions; and instrument vendors should not suggest to potential customers that surface characterization, e.g., by XPS, is trivial or simple.
Carmeli I., Bounioux C. M., Mickel P., Richardson M. B., Templeman Y., Scofield J. M. P., Qiao G. G., Rosen B. A., Yusupov Y., Meshi L., Voelcker N. H., Diéguez O., Miloh T., Král P., Cohen H. & Richter S. E. (2023) Nature Communications. 14, 1, 2869
Biological and synthetic molecular motors, fueled by various physical and chemical means, can perform asymmetric linear and rotary motions that are inherently related to their asymmetric shapes. Here, we describe silver-organic micro-complexes of random shapes that exhibit macroscopic unidirectional rotation on water surface through the asymmetric release of cinchonine or cinchonidine chiral molecules from their crystallites asymmetrically adsorbed on the complex surfaces. Computational modeling indicates that the motor rotation is driven by a pH-controlled asymmetric jet-like Coulombic ejection of chiral molecules upon their protonation in water. The motor is capable of towing very large cargo, and its rotation can be accelerated by adding reducing agents to the water.
Song X., Cohen H., Yin J., Li H., Wang J., Yuan Y., Huang R., Cui Q., Ma C., Liu S. F., Hodes G. & Zhao K. (2023) Small. 19, 30, 2300892
Metal-free halide perovskites (MFHaPs) have garnered significant attention in recent years due to their desirable properties, such as low toxicity, light weight, chemical versatility, and potential for optoelectronics. MFHaPs with the formula A2+B+X−3 (where A is a large organic divalent cation, B+ is typically NH4+, and X is a halide) have been studied extensively, but few studies have examined alternative cations at the B position. This paper reports the synthesis of three MFHaP-related single crystals, DABCO-N2H5-X3 (DABCO = N-N-diazabicyclo[2.2.2]octonium, X = Br and I) and (DABCO)3-N2H5(NH4)2Cl9, which feature hydrazinium (N2H5) at the B position. The crystals have a perovskite-like, one-dimensional, edge-connected structure and exhibit optical and band structure properties. The crystals were then tested as X-ray detectors, where they showed excellent photoresponsivity, stability, and low background noise, owing to the large semi-gap that dictates long lifetimes. The detectors exhibited sensitivity as high as 1143 ± 10 µC Gyair−1 cm−2 and a low detection limit of 2.68 µGy s−1 at 10 V. The researchers suggest that the stronger hydrogen bonding in N2H5+ compounds compared to NH4+ MFHaPs may contribute to the detectors enhanced stability.
Singh A. K., Chakrabarti S., Vilan A., Smogunov A. & Tal O. (2023) Nano Letters. 23, 17, p. 7775-7781
Forming atomic-scale contacts with attractive geometries and material compositions is a long-term goal of nanotechnology. Here, we show that a rich family of bimetallic atomic-contacts can be fabricated in break-junction setups. The structure and material composition of these contacts can be controlled by atomically precise electromigration, where the metal types of the electron-injecting and sink electrodes determine the type of atoms added to, or subtracted from, the contact structure. The formed bimetallic structures include, for example, platinum and aluminum electrodes bridged by an atomic chain composed of platinum and aluminum atoms as well as iron-nickel single-atom contacts that act as a spin-valve break junction without the need for sophisticated spin-valve geometries. The versatile nature of atomic contacts in bimetallic junctions and the ability to control their structure by electromigration can be used to expand the structural variety of atomic and molecular junctions and their span of properties.
Toren Y., Vilan A. & Amdursky N. (2023) Small. 19, 26, 2301371
Carbon dots (CDs) are a new class of nanoparticles that gained widespread attention recently because of their easy preparation, water solubility, biocompatibility, and bright luminescence, leading to their integration in various applications. Despite their nm-scale and proven electron transfer capabilities, the solid-state electron transport (ETp) across single CDs was never explored. Here, a molecular junction configuration is used to explore the ETp across CDs as a function of their chemical structure using both DC-bias currentvoltage and AC-bias impedance measurements. CDs are used with Nitrogen and Sulfur as exogenous atoms and doped with small amounts of Boron and Phosphorous. It is shown that the presence of P and B highly improves the ETp efficiency across the CDs, yet without an indication of a change in the dominant charge carrier. Instead, structural characterizations reveal significant changes in the chemical species across the CDs: the formation of sulfonates and graphitic Nitrogen. Temperature-dependent measurements and normalized differential conductance analysis reveal that the ETp mechanism across the CDs behaves as tunneling, which is common to all CDs used here. The study shows that the conductivity of CDs is on par with that of sophisticated molecular wires, suggesting CDs as new green candidates for molecular electronics applications.
Guo C., Gavrilov Y., Gupta S., Bendikov T., Levy Y., Vilan A., Pecht I., Sheves M. & Cahen D. (2022) Physical Chemistry Chemical Physics. 24, 47, p. 28878-28885
A way of modulating the solid-state electron transport (ETp) properties of oligopeptide junctions is presented by charges and internal hydrogen bonding, which affect this process markedly. The ETp properties of a series of tyrosine (Tyr)-containing hexa-alanine peptides, self-assembled in monolayers and sandwiched between gold electrodes, are investigated in response to their protonation state. Inserting a Tyr residue into these peptides enhances the ETp carried
their junctions. Deprotonation of the Tyr-containing peptides causes a further increase of ETp efficiency that depends on this residue's position. Combined results of molecular dynamics simulations and spectroscopic experiments suggest that the increased conductance upon deprotonation is mainly a result of enhanced coupling between the charged C-terminus carboxylate group and the adjacent Au electrode. Moreover, intra-peptide hydrogen bonding of the Tyr hydroxyl to the C-terminus carboxylate reduces this coupling. Hence, the extent of such a conductance change depends on the Tyr-carboxylate distance in the peptide's sequence.
Itzhak A., Keller D., Bendikov T., Kama A., Millo O., Balberg I. & Cahen D. (2022) Journal of Physics D: Applied Physics. 55, 45, 455107
Most of the charge transport properties in halide perovskite (HaP) absorbers are measured by transient measurements with pulsed excitations; however, most solar cells in real life function in steady-state conditions. In contrast to working devices that include selective contacts, steady-state measurements need as high as possible photoconductivity (sigma ph), which is typically restricted to the absorber alone. In this paper, we enabled steady-state charge transport measurement using atomic layer deposition (ALD) to grow a conformal, ultra-thin (similar to 4 nm) ZnO electron transport layer that is laterally insulating due to its thickness. Due to the highly alkaline behavior of the ZnO surfaces, it readily reacts with halide Perovskites. ALD process was used to form an Aluminum oxynitride (AlON) thin (similar to 2 nm) layer that passivates the ZnO-HaP interface. We show that the presence of the AlON layer prevents HaP degradation caused by the interaction with the ZnO layer, improves the HaP sigma ph, and doubles the HaP carrier diffusion lengths.
Budniak A. K., Zelewski S. J., Birowska M., Woźniak T., Bendikov T., Kauffmann Y., Amouyal Y., Kudrawiec R. & Lifshitz E. (2022) Advanced Optical Materials. 10, 7, 2102489
Lamellar structures of transition metal phosphorus trisulfides possess strong intralayer bonding, albeit adjacent layers are held by weak van der Waals interactions. Those compounds received enormous interest due to their unique combination of optical and long-range magnetic properties. Among them, iron phosphorus trisulfide (FePS<sub>3</sub>) gathered special attention for being a semiconductor with an absorption edge in the near-infrared, as well as showing an Ising-like anti-ferromagnetism. A successful growth of centimeter size bulk FePS<sub>3</sub> crystals with a chemical yield above 70% is reported, whose crystallographic structure and composition are carefully identified by advanced electron microscopy methodologies, including atomic resolution elemental mapping, along with photoelectron spectroscopy. The knowledge on the optical activity of FePS<sub>3</sub> is extended utilizing temperature-dependent absorption and photoacoustic spectroscopies, while measurements are corroborated with density-functional theory calculations. Temperature-dependent experiments show a small and monotonic band-edge energy shift down to 115 K and expose the interconnected importance of electron-phonon coupling. Most of all, the correlation between the optical behavior and the magnetic phase transition is revealed, which shows the practical utilization of temperature-dependent optical absorption to investigate magnetic interactions.
Nasi H., Chiara di Gregorio M., Wen Q., Shimon L. J. W., Kaplan-Ashiri I., Bendikov T., Leitus G., Kazes M., Oron D., Lahav M. & van der Boom M. E. (2022) Angewandte Chemie - International Edition. 61, 34, e202205238
We show that metalorganic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal-to-crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese-based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy-dispersive X-ray spectroscopy on microtome-sectioned crystals. The observed cation exchange is in excellent agreement with the IrvingWilliams series (Mn
Zhai H., Bendikov T. & Gal A. (2022) Angewandte Chemie - International Edition. 61, 17, e202115930
In nature, simple organisms evolved mechanisms to form intricate biosilica nanostructures, far exceeding current synthetic manufacturing. Based on the properties of extracted biomacromolecules, polycation-polyanion pairs were suggested as moderators of biosilica formation. However, the chemical principles of this polymer-induced silicification remain unclear. Here, we used a biomimetic polycation-polyanion system to study polymer-induced silicification. We demonstrate that it is the polymer phase separation process, rather than silica-polymer interactions, which controls silica precipitation. Since ionic strength controls this electrostatic phase separation, it can be used to tune the morphology and structure of the precipitates. In situ cryo electron microscopy highlights the pivotal role of the hydrated polymer condensates in this process. These results pave the road for developing nanoscale morphologies of bioinspired silica based on the chemistry of liquid-liquid phase separation.
Malik N., Bendikov T., Lahav M. & van der Boom M. E. (2022) Energy Technology. 10, 2, 2100769
Herein, two different functionalities of one metallo-organic film assembled on the surface of a transparent metal-oxide electrode are described. Two redox-active elements, electrochromic ironpolypyridyl complexes and catalytically active palladium centers, operate by applying different potentials in aqueous solutions. The color of the material can be cycled 1500 times from dark purple to colorless by electrochemically addressing the Fe<sup>2+/3+</sup> centers at 0.01.0 V (vs Ag/Ag<sup>+</sup>). The differences between the transmittance of these two states is high: ΔT = 52%. Catalytic water oxidation can occur by palladium oxide particles that form in situ by applying a higher potential (1.222.0 V vs Ag/Ag<sup>+</sup>), resulting in the formation of hydrogen and oxygen. The catalyst is active for at least 7 h in an aqueous electrolyte at pH = 6.9, with a Faradaic efficiency of ≈70%.
Cohen A., Cohen H., Cohen S., Khodorov S., Feldman Y., Kossoy A., Kaplan-Ashiri I., Frenkel A., Wachtel E., Lubomirsky I. & Ehre D. (2022) Sensors (Basel, Switzerland). 22, 18, 7041
A protocol for successfully depositing [001] textured, 23 µm thick films of Al<sub>0.75</sub>Sc<sub>0.25</sub>N, is proposed. The procedure relies on the fact that sputtered Ti is [001]-textured (Formula presented.) -phase (hcp). Diffusion of nitrogen ions into the α-Ti film during reactive sputtering of Al<sub>0.75</sub>,Sc<sub>0.25</sub>N likely forms a [111]-oriented TiN intermediate layer. The lattice mismatch of this very thin film with Al<sub>0.75</sub>Sc<sub>0.25</sub>N is ~3.7%, providing excellent conditions for epitaxial growth. In contrast to earlier reports, the Al<sub>0.75</sub>Sc<sub>0.25</sub>N films prepared in the current study are Al-terminated. Low growth stress (
Suzuki V. Y., Amorin L. H. C., Fabris G. S. L., Dey S., Sambrano J. R., Cohen H., Oron D. & La Porta F. A. (2022) Catalysts. 12, 7, 692
Well-defined Zn2GeO4/g-C3N4 nanocomposites with a band alignment of type-I were prepared by the ultrasound-assisted solvent method, starting from g-C3N4 nanosheets and incorporating 0, 10, 20, and 40 wt% of Zn2GeO4. In this study, we have investigated in-depth the photoluminescence emission and photocatalytic activity of these nanocomposites. Our experimental results showed that an increased mass ratio of Zn2GeO4 to g-C3N4 can significantly improve their photoluminescence and photocatalytic responses. Additionally, we have noted that the broadband photoluminescence (PL) emission for these nanocomposites reveals three electronic transitions; the first two well-defined transitions (at ca. 450 nm and 488 nm) can be attributed to π*→ lone pair (LP) and π*→π transitions of g-C3N4, while the single shoulder at ca. 532 nm is due to the oxygen vacancy (Vo) as well as the hybridization of 4s and 4p orbital states in the Zn and Ge belonging to Zn2GeO4. These experimental findings are also supported by theoretical calculations performed under periodic conditions based on the density functional theory (DFT) fragment. The theoretical findings for these nanocomposites suggest a possible strain-induced increase in the Zn-O bond length, as well as a shortening of the Ge-O bond of both tetrahedral [ZnO4] and [GeO4] clusters, respectively. Thus, this disordered structure promotes local polarization and a charge gradient in the Zn2GeO4/g-C3N4 interface that enable an efficient separation and transfer of the photoexcited charges. Finally, theoretical results show a good correlation with our experimental data.
Sreedhara M. B., Bukvišová K., Khadiev A., Citterberg D., Cohen H., Balema V., K. Pathak A., Novikov D., Leitus G., Kaplan-Ashiri I., Kolíbal M., Enyashin A. N., Houben L. & Tenne R. (2022) Chemistry of Materials. 34, 4, p. 1838-1853
Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5d z 2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 104 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.
Gupta N. K., Karuppannan S. K., Pasula R. R., Vilan A., Martin J., Xu W., May E. M., Pike A. R., Astier H. P., Salim T., Lim S. & Nijhuis C. A. (2022) ACS Applied Materials and Interfaces. 14, 39, p. 44665-44675
Understanding the mechanisms of charge transport (CT) across biomolecules in solid-state devices is imperative to realize biomolecular electronic devices in a predictive manner. Although it is well-Accepted that biomolecule-electrode interactions play an essential role, it is often overlooked. This paper reveals the prominent role of graphene interfaces with Fe-storing proteins in the net CT across their tunnel junctions. Here, ferritin (AfFtn-AA) is adsorbed on the graphene by noncovalent amine-graphene interactions confirmed with Raman spectroscopy. In contrast to junctions with metal electrodes, graphene has a vanishing density of states toward its intrinsic Fermi level ("Dirac point"), which increases away from the Fermi level. Therefore, the amount of charge carriers is highly sensitive to temperature and electrostatic charging (induced doping), as deduced from a detailed analysis of CT as a function of temperature and iron loading. Remarkably, the temperature dependence can be fully explained within the coherent tunneling regime due to excitation of hot carriers. Graphene is not only demonstrated as an alternative platform to study CT across biomolecular tunnel junctions, but it also opens rich possibilities in employing interface electrostatics in tuning CT behavior.
Chakrabarti S., Vilan A., Deutch G., Oz A., Hod O., Peralta J. E. & Tal O. (2022) Nature Communications. 13, 4113
When reducing the size of materials towards the nanoscale, magnetic properties can emerge due to structural variations. Here, we show the reverse effect, where the structure of nanomaterials is controlled by magnetic manipulations. Using the break-junction technique, we find that the interatomic distance in platinum atomic wires is shorter or longer by up to ∼20%, when a magnetic field is applied parallel or perpendicular to the wires during their formation, respectively. The magnetic field direction also affects the wire length, where longer (shorter) wires are formed under a parallel (perpendicular) field. Our experimental analysis, supported by calculations, indicates that the direction of the applied magnetic field promotes the formation of suspended atomic wires with a specific magnetization orientation associated with typical orbital characteristics, interatomic distance, and stability. A similar effect is found for various metal and metal-oxide atomic wires, demonstrating that magnetic fields can control the atomistic structure of different nanomaterials when applied during their formation stage.
Santra K., Bhowmick D., Zhu Q., Bendikov T. & Naaman R. (2021) Journal of Physical Chemistry C. 125, 31, p. 17530-17536
We present a method for enantioselective separation of a racemic mixture of thiolated molecules by applying ferromagnetic surfaces. Depending on the direction of the magnetization on the surface, up or down relative to the surface normal, one enantiomer is adsorbed and the other one is extracted. After an electric field is applied, the adsorbed enantiomer is released and collected. The same ferromagnetic surfaces can be used, in principle, for all thiolated chiral molecules; the method operates at low pressure and allows both enantiomers to be retrieved. Because the thiol group can be easily attached to many molecules, this method can be widely applied.
Ceratti D. R., Cohen A. V., Tenne R., Rakita Y., Snarski L., Jasti N. P., Cremonesi L., Cohen R., Weitman M., Rosenhek-Goldian I., Kaplan-Ashiri I., Bendikov T., Kalchenko V., Elbaum M., Potenza M. A. C., Kronik L., Hodes G. & Cahen D. (2021) Materials Horizons. 8, 5, p. 1570-1586
We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.
Modak A., Mohan R., Rajavelu K., Cahan R., Bendikov T. & Schechter A. (2021) ACS Applied Materials and Interfaces. 13, 7, p. 8461-8473
The electrochemical urea oxidation reaction (UOR) is considered as a promising renewable source for harvesting energy from waste. We report a new synthetic design approach to produce an ironnickel alloy nanocatalyst from a metalorganic polymer (MOP) by a single-step carbonization process at 500 °C, thus forming a coreshell of ironnickel-coated carbon (C@FeNi) nanostructures wired by embedded carbon nanotubes (CNTs) (CNT/C@FeNi). Powder X-ray diffraction confirmed the formation of metallic FeNi3 alloy nanoparticles (∼20 to 28 nm). Our experimental results showed that MOP containing CNTs acquired an interconnected hierarchical topology, which prevented the collapse of its microstructure during pyrolysis. Hence, CNT/C@FeNi shows higher porosity (10 times) than C@FeNi. The electrochemical UOR in alkaline electrolytes on these catalysts was studied using cyclic voltammetry (CV). The result showed a higher anodic current (3.5 mA cm2) for CNT/C@FeNi than for C@FeNi (1.1 mA cm2) at 1.5 V/RHE. CNT/C@FeNi displayed good stability in chronoamperometry experiments and a lower Tafel slope (33 mV dec1) than C@FeNi (41.1 mV dec1). In this study, CNT/C@FeNi exhibits higher exchange current density (3.2 μA cm2) than does C@FeNi (2 μA cm2). The reaction rate orders of CNT/C@FeNi and C@FeNi at a kinetically controlled potential of 1.4 V/RHE were 0.5 and 0.9, respectively, higher than the 0.26 of β-Ni(OH)2, Ni/Ni(OH)2 electrodes. The electrochemical impedance result showed a lower charge-transfer resistance for CNT/C@FeNi (61 Ω·cm2) than for C@FeNi (162 Ω·cm2), due to faster oxidation kinetics associated with the CNT linkage. Moreover, CNT/C@FeNi exhibited a lower Tafel slope and resistance and higher heterogeneity (25.2 × 105 cm s1), as well as relatively high faradic efficiency (68.4%) compared to C@FeNi (56%). Thus, the carbon-coated FeNi3 core connected by CNT facilitates lower charge-transfer resistance and reduces the UOR overpotential.
Rahamim M., Cohen H. & Edri E. (2021) ACS Applied Materials and Interfaces. 13, 41, p. 49423-49432
Zinc oxide, a wide-band-gap semiconductor, shows intriguing optoelectronic properties when coupled with Ag. Specifically, an absorbance band in the visible range that is not apparent in the separated materials emerges when the interface is formed. Interestingly, photoexcitation of this "interface band"or band-to-band results in a counterintuitive photovoltaic response when a supra/sub-band-gap light is shone. To investigate the origin of this absorbance band and photovoltaic response, we studied in detail the energy-band alignment of ultrathin layers of ZnO (3-60 nm) with Ag. Our analysis indicated that an electrostatic potential cliff' is formed within the first 1-2 nm of ZnO. In addition, oxygen vacancies, presumably generated by AgxO-Zn bonds, form mid-gap acceptor states within these first few nm. Both effects facilitate a valence band-to-defect state optical transition that is confined to the interface region. The second type of defects - hole-trap states associated with zinc hydroxide - are spread throughout the ZnO layer and dominate the supra-band-gap photovoltaic response. These findings have potential implications in emerging technologies such as photocatalytic Ag/ZnO heterostructures that will utilize the long-lived charges for chemical work or other optoelectronic applications.
Yu H., Cohen H. & Neumann R. (2021) Chemistry : a European journal. 27, 54, p. 13513-13517
Research on the photoreduction of CO<sub>2</sub> often has been dominated by the use of sacrificial reducing agents. A pathway that avoids this problem would be the development of photocathodes for CO<sub>2</sub> reduction that could then be coupled to a photoanodic oxygen evolution reaction. Here, we present the use of copper-substituted graphitic carbon nitride (Cu−CN) on a fluorinated tin oxide (FTO) electrode for the photoelectrochemical two-electron reduction of CO<sub>2</sub> to CO as a major product (>95 %) and formic acid (
Aggarwal N., Eliaz D., Cohen H., Rosenhek-Goldian I., Cohen S. R., Kozell A., Mason T. O. & Shimanovich U. (2021) Communications Chemistry. 4, 1, 62
The process of amyloid nanofibril formation has broad implications including the generation of the strongest natural materials, namely silk fibers, and their major contribution to the progression of many degenerative diseases. The key question that remains unanswered is whether the amyloidogenic nature, which includes the characteristic H-bonded β-sheet structure and physical characteristics of protein assemblies, can be modified via controlled intervention of the molecular interactions. Here we show that tailored changes in molecular interactions, specifically in the H-bonded network, do not affect the nature of amyloidogenic fibrillation, and even have minimal effect on the initial nucleation events of self-assembly. However, they do trigger changes in networks at a higher hierarchical level, namely enhanced 2D packaging which is rationalized by the 3D hierarchy of β-sheet assembly, leading to variations in fibril morphology, structural composition and, remarkably, nanomechanical properties. These results pave the way to a better understanding of the role of molecular interactions in sculpting the structural and physical properties of protein supramolecular constructs.
Adler S., Motiei L., Mankovski N., Cohen H. & Margulies D. (2021) Israel Journal of Chemistry. 61, 3-4, p. 239-243
A method for decorating bacteria with synthetic receptors while they are attached to a solid support is described. This approach involves two steps. First, E. coli expressing a His-tagged outer membrane protein C (His-OmpC) are attached to a monolayer of DNA duplexes appended with a tri-nitrilotriacetic acid (tri-NTA) group. The remaining His-tags, which are not involved in the interaction with the monolayer, can then be modified with additional tri-NTA-appended duplexes that bear different functionalities. By labelling the surface-bound bacteria with an oligodeoxynucleotide (ODN) appended with a fluorescent dye and by turning off its emission using a complementary, quencher-bearing strand, we demonstrate the versatility of this approach, as well as a means to modify cell membrane proteins on a solid support using simple self-assembly processes.
Gupta N. K., Wilkinson E. A., Karuppannan S. K., Bailey L., Vilan A., Zhang Z., Qi D., Tadich A., Tuite E. M., Pike A. R., Tucker J. H. R. & Nijhuis C. A. (2021) Journal of the American Chemical Society. 143, 48, p. 20309-20319
Deoxyribonucleic acid (DNA) has been hypothesized to act as a molecular wire due to the presence of an extended π-stack between base pairs, but the factors that are detrimental in the mechanism of charge transport (CT) across tunnel junctions with DNA are still unclear. Here we systematically investigate CT across dense DNA monolayers in large-area biomolecular tunnel junctions to determine when intrachain or interchain CT dominates and under which conditions the mechanism of CT becomes thermally activated. In our junctions, double-stranded DNA (dsDNA) is 30-fold more conductive than single-stranded DNA (ssDNA). The main reason for this large change in conductivity is that dsDNA forms ordered monolayers where intrachain tunneling dominates, resulting in high CT rates. By varying the temperature T and the length of the DNA fragments in the junctions, which determines the tunneling distance, we reveal a complex interplay between T, the length of DNA, and structural order on the mechanism of charge transport. Both the increase in the tunneling distance and the decrease in structural order result in a change in the mechanism of CT from coherent tunneling to incoherent tunneling (hopping). Our results highlight the importance of the interplay between structural order, tunneling distance, and temperature on the CT mechanism across DNA in molecular junctions.
Yelin T., Chakrabarti S., Vilan A. & Tal O. (2021) Nanoscale. 13, 44, p. 18434-18440
In the field of molecular electronics, the interplay between molecular orientation and the resulting electronic transport is of central interest. At the single molecule level, this topic is extensively studied with the aid of break junction setups. In such experiments, two metal electrodes are brought into contact, and the conductance is typically measured when the electrodes are pulled apart in the presence of molecules, until a molecule bridges the two electrodes. However, the molecular junctions formed in this pull process reflect only part of the rich possible junction configurations. Here, we show that the push process, in which molecular junctions are formed by bringing the electrodes towards each other, allows the fabrication of molecular junction structures that are not necessarily formed in the pull process. We also find that in the extreme case, molecular junctions can be formed only in the push process that is typically ignored. Our findings demonstrate that tracking the two inverse processes of molecular junction formation, reveals a more comprehensive picture of the variety of molecular configurations in molecular junctions.
Gupta N. K., Schultz T., Karuppannan S. K., Vilan A., Koch N. & Nijhuis C. A. (2021) Physical Chemistry Chemical Physics. 23, 24, p. 13458-13467
The energy level alignment after the formation of a molecular tunnel junction is often poorly understood because spectroscopy inside junctions is not possible, which hampers the rational design of functional molecular junctions and complicates the interpretation of the data generated by molecular junctions. In molecular junction platforms where the top electrode-molecule interaction is weak; one may argue that the energy level alignment can be deduced from measurements with the molecules supported by the bottom electrode (sometimes referred to as \u201chalf junctions\u201d). This approach, however, still relies on a series of assumptions, which are challenging to address experimentally due to difficulties in studying the molecule-top electrode interaction. Herein, we describe top electrode-molecule junctions with a liquid metal alloy top electrode of EGaIn (which stands for eutectic alloy of Ga and In) interacting with well-characterised ferrocene (Fc) moieties. We deposited a ferrocene derivative on films of EGaIn, coated with its native GaO<sub>x</sub>layer, and studied the energy level alignment with photoelectron spectroscopy. Our results reveal that the electronic interaction between the Fc and GaO<sub>x</sub>/EGaIn is very weak, resembling physisorption. Therefore, investigations of \u201chalf junctions\u201d for this system can provide valuable information regarding the energy level alignment of complete EGaIn junctions. Our results help to improve our understanding of the energy landscape in weakly coupled molecular junctions and aid to the rational design of molecular electronic devices.
Shalom H., Bendikov T., Feldman Y., Lachman N., Zak A. & Tenne R. (2020) Chemical Physics Letters. 761, 138052
Tungsten disulfide is an inorganic compound with layer structure. Nanoparticles of WS<sub>2</sub> were found to be excellent candidates for reinforcing various polymer materials. However, long storage times in ambient conditions lead to sorption of water molecules and slow oxidation of defect sites. In the present study, the influence of various treatments on the surface composition and reactivity of several kinds of WS<sub>2</sub> nanoparticles, were investigated. This study reveals some complex interplay between water and organic moieties desorption and the subsequent oxidation of \u201cnascant\u201d surfaces. Remarkably, simple immersion in HF solution seems to produce the ultimately cleanest surface of these particles.
Fereiro J. A., Bendikov T., Pecht I., Sheves M. & Cahen D. (2020) Journal of the American Chemical Society. 142, 45, p. 19217-19225
We observe reversible, bias-induced switching of conductance via a blue copper protein azurin mutant, N42C Az, with a nearly 10-fold increase at |V| > 0.8 V than at lower bias. No such switching is found for wild-type azurin, WT Az, up to |1.2 V|, beyond which irreversible changes occur. The N42C Az mutant will, when positioned between electrodes in a solid-state AuproteinAu junction, have an orientation opposite that of WT Az with respect to the electrodes. Current(s) via both proteins are temperature-independent, consistent with quantum mechanical tunneling as dominant transport mechanism. No noticeable difference is resolved between the two proteins in conductance and inelastic electron tunneling spectra at
Hartmann C., Gupta S., Bendikov T., Kozina X., Kunze T., Felix R., Hodes G., Wilks R. G., Cahen D. & Bar M. (2020) ACS Applied Materials and Interfaces. 12, 10, p. 12353-12361
We report on the chemical and electronic structure of cesium tin bromide (CsSiiBr(3)) and how it is impacted by the addition of 20 mol % tin fluoride (SnF2) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF2 improves coverage of the titanium dioxide substrate by CsSnBr3 and decreases the oxidation of Sn-II to Sn-IV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF2-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn-II/Sn-IV ratio. Notwithstanding some variability in the data, we conclude that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level, which we explain by defect chemistry considerations.
Gerlitz I., Fiegenbaum-Raz M., Bar-Sadan M., Cohen H., Ismach A. & Rosen B. A. (2020) ChemElectroChem. 7, 20, p. 4224-4232
Hybrid electrodes containing MoS<sub>2</sub> with metals active for the hydrogen evolution reaction (HER) are highly desirable as they can provide a cost-effective and active route for producing hydrogen. It is well known that the technique used to facilitate such hybrid formation plays a critical role in the activity and stability of the electrode. Here we utilized a novel method for hybridization based on electron-beam deposition of Ag, Pd, and Mo to make precursor films which, upon sulfurization, formed hybrid electrodes based on vertically aligned molybdenum disulfide (VA-MoS<sub>2</sub>). We further modified the surface of these electrodes by exploiting galvanic replacement of palladium. Surprisingly, both hybridization techniques showed a synergistic effect between Pd and Ag, and enhanced activity was enabled by the presence of both metals. XPS, XRD, Raman spectroscopy, and FIB-EM were used to explore the root cause of the synergy. It appears that enhanced performance is based partially on the mutual solubility of Pd−Ag and the facile diffusion of Ag through the MoS<sub>2</sub> film. Ag/VA-MoS<sub>2</sub> modified by galvanic replacement provided 10 mA/cm<sup>2</sup> at 210 mV of overpotential and was stable for up to 4000 cycles, surpassing the stability of previously reported Pd/MoS<sub>2</sub> hybrids, making these techniques of interest for future generations of hybrid HER electrodes based on MoS<sub>2</sub>.
Song X., Cui Q., Liu Y., Xu Z., Cohen H., Ma C., Fan Y., Zhang Y., Ye H., Peng Z., Li R., Chen Y., Wang J., Sun H., Yang Z., Liu Z., Yang Z., Huang W., Hodes G., Liu S. (. & Zhao K. (2020) Advanced Materials. 32, 42, 2003353
Metal-free halide perovskites, as a specific category of the perovskite family, have recently emerged as novel semiconductors for organic ferroelectrics and promise the wide chemical diversity of the ABX(3)perovskite structure with mechanical flexibility, light weight, and eco-friendly processing. However, after the initial discovery 17 years ago, there has been no experimental information about their charge transport properties and only one brief mention of their optoelectronic properties. Here, growth of large single crystals of metal-free halide perovskite DABCO-NH4Br3(DABCO =N-N '-diazabicyclo[2.2.2]octonium) is reported together with characterization of their instrinsic optical and electronic properties and demonstration, of metal-free halide perovskite optoelectronics. The results reveal that the crystals have an unusually large semigap of approximate to 16 eV and a specific band nature with the valence band maximum and the conduction band minimum mainly dominated by the halide and DABCO(2+), respectively. The unusually large semigap rationalizes extremely long lifetimes approaching the millisecond regime, leading to very high charge diffusion lengths (tens of mu m). The crystals also exhibit high X-ray attenuation as well as being lightweight. All these properties translate to high-performance X-ray imaging with sensitivity up to 173 mu C Gy(air)(-1)cm(-2). This makes metal-free perovskites novel candidates for the next generation of optoelectronics.
Haviv E., Chen B., Carmielli R., Houben L., Cohen H., Leitus G., Avram L. & Neumann R. (2020) Journal of the American Chemical Society. 142, 34, p. 14504-14512
Host-guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate type molybdenum oxide based porous, spherical clus-ters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here we demonstrate the synthetically controlled encapsula-tion of first row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate ani-ons, {Mo132PO4}. The resulting M2+x{Mo132PO4} host-guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encap-sulation of up to 10 equivalents showed little magnetic interaction between the encapsulated metals, indicating that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal was possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photo-initiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.
Baer D. R., Artyushkova K., Cohen H., Easton C. D., Engelhard M., Gengenbach T. R., Greczynski G., Mack P., Morgan D. J. & Roberts A. (2020) Journal of Vacuum Science and Technology A. 38, 3, 031204
This guide deals with methods to control surface charging during XPS analysis of insulating samples and approaches to extracting useful binding energy information. The guide summarizes the causes of surface charging, how to recognize when it occurs, approaches to minimize charge buildup, and methods used to adjust or correct XPS photoelectron binding energies when charge control systems are used. There are multiple ways to control surface charge buildup during XPS measurements, and examples of systems on advanced XPS instruments are described. There is no single, simple, and foolproof way to extract binding energies on insulating material, but advantages and limitations of several approaches are described. Because of the variety of approaches and limitations of each, it is critical for researchers to accurately describe the procedures that have been applied in research reports and publications.
Teich J., Dvir R., Henning A., Hamo E. R., Moody M. J., Jurca T., Cohen H., Marks T. J., Rosen B. A., Lauhon L. J. & Ismach A. (2020) Nanoscale. 12, 4, p. 2715-2725
Multi-component 3D porous structures are highly promising hierarchical materials for numerous applications. Herein we show that atomic-layer deposition (ALD) of MoS
<sub>2</sub> on graphene foams with variable pore size is a promising methodology to prepare complex 3D heterostructures to be used as electrocatalysts for the hydrogen evolution reaction (HER). The effect of MoS
<sub>2</sub> crystallinity is studied and a trade-off between the high density of defects naturally presented in amorphous MoS
<sub>2</sub> coatings and the highly crystalline phase obtained after annealing at 800 °C is established. Specifically, an optimal annealing at 500 °C is shown to yield improved catalytic performance with an overpotential of 180 mV, a low Tafel slope of 47 mV dec
<sup>-1</sup>, and a high exchange current of 17 μA cm
<sup>-2</sup>. The ALD deposition is highly conformal, and thus advantageous when coating 3D porous structures with small pore sizes, as required for real-world applications. This approach is enabled by conformal thin film deposition on porous structures with controlled crystallinity by tuning the annealing temperature. The results presented here therefore serve as an effective and general platform for the design of chemically and structurally tunable, binder-free, complex, lightweight, and highly efficient 3D porous heterostructures to be used for catalysis, energy storage, composite materials, sensors, water treatment, and more.
Mukhopadhyay S., Karuppannan S. K., Guo C., Fereiro J. A., Bergren A., Mukundan V., Qiu X., Ocampo O. E. C., Chen X., Chiechi R. C., McCreery R., Pecht I., Sheves M., Pasula R. R., Lim S., Nijhuis C. A., Vilan A. & Cahen D. (2020) iScience. 23, 5, 101099
Successful integration of proteins in solid-state electronics requires contacting them in a non-invasive fashion, with a solid conducting surface for immobilization as one such contact. The contacts can affect and even dominate the measured electronic transport. Often substrates, substrate treatments, protein immobilization, and device geometries differ between laboratories. Thus the question arises how far results from different laboratories and platforms are comparable and how to distinguish genuine protein electronic transport properties from platform-induced ones. We report a systematic comparison of electronic transport measurements between different laboratories, using all commonly used large-area schemes to contact a set of three proteins of largely different types. Altogether we study eight different combinations of molecular junction configurations, designed so that A(geo) of junctions varies from 10(5) to 10(-3) mu m(2). Although for the same protein, measured with similar device geometry, results compare reasonably well, there are significant differences in current densities (an intensive variable) between different device geometries. Likely, these originate in the critical contact-protein coupling (similar to contact resistance), in addition to the actual number of proteins involved, because the effective junction contact area depends on the nanometric roughness of the electrodes and at times, even the proteins may increase this roughness. On the positive side, our results show that understanding what controls the coupling can make the coupling a design knob. In terms of extensive variables, such as temperature, our comparison unanimously shows the transport to be independent of temperature for all studied configurations and proteins. Our study places coupling and lack of temperature activation as key aspects to be considered in both modeling and practice of protein electronic transport experiments.
Karuppannan S. K., Neoh E. H. L., Vilan A. & Nijhuis C. A. (2020) Journal of the American Chemical Society. 142, 7, p. 3513-3524
A major obstacle for transforming large-area molecular junctions into a viable technology is the deposition of a top, metallic contact over the self-assembled monolayer (SAM) without chemically damaging the molecules and preventing an interface-limited charge transport. Often a thin conducting layer is softly deposited over the SAM to protect it during the deposition of the metal electrode which requires conditions under which organic molecules are not stable. We report a new protective layer based on carbon paint which is highly conductive and has metallic- like behavior. Junctions made of SAMs of n-alkanethiolates supported by Au were characterized with both dc and ac techniques, revealing that carbon paint protective layers provide a solution to three well-known challenges in molecular junctions: series resistance of the leads, poor interface conductance, and low effective contact area related to the roughness of the interfaces. Transport is constant with coherent tunneling down to 10 K, indicating the carbon paint does not add spurious thermally activated components. The junctions have both high reproducibility and good stability against bias stressing. Finally, normalized differential conductance analysis of the tunneling characteristics of the junctions as a function of molecular length reveals that the scaling voltage changes with molecular length, indicating a significant voltage drop on the molecules rather than on the molecule-electrode interface. There is a clear inverse dependence of the scaling voltage on length, which we deduced has a tunneling barrier height of close to 2 eV. The paper establishes the reliability of carbon paint protective layers and provides a procedure for discriminating genuine molecular effects from interfacial contributions.
Vilan A. (2020) Nature Materials. 19, 3, p. 262-263
Fullerene-triethylene glycol monomers are shown to form self-assembled bilayers on Au with an exchangeable upper layer. These structures are as versatile as thiol-based self-assembled monolayers while showing enhanced robustness.
Kayser B., Fereiro J. A., Bhattacharyya R., Cohen S. R., Vilan A., Pecht I., Sheves M. & Cahen D. (2020) Journal of Physical Chemistry Letters. 11, 1, p. 144-151
Solid-state electronic transport (ETp) via the electron-transfer copper protein azurin (Az) was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help when observing fine features of electronic transport, but it also limits possible electron transport mechanisms. Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in conducting-probe atomic force microscopy measurements, served, between room temperature and the protein's denaturation temperature (similar to 323 K), to check that conductance behavior is independent of device configuration or contact nature and thus is a property of the protein itself. Temperature-independent currents were observed from similar to 320 to 4 K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values and to compare data sets. Our results strongly support that quantum tunneling, rather than hopping, dominates ETp via Az.
Muench F., Solomonov A., Bendikov T., Molina-Luna L., Rubinstein I. & Vaskevich A. (2019) Applied Bio Materials. 2, 2, p. 856-864
To facilitate the implementation of biosensors based on the localized surface plasmon resonance (LSPR) of metal nanostructures, there is a great need for cost-efficient, flexible, and tunable methods for producing plasmonic coatings. Due to its simplicity and excellent conformity, electroless plating (EP) is well suited for this task. However, it is traditionally optimized to produce continuous metal films, which cannot be employed in LSPR sensors. Here, we outline the development of an EP strategy for depositing island-like silver nanoparticle (NP) films on glass with distinct LSPR bands. The fully wet-chemical process only employs standard chemicals and proceeds within minutes at room temperature. The key step for producing spread-out NP films is an accelerated ripening of the silver seed layer in diluted hydrochloric acid, which reduces the nucleation density during plating. The reaction kinetics and mechanisms are investigated with scanning (transmission) electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and UVvis spectroscopy, with the latter enabling a convenient live monitoring of the deposition, allowing its termination at a stage of desired optical properties. The sensing capabilities of chemically deposited NP films as LSPR transducers are exemplified in DNA biosensing. To this end, a sensing interface is prepared using layer-by-layer (LbL) buildup of polyelectrolytes (PE), followed by adsorption and covalent immobilization of ssDNA. The obtained LSPR transducers demonstrate robustness and selectivity in sensing experiments with binding complementary and unrelated DNA strands.
Zhang Q., Cohen S. R., Rosenhek-Goldian I., Amgar D., Bar-Elli O., Tsarfati Y., Bendikov T., Shimon L. J. W., Feldman Y., Iron M. A., Weissman H., Levine I., Oron D. & Rybtchinski B. (2019) Journal of Physical Chemistry C. 123, 41, p. 25031-25041
Organic photovoltaics enable cost-efficient, tunable, and flexible platforms for solar energy conversion, yet their performance and stability are still far from optimal. Here, we present a study of photoinduced charge transfer processes between electron donor and acceptor organic nanocrystals as part of a pathfinding effort to develop robust and efficient organic nanocrystalline materials for photovoltaic applications. For this purpose, we utilized nanocrystals of perylenediimides as the electron acceptors and nanocrystalline copper phthalocyanine as the electron donor. Three different configurations of donor-acceptor heterojunctions were prepared. Charge transfer in the heterojunctions was studied with Kelvin probe force microscopy under laser or white light excitation. Moreover, detailed morphology characterizations and time-resolved photoluminescence measurements were conducted to understand the differences in the photovoltaic processes of these organic nanocrystals. Our work demonstrates that excitonic properties can be tuned by controlling the crystal and interface structures in the nanocrystalline heterojunctions, leading to a minimization of photovoltaic losses.
Dey S., Cohen H., Pinkas I., Lin H., Kazes M. & Oron D. (2019) Journal of Chemical Physics. 151, 17, 174704
Formation of a p-n junction-like with a large built-in field is demonstrated at the nanoscale, using two types of semiconducting nanoparticles, CsPbBr<sub>3</sub> nanocrystals and CdSe nanoplatelets, capped with molecular linkers. By exploiting chemical recognition of the capping molecules, the two types of nanoparticles are brought into mutual contact, thus initiating spontaneous charge transfer and the formation of a strong junction field. Depending on the choice of capping molecules, the magnitude of the latter field is shown to vary in a broad range, corresponding to an interface potential step as large as ∼1 eV. The band diagram of the system as well as the emergence of photoinduced charge transfer processes across the interface is studied here by means of optical and photoelectron based spectroscopies. Our results propose an interesting template for generating and harnessing internal built-in fields in heterogeneous nanocrystal solids.
Meirzadeh E., Christensen D., Makagon E., Cohen H., Rosenhek-Goldian I., Morales E. H., Bhowmik A., Lastra J. M. G., Rappe A. M., Ehre D., Lahav M., Pryds N. & Lubomirsky I. (2019) Advanced Materials. 31, 44, 1904733
Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2-terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.
Alpern H., Yavilberg K., Dvir T., Sukenik N., Klang M., Yochelis S., Cohen H., Grosfeld E., Steinberg H., Paltiel Y. & Millo O. (2019) Nano Letters. 19, 8, p. 5167-5175
Hybrid ferromagnetic/superconducting systems are well-known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin-polarized Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities on a superconducting surface. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor by chemisorbing nonmagnetic chiral molecules. Conductance spectra measured on NbSe2 flakes over which chiral alpha-helix polyalanine molecules were adsorbed exhibit, in some cases, in gap states nearly symmetrically positioned around zero bias that shift with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional, possibly triplet superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance that diminishes, but does not split, with magnetic field. The transition between these two scenarios appears to be governed by the density of adsorbed molecules.
Pal A. N., Li D., Sarkar S., Chakrabarti S., Vilan A., Kronik L., Smogunov A. & Tal O. (2019) Nature Communications. 10, 1, 5565
Key spin transport phenomena, including magnetoresistance and spin transfer torque, cannot be activated without spin-polarized currents, in which one electron spin is dominant. At the nanoscale, the relevant length-scale for modern spintronics, spin current generation is rather limited due to unwanted contributions from poorly spin-polarized frontier states in ferromagnetic electrodes, or too short length-scales for efficient spin splitting by spin-orbit interaction and magnetic fields. Here, we show that spin-polarized currents can be generated in silver-vanadocene-silver single molecule junctions without magnetic components or magnetic fields. In some cases, the measured spin currents approach the limit of ideal ballistic spin transport. Comparison between conductance and shot-noise measurements to detailed calculations reveals a mechanism based on spin-dependent quantum interference that yields very efficient spin filtering. Our findings pave the way for nanoscale spintronics based on quantum interference, with the advantages of low sensitivity to decoherence effects and the freedom to use non-magnetic materials.
Fereiro J. A., Kayser B., Romero-Muniz C., Vilan A., Dolgikh D. A., Chertkova R., Cuevas J. C., Zotti L. A., Pecht I., Sheves M. & Cahen D. (2019) Angewandte Chemie - International Edition. 58, 34, p. 11852-11859
A sample-type protein monolayer, that can be a stepping stone to practical devices, can behave as an electrically driven switch. This feat is achieved using a redox protein, cytochrome C (CytC), with its heme shielded from direct contact with the solid-state electrodes. Ab initio DFT calculations, carried out on the CytC-Au structure, show that the coupling of the heme, the origin of the protein frontier orbitals, to the electrodes is sufficiently weak to prevent Fermi level pinning. Thus, external bias can bring these orbitals in and out of resonance with the electrode. Using a cytochrome C mutant for direct S-Au bonding, approximately 80 % of the Au-CytC-Au junctions show at greater than 0.5 V bias a clear conductance peak, consistent with resonant tunneling. The on-off change persists up to room temperature, demonstrating reversible, bias-controlled switching of a protein ensemble, which, with its built-in redundancy, provides a realistic path to protein-based bioelectronics.
Karuppannan S. K., Hu Hongting, Troadec C., Vilan A. & Nijhuis C. A. (2019) Advanced Functional Materials. 29, 38, 1904452
In molecular electronics, it is critical to minimize the sources that can result in defective electrodes, such as contaminations related to the fabrication process (photoresist and organic residues) or roughening of the electrode during etching, because these defects hamper the formation of well-organized molecular structures. Junctions based on micropores are desirable as they are scalable, but micropores are not fabricated on ultrasmooth template-stripped electrodes, and may suffer from stray capacitances and leakage currents across the insulating matrix. A method is reported to fabricate micropores in AlOx on template-stripped Au based on a two-step etch process so that the Au surface is not in direct contact with photoresistance during the fabrication process. These junctions do not suffer from stray capacitances or leakage currents, enable temperature variable measurements down to 8.5 K, have excellent current retention characteristics, and are stable for at least 2 months. By analyzing the normalized differential conductance curves and detailed comparison against junctions with cone-shaped tips of EGaIn and EGaIn stabilized in a through-hole in polydimethylsiloxane, how the surface roughness of top electrodes affects the effective contact area, influences the symmetry of the response of the junctions, and how the electrical characteristics scale with molecular length are established.
Popov I., Moshkovich A., Bendikov T. & Rapoport L. (2018) Tribology Letters. 66, 3, 78
Friction and wear are accompanied by severe plastic deformation of surface layers similar to that observed under different processes of grain refinement. Recently, a dependence of tribological properties for four fcc metals (Ag, Cu, Ni, and Al) under friction in boundary lubrication was reported. A connection between friction and wear properties and stacking fault energy (SFE) of the studied metals was considered. Steady-state friction is mainly determined by deformation microstructure and chemical interaction in thin surface layers. The objectives of this work are to study the evolution of the microstructure with the depth of plastic deformation after friction of Al and Cu, to evaluate the effect of SFE on microstructure and wear loss, and to analyze the chemical composition of thin surface layers of Cu and Al and their correlation with the microstructure and wear properties of the studied metals. Friction surfaces were examined with a field emission scanning electron microscope. The cross-sectional TEM lamellae were prepared using focused ion beam. Evolution of deformation microstructure and chemistry of surface layers of Cu and Al after friction in the lubricant conditions were estimated by STEM, TEM, and XPS analyses.
Elianov O., Garusi S., Rosentsveig R., Cohen S. R., Feldman Y., Pinkas B., Bendikov T., Kaplan-Ashiri, Moshkovich A., Perfilyev V., Rapoport L., Moshonov J., Tenne R. & Shay B. (2018) Surface & Coatings Technology. 353, p. 116-125
Self-lubricating films are of immense importance in various tribological applications. Nanoparticles are often used as the lubricating component in such films. Inorganic fullerene-like (IF) nanoparticles of WS2 have been used in the past for variety of tribological applications including for self-lubricating polymer and metallic films. IF nanoparticles from MoS2 with superior tribological behavior were reported in the past. However, such nanoparticles, which are available in minute amounts, were not applied for self-lubricating coatings in the past. In the current work, inorganic fullerene-like nanoparticles of MoS2 (IF-MoS2) were co-evaporated with titanium (cobalt) on metal substrates. The films were characterized by different techniques and were shown to have good adhesion to the underlying substrate. Furthermore, tribological tests indicated that such coatings exhibit improved friction coefficient (roughly 0.1) and very small wear under relatively high load.
Muench F., Popovitz-Biro R., Bendikov T., Feldman Y., Hecker B., Oezaslan M., Rubinstein I. & Vaskevich A. (2018) Advanced Materials. 30, 51, 1805179
Due to their distinctive electronic, optical, and chemical properties, metal nanoplates represent important building blocks for creating functional superstructures. Here, a general deposition method for synthesizing Ag nanoplate architectures, which is compatible with a wide substrate range (flexible, curved, or recessed; consisting of carbon, silicon, metals, oxides, or polymers) is reported. By adjusting the reaction conditions, nucleation can be triggered in the bulk solution, on seeds and by electrodeposition, allowing the production of nanoplate suspensions as well as direct surface modification with open-porous nanoplate films. The latter are fully percolated, possess a large, easily accessible surface, a defined nanostructure with {111} basal planes, and expose defect-rich, particularly reactive edges in high density, making them compelling platforms for heterogeneous catalysis, and electro- and flow chemistry. This potential is showcased by exploring the catalytic performance of the nanoplates in the reduction of carbon dioxide, 4-nitrophenol, and hydrogen peroxide, devising two types of microreactors, and by tuning the nanoplate functionality with derivatization reactions.
Garg K., Raichlin S., Bendikov T., Pecht I., Sheves M. & Cahen D. (2018) ACS Applied Materials and Interfaces. 10, 48, p. 41599-41607
Different batches of Si wafers with nominally the same specifications were found to respond differently to identical chemical surface treatments aimed at regrowing Si oxide on them. We found that the oxides produced on different batches of wafer differ electrically, thereby affecting solid-state electron transport (ETp) via protein films assembled on them. These results led to the another set of experiments, where we studied this phenomenon using two distinct chemical methods to regrow oxides on the same batch of Si wafers. We have characterized the surfaces of the regrown oxides and of monolayers of linker molecules that connect proteins with the oxides and examined ETp via ultrathin layers of the protein bacteriorhodopsin, assembled on them. Our results illustrate the crucial role of (near) surface charges on the substrate in defining the ETp characteristics across the proteins. This is expressed most strikingly in the observed current's temperature dependences, and we propose that these are governed by the electrostatic landscape at the electrode-protein interface rather than by intrinsic protein properties. This study's major finding, relevant to protein bioelectronics, is that protein-electrode coupling in junctions is a decisive factor in ETp across them. Hence,surface electrostatics can create a barrier that dominates charge transport and controls the transport mode across the junction. Our findings' wider importance lies in their relevance to hybrid junctions of Si with (polyelectrolyte) biomolecules, a likely direction for future bioelectronics. A remarkable corollary of presented results is that once an electron is injected into the protein, transport within the proteins is so efficient that it does not encounter a measurable barrier down to 160 K.
Fereiro J. A., Porat G., Bendikov T., Pecht I., Sheves M. & Cahen D. (2018) Journal of the American Chemical Society. 140, 41, p. 13317-13326
Making biomolecular electronics a reality will require control over charge transport across biomolecules. Here we show that chemical modulation of the coupling between one of the electronic contacts and the biomolecules in a solid-state junction allows controlling electron transport (ETp) across the junction. Employing the protein azurin (Az), we achieve such modulation as follows: Az is covalently bound by Au-S bonding to a lithographically prepared Au electrode (Au-Az). Au nanowires (AuNW) onto which linker molecules, with free carboxylic group, are bound via Au-S bonds serve as top electrode. Current voltage plots of AuNW-linkerCOOH//Az-Au junctions have been shown earlier to exhibit step-like features, due to resonant tunneling through discrete Az energy levels. Forming an amide bond between the free carboxylic group of the AuNW-bound linker and Az yields AuNW-linkerCO-NH-Az-Au junctions. This Az-linker bond switches the ETp mechanism from resonant to off-resonant tunneling. By varying the extent of this amide bonding, the current voltage dependence can be controlled between these two mechanisms, thus providing a platform for altering and controlling the ETp mechanism purely by chemical modification in a two-terminal device, i.e., without a gate electrode. Using results from conductance, including the energy barrier and electrode molecule coupling parameters extracted from current voltage fitting and normalized differential conductance analysis and from inelastic-electron-tunneling and photoelectron spectroscopies, we determine the Az frontier orbital energies, with respect to the Au Fermi level, for four junction configurations, differing only in electrode-protein coupling. Our approach and findings open the way to both qualitative and quantitative control of biomolecular electronic junctions.
Bublil S., Fayena-Greenstein M., Talyanker M., Solomatin N., Tsubery M. N., Bendikov T., Penki T. R., Grinblat J., Duran I. B., Grinberg I., Ein-Eli Y., Elias Y., Hartmann P. & Aurbach D. (2018) Journal of Materials Chemistry A. 6, 30, p. 14816-14827
Layered cathode materials of the type Na1+xLi0.05Mn0.54Co0.13NiyO2-z(0
Cohen E., Soffer Y., Weissman H., Bendikov T., Schilt Y., Raviv U. & Rybtchinski B. (2018) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 57, 29, p. 8871-8874
An amphiphile based on polyethylene glycol (PEG) polymer and two molecular moieties (perylene diimide and C-7 fluoroalkyl, PDI and C7F) attached to its termini assembles into crystalline films with long-range order. The films reversibly switch from crystalline to amorphous above the PEG melting temperature. The adaptive behavior stems from the responsiveness of the PEG domain and the robustness of the PDI and C7F assemblies. The hydrophobicity of the film can be controlled by heating, resulting in switching from highly hydrophobic to superhydrophilic. The long-range order, reversible crystallinity switching, and the temperature-controlled wettability demonstrate the potential of block copolymer analogues based on simple polymeric/molecular hybrids.
Xie Y., Hu P., Bendikov T. & Milstein D. (2018) Catalysis Science and Technology. 8, 11, p. 2784-2788
We report the heterogeneously catalyzed hydrogenation of amides to form alcohols and amines. This C-N bond cleavage reaction is catalyzed by silver on -alumina, proceeds smoothly in high yields and 100% selectivity, and is effective for both benzamides and aliphatic amides. This is in contrast to amide hydrogenation catalyzed by heterogeneous catalysts, which generally proceeds via C-O bond cleavage. Recycling experiments show that the catalyst Ag/-Al2O3 can be easily recovered and regenerated without decrease in catalytic activity.
Guo C., Sarkar S., Refaely-Abramson S., Egger D. A., Bendikov T., Yonezawa K., Suda Y., Yamaguchi T., Pecht I., Kera S., Ueno N., Sheves M., Kronik L. & Cahen D. (2018) Physical Chemistry Chemical Physics. 20, 10, p. 6860-6867
Peptide-based molecular electronic devices are promising due to the large diversity and unique electronic properties of biomolecules. These electronic properties can change considerably with peptide structure, allowing diverse design possibilities. In this work, we explore the effect of the side-chain of the peptide on its electronic properties, by using both experimental and computational tools to detect the electronic energy levels of two model peptides. The peptides include 2Ala and 2Trp as well as their 3-mercaptopropionic acid linker which is used to form monolayers on an Au surface. Specifically, we compare experimental ultraviolet photoemission spectroscopy measurements with density functional theory based computational results. By analyzing differences in frontier energy levels and molecular orbitals between peptides in gas-phase and in a monolayer on gold, we find that the electronic properties of the peptide side-chain are maintained during binding of the peptide to the gold substrate. This indicates that the energy barrier for the peptide electron transport can be tuned by the amino acid compositions, which suggests a route for structural design of peptide-based electronic devices.
Alon H., Garrick R., Pujari S. P., Toledano T., Sinai O., Kedem N., Bendikov T., Baio J. E., Weidner T., Zuilhof H., Cahen D., Kronik L., Sukenik C. N. & Vilan A. (2018) Journal of Physical Chemistry C. 122, 6, p. 3312-3325
Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface's electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (PhO), aryl vinyl sulfide (PhS), and the corresponding molecule with a CH2 group allyl benzene (PhC). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current voltage and capacitance voltage analysis of Hg/PhX (X = C, 0, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor's effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact.
Muench F., Vaskevich A., Popovitz-Biro R., Bendikov T., Feldman Y. & Rubinstein I. (2018) Electrochimica Acta. 264, p. 233-243
Metal nanoplatelets (NPLs) display an important class of nanomaterials, whose morphological diversity is limited due to the uniform mechanisms governing their growth. Here, we introduce a new electroless plating (EP) strategy based on a metal complex surfactant, which is capable of producing a variety of previously undescribed silver NPL architectures. Our NPL formation can be understood as a unique example of space-filling dendritic growth. Likewise to conventional colloidal syntheses, in-plane growth is promoted by stacking faults, while out-of-plane growth on {111} is efficiently suppressed. Remarkably, the passivation extends to the nanostructure edges, limiting in-plane growth to random, localized nucleation events, which constantly change direction within a privileged set of six vectors. Out-of-plane growth proceeds discontinuously, by the formation of separate NPL layers with the same morphology. The nucleation probabilities change throughout the reaction, causing the deposit morphology to transition from individual nanobelts (NBs) over NPLs to porous dendrite-like sheets, under retention of the internal nanostructure. The synthesis is facile, scalable, tunable and applicable to various substrate materials, and thus represents a powerful tool for the direct modification of surfaces with anisotropic nanostructures. (c) 2018 Elsevier Ltd. All rights reserved.
Niazov-Elkan A., Weissman H., Dutta S., Cohen S. R., Iron M. A., Pinkas I., Bendikov T. & Rybtchinski B. (2018) Advanced Materials. 30, 2, 1705027
Organic crystalline materials are used as dyes/pigments, pharmaceuticals, and active components of photonic and electronic devices. There is great interest in integrating organic crystals with inorganic and carbon nanomaterials to create nanocomposites with enhanced properties. Such efforts are hampered by the difficulties in interfacing organic crystals with dissimilar materials. Here, an approach that employs organic nanocrystallization is presented to fabricate solution-processed organic nanocrystal/carbon nanotube (ONC/CNT) hybrid materials based on readily available organic dyes (perylene diimides (PDIs)) and carbon nanotubes. The hybrids are prepared by self-assembly in aqueous media to afford free-standing films with tunable CNT content. These exhibit excellent conductivities (as high as 5.78 ± 0.56 S m<sup>−1</sup>), and high thermal stability that are superior to common polymer/CNT hybrids. The color of the hybrids can be tuned by adding various PDI derivatives. ONC/CNT hybrids represent a novel class of nanocomposites, applicable as optoelectronic and conductive colorant materials.
Sharma R., Birojud R. K., Sinai O., Cohen H., Sahoo K. R., Artel V., Alon H., Levi A., Subrahmanyam A., Theis W., Naveh D. & Narayanan T. N. (2018) Applied Materials Today. 13, p. 387-395
Fluorographene is one of the most interesting 2D materials owing to its span of electronic properties, from a conductor to wide-gap insulator, controlled by the compositional carbon to fluorine ratio. Unlike the chemically inert graphene, fluorographene is recognized for its rich chemistry, particularly at ambient, allowing tailoring its physical properties. Here, we report on single step, catalyst free, wafer-scale synthesis of fluorographene oxide (FGO) ultra-thin films (similar to 4 nm thickness) by physical vapour deposition. The FGO, possessing 7% fluorine content, comprises few-nanometer domains of sp(2)-sp(3) carbon with high thermal stability, as confirmed by several analytical methods. We show that FGO can be utilized as an active hetero-layer on a few-layer MoS2 field effect transistor (FET), significantly improving the performance of MoS2 optoelectronic devices with an extended spectral response towards the near infrared and responsivity of up to 6 A/W. The FGO-MoS2 band alignment, as derived from the measured work function of FGO (4.69 eV), indicates a plausible photoconductive gain mechanism with a fast transit time of holes mediated by FGO quasi-continuous defect states. (C) 2018 Elsevier Ltd. All rights reserved.
Givon A., Steirer K. X., Segre E. & Cohen H. (2018) Applied Physics Letters. 113, 9, 091602
Chemically resolved electrical measurements of zinc oxysulfide over-layers on gold show very poor conductance under either electrical or optical input signals, whereas simultaneous application of the two yields extremely high sample currents. The effect and its dependence on the wavelength and electrical parameters are explained by the in-situ derived band diagram, in which a buffer level of charge traps cannot contribute directly to conductance, while yet amplifying the photoconductance by orders of magnitudes under sub-bandgap illumination. This AND-type doubly triggered response proposes interesting applications and an answer to problems encountered in related optoelectronic devices.
Kantor-Uriel N., Roy P., Lerman K., Sukenik C. N. & Cohen H. (2018) Journal Of Vacuum Science & Technology B. 36, 4, ARTN 04H10
Dithiol self-assembled molecular layers (SAMs) are exploited as controllable spacers between CdSe light absorbers and a gold collector. Using two sets of molecules, alkanes with different chain lengths and biphenyl derivatives with varied conjugation, the charge transport across corresponding SAMs is investigated by chemically resolved electrical measurements and photoluminescence (PL). Complementarity is found between the photovoltage and the PL intensity as a function of spacer thickness. However, it is not obeyed for varied conjugation, a feature shown to arise from reduced charge selectivity in the transport efficiencies. Interplay between dark and photo-induced mechanisms are revealed, where the emergence of charge traps by itself becomes spacer dependent via a remote effect of the substrate on surface chemical activity. Published by the AVS.
Curland S., Meirzadeh E., Cohen H., Ehre D., Maier J., Lahav M. & Lubomirsky I. (2018) Angewandte Chemie - International Edition. 57, 24, p. 7076-7079
The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.
Udayabhaskararao T., Houben L., Cohen H., Menahem M., Pinkas I., Avram L., Wolf T., Teitelboim A., Leskes M., Yaffe O., Oron D. & Kazes M. (2018) Chemistry of Materials. 30, 1, p. 84-93
Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.
Although the importance of energy-alignment at hetero-interface is undeniable, the mechanism dictating it is still debatable. Confusion is even larger for molecular monolayers, because molecules are mostly insulators with no energy-levels in the vicinity of the Fermi-level. This rules out molecular ionization as a means to establish electronic equilibrium. Still, there is a growing evidence for energy alignment between adsorbed molecules and the Fermilevel of their supporting substrate. This can be understood as an equilibrium between the Fermi level of the contact(s) and the chemical potential of the electrons in the molecule, established via polarization of the bound electrons in the immediate vicinity of the interface. The extent of this polarization relates to the notion of 'chemical hardness' and is relevant for both chemical binding and physical contact. This concept is demonstrated for several examples of molecular monolayers on Si and on Au.
Pal A. N., Klein T., Vilan A. & Tal O. (2018) Beilstein Journal of Nanotechnology. 9, 1, p. 1471-1477
Single-molecule junctions are versatile test beds for electronic transport at the atomic scale. However, not much is known about the early formation steps of such junctions. Here, we study the electronic transport properties of premature junction configurations before the realization of a single-molecule bridge based on vanadocene molecules and silver electrodes. With the aid of conductance measurements, inelastic electron spectroscopy and shot noise analysis, we identify the formation of a single-molecule junction in parallel to a single-atom junction and examine the interplay between these two conductance pathways. Furthermore, the role of this structure in the formation of single-molecule junctions is studied. Our findings reveal the conductance and structural properties of premature molecular junction configurations and uncover the different scenarios in which a single-molecule junction is formed. Future control over such processes may pave the way for directed formation of preferred junction structures.
Sedova A., Leitus G., Feldman Y., Bendikov T., Popovitz-Biro R., Khodorov S., Dodiuk H., Kenig S. & Tenne R. (2017) Journal of Materials Science. 52, 11, p. 6376-6387
Owing to their unique properties such as mechanical, optical, magnetic, nanomaterials attracted a great interest over the last two decades. Inorganic nanotubes, e.g. WS2, make an important class of nanomaterials with numerous potential applications. In the current work, a new synthetic strategy is developed to decorate the surface of WS2 nanotubes with FeWO4 nanoparticles. The FeWO4 nanoparticles were produced by first depositing amorphous iron oxide film onto the WS2 nanotubes' surface and, subsequently, high-temperature annealing (600 C-degrees). Careful analysis by electron microscopy; X-ray diffraction and other techniques were carried out. Based on these analyses, the growth mechanism of the hybrid nanostructures was elucidated. Magnetic measurements were employed to shed light on the magnetic behavior of the hybrid nanostructures. The orientation and position of the WS2 nanotubes decorated with the FeWO4 nanoparticles could be partially affected by applying a magnetic field using non-viscous solvents, like ethanol.
Elool Dov N., Shankar S., Cohen D., Bendikov T., Rechav K., Shimon L. J. W., Lahav M. & van der Boom M. E. (2017) Journal of the American Chemical Society. 139, 33, p. 11471-11481
In this study, we demonstrate a versatile approach for the formation of electrochromic nanoscale assemblies on transparent conductive oxides on both rigid and flexible substrates. Our method is based on the application of alternating spin-coated layers of well-defined metal polypyridyl complexes and a palladium(II) salt to form electrochemically addressable films with a high chromophore density. By varying the central metal ion of the polypyridyl complexes (Os, Ru, and" Fe) and their ligands and by mixing these complexes, coatings with a wide range of colors can be achieved. These coatings cover a large area of RGB color space. The coloration intensities of these nanoscale films can be tuned by "the number of deposition steps. The materials have very attractive ON/OFF ratios, electrochemical stabilities, and coloration efficiencies. Reversible color-to-colorless and color-to-color transitions were demonstrated, and the films were further integrated into sandwich cells.
Susman M. D., Feldman Y. (., Bendikov T. A., Vaskevich A. & Rubinstein I. (2017) Nanoscale. 9, 34, p. 12573-12589
Oxidation and corrosion reactions have a major effect on the application of non-noble metals. Kinetic information and simple theoretical models are often insufficient for describing such processes in metals at the nanoscale, particularly in cases involving formation of internal voids (nano Kirkendall effect, NKE) during oxidation. Here we study the kinetics of solid-state oxidation of chemically-grown copper nanoparticles (NPs) by in situ localized surface plasmon resonance (LSPR) spectroscopy during isothermal annealing in the range 110-170 °C. We show that LSPR spectroscopy is highly effective in kinetic studies of such systems, enabling convenient in situ real-time measurements during oxidation. Change of the LSPR spectra throughout the oxidation follows a common pattern, observed for different temperatures, NP sizes and substrates. The well-defined initial Cu NP surface plasmon (SP) band red-shifts continuously with oxidation, while the extinction intensity initially increases to reach a maximum value at a characteristic oxidation time τ, after which the SP intensity continuously drops. The characteristic time τ is used as a scaling parameter for the kinetic analysis. Evolution of the SP wavelength and extinction intensity during oxidation at different temperatures follows the same kinetics when the oxidation time is normalized to τ, thus pointing to a general oxidation mechanism. The characteristic time τ is used to estimate the activation energy of the process, determined to be 144 ± 6 kJ mol<sup>-1</sup>, similar to previously reported values for high-temperature Cu thermal oxidation. The central role of the NKE in the solid-state oxidation process is revealed by electron microscopy, while formation of Cu<sub>2</sub>O as the major oxidation product is established by X-ray diffraction, XPS, and electrochemical measurements. The results indicate a transition of the oxidation mechanism from a Valensi-Carter (VC) to NKE mechanism with the degree of oxidation. To interpret the optical evolution during oxidation, Mie scattering solutions for metal core-oxide shell spherical particles are computed, considering formation of Kirkendall voids. The model calculations are in agreement with the experimental results, showing that the large red-shift of the LSPR band during oxidation is the result of Kirkendall voiding, thus establishing the major role of the NKE in determining the optical behavior of such systems.
Cohen H. & Amir R. (2017) Plant Cell Reports. 36, 5, p. 719-730
Key message: Higher methionine levels in transgenic Arabidopsis seeds trigger the accumulation of stress-related transcripts and primary metabolites. These responses depend on the levels of methionine within seeds. Abstract: Methionine, a sulfur-containing amino acid, is a key metabolite in plant cells. To reveal the regulatory role of the Arabidopsis thaliana CYSTATHIONINE γ-SYNTHASE (AtCGS), methionine main regulatory enzyme, in the synthesis of methionine in seeds, we generated transgenic RNAi seeds with targeted repression of AtCGS during late developmental stages of seeds. Unexpectedly, these seeds accumulated 2.5-fold more methionine than wild-type seeds. To study the nature of these seeds, transcriptomic and primary metabolite profiling were employed using Affymetrix ATH1 microarray and gas chromatographymass spectrometry analyses, respectively. The results were compared to transgenic Arabidopsis seeds expressing a feedback-insensitive form of AtCGS (named SSE-AtD-CGS) that were previously showed to accumulate up to sixfold more soluble methionine than wild-type seeds. Statistical assessments showed that the nature of transcriptomic and metabolic changes that occurred in RNAi::AtCGS seeds were relatively similar, but to lesser extents, to those previously reported for SSE-AtD-CGS seeds, and linked to the induction of global transcriptomic and metabolic responses associated with stronger desiccation stress. As transgenic seeds obtained by both manipulations exhibited higher, but different methionine levels, the data strongly suggest that these changes depend on the absolute amounts of methionine within seeds and much less to the expression level of AtCGS.
Cohen H., Salmon A., Tietel Z., Hacham Y. & Amir R. (2017) Plant Cell Reports. 36, 5, p. 731-743
Key message: Enzymes operating in theS-methylmethionine cycle make a differential contribution to methionine synthesis in seeds. In addition, mutual effects exist between theS-methylmethionine cycle and the aspartate family pathway in seeds. Abstract: Methionine, a sulfur-containing amino acid, is a key metabolite in plant cells. The previous lines of evidence proposed that the S-methylmethionine (SMM) cycle contributes to methionine synthesis in seeds where methionine that is produced in non-seed tissues is converted to SMM and then transported via the phloem into the seeds. However, the relative regulatory roles of the S-methyltransferases operating within this cycle in seeds are yet to be fully understood. In the current study, we generated transgenic Arabidopsis seeds with altered expression of three HOMOCYSTEINE S-METHYLTRANSFERASEs (HMTs) and METHIONINE S-METHYLTRANSFERASE (MMT), and profiled them for transcript and metabolic changes. The results revealed that AtHMT1 and AtHMT3, but not AtHMT2 and AtMMT, are the predominant enzymes operating in seeds as altered expression of these two genes affected the levels of methionine and SMM in transgenic seeds. Their manipulations resulted in adapted expression level of genes participating in methionine synthesis through the SMM and aspartate family pathways. Taken together, our findings provide new insights into the regulatory roles of the SMM cycle and the mutual effects existing between the two methionine biosynthesis pathways, highlighting the complexity of the metabolism of methionine and SMM in seeds.
Artel V., Guo Q., Cohen H., Gasper R., Ramasubramaniam A., Xia F. & Naveh D. (2017) npj 2D Materials and Applications. 1, 1, p. 1-5, 6
Black phosphorous is a fascinating layered material, with extraordinary anisotropic mechanical, optical and electronic properties. However, the sensitivity of black phosphorous to oxygen and moisture poses significant challenges for technological applications of this unique material. Here, we report a viable solution that overcomes degradation of few-layer black phosphorous by passivating the surface with self-assembled monolayers of octadecyltrichlorosilane that provide long-term stability in ambient conditions. Importantly, we show that this treatment does not cause any undesired carrier doping of the bulk channel material, thanks to the emergent hierarchical interface structure. Our approach is compatible with conventional electronic materials processing technologies thus providing an immediate route toward practical applications in black phosphorous devices.
Somekh M., Cohen H., Diskin-Posner Y., Shimon L. J. W., Carmieli R., Rosenberg J. N. & Neumann R. (2017) ACS Catalysis. 7, 4, p. 2725-2729
The valorization of alkanes is possible via carbon carbon coupling reactions. A series of dialkyl cobalt complexes RRCH2)(2)Co-III(bpy)(2)]ClO4 (R = H, Me, Et, and Ph) were reacted with the H5PV2Mo10O40 polyoxometalate as a catalyst, leading to a selective oxidative carbon carbon bond coupling reaction. The reaction is initiated by electron transfer from [(RCH2)(2)Co-III(bpy)(2)](+) to (H5PV2Mo10O40)-Mo-V to yield an intermediate [(RCH2)(2)Co-IV(bpy)(2)](2+)-(H5PVVMo10O40)-V-IV-Mo-V, as identified by a combination of EPR and X-ray photoelectron spectroscopy experiments. The reaction is catalytic with O-2 as terminal oxidant representing an aerobic C-C bond coupling reaction.
Beilis E., Horowitz Y., Givon A., Sornorjai G. A., Cohen H. & Richter S. (2017) Journal of Physical Chemistry C. 121, 17, p. 9579-9586
Direct exploration of the mutually interfering morphological and charge-transport characteristics in self-assembled monolayers (SAMs) is reported. These strongly coupled properties are addressed by means of surface spectroscopy techniques, combined so as to consistently account at high sensitivity for a broad range of surface properties without any use of a top contact. Applied to doped bovine serum albumin (BSA) SAMs, we show how the BSA conformation, its dehydration, and monolayer assembly are all correlated. Moreover, the electrical properties, transport, and charge trapping are highly affected by the SAM compositional and structural state, with a specific role of water molecules. Our results reveal and further demonstrate a useful approach to the complex challenges presented by (bio) molecular electronics.
Filip-Granit N., Goldberg E., Samish I., Ashur I., van der Boom M. E., Cohen H. & Scherz A. (2017) Journal Of Physical Chemistry B. 121, 29, p. 6981-6988
Redox reactions play key roles in fundamental biological processes. The related spatial organization of donors and acceptors is assumed to undergo evolutionary optimization facilitating charge mobilization within the relevant biological context. Experimental information from submolecular functional sites is needed to understand the organization strategies and driving forces involved in the self-development of structure-function relationships. Here we exploit chemically resolved electrical measurements (CREM) to probe the atom-specific electrostatic potentials (ESPs) in artificial arrays of bacteriochlorophyll (BChl) derivatives that provide model systems for photoexcited (hot) electron donation and withdrawal. On the basis of computations we show that native BChl's in the photosynthetic reaction center (RC) self-assemble at their ground-state as aligned gates for functional charge transfer. The combined computational and experimental results further reveal how site-specific polarizability perpendicular to the molecular plane enhances the hot-electron transport. Maximal transport efficiency is predicted for a specific, ∼5 Å, distance above the center of the metalized BChl, which is in remarkably close agreement with the distance and mutual orientation of corresponding native cofactors. These findings provide new metrics and guidelines for analysis of biological redox centers and for designing charge mobilizing machines such as artificial photosynthesis.
Vilan A., Aswal D. & Cahen D. (2017) Chemical Reviews. 117, 5, p. 4248-4286
We review charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research and to single-molecule junctions (1mJ) in addition to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MolEl, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead, we focus on how to analyze the net electrical performance of molecular junctions, from a functional device perspective. Finally, we point to creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes. This review is complemented by a another review that covers metal-molecule-semiconductor junctions and their unique hybrid effects.
Vilan A. & Cahen D. (2017) Chemical Reviews. 117, 5, p. 4624-4666
Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current-voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers. Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another one, concerning the formation of large-area molecular junctions and charge transport that is dominated solely by molecules.
Pawlicki A. A., Vilan A., Jurow M., Drain C. M. & Batteas J. D. (2017) Faraday Discussions. 204, p. 349-366
Here we report on the effect of local molecular organization or "tertiary structure" on the charge transport properties of thiol-tethered tetraphenylporphyrin (ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH) nanoscale clusters of ca. 5 nm in lateral dimension embedded within a dodecanethiol (C12) monolayer on Au(111). The structure of the clusters in the mixed monolayers and their resulting transport properties were monitored by Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). The mixed films were deposited on Au(111) for a period of one to five days, during which the lateral dimensions of the ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH islands that were formed after one day reduced by nearly 35% on average by five days, accompanied by a noticeable depletion of the surrounding C12 monolayer. These subtle changes in mixed monolayer morphology were accompanied by drastic differences in conductance. The ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH clusters assembled for one day exhibited highly reproducible I-V spectra with simple tunneling behavior. By three days, this evolved into bias-induced switching of conductance, with a ∼100-1000 fold increase. Furthermore, current fluctuations started to become significant, and then dominated transport across the ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH clusters assembled over five days. Our data suggests that this evolution can be understood by slow surface diffusion, enabling the ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH molecules to overcome initial steric hindrance in the early stages of island formation in the C12 monolayer (at day one), to reach a more energetically-favored, close-packed organization, as noted by the decrease in island size (by day three). However, when desorption of the supporting matrix of C12 became pronounced (by day five), the ZnTPPF<sub>4</sub>-SC<sub>5</sub>SH clusters began to lose stabilization, and stochastic switching was then observed to dominate transport in the clusters, illustrating the critical nature of the local organization on these transport properties.
Vilan A. (2017) Physical Chemistry Chemical Physics. 19, 40, p. 27166-27172
This paper presents the use of normalized differential conductance (NDC) for exploring the effect of applied voltage on charge tunnelling across molecular junctions. It shows the power of NDC in revealing subtle features characteristic of specific tunnelling models and considers its theoretical basis in view of the Taylor-expansion (parabolic approximation) for tunnelling. A practical application of NDC is illustrated by analyzing the current-voltage relations across a short alkyl-thiol monolayer on smooth compared with rough Ag substrates.
Itzhaik Y., Bendikov T., Hines D., Kamat P. V., Cohen H. & Hodes G. (2016) Journal of Physical Chemistry C. 120, 1, p. 31-41
Thiocyanate ion treatment, usually either LiSCN or KSCN, of the absorbing semiconductor before deposition of a CuSCN hole conducting layer is known to improve the performance of extremely thin absorber (ETA) solar cells by reducing the cell resistivity. However, in spite of several hypotheses, the mechanism behind this treatment outcome remains elusive. In this study, the interface between Sb2S3 and CuSCN in an ETA cell is investigated with surface spectroscopy and transient absorption spectroscopy to establish the mechanistic aspects of the KSCN treatment and the role it plays in improving the photovoltaic performance. The prominent factors that dictate the cell performance are (a) doping the interfacial CuSCN and thus preventing the formation of a sub-mu m depleted layer and (b) passivating charge traps at the Sb2S(O)(3) surface, which increases the rate of hole transfer from the absorber to the hole conductor. We further show that the treatment works just as well in improving photovoltaic performance when carried out after CuSCN deposition (post-treatment).
Sarkar D., Ishchuk S., Taffa D. H., Kaynan N., Berke B. A., Bendikov T. & Yerushalmi R. (2016) Journal of Physical Chemistry C. 120, 7, p. 3853-3862
Molecular layer deposition (MLD) of TiCl4 and ethylene-glycol (Ti-EG) was recently demonstrated as a vapor phase synthetic route for preparation of photocatalytic thin films via hybrid organic inorganic thin films. The organic moieties of the hybrid material function as sacrificial components that undergo controlled decomposition during thermal annealing. Anneal temperature was shown to be an important factor determining the overall photocatalytic performances of the treated films with 650 degrees C optimal for photodegradation of dye molecules and anneal at 520 degrees C showing optimal performance for the direct photocatalytic production of H2O2. Both systems exhibit activities that are not typically attainable by titania, yet a fundamental understanding of the underlying details leading to these improved reactivities for the specific cases is still lacking. Here we demonstrate that thermal anneal of hybrid organic inorganic thin films prepared by MLD yield oxygen-deficient titania with controllable levels of oxygen vacancies (OVs) and defect states that are adjusted by the temperature of the anneal process, performed under air. The anneal process result in nonstoichiometric oxide films with unique electronic properties including the tuning of band positions, accessibility to significantly deeper valence band position and controlled formation of electronic defect states that assist in charge separation for Au-titania catalyst. We correlate the oxygen deficiency and electronic structure of the annealed film with the photocatalytic activity for shedding light on the details that lead to the improved reactivity. These results extend the scope of using MLD in the context of photocatalysis with new routes for obtaining nonstoichiometric oxides which are key for enhancing and tailoring the reactivity of metal oxide (photo)catalysis.
Morozov M., Bendikov T., Evmenenko G., Dutta P., Lahav M. & van der Boom M. E. (2016) Chemical Communications. 52, 13, p. 2683-2686
We demonstrate a process that results in the formation of palladium nanoparticles during the assembly of molecular thin films. These nanoparticles are embedded in the films and are generated by a chemical reaction of the counter anions of the molecular components with the metal salt that is used for cross-linking these components.
Balgley R., de Ruiter G., Evmenenko G., Bendikov T., Lahav M. & van der Boom M. E. (2016) Journal of the American Chemical Society. 138, 50, p. 16398-16406
In this paper, we demonstrate how photo chemically enhancing the permeability of metal-organic assemblies results in a significant enhancement of the electrochemical activity of metal complexes located within the assembly. The molecular assemblies consist of different layers of redox-active metal complexes ([M(mbpy-py)(3)][PF6](2); M = Ru or Os) that are separated by redox-inactive spacers consisting of 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPEB) and PdCl2 of variable thicknesses (0-13.4 nm). UV-irradiation (lambda = 254 nm) of our assemblies induces a photochemical reaction in the redox-inactive spacer increasing the permeability of the assembly. The observed increase was evident by trapping organic ((Bu4NBF4)-Bu-n) and inorganic (NiCl2) salts inside the assemblies, and by evaluating the electrochemical response of quinones absorbed inside the molecular assemblies before and after UV irradiation. The increase in permeability is reflected by higher currents and a change in the directionality of electron transfer, i.e., from mono- to bidirectional, between the redox-active metal complexes and the electrode surface. The supramolecular structure of the assemblies dominates the overall electron transfer properties and overrules possible electron transfer mediated by the extensive pi-conjugation of its individual organic components.
Gupta S., Bendikov T., Hodes G. & Cahen D. (2016) ACS Energy Letters. 1, 5, p. 1028-1033
Solar cells based on "halide perovskites" (HaPs) have demonstrated unprecedented high power conversion efficiencies in recent years. However, the well-known toxicity of lead (Pb), which is used in the most studied cells, may affect its widespread use. We explored an all-inorganic lead-free perovskite option, cesium tin bromide (CsSnBr<sub>3</sub>), for optoelectronic applications. CsSnBr<sub>3</sub>-based solar cells exhibited photoconversion efficiencies (PCEs) of 2.1%, with a short-circuit current (J<sub>SC</sub>) of ∼9 mA cm<sup>-2</sup>, an open circuit potential (V<sub>OC</sub>) of 0.41 V, and a fill factor (FF) of 58% under 1 sun (100 mW cm<sup>-2</sup>) illumination, which, even though meager compared to the Pb analogue-based cells, are among the best reported until now. As reported earlier, addition of tin fluoride (SnF<sub>2</sub>) was found to be beneficial for obtaining good device performance, possibly due to reduction of the background carrier density by neutralizing traps, possibly via filling of cation vacancies. The roles of SnF<sub>2</sub> on the properties of the CsSnBr<sub>3</sub> were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analysis.
Balgley R., Drees M., Bendikov T., Lahav M., Facchetti A. & van der Boom M. E. (2016) Journal of Materials Chemistry C. 4, 21, p. 4634-4639
The fundamental science behind the design of organic photovoltaic (OPV) cells lies in the formation of energy level gradients for efficient charge separation and collection. Tuning the energy levels at the device electrodes by the right choice of the components is a key requirement for achieving enhanced characteristics. Here we demonstrate control and optimization of OPV cell performance by using a set of polypyridyl complexes based on iron, ruthenium, and osmium centers with tunable frontier orbital energies as interlayers for inverted bulk heterojunction solar cells. We found that changing the metal center of isostructural transition-metal complexes results in evident shifts of the HOMO and LUMO energy levels and the work functions of the corresponding interlayers, which has a prominent effect on the device performance. We generalize our approach by combining the interlayers with different sets of photoactive materials to test the electron transporting as well as the hole blocking characteristics of the interlayers.
Guo C., Yu X., Refaely-Abramson S., Sepunaru L., Bendikov T., Pecht I., Kronik L., Vilan A., Sheves M. & Cahen D. (2016) Proceedings of the National Academy of Sciences of the United States of America. 113, 39, p. 10785-10790
Charge migration for electron transfer via the polypeptide matrix of proteins is a key process in biological energy conversion and signaling systems. It is sensitive to the sequence of amino acids composing the protein and, therefore, offers a tool for chemical control of charge transport across biomaterial-based devices. We designed a series of linear oligoalanine peptides with a single tryptophan substitution that acts as a "dopant," introducing an energy level closer to the electrodes' Fermi level than that of the alanine homopeptide. We investigated the solid-state electron transport (ETp) across a selfassembled monolayer of these peptides between gold contacts. The single tryptophan "doping" markedly increased the conductance of the peptide chain, especially when its location in the sequence is close to the electrodes. Combining inelastic tunneling spectroscopy, UV photoelectron spectroscopy, electronic structure calculations by advanced density-functional theory, and dc current-voltage analysis, the role of tryptophan in ETp is rationalized by charge tunneling across a heterogeneous energy barrier, via electronic states of alanine and tryptophan, and by relatively efficient direct coupling of tryptophan to a Au electrode. These results reveal a controlled way of modulating the electrical properties of molecular junctions by tailormade "building block" peptides.
Rakita Y., Meirzadeh E., Bendikov T., Kalchenko V. (., Lubomirsky I., Hodes G., Ehre D. & Cahen D. (2016) APL Materials. 4, 5, 051101
To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr<sub>3</sub>). We find that MAPbBr<sub>3</sub> is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr<sub>3</sub> semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr<sub>3</sub> solar cells.
Kaplan V., Bendikov T., Feldman Y. (., Gartsman K., Wachtel E. & Lubomirsky I. (2016) Journal of Power Sources. 301, p. 271-276
An electrochemical cell containing molten Li<sub>2</sub>CO<sub>3</sub>-Li<sub>2</sub>O at 900 °C has been proposed for the conversion of the greenhouse gas CO<sub>2</sub> to CO for chemical energy storage. In the current work, we have examined the corrosion resistance of zirconia, beryllia and magnesia ceramics at 900 °C in the Li<sub>2</sub>CO<sub>3</sub>-Li<sub>2</sub>O and Li-Na-K carbonate eutectic mixtures to identify suitable electrically insulating materials. Conclusions regarding material stability were based on elemental analysis of the melt, primarily via X-ray photoelectron spectroscopy, a particularly sensitive technique. It was found that magnesia is completely stable for at least 33 h in a Li<sub>2</sub>CO<sub>3</sub>-Li<sub>2</sub>O melt, while a combined lithium titanate/lithium zirconate layer forms on the zirconia ceramic as detected by XRD. Under the same melt conditions, beryllia shows considerable leaching into solution. In a Li-Na-K carbonate eutectic mixture containing 10.2 mol% oxide at 900 °C under standard atmospheric conditions, magnesia showed no signs of degradation. Stabilization of the zirconia content of the eutectic mixture at 0.01-0.02 at% after 2 h is explained by the formation of a lithium zirconate coating on the ceramic. On the basis of these results, we conclude that only magnesia can be satisfactorily used as an insulating material in electrolysis cells containing Li<sub>2</sub>CO<sub>3</sub>-Li<sub>2</sub>O melts.
Siram R. B. K., Das J., Mukhopadhyay S., Brenner T. M., Kedem N., Kulbak M., Bendikov T., Cahen D., Hodes G. & Rybtchinski B. (2016) Advanced Materials Interfaces. 3, 23, 1600506
Simple organic salts are used as a cheap alternative for hole-conducting materials in methylammonium lead bromide perovskite solar cells and obtaining power conversion efficiency of 4.4%. The findings suggest that the polar organic salts interact with the perovskite surface, leading to formation of a surface dipole or change of an existing one on the perovskites that changes its effective work function.
Kulbak M., Gupta S., Kedem N., Levine I., Bendikov T., Hodes G. & Cahen D. (2016) The journal of physical chemistry letters. 7, 1, p. 167-172
Direct comparison between perovskite-structured hybrid organic-inorganic methylammonium lead bromide (MAPbBr<sub>3</sub>) and all-inorganic cesium lead bromide (CsPbBr<sub>3</sub>), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr<sub>3</sub> demonstrates a higher thermal stability than MAPbBr<sub>3</sub>. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr<sub>3</sub> or CsPbBr<sub>3</sub>, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation.
Cohen H., Shir O. M., Yu Y., Hou W., Sun S., Han T. & Amir R. (2016) Plant Cell And Environment. 39, 8, p. 1805-1817
The metabolic profiles and composition of storage reserves of agricultural crop seeds are strongly regulated by heritable and environmental factors. Yet, very little is known about the genetic and environmental determinants of adaptive metabolic variation amongst wild type as well as transgenic seed populations derived from the same genetic background, grown under natural field conditions. The goal of the current study was to investigate the effects of natural environmental conditions on wild type and transgenic soybean seeds expressing a feedback-insensitive form of cystathionine γ-synthase, a methionine main regulatory enzyme. The seeds were grown in four geographically distinct habitats in China and then assayed for primary metabolic profiles using gas chromatography mass spectrometry, morphological traits and storage reserve accumulation. The analyses revealed changes in the levels of primary metabolites which evidently exhibited high correlation to methionine regardless of changes in environmental conditions. The environment, however, constituted a major determinant of metabolic profiles amongst seeds, as much more metabolites were observed to be affected by this variable, particularly along the north-to-south latitudinal gradient. The observations suggest that metabolic variation amongst seeds grown under natural field conditions depends upon the complex relationships existing amongst their genetic background and the environmental conditions characterizing their cultivation areas.
Cohen H., Pajak A., Pandurangan S., Amir R. & Marsolais F. (2016) Amino Acids. 48, 6, p. 1413-1422
Previous in vitro studies demonstrate that exogenous application of the sulfur-containing amino acid methionine into cultured soybean cotyledons and seedlings reduces the level of methionine-poor storage proteins and elevates those that are methionine-rich. However, the effect of higher endogenous methionine in seeds on the composition of storage products in vivo is not studied yet. We have recently produced transgenic Arabidopsis seeds having significantly higher levels of methionine. In the present work we used these seeds as a model system and profiled them for changes in the abundances of 12S-globulins and 2S-albumins, the two major groups of storage proteins, using 2D-gels and MALDI-MS detection. The findings suggest that higher methionine affects from a certain threshold the accumulation of several subunits of 12S-globulins and 2S-albumins, regardless of their methionine contents, resulting in higher total protein contents. The mRNA abundances of most of the genes encoding these proteins were either correlated or not correlated with the abundances of these proteins, implying that methionine may regulate storage proteins at both transcriptional and post-transcriptional levels. The elevations in total protein contents resulted in reduction of total lipids and altered the fatty acid composition. Altogether, the data provide new insights into the regulatory roles of elevated methionine levels on seed composition.
Cohen H., Eli S. E., Jõgi M. & Suss M. E. (2016) ChemSusChem. 9, 21, p. 3045-3048
Carbon-based suspension electrodes are currently intensively investigated for emerging electrochemical systems, such as flow batteries, flow capacitors, and capacitive deionization cells. The main limitation of such electrodes is their low electric conductivity, which is typically orders of magnitude lower than that of traditional static carbon electrodes. Two main categories of suspension electrodes exist: 1) slurry electrodes where particles are not significantly affected by gravity, and 2) fluidized bed electrodes where particles are affected by gravity. We introduce a novel category that we term \u201ccombined\u201d suspension electrodes, which combine dilute slurries and dense fluidized beds. We present experimental measurements of the electrochemical impedance and electric conductivity of two combined electrodes. For one set of materials, the measured electric conductivity of the combined electrode is at least an order of magnitude above the fluidized bed and slurry components alone, demonstrating that a synergetic effect can be achieved when adding dilute slurry to dense fluidized bed. For a second set of materials, results show that the combined electrode conductivity is lower than the slurry component alone, a counter-intuitive result, demonstrating that increasing electrode carbon loading does not always enhance the electric conductivity.
Walther R., Fritz S., Mueller E., Schneider R., Gerthsen D., Sigle W., Maniv T., Cohen H., Matyssek C. & Busch K. (2016) ACS Photonics. 3, 5, p. 836-843
Electron energy loss spectroscopy (EELS) in a monochromated transmission electron microscope is applied to probe standing-wave-like cavity modes hybridized with surface plasmon polaritons (SPP) in rectangular submicron slits in a thin gold film. Coupling of hybridized SPP-cavity modes between two adjacent slits is studied by systematically varying the width of the metal bar d that separates the identical slits in a two-slit system. Measurements on two-slit systems with different slit lengths L and fixed width reveal energy shifts and mode splitting of the fundamental SPP cavity mode which can be generally described as a function of a dimensionless scaling parameter L/d. Numerical simulations with the Discontinuous Galerkin Time-Domain (DGTD) method confirm the experimental data and reveal insights into the underlying complex coupling mechanisms.
Barad H., Ginsburg A., Cohen H., Rietwyk K. J., Keller D. A., Tirosh S., Bouhadana Y., Anderson A. Y. & Zaban A. (2016) Advanced Materials Interfaces. 3, 7, 1500789
The present work reports a simple and direct sputtering deposition to form solid state TiO2 vertical bar Ag independent plasmonic solar cells. The independent plasmonic solar cells are based on a Schottky barrier between two materials, TiO2 and Ag. The Ag functions as the absorber generating "hot" electrons, as well as the contact for the solar cell. The Ag sputtering is performed for different durations, to form Ag nanoparticles with a wide size distribution on the surface of rough spray pyrolysis deposited TiO2. Incident photon to current efficiency (IPCE) measurements show photovoltaic activity below the TiO2 bandgap, which is caused by the silver nanoparticles that have a wide plasmonic band, leading to the generation of "hot" electrons. X-ray photoelectron spectroscopy analysis supports the "hot" electron injection mechanism by following the Ag plasmon band and detecting local photovoltages. The measurements show that electrons are formed in the Ag upon illumination and are injected into the TiO2, producing photovoltaic activity. J-V measurements show photocurrents up to 1.18 mA cm(-2) and photovoltages up to 430 mV are achieved, with overall effi ciencies of 0.2%. This is, to our knowledge, the highest performance reported for such independent plasmonic solar cells.
Sui X., Greenfeld I., Cohen H., Zhang X., Li Q. & Wagner H. D. (2016) Composites Science and Technology. 137, p. 35-43
Multilevel hierarchical structures built up from nanoscale to macroscale are common in nature, but their potential has not been achieved by man-made composites. The presented multilevel structure consists of carbon nanotube fibers (CNTFs) embedded in epoxy matrix. This structure exploits the supreme mechanical properties of individual CNTs together with the manageability of the microscale CNTFs, and has the potential to overcome the implementation difficulties associated with nanocomposites. Using different chemical treatments (ethylene glycol or nitric acid solvents), the CNTFs are densified and the amount of epoxy penetration inside the CNTFs is controlled, creating an interphase between the single CNTs. The strength and adhesion properties of individual CNTFs in epoxy are measured by continuously monitored fragmentation tests and characterized by electron microscopy. A modified Cottrell-Kelly-Tyson model is applied to account for the CNTF unique cross-sectional geometry, comprising millions of individual multiwalled CNTs, and for the effect of matrix penetration. The composite strength and toughness are found to be strongly dependent on and improved by the extent of penetration, suggesting that the composite mechanical properties would be tunable by controlling the interphase. The presented integrative analysis shows that CNTF based composites are an excellent potential choice for strong and tough structures, as well as for bio-engineering. (C) 2016 Elsevier Ltd. All rights reserved.
Yamin T., Wissberg S., Cohen H., Cohen-Taguri G. & Sharoni A. (2016) ACS Applied Materials and Interfaces. 8, 23, p. 1486314870
The metal-insulator transition (MIT) properties of correlated oxides thin films, such as VO<sub>2</sub>, are dramatically affected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO<sub>2</sub> grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO<sub>2</sub> films by postdeposition thinning of relaxed films, overcoming the need for thickness-dependent strain-engineered substrates. This is possible only if thinning does not alter the films' electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO<sub>2</sub> films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.
Meirzadeh E., Sapir L., Cohen H., Cohen S., Ehre D., Harries D., Lahav M. & Lubomirsky I. (2016) Journal of the American Chemical Society. 138, 44, p. 14756-14763
The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 μC·cm<sup>-2</sup>, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.
Mondal P. C., Roy P., Kim D., Fullerton E. E., Cohen H. & Naaman R. (2016) Nano Letters. 16, 4, p. 2806-2811
The combination of photonics and spintronics opens new ways to transfer and process information. It is shown here that in systems in which organic molecules and semiconductor nanoparticles are combined, matching these technologies results in interesting new phenomena. We report on light induced and spin-dependent charge transfer process through helical oligopeptide-CdSe nanoparticles' (NPs) architectures deposited on ferromagnetic substrates with small coercive force (∼100-200 Oe). The spin control is achieved by the application of the chirality-induced spin-dependent electron transfer effect and is probed by two different methods: spin-controlled electrochemichemistry and photoluminescence (PL) at room temperature. The injected spin could be controlled by excitation of the nanoparticles. By switching the direction of the magnetic field of the substrate, the PL intensity could be alternated.
Walther R., Fritz S., Mueller E., Schneider R., Maniv T., Cohen H., Matyssek C., Busch K. & Gerthsen D. (2016) Physical Review B. 93, 24, 245417
The excitation of cavity standing waves in double-slit structures in thin gold films, with slit lengths between 400 and 2560 nm, was probed with a strongly focused electron beam in a transmission electron microscope. The energies and wavelengths of cavity modes up to the 11th mode order were measured with electron energy loss spectroscopy to derive the corresponding dispersion relation. For all orders, a significant redshift of mode energies accompanied by a wavelength elongation relative to the expected resonator energies and wavelengths is observed. The resultant dispersion relation is found to closely follow the well-known dispersion law of surface-plasmon polaritons (SPPs) propagating on a gold/air interface, thus providing direct evidence for the hybridized nature of the detected cavity modes with SPPs.
Rez P., Aoki T., March K., Gur D., Krivanek O. L., Dellby N., Lovejoy T. C., Wolf S. G. & Cohen H. (2016) Nature Communications. 7, 10945
Vibrational spectroscopy in the electron microscope would be transformative in the study of biological samples, provided that radiation damage could be prevented. However, electron beams typically create high-energy excitations that severely accelerate sample degradation. Here this major difficulty is overcome using an 'aloof' electron beam, positioned tens of nanometres away from the sample: high-energy excitations are suppressed, while vibrational modes of energies
Chouhan D. K., Patro T. U., Harikrishnan G., Kumar S., Gupta S., Kumar G. S., Cohen H. & Wagner H. D. (2016) Applied Clay Science. 132-133, p. 105-113
A simple method for preparation of hybrid of graphene oxide (GO) and Laponite (Lap), obtained by solvent evaporation from their highly stable aqueous dispersions is reported. The dispersion up to ~ 1 mg/ml of GO in 1% Lap dispersion, i.e., 10:1 of Lap:GO was found to be stable without flocculation for several months; lower mass ratios of Lap to GO than this showed marginal flocculation with time. The electrostatic interaction between cations present in the interlayers of Lap and the functional groups of GO is envisaged to be the cause for the stable dispersion, which was confirmed by the presence of cations; viz., Na<sup>+</sup> and small amounts of K<sup>+</sup> and Mg<sup>2 +</sup> in the aqueous filtrate of the hybrid. Their interaction was further confirmed by higher absorption of GO in aqueous Lap dispersion than that in water using UVvis spectroscopy. The resulting hybrid material was found to be partially reduced and self-assembled to form layered structure in its dry state. The hybrids further showed improved electrical conductivity (~ 0.01 S/cm) upon chemical reduction. The present study demonstrates a facile method for preparation of a new hybrid material and greener pathway for GO reduction; though partially. This hybrid has potential as multifunctional filler for clay polymer nanocomposites.
Maoz R., Burshtain D., Cohen H., Nelson P., Berson J., Yoffe A. & Sagiv J. (2016) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 55, 40, p. 12366-12371
Effective control of chemistry at interfaces is of fundamental importance for the advancement of methods of surface functionalization and patterning that are at the basis of many scientific and technological applications. A conceptually new type of interfacial chemical transformations has been discovered, confined to the contact surface between two solid materials, which may be induced by exposure to X-rays, electrons or UV light, or by the application of electrical bias. One of the reacting solids is a removable thin film coating that acts as a reagent/catalyst in the chemical modification of the solid surface on which it is applied. Given the diversity of thin film coatings that may be used as solid reagents/catalysts and the lateral confinement options provided by the use of irradiation masks, conductive AFM probes or stamps, and electron beams in such solid-phase reactions, this approach is suitable for precise targeting of different desired chemical modifications to predefined surface sites spanning the macro- to nanoscale.
Zohar A., Kedem N., Levine I., Zohar D., Vilan A., Ehre D., Hodes G. & Cahen D. (2016) The journal of physical chemistry letters. 7, 1, p. 191-197
Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH<sub>3</sub>NH<sub>3</sub>]PbI<sub>3</sub>(Cl) ("perovskite") films in vacuo, and in N<sub>2</sub>, air, and O<sub>2</sub>, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance in vacuo or post-vacuo N<sub>2</sub> exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O<sub>2</sub>-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material's electrical properties and electrochemical reactivity, which can also affect material stability.
Pawlicki A., Avery E., Jurow M., Ewers B., Vilan A., Drain C. M. & Batteas J. (2016) Journal Of Physics-Condensed Matter. 28, 9, 094013
Directing molecular devices into pre-designed integrated electronic circuits while enforcing selectivity and hierarchy is an inherent challenge for molecular electronics. Here we explore ways to direct the assembly of electrically-active molecular monolayers into specific locations as well as controlling their internal organization. We have accomplished this by two consecutive surface reactions: (1) forming pentanedithiol (C5DT) domains within an inert alkanethiol self-assembled monolayer (SAM) on Au; and (2) selectively binding porphyrin derivatives to the C5DT domains. The C5DT domains were fabricated by phase segregation during co-adsorption from a mixed C5DT/dodecanethiol (C12) solution and nanografting with Atomic Force Microscopy (AFM). AFM revealed that co-absorbed and nanografted C5DT domains were in a standing-up phase and scanning tunneling microscopy (STM) showed that their molecular organization within about 5 nm, 40 nm, 50 nm and 120 nm domains, was dependent upon the size of the domain, such that structure of the C5DT transitions from ( × ) R30°, to (2 × 2), and ultimately to a disordered phase with increasing domain size. This is due to the varying degrees of influence of the surrounding C12; providing sufficient van der Waals interactions as well as a geometric confinement to stabilize the standing-up phase of the C5DT. Understanding the molecular configuration of dithiol SAMs affords their use as a reactive template to subsequently bind active head groups. As a proof of principle, porphyrins with a pendant pentafluorophenyl ring were attached to the C5DT domains by a 'click' reaction between the fluorinated ring and the free thiol on the surface. From AFM and STM, these porphyrin derivatives reacted selectively with the C5DT domains with some porphyrins binding directly to the C5DT, subsequently allowing additional localized porphyrin deposition through pi-stacking.
Vilan A. (2016) Journal of Applied Physics. 119, 1, 014504
Molecular electronics studies how the molecular nature affects the probability of charge carriers to tunnel through the molecules. Nevertheless, transport is also critically affected by the contacts to the molecules, an aspect that is often overlooked. Specifically, the limited ability of non-metallic contacts to maintain the required charge balance across the fairly insulating molecule often have dramatic effects. This paper shows that in the case of lead/organic monolayer-silicon junctions, a charge balance is responsible for an unusual current scaling, with the junction diameter (perimeter), rather than its area. This is attributed to the balance between the 2D charging at the metal/insulator interface and the 3D charging of the semiconductor space-charge region. A derivative method is developed to quantify transport across tunneling metal-insulator-semiconductor junctions; this enables separating the tunneling barrier from the space-charge barrier for a given current-voltage curve, without complementary measurements. The paper provides practical tools to analyze specific molecular junctions compatible with existing silicon technology, and demonstrates the importance of contacts' physics in modeling charge transport across molecular junctions.
Butschke B., Fillman K. L., Bendikov T., Shimon L. J. W., Diskin Posner Y., Leitus G., Gorelsky S. I., Neidig M. L. & Milstein D. (2015) Inorganic Chemistry. 54, 10, p. 4909-4926
Herein we present a series of new α-iminopyridine-based iron-PNN pincer complexes [FeBr<sub>2</sub>L<sub>PNN</sub>] (1), [Fe(CO)<sub>2</sub>L<sub>PNN</sub>] (2), [Fe(CO)<sub>2</sub>L<sub>PNN</sub>](BF<sub>4</sub>) (3), [Fe(F)(CO)<sub>2</sub>L<sub>PNN</sub>](BF<sub>4</sub>) (4), and [Fe(H)(CO)<sub>2</sub>L<sub>PNN</sub>](BF<sub>4</sub>) (5) with formal oxidation states ranging from Fe(0) to Fe(II) (L<sub>PNN</sub> = 2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine). The complexes were characterized by a variety of methods including <sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N, and <sup>31</sup>P NMR, IR, Mössbauer, and X-ray photoelectron spectroscopy (XPS) as well as electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy, SQUID magnetometry, and X-ray crystallography, focusing on the assignment of the metal oxidation states. Ligand structural features suggest that the α-iminopyridine ligand behaves as a redox non-innocent ligand in some of these complexes, particularly in [Fe(CO)<sub>2</sub>L<sub>PNN</sub>] (2), in which it appears to adopt the monoanionic form. In addition, the NMR spectroscopic features (<sup>13</sup>C, <sup>15</sup>N) indicate the accumulation of charge density on parts of the ligand for 2. However, a combination of spectroscopic measurements that more directly probe the iron oxidation state (e.g., XPS), density functional theory (DFT) calculations, and electronic absorption studies combined with time-dependent DFT calculations support the description of the metal atom in 2 as Fe(0). We conclude from our studies that ligand structural features, while useful in many assignments of ligand redox non-innocence, may not always accurately reflect the ligand charge state and, hence, the metal oxidation state. For complex 2, the ligand structural changes are interpreted in terms of strong back-donation from the metal center to the ligand as opposed to electron transfer. (Chemical Equation Presented).
Barats-Damatov D., Shimon L. J., Feldman Y., Bendikov T. & Neumann R. (2015) Inorganic Chemistry. 54, 2, p. 628-634
The crystal packing and secondary structure of H<sub>5</sub>PV<sub>2</sub>Mo<sub>12</sub>O<sub>40</sub> was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H<sub>5</sub>PV<sub>2</sub>Mo<sub>12</sub>O<sub>40</sub> becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H<sub>5</sub>PV<sub>2</sub>Mo<sub>12</sub>O<sub>40</sub> to crystalline MoO<sub>3</sub> and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO<sub>3</sub>. Importantly, H<sub>5</sub>PV<sub>2</sub>Mo<sub>12</sub>O<sub>40</sub> can be easily recovered by dissolution in water at 80 °C.
Toledano T., Garrick R., Sinai O., Bendikov T., Haj-Yahia A., Lerman K., Alon H., Sukenik C., Vilan A., Kronik L. & Cahen D. (2015) Journal of Electron Spectroscopy and Related Phenomena. 204, Part A, p. 149-158
We report a combined ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) study of the electronic structure of aromatic self-assembled monolayers covalently bound to Si, using several different aromatic groups (phenyl, biphenyl, and fluorene) and binding groups (O, NH, and CH<sub>2</sub>). We obtain excellent agreement between theory and experiment, which allows for a detailed interpretation of the experimental results. Our analysis reveals a significant effect of the binding group on state hybridization at the organic/inorganic interface. Specifically, it highlights that lone-pair electrons in the binding atom facilitate hybridization between the aromatic system and the Si substrate, resulting in a significant induced density of interface states (IDIS). These interface states are manifested as a broadened HOMO peak in the experimental UPS data and are clearly observed in a theoretical spatially-resolved density of states map. This provides means to control the degree of coupling between substrate and molecule, which may prove useful in the design of transport across organic/inorganic interfaces.
Frank A., Cohen H., Hoffman D. & Amir R. (2015) Amino Acids. 47, 3, p. 497-510
Methionine is a nutritionally essential sulfur-containing amino acid found at low levels in plant tissues. Yet, the factors that regulate its synthesis and accumulation in seeds are not fully known. Recent genetic studies demonstrate that Arabidopsis seeds are able to synthesize methionine de novo through the aspartate family pathway similarly to vegetative tissues; however, additional biochemical studies suggest that the S-methylmethionine (SMM) cycle also plays a major role in methionine synthesis in seeds. To better understand the contribution of these two pathways to methionine synthesis, we have sampled various vegetative and reproductive tissues during the Arabidopsis life cycle and determined the contents of soluble and protein-incorporated methionine, SMM, as well as the expression levels of the key genes involved in these two pathways. Our results strengthen the hypothesis that SMM that is produced in the rosette leaves from methionine contributes to methionine accumulation in seeds. However, the SMM cycle may have additional functions in plant tissues since its key genes were expressed in all of the examined tissues, although at different rates. The accumulation patterns of soluble and protein-incorporated methionine during the Arabidopsis life cycle were found to be similar to most of the other amino acids, especially to those belonging to the branched-chain and aromatic amino acids that are produced in chloroplasts together with methionine. This indicates that similar factors regulate the levels of amino acids during development.
Afek A., Cohen H., Barber-Zucker S., Gordân R. & Lukatsky D. B. (2015) PLoS Computational Biology. 11, 8, e1004429
Recent genome-wide experiments in different eukaryotic genomes provide an unprecedented view of transcription factor (TF) binding locations and of nucleosome occupancy. These experiments revealed that a large fraction of TF binding events occur in regions where only a small number of specific TF binding sites (TFBSs) have been detected. Furthermore, in vitro protein-DNA binding measurements performed for hundreds of TFs indicate that TFs are bound with wide range of affinities to different DNA sequences that lack known consensus motifs. These observations have thus challenged the classical picture of specific protein-DNA binding and strongly suggest the existence of additional recognition mechanisms that affect protein-DNA binding preferences. We have previously demonstrated that repetitive DNA sequence elements characterized by certain symmetries statistically affect protein-DNA binding preferences. We call this binding mechanism nonconsensus protein-DNA binding in order to emphasize the point that specific consensus TFBSs do not contribute to this effect. In this paper, using the simple statistical mechanics model developed previously, we calculate the nonconsensus protein-DNA binding free energy for the entire C. elegans and D. melanogaster genomes. Using the available chromatin immunoprecipitation followed by sequencing (ChIP-seq) results on TF-DNA binding preferences for ~100 TFs, we show that DNA sequences characterized by low predicted free energy of nonconsensus binding have statistically higher experimental TF occupancy and lower nucleosome occupancy than sequences characterized by high free energy of nonconsensus binding. This is in agreement with our previous analysis performed for the yeast genome. We suggest therefore that nonconsensus protein-DNA binding assists the formation of nucleosome-free regions, as TFs outcompete nucleosomes at genomic locations with enhanced nonconsensus binding. In addition, here we perform a new, large-scale analysis using in vitro TF-DNA preferences obtained from the universal protein binding microarrays (PBM) for ~90 eukaryotic TFs belonging to 22 different DNA-binding domain types. As a result of this new analysis, we conclude that nonconsensus protein-DNA binding is a widespread phenomenon that significantly affects protein-DNA binding preferences and need not require the presence of consensus (specific) TFBSs in order to achieve genome-wide TF-DNA binding specificity.
Givon A., Piao H., McMahon J., Zorn G. & Cohen H. (2015) Applied Physics Letters. 107, 17, 173101
Probing depth electrostatic potential profiles at sub-nm resolution is a major characterization challenge. An answer is frequently proposed by chemically resolved electrical measurements (CREM); yet, CREM is limited in extracting the profile details within compositionally uniform domains. Here, we show that this principal limitation can be overcome and the CREM resolution be improved significantly. Applied to nanometric SiO2 layers on SiC, hidden impurity concentration profiles are revealed and the inner fields, before and during dielectric collapse, are quantified. With this leap improvement in resolution and sensitivity, our advanced CREM analysis promises diverse applications in device contact-free electrical studies. (C) 2015 AIP Publishing LLC.
Visic B., Cohen H., Popovitz-Biro R., Tenne R., Sokolov V. I., Abramova N. V., Buyanovskaya A. G., Dzvonkovskii S. L. & Lependina O. L. (2015) Chemistry, an Asian journal. 10, 10, p. 2234-2239
Palladium nanoparticles were deposited on multiwall WS<sub>2</sub> nanotubes. The composite nanoparticles were characterized by a variety of techniques. The Pd nanoparticles were deposited uniformly on the surface of WS<sub>2</sub> nanotubes. An epitaxial relationship between the (111) plane of Pd and the (013) plane of WS<sub>2</sub> was mostly observed. The composite nanoparticles were found to perform efficiently as catalysts for cross-coupling (Heck and Suzuki) reactions. The role of the nanotubes support in the catalytic process is briefly discussed.
Fontanesi C., Tassinari F., Parenti F., Cohen H., Mondal P. C., Vankayala K., Giglia A., Pasquali L. & Naaman R. (2015) Langmuir. 31, 11, p. 3546-3552
This article reports on a facile and fast strategy for the self-assembled monolayer (SAM) functionalization of nickel surfaces, employing cyclic voltammetry (CV) cycling of a suitable tailored solution containing the species to be adsorbed. Results are presented for ultrathin films formed on Ni by 1-hexadecanethiol (C16), l-cysteine (l-cys), and the poly{methyl (2R)-3-(2,2'-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate} (PCT-L) thiophene-based chiral polymer. The effective formation of high-quality ultrathin organic films on the nickel was verified both electrochemically and by exploiting typical surface characterization techniques such as contact angle, ellipsometry, atomic force microscopy (AFM), polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS).
Kirmayer S., Edri E., Hines D., Klein Kedem N., Cohen H., Niitsoo O., Pinkas I., Kamat P. V. & Hodes G. (2015) Advanced Materials Interfaces. 2, 1, 1400346
TiO<sub>2</sub>/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 °C). Annealing in N<sub>2</sub> produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N<sub>2</sub>-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N<sub>2</sub>-annealed systems, correlated with the formation of Cd-O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO<sub>2,</sub> but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers. Upon annealing in air, extremely thin absorber solar cells based on CdSe-sensitized titania show relatively high values of open-circuit voltage but low currents. This stems from the fact that while oxidation impedes electron injection from the CdSe to the TiO<sub>2,</sub> the balance between hole extraction and recombination is improved in favor of the former.
Schreiber R., Cohen H., Leitus G., Wolf S. G., Zhou A., Que L. & Neumann R. (2015) Journal of the American Chemical Society. 137, 27, p. 8738-8748
Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH<sub>2</sub>(Mn-L)W<sub>17</sub>F<sub>6</sub>O<sub>55</sub>]<sup>q-</sup>, has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O<sub>2</sub>, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K<sup>+</sup>, Rb<sup>+</sup>, and Cs<sup>+</sup>) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li<sup>+</sup> and Na<sup>+</sup> did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O<sub>2</sub> formation is fastest in the presence of Cs<sup>+</sup> and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O<sub>2</sub> formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O<sub>2</sub> formation.
Givon A. & Cohen H. (2015) Surface and Interface Analysis. 47, 5, p. 607-611
The role of angular broadening in quantitative core-electron spectroscopy is investigated using an analytic approach. It is shown why, practically, this effect remains relatively small for a broad range of parameters. A correction factor is derived, suggesting that the replacement of inelastic mean free path by an effective attenuation-length parameter is not necessarily an optimal choice. The derived expression further proposes useful insight on the contribution of leading experimental parameters and, in particular, on the sharp increase of elastic-scattering corrections above a (depth dependent) critical angle.
Jiang L., Sangeeth C. S., Wan A., Vilan A. & Nijhuis C. A. (2015) Journal of Physical Chemistry C. 119, 2, p. 960-969
Although the tunneling rates decrease exponentially with a decay coefficient β close to 1.0 n<sub>C</sub><sup>-1</sup> across n-alkanethiolate (SC<sub>n</sub>) monolayer based tunneling junctions determined over a multitude of test beds, the origins of the large spread of injection current densities - the hypothetical current density, J<sub>0</sub> (in A/cm<sup>2</sup>), that flows across the junction when n = 0 - of up to 12 orders of magnitude are unclear. Every type of junction contains a certain distribution of defects induced by, for example, defects in the electrode materials or impurities. This paper describes that the presence of defects in the junctions is one of the key factors that cause an increase in the observed values of J<sub>0</sub>. We controlled the number of defects in Ag<sup>TS</sup>-SC<sub>n</sub>//GaO<sub>x</sub>/EGaIn junctions by varying the geometrical contact area (Ageo) of the junction. The value of J<sub>0</sub> (∼10<sup>2</sup> A/cm<sup>2</sup>) is independent of the junction size when A<sub>geo</sub> is small (
Yu X., Lovrincic R., Sepunaru L., Li W., Vilan A., Pecht I., Sheves M. & Cahen D. (2015) ACS Nano. 9, 10, p. 9955-9963
Surprisingly efficient solid-state electron transport has recently been demonstrated through "dry" proteins (with only structural, tightly bound H<sub>2</sub>O left), suggesting proteins as promising candidates for molecular (bio)electronics. Using inelastic electron tunneling spectroscopy (IETS), we explored electron-phonon interaction in metal/protein/metal junctions, to help understand solid-state electronic transport across the redox protein azurin. To that end an oriented azurin monolayer on Au is contacted by soft Au electrodes. Characteristic vibrational modes of amide and amino acid side groups as well as of the azurin-electrode contact were observed, revealing the azurin native conformation in the junction and the critical role of side groups in the charge transport. The lack of abrupt changes in the conductance and the line shape of IETS point to far off-resonance tunneling as the dominant transport mechanism across azurin, in line with previously reported (and herein confirmed) azurin junctions. The inelastic current and hence electron-phonon interaction appear to be rather weak and comparable in magnitude with the inelastic fraction of tunneling current via alkyl chains, which may reflect the known structural rigidity of azurin.
Fadida T., Kroupitski Y., Peiper U. M., Bendikov T., Sela Saldinger S. & Poverenov E. (2014) Colloids And Surfaces B-Biointerfaces. 122, p. 294-300
Air-ozonolysis was revealed as an accessible and effective approach for surface activation and further functionalization of hydrocarbon polymers. Antimicrobial contact active polyethylene (PE) and polystyrene (PS) were designed by generation on their surfaces OH-functional groups and covalent graft of dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (C<sub>18</sub>-TSA) quaternary ammonium salt. The shortened analog, trimethyl [3-(trimethoxysilyl) propyl] ammonium chloride (C<sub>1</sub>-TSA), was also covalently attached to the activated PE and PS surfaces. X-ray photoelectron spectroscopy (XPS) and FTIR confirmed the surface modifications. Scanning electron (SEM) and confocal microscopy were utilized to monitor surface morphology and bacteria interactions. The antimicrobial effect of the C<sub>18</sub>-TSA grafted polymer surfaces was demonstrated on Gram-negative and Gram-positive bacteria species including human pathogen, Salmonella enterica. The shorter C<sub>1</sub>-TSA grafted polymers did not demonstrate bactericidal activity, suggesting the critical role of the alkyl chain length. The described strategy may establish a new general and safe platform for future development and application of contact active antimicrobial polymers.
Zell T., Milko P., Fillman K. L., Diskin Posner Y., Bendikov T., Iron M. A., Leitus G., Ben-David Y., Neidig M. L. & Milstein D. (2014) Chemistry - A European Journal. 20, 15, p. 4403-4413
A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (<sup>1</sup>H, <sup>13</sup>C, <sup>15</sup>N, <sup>31</sup>P), IR spectroscopy, cyclic voltammetry, <sup>57</sup>Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)<sub>2</sub>] (5: R-PNN=tBu-PNN=6-[(di-tert- butylphosphino)methyl]-2,2-bipyridine, 6: R-PNN=iPr-PNN=6- [(diisopropylphosphino)methyl]-2,2-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)<sub>2</sub>] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO. The analogous Ph-substituted complex 7 was synthesized by a reaction of the free ligand with iron pentacarbonyl. The low-spin complexes 5-7 (S=0) are diamagnetic and have distorted trigonal bipyramidal structures in solution, whereas in the solid state the geometries around the iron are best described as distorted square pyramidal. Compared to other structurally characterized complexes with these PNN ligands, shortened interpyridine C-C bonds of about 1.43 Å were measured. A comparison with known examples, theoretically described as metal complexes bearing bipyridine π-radical anions (bpy <sup>.-</sup>), suggests that the complexes can be described as Fe<sup>I</sup> complexes with one electron antiferromagnetically coupled to the ligand-based radical anions. However, computational studies, at the NEVPT2/CASSCF level of theory, reveal that the shortening of the C-C bond is a result of extensive π-backbonding of the iron center into the antibonding orbital of the bpy unit. Hence, the description of the complexes as Fe <sup>0</sup> complexes with neutral bipyridine units is the favorable one. Innocent till proved guilty! Metrical parameters for the assignment of oxidation states of bipyridine ligands are challenged. A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and fully characterized by various methods. Unusually short interpyridine C-C bonds were derived by X-ray diffraction (see scheme). The question if this is an effect of an intramolecular electron transfer or an effect of classical π-backbonding is addressed.
Barnea-Nehoshtan L., Nayak P. K., Shu A., Bendikov T., Kahn A. & Cahen D. (2014) ACS Applied Materials and Interfaces. 6, 4, p. 2317-2324
The alignment between the energy levels of the constituents of an organic solar cell plays a central role in determining the open-circuit voltage. However, tuning the energy levels of electrodes and/or active components via molecular modifiers placed at interfaces is not straightforward. The morphology of organic materials is commonly controlled by the substrate onto which they are deposited, and differences in morphology often lead to differences in energetics. Such a change in morphology may reduce the effect of surface modifications, as the modified surface is part of an interface with the organic material. Here we show, in an experimental model system, that by using binary molecular monolayers, in which dipolar molecules are buried in a protective nonpolar matrix, we can transform changes in the electrode surface dipole into interface dipole changes without significantly affecting the growth of pentacene onto the molecular layer, thus enabling the use of the full range of dipolar-induced open-circuit-voltage tuning.
Toledano T., Sazan H., Mukhopadhyay S., Alon H., Lerman K., Bendikov T., Major D. T., Sukenik C. N., Vilan A. & Cahen D. (2014) Langmuir. 30, 45, p. 13596-13605
A distinct odd-even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H<sub>2</sub>C=CH-(CH<sub>2</sub>)<sub>n</sub>-phenyl, with n = 2-5, were adsorbed onto Si-H and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π-π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd-even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics.
Cohen H., Israeli H., Matityahu I. & Amir R. (2014) Plant Physiology. 166, 3, p. 1575-1592
With an aim to elucidate novel metabolic and transcriptional interactions associated with methionine (Met) metabolism in seeds, we have produced transgenic Arabidopsis (Arabidopsis thaliana) seeds expressing a feedback-insensitive form of CYSTATHIONINE-g- SYNTHASE, a key enzyme of Met synthesis. Metabolic profiling of these seeds revealed that, in addition to higher levels of Met, the levels of many other amino acids were elevated. The most pronounced changes were the higher levels of stress-related amino acids (isoleucine, leucine, valine, and proline), sugars, intermediates of the tricarboxylic acid cycle, and polyamines and lower levels of polyols, cysteine, and glutathione. These changes reflect stress responses and an altered mitochondrial energy metabolism. The transgenic seeds also had higher contents of total proteins and starch but lower water contents. In accordance with the metabolic profiles, microarray analysis identified a strong induction of genes involved in defense mechanisms against osmotic and drought conditions, including those mediated by the signaling cascades of ethylene and abscisic acid. These changes imply that stronger desiccation processes occur during seed development. The expression levels of transcripts controlling the levels of Met, sugars, and tricarboxylic acid cycle metabolites were also significantly elevated. Germination assays showed that the transgenic seeds had higher germination rates under salt and osmotic stresses and in the presence of ethylene substrate and abscisic acid. However, under oxidative conditions, the transgenic seeds displayed much lower germination rates. Altogether, the data provide new insights on the factors regulatingMet metabolism in Arabidopsis seeds and on the mechanisms by which elevated Met levels affect seed composition and behavior.
Beilis E., Belgorodsky B., Fadeev L., Cohen H. & Richter S. (2014) Journal of the American Chemical Society. 136, 17, p. 6151-6154
Evidence for considerable stabilization of doped bovine serum albumin (BSA) molecules upon adsorption on gold surfaces is provided. This is compared to the surface-induced conformational changes of the bare BSA and its corresponding monolayer. The BSA unfolding phenomenon is correlated with dehydration, which in turn enables improved monolayer coverage. The stabilization mechanism is found to be partially controllable via nanodoping of the BSA molecules, upon which the dehydration process is suppressed and molecular rigidity can be varied. Our experimental data and calculations further point to the intermixing of structural characteristics and inherent molecular properties in studies of biological monolayers.
Edri E., Kedem N., Cohen H., Barnes P. & Hodes G. (2014) Journal of Physical Chemistry C. 118, 30, p. 16884-16891
Solid-state semiconductor-sensitized solar cells require a thin, dense hole-blocking layer at the conducting glass substrate (F-doped tin oxide (FTO)) to prevent shorting beween the FTO and hole conductor. We found that by adding a small amount of Sb ions to a ZnO chemical deposition bath a thin (few tens of nanometers thick) dense and uniform layer of Sb-incorporated ZnO forms. Here we investigate the electronic properties of this layer in comparison to the continuous ZnO layer at the base of the ZnO rods formed in the standard preparation. Devices incorporating the Sb-incorporated dense layer followed by a standard ZnO nanorod growth, onto which CdS or CdSe was grown followed by a CuSCN hole conductor, showed 100-200 mV higher photovoltage together with occasional improvement in the short-circuit current. Electrochemical and electrical measurements indicated complete coverage of the FTO substrate by both preparations; however, the shunt resistance (resistance to a reverse leakage current) in the cells (and films) made using the Sb-incorporated ZnO layer is dramatically increased. Using bias-dependent incident photon-to-electron conversion efficiency studies, we found that an increased dark or leakage current develops in the cell on illumination with UV light together with application of a forward bias. This can be explained by the presence of a "Schottky junction" at the FTO\ZnO interface. This increased leakage current is significantly larger in cells without the Sb-incorporated ZnO compact layer.
Walther R., Carmeli I., Schneider R., Gerthsen D., Busch K., Matyssek C., Shvarzman A., Maniv T., Richter S. & Cohen H. (2014) Journal of Physical Chemistry C. 118, 20, p. 11043-11049
One-dimensional arrays of submicrometer rectangular holes in 200 nm thin gold films are investigated using electron energy loss spectroscopy combined with scanning transmission electron microscopy (STEM-EELS). Improved energy resolution, down to 0.11 eV, is accomplished in our monochromated transmission electron microscope, allowing the reliable quantification of signals at loss energies as small as 0.43 eV. The standing-wave resonances of individual holes are thus investigated at nanometer-scale spatial resolution, focusing in particular on the effect of neighboring holes. We show how the coupling between holes is facilitated by surface plasmon polaritons (SPPs) propagating on the top and bottom surfaces of the separating metal-film strips. Thus, complex spatiotemporal coupling dynamics emerges, characterized by strong interslit interactions and a phase that can be controlled by varying the width of the metal strip between adjacent holes. Applying real-space real-time numerical simulations and exploiting the short interaction time of 300 keV electrons with the thin gold film, we reveal intriguing features of these ultrafast coupling mechanisms, including unusual line-narrowing and marked SPP signal enhancements in the corresponding EEL spectra. Complementary aspects of the far-field and near-field components in the SPP eigenstates are further demonstrated. Our combined analysis effectively equips STEM-EELS with an excellent temporal resolution and further yields a consistent description of the entangled femtosecond-scale SPP dynamics.
Sarkar D., Taffa D. H., Ishchuk S., Hazut O., Cohen H., Toker G., Asscher M. & Yerushalmi R. (2014) Chemical Communications. 50, 65, p. 9176-9178
Tailor-made metal oxide (MO) thin films with controlled compositions, electronic structures, and architectures are obtained via molecular layer deposition (MLD) and solution treatment. Step-wise formation of permeable hybrid films by MLD followed by chemical modification in solution benefits from the versatility of gas phase reactivity on surfaces while maintaining flexibility which is more common at the liquid phase.
Kedem N., Blumstengel S., Henneberger F., Cohen H., Hodes G. & Cahen D. (2014) Physical Chemistry Chemical Physics. 16, 18, p. 8310-8319
The work function (WF) of ZnO is modified by two types of dipole-bearing phenylphosphonate layers, yielding a maximum WF span of 1.2 eV. H <sub>3</sub>CO-phenyl phosphonate, with a positive dipole (positive pole pointing outwards from the surface), lowers the WF by ∼350 meV. NC-phenyl phosphonate, with a negative dipole, increases the WF by ∼750 meV. The WF shift is found to be independent of the type of ZnO surface. XPS data show strong molecular dipoles between the phenyl and the functionalizing (CN and OMe) tail groups, while an opposite dipole evolves in each molecular layer between the surface and the phenyl rings. The molecular modification is found to be invariant to supra-bandgap illumination, which indicates that the substrate's space charge-induced built-in potential is unlikely to be the reason for the WF difference. ZnO, grown by several different methods, with different degrees of crystalline perfection and various morphologies and crystallite dimensions, could all be modified to the same extent. Furthermore, a mixture of opposite dipoles allows gradual and continuous tuning of the WF, varying linearly with the partial concentration of the CN-terminated phosphonate in the solution. Exposure to the phosphonic acids during the molecular layer deposition process erodes a few atomic layers of the ZnO. The general validity of the treatment and the fine-tuning of the WF of treated interfaces are of interest for solar cells and LED applications.
Yu X., Lovrincic R., Kraynis O., Man G., Ely T., Zohar A., Toledano T., Cahen D. & Vilan A. (2014) Small. 10, 24, p. 5151-5160
Reproducible molecular junctions can be integrated within standard CMOS technology. Metal-molecule-semiconductor junctions are fabricated by direct Si-C binding of hexadecane or methyl-styrene onto oxide-free H-Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO<sub>2</sub> well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log( J ) of 0.2 over a junction diameter change from 3 to 100 μm. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n-Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium-doped n-Si, the molecular tunneling barrier can be separated from the Si-Schottky one, which is a 0.47 eV, in agreement with the molecular-modified surface dipole and quite different from the bare Si-H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si-based molecular electronics.
Haj Yahia Y. A. E., Yaffe O., Bendikov T., Cohen H., Feldman Y. (., Vilan A. & Cahen D. (2013) Advanced Materials. 25, 5, p. 702-706
An eight-orders of magnitude enhancement in current across Hg/X-styrene-Si junctions is caused by merely altering a substituent, X. Interface states are passivated and, depending on X, the Si Schottky junction encompasses the full range from Ohmic to strongly rectifying. This powerful electrostatic molecular effect has immediate implications for interface band alignment and sensing.
Chaikin Y., Bendikov T. A., Cohen H., Vaskevich A. & Rubinstein I. (2013) Journal of Materials Chemistry C. 1, 22, p. 3573-3583
The first case of phosphonic acid terminated, environmentally friendly silver nanoparticles (NPs) is described. The NPs are produced by a simple one-step synthesis using commercially available reagents, furnishing stable, rather monodisperse, size-controlled, aqueous-based Ag NPs, stabilized by aminomethylene phosphonic acid (AMP) molecules. In this synthesis the commercial reagent ethylenediamine tetra(methylene phosphonic acid) (EDTMP) serves as a reducing agent for Ag<sup>+</sup> ions, while its oxidation product AMP is the stabilizer of the generated Ag NPs. The negatively charged, phosphonate- stabilized NPs were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Variation of the EDTMP/AgNO<sub>3</sub> molar ratio enables simple and efficient control of the average particle diameter in the range ∼5.5 to 15 nm. The AMP-stabilized Ag NPs are stable in water under an inert atmosphere for at least 2 months with no observed aggregation. Self-assembled layers of phosphonate-coated Ag NPs were prepared on substrates primed with positively charged molecular self-assembled monolayers (SAMs) or with polyelectrolyte (PE) layers. The NP films were studied by UV-vis spectroscopy, polarization modulation Fourier-transform infrared reflection-absorption spectroscopy (PM-IRRAS), and high-resolution scanning electron microscopy (HRSEM). While NP monolayers commonly undergo extensive aggregation upon drying, the present phosphonate-stabilized Ag NP monolayers display homogeneously dispersed, mostly isolated NPs over large areas after adsorption and drying. The NP distribution and degree of aggregation can be modulated by the substrate type (gold, glass), the colloid solution pH, the nature of the primer layer (charged molecules, PEs), and the surface charge density. Phosphonate-terminated Ag NPs provide unique physical and chemical properties, including a negatively charged surface in a wide pH range, long-term stability, size control, and the possibility of participating in electrostatic and coordination binding processes.
Voldman A., Zbaida D., Cohen H., Leitus G. & Tenne R. (2013) Macromolecular Chemistry and Physics. 214, 18, p. 2007-2015
Polymer-inorganic nanoparticle composites are an area of great research interest. Inorganic fullerene-like (IF) structures and inorganic nanotubes (INT) are used for producing composite materials with specific goals of reinforcement of the polymer and ameliorating its thermal stability. Here, a composite material containing INT and conducting polymer (polyaniline (PANI)) is synthesized by an in situ oxidative polymerization of the monomer in the presence of WS<sub>2</sub> nanotubes. The structure and electrical behavior of PANI/INT composite is studied and compared with the results of pristine PANI (synthesized under the same conditions without INT). Most remarkably, INT-WS<sub>2</sub> are found to play the role of an active doping agent. The highest conductivity is obtained for 0.85 wt% (ca. 0.06 at%) nanotubes content, two orders of magnitude higher than that of pristine PANI. This work suggests a new approach to control the host-guest interaction in the polymer nanocomposite via (Lewis) acid-base equilibrium. A composite material containing WS <sub>2</sub> nanotubes (INT-WS<sub>2</sub>) and conducting polymer (polyaniline (PANI)) is obtained by an in situ oxidative polymerization of the monomer in the presence of the INT-WS<sub>2</sub>. Electron transfer from PANI to the INT-WS<sub>2</sub> is shown to produce a peak in the conductivity at 0.85 wt% (0.2 at%) of the nanotubes.
Yaffe O., Ely T., Har Lavan L. R., Egger D. A., Johnston S., Cohen H., Kronik L., Vilan A. & Cahen D. (2013) Journal of Physical Chemistry C. 117, 43, p. 22351-22361
We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S<sub>N</sub>) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S<sub>N</sub> reaction, whereas with UV irradiation RCR dominates, with S<sub>N</sub> as a secondary path. We show that the site-sensitive S<sub>N</sub> reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S<sub>N</sub> and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.
Yaffe O., Pujari S., Sinai O., Vilan A., Zuilhof H., Kahn A., Kronik L., Cohen H. & Cahen D. (2013) Journal of Physical Chemistry C. 117, 43, p. 22422-22427
The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.
Ehre D. & Cohen H. (2013) Applied Physics Letters. 103, 5, 052901
Non-contact pyroelectricity measurements based on x-ray photoelectron spectroscopy (XPS) are presented. Applied to Lithium Tantalate crystals, we demonstrate how the XPS-derived surface potential provides a simple probe of the desired property, free of all top-contact related difficulties. In particular, the increase in Lithium Tantalate spontaneous polarization under cooling, an experimentally challenging feature, is evaluated. We further inspect the roll of surface contaminants and the control over trapped surface charge in the XPS vacuum environment. Our approach can be extended to other non-contact probes, as well as to measuring additional electrical properties, such as piezoelectricity and ferroelectricity.
Edri E., Cohen H. & Hodes G. (2013) ACS Applied Materials and Interfaces. 5, 11, p. 5156-5164
In all solar cells, and especially in extremely thin absorber (ETA) solar cells, proper energy band alignment is crucial for efficient photovoltaic conversion. However, available tabulated data usually do not agree with actual results, and in most cases, V<sub>oc</sub> values lower than expected are achieved. In fact, ETA cells suffer from a very low V<sub>oc</sub>/E <sub>gap</sub> ratio, such as in ZnO/CdS/CuSCN cells. Here, we investigate limiting factors of ZnO/CdS/CuSCN ETA cells, applying X-ray photoelectron spectroscopy (XPS), chemically resolved electrical measurement (CREM), Kelvin probe, and I-V characterization. We show that electric fields are gradually developed in the cell upon increased absorber thickness. Moreover, an accumulation layer, unfavorable for the solar cell function, has been revealed at the oxide-absorber interface An effective chemical treatment to prevent formation of this accumulation layer is demonstrated.
Vilan A., Cahen D. & Kraisler E. (2013) ACS Nano. 7, 1, p. 695-706
Transition voltage spectroscopy (TVS) has become an accepted quantification tool for molecular transport characteristics, due to its simplicity and reproducibility. Alternatively, the Taylor expansion view, TyEx, of transport by tunneling suggests that conductance-voltage curves have approximately a generic parabolic shape, regardless of whether the tunneling model is derived from an average medium view (e.g., WKB) or from a scattering view (e.g., Landauer). Comparing TVS and TyEx approaches reveals that TVS is closely related to a bias-scaling factor, V<sub>0</sub>, which is directly derived from the third coefficient of TyEx, namely, the second derivative of the conductance with respect to bias at 0 V. This interpretation of TVS leads to simple expressions that can be compared easily across primarily different tunneling models. Because the basic curve shape is mostly generic, the quality of model fitting is not informative on the actual tunneling model. However internal correlation between the conductance near 0 V and V<sub>0</sub> (TVS) provides genuine indication on fundamental tunneling features. Furthermore, we show that the prevailing concept that V<sub>0</sub> is proportional to the barrier height holds only in the case of resonant tunneling, while for off-resonant or deep tunneling, V<sub>0</sub> is proportional to the ratio of barrier height to barrier width. Finally, considering TVS as a measure of conductance nonlinearity, rather than as an indicator for energy level spectroscopy, explains the very low TVS values observed with a semiconducting (instead of metal) electrode, where transport is highly nonlinear due to the relatively small, bias-dependent density of states of the semiconducting electrode.
Lovrincic R., Kraynis O., Har Lavan L. R., Haj-Yahya A., Li W., Vilan A. & Cahen D. (2013) Journal of Physical Chemistry Letters. 4, 3, p. 426-430
Thermally evaporated Pb preserves the electronic properties of an organic monolayer (ML) on Si and surface passivation of the Si surface itself. The obtained current-voltage characteristics of Pb/ML/Si junctions agree with results obtained with the well-established Hg contact and preserve both the molecule-induced dipole effect on, and length-attenuation of, the current. We rationalize our findings by the lack of interaction between the Pb and the Si substrate. This method is fast, scalable, and compatible with standard semiconductor processing, results in close to 100% yield, and can help the development of large-scale utilization of silicon-organic hybrid electronics. Our experimental data show a dependence of the transport across the molecules on the substrate orientation, expressed in the smaller distance decay parameter with Si(100) than that with Si(111).
Ely T., Das S., Li W., Kundu P. K., Tirosh E., Cahen D., Vilan A. & Klajn R. (2013) Synlett. 24, 18, p. 2370-2374, ST-2013-B0
We report a method for preparing electrode-molecule-electrode junctions that incorporate nonsymmetrical azobenzene dithiols. Our approach is based on sequential deprotection of thiol moieties originally carrying two different protecting groups. The azobenzene derivatives retained their switching properties within monolayers and permitted the photocontrol of electrical conductance.
Buzaglo M., Shtein M., Kober S., Lovrincic R., Vilan A. & Regev O. (2013) Physical Chemistry Chemical Physics. 15, 12, p. 4428-4435
Dispersing graphite into few-layers graphene sheets (GS) in water is very appealing as an environmental-friendly, low-cost, low-energy method of obtaining graphene. Very high GS concentrations in water (0.7 mg mL<sup>-1</sup>) were obtained by optimizing the nature of dispersant and the type of ultra-sonic generator. We find that a multi-step sonication procedure involving both tip and bath sources considerably enhances the yield of exfoliated GS. Raman and transmission electron microscopy indicate few-layers graphene patches with typical size of ∼0.65 µm in one dimension and ∼0.35 µm in the other. These were further employed in combination with water-dispersed CNTs to fabricate conductive transparent electrodes for a molecularly-controlled solar-cell with an open-circuit voltage of 0.53 V.
Toledano T., Biller A., Bendikov T., Cohen H., Vilan A. & Cahen D. (2012) Journal of Physical Chemistry C. 116, 21, p. 11434-11443
Good passivation of Si, both electrically and chemically, is achieved by monolayers of 1,9-decadiene, directly bound to an oxide-free Si surface. The terminal C - C bond of the decadiene serves for further in situ reaction, without harming the surface passivation, to -OH- or -Br-terminated monolayers that have different dipole moments. Such a two-step procedure meets the conflicting requirements of binding mutually repelling dipolar groups to a surface, while chemically blocking all surface reactive sites. We demonstrate a change of 0.15 eV in the Si surface potential, which translates into a 0.4 eV variation in the Schottky barrier height of a Hg junction to those molecularly modified n-Si surfaces. Charge transport across such junctions is controlled both by tunneling across the molecular monolayer and by the Si space charge. For reliable insight into transport details, we resorted to detailed numerical simulations, which reveal that the Si space charge and the molecular tunneling barriers are coupled. As a result, attenuation due to the molecular tunneling is much weaker than in metal/molecule/metal molecular junctions. Simulation shows also that some interface states are present but that they have a negligible effect on Fermi level pinning. These states are efficiently decoupled from the metal (Hg) and interact mostly with the Si.
Lachman N., Sui X., Bendikov T., Cohen H. & Wagner D. H. (2012) Carbon. 50, 5, p. 1734-1739
Structural changes in multi-walled carbon nanotubes (CNTs) resulting from covalent functionalization during oxidation were studied by transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Strong correlation is found between the oxidation progress (i.e. increase in the amount of oxygen and oxidized carbon) and the degradation in inter-atomic conjugation, the latter being inferred from the intensity of satellite ('shake-up') lines in the C1s XPS spectrum. Moreover, defects appear to cluster rather than distribute at random along the CNTs. Hints on the sites most vulnerable to the progress of the chemical attack are provided. Defect clustering leads as well to deterioration of inner layers of nanotubes at relatively early stages of the chemical attack. As a consequence, the mechanical strength of the CNTs degrades but the resultant interface with a host matrix can become physically intermittent, which may positively contribute to the composite strength and toughness.
Piperno S., Cohen H., Bendikov T., Lahav M. & Lubomirsky I. (2012) Physical Chemistry Chemical Physics. 14, 16, p. 5551-5557
It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacrylate (PMMA) or Nylon as well as non-rubbed PMMA and Nylon induce "redox" reactions, including those of the reduction of Pd <sup>+2</sup> and Cu <sup>+2</sup> ions. On this basis, it was deduced that these dielectric materials may hold ≅10 <sup>13</sup>-10 <sup>14</sup> of "hidden" electrons cm <sup>-2</sup>, a value at least three orders of magnitude higher than the charge that a dielectric surface can accumulate without being discharged in air. The "hidden" electrons were termed "cryptoelectrons". In variance to these reports, we offer here an alternative interpretation. Our model is supported by X-ray photoelectron spectroscopy, contact angle and vibrating electrode (modified Kelvin probe) measurements performed on representative examples. Rubbing of the polymers was found to transfer polymer fragments between the rubbed surfaces altering their physical properties. The transferred polymer fragments promote adsorption of Cu <sup>2+</sup> and Pd <sup>2+</sup> ions. It was found that Teflon and PE rubbed with PMMA and Nylon, and non-rubbed PMMA and non-rubbed Nylon do not induce "redox" reactions of Cu <sup>2+</sup> and Pd <sup>2+</sup> ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd <sup>2+</sup> to Pd <sup>0</sup> by electrons, as detected by catalytic activity of Pd <sup>0</sup> in a Cu-plating bath, can be alternatively explained by reduction of adsorbed Pd <sup>2+</sup> by the reducing agents of the bath itself. Based on these findings, we support the hypothesis that charging of dielectric polymers is due to ions or free radicals rather than electrons and there is no evidence to invoke a hypothesis of "cryptoelectrons".
Piperno S., Cohen H., Bendikov T., Lahav M. & Lubomirsky I. (2012) Physical Chemistry Chemical Physics. 14, 31, p. 11185-11186
Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu <sup>2+</sup> and Pd <sup>2+</sup> ions can be absorbed but not chemically reduced by a Teflon surface rubbed by PMMA (Phys. Chem. Chem. Phys., 2012, 14, 5551). In response, we show that this treatment does not affect the adsorption of Cu <sup>2+</sup> and Pd <sup>2+</sup>. We reaffirm our statement that Cu <sup>2+</sup> and Pd <sup>2+</sup> ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.
Levine I., Weber S. M., Feldman Y. (., Bendikov T., Cohen H., Cahen D. & Vilan A. (2012) Langmuir. 28, 1, p. 404-415
A combined electronic transport-structure characterization of self-assembled monolayers (MLs) of alkyl-phosphonate (AP) chains on Al-AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that "long" APs (C14 and C16) form much denser MLs than do "short" APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed "short" AP MLs attenuate the current relatively more efficiently than those with densely packed, "long" ones. Furthermore, "long" AP ML junctions showed strong bias variation of the length decay coefficient, β, while for "short" AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed.
Sheberla D., Patra S., Sharma S., Bendikov T., Sheynin Y. & Bendikov M. (2012) Chemical Communications. 48, 54, p. 6776-6778
We show that interactions between the electrode surface and the transition metal during the initial step of metal nanoparticle formation can be utilized to control the formation and size of metal nanoparticles deposited on a conducting surface. Pt nanoparticles formed on the PEDOS surface are of smaller size compared to the PEDOT surface.
Yoffe A., Cohen H. & Shelukhin V. (2012) Technical Physics. 57, 1, p. 134-136
A procedure for the production of metal-coated quasi-amorphous pyroelectric thin SrTiO <sub>3</sub> films is described. The films are grown in a modified 306 Edwards magnetron sputtering setup under controlled thermal conditions and stabilized high-accuracy vacuum and gas pressure conditions. Three-layer 200 nm metal-100 nm SrTiO <sub>3</sub>-100 nm metal films are studied. The pyroelectric nature of the electric response of these films to heating is directly established, since metallic contacts made from the same material do not distort the measurement results, which excludes the effect of a contact potential difference on the measurement results.
Mentovich E., Belgorodsky B., Gozin M., Richter S. & Cohen H. (2012) Journal of the American Chemical Society. 134, 20, p. 8468-8473
Control over molecular scale electrical properties within nano junctions is demonstrated, utilizing site-directed C<sub>60</sub> targeting into protein macromolecules as a doping means. The protein molecules, self-assembled in a miniaturized transistor device, yield robust and reproducible operation. Their device signal is dominated by an active center that inverts affinity upon guest incorporation and thus controls the properties of the entire macromolecule. We show how the leading routes of electron transport can be drawn, spatially and energetically, on the molecular level and, in particular, how the dopant effect is dictated by its "strategic" binding site. Our findings propose the extension of microelectronic methodologies to the nanometer scale and further present a promising platform for ex situ studies of biochemical processes.
Moshkovich A., Perfilyev V., Meshi L., Samuha S., Cohen S., Cohen H., Laikhtman A. & Rapoport L. (2012) Tribology International. 46, 1, p. 154-160
Friction and wear of copper rubbed with lubrication in wide range of loads and sliding velocities were studied. The results of friction and wear experiments are presented as the Stribeck curve where the boundary lubrication (BL), mixed (ML) and elasto-hydrodynamic lubrication (El-IL) regions are considered. The structural state of subsurface layers in different lubricant regions is studied by X-ray photoelectron spectroscopy, optical, transmission and scanning microscopy analysis. Dislocation density of dislocations in El-IL and BL lubricant regimes was determined. Nanohardness at thin surface layers rubbed under different lubricant regimes is compared. The dominant friction and wear mechanisms in different lubrications regions are discussed.
Shpaisman H., Cohen H., Har Lavan L. R., Azulai D., Stein N. & Cahen D. (2012) Nanoscale. 4, 10, p. 3128-3134
Electrical transport studies across nm-thick dielectric films can be complicated, and datasets compromised, by local electrical breakdown enhanced by nm-sized features. To avoid this problem we need to know the minimal voltage that causes the enhanced electrical breakdown, a task that usually requires numerous measurements and simulation of which is not trivial. Here we describe and use a model system, using a "floating" gold pad to contact Au nanoparticles, NPs, to simultaneously measure numerous junctions with high aspect ratio NP contacts, with a dielectric film, thus revealing the lowest electrical breakdown voltage of a specific dielectric-nanocontact combination. For a 48 ± 1.5 Å SiO <sub>2</sub> layer and a ∼7 Å monolayer of organic molecules (to link the Au NPs) we show how the breakdown voltage decreases from 4.5 ± 0.4 V for a flat contact, to 2.4 ± 0.4 V if 5 nm Au NPs are introduced on the surface. The fact that larger Au NPs on the surface do not necessarily result in significantly higher breakdown voltages illustrates the need for combining experiments with model calculations. This combination shows two opposite effects of increasing the particle size, i.e., increase in defect density in the insulator and decrease in electric field strength. Understanding the process then explains why these systems are vulnerable to electrical breakdown as a result of spikes in regular electrical grids. Finally we use XPS-based chemically resolved electrical measurements to confirm that breakdown occurs indeed right below the nm-sized features.
Gu X. W., Shadmi N., Yarden T. S., Cohen H. & Joselevich E. (2012) Journal of Physical Chemistry C. 116, 37, p. 20121-20126
Semiconducting nanowires frequently have enhanced properties and unique functionality compared to their bulk counterparts. Controlling the geometry of nanowires is crucial for their integration into nanoscale devices because the shape of a device component can dictate its functionality, such as in the case of a mechanical spring or an antenna. We demonstrate a novel synthetic method for making polycrystalline CdSe nanowires with controlled geometries by using self-organized single-walled carbon nanotubes as a template for the selective electrodeposition of nanowires. Nanowires of up to hundreds of micrometers in length are formed as high-density straight arrays, as well as in the shape of serpentines and loops. These nanowires exhibit significant photoluminescence and photoconductivity applicable to photodetectors and respond to illumination up to 2 orders of magnitude faster than single crystalline CdSe.
Har Lavan L. R., Yaffe O., Joshi P., Kazaz R., Cohen H. & Cahen D. (2012) AIP Advances. 2, 1, 012164
We report near-perfect transfer of the electrical properties of oxide-free Si surface, modified by a molecular monolayer, to the interface of a junction made with that modified Si surface. Such behavior is highly unusual for a covalent, narrow bandgap semiconductor, such as Si. Short, ambient atmosphere, room temperature treatment of oxide-free Si(100) in hydroquinone (HQ)/alkyl alcohol solutions, fully passivates the Si surface, while allowing controlled change of the resulting surface potential. The junctions formed, upon contacting such surfaces with Hg, a metal that does not chemically interact with Si, follow the Schottky-Mott model for metal-semiconductor junctions closer than ever for Si-based junctions. Two examples of such ideal behavior are demonstrated: a) Tuning the molecular surface dipole over 400 mV, with only negligible band bending, by changing the alkyl chain length. Because of the excellent passivation this yields junctions with Hg with barrier heights that follow the change in the Si effective electron affinity nearly ideally. b) HQ/methanol passivation of Si is accompanied by a large surface dipole, which suffices, as interface dipole, to drive the Si into strong inversion as shown experimentally via its photovoltaic effect. With only ∼0.3 nm molecular interlayer between the metal and the Si, our results proves that it is passivation and prevention of metal-semiconductor interactions that allow ideal metal-semiconductor junction behavior, rather than an insulating transport barrier.
Carmeli I., Itskovsky M. A., Kauffmann Y., Shaked Y., Richter S., Maniv T. & Cohen H. (2012) Physical Review B. 85, 4, 041405(R)
Electromagnetic far- and near-field excitations of rectangular microholes in gold films are investigated by means of a focused e beam. Radiative cavity modes, well below the surface plasmon (SP) frequency, are detected at exceptionally large distances and are shown to be strongly enhanced at near-field regions of selected slit walls due to hybridization with metal-supported SP polaritons. The proposed enhancement mechanism of such hybridized modes, found here to preserve the cutoff frequencies and symmetry characteristics of the pure waveguide modes, sheds light on the intriguing phenomenon of extraordinary optical transmission through subwavelength apertures in metallic films.
Yaffe O., Qi Y., Scheres L., Puniredd S. R., Segev L., Ely T., Haick H., Zuilhof H., Vilan A., Kronik L., Kahn A. & Cahen D. (2012) Physical Review B. 85, 4, 045433
We compare the charge transport characteristics of heavy-doped p <sup>++</sup>- and n <sup>++</sup>-Si-alkyl chain/Hg junctions. Based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction we suggest that for both p <sup>++</sup>- and n <sup>++</sup>-type junctions, the energy difference between the Fermi level and lowest unoccupied molecular orbital (LUMO), i.e., electron tunneling, controls charge transport. This conclusion is supported by results from photoelectron spectroscopy (ultraviolet photoemission spectroscopy, inverse photoelectron spectroscopy, and x-ray photoemission spectroscopy) for the molecule-Si band alignment at equilibrium, which clearly indicate that the energy difference between the Fermi level and the LUMO is much smaller than that between the Fermi level and the highest occupied molecular orbital (HOMO). Furthermore, the experimentally determined Fermi level - LUMO energy difference, agrees with the non-resonant tunneling barrier height, deduced from the exponential length attenuation of the current.
Levine I., Yoffe A., Salomon A., Li W., Feldman Y. (. & Vilan A. (2012) Journal of Applied Physics. 111, 12, 124320
Aluminum thin films are known for their extremely rough surface, which is detrimental for applications such as molecular electronics and photonics, where protrusions cause electrical shorts or strong scattering. We achieved atomically flat Al films using a highly non-equilibrium approach. Ultra-fast thermal deposition (UFTD), at rates >10 nm/s, yields RMS roughness of 0.4 to 0.8 nm for 30-50 nm thick Al films on variety of substrates. For UFTD on Si(111) substrates, the top surface follows closely the substrate topography (etch pits), indicating a 2D, layer-by-layer growth. The Al film is a mixture of (100) and (111) grains, where the latter are commensurate with the in-plane orientation of the underlying Si (epitaxy). We show the use of these ultra-smooth Al films for highly reproducible charge-transport measurements across a monolayer of alkyl phosphonic acid as well as for plasmonics applications by directly patterning them by focused ion beam to form a long-range ordered array of holes. UFTD is a one-step process, with no need for annealing, peeling, or primer layers. It is conceptually opposite to high quality deposition methods, such as MBE or ALD, which are slow and near-equilibrium processes. For Al, though, we find that limited diffusion length (and good wetting) is critical for achieving ultra-smooth thin films.
Shpaisman H., Seitz O., Yaffe O., Roodenko K., Scheres L., Zuilhof H., Chabal Y. J., Sueyoshi T., Kera S., Ueno N., Vilan A. & Cahen D. (2012) Chemical Science. 3, 3, p. 851-862
Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to ∼80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron-phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids.
Lavi A., Cohen H., Bendikov T., Vilan A. & Cahen D. (2011) Physical Chemistry Chemical Physics. 13, 4, p. 1293-1296
We show that electronic transport quality alkyl chain mono-layers can be prepared from dilute solution, rather than from neat alkanes, and on Si (100) instead of (111) surfaces. High monolayer quality was deduced from XPS and from comparing current-voltage curves of Hg/alkyl/Si junctions with those for junctions with monolayers made from neat alkanes. XPS shows that limited surface oxidation does not harm the integrity of the monolayer. Solution preparation significantly widens the range of molecules that can be used for transport studies.
Tesler A. B., Chuntonov L., Karakouz T., Bendikov T. A., Haran G., Vaskevich A. & Rubinstein I. (2011) Journal of Physical Chemistry C. 115, 50, p. 24642-24652
Gold island films displaying localized plasmon properties were prepared by evaporation of just-percolated Au films onto glass substrates followed by annealing at ≥550 °C. Annealing induces depercolation and formation of large, single-crystalline, well-separated islands, partially embedded in the glass. Two dewetting mechanisms were identified, depending on the initial film morphology. The variability of island sizes and shapes provides effective means of tuning the position of the localized surface plasmon resonance (LSPR) band in a wide wavelength range. With an increase in the Au nominal thickness a transition occurs from transducers dominated by absorbance to ones dominated by scattering. Numerical simulations taking into account the shape and size distribution in actual island samples are in agreement with the experimental spectra. Refractive index sensitivity (RIS) measurements at a constant wavelength or at a constant extinction, tailored to the specific transducer, provide superior sensitivity to refractive index change, up to ca. 600 nm RIU <sup>-1</sup> in wavelength shift.
Kossoy A., Cohen H., Bendikov T., Wachtel E. & Lubomirsky I. (2011) Solid State Ionics. 194, 1, p. 1-4
The presence of water at the surface of sputtered thin films of Ce <sub>0.8</sub>Gd<sub>0.2</sub>O<sub>1.9</sub>, which display elastic anomalies as a function of thermal treatment, and of stoichiometric CeO<sub>2</sub> films, which do not, was monitored using X-ray photoelectron spectroscopy. We find that considerably more water is strongly bound at the surface of the Ce <sub>0.8</sub>Gd<sub>0.2</sub>O<sub>1.9</sub> films than of the CeO<sub>2</sub> films. This supports the theoretical prediction that water binds preferentially at the oxygen vacancy sites. In addition, all films were treated according to the protocol which has been shown to produce inelastic behavior in the doped films: annealed at 500 °C, exposed to ambient atmosphere for one month and then heated to 250 °C in ultra-high vacuum. Neither the Ce <sub>0.8</sub>Gd<sub>0.2</sub>O<sub>1.9</sub> nor the CeO<sub>2</sub> films show any change in the amount of adsorbed water. We therefore conclude that changes in the amount of surface adsorbed water do not play a role in the elastic anomalies observed as a function of thermal treatment of Ce<sub>0.8</sub>Gd <sub>0.2</sub>O<sub>1.9</sub> films.
Piperno S., Cohen H., Bendikov T., Lahav M. & Lubomirsky I. (2011) Angewandte Chemie - International Edition. 50, 25, p. 5654-5657
Rub-a-dub-dub: The hypothesis of contact electrification through the transfer of cryptoelectrons was tested by scrutinizing the evidence for the reduction of Pd<sup>2+</sup> and Cu<sup>2+</sup> by static charges on rubbed Teflon. X-ray photoelectron spectroscopy studies indicated that neither of these ions is reduced (see picture; black: Pd<sup>2+</sup> adsorbed, red: Pd <sup>0</sup> adsorbed and then reduced by formadehyde, green: mixture of Pd <sup>2+</sup> and Pd<sup>0</sup> arising from partial reduction after 2 h under the XPS probe) by the static charge. The presented alternative interpretation challenges the role of cryptoelectrons.
Aqua T., Cohen H., Sinai O., Frydman V., Bendikov T., Krepel D., Hod O., Kronik L. & Naaman R. (2011) Journal of Physical Chemistry C. 115, 50, p. 24888-24892
Self-assembled organic monolayers serve for modifying the work function of inorganic substrates. We examine the role of the molecular backbone in determining monolayer-adsorbed work function, by considering the adsorption of dithiols with either a partially conjugated or a saturated backbone on the GaAs(001) surface. Using a combination of chemically resolved electrical measurements based on X-ray photo-electron spectroscopy and contact potential difference, together with first principles electronic structure calculations, we are able to distinguish quantitatively between the contributions of the band bending and surface dipole components. We find that the substrates coated by partially conjugated layers possess a larger band-bending, relative to that of the substrates coated by saturated layers. This is associated with an increased density of surface states, likely related to the presence of oxygen. At the same time, the samples coated by partially conjugated layers also possess a larger bond-dipole, with the difference found to result primarily from an extended charge rearrangement on the molecular backbone. The two effects are, in this case, of opposite sign, but a significant net change in work function is still found. Thus, design of the molecular backbone emerges as an additional and important degree of freedom in the design of potential profiles and charge injection barriers in monolayer-based structures and devices.
The present invention relates to a novel system and method for the determination of depth profiling with improved accuracy and reliability. The method comprises obtaining spectroscopic data from the sample while under at least two different elec. conditions of the sample, the spectroscopic data comprising a signal of charged particles emitted from the sample, and being indicative of a change in amplitude, spectral position and spectral shape of the signal from the sample while under different elec. conditions of the sample, the change being indicative of the compositional profile and spatial distribution for at least one chem. element in the sample along a direction through the sample.
Itzhaik Y., Hodes G. & Cohen H. (2011) Journal of Physical Chemistry Letters. 2, 22, p. 2872-2876
The internal fields and band offsets developing at individual interfaces, a critical aspect of device performance, are generally inaccessible by standard electrical tools. To address this problem, we propose chemically resolved electrical measurements (CREM) capable of resolving the internal details layer-by-layer. Applied to nanoporous photovoltaic cells, we thus extract a realistic band diagram for the multi-interfacial structure and, in particular, resolve the two p-n-like junction fields built spontaneously in the device. The lack of homogeneity common to many of these nanoporous cells is exploited here to "see" deep into the cell structure, beyond the typical depth limitations of the surface-sensitive technique. Further information on the cell operation under "real" working conditions is achieved by studying the charge trapping at each specific layer under optical and electrical stimuli. Our methodology overcomes a missing link in device characterization and in fundamental studies of nanoscale solid-state devices.
Gershi H., Geclanken A., Keppner H. & Cohen H. (2011) Carbon. 49, 4, p. 1067-1074
The dependence of the macroscopic shape of pure carbon on the precursor structure is observed using Reactions under Autogenic Pressure at Elevated Temperatures (RAPET) for the thermal dissociation of several precursors, including stearic acid, oleic acid, linoleic acid, methyl 3 butenoate, methyl butyrate, octadecane, octadecene, octane, octene and acrolein. The precursors are dissociated under their autogenic pressure developed at 700 °C to create a range of pure carbon microstructures. Prolate spheroidal-shaped carbon (PSSHC) is prepared by heating octene, among others, at 700 °C in a closed cell in a one-step process. The dimensions of the carbon bodies were 3-5 μm for the polar diameter and 6-8 μm for the equatorial diameter. Obtaining the PSSHC from octene is in contrast to a previous work, which required a long hydrocarbon chain and the presence of oxygen for the formation of PSSHC upon thermolysis under identical autogenic pressure. The products of the RAPET reaction of octene showed a strong ESR signal, resulting from nonbonding dangling electrons on the carbon surface. However, treating the carbons with a beam of hydrogen atoms has almost completely eliminated the ESR signal. The question why the thermal dissociation of some precursors yield PSSHC, while other precursors yield other morphologies, is discussed.
Chen M., Briscoe W. H., Armes S. P., Cohen H. & Klein J. (2011) European Polymer Journal. 47, 4, p. 511-523
Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature's benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.
Qi Y., Yaffe O., Tirosh E., Vilan A., Cahen D. & Kahn A. (2011) Chemical Physics Letters. 511, 4-6, p. 344-347
The electronic structure of the prototypical self-assembled monolayer (SAM) system, i.e. alkanethiol molecules on Au, is investigated via ultraviolet and inverse photoemission spectroscopy measurements. The determination of the density of filled and empty states of the system reveals that the metal Fermi level is significantly closer to the lowest unoccupied molecular orbital (LUMO) of the molecules than to their highest occupied molecular orbital (HOMO). The results suggest that charge carrier tunneling is controlled by the LUMO, rather than by the HOMO, in contrast to what is commonly assumed.
Kedem N., Edri E., Kokotov M., Cohen H., Bendikov T., Popovitz-Biro R., Von Huth H. P., Ginley D. & Hodes G. (2010) Crystal Growth & Design. 10, 10, p. 4442-4448
Low concentrations (0.1-1% of the Zn concentration) of Sb ions in an alkaline ZnO chemical bath deposition solution were found to lead to pronounced changes in the ZnO film morphology. The tapered nanorods obtained in the absence of Sb become flat-topped and more closely packed when Sb is present, and the nanorod diameter changes, the direction of these changes depending on other bath parameters. An initial compact layer that was comprised of very small nanocrystals is formed. Sb was found to be present in the films, mostly in the compact initial layer. We postulate that initial Sb-rich nuclei promote ZnO nucleation but retard subsequent ZnO crystal growth. As the Sb concentration (relative to the Zn) in the bath drops, ZnO nanorods can then grow, but with altered morphology because of preferential adsorption of the low levels of Sb in solution on (002) ZnO crystal faces. These films were found to be very suitable for solid state semiconductor-sensitized solar cells. Such cells normally require a separate deposition of a blocking compact layer before the nanoporous layer. The initial in situ compact layer in our films is very efficient for this purpose, giving much more reproducible performance.
Shahar C., Zbaida D., Rapoport L., Cohen H., Bendikov T., Tannous J., Dassenoy F. & Tenne R. (2010) Langmuir. 26, 6, p. 4409-4414
WS<sub>2</sub> belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures - onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS<sub>2</sub> surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS<sub>2</sub> nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.
Moshkovich A., Perfilyev V., Bendikov T., Lapsker I., Cohen H. & Rapoport L. (2010) Acta Materialia. 58, 14, p. 4685-4692
Friction and wear of copper rubbed in a wide range of loads and sliding velocities were studied. The results of friction and wear experiments in PAO-4 lubricant are presented as the Stribeck curve where the boundary, mixed and elasto-hydrodynamic lubrication regimes are considered. The structural state of surface layers in different lubricant regimes is studied by optical and scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The dominant friction and wear mechanisms in different lubrication regimes are discussed. Severe plastic deformation of subsurface layers under friction is correlated with the nanocrystalline structure obtained by different methods of grain refinement.
Edri E., Rabinovich E., Niitsoo O., Cohen H., Bendikov T. & Hodes G. (2010) Journal of Physical Chemistry C. 114, 30, p. 13092-13097
ZnO nanorod films are increasingly used as an alternative to nanoporous TiO<sub>2</sub> as the electron conductor in semiconductor-sensitized nanoporous solar cells (SSSCs). Semiconductor light absorbers deposited onto ZnO by chemical bath deposition (CBD) tend to form poorly covering deposits of isolated clusters. We show how a short solution pretreatment of the ZnO nanorod films with a sulfide solution, which forms a thin ZnS film at the ZnO surface, results in a very large improvement of the coverage and uniformity of the CBD semiconductor coatings. These composite films give large improvements in solar cell efficiency. Several factors can be responsible for this improvement: better coverage of the ZnO nanorods, less shorting between hole conductor and conducting substrate, and the formation of a ZnS buffer layer.
Tenne R., Deepak F. L., Cohen H., Cohen S. R., Rosentsveig R. & Yadgarov L. (2010) .
A nanostructure, being either an Inorganic Fullerene-like (IF) nanostructure or an Inorganic Nanotube (INT), A1-x-Bx-chalcogenide are described. A being a metal or transition metal or an alloy of metals and/or transition metals, B being a metal or transition metal B different from that of A and x being ≤0.3. A process for their manufacture and their use for modifying the electronic character of A-chalcogenide are described.
Kopnov G., Umansky V. Y., Cohen H., Shahar D. & Naaman R. (2010) Physical Review B. 81, 4, 045316
The effect of the surface treatments on the transport properties of a two-dimensional electron gas was studied at the quantum limit. The surface of the Al0.36 Ga0.64 As/GaAs heterostructure was either coated with gold or etched with HCl solution, or etched and then coated by a self-assembled monolayer (SAM) of either phosphonated (ODP-C18 H39 PO3) or thiolated (ODT-C18 H37 S) molecules. The etching process was found to reduce significantly both the mobility and the charge density. This effect was reversed upon sequential adsorption of the phosphonated SAM. We propose fine tuning of the device performance by the flexible chemistry of the assembled molecules, two of them demonstrated here. The results indicate that the surface oxidation does not necessarily play the dominant role in this respect and, in particular, that octadecane phosphonic acid (ODP) can protect the substrate from both oxidation and the formation of a passivating carbon layer. In contrast, octadecanethiol (ODT) is not stable enough and is not effective in eliminating surface states, as a result devices covered with ODT behave like those with etched surfaces.
Cohen H. (2010) Journal of Electron Spectroscopy and Related Phenomena. 176, 1-3, p. 24-34
The spectroscopy of charged particles, e.g. X-ray photoelectron spectroscopy (XPS), proposes unique capabilities for electrical studies of nanometric heterostructures. By turning the problem of surface charging into a consistent and general method for chemically resolved electrical measurements (CREM), selected surface and sub-surface regions can be probed in a non-contact manner, with useful advantages over standard electrical tools. When applied to organic systems, CREM can read electrical information out of atomic sites within molecular assemblies, and hence, map the electrostatic potential variations under external stimuli, approaching atomic resolution. This paper reviews CREM studies of self-assembled monolayers (SAMs), with an emphasis on questions regarding the surface charge distribution. Surprising observations in work function changes under self-assembly of dipolar molecules are presented, as well as the direct detection of sub-molecular polarization effects, and photo-induced charge redistribution across SAMs that are situated between semiconducting media. The review concludes by suggesting future applications of CREM and further challenges that may arise.
Vilan A. & Cahen D. (2010) AIP Conference Proceedings. 1313, p. 30-33
Molecular electronics is a flourishing area of nano-science and -technology, with a promise for cheap electronics of novel functionality. Here we outline the major challenges for molecular electronics becoming an established scientific discipline, including models with predictive power.
Vilan A., Yaffe O., Biller A., Salomon A., Kahn A. & Cahen D. (2010) Advanced Materials. 22, 2, p. 140-159
Basic scientific interest in using a semiconducting electrode in moleculebased electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power. To consider these issues and questions we will, in this Progress Report, o review junctions based on direct bonding of molecules to oxide-fřee Si. o describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified, o investigate to what extent imperfections in the monolayer are important for transport across the monolayer, o revisit the concept of energy levels in such hybrid systems.
Yaffe O., Scheres L., Segev L., Biller A., Ron I., Salomon E., Giesbers M., Kahn A., Kronik L., Zuilhof H., Vilan A. & Cahen D. (2010) Journal of Physical Chemistry C. 114, 22, p. 10270-10279
Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.
Stein N., Korobko R., Yaffe O., Lavan R. H., Shpaisman H., Tirosh E., Vilan A. & Cahen D. (2010) Journal of Physical Chemistry C. 114, 29, p. 12769-12776
One of the major problems in molecular electronics is how to make electronically conducting contact to the "soft" organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current-voltage characteristics of MOMS devices.
Shpaisman H., Har Lavan L. R., Stein N., Yaffe O., Korobko R., Seitz O., Vilan A. & Cahen D. (2010) Advanced Functional Materials. 20, 13, p. 2181-2188
Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a "ready-made" metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal-insulator-semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal-semiconductor (MS) structure. By comparing a molecule-free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non-invasive ways to make electronic contact to soft matter.
Mentovich E. D., Belgorodsky B., Kalifa I., Cohen H. & Richter S. (2009) Nano Letters. 9, 4, p. 1296-1300
We suggest a universal method for the mass production of nanometer-sized molecular transistors. This vertical-type device was fabricated using conventional photolithography and self-assembly methods and was processed in parallel fashion. We used this transistor to investigate the transport properties of a single layer of bovine serum albumin protein. This 4-nm-channel device exhibits low operating voltages, ambipolar behavior, and high gate sensitivity. The operation mechanism of this new device is suggested, and the charge transfer through the protein layer was explored.
Rozenblat A., Rosenwaks Y. & Cohen H. (2009) Applied Physics Letters. 94, 21, 213501
We use the recently developed chemically resolved electrical measurements (CREM) to sensitively measure hot-electron transport characteristics in thin dielectric layers. By comparing bare gate-oxide layers, SiO2 and SiON, pronounced differences are revealed that are absent from standard contact measurements and from CREM conducted on top metallic pads. The "on pad" and standard measurements obey a similar defect-assisted "Poole-Frenkel" transport, whereas I∼ Vα characterizes the hot-electron transport through the bare overlayer, with a clear thickness dependence of α. These unique CREM features offer useful advantages in gate-oxide characterization.
Rozenblat A., Rosenwaks Y., Segev L. & Cohen H. (2009) Applied Physics Letters. 94, 5, 053116
The internal structure of SiON films is extracted electrically, demonstrating an efficient, noncontact, nondestructive means for depth compositional analysis in gate oxides. The electrical data, obtained using x-ray photoelectron spectroscopy (XPS) based controlled surface charging (CSC), are compared with independent time of flight secondary ion mass spectroscopy and angle resolved XPS data. Inhomogeneous composition with significant nitrogen enrichment at the top of the oxide layer is observed. Capabilities of the CSC method in treating heterostructures of poor chemical contrast are discussed.
Shpaisman H., Salomon E., Nesher G., Vilan A., Cohen H., Kahn A. & Cahen D. (2009) Journal of Physical Chemistry C. 113, 8, p. 3313-3321
We report on electronic transport measurements through dense monolayers of CH <sub>3</sub>(CH <sub>2</sub>) <sub>n</sub>PO <sub>3</sub>H <sub>2</sub> molecules of varying chain lengths, with a strong and stable bond through the phosphonic acid end group to a GaAs surface and a Hg top contact. The monolayers maintain their high quality during and after the electrical measurements. Analyses of the electronic transport measurements of junctions, and of UV and inverse photoemission spectroscopy data on band alignments of free surfaces, yield insight about the electrical transport mechanism. Transport characteristics for n-GaAs junctions at low forward bias are identical for different chain lengths, a strong indication of high-quality monolayers. Tunneling barrier and carrier effective mass values for n- and p-GaAs samples were deduced from the transport data. In this way we find a tunneling barrier for n-GaAs of 1.3 eV, while UPS data for the lowest unoccupied system orbital (LUSO) point to a 2.4 eV barrier. This discrepancy can be understood by invoking states, closer to the Fermi level than the LUSO state, that contribute to charge transport. Such states lead to a manifold of transitions, each having a different probability, both because of differences in the tunnel barrier and because of differences in density of these interface-induced states; i.e., the single barrier, deduced from J-V measurements, is an effective value only.
Sherratt P. A. J., Cohen H. & Seideman T. (2009) Journal of Chemical Physics. 131, 23, 234701
We point to the ability of noncontact measurements of electron transport via self-assembled monolayers to provide chemical, Å-resolved information about the underlying molecule. A conceptual framework is presented to model a current flow of soft electrons through a molecular monolayer to a substrate and explore the information content of this and other emerging noncontact measurements. A numerical scheme is developed where advantage is taken of the split-operator formalism to propagate the incident electronic wave function over a suitable periodic potential energy surface representing the self-assembled monolayer. The (experimentally observable) potential difference introduced by the transmitted electrons is extracted from the time-averaged electron density using the Poisson equation of classical electrostatics.
Noncontact examination of dielectric breakdown in thin silica layers reveals the stochastic appearance of coexisting stable and "broken" surface domains. Using chemically resolved electrical measurements for resolving electrically these domains, a power law evolution of the hot channels above the percolation critical point is found, ∼(V- V<sub>c</sub>)<sup>213</sup>, accounted for by a model for discharge across a two-phase medium. Consistency with the electrical data is shown, where voltage dependent percolation paths are scanned under controllable termination of the sparklike avalanche processes. Interface effects are found to qualitatively alter the V - V characteristics of high quality layers. Useful applications in studies of stressed insulating components are expected.
Caspi Y., Zbaida D., Cohen H. & Elbaum M. (2009) Macromolecules. 42, 3, p. 760-767
Passage of polymers through pores narrower than the hydrodynamic diameter is impeded by an entropic penalty for their confinement. This might be balanced by an attractive interaction with the pore walls. We found that the hydrogen-bonding polymer, poly(isopropylacrylamide) (pNIPAM), diffused readily through narrow pores in polycarbonate track-etched membranes. The trans side accumulation of pNIPAM followed a stretched exponential behavior. By contrast, a much smaller dextran diffused at a comparable or slower rate and showed ordinary Fick-like behavior. Comparison between the influence of pNIPAM surface adsorption and chemical grafting to the pores points to weak interpolymeric bonds as the source for the transport-accelerating surface interactions. We interpret the results as evidence for anomalous diffusion of pNIPAM inside the pores.
Yaffe O., Scheres L., Puniredd S. R., Stein N., Biller A., Lavan R. H., Shpaisman H., Zuilhof H., Haick H., Cahen D. & Vilan A. (2009) Nano Letters. 9, 6, p. 2390-2394
Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain length from 18 down to 1 or 2 carbons This and further recent results indicate that electron transport is minority, rather than majority carrier dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission, and, as such, is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.
Ruehle S., Segal A., Vilan A., Kurtz S. R., Grinis L., Zaban A., Lubomirsky I. & Cahen D. (2009) Journal of Renewable and Sustainable Energy. 1, 1
A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell (pc-Si). With the dichroic mirror simultaneous excitation of a medium-efficient (11.1%) commercial pc-Si and a custom-made high band gap GaInP cell (12.3%), yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3081510]
Cohen Y. S., Vilan A., Ron I. & Cahen D. (2009) Journal of Physical Chemistry C. 113, 15, p. 6174-6181
Bromine-terminated alkyl-chain monolayers, bound to oxide-free Si substrates, were prepared by self-assembly. Infrared spectroscopy and atomic force microscopy imply that monolayer packing density improves after hydrolysis, despite an increase in the presence of oxide. The probable reason is that OH-mediated intermolecular H-bonding along the monolayer emerges after hydrolysis and rearranges the molecular components of the insulating layer. Current - voltage and differential capacitance measurements show that also the interfacial electronic properties of these junctions are changed by hydrolysis of the Br groups. This is expressed in an increased effective Schottky barrier height and a decreased junction ideality factor. We correlate the proposed structural changes of the monolayer with the change in the interfacial electronic properties, with the help of the inhomogeneous Schottky barrier height model. The role of oxide in the charge transport through the monolayer is discussed, as well.
Rubinstein I., Vaskevich A., Ruach-Nir I. & Bendikov T. (2008) .
A device is presented comprising a multi-layer structure with a plurality of metal islands on the surface of a solid transparent substrate. The islands are coated by an ultrathin sol-gel derived silica film via self-assembled monolayer.
Vilan A., Bendikov T. A. & Cohen H. (2008) Journal of Electron Spectroscopy and Related Phenomena. 162, 2, p. 99-105
Secondary electron emission (SEE) is a major player in surface charging during X-ray photoelectron spectroscopy (XPS); its characteristics and applicability as a current source for electrical measurements are studied. We employ sample biasing and a top retarding grid to control the photoelectron current, and further compare their I-V characteristics with direct spectroscopy of the secondary electrons. Using silica-coated gold substrates, the effect of sample work function on the emitted secondary electrons is shown and fine control over the surface potential gradients, in the range of 10-100 meV, is achieved. XPS-based chemically resolved electrical measurements (CREM) can thus be extended to the positive current regime.
Karakouz T., Tesler A. B., Bendikov T. A., Vaskevich A. & Rubinstein I. (2008) Advanced Materials. 20, 20, p. 3893-3899
A study was conducted to obtain highly stable localized plasmon transducers by thermal embedding of gold (Au) island films on glass. The Au island films were directly evaporated on glass substrates and subjected to post-deposition annealing at a temperature T near the glass substrate glass transition T <sub>g</sub> undergoing partial embedding in the glass substrate. This process was accompanied by formation of a glass rim around the islands. The obtained island films showed hexagonal shape, with automatically flat top surfaces characteristic of the crystallographic plane. It was observed that the films are significantly stable toward immersion in solvents, drying,and binding of organic and biological molecules, while passing successfully the adhesive tape test. The superior adhesion and stability, obtained without the use of an intermediate adhesion layer, or a protective overlayer, was attributed to the island embedding and rim formation.
Bendikov T. A., Rabinkov A., Karakouz T., Vaskevich A. & Rubinstein I. (2008) Analytical Chemistry. 80, 19, p. 7487-7498
Discontinuous, island-type gold films (typically ≤10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.
Kopnov F., Tenne R., Späth B., Jägermann W., Cohen H., Feldman Y., Zak A., Moshkovich A. & Rapoport L. (2008) . p. 51-59
The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS<sub>2</sub> were investigated using X-ray photo-electron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed on the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.
Kopnov F., Tenne R., Spaeth B., Jaegermann W., Cohen H., Feldman Y. (., Zak A., Moshkovich A. & Rapoport L. (2008) . p. 51-59
The temporal chemical changes occurring at the surface of fullerence-like (IF) nanoparticles of WS2 were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder (2H) of the same material. It is possible to follow the long term (surface oxidation and carbonization) occurring at defects on the outermost surface (0001) layer of the fullerene-like nanoparticles. Similar but perhaps more distinctive changes are observed oil the prismatic (hk0) surfaces of the 2H powder. Vacuum annealing is shown to remove most of these changes and bring the surface close to its stoichiometric composition. In accordance with previous measurements, further evidence is obtained for the existence of water molecules which are entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. They are also shown to be removed by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS show that the vacuum annealed IF samples become more intrinsic. Finally, tribological measurements show that the vacuum annealed IF samples perform better as an additive to oil than the non-annealed IF samples and the bulk (2H) platelets powder.
Wen K., Maoz R., Cohen H., Sagiv J., Gibaud A., Desert A. & Ocko B. M. (2008) ACS Nano. 2, 3, p. 579-599
Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic add function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectmscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3 D molecular arrangement of the initial alcohol-silane multilayer stack 6 well preserved upon virtually quantitative conversion of the alcohol to carboxyric add and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.
Itskovsky M. A., Cohen H. & Maniv T. (2008) Physical Review B. 78, 4, 045419
A quantum-mechanical scattering theory for relativistic, highly focused electron beams in the vacuum near nanoscopic platelets is presented, revealing an excitation mechanism due to the electron wave scattering from the platelet edges. Radiative electromagnetic excitations within the light cone are shown to arise, allowed by the breakdown of momentum conservation along the beam axis in the inelastic-scattering process. Calculated for metallic (silver and gold) and insulating (SiO2 and MgO) nanoplatelets, radiative features are revealed above the main surface-plasmon-polariton peak, and dramatic enhancements in the electron-energy-loss probability at gaps of the "classical" spectra are found. The corresponding radiation should be detectable in the vacuum far-field zone, with e beams exploited as sensitive "tip detectors" of electronically excited nanostructures.
Kopnov F., Feldman Y. (., Popovitz-Biro R., Vilan A., Cohen H., Zak A. & Tenne R. (2008) Chemistry of Materials. 20, 12, p. 4099-4105
WS<sub>2</sub> inorganic fullerene-like (IF) nanoparticles were subjected to intercalation with potassium, sodium, and rubidium atoms in heated sealed ampules. The product of the intercalation process was not pure and was composed of both intercalated and nonintercalated phases. X-ray diffraction measurements under inert conditions of the intercalated powders showed that the interlayer expansion was correlated with the alkali metal radius. Small increase of the a-axis was observed as well and was explained on the grounds of the WS<sub>2</sub> band structure. The XPS analysis of the rubidium intercalated material showed a rise in the Fermi energy as a result of the intercalation, endowing the originally p-type nanoparticles an n-type character.
Sarkar S. K., Hodes G., Kronik L. & Cohen H. (2008) Journal of Physical Chemistry C. 112, 16, p. 6564-6570
Investigations of charge transport mechanisms in thin CdSe nanoparticles films deposited on silicon, using surface photovoltage and chemically resolved electrical measurements, reveal a strongly nonlinear optical response and a negative differential resistance. Here, both phenomena are rationalized within a phenomenological model consisting of two spatially separated types of traps, one related to the CdSe nanoparticles and the other to the nanoparticle/ substrate interface. The model successfully explains a broad range of both old and new experimental observations and is used as a numerical framework for showing how the interplay between hole and electron processes dominates the photoelectrical properties of nanoparticle films.
Seitz O., Vilan A., Cohen H., Hwang J., Haeming M., Schoell A., Umbach E., Kahn A. & Cahen D. (2008) Advanced Functional Materials. 18, 14, p. 2102-2113
n-Si/C<sub>n</sub>H<sub>2n+1</sub>/Hg junctions (n = 12, 14, 16 and 18) can be prepared with sufficient quality to assure that the transport characteristics are not anymore dominated by defects in the molecular monolayers. With such organic monolayers we can, using electron, UV and X-ray irradiation, alter the charge transport through the molecular junctions on n- as well as on p-type Si. Remarkably, the quality of the self-assembled molecular monolayers following irradiation remains sufficiently high to provide the same very good protection of Si from oxidation in ambient atmosphere as provided by the pristine films. Combining spectroscopic (UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Auger, near edge-X-ray absorption fine structure (NEXAFS)) and electrical transport measurements, we show that irradiation induces defects in the alkyl films, most likely C=C bonds and C-C crosslinks, and that the density of defects can be controlled by irradiation dose. These altered intra- and intermolecular bonds introduce new electronic states in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the alkyl chains and, in the process, dope the organic film. We demonstrate an enhancement of 1-2 orders of magnitude in current. This change is clearly distinguishable from the previous observed difference between transport through high quality and defective monolayers. A detailed analysis of the electrical transport at different temperatures shows that the dopants modify the transport mechanism from tunnelling to hopping. This study suggests a way to extend significantly the use of monolayers in molecular electronics.
Thieblemont F., Seitz O., Vilan A., Cohen H., Salomon E., Kahn A. & Cahen D. (2008) Advanced Materials. 20, 20, p. 3931-3936
The preparation of alkoxy monolayers on oxide-free Si (100) and on electronic current transport measurements through junctions that are made up of these monolayers, sandwiched between Si as one electrode and a Hg drop as the other electrode, was reported. Based on the polarized IR spectra, the average tilt angles of the alkoxy and alkyl chains with respect to the surface normal are found to be very similar at 28° and 27° respectively. The monolayer thickness value for the OC12 layer, calculated using an inelastic mean free path of 3.3 nm, is 15∓2Å. The atomic force microscopy (AFM) and contact angle measurements indicate that neither binding chemistry nor molecular length alter the monlayer density. The X-ray photoelectron spectroscopy (XPS) binding energy of the C atom closets to the Si is lower than that of the C atom of the alkyl backbone.
Spaeth B., Kopnov F., Cohen H., Zak A., Moshkovich A., Rapoport L., Jaegermann W. & Tenne R. (2008) Physica Status Solidi (B): Basic Research. 245, 9, p. 1779-1784
The time dependent chemical changes occurring at the surface of inorganic fullerene-like (IF) nanoparticles of WS<sub>2</sub> were investigated using X-ray photoelectron spectroscopy (XPS) and compared to those of bulk powder, 2H-WS<sub>2</sub>. It was possible to follow the long term surface oxidation and carbonization occurring at defects on the outermost surface (0001) molecular layers of the inorganic fullerene-like nanoparticles. Vacuum annealing was shown to remove most of these contaminants and bring the surface close to its pristine stoichiometric composition. In accordance with previous measurements, further evidence was obtained for the existence of water molecules, which were entrapped in the hollow core and interstitial defects of the fullerene-like nanoparticles during the synthesis. These water molecules were also shown to be removable by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS showed that the IF samples had become less p-type after the vacuum annealing. Finally, tribological measurements showed that the vacuum annealed IF samples performed better as an oil additive than the non-annealed IF samples and the 2H-WS<sub>2</sub> powder.
Ehre D., Cohen H., Lyahovitskaya V. & Lubomirsky I. (2008) Physical Review B. 77, 18, 184106
Nonpolar amorphous and polar quasiamorphous phases of substrate-supported BaTi O3 and SrTi O3 were studied with x-ray photoelectron spectroscopy (XPS) to characterize the structural and chemical changes accompanying the transformation of the former into the latter. It was found that there are two spectral features distinguishing the amorphous from the quasiamorphous films: (1) an extra peak in the valence band spectra of amorphous films and (2) a satellite line in the XPS O 1s spectra of the amorphous films. On the basis of literature data, we suggest that both these features may be interpreted as originating from an oxygen-oxygen chemical bond. During the thermally driven transformation of an amorphous into a polar quasiamorphous phase, the oxygen-oxygen chemical bond breaks, leading to volume expansion and the development of inhomogeneous in-plane mechanical stress.
Caspi Y., Zbaida D., Cohen H. & Elbaum M. (2008) Nano Letters. 8, 11, p. 3728-3734
The nuclear pore complex is a large protein channel present universally in eukaryotic cells. It generates an essential macromolecular separation between the nucleus and cytoplasm. The transport mechanism relies on recognition of molecular cargos by receptor proteins, and on specific interaction between the receptors and the pores. We present a chemical mimic of this "receptor-mediated" transport using modified nanoporous membrane filters, polyisopropylacrylamide as the carrier molecule, or receptor, and single-stranded DNA as the cargo. We show that a complex of ssDNA and polyisopropylacrylamide diffuses faster through the modified pores than does the bare ssDNA, in spite of the larger size of the complex. The mobile polymer thus acts as a soluble receptor to usher a macromolecular cargo specifically through the pores.
Deepak F. L., Popovitz-Biro R., Feldman Y. (., Cohen H., Enyashin A., Seifert G. & Tenne R. (2008) Chemistry-An Asian Journal. 3, 8-9, p. 1568-1574
Inorganic fullerene-like (IF) Mo<sub>1-x</sub>Re<sub>x</sub>S<sub>2</sub> and W<sub>1-x</sub>Re<sub>x</sub>S<sub>2</sub> nanoparticles have been synthesized by a gasphase reaction involving the respective metal halides with H<sub>2</sub>S. The IF-Mo(W)<sub>1-x</sub>Re<sub>x</sub>S<sub>2</sub> nanoparticles, contain - ing up to 5% Re, were characterized by a variety of experimental techniques. Analyses of the X-ray powder diffraction and different electron microscopy techniques show that the Re is doped in the MoS<sub>2</sub> host lattice. Interestingly, Re-dop ed MoS<sub>2</sub> nanotubes are present as well, although in small quantities (∼5%). XPS results confirm the nanoparticles to be more n-type arising from the effect of Re doping. Additionally, density-functional tight-binding (DFTB) calculations support the observed n-type behavior.
Vilan A. & Hikmet R. A. (2008) Journal of Physical Chemistry C. 112, 1, p. 269-281
A ready-made procedure for the preparation of molecular junctions at ambient conditions is reported. Junctions were constructed from bilayers of alkyl thiols at the interfaces between gold flakes and stationary gold strips. Addition of an amine or carboxylic acid end group to the alkyl thiols drastically increased the resistance (up to TΩ over a couple of nanometers only) and the breakdown voltage of the bilayer junction. These junctions pose a severe quantification challenge because of the large uncertainty regarding their microscopic morphology. A novel quantification procedure is proposed that replaces highly nonlinear tunneling or super-exchange current voltage relations by an effective analytical relation of two characteristic parameters: equilibrium conductance and shape factor. Correlating these factors over a series of systematically varying junctions allows us to evaluate the effective contact area for transfer. This approach was also extended to the field-emission regime. Within the large spreading of the data, our analyses show that low-bias transfer occurs via most of the contact area while field emission is limited to "hot spots" of a few nanometers wide and only couple of angstroms long. The extracted length for charge transfer was only 1/4 to 1/3 of the nominal bilayer thickness, except for the polar interfaces, which were considerably thicker. The effect of polar end groups on the bilayer thickness is presumably due to weak repulsive forces at the bilayer interface, preventing the collapse of the bilayer.
Yerushalmi R., van der Boom M. E. & Cohen H. (2007) Journal of Physical Chemistry C. 111, 37, p. 13652-13654
The study of molecular electric properties is an intriguing, rapidly developing field in which technological and basic scientific challenges and developments are evolving. Nevertheless, understanding of the interplay of intermolecular interactions, substrate effects, and electrode contacts remains challenging. Here, we present noncontact chemically resolved electrical measurements (CREM) of halide-terminated molecular layers and a straightforward model for quantitative analysis of submolecular chemical site capacitance. We demonstrate that under low current densities, the main electronic effects can be accounted for by considering the (sub)molecular properties of the monolayers, whereas the excess potential due to charge injection can be described as site capacitance corresponding to chemically identifiable molecular sites.
Ray S. G., Daube S. S., Cohen H. & Naaman R. (2007) Israel Journal of Chemistry. 47, 2, p. 149-159
Many of the mutagenic or lethal effects of ionization radiation can be attributed to damage caused to the DNA by low-energy electrons. In order to gain insight on the parameters affecting this process, we measured the low-energy electron (
Sarkar S. K., Kababya S., Vega S., Cohen H., Woicik J. C., Frenkel A. I. & Hodes G. (2007) Chemistry of Materials. 19, 4, p. 879-888
Chemical bath deposited PbSe films were subjected to postdeposition treatment with aqueous (typically 0.25-0.5 M) KOH. For films deposited using a citrate complex, this treatment resulted in dissolution of surface lead oxides (seen from XPS and EXAFS measurements) and growth of the nanocrystals (from ca. 5 to as much as 20 nm, measured by XRD and TEM) by an Ostwald ripening mechanism and formation of a porous network. For films deposited using KOH-complexed Pb, this growth did not occur. The latter films are made up of PbSe crystals (ca. 4 nm) embedded in an amorphous matrix of lead oxide. Successful etching of the crystallite surface passivation is found to be critical for the growth progress. While the KOH treatment removed most of this matrix, the individual crystals of PbSe still remained passivated with a surface where Pb was apparently bonded to both O and Se. With use of a concentrated KOH solution (3 M) for long periods of time (> 1 h), this surface could be removed and crystal growth occurred to give a network of PbSe crystals several tens of nanometers in size. This study, besides explaining the very different chemical behaviors of the two types of PbSe films, demonstrates the important role of what appear to be small differences in surface chemistries in determining the chemical properties of nanocrystals.
Seitz O., Vilan A., Cohen H., Chan C., Hwang J., Kahn A. & Cahen D. (2007) Journal of the American Chemical Society. 129, 24, p. 74947495
Electron irradiation can alter electronic charge transport through Si-CH<sub>2</sub>(CH<sub>2</sub>)<sub>12</sub>-CH<sub>3</sub>//Hg molecular junctions. Applying UPS, XPS, Auger, NEXAFS, and electrical transport measurements, we show that irradiation induces defects, most likely C=C bonds and C-C cross-links, which introduce new electronic states into the HOMO-LUMO gap of the alkyl chains, and, hence, effectively dope these layers. We demonstrate a 1-2 order of magnitude enhancement in current, clearly distinguishable from that of defects in as-prepared layers.
Friedman H., Eidelman O., Feldman Y. (., Moshkovich A., Perfiliev V., Rapoport L., Cohen H., Yoffe A. & Tenne R. (2007) Nanotechnology. 18, 11, 115703
Composite coatings of Co + fullerene-like WS<sub>2</sub> nanoparticles on stainless steel substrate were obtained through electroless deposition, using DMAB (dimethyl borane complex, 97%) as the reducing agent, and by electroplating in acidic solution. Phase analysis results show that the coatings consist of Co and the fullerene-like WS<sub>2</sub> nanoparticles alone. Tribological measurements show reduced wear and friction of the composite coatings as compared with the pure cobalt film or the stainless steel substrate.
Gunanathan C., Pais A., Furman-Haran E., Seger D., Eyal E., Mukhopadhyay S., Ben-David Y., Leitus G., Cohen H., Vilan A., Degani H. & Milstein D. (2007) Bioconjugate Chemistry. 18, 5, p. 1361-1365
Novel estrogen-conjugated pyridine-containing Gd(III) and Eu(III) contrast agents (EPTA-Gd/Eu) were designed and effectively synthesized. Convenient to administration and MRI experiments, both EPTA-Gd and EPTA-Eu are soluble in water. The EPTA-Gd selectively binds with a micromolar affinity to the estrogen receptor and induces proliferation of human breast cancer cells. The EPTA-Gd is not lethal and does not cause any adverse effects when administrated intravenously. It enhances T<sub>1</sub> and T<sub>2</sub> nuclear relaxation rates of water and serves as a selective contrast agent for localizing the estrogen receptor by MRI.
Ehre D., Cohen H., Lyahovitskaya V., Tagantsev A. & Lubomirsky I. (2007) Advanced Functional Materials. 17, 7, p. 1204-1208
A model of structural transformations of amorphous into quasi-amorphous BaTiO<sub>3</sub> is suggested. The model is based on previously published data and on X-ray photoelectron spectroscopy data presented in the current report Both amorphous and quasi-amorphous phases of BaTiO<sub>3</sub> are made up of a network of slightly distorted TiO<sub>6</sub> octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical-charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO<sub>6</sub> network. Upon heating, the edge-to-edge and face-to-face connections between TiO<sub>6</sub> octahedra are severed and then reconnected via apices. Severing the connections between TiO<sub>6</sub> octahedra requires a volume increase, suppression of which keeps some of the edge-to-edge and face-to-face connections intact. Transformation of the amorphous thin films into the quasi-amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO<sub>6</sub> octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non-centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi-amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.
Aqua T., Cohen H., Vilan A. & Naaman R. (2007) Journal of Physical Chemistry C. 111, 44, p. 16313-16318
The reactivity of the tail group of molecules absorbed in a self-assembled monolayer is affected significantly by the substrate through long-range charge redistribution occurring during the adsorption. Alkyl dithiol monolayers on GaAs are highly stable as compared to monolayers of monothiols on GaAs or dithiols on gold. X-ray photoelectron spectroscopy (XPS) measurements reveal fairly weak binding of monothiol layers on GaAs, prone to rapid oxidation at the molecule-substrate interface. This is in contrast with the high stability of monothiols on gold. However, in the case of dithiols, the situation is reversed. When adsorbed on gold, the top thiol group tends to oxidize, whereas on GaAs, it does not. Furthermore, the monolayer was found to be stable in ambient for months. Contact potential difference (CPD) measurements showed a significant difference in charge distribution on the monolayers adsorbed on the two substrates, gold and GaAs. The change in reactivity and stability is attributed to the difference in the substrate-induced charge distribution across the adsorbed molecules.
Chen M., Briscoe W. H., Armes S. P., Cohen H. & Klein J. (2007) ChemPhysChem. 8, 9, p. 1303-1306
Robust polymer brushes are grown from a planar mica surface (see figure) using atom-transfer radical polymerization. The brushes are characterized by X-ray photoelectron spectroscopy, X-ray reflectometry, atomic force microscopy, optical interferometry, and surface forces. (Figure Presented).
Deepak F. L., Cohen H., Cohen S., Feldman Y. (., Popovitz-Biro R., Azulay D., Millo O. & Tenne R. (2007) Journal of the American Chemical Society. 129, 41, p. 12549-12562
IF-MO<sub>1-x</sub>Nb<sub>x</sub>S<sub>2</sub> nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H<sub>2</sub>S. The IF-MO<sub>1-x</sub>Nb<sub>x</sub>S<sub>2</sub> nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS<sub>2</sub> within the MoS<sub>2</sub> host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS<sub>2</sub> host lattice. Very few Nb atoms, if any, are intercalated between the MoS<sub>2</sub> layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS<sub>2</sub> (whereas unsubstituted IF-MoS<sub>2</sub> nanoparticles are semiconducting). Furthermore the IF-MO<sub>1-x</sub>Nb<sub>x</sub>S<sub>2</sub> nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.
Vilan A. (2007) Journal of Physical Chemistry C. 111, 11, p. 4431-4444
Use of the Simmons model for analyzing tunneling transport across molecular junctions is reviewed, and its inherent limitations are examined, specifically for cases where there are no molecular length-dependent data (to extract a decay parameter), be it for experimental reasons or because of changes of the molecular energetics or packing with molecular length. The potential barrier across a molecular junction is shown to be strongly bias-dependent, much more so than is assumed in the commonly used version of the Simmons model. The means to distinguish true tunneling from conduction via pinholes (or hot spots) are also considered. Power expansion to the Simmons model shows that I/V vs V <sup>2</sup> plots should be linear over that range, thus providing a simple and standardized parameter extraction. From such plots, we can extract values for the equilibrium conductance and for the "shape factor", a complementary parameter that describes the shape of the I- V relations. The applicability of these two parameters for describing actual transport is illustrated by analyzing data for three different types of molecular junctions. The linearity of the I/V vs V<sup>2</sup> plots can be used to evaluate if, and if so, at which bias (and if at all) direct tunneling occurs and under which conditions this is not the case. The extracted equilibrium conductance and the "shape factor" provide an empirical method for quantifying electronic transport across practical molecular junctions where the exact packing of the molecules is rather uncertain. As such, the analysis can be used to weigh and classify the effect of chemical modifications to molecular junctions or compare contacting methods, so as to allow a deeper understanding of transport via molecular junctions.
A method and device are presented for measuring the electrical properties of a specimen. The specimen is excited with high energy radiation to cause emission of internal charged particles from the specimen. Electrical power is supplied to a circuit, that is formed by the specimen and any added component connected to a back contact of the specimen. The electric power supply includes at least one of the following: irradiating the circuit with low energy charged particles; subjecting the circuit to an external field of the kind affecting the flux of emitted internal charged particles, and supplying a bias voltage to the back contact of the specimen. During the power supply to the specimen, at least one of the following is carried out: an electric current through the specimen is measured, and the emitted charged particles are analyzed versus their energy (using a contactless voltmeter) which provides local potential values at chemical entities of the specimen. This technique enables determination of rich, chemically resolved, electrical properties of a specimen, such as IV characteristic, and/or evaluation of a work function characteristic, and/or characterization of electric leakage or breakdown conditions of the sample, and/or characterization of accumulation of charge within at least one region of the sample, and/or chemically resolved photovoltaic characteristics (photovoltage and/or photocurrent) of the sample.
Nesher G., Vilan A., Cohen H., Cahen D., Amy F., Chan C., Hwang J. & Kahn A. (2006) Journal Of Physical Chemistry B. 110, 29, p. 14363-14371
A series of p- and n-GaAs-S-C<sub>n</sub>H2<sub>n+1</sub> ∥ Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5-1.6m<sub>e</sub> for the electronic carriers that cross the junctions, We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3-1.4 eV above and 2.0-2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p-GaAs system.
Sfez R., Peor N., Cohen S., Cohen H. & Yitzchaik S. (2006) Journal of Materials Chemistry. 16, 41, p. 4044-4050
In this contribution we report on an in situ enzymatic self-assembly of a polyaniline (PAN) monolayer on modified hydroxyl-terminated surfaces. The consecutive assembly steps consist of a chemical deposition of 3-aminopropyltrimethoxysilane (APT) as a coupling agent with a positively-charged amine end group. The next step involved an electrostatic adhesion of sulfonated polystyrene (SPS) followed by electrostatic adhesion of anilinium. In situ enzymatic polymerization of the anilinium monolayer took place using horseradish peroxidase (HRP) enzyme and its substrate H <sub>2</sub>O<sub>2</sub>. The assembly steps were characterized by variable angle spectroscopic ellipsometry (VASE), UV-Vis-NIR spectroscopy, contact angle measurements, X-ray photoelectron spectroscopy (XPS), contact potential difference (CPD) and scanning force microscopy (SFM). The SFM measurements were divided between ex situ analysis in which morphology of the obtained layers was determined, and in situ analysis which provided information on the dynamic process of the enzymatic polymerization.
Filip-Granit N., van der Boom M. E., Yerushalmi R., Scherz A. & Cohen H. (2006) Nano Letters. 6, 12, p. 2848-2851
Potential profiles across molecular layers are constructed by means of noncontact electrically stimulated photoelectron spectroscopy, probing for the first time the molecule-substrate interface potential and resolving local screening effects across inner phenyl groups.
Cohen H., Maoz R. & Sagiv J. (2006) Nano Letters. 6, 11, p. 2462-2466
Charge accumulation in an organosilane monolayer self-assembled on silicon is studied using electron-spectroscopy-based chemically resolved electrical measurements (CREM). By resolving the net electrical response of the organic layer, a significant capability of holding extra charge is indicated. Quantum size effects at a molecularly thin layer and the role of competing discharge mechanisms, including defect-assisted leakage currents, are discussed.
Cohen H., Sarkar S. K. & Hodes G. (2006) Journal Of Physical Chemistry B. 110, 50, p. 25508-25513
Light-induced chemically resolved electrical measurements (CREM) under controlled electrical conditions are used to study photovoltaic effects at selected regions in nanocrystalline CdSe-based films. The method, based on X-ray photoelectron spectroscopy (XPS), possesses unique capabilities for exploring charge trapping and charge transport mechanisms, combining spectrally filtered input signals with photocurrent detection and with a powerful, site-selective, photovoltage probe.
Cohen H., Zenkina O. V., Shukla A. D. & van der Boom M. E. (2006) Journal Of Physical Chemistry B. 110, 4, p. 1506-1508
Submolecular electrical information is successfully derived by applying element-specific, chemically resolved electrical measurements to a covalently bound stilbazole-based monolayer on a silicon substrate. Pronounced affinity effects are found in the response of adjacent atomic sites to external charge injection, accompanied by intramolecular polarization variations. These noncontact electrical read-out capabilities may provide a first entry toward the realization of organic devices based on submolecular electrical units.
Wanunu M., Popovitz-Biro R., Cohen H., Vaskevich A. & Rubinstein I. (2005) Journal of the American Chemical Society. 127, 25, p. 9207-9215
Gold nanoparticle (NP) mono- and multilayers were constructed on gold surfaces using coordination chemistry. Hydrophilic Au NPs (6.4 nm average core diameter), capped with a monolayer of 6-mercaptohexanol, were modified by partial substitution of bishydroxamic acid disulfide ligand molecules into their capping layer. A monolayer of the ligand-modified Au NPs was assembled via coordination with Zr<sup>4+</sup> ions onto a semitransparent Au substrate (15 nm Au, evaporated on silanized glass and annealed) precoated with a self-assembled monolayer of the bishydroxamate disulfide ligand. Layer-by-layer construction of NP multilayers was achieved by alternate binding of Zr <sup>4+</sup> ions and ligand-modified NPs onto the first NP layer. Characterization by atomic force microscopy (AFM), ellipsometry, wettability, transmission UV-vis spectroscopy, and cross-sectional transmission electron microscopy showed regular growth of NP layers, with a similar NP density in successive layers and gradually increased roughness. The use of coordination chemistry enables convenient step-by-step assembly of different ligand-possessing components to obtain elaborate structures. This is demonstrated by introducing nanometer-scale vertical spacing between a NP layer and the gold surface, using a coordination-based organic multilayer. Electrical characterization of the NP films was carried out using conductive AFM, emphasizing the barrier properties of the organic spacer multilayer. The results exhibit the potential of coordination self-assembly in achieving highly controlled composite nanostructures comprising molecules, NPs, and other ligand-derivatized components.
Wanunu M., Vaskevich A., Cohen S., Cohen H., Arad-Yellin R., Shanzer A. & Rubinstein I. (2005) Journal of the American Chemical Society. 127, 50, p. 17877-17887
A C<sub>3</sub>-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic coordination using Zr<sup>4+</sup> ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr <sup>4+</sup>-based assemblies to Hf<sup>4+</sup> ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.
Remskar M., Mrzel A., Jesih A., Kovac J., Cohen H., Sanjines R. & Levy F. (2005) Advanced Materials. 17, 7, p. 911-914
The growth of MoS<sub>2</sub>-C<sub>60</sub> composite crystals by a catalyzed transport reaction involving fullerene C<sub>60</sub> was discussed. The domain of the short-range-ordered C<sub>60</sub> molecules in the high-resolution TEM (HRTEM) image showed a 3×3 superstructure with regard to the MoS<sub>2</sub> lattice. It was found that the intensitites of the superlattice spot increased with the electron-irradiation time. Analysis shows that the large distance between two neighboring C<sub>60</sub> molecules and their inhomogeneous distribution together with the incommensurate relationship of the sublattice can both contribute to the deformation of the nanometer-scale structure forming the dislocation loop assemblies.
Ray S., Cohen H., Naaman R., Liu H. & Waldeck D. (2005) Journal Of Physical Chemistry B. 109, 29, p. 14064-14073
The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.
Ray S., Cohen H., Naaman R. & Rabin Y. (2005) Journal of the American Chemical Society. 127, 49, p. 17138-17139
Monolayers of single-stranded DNA (ssDNA) immobilized on surfaces form the basis of a number of important biotechnology applications, including DNA microarrays and biosensors. The organization of ssDNA as layer on a solid substrate allows one to investigate various properties of the DNA in a controlled manner and to use DNA for analytical applications as well as for exploring futuristic schemes for molecular electronics. It is commonly assumed that the adsorbed DNA layer contains some structural water and the cations. Here we show, based on XPS studies, that when monolayers of ssDNA are formed from sodium phosphate buffer and washed thoroughly, no Na<sup>+</sup> signal is detected. A finite concentration of ions is observed when the DNA is made from a solution of Mg<sup>2+</sup> ions, but it is still only a fifth of what it would be if all the phosphate ions were fully neutralized by the metal cations.
Sarkar S., Cohen H. & Hodes G. (2005) Journal Of Physical Chemistry B. 109, 1, p. 182-187
If a thin film (tens of nm) of CdSe quantum dots (4 nm diameter) is deposited by chemical bath deposition onto various substrates, the films, although essentially intrinsic, behave as if they were n-type with respect to charge separation. However, films deposited under certain deposition conditions on Si (both n<sup>+</sup>- and p<sup>+</sup>-type) behave as if they were p-type. In this case, we show that it is possible to switch this p-type photoresponse by either light illumination intensity or injection of electrons from an external filament. Using both surface photovoltage spectroscopy and a novel adaptation of X-ray photoelectron spectroscopy, we show how this behavior results from a Cd(OH)<sub>2</sub> layer adsorbed at the Si surface at the beginning of the deposition. This response is explained by a competition between a high concentration of relatively shallow hole traps in the CdSe and a lower concentration of deeper electron traps in the Cd(OH)<sub>2</sub>. The relative occupancies of these traps determine the fields in the film and their response to external parameters.
Haick H., Ghabboun J., Niitsoo O., Cohen H., Cahen D., Vilan A., Hwang J., Wan A., Amy F. & Kahn A. (2005) Journal Of Physical Chemistry B. 109, 19, p. 9622-9630
Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.
Doron-Mor I., Cohen H., Barkay Z., Shanzer A., Vaskevich A. & Rubinstein I. (2005) Chemistry-A European Journal. 11, 19, p. 5555-5562
The distance dependence of the localized surface plasmon (SP) extinction of discontinuous gold films is a crucial issue in the application of transmission surface plasmon resonance (T-SPR) spectroscopy to chemical and biological sensing. This derives from the usual sensing configuration, where-by an analyte binds to a selective receptor layer on the gold film at a certain distance from the metal surface. In the present work the distance sensitivity of T-SPR spectroscopy of 1.0-5.0 nm (nominal thickness) gold island films evaporated on silanized glass substrates is studied by using coordination-based self-assembled multilayers, offering thickness tuning in the range from ∼1 to ∼15nm. The morphology, composition and optical properties of the Au/multilayer systems were studied at each step of multilayer construction. High-resolution scanning electron microscopy (HRSEM) showed no apparent change in the underlying Au islands, while atomic force microscopy (AFM) indicated flattening of the surface topography during multilayer construction. A regular growth mode of the organic layers was substantiated by X-ray photoelectron spectroscopy (XPS). Transmission UV-visible spectra showed an increase of the extinction and a red shift of the maximum of the SP band upon addition of organic layers, establishing the distance dependence of the Au SP absorbance. The distance sensitivity of T-SPR spectroscopy can be varied by using characteristic substrate parameters, that is, Au nominal thickness and annealing. In particular, effective sensitivity up to a distance of at least 15 nm is demonstrated with 5 nm annealed Au films. It is shown that intensity measurements, particularly in the plasmon intensity change (PIC) presentation, provide an alternative to the usually measured plasmon band position, offering good accuracy and the possibility of measuring at a single wavelength. The present distance sensitivity results provide the basis for further development of T-SPR transducers based on receptor-coated Au island films.
Cohen H., Nogues C., Zon I. & Lubomirsky I. (2005) Journal of Applied Physics. 97, 11, 113701
A method for work-function evaluation is proposed, based on recording the shift of x-ray photoelectron signals from a surface irradiated by low-energy electrons. The method is capable of measuring samples with very low conductivity, poor back contacts, and high dielectric constants. The method is also applicable to magnetic materials and can be particularly effective for studies of multilayer and heterogeneous systems.
Iron M. A., Lucassen A. C. B., Cohen H., van der Boom M. E. & Martin J. M. L. (2004) Journal of the American Chemical Society. 126, 37, p. 11699-11710
Various metallabenzene complexes, analogues of benzene where one CH unit has been replaced by an organometallic fragment, have been reported in the literature. A detailed theoretical investigation on the chemistry of these complexes is presented here. This includes an evaluation of their aromaticity, the mechanisms of formation of osmium, iridium, and platinum metallabenzene complexes, and one intriguing aspect of their chemistry, the formation of cyclopentadienyl (Cp) complexes. X-ray photoelectron spectroscopy (XPS) measurements on two osmabenzene examples are also presented. In addition, diffuse functions for use with the SDD and SDB-cc-pVDZ basis set-RECP combinations are presented for the transition metals.
Shukla A. D., Strawser D., Lucassen A. C. B., Freeman D., Cohen H., Amilan Jose D., Das A., Evmenenko G., Dutta P. & van der Boom M. E. (2004) Journal Of Physical Chemistry B. 108, 45, p. 17505-17511
A new series of stilbene-based chromophores have been used to prepare structurally related siloxane-based monolayers in order to determine which factors control the intermolecular chromophore-chromophore interactions in the solid state. The reaction of chromophore precursors 4-styrylpyridine (1), 4-[2-(4-bromophenyl)-vinyl]-pyridine (2), 4-(2-naphthalen-1-ylvinyl)-pyridine (3), 4-(2-anthracen-9-ylvinyl)-pyridine (4), and 4- (2-pyren-2-ylvinyl)-pyridine (5) with excess 3-iodo-n-propyl-1-trimethoxysilane resulted in the corresponding salts 6-10 in quantitative yield. The assembly of chromophores 6-10 on hydrophilic substrates from solution resulted in the formation of densely packed monolayers with a film thickness of similar to1 nm. The average chromophore density (similar to1 chromophore/50 Angstrom(2)) is well within the range that allows pi-pi stacking to occur. Transmission UV-vis spectroscopy of the siloxane-based films shows that the intermolecular interactions are a function of the aryl groups (e.g., phenyl, bromophenyl, naphthalene, anthracene, and pyrene). Relatively weak electronic interactions occur between the surface-bound chromophores 6, 7, and 10, whereas strong electronic interactions occur between surface-bound chromophores 8 and 9. The series of monolayers on sodium lime glass and polished silicon is characterized by a combination of physicochemical methods including X-ray photoelectron spectroscopy, advancing aqueous contact angle measurements, optical spectroscopy, atomic force microscopy, and synchrotron X-ray reflectivity.
Haimov A., Cohen H. & Neumann R. (2004) Journal of the American Chemical Society. 126, 38, p. 11762-11763
Combinations of alkylated polyethylenimine and polyoxometalates yield water-soluble synzymes with hydrophobic regions that allowed the aqueous biphasic selective oxidation of very hydrophobic, water-insoluble substrates with hydrogen peroxide. With the alkylated PEI/{PO<sub>4</sub>[WO(O<sub>2</sub>)<sub>2</sub>]<sub>4</sub>}<sup>3</sup>- highly effective C-C bond cleavage of alkenes to aldheydes was observed. The synzymes have both tertiary and quaternary amine centers as shown by a combination of <sup>15</sup>N-<sup>1</sup>H HMBC and XPS measurements. The existence of hydrophobic regions was concluded from the measurement of contact angles and a hypsochromic shift of a fluorescent probe.
Doron-Mor I., Cohen H., Cohen S., Popovitz-Biro R., Shanzer A., Vaskevich A. & Rubinstein I. (2004) Langmuir. 20, 24, p. 10727-10733
Coordination self-assembly of bishydrpxamate-based metal-organic multilayers on gold employing a layer-by-layer (LbL) approach was investigated. It is shown that the solution chemistry of the participating metal ion has a marked influence on the composition and properties of the multilayers. Use of Ce<sup>4+</sup> and particularly zirconium(IV) acetylacetonate (Zr(acac) <sub>4</sub>) solutions in the ion-binding step of multilayer construction leads to multilayers with a near-stoichiometric metal ion-to-ligand ratio, suggesting a structure close to that predicted by a simple coordination self-assembly scheme. On the other hand use of a ZrCl<sub>4</sub> solution as the source of metal ions in the multilayer construction leads to a multilayer with greater thickness and a large excess of Zr(IV), evenly distributed between the organic layers. In the latter case, a ratio of ca. 1:2 between the excess Zr and oxygen, as well as long-term Zr<sup>4+</sup> binding experiments showing deposition of ZrO<sub>2</sub>, suggest the formation of a zirconia-type nanophase between the bishydroxamate organic repeat units during multilayer self-assembly. Hence, while the multilayer prepared using Zr(acac)<sub>4</sub> solution appears to represent a "true" coordination-based structure, the one prepared using ZrCl<sub>4</sub> is best described as a composite organic-ceramic multilayer. Composite multilayers prepared in this way display different properties from those of the stoichiometric ones, such as improved dielectric behavior and higher stiffness. Even greater mechanical stability is obtained with multilayers constructed using alternate binding of ZrCl<sub>4</sub> and Ce<sup>4+</sup>, The concept of LbL formation of coordination-based composite organic-ceramic structures may be useful in obtaining nanometer-scale structures with tunable properties.
Band A., Albu-Yaron A., Livneh T., Cohen H., Feldman Y. (., Shimon L. J. W., Popovitz-Biro R., Lyahovitskaya V. & Tenne R. (2004) Journal Of Physical Chemistry B. 108, 33, p. 12360-12367
Cesium oxides are materials of great interest to the photodetection industry because of their relatively low work function (∼1 eV). Used mainly as coating films for photoemissive devices, they provide high wavelength thresholds and high photocurrents. However, they are unstable, air-sensitive, and hygroscopic, rendering them short-lived and limiting their applications. Although the technology of these devices is highly developed, their characterization on the micro- and nanoscale suffers from their poor chemical stability and poor crystallinity. In the present study, cesium oxides were synthesized from the elements and were characterized using a combination of chemical and structural analysis techniques. Because the reaction products were extremely sensitive to humidity, sample analysis without atmospheric exposure was essential, and techniques were developed for the transfer of the samples to the measurements systems. Extensive data obtained from X-ray energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and Raman microscopy were obtained. Raman spectra with bands at 103, 742, and 1134 cm<sup>-1</sup> strongly confirmed the presence of the oxide, peroxide, and Superoxide ions, respectively, as well as the absence of carbonate as an impurity. The A<sub>1g</sub> mode of Cs<sub>2</sub>O was detected as an anti-Stokes band at 103 cm<sup>-1</sup>. This study provides further insight into the reactivity of the various cesium oxides.
Cohen H. (2004) Applied Physics Letters. 85, 7, p. 1271-1273
Noncontact chemically resolved electrical measurements, which are capable of probing selected regions within fine heterostructures, using x-ray photoelectron spectroscopy (XPS) were analyzed. A modified x-ray photoelectron spectrometer was used to measure I-V curves of molecular layers, free of substrate and contact contributions. The use of a high-quality self-assembled monolayer (SAM), NH<sub>2</sub>C<sub>20</sub>Si, on p-Si, for the improvement in the performance of this technique was also discussed. The results indicate the efficient applications of this approach in conductance measurements through molecular layers.
Feldman Y. (., Zak A., Tenne R. & Cohen H. (2003) JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A. 21, 5, p. 1752-1757
Pronounced surface diffusion is observed during x-ray photoelectron spectroscopy measurements of 2H platelets and inorganic fullerene-like (IF) MS2 (M=WMo) powders, intercalated with alkaline (A=K,Na) elements. Using controlled surface charging the intercalants migrate towards the surface, where they oxidize. This dry deintercalation is controllable via external charging parameters, yet showing that internal chemical and structural parameters play an important role in the process. Diffusion rates out of 214 matrixes are generally higher than in corresponding IF samples. Clear differences are also found between Mo and W-based systems. Application of this approach into surface modification and processing is, proposed. (C) 2003 American Vacuum Society.
Lembrikov B., Itskovsky M., Cohen H. & Maniv T. (2003) Physical Review B. 67, 8, 085401
An electromagnetic theory of valence electron excitations by an external electron beam in the near field regions of uniaxial anisotropic nanoplatelets is presented. It is shown that in an interface of high symmetry (i.e., perpendicular to the symmetry axis) only extraordinary waves can be excited by the electron beam, usually as surface-plasmon-polariton modes. However, the anisotropy also allows excited extraordinary waves to propagate as waveguide modes. In an interface of low symmetry a mixture of both waves is inseparably excited. Application is made to directional near-field electron energy loss spectroscopy of uniaxial dielectric nanoplatelets. It is found that all relevant length parameters in this spectroscopy happen to fall in the same range, giving rise to enhanced sensitivity of the electron energy loss (EEL) signal to the size and geometry of the detected nanoparticle. The breakdown of momentum conservation in the electron-plasmon scattering event, associated with the finite size of the platelet along the beam direction, strongly changes the EEL signal pattern.
Samokhvalov A., Gurney R., Lahav M., Cohen S. R., Cohen H. & Naaman R. (2003) Journal Of Physical Chemistry B. 107, 18, p. 4245-4252
CdS quantum particles (QPs) assembled at predetermined distances from a gold substrate are prepared within a Langmuir-Blodgett film that forms an organic host matrix. The system is characterized by controlled surface charging (CSC) in X-ray photoelectron spectroscopy (XPS) and complementary methods, successfully resolving the discrete QP layers. A quantitative study of substrate-QPs charge-transfer channels is provided by laser-intensity dependent contact potential difference (CPD) measurements. The extracted electron-transfer rate constants exhibit marked differences in electron transfer from the film toward the substrate versus the backward process. The charging of the hybrid film was found to be either negative or positive depending on the intensity of the laser that photoexcites the QPs.
Shabtai K., Cohen S., Cohen H. & Rubinstein I. (2003) Journal Of Physical Chemistry B. 107, 23, p. 5540-5546
A composite surface comprising evenly distributed silicon oxide islands on a gold substrate is described. The composite surface is prepared by evaporation of a thin (50 nm) gold layer on oxide-free (H-passivated) silicon, followed by thermal diffusion of Si through the Au layer, gradually forming islands of SiO<sub>2</sub> on the Au surface. The rate of Si diffusion through the Au, and hence, the rate of SiO<sub>2</sub> island formation, is controlled by the annealing temperature, the Si crystallographic face, the Au film thickness, and the contacting atmosphere. The Au-SiO<sub>2</sub> composite surfaces can be used in applications requiring substrates patterned on a mesoscopic scale, while exposing two chemically dissimilar phases. One such application is shown here, namely, the formation of thiol-silane monolayers, for which the distribution of the different molecules in the resultant monolayer is determined by the substrate composition. The XPS controlled surface charging (CSC) method is used to establish a site-selective adsorption. The SiO<sub>2</sub> islands are found to be rather labile, shifting and aggregating upon self-assembly of alkanethiol molecules on the Au exposed areas. Pretreatment of the islands with a long-chain silane stabilizes the morphology.
Frey G., Reynolds K., Friend R., Cohen H. & Feldman Y. (. (2003) Journal of the American Chemical Society. 125, 19, p. 5998-6007
The development of low-cost, large-area electronic applications requires the deposition of active materials in simple and inexpensive techniques at room temperature, properties usually associated with polymer films. In this study, we demonstrate the integration of solution-processed inorganic films in light-emitting diodes. The layered transition metal dichalcogenide (LTMDC) films are deposited through Li intercalation and exfoliation in aqueous solution and partially oxidized in an oxygen plasma generator. The chemical composition and thickness of the LTMDC and corresponding transition metal oxide (TMO) films are investigated by X-ray photoelectron spectroscopy. The morphology and topography of the films are studied by atomic force microscopy. X-ray powder diffraction is used to determine the orientation of the LTMDC film. Finally, the LTMDC and their corresponding oxides are utilized as hole-injecting and electron-blocking materials in polymer light-emitting diodes with the general structure ITO/LTMDC/TMO/polyfluorene/Ca/Al. Efficient hole injection and electron blocking by the inorganic layers result in outstanding device performance and high efficiency.
Gofer Y., Turgeman R., Cohen H. & Aurbach D. (2003) Langmuir. 19, 6, p. 2344-2348
Metallic magnesium can be reversibly deposited from ethereal solutions of magnesium - aluminum complex salts of the general structure Mg(AlCl<sub>4-n</sub>R<sub>n</sub>)<sub>2</sub>, where R = alkyl. In contrast to the case of lithium surfaces, al ready thoroughly investigated, the surface state of magnesium electrodes in contact with organic solutions is yet unclear. In this paper we report on a systematic surface analysis of magnesium electrodes in contact with various organic solutions, using XPS. We find in both clean tetrahydrofurane (THF) and THF solutions of dibutylmagnesium or butylmagnesium chloride that the metal surface consists of magnesium oxide and hydroxide (probably developed during manipulation and sample transfer); however, it does not develop thick passivation layers. In THF solutions containing Mg(AlEtBuCl<sub>2</sub>)<sub>2</sub>, surface residuals of carbon, aluminum, and chlorine are detected yet are restricted to the outermost part of the surface, as physically adsorbed species. From their concentration one deduces that both the complex salt and the ether are not reduced at the magnesium surface but precipitate as an insoluble film. Metallic magnesium deposited from THF/Mg(AlEtBuCl<sub>2</sub>)<sub>2</sub> solution on gold electrodes shows a very similar surface chemistry, providing an additional proof that, even in the most frail conditions available during electrochemical deposition, pure magnesium is deposited. The surface chemistry of magnesium in contact with propylene carbonate (PC) exhibits as well layered surface chemistry, most of it composed of magnesium oxide and hydroxide, but no evidence is found for reduction products of PC. It is concluded that the magnesium metal behaves like a surface film-free electrode in organo-haloaluminate/THF solutions. Our conclusions support several other studies on the properties of magnesium in such solutions.
Artzi R., Daube S., Cohen H. & Naaman R. (2003) Langmuir. 19, 18, p. 7392-7398
GaAs-based electronic devices have interesting applications in spintronics and as sensors. In the past, methods were developed to stabilize the surface of GaAs, since it is known to be highly sensitive and unstable. It turns out, however, that these particular properties can be used for controlling the electronic characteristics of the devices, by adsorbing molecules that affect the surface properties. Here, we concentrate on the adsorption of molecules that can be bound to GaAs through their phosphate group. Phosphate functional groups can be found in many biological molecules; therefore, the binding of organic phosphate to a semiconductor surface can provide the first step toward a new line of hybrid bioorganic/inorganic electronic devices. We investigated the adsorption of tridecyl phosphate (TDP) and compared its adsorption to that of dodecanoic acid (lauric acid), which contains a carboxylic binding group. The alkyl phosphate monolayer is found to bind to the GaAs surface more strongly than any other functional group known to date. In addition, we show that the adsorption of a DNA nucleotide (5-AMP), as well as single-stranded DNA (ssDNA), on the GaAs surface occurs through the phosphate groups. Hence, DNA can be bound to these surfaces with no need for chemical modifications.
Bar-Nahum I., Cohen H. & Neumann R. (2003) Inorganic Chemistry. 42, 11, p. 3677-3684
Hybrid compounds with two functional centers consisting of a metallosalen moiety (M-salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen-polyoxometalate compounds (M-salen-POM) it was shown by the use of a combination of UV-vis, <sup>1</sup>H NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn-salen-POM, the metallosalen center is best described as a metal-salen cation radical species; that is, a localized "hole" is formed on the salen ligand. For the other M-salen-POM compounds, the metallosalen moiety can be described as a hybrid of a metal-salen cation radical species and an oxidized metal-salen species, that is, a delocalized "hole" is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor-polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers.
Cohen H., Lembrikov B., Itskovsky M. & Maniv T. (2003) Nano Letters. 3, 2, p. 203-206
Pronounced electromagnetic quantum-size effects are observed in directional near-field electron energy loss spectroscopy of rectangular nanocrystals. A theoretical analysis, exploiting a unique coincidence of all relevant length parameters in this spectroscopy and the momentum space filtering imposed by the exponentially decaying electron-plasmon interaction in the vacuum, reveals strong sensitivity of the EEL signal to the size and shape of the nanocrystal. The breakdown of momentum conservation along the beam direction, associated with the finite size of the nanocrystal, drastically alters the EEL signal pattern.
A method of examining a sample, including: performing a 1st spectroscopic anal. of a surface portion of the sample when the sample surface portion is in a 1st elec. charge state; placing the sample surface portion in a 2nd elec. charge state that is different from the 1st elec. charge state and performing a 2nd spectroscopic anal. of the surface portion of the sample when the sample surface portion is in the 2nd elec. charge state; and comparing the 1st spectroscopic anal. result with the 2nd spectroscopic anal. result to obtain at least one of structural and elec. information about the sample.
Zak A., Feldman Y. (., Lyakhovitskaya V., Leitus G., Popovitz-Biro R., Wachtel E., Cohen H., Reich S. & Tenne R. (2002) . 633, p. 67-70
Layered metal disulfides - MoS2 and WS2 in the form of fullerene-like (IF) nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). X-ray powder diffraction (XRD) analysis and transmission electron microscopy (TEM) of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 Angstrom) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. The modifications in magnetic and transport properties of the intercalated materials were investigated, and are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Restacking of the MS2 layers after prolonged exposure to the atmosphere and recovery of the pristine compound properties were observed as a result of deintercalation of the metal atoms.
Zak A., Feldman Y. (., Lyakhovitskaya V., Leitus G., Popovitz-Biro R., Wachtel E., Cohen H., Reich S. & Tenne R. (2002) Journal of the American Chemical Society. 124, 17, p. 4747-4758
Layered metal disulfides-MS<sub>2</sub> (M = Mo, W) in the form of fullerene-like nanoparticles and in the form of platelets (crystallites of the 2H polytype) have been intercalated by exposure to alkali metal (potassium and sodium) vapor using a two-zone transport method. The composition of the intercalated systems was established using X-ray energy dispersive spectrometer and X-ray photoelectron spectroscopy (XPS). The alkali metal concentration in the host lattice was found to depend on the kind of sample and the experimental conditions. Furthermore, an inhomogeneity of the intercalated samples was observed. The product consisted of both nonintercalated and intercalated phases. X-ray diffraction analysis and transmission electron microscopy of the samples, which were not exposed to the ambient atmosphere, showed that they suffered little change in their lattice parameters. On the other hand, after exposure to ambient atmosphere, substantial increase in the interplanar spacing (3-5 Å) was observed for the intercalated phases. Insertion of one to two water molecules per intercalated metal atom was suggested as a possible explanation for this large expansion along the c-axis. Deintercalation of the hydrated alkali atoms and restacking of the MS<sub>2</sub> layers was observed in all the samples after prolonged exposure to the atmosphere. Electric field induced deintercalation of the alkali metal atoms from the host lattice was also observed by means of the XPS technique. Magnetic moment measurements for all the samples indicate a diamagnetic to paramagnetic transition after intercalation. Measurements of the transport properties reveal a semiconductor to metal transition for the heavily K intercalated 2H-MOS<sub>2</sub>. Other samples show several orders of magnitude decrease in resistivity and two- to five-fold decrease in activation energies upon intercalation. These modifications are believed to occur via charge transfer from the alkali metal to the conduction band of the host lattice. Recovery of the pristine compound properties (diamagnetism and semiconductivity) was observed as a result of deintercalation.
Hensel V., Godt A., Popovitz-Biro R., Cohen H., Jensen T., Kjaer K., Weissbuch I., Lifshitz E. & Lahav M. (2002) Chemistry-A European Journal. 8, 6, p. 1413-1423
Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of π-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6(Cd), 8(Cd) or Pb salts 6(Pb), 8(Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6(H), 8(H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H<sub>2</sub>S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.
Leitus G., Cohen H. & Reich S. (2002) Physica C-Superconductivity And Its Applications. 371, 4, p. 321-329
It is known that Cs<sub>x</sub>WO<sub>3</sub>, 0.3 ≥ x ≥ 0.19, is a 3D superconductor. Below x = 0.19 a transition from a metal to an insulator occurs and for x = 0.10, for example, no superconductivity is observed. We find, however, that for x ≤ 0.05 superconductivity reappears. For x = 0.005 nominal concentration we observe a critical transition T<sub>c</sub> to the superconducting state at 5.9 K. This is observed in magnetic measurements, where a pronounced Meissner effect is present. X-ray photoelectron spectroscopy and transport measurements indicate that the SC state is confined to mesoscopic islands, some 10 nm thick on the surface of the Cs<sub>x</sub>WO<sub>3</sub> doped crystals. The bulk of the crystal is practically pure WO<sub>3</sub>. Thus during the high temperature crystal growth process, there is a redistribution of the Cs dopant as it migrates to the surface of the WO<sub>3</sub> crystals. From these observations, we conclude that upon reduction of the Cs nominal concentration below x = 0.05, a 2D superconductor is formed, while for x ≥ 0.19, the Cs<sub>x</sub>WO<sub>3</sub> bronze is a 3D superconductor.
Lyahovitskaya V., Feldman Y. (., Gartsman K., Cohen H., Cytermann C. & Cahen D. (2002) Journal of Applied Physics. 91, 7, p. 4205-4212
The effect of sodium on the performance of CuInSe<sub>2</sub>-based solar cells has been under discussion for already a decade. We present experimental evidence using secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS) and other, complementary physical characterization methods, which indicate that, after exposure to an external Na source, no significant amounts of sodium, beyond the residual amount, found in as-grown samples, enter intact crystals, except via defects such as grain boundaries. However, after such exposure, sodium is found in significant concentrations on crystal surfaces, something that is accompanied by an increase in oxygen concentration, as judged by XPS. As expected metallic Na attacks the crystals and can destroy them or at least introduce significant defect densities. Adding Se<sup>0</sup> is found, via Na<sub>2</sub>Se formation, to temper Na activity specifically its effects on crystal disintegration. This is different from the effect of Se<sup>0</sup> along where annealing (of n-type) crystals results in n to p type conversion by Cu outdiffusion.
Buller R., Cohen H., Minkin E., Popovitz-Biro R., Lifshitz E. & Lahav M. (2002) Advanced Functional Materials. 12, 10, p. 713-718
Elliptically shaped (Pb<sub>1-x</sub>Cd<sub>x</sub>)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p-phenylene-ethynylene) dicarboxylate]Pb<sub>0.9</sub>Cd<sub>0.1</sub> salt matrix, with gaseous H<sub>2</sub>S. A significantly long emission lifetime, with multi-exponential behavior, is detected in time-resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co-existence of Cd and Pb within the same particle is provided by light-induced X-ray photoelectron spectroscopy.
Zelner M., Minti H., Reisfeld R., Cohen H., Feldman Y. (., Cohen S. & Tenne R. (2001) Journal of Sol-Gel Science and Technology. 20, 2, p. 153-160
Two methods for the synthesis of CdTe nanoparticles in zirconia sol-gel films are demonstrated. The nanoparticles were obtained by chemical reduction of Te(IV) using reducing agent (hydrazine) or tin chloride. Particle sizes ranging from 6 to 20 nm in diameter could be prepared by varying the experimetal parameters. The size and crystalline structure of the particles were characterized by optical absorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The film morphology was characterized by scanning force microscopy. The film obtained by SnCl<sub>2</sub> method is smooth and homogeneous. The dense structure of CdTe nanoparticles of a few nm in diameter is revealed. The films prepared with hydrazine are porous as a result of evolution of the decomposition gaseous products during the reduction. Advantages and disadvantages of the two methods are discussed.
Buller R., Cohen H., Jensen T., Kjaer K., Lahav M. & Leiserowitz L. (2001) Journal Of Physical Chemistry B. 105, 46, p. 11447-11455
Five bifunctional α- amino acid-∈-carboxy bolaamphiphiles [(2-R,S, or RS)-(HOOC)-(CH<sub>2</sub>)<sub>m</sub>-CONH-(CH<sub>2</sub>)n-CH(COOH)(NH<sub>2 </sub>) m = 20,22, n = 3,4 labeled (l), (d,l)-C<sub>22</sub>Orn (l), (d), (d,l)-C<sub>22</sub>Lys, (d,l)-C<sub>24</sub>Lys respectively] were synthesized. These molecules were deposited on different aqueous subphases, and studied by means of grazing incidence X-ray diffraction (GIXD). On deionized water, the bolaamphiphiles (d,l)-C<sub>22</sub>Lys yield a mixture of crystallites: both a monolayer, in which the chains are tilted from the normal, and a multilayer in which the molecules lie parallel to the water. On the other hand, when deposited on mono- or bi-metal ionic subphases, they self-assemble into crystalline multilayer films in which the molecules lie parallel to the aqueous solution surface, linked head-to-head and tail-to-tail in the form of extended chains. The latter are juxtaposed such that the metal ions form sheets separated by the organic molecules, aligned perpendicular to the plane of the aqueous solution. Deposition of either the enantiomerically pure or racemic bifunctional bolaamphiphiles on an aqueous solution of mixed Cu(Acetate)<sub>2</sub> and Pb(Acetate)<sub>2</sub>, yields self-assembled crystalline films composed of the two different metal cations, arranged in alternating sheets, separated by the organic spacer. Both GIXD and X-ray-photoelectric-spectroscopy (XPS) studies (after deposition on solid support) demonstrate that the structures of these films differ form the ones formed on either pure Pb(Ac)<sub>2</sub> or on Cu(Ac)<sub>2</sub> solutions, thus excluding the simultaneous formation of the two monometallic crystalline phases.
Weissbuch I., Baxter P., Kuzmenko I., Cohen H., Cohen S. R., Kjaer K., Howes P. B., Als-Nielsen J., Lehn J., Leiserowitz L. & Lahav M. (2000) Chemistry-A European Journal. 6, 4, p. 725-734
Oriented crystalline monolayers, ~ 14 Å thick, of a 2 x 2 Ag<sup>+</sup> grid complex, self-assembled at the air-solution interface starting from an waterinsoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counterions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial selfassembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.
Shabtai K., Rubinstein I., Cohen S. & Cohen H. (2000) Journal of the American Chemical Society. 122, 20, p. 4959-4962
X-ray photoelectron spectroscopy (XPS), an essentially macroscopic probe, is used to analyze mesoscopic systems at a lateral resolution given by the substrate structure. The method is based on controlled differential charging of multi-component surfaces, using a simple, commonly available XPS function, the electron flood gun. This new approach is applied here to a novel composite surface comprising SiO<sub>2</sub> clusters on a {111} gold substrate, onto which different molecules are self-assembled to form a mixed organic monolayer. The method allows direct correlation of adsorbed molecules with surface sites, by analyzing XPS line shifts, which reflect local potential variations resulting from differential surface conductivity. This provides a powerful tool for resolving complex ultrathin films on heterogeneous substrates, on a length scale much smaller than the probe size.
Haran A., Cohen H. & Naaman R. (2000) Superlattices and Microstructures. 28, 4, p. 279-288
The effect of the structure of organic films on their electronic band above the vacuum level is investigated. The ballistic transmission probability of secondary electrons emitted from metal substrates through organized organic thin films is found to decrease for electrons with kinetic energy higher than ca. 1 eV. The thicker and more ordered the adsorbed film is, the better defined is its band structure and the sharper is its transmission resonance.
Guo S., Konopny L., Popovitz-Biro R., Cohen H., Sirota M., Lifshitz E. & Lahav M. (2000) Advanced Materials. 12, 4, p. 302-306
CdS and Cd<sub>1-x</sub>Mn<sub>x</sub>S nanoparticles arranged in patterns within acrylamide were prepared via the topotactic reaction of the corresponding metal thioalkanoates with gaseous ammonia. The organization of the particles was improved in some systems and induced in others in the presence of alkanoates that act as site-directing nucleating centers.
Doron-Mor I., Hatzor A., Vaskevich A., van der Boom-Moav T., Shanzer A., Rubinstein I. & Cohen H. (2000) Nature. 406, 6794, p. 382-385
Probing the structure of material layers just a few nanometres thick requires analytical techniques with high depth sensitivity. X-ray photoelectron spectroscopy(1) (XPS) provides one such method, but obtaining vertically resolved structural information from the raw data is not straightforward. There are several XPS depth-profiling methods, including ion etching(2), angle-resolved XPS (ref. 2) and Tougaard's approach(3), but all suffer various limitations(2-5). Here we report a simple, non-destructive XPS depth-profiling method that yields accurate depth information with nanometre resolution. We demonstrate the technique using self-assembled multilayers on gold surfaces; the former contain 'marker' monolayers that have been inserted at predetermined depths. A controllable potential gradient is established vertically through the sample by charging the surface of the dielectric overlayer with an electron flood gun. The local potential is probed by measuring XPS line shifts, which correlate directly with the vertical position of atoms. We term the method 'controlled surface charging', and expect it to be generally applicable to a large variety of mesoscopic heterostructures.
Rapopor L., Feldman Y., Homyonfer M., Cohen H., Cohen S. & Tenne R. (1999) Tribology Series. 36, p. 567-573
Friction and wear behavior of new nanomaterial - inorganic fullerene-like (IF) WS2 supramolecules has been compared with layered solid lubricants MoS2 and WS2. Lubrication mechanism of IF nanoparticles and tribochemistry of contact have been studied. The main advantages of IF nanoparticles lies in their round shape and the absence of dangling bonds. Velocity accommodation modes under friction with solid lubricant powders were considered. The main lubrication mechanism of contact with IF nanoparticles appears to be rolling friction.
Rapoport L., Feldman Y. (., Homyonfer M., Cohen H., Sloan J., Hutchison J. & Tenne R. (1999) Wear. 225-229, part 2, p. 975-982
Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX<sub>2</sub> (M = Mo, W, etc.; X = S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. The main goal of the present work was to study the tribological properties of IF-WS<sub>2</sub> in comparison to 2H-WS<sub>2</sub> and MoS<sub>2</sub> platelets over a wide range of loads and sliding velocities. The size and shape distributions of the nanoparticles were studied by transmission electron microscopy (TEM). The average size of the IF-WS<sub>2</sub> particles was 120 nm, while that of 2H-WS<sub>2</sub> and 2H-MoS<sub>2</sub> was 0.5 μm and 4 μm. The chemical reactivity of the different powders in an oxygenated atmosphere was verified by heating the powders in ambient atmosphere. The friction experiments were performed in laboratory atmosphere (humidity ∼ 50%) using a ring-block tester. Complementary information on the state of wear of the powders in the lubricating fluid and on the metal surfaces of the specimens was obtained using a combination of TEM, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It has been established that IF-WS<sub>2</sub> nanoparticles appear to have excellent tribological properties in definite loading range in comparison to typical metal dichalcogenides. The oxidation of the IF particles and the wear track was essentially less than with solid lubricants made of platelets of the same chemical compound (WS<sub>2</sub>). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.
Eliyahu A., Buehler J., Ben-Chorin M., Cohen H. & Prior Y. (1999) Diamond and Related Materials. 8, 2-5, p. 146-149
We present a new procedure for pretreatment seeding by ultrasonic agitation of silicon substrates in diamond nano-powder suspensions to which HF and KOH were added X-ray photoelectron spectroscopy (XPS) was used to measure the surface coverage by diamond nuclei immediately after the pretreatment. Coverage percentages of 70, 40 and 55% were obtained for the HF, KOH and the original diamond slurry, respectively. The seeding density (S<sub>D</sub>) was calculated from the known nano-particles size, determined independently from X-ray diffraction of the powder. For nano-particle size of ~6 nm, we obtain nominal seeding densities of the order ~10<sup>12</sup> cm<sup>-2</sup>. The advantage of the high coverage was most evident for films deposited at low substrate temperature (570 °C). The potential of the new seeding procedure and the XPS characterization method are discussed.
Cohen S., Feldman Y. (., Cohen H. & Tenne R. (1999) Applied Surface Science. 144-145, p. 603-607
Recently, new structural formulations of metal dichalcogenides have resulted in greatly enhanced lubricant properties, placing the mechanisms of lubrication for this class of materials under closer scrutiny. In this work, scanned probe microscope techniques have been used to probe the surface on a nanoscale, comparing friction and adhesion at specific surface sites. In addition, the role of interlayer shear and subsurface defects in moderating friction are examined. (C) 1999 Elsevier Science B.V. All rights reserved.
Ranjit K., Cohen H., Willner I., Bossmann S. & Braun A. (1999) Journal of Materials Science. 34, 21, p. 5273-5280
A series of lanthanide oxide-doped titanium dioxide photocatalysts Ln<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> (Ln<sup>3+</sup> = Eu<sup>3+</sup>, Pr<sup>3+</sup>, Yb<sup>3+</sup>) were prepared. The photocatalysts reveal a substantially enhanced activity for the degradation of organic pollutants, as compared to undoped TiO<sub>2</sub>. The photodegradation processes of p-nitrobenzoic acid, (1), p-chlorophenoxyacetic acid, (2), aniline, (3), salicylic acid, (4), and trans-cinnamic acid, (5), with the different photocatalysts was examined. The photodegradation of (1)-(5) is significantly faster with Ln<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> photocatalysts and leads to complete mineralization of the organic compounds. The high activity of the Ln<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> photocatalysts is attributed to the enhanced association of the functionalized organic pollutants to lanthanide-ion surface sites.
Schechter A., Aurbach D. & Cohen H. (1999) Langmuir. 15, 9, p. 3334-3342
Li samples were freshly prepared (shearing) and stored (2 days) in dimethyl carbonate (DMC), and ethyl carbonate-diethyl carbonate (EC-DEC), and dry (20 ppm of H<sub>2</sub>O) and wet (500 ppm of H<sub>2</sub>O) EC-DMC solutions of LiAsF<sub>6</sub> (1 M), and were then studied by X-ray photoelectron spectroscopy (XPS). The XPS analysis, including depth profiling of these surface films, appears to be reliable on the qualitative level only, because both the X-ray beam and sputtering should be suspected as being partially destructive to the surface films on lithium. These studies basically confirm previous conclusions obtained by Fourier transform infrared spectroscopy spectroscopic studies of Li surfaces. Surface films formed on Li in alkyl carbonate solutions of LiAsF<sub>6</sub> are comprised of ROCO<sub>2</sub>Li, Li<sub>2</sub>CO<sub>3</sub>, LiF, Li<sub>x</sub>AsF<sub>y</sub>, and Li oxides. XPS could also detect surface species with Li-C bonds (e.g., LiCH<sub>2</sub>CH<sub>2</sub>OCO<sub>2</sub>Li). When EC is present, its reduction dominates the surface film formation. The presence of water suppresses both solvent and salt anion reduction and enriches the surface films with Li<sub>2</sub>CO<sub>3</sub> (because of secondary reactions of water with surface species), LiOH, and Li<sub>2</sub>O. These studies also confirm that the surface films formed on Li have a multilayer structure.
Guo S., Konopny L., Popovitz-Biro R., Cohen H., Porteanu H., Lifshitz E. & Lahav M. (1999) Journal of the American Chemical Society. 121, 41, p. 9589-9598
A method is described for the preparation of hybrid organic/inorganic structures where the inorganic component comprises semiconductor nanoparticles aligned in periodic layers within three-dimensional (3-D) crystalline powders and Langmuir-Blodgett (LB) films. The preparation process comprises the organization of metal ions in the form of periodic arrays within 3-D crystals or the LB films, followed by a topotactic gas/solid reaction. The method is illustrated for the organization of CdS nanoparticles within alkanoic acids. The order of the nanoparticles is achieved by introducing site directing nucleation centers of Cd thioalkanoates within Cd alkanoates, in the form of solid solutions. The formed particles are attached to the organic matrix via -C(O)S-Cd-S- bonds. The structure of those supramolecular architectures has been characterized by a variety of complementary methods, including transmission electron microscopy (TEM) and electron diffraction (ED), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other spectroscopic measurements.
Behar D., Rubinstein I., Hodes G., Cohen S. & Cohen H. (1999) Superlattices and Microstructures. 25, 4, p. 601-613
CdS quantum dots (QDs) have been electrodeposited onto textured gold substrates from a nonaqueous electrolyte containing Cd(ClO<sub>4</sub>)<sub>2</sub> and elemental S. The initial deposit consisted of very small (about 3 nm) nanocrystals of CdS which were partially oriented with the Au substrate. With increasing deposit thickness, the crystal size increased and the degree of orientation decreased. Photocurrent spectroscopy and I-V spectroscopy, using a conducting scanning force microscope tip, were used to measure the CdS bandgap variations due to size quantization. The latter method also revealed room temperature conductivity peaks assigned to Coulomb charging of the QDs and evidence for charge tunneling into higher discrete energy levels.
Brami Y., Herut B., Shemesh A. & Cohen H. (1999) Environmental Science and Technology. 33, 2, p. 276-281
The surface (0.2-0.5 nm) chemical characteristics of coal fly ash (CFA) before and after interaction with Mediterranean deep seawater was studied by X-ray photoelectron spectroscopy (XPS). Significantly lower values of Si, Ca, and S and higher values of Mg and CI were found in the retrieved CFA as compared to fresh CFA. It is suggested that hydrolysis of the oxide matrixes results in an alkaline environment which rapidly leads to several chemical reactions. The two most important are (a) dissolution of the amorphous silicate and the calcium phases and (b) precipitation of Mg(OH)<sub>2</sub>-brucite. A depth profile of the retrieved CFA was measured by both line-shape analysis of the XPS spectra and by consecutive cycle of sputtering. The thickness of the brucite layer is estimated to be 1.3 nm.
Berfeld M., Kuzmenko I., Weissbuch I., Cohen H., Howes P., Kjaer K., Als-Nielsen J., Leiserowitz L. & Lahav M. (1999) Journal Of Physical Chemistry B. 103, 33, p. 6891-6899
To provide more direct information on the role played by "tailor-made" auxiliary molecules in the early stages of crystal nucleation, the interplay between clusters of polar headgroups of monolayers of the copper complexes 5-Cu-S and S-Cu-R and water-soluble copper complexes S-Cu-S and R-Cu-R were investigated [where S represents enantiomerically pure (S)-palmitoyl-N<sup>ε</sup>-lysine, and S and R represent chiral resolved (S) and (R) forms of alanine, serine, or valine]. The different monolayers were formed by spreading the amphiphilic (R) or (S) α-amino acid on an aqueous solution of copper acetate followed by injection of the water-soluble (S) or (R) α-amino acid into the subphase. The surface pressure-molecular area isotherms of the Langmuir monolayers of the two type of complexes (S-Cu-S and S-Cu-R) are different, the former being substantially more expanded. The polar headgroups of the S-Cu-S and of the S-Cu-R monolayers transferred onto a solid support assume a trans and cis configuration, respectively, according to comparative X-ray photoelectron spectroscopy (XPS) studies with appropriate cis and trans three-dimensional (3-D) α-amino acid Cu complexes. A grazing incidence X-ray diffraction (GIXD) analysis demonstrated that the S-Cu-S and S-Cu-R monolayers have different 2-D crystal structures, in keeping with the XPS results. A model is presented suggesting that the water-soluble S-Cu-S copper complexes are enantioselectively bound to the periphery of the domains of the cis S-Cu-S monolayers, but not to the domains of the trans S-Cu-R monolayers. By symmetry, the same principal holds for the monolayers and water soluble copper complexes of α-amino acids of the opposite handedness.
Weissbuch I., Baxter P., Cohen S. R., Cohen H., Kjaer K., Howes P. B., Als-Nielsen J., Hanan G., Schubert U., Lehn J., Leiserowitz L. & Lahav M. (1998) Journal of the American Chemical Society. 120, 19, p. 4850-4860
Oriented crystalline films, ~11-20 Å thick, of metal ion complexes of the grid type [Co<sub>4</sub>L<sub>4</sub>]<sup>8+</sup>·8PF<sub>6</sub><sup>-</sup> and [Ag<sub>9</sub>L<sub>6</sub>]<sup>9+</sup> 9·CF<sub>3</sub>SO<sub>3</sub><sup>-</sup>, based on various ligands L, were prepared in-situ at the air-aqueous solution interface by the interaction of the free ligand molecules spread onto aqueous solutions containing Co<sup>2+</sup> or Ag<sup>+</sup> ions. The structure of the complex architectures composed of a 2 x 2 Co<sup>2+</sup> grid coordinated to four ligand molecules and a 3 x 3 Ag<sup>+</sup> grid coordinated to six ligand molecules as well as their molecular organization in thin films were characterized by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity (XR) measurements performed at the air-aqueous solution interface and by UV, X-ray photoelectron spectroscopy (xPs), and scanning force microscopy (SFM) after film transfer onto various solid supports. The results open perspectives toward an implementation of the air-water interface for the self-assembly and subsequent deposition of organized arrays of complex inorganic architectures onto solid support.
Maoz R., Cohen H. & Sagiv J. (1998) Langmuir. 14, 21, p. 5988-5993
We demonstrate a new approach to the study and application of microwave-induced chemical reactions, using a purpose-designed molecular film as a discrete ultrathin antenna that can utilize absorbed electromagnetic energy to directly drive a specific chemical transformation. Exposure of such a bilayer "antenna" self-assembled on silicon to common microwave radiation in a domestic oven is shown to reproducibly generate a depleted top monolayer structure with molecular-size vacancies that can incorporate and control the further surface manipulation of various gap-fitting guest species. The nonthermal nature of this process is unequivocally demonstrated, as the irradiated bilayer cannot store heat, while conventional heating causes its irreversible structural deterioration, before an equivalent thermally activated transformation could occur. These findings shed new light on the much disputed issue of nonthermal microwave effects, suggesting, beyond obvious implications for basic research in this area, that microwave radiation could be rationally utilized to achieve specific (nondestructive) transformations in properly designed supramolecular systems and could be utilized particularly as an attractive new synthetic tool in molecular surface engineering.
Moav T., Hatzor A., Cohen H., Libman J., Rubinstein I. & Shanzer A. (1998) Chemistry-A European Journal. 4, 3, p. 502-507
A novel type of bilayer on a gold surface, based upon metal-ion coordination to hydroxamate moieties, is described. Tailor-made bifunctional ligands containing hydroxamate groups (for metal coordination) and a cyclic disulfide residue (for surface attachment) have been prepared. The bishydroxamate binding site forms 2:1 ligand/metal complexes with octacoordinating metal ions such as Zr(IV), Ce(IV), and Ti(IV); the cyclic disulfide moiety anchors the complex to the gold surface. Two routes to bilayer formation are demonstrated: i) a one-step process from preformed 2:1 complexes, and ii) a stepwise process including formation of the ligand monolayers followed by binding of a guest ion and a second layer of ligand molecules. The former approach allows full characterizations of the complexes before assembly, whereas the latter enables construction of either symmetric (identical) or asymmetric (nonidentical) bilayers. Both types of bilayers were characterized by ellipsometry contact angle, and XPS measurements. Symmetric bilayers obtained by the two processes have similar properties.
Ron H., Cohen H., Matlis S., Rappaport M. & Rubinstein I. (1998) Journal Of Physical Chemistry B. 102, 49, p. 9861-9869
Self-assembly (SA) of long-chain alkanethiols on copper was studied. Two factors were found to have substantial influence on the SA process: (i) the chemical reactivity of Cu toward substances present in the adsorption solution, particularly the solvent, and (ii) surface pretreatment, which influences the amount of oxide and the surface morphology. Both factors are less important in the case of SA onto gold because of its chemical inertness. Monolayers of octadecanethiol (C18SH) were adsorbed from different solvents (ethanol, toluene, and bicyclohexyl) at various thiol concentrations onto Cu surfaces pretreated in several different ways. The monolayers were characterized by contact-angle measurements, grazing-incidence Fourier transform infrared spectroscopy, and scanning force microscopy. Ethanol, the most common solvent for alkanethiol SA, is found to have a negative effect on monolayer SA apparently because of its chemical reactivity toward copper. With toluene as a solvent, better oriented and more crystalline monolayers are obtained provided that a higher thiol concentration is used to compensate for the higher solubility of thiols in toluene. Treatment of the Cu surface prior to SA is shown to significantly improve the SA by reducing the amount of surface oxide and the surface corrugation. The effect of the solvent is more critical than surface oxidation; hence, high-quality monolayers are formed in the presence of thin oxide layers on Cu surfaces. Superior C18SH monolayers, in terms of organization and crystallinity, are obtained by SA from toluene onto Cu surfaces sputtered-annealed in high vacuum, even when the Cu surface is subjected to short exposure to air before SA.
Guo S., Popovitz-Biro R., Weissbuch I., Cohen H., Hodes G. & Lahav M. (1998) Advanced Materials. 10, 2, p. 121-125
Quantum dots of CdS and PbS can be generated in thin films and 3D crystalline powders by taking advantage of the packing arrangement of metal alpha,omega-alkanedicarboxylates (see previous communication). The Figure is a transmission electron micrograph of the CdS particles formed in the 3D crystalline powder of cadmium C-12 dicarboxylate.
Hatzor A., Moav T., Cohen H., Matlis S., Libman J., Vaskevich A., Shanzer A. & Rubinstein I. (1998) Journal of the American Chemical Society. 120, 51, p. 13469-13477
A new kind of multilayers based on metal-ion coordination was constructed on gold surfaces, where molecular layers are successively added using a highly controlled step-by-step procedure. A bifunctional ligand is used as the base layer, bearing a cyclic disulfide group to attach to the gold surface and a bishydroxamate group capable of ion binding. An 8- coordinating metal ion such as Zr<sup>4+</sup> or Ce<sup>4+</sup> is then coordinated tO the bishydroxamate site, followed by exposure to a second ligand possessing four hydroxamate groups. The tetrahydroxamate molecule ligates to the metal ion (bound to the base layer) using two of its four hydroxamate groups and is free to bind a second metal ion at its other end. A sequence of adsorption steps using metal ions and tetrahydroxamate ligands was carried out, resulting in an ordered metal-organic multilayer. Multilayer structures comprising up to 10 tetrahydroxamate/metal ion layers were constructed, with full characterization at each step of multilayer formation using ellipsometry, contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The multilayer morphology and mechanical properties were studied by scanning force microscopy. It is shown that different base ligands induce dramatic differences in the morphology and stiffness of the final multilayer. The possibility to construct segmented multilayers containing Zr<sup>4+</sup> and Ce<sup>4+</sup> ions at defined locations is presented.
Cohen H., Maniv T., Tenne R., Hacohen Y., Stephan O. & Colliex C. (1998) Physical review letters. 80, 4, p. 782-785
Near-field spectroscopy of nanoparticles was performed with a subnanometer probe size, using a scanning transmission electron microscope. At nonintersecting beam-particle configuration, surface collective modes are isolated from the entire excitation spectrum. The relativistic narrow beam generates a new type of spatial dispersion and may induce an additional spectral quantization. Line intensity variations during the mapping can be accounted for quantitatively by using a relatively simple theoretical model. This method is also suggested as a new way for transmission electron microscopy studies of beam sensitive samples.
Cohen S., Rapoport L., Ponomarev E., Cohen H., Tsirlina T., Tenne R. & Levy-Clement C. (1998) Thin Solid Films. 324, 1-2, p. 190-197
The tribological properties of textured WS<sub>2</sub>, MoS<sub>2</sub> and WSe<sub>2</sub> films, which were prepared using an ultra-thin interlayer of Ni/Cr (van der Waals rheotaxy technique) on quartz substrate, were determined in ambient conditions. Using scanning force microscope adapted for tribological measurements, very low (0.04 and below) friction coefficients and little wear were measured on flat areas of the films. Macroscopic (engineering) tribological measurements, using the reciprocating ball on flat tribometer, exhibit somewhat higher friction coefficients. Compactization of the films under the load and little wear were observed for the films even after a few hundred cycles. X-ray photoelectron spectroscopy of the WS<sub>2</sub> film after the wear experiment confirmed that some oxidation took place within the wear track, but the overall integrity of the film was preserved. These measurements indicate that highly textured films of this kind are promising candidates for tribological coatings, where oil-free lubrication is required.
Zelner M., Minti H., Reisfeld R., Cohen H. & Tenne R. (1997) Chemistry of Materials. 9, 11, p. 2541-2543
A new method for the synthesis of sol-gel films of zirconia-containing nanoparticles of CdS is demonstrated. The method consists of dip-coating of the substrate from a preprepared mother solution containing zirconia tetrapropoxide, cadmium acetate, and ammonium thiocyanate and subsequent annealing of the dried film at elevated temperatures. The films were characterized by optical absorption spectra, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The satellite spectrum of the Cd core levels was analyzed, and thereby the plasmonic structure of the film was resolved. The potential of the process for the fabrication of a class of sol-gel films with semiconductor nanoparticles having optical quality is discussed.
Kataby G., Prozorov T., Koltypin Y., Cohen H., Sukenik C., Ulman A. & Gedanken A. (1997) Langmuir. 13, 23, p. 6151-6156
Self-assembled coatings of long chain thiols on nanoparticles of amorphous iron as well as on amorphous Fe<sub>2</sub>O<sub>3</sub> were formed. The thermal stability of these coatings was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the particles and by mass spectrometric analysis of the molecules desorbed from the surface by heating the amorphous substrates. The results showed a sharp weight loss centered at about 200 °C for the thiol-coated particles. The mass spectrometric study of the molecules removed from the surface of the amorphous iron revealed the formation of a dialkyl thioether. For the amorphous Fe<sub>2</sub>O<sub>3</sub> surfaces, a dialkyl disulfide was removed from the surface. The desorptions from the Fe<sub>2</sub>O<sub>3</sub> surface occurred at higher temperatures than those for the amorphous iron. The different mechanisms responsible for these desorption reactions are discussed.
Frydman E., Cohen H., Maoz R. & Sagiv J. (1997) Langmuir. 13, 19, p. 5089-5106
The damage caused to amphiphilic n-alkane monolayers under XPS measurement conditions was assessed in a combined XPS-FTIR study supplemented by additional AFM imaging and contact angle measurements. Nine different self-assembled monolayer/substrate systems were examined, comprising a long chain silane (C<sub>18</sub>, OTS), a short chain silane (C<sub>1</sub>, MTS), a functional (COOH-terminated) long chain silane (C<sub>18</sub>, NTS<sub>ox</sub>), a long chain carboxylic acid (C<sub>20</sub>, AA), and four different solid substrates (silicon, quartz, glass, and ZnSe). Significant differences were observed in the behavior of the various examined monolayer systems under identical X-ray irradiation conditions. These are interpreted in terms of effects associated with the specific mode of layer-to-surface and intralayer coupling, the size of the monolayer hydrocarbon core, and the presence of radiation-sensitive functional groups in the layer. All these factors and their influence on the degradation path followed by a particular monolayer upon exposure to the X-rays were found to be interrelated, giving rise to a variety of possible damage patterns, including an unexpected overall stabilization effect initiated by the preferential rapid loss of a labile top functional group (NTS<sub>ox</sub>). XPS is shown to be insufficient as a tool for the evaluation of the radiation-induced damage in such ultrathin films, because of its insensitivity to loss of hydrogen and to structural transformations that occur without a net loss of carbon from the surface. Independent methods of surface analysis (mainly FTIR), applied in conjunction with XPS, provide a more comprehensive picture of the induced damage, thus permitting a realistic interpretation of the XPS experimental data as well as the design of improved data acquisition procedures. This could also assist in the tailoring of monolayers with predetermined degradability, for specific purposes. Finally, results of combined AFM-XPS-FTIR-contact angle measurements suggest the possible formation of a "diamond-like" surface film upon extensive X-ray irradiation of an OTS/Si monolayer.
Haran A., Kadyshevitch A., Cohen H., Naaman R., Evans D., Seideman T. & Nitzan A. (1997) Chemical Physics Letters. 268, 5-6, p. 475-480
Direct evidence for the electronic band structure of thin organized organic layers is presented. The experimental results indicate that the electron-organic film system has to be described in quantum mechanical terms and that classical concepts fail. Quantum mechanical simulations on a generic system are also presented. They indicate that this type of simulation provides insight into the system studied experimentally.
Dhas N., Cohen H. & Gedanken A. (1997) Journal Of Physical Chemistry B. 101, 35, p. 6834-6838
Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-μ-[dibenzylideneacetone]-dipalladium [(φ-CH=CH-CO-CH=CH-φ)<sub>3</sub>Pd<sub>2</sub>] in mesitylene. Characterization by elemental analysis, transmission electron microscopy, differential scanning calorimetry, X-ray diffraction, X-ray photoelectron spectroscopy, and BET surface area measurements shows that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 Å. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated.
Tsirlina T., Cohen S. R., Cohen H., Sapir L., Peisach M., Tenne R., Matthaeus A., Tiefenbacher S., Jaegermann W., Ponomarev E. & Levy-Clement C. (1996) Solar Energy Materials and Solar Cells. 44, 4, p. 457-470
Thin films of metal dichalcogenide compounds with a layered structure, such as MoS<sub>2</sub> (WSe<sub>2</sub>), play an important role in a number of technologies, like solid lubrication, experimental photovoltaic cells, etc. Such films usually adopt a type-I texture, in which case the c-axis of the crystallites is parallel to the substrate plane. However, for the aforementioned applications, type-II texture, where the c-axis of the crystallite is perpendicular to the substrate, is required. We have recently demonstrated a novel growth technique (Van der Waals rheotaxy, VdWR) which yields a crystalline film having exclusively type-II texture on amorphous, (quartz) substrate. In the present work superior crystalline, optical and electronic properties of the overlying WSe<sub>2</sub> (WS<sub>2</sub>) film together with an improved adhesion of the film to the quartz substrate are obtained by replacing the ultra thin Ni film with a Ni/Cr film.
Feldman Y. (., Frey G., Homyonfer M., Lyakhovitskaya V., Margulis L., Cohen H., Hodes G., Hutchison J. & Tenne R. (1996) Journal of the American Chemical Society. 118, 23, p. 5362-5367
Recently, milligram quantities of MoS<sub>2</sub> fullerene-like nanotubes and negative curvature polyhedra (generically called inorganic fullerene-like material, IF), were reproducibly obtained by a gas phase reaction from an oxide precursor (Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. Srolovitz, D. J.; Safran, S. A.; Homyonfer, M.; Tenne, R. Phys. Rev. Lett. 1995, 74, 1778). The present work focuses on the mechanism of the synthesis of IF-MS<sub>2</sub> (M = W, Mo). The IF material is obtained from oxide particles smaller than ca. 0.2 μm, while larger oxide particles result in 2H-MS<sub>2</sub> platelets. The key step in the reaction mechanism is the formation of a closed layer of MS<sub>2</sub>, which isolates the nanoparticle from its surroundings and prevents its fusion into larger particles. Subsequently, the oxide core of the nanoparticle is progressively converted into a sulfide nanoparticle with an empty core (IF). Taking advantage of this process, we report here a routine for the fabrication of macroscopic quantities of a pure IF-WS<sub>2</sub> phase with a very high yield. As anticipated, the size distribution of the IF material is determined by the size distribution of the oxide precursor. The present synthesis paves the way for a systematic study of these materials which are promising candidates for, e.g., solid lubrication.
Aurbach D., Markovsky B., Shechter A., Ein-Eli Y. & Cohen H. (1996) Journal of the Electrochemical Society. 143, 12, p. 3809-3820
This work entails a comparative study of both Li and synthetic graphite electrodes in electrolyte solutions based on ethylene and dimethyl carbonates (EC-DMC) and the impact of the salt used [from the LiAsF<sub>6</sub>, LiClO<sub>4</sub>, LiPF<sub>6</sub>, LiBF<sub>4</sub>, and LiN(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub> list]. The presence of some additives in solutions (e.g., Li<sub>2</sub>CO<sub>3</sub>, CO<sub>2</sub>, tributylamine) and the effect of the particle size of the carbon on the electrode's behavior were investigated. The correlation between the surface chemistry, the morphology, and the performance of Li and graphite electrodes was explored using surface sensitive Fourier transform infrared and x-ray and photoelectron spectroscopies, impedance spectroscopy, x-ray diffraction and scanning electron microscopy in conjunction with standard electrochemical techniques. Synthetic graphite anodes could be cycled (Li intercalation-deintercalation) hundreds of times at a capacity close to the optimal (x → 1 in Li<sub>x</sub>C<sub>6</sub>) in EC-DMC solutions due to the formation of highly stable and passivating surface films in which EC reduction products such as (CH<sub>2</sub>OCO<sub>2</sub>Li)<sub>2</sub> are the major constituents. The cycling efficiency of Li metal anodes in these solutions, however, is lower than that obtained in ethereal solutions and seems to be too low for Li-metal liquid electrolyte, rechargeable battery application. The connection between the solution composition and the electrode's performance is discussed.
Aurbach D., Weissman I., Schechter A. & Cohen H. (1996) Langmuir. 12, 16, p. 3991-4007
Li electrodes prepared in situ in solutions and then stored in them for different periods were studied by X-ray photoelectron spectroscopy (XPS) including depth profiling performed by argon sputtering followed by XPS. A set of solvents, propylene carbonate (PC), ethylene carbonate (EC)-dimethyl carbonate (DMC) mixtures, and 1,3-dioxolane, and a set of salts, LiAsF<sub>6</sub>, LiBF<sub>4</sub>, LiPF<sub>6</sub>, LiN(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>, and LiC(SO<sub>2</sub>CF<sub>3</sub>)<sub>3</sub>, were investigated with respect to the effect of storage time. The results of this study were compared with previous studies of Li electrodes in the same solutions by in situ and ex situ Fourier transform infrared spectroscopy. Basically, the results thus obtained are in line with the previous studies. The Li surface chemistry is dominated by solvent reactions. However, all the above salt anions are also reduced to form insoluble species which also contribute to the build-up of the surface films (e.g., the salt anions of the type MF<sub>y</sub><sup>-</sup> (M = As, P, B) are reduced to LiF and species of the Li<sub>x</sub>MF<sub>z</sub> type). The surface reactions of these solvents and the salts on Li are discussed in detail. Depth profiling of the surface films formed on Li in solutions indicates that they have a multilayer structure. The concentration of the organic salts in the surface layers decreases as the layer is closer to the Li-film interface.
REUBEN C., Galun E., Cohen H., Tenne R., KALISH R., MURAKI Y., HASHIMOTO K., FUJISHIMA A., BUTLER J. & LEVYCLEMENT C. (1995) Journal of Electroanalytical Chemistry. 396, 1-2, p. 233-239
Boron-doped diamond films were used as electrodes for the reduction of nitrate and nitrite ions to ammonia. Prior to the electrochemical investigation, some in-depth analysis of the diamond films, was undertaken using various techniques. It was found that the plasma-assisted chemical vapor deposition process is superior to the chemical deposition process in controlling the (boron) doping process. The analysis also indicates that some of the boron is occluded in the film along the channels (grain boundaries) between the diamond crystallites. The faradaic efficiency (FE) of reduction of the nitrite (nitrate) to ammonia was found to be greater than unity and could be partially accounted for by the dissolution of the Si substrate itself. Further experiments with free-standing diamond films showed a rather smaller FE; nevertheless it was still greater than unity. A submonolayer of bound nitrogen was found on the diamond surface after the electrolysis. The present results are attributed to a catalytic mechanism for the multi-electron process described here.
Galun E., Cohen H., MARGULIS L., Vilan A., TSIRLINA T., Hodes G., Tenne R., HERSHFINKEL M., JAEGERMANN W. & ELLMER K. (1995) Applied Physics Letters. 67, p. 3474
Anisotropic materials with layered structure, like MoS<sub>2</sub> and WSe<sub>2</sub>, play an important role in a number of technologies. Some of these applications (lubrication, photovoltaics) require polycrystalline films oriented with their c axis perpendicular to the substrate surface (type-II texture), which is the thermodynamically favorable texture. However, films with the substrate ∥c (type-I texture) are usually obtained. We report that an ultrathin (
COHEN H., KOLODNEY E., HEBARD A., MANIV T. & FOLMAN M. (1994) Europhysics Letters. 27, 7, p. 519-524
We report the observation of well-resolved fine structures in the main plasmonic band multiples of solid C60, as measured by reflection EELS. The various peak energies are correlated as pure overtones and combinations of a fundamental set of plasmonic features, showing no evidence for anharmonicity. Focusing on the relative intensity of combinations and overtones, a pronounced dependence on crystalline order is found. This effect can be explained by the localization of some collective modes within the granular structures.
