Publications

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.

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.

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 WS₂ nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO₂ nanoparticles anchor within the WS₂ 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 WS₂ nanotubes, as demonstrated herein.

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 WSe₂ 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 WSe₂ and WS₂ 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 WSe₂ and WS₂. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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⁺ and small amounts of K⁺ and Mg^2 + 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.

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.

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.

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.

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.

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.

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.

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, [NaH₂(Mn-L)W₁₇F₆O₅₅]^q-, 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 O₂, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K⁺, Rb⁺, and Cs⁺) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li⁺ and Na⁺ 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 O₂ formation is fastest in the presence of Cs⁺ and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O₂ formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O₂ formation.

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).

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.

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 ₃CO-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.

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.

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.

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⁺ 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/AgNO₃ 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.

Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu ²⁺ and Pd ²⁺ 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 ²⁺ and Pd ²⁺. We reaffirm our statement that Cu ²⁺ and Pd ²⁺ ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.

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.

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.

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.

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.

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.

The presence of water at the surface of sputtered thin films of Ce _0.8Gd_0.2O_1.9, which display elastic anomalies as a function of thermal treatment, and of stoichiometric CeO₂ 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 _0.8Gd_0.2O_1.9 films than of the CeO₂ 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 _0.8Gd_0.2O_1.9 nor the CeO₂ 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_0.8Gd _0.2O_1.9 films.

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.

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.

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.

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.

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.

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.

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.

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.

The temporal chemical changes occurring at the surface of fullerene-like (IF) nanoparticles of WS₂ 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.

n-Si/C_nH_2n+1/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.

WS₂ 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 WS₂ 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.

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.

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.

IF-MO_1-xNb_xS₂ nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H₂S. The IF-MO_1-xNb_xS₂ 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 NbS₂ within the MoS₂ host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS₂ host lattice. Very few Nb atoms, if any, are intercalated between the MoS₂ 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 MoS₂ (whereas unsubstituted IF-MoS₂ nanoparticles are semiconducting). Furthermore the IF-MO_1-xNb_xS₂ nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures.

Electron irradiation can alter electronic charge transport through Si-CH₂(CH₂)₁₂-CH₃//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.

A model of structural transformations of amorphous into quasi-amorphous BaTiO₃ 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 BaTiO₃ are made up of a network of slightly distorted TiO₆ 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 TiO₆ network. Upon heating, the edge-to-edge and face-to-face connections between TiO₆ octahedra are severed and then reconnected via apices. Severing the connections between TiO₆ 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 TiO₆ 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.

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.

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.

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.

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.

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⁺ signal is detected. A finite concentration of ions is observed when the DNA is made from a solution of Mg²⁺ 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.

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.

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.

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⁺- and p⁺-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)₂ 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)₂. The relative occupancies of these traps determine the fields in the film and their response to external parameters.

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.

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/{PO₄[WO(O₂)₂]₄}³- 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 ¹⁵N-¹H 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.

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.

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, ¹H 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.

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.

MoS_2-xI_y (x≈0,y

Elliptically shaped (Pb_1-xCd_x)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_0.9Cd_0.1 salt matrix, with gaseous H₂S. 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.

Layered metal disulfides-MS₂ (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 MS₂ 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-MOS₂. 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.

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 H₂S 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.

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.

Oriented crystalline monolayers, ~ 14 Å thick, of a 2 x 2 Ag⁺ 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.

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.

CdS quantum dots (QDs) have been electrodeposited onto textured gold substrates from a nonaqueous electrolyte containing Cd(ClO₄)₂ 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.

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 H₂O) and wet (500 ppm of H₂O) EC-DMC solutions of LiAsF₆ (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 LiAsF₆ are comprised of ROCO₂Li, Li₂CO₃, LiF, Li_xAsF_y, and Li oxides. XPS could also detect surface species with Li-C bonds (e.g., LiCH₂CH₂OCO₂Li). 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 Li₂CO₃ (because of secondary reactions of water with surface species), LiOH, and Li₂O. These studies also confirm that the surface films formed on Li have a multilayer structure.

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)₂-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.

Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX₂ (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-WS₂ in comparison to 2H-WS₂ and MoS₂ 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-WS₂ particles was 120 nm, while that of 2H-WS₂ and 2H-MoS₂ 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-WS₂ 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 (WS₂). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.

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 Zr⁴⁺ or Ce⁴⁺ 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 Zr⁴⁺ and Ce⁴⁺ ions at defined locations is presented.

The tribological properties of textured WS₂, MoS₂ and WSe₂ 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 WS₂ 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.

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.

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.

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)₃Pd₂] 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.

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 LiAsF₆, LiClO₄, LiPF₆, LiBF₄, and LiN(SO₂CF₃)₂ list]. The presence of some additives in solutions (e.g., Li₂CO₃, CO₂, 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_xC₆) in EC-DMC solutions due to the formation of highly stable and passivating surface films in which EC reduction products such as (CH₂OCO₂Li)₂ 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.

Electronic excitations of solid C60 have been studied using the reflection EELS technique with a 0.2 eV energy resolution. A comparison with graphite revealed an overall similarity of the spectral gross features, while significant differences were observed in the fine details, mainly at low loss energies. A similar dominant plasmon was observed in C60 at a slightly shifted energy of 23.2 eV. For C60, in contrast to graphite, a clear energy loss onset was found at 1.7 eV and up to 6 eV six loss peaks were clearly observed. It was found that minima in the loss spectrum fit well with reported optical transitions. This observation supports an interpretation which attributes a significant collective character to the loss peaks.