Publications

The chirality-induced spin selectivity (CISS) effect is the capability of chiral molecules to act as spin filters, i.e. to selectively sort flowing electrons based on their spin states. The application of this captivating phenomenon holds great promise in the realm of molecular spintronics, where the primary focus lies in advancing technologies based on chiral molecules to regulate the injection and coherence of spin-polarized currents. In this context, we conducted a study to explore the spin filtering capabilities of a monolayer of the thia-bridged triarylamine hetero[4]helicene radical cation chemisorbed on a metallic surface. Magnetic-conductive atomic force microscopy revealed efficient electron spin filtering at exceptionally low potentials. Furthermore, we constructed a spintronic device by incorporating a monolayer of these molecules in between two electrodes, obtaining an asymmetric magnetoresistance trend with signal inversion in accordance with the handedness of the enantiomer involved, indicative of the presence of the CISS effect. Our findings underscore the significance of thia[4]azahelicene organic radicals as promising candidates for the development of quantum information operations based on the CISS effect as a tool to control the molecular spin states.

Chiral oligopeptide monolayers are adsorbed on a ferromagnetic surface and their magnetoresistance is measured as a function of the angle between the magnetization of the ferromagnet and the surface normal. These measurements are conducted as a function of temperature for both enantiomers. The angle dependence is found to follow a changing trend with a period of 360°. Quantum simulations reveal that the angular distribution can be obtained only if the monolayer has significant effective spin orbit coupling (SOC), that includes contribution from the vibrations. The model shows that SOC only in the leads cannot reproduce the observed angular dependence. The simulation can reproduce the experiments if it included electron\u2013phonon interactions and dissipation.

Since the initial landmark study on the chiral induced spin selectivity (CISS) effect in 1999, considerable experimental and theoretical efforts have been made to understand the physical underpinnings and mechanistic features of this interesting phenomenon. As first formulated, the CISS effect refers to the innate ability of chiral materials to act as spin filters for electron transport; however, more recent experiments demonstrate that displacement currents arising from charge polarization of chiral molecules lead to spin polarization without the need for net charge flow. With its identification of a fundamental connection between chiral symmetry and electron spin in molecules and materials, CISS promises profound and ubiquitous implications for existing technologies and new approaches to answering age old questions, such as the homochiral nature of life. This review begins with a discussion of the different methods for measuring CISS and then provides a comprehensive overview of molecules and materials known to exhibit CISS-based phenomena before proceeding to identify structure-property relations and to delineate the leading theoretical models for the CISS effect. Next, it identifies some implications of CISS in physics, chemistry, and biology. The discussion ends with a critical assessment of the CISS field and some comments on its future outlook.

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin-orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Self-assembling features, chiroptical activity, and spin filtering properties are reported for 2,15- and 4,13-disubstituted [6]helicenes decorated in their periphery with 3,4,5-tris(dodecyloxy)-N-(4-ethynylphenyl)benzamide moieties. The weak non-covalent interaction between these units conditions the corresponding circularly polarized luminescence and spin polarization. The self-assembly is overall weak for these [6]helicene derivatives that, despite the formation of H-bonding interactions between the amide groups present in the peripheral moieties, shows very similar chiroptical properties both in the monomeric or aggregated states. This effect could be explained by considering the steric effect that these groups could generate in the growing of the corresponding aggregate formed. Importantly, the self-assembling features also condition chiral induced spin selectivity (CISS effect), with experimental spin polarization (SP) values found between 35\u201340 % for both systems, as measured by magnetic-conducting atomic force microscopy (AFM) technique.

Metal-reducing bacteria have adapted the ability to respire extracellular solid surfaces instead of soluble oxidants. This process requires an electron transport pathway that spans from the inner membrane, across the periplasm, through the outer membrane, and to an external surface. Multiheme cytochromes are the primary machinery for moving electrons through this pathway. Recent studies show that the chiral-induced spin selectivity (CISS) effect is observable in some of these proteins extracted from the model metal-reducing bacteria, Shewanella oneidensis MR-1. It was hypothesized that the CISS effect facilitates efficient electron transport in these proteins by coupling electron velocity to spin, thus reducing the probability of backscattering. However, these studies focused exclusively on the cell surface electron conduits, and thus, CISS has not been investigated in upstream electron transfer components such as the membrane-associated MtrA, or periplasmic proteins such as small tetraheme cytochrome (STC). By using conductive probe atomic force microscopy measurements of protein monolayers adsorbed onto ferromagnetic substrates, we show that electron transport is spin selective in both MtrA and STC. Moreover, we have determined the spin polarization of MtrA to be ∼77% and STC to be ∼35%. This disparity in spin polarizations could indicate that spin selectivity is length dependent in heme proteins, given that MtrA is approximately two times longer than STC. Most significantly, our study indicates that spin-dependent interactions affect the entire extracellular electron transport pathway.

The oxygen reduction reaction (ORR) is the key for oxygen-based respiration and the operation of fuel cells. It involves the transmission of two pairs of electrons. We probed what type of interaction between the electrons is required to enable their efficient transfer into the oxygen. We show experimentally that the transfer of the electrons is controlled by the "hidden property" and present a theoretical model suggesting that it is related to coherent phase relations between the two electrons. Using spin polarization electrochemical measurements, with electrodes coated with different thicknesses of chiral coating, we confirm the special relation between the electrons. This relation is destroyed by multiple scattering events that result in the formation of hydrogen peroxide, which indicates a reduction in the ORR efficiency. Another indication for the possible role of coherence is the fluctuations in the reaction efficiency as a function of thickness of the chiral coated electrode.

Metal-organic Co(ii)-phenylalanine crystals were studied and were found to possess magnetic properties and long-range spin transport. Magnetic measurements confirmed that in the crystals there are antiferromagnetic interactions between Co(ii) and the lattice. The metal-organic crystals (MOCs) also present the chirality-induced spin selectivity (CISS) effect at room temperature. A long-range spin polarization is observed using a magnetic conductive-probe atomic force microscope. The spin polarization is found to be in the range of 35-45%.

Molecular spins are promising building blocks of future quantum technologies thanks to the unparalleled flexibility provided by chemistry, which allows the design of complex structures targeted for specific applications. However, their weak interaction with external stimuli makes it difficult to access their state at the single-molecule level, a fundamental tool for their use, for example, in quantum computing and sensing. Here, an innovative solution exploiting the interplay between chirality and magnetism using the chirality-induced spin selectivity effect on electron transfer processes is foreseen. It is envisioned to use a spin-to-charge conversion mechanism that can be realized by connecting a molecular spin qubit to a dyad where an electron donor and an electron acceptor are linked by a chiral bridge. By numerical simulations based on realistic parameters, it is shown that the chirality-induced spin selectivity effect could enable initialization, manipulation, and single-spin readout of molecular qubits and qudits even at relatively high temperatures.

The adsorption of oxidatively damaged DNA onto ferromagnetic substrates was investigated. Both confocal fluorescence microscopy and quartz crystal microbalance methods show that the adsorption rate and the coverage depend on the magnetization direction of the substrate and the position of the damage site on the DNA relative to the substrate. SQUID magnetometry measurements show that the subsequent magnetic susceptibility of the DNA-coated ferromagnetic film depends on the direction of the magnetic field that was applied to the ferromagnetic film as the molecules were adsorbed. This study reveals that (i) the spin and charge polarization in DNA molecules is changed significantly by oxidative damage in the G bases and (ii) the rate of adsorption on a ferromagnet, as a function of the direction of the magnetic dipole of the surface, can be used as an assay to detect oxidative damage in the DNA.

The oxygen reduction reaction (ORR) is of high importance, among others, because of its role in cellular respiration and in the operation of fuel cells. Recently, a possible relation between respiration and general anesthesia has been found. This work aims to explore whether anesthesia related gases affect the ORR. In ORR, oxygen which is in its triplet ground state is reduced to form products that are all in the singlet state. While this process is "in principle" forbidden because of spin conservation, it is known that if the electrons transferred in the ORR are spin-polarized, the reaction occurs efficiently. Here we show, in electrochemical experiments, that the efficiency of the oxygen reduction is reduced by the presence of general anesthetics in solution. We suggest that a spin-orbit coupling to the anesthetics depolarizes the spins. This causes both a reduction in reaction efficiency and a change in the reaction products. The findings may point to a possible relation between ORR efficiency and anesthetic action.

Considerable electric fields are present within living cells, and the role of bioelectricity has been well established at the organismal level. Yet much remains to be learned about electric-field effects on protein function. Here, we use phototriggered charge injection from a site-specifically attached ruthenium photosensitizer to directly demonstrate the effect of dynamic charge redistribution within a protein. We find that binding of an antibody to phosphoglycerate kinase (PGK) is increased twofold under illumination. Remarkably, illumination is found to suppress the enzymatic activity of PGK by a factor as large as three. These responses are sensitive to the photosensitizer position on the protein. Surprisingly, left (but not right) circularly polarized light elicits these responses, indicating that the electrons involved in the observed dynamics are spin polarized, due to spin filtration by protein chiral structures. Our results directly establish the contribution of electrical polarization as an allosteric signal within proteins. Future experiments with phototriggered charge injection will allow delineation of charge rearrangement pathways within proteins and will further depict their effects on protein function.

Controlled reduction of oxygen is important for developing clean energy technologies, such as fuel cells, and is vital to the existence of aerobic organisms. The process starts with oxygen in a triplet ground state and ends with products that are all in singlet states. Hence, spin constraints in the oxygen reduction must be considered. Here, we show that the electron transfer efficiency from chiral electrodes to oxygen (oxygen reduction reaction) is enhanced over that from achiral electrodes. We demonstrate lower overpotentials and higher current densities for chiral catalysts versus achiral ones. This finding holds even for electrodes composed of heavy metals with large spin-orbit coupling. The effect results from the spin selectivity conferred on the electron current by the chiral assemblies, the chiral-induced spin selectivity effect.

We report on the synthesis and self-assembly of 2,15- and 4,13-disubstituted carbo[6]helicenes 1 and 2 bearing 3,4,5-tridodecyloxybenzamide groups. The self-assembly of these [6]helicenes is strongly influenced by the substitution pattern in the helicene core that affects the mutual orientation of the monomeric units in the aggregated form. Thus, the 2,15-substituted derivative 1 undergoes an isodesmic supramolecular polymerization forming globular nanoparticles that maintain circularly polarized light (CPL) with glum values as high as 2 × 10\u20132. Unlike carbo[6]helicene 1, the 4,13-substituted derivative 2 follows a cooperative mechanism generating helical one-dimensional fibers. As a result of this helical organization, [6]helicene 2 exhibits a unique modification in its ECD spectral pattern showing sign inversion at low energy, accompanied by a sign change of the CPL with glum values of 1.2 × 10\u20133, thus unveiling an example of CPL inversion upon supramolecular polymerization. These helical supramolecular structures with high chiroptical activity, when deposited on conductive surfaces, revealed highly efficient electron-spin filtering abilities, with electron spin polarizations up to 80% for 1 and 60% for 2, as measured by magnetic conducting atomic force microscopy.

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Enantiospecific biorecognition interactions are key to many biological events. Commonly, bio-affinity values, measured in these processes, are higher than those calculated by available methods. We report here the first direct measurement of the interaction force between two chiral peptides (right- and left-handed helical polyalanine peptides) and the quantification of difference in the interaction force between homochiral and heterochiral pairs of molecules using atomic force microscope (AFM), together with supportive calculations based on a simple theoretical model. A force difference of 70 pN between same and opposite enantiomer interactions is measured. Additional measurements show spin dependency and fast decay of the interaction term, consistent with spin exchange interactions. This short range enantiospecific interaction term is especially relevant in crowded biological systems. The results shed light on the importance of spin and exchange interactions in biological processes.

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.

The steady-state charge and spin transfer yields were measured for three different Ru-modified azurin derivatives in protein films on silver electrodes. While the charge-transfer yields exhibit weak temperature dependences, consistent with operation of a near activation-less mechanism, the spin selectivity of the electron transfer improves as temperature increases. This enhancement of spin selectivity with temperature is explained by a vibrationally induced spin exchange interaction between the Cu(II) and its chiral ligands. These results indicate that distinct mechanisms control charge and spin transfer within proteins. As with electron charge transfer, proteins deliver polarized electron spins with a yield that depends on the protein's structure. This finding suggests a new role for protein structure in biochemical redox processes.

We overview experiments performed on the chiral induced spin selectivity (CISS) effect using various materials and experimental configurations. Through this survey of different material systems that manifest the CISS effect, we identify several attributes that are common to all the systems. Among these are the ability to observe spin selectivity for two point contact configurations, when one of the electrodes is magnetic, and the correlation between the optical activity of the chiral systems and a material\u2019s spin filtering properties. In addition, recent experiments show that spin selectivity does not require pure coherent charge transport and the electron spin polarization persists over hundreds of nanometers in an ordered medium. Finally, we point to several issues that still have to be explored regarding the CISS mechanism. Among them is the role of phonons and electron\u2013electron interactions.

Protein function may be modulated by an event occurring far away from the functional site, a phenomenon termed allostery. While classically allostery involves conformational changes, we recently observed that charge redistribution within an antibody can also lead to an allosteric effect, modulating the kinetics of binding to target antigen. In the present work, we study the association of a polyhistidine tagged enzyme (phosphoglycerate kinase, PGK) to surface-immobilized anti-His antibodies, finding a significant Charge-Reorganization Allostery (CRA) effect. We further observe that PGK\u2019s negatively charged nucleotide substrates modulate CRA substantially, even though they bind far away from the His-tag\u2013antibody interaction interface. In particular, binding of ATP reduces CRA by more than 50%. The results indicate that CRA is affected by the binding of charged molecules to a protein and provide further insight into the significant role that charge redistribution can play in protein function.

A new mechanism of allostery in proteins, based on charge rather than structure, is reported. We demonstrate that dynamic redistribution of charge within a protein can control its function and affect its interaction with a binding partner. In particular, the association of an antibody with its target protein antigen is studied. Dynamic charge shifting within the antibody during its interaction with the antigen is enabled by its binding to a metallic surface that serves as a source for electrons. The kinetics of antibody–antigen association are enhanced when charge redistribution is allowed, even though charge injection happens at a position far from the antigen binding site. This observation points to charge-reorganization allostery, which should be operative in addition or parallel to other mechanisms of allostery, and may explain some current observations on protein interactions.

Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.

The electro-oxidative polymerization mechanism of an enantiopure chiral EDOT monomer, performed using spin polarized currents, is shown to depend on the electron spin orientation. The spin-polarized current is shown to influence the initial nucleation rate of the polymerization reaction. This observation is rationalized in the framework of the Chiral Induced Spin Selectivity effect.

The effect of spin polarization in conduction and electric field-induced polarization was measured for double-stranded DNA oligonucleotides and oligopeptides of different lengths. The measurements were conducted using magnetic contact AFM, spin-dependent electrochemistry, spin-dependent polarization, and magnetoresistance studies. It was established that the spin-dependent conduction through chiral molecules depends on the voltage applied with a power of d, when d is larger than unity, and that there is a different voltage threshold for conducting each of the spin polarizations. In addition, there is no spin flipping during the conduction through the chiral system. The spin polarization depends linearly on the length, within the range of lengths studied, and it seems to scale like the optical activity. These results suggest the importance of the electric polarizability in the chiral-induced spin selectivity process.

This perspective discusses recent experiments that bear on the Chiral Induced Spin Selectivity (CISS) mechanism and its manifestation in electronic and magnetic properties of chiral molecules and materials. Although the discussion emphasizes newer experiments, such as the magnetization dependence of chiral molecule interactions with ferromagnetic surfaces, early experiments, which reveal the nonlinear scaling of the spin filtering with applied potential, are described also. In many of the theoretical studies, one has had to invoke unusually large spin-orbit couplings in order to reproduce the large spin-filtering observed in experiments. Experiments imply that exchange interactions and Pauli Exclusion constraints are an important aspect of CISS. They also demonstrate the spin-dependent charge flow between a ferromagnetic substrate and chiral molecules. With these insights in mind, a simplified model is described in which the chiral molecule’s spin polarization is enhanced by a spin blockade effect to generate large spin filtering.

Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet's magnetization direction and the molecules' chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.

Multiheme cytochromes, located on the bacterial cell surface, function as long-distance (>10 nm) electron conduits linking intracellular reactions to external surfaces. This extracellular electron transfer process, which allows microorganisms to gain energy by respiring solid redox-active minerals, also facilitates the wiring of cells to electrodes. While recent studies have suggested that a chiral indexed spin selectivity effect is linked to efficient electron transmission through biomolecules, this phenomenon has not been investigated in extracellular electron conduits. Using magnetic conductive probe atomic force microscopy, Hall voltage measurements, and spin-dependent electrochemistry of the decaheme cytochromes MtrF and OmcA from the metal-reducing bacterium Shewanella oneidensis MR-1, we show that electron transport through these extracellular conduits is spin-selective. Our study has implications for understanding how spin-dependent interactions and magnetic fields may control electron transport across biotic-abiotic interfaces in both natural and biotechnological systems.

Spin based properties, applications, and devices are typically related to inorganic ferromagnetic materials. The development of organic materials for spintronic applications has long been encumbered by its reliance on ferromagnetic electrodes for polarized spin injection. The discovery of the chirality-induced spin selectivity (CISS) effect, in which chiral organic molecules serve as spin filters, defines a marked departure from this paradigm because it exploits soft materials, operates at ambient temperature, and eliminates the need for a magnetic electrode. To date, the CISS effect has been explored exclusively in molecular insulators. Here we combine chiral molecules, which serve as spin filters, with molecular wires that despite not being chiral, function to preserve spin polarization. Self-assembled monolayers (SAMs) of right-handed helical (L-proline)(8) (Pro(8)) and corresponding peptides, N-terminal conjugated to (porphinato)zinc or meso-to-meso ethyne-bridged (porphinato)zinc structures (Pro(8)PZn(n)), were interrogated via magnetic conducting atomic force microscopy (mC-AFM), spin-dependent electrochemistry, and spin Hall devices that measure the spin polarizability that accompanies the charge polarization. These data show that chiral molecules are not required to transmit spin-polarized currents made possible by the CISS mechanism. Measured Hall voltages for Pro(8)PZn(1-3) substantially exceed that determined for the Pro(8) control and increase dramatically as the conjugation length of the achiral PZnn component increases; mC-AFM data underscore that measured spin selectivities increase with an increasing Pro(8)PZn(1-3) N-terminal conjugation. Because of these effects, spin-dependent electrochemical data demonstrate that spin-polarized currents, which trace their genesis to the chiral Pro(8) moiety, propagate with no apparent dephasing over the augmented Pro(8)PZn(n) length scales, showing that spin currents may be transmitted over molecular distances that greatly exceed the length of the chiral moiety that makes possible the CISS effect.

The functionality of many biological systems depends on reliable electron transfer with minimal heating. Interestingly, nature realizes electron transport via insulating molecules, in contrast to man-made electronic devices which are based on metals and semiconductors. The high efficiency of electron transfer through these organic molecules is unexpected for tunneling-based transport, and it is one of the most compelling questions in the field. Furthermore, it has been shown that the electron tunneling probability is strongly spin-dependent. Here, we demonstrate that the chiral structure of these molecules gives rise to robust coherent electron transfer. We introduce spin into the analysis of tunneling through organic helical molecules and show that they support strong spin filtering accompanied by enhanced transmission. Thus, our study resolves two key questions posed by transport measurements through organic molecules.

Enantiospecific crystallization of the three amino acids asparagine (Asn), glutamic acid hydrochloride (Glu·HCl) and threonine (Thr), induced by ferromagnetic (FM) substrates, is reported. The FM substrates were prepared by evaporating nickel capped with a thin gold layer on standard silicon wafers. Magnets were positioned underneath the substrate with either their North (N) or South (S) poles pointing up. Asymmetric induction, controlled by the magnetic substrates, was demonstrated for the crystallization of the pure enantiomers and was then extended for the racemic mixtures of Asn and Glu·HCl. In the case of the solution of the pure enantiomers, the l enantiomer was crystallized preferentially at one pole of the magnet and the d enantiomer at the other. Consequently, the racemates of Asn and Glu·HCl undergo separation under the influence of the magnetic substrate. With Thr, however, despite the enantiospecific interactions of the pure enantiomers with the FM, no separation of the emerging crystals could be achieved with the racemates, although they crystallize as conglomerates, implying differences taking place in the crystallization step. The results reported here are not directly related to the magnetic field, but rather to the aligned spins within the ferromagnets. The findings provide a novel method for resolving enantiomers by crystallization and offer a new perspective for a possible role played by magnetic substrates regarding the origin of chirality in nature.

The electron's spin is essential to the stability of matter, and control over the spin opens up avenues for manipulating the properties of molecules and materials. The Pauli exclusion principle requires that two electrons in a single spatial eigenstate have opposite spins, and this fact dictates basic features of atomic states and chemical bond formation. The energy associated with interacting electron clouds changes with their relative spin orientation, and by manipulating the spin directions, one can guide chemical transformations. However, controlling the relative spin orientation of electrons located on two reactants (atoms, molecules or surfaces) has proved challenging. Recent developments based on the chiral-induced spin selectivity (CISS) effect show that the spin orientation is linked to molecular symmetry and can be controlled in ways not previously imagined. For example, the combination of chiral molecules and electron spin opens up a new approach to (enantio) selective chemistry. This Review describes the theoretical concepts underlying the CISS effect and illustrates its importance by discussing some of its manifestations in chemistry, biology and physics. Specifically, we discuss how the CISS effect allows for efficient long-range electron transfer in chiral molecules and how it affects biorecognition processes. Several applications of the effect are presented, and the importance of controlling relative spin orientations in multi-electron processes, such as electrochemical water splitting, is emphasized. We describe the enantiospecific interaction between ferromagnetic substrates and chiral molecules and how it enables the separation of enantiomers with ferromagnets. Lastly, we discuss the relevance of CISS effects to biological electron transfer, enantioselectivity and CISS-based spintronics applications.

We study GaAs/AlGaAs devices hosting a two-dimensional electron gas and coated with a monolayer of chiral organic molecules. We observe clear signatures of room-temperature magnetism, which is induced in these systems by applying a gate voltage. We explain this phenomenon as a consequence of the spin-polarized charges that are injected into the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the megahertz range. Thus, our devices implement an efficient, electric field-controlled magnetization, which has long been desired for their technical prospects.

The rapid growth in demand for data and the emerging applications of Big Data require the increase of memory capacity. Magnetic memory devices are among the leading technologies for meeting this demand; however, they rely on the use of ferromagnets that creates size reduction limitations and poses complex materials requirements. Usually magnetic memory sizes are limited to 30-50 nm. Reducing the size even further, to the approximate to 10-20 nm scale, destabilizes the magnetization and its magnetic orientation becomes susceptible to thermal fluctuations and stray magnetic fields. In the present work, it is shown that 10 nm single domain ferromagnetism can be achieved. Using asymmetric adsorption of chiral molecules, superparamagnetic iron oxide nanoparticles become ferromagnetic with an average coercive field of approximate to 80 Oe. The asymmetric adsorption of molecules stabilizes the magnetization direction at room temperature and the orientation is found to depend on the handedness of the chiral molecules. These studies point to a novel method for the miniaturization of ferromagnets (down to approximate to 10 nm) using established synthetic protocols.

A novel Hall circuit design that can be incorporated into a working electrode, which is used to probe spin-selective charge transfer and charge displacement processes, is reviewed herein. The general design of a Hall circuit based on a semiconductor heterostructure, which forms a shallow 2D electron gas and is used as an electrode, is described. Three different types of spin-selective processes have been studied with this device in the past: i) photoinduced charge exchange between quantum dots and the working electrode through chiral molecules is associated with spin polarization that creates a local magnetization and generates a Hall voltage; ii) charge polarization of chiral molecules by an applied voltage is accompanied by a spin polarization that generates a Hall voltage; and iii) cyclic voltammetry (current-voltage) measurements of electrochemical redox reactions that can be spin-analyzed by the Hall circuit to provide a third dimension (spin) in addition to the well-known current and voltage dimensions. The three studies reviewed open new doors into understanding both the spin current and the charge current in electronic materials and electrochemical processes.

There is an increasing demand for the development of a simple Si-based universal memory device at the nanoscale that operates at high frequencies. Spin-electronics (spintronics) can, in principle, increase the efficiency of devices and allow them to operate at high frequencies. A primary challenge for reducing the dimensions of spintronic devices is the requirement for high spin currents. To overcome this problem, a new approach is presented that uses helical chiral molecules exhibiting spin-selective electron transport, which is called the chiral-induced spin selectivity (CISS) effect. Using the CISS effect, the active memory device is miniaturized for the first time from the micrometer scale to 30 nm in size, and this device presents memristor-like nonlinear logic operation at low voltages under ambient conditions and room temperature. A single nanoparticle, along with Au contacts and chiral molecules, is sufficient to function as a memory device. A single ferromagnetic nanoplatelet is used as a fixed hard magnet combined with Au contacts in which the gold contacts act as soft magnets due to the adsorbed chiral molecules.

It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Spin-polarized electrons are injected from an electrochemical cell through a chiral self-assembled organic monolayer into a AlGaN/GaN device in which a shallow two-dimensional electron gas (2DEG) layer is formed. The injection is monitored by a microwave signal that indicates a coherent spin lifetime that exceeds 10 ms at room temperature. The signal was found to be magnetic field independent; however, it depends on the current of the injected electrons, on the length of the chiral molecules, and on the existence of 2DEG.

Over the last decade, we have developed a molecular-controlled semiconductor resistor (MOCSER) device that is highly sensitive to variations in its surface potentials. This device was applied as a molecular sensor both in the gas phase and in solutions. The device is based on an AlGaAs/ GaAs structure. In the current work, we developed an electronic biosensor for real-time, label-free monitoring of cellular metabolic activity by culturing HeLa cells directly on top of the device's conductive channel. Several properties of GaAs make it attractive for developing biosensors, among others its high electron mobility and ability to control the device's properties by proper epitaxial growing. However, GaAs is very reactive and sensitive to oxidation in aqueous solutions, and its arsenic residues are highly toxic. Nevertheless, we have managed to overcome this inherent chemical instability by developing a surface-protecting layer using polymerized (3-mercaptopropyl)-trimethoxysilane (MPTMS). To improve cell adhesion and biocompatibility, the MPTMS-coated devices were further modified with an additional layer of (3-aminopropyl)-trimethoxysilane (APTMS). HeLa cells were found to grow successfully on these devices, and MOCSER devices cultured with these cells were stable and sensitive to cellular metabolic activity. The sensitivity of the MOCSER device results from the sensing of extracellular acidification in the microenvironment of the cell-MOCSER interspace. We have found that this sensitivity is maintained only when the device is partially covered with the cellular layer, whereas at full coverage the sensitivity is lost. This phenomenon is related to the negatively charged cellular membrane potentials that lead to a reduction in the channel's conductivity. We propose that the coated MOCSER device can be applied for real-time and continuous monitoring of cellular viability and activity.

Electron spin states play an important role in many chemical processes. Most spin-state studies require the application of a magnetic field. Recently it was found that the transport of electrons through chiral molecules also depends on their spin states and may also play a role in enantiorecognition. Electrochemistry is an important tool for studying spin-specific processes and enantioseparation of chiral molecules. A new device is presented, which serves as the working electrode in electrochemical cells and is capable of providing information on the correlation of spin selectivity and the electrochemical process. The device is based on the Hall effect and it eliminates the need to apply an external magnetic field. Spin-selective electron transfer through chiral molecules can be monitored and the relationship between the enantiorecognition process and the spin of electrons elucidated.

Efficient photo-electrochemical production of hydrogen from water is the aim of many studies in recent decades. Typically, one observes that the electric potential required to initiate the process significantly exceeds the thermodynamic limit. It was suggested that by controlling the spins of the electrons that are transferred from the solution to the anode, and ensuring that they are coaligned, the threshold voltage for the process can be decreased to that of the thermodynamic voltage. In the present study, by using anodes coated with chiral conductive polymer, the hydrogen production from water is enhanced, and the threshold voltage is reduced, as compared with anodes coated with achiral polymer. When CdSe quantum dots were embedded within the polymer, the current density was doubled. These new results point to a possible new direction for producing inexpensive, environmentally friendly, efficient water-splitting photo-electrochemical cells. (Graph Presented).

Noncovalent interactions between molecules are key for many biological processes. Necessarily, when molecules interact, the electronic charge in each of them is redistributed. Here, we show experimentally that, in chiral molecules, charge redistribution is accompanied by spin polarization. We describe how this spin polarization adds an enantioselective term to the forces, so that homochiral interaction energies differ from heterochiral ones. The spin polarization was measured by using a modified Hall effect device. An electric field that is applied along the molecules causes charge redistribution, and for chiral molecules, a Hall voltage is measured that indicates the spin polarization. Based on this observation, we conjecture that the spin polarization enforces symmetry constraints on the biorecognition process between two chiral molecules, and we describe how these constraints can lead to selectivity in the interaction between enantiomers based on their handedness. Model quantum chemistry calculations that rigorously enforce these constraints show that the interaction energy for methyl groups on homochiral molecules differs signif-icantly from that found for heterochiral molecules at van der Waals contact and shorter ( i. e., similar to 0.5 kcal/mol at 0.26 nm).

In clinical applications, inspection of the hemorrhagic spots in the gastrointestinal (GI) tract is a challenging task. The hybrid GaAs-based device, developed by us, has been successfully used for sensing hemoglobin (Hb) in biological solutions. In this paper, a new method and apparatus were applied for using the GaAs-based sensor to detect Hb in a mimicked gastrointestinal circumstance. A surface protection layer of polymerized thiolated silanes was deposited on the top of the device to obtain a chemical passivation coating against surface etching and to achieve bio-compatibility. A selective device functionalization was achieved by the subsequent adsorption of Hb antibodies on the top of the protection layer. An integrated sensor containing two different antibodies enabled examining its selectivity to purely Hb. In vitro testing of the sensor indicated that it is capable of discriminating fasted-state simulated intestinal fluids when the concentration of Hb was above 10 mu g/mL. Moreover, the sensor was capable of detecting Hb in swine intestinal fluids with the same sensitivity. This paper verified the feasibility to apply the sensor in clinical use, by embedding it on different types of endoscopes, in order to localize GI bleeding with high precision and sensitivity.

Usually, when one refers to chirality, one focuses on the structural properties of the molecules. This review focuses on the interesting electronic properties of these molecules, and specifically, on the interrelation between the spin of the electrons transmitted through molecules and the chiral structure of these molecules. This relation is expressed in the chiral-induced spin selectivity (CISS) effect, which is described, including some of its implications. Specifically, several applications are described, in spintronics, in spin-specific reactivity, and in spin-controlled multiple-electron oxidation processes. In the latter, experimental results are described that demonstrate that by coating the anode of a photoelectrochemical cell with chiral molecules, the overpotential of the water-splitting process can be reduced.

This work demonstrates that chiral imprinted CdSe quantum dots (QDs) can act as spin selective filters for charge transport. The spin filtering properties of chiral nanoparticles were investigated by magnetic conductive-probe atomic force microscopy (mCP-AFM) measurements and magnetoresistance measurements. The mCP-AFM measurements show that the chirality of the quantum dots and the magnetic orientation of the tip affect the current-voltage curves. Similarly, magnetoresistance measurements demonstrate that the electrical transport through films of chiral quantum dots correlates with the chiroptical properties of the QD. The spin filtering properties of chiral quantum dots may prove useful in future applications, for example, photovoltaics, spintronics, and other spin-driven devices.

Spin-dependent photoluminescence (PL) quenching of CdSe nanoparticles (NPs) has been explored in the hybrid system of CdSe NP purple membrane, wild-type bacteriorhodopsin (bR) thin film on a ferromagnetic (Ni-alloy) substrate. A significant change in the PL intensity from the CdSe NPs has been observed when spin-specific charge transfer occurs between the retinal and the magnetic substrate. This feature completely disappears in a bR apo membrane (wild-type bacteriorhodopsin in which the retinal protein covalent bond was cleaved), a bacteriorhodopsin mutant (D96N), and a bacteriorhodopsin bearing a locked retinal chromophore (isomerization of the crucial C13=C14 retinal double bond was prevented by inserting a ring spanning this bond). The extent of spin-dependent PL quenching of the CdSe NPs depends on the absorption of the retinal, embedded in wild-type bacteriorhodopsin. Our result suggests that spin-dependent charge transfer between the retinal and the substrate controls the PL intensity from the NPs.

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic-inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material.

This study presents results on the charge transfer between CdSe nanoparticles (NPs) and a gold substrate, when the NPs are attached to the gold via self-assembled monolayers of alkanedithiols (DT) of various lengths. The study examines the dependence of the photoinduced charge transfer on the DT length. Two methods were applied for measuring the charge transfer yield, surface photovoltage (SPV) and temperature dependent photoluminescence. The results demonstrate a net transfer of electrons from the NPs to the gold, under constant illumination. Interestingly, the data reveal that the monolayer composed of 10 carbon methylene chains displays an unusually efficient electron transfer, which is attributed to a high local ligand density resulting in multiple links between the NPs and the substrate.

The search to understand the origin of homochirality in nature has been ongoing since the time of Pasteur. Previous work has shown that DNA can act as a spin filter for low-energy electrons and that spin-polarized secondary electrons produced by X-ray irradiation of a magnetic substrate can induce chiral selective chemistry. In the present work it is demonstrated that secondary electrons from a substrate that are transmitted through a chiral overlayer cause enantiomeric selective chemistry in an adsorbed adlayer. We determine the quantum yields (QYs) for dissociation of (R)- or (S)-epichlorohydrin adsorbed on a chiral self-assembled layer of DNA on gold and on bare gold (for control). The results show that there is a significant difference in the QYs between the two enantiomers when adsorbed on DNA, but none when they are adsorbed on bare Au. We propose that the effect results from natural spin filtering effects cause by the chiral monolayer.

We design sensors where information is transferred between the sensing event and the actuator via quantum relaxation processes, through distances of a few nanometers. We thus explore the possibility of sensing using intrinsically quantum mechanical phenomena that are also at play in photobiology, bioenergetics, and information processing. Specifically, we analyze schemes for sensing based on charge transfer and polarization (electronic relaxation) processes. These devices can have surprising properties. Their sensitivity can increase with increasing separation between the sites of sensing (the receptor) and the actuator (often a solid-state substrate). This counterintuitive response and other quantum features give these devices favorable characteristics, such as enhanced sensitivity and selectivity. Using coherent phenomena at the core of molecular sensing presents technical challenges but also suggests appealing schemes for molecular sensing and information transfer in supramolecular structures.

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).

Chiral conducting polymer films are self‐assembled on ferromagnetic materials and exhibit an efficient spin filtering at room temperature. In addition, the present system displays asymmetric magnetoresistance with respect to zero magnetic field.

A biosensor for ammonia is developed aimed at detecting the presence of H. pylori bacteria in gastric fluids. The sensor is based on a GaAs device coated with a unique functional polymer that enables high device sensitivity to low concentrations of ammonia and long-term protection in harsh environments. The detection of ammonia in gastric fluids taken from patients is possible by covering the device with a dialysis membrane, thus enabling the diffusion of only small molecules to the sensing area, while preventing agglomeration of macromolecules on the surface of the device. The mechanism by which ammonia is detected is investigated and an analytical expression is provided relating the response of the detector to the ammonia concentration and the pH of the solution.

Photosystem I (PSI) is one of the most studied electron transfer (ET) systems in nature; it is found in plants, algae, and bacteria. The effect of the system structure and its electronic properties on the electron transfer rate and yield was investigated for years in details. In this work we show that not only those system properties affect the ET efficiency, but also the electrons' spin. Using a newly developed spintronic device and a technique which enables control over the orientation of the PSI monolayer relative to the device (silver) surface, it was possible to evaluate the degree and direction of the spin polarization in ET in PSI. We find high-spin selectivity throughout the entire ET path and establish that the spins of the electrons being transferred are aligned parallel to their momenta. The spin selectivity peaks at 300 K and vanishes at temperatures below about 150 K. A mechanism is suggested in which the chiral structure of the protein complex plays an important role in determining the high-spin selectivity and its temperature dependence. Our observation of high light induced spin dependent ET in PSI introduces the possibility that spin may play an important role in ET in biology.

We present an experimental and theoretical study of the kinetic energy dependence of spin filtering of electrons by organized layers of DNA adsorbed on a gold substrate. When Au 4f_7/2,5/2 levels are ionized by circularly polarized X-rays, the emitted electrons will be spin polarized. The spin distribution depends on the particular sublevel and is opposite for right versus left circularly polarized light. If the DNA overlayer preferentially attenuates one spin over another, then there should be a circular dichroism (CD) in the X-ray photoelectron spectroscopy (XPS) spectra observed with the different polarizations. Using synchrotron radiation excitation, XPS CD measurements were made of electrons with kinetic energies in the range 30 to 760 eV. In all cases there was no evidence of any significant dichroism. These results are explained by a model in which the longitudinal polarization is strongly dependent on the k-vector, and hence the energy or the de Broglie wavelength, which are simply connected to the magnitude of this vector of the incoming electrons. For a helix with a fixed number of turns, this dependence is due to a coherent process associated with multiple scattering. This model predicts that there is a window of energies where changes in the polarization should be expected. Two competing effects determine this window: The energy has to be small enough to allow for at least double scattering, but large enough so that the de Broglie wavelength probes the chiral structure. Also at very low energies the spin-orbit interaction weakens and no polarization results.

A coating scheme was developed for enabling the operation of a GaAs-based Molecular Controlled Semiconductor Resistor (MOCSER) under biological conditions. Usually GaAs is susceptible to etching in an aqueous environment. Several methods of protecting the semiconductor based devices were suggested previously. However, even when protected, it is very difficult to ensure the operation of a GaAs-based electronic sensor in aqua solution for long periods. We developed a new depositing scheme of (3-mercaptopropyl)-trimethoxysilane (MPTMS) on GaAs substrate consisting of two separate steps. The first involves chemisorption of a dense primary MPTMS layer on the substrate, whereas in the second, a thin MPTMS polymer layer is deposited on the already adsorbed layer, resulting in a 15 - 29 nm thick coating. We show that applying the new MPTMS deposition procedure to GaAs-based MOCSER devices allows up to 15 hours of continuous electrical measurements and stable performance of the sensing device in harsh biological environment. The new protection allows implementing GaAs technology in bioelectronics, particularly in biosensing.

Microelectronic-based sensors are ideal for real-time continuous monitoring of health states due to their low cost of production, small size, portability, and ease of integration into electronic systems. However, typically semiconductor-based devices cannot be operated in aqueous solutions, especially in solutions with a low pH. However, in this work we overcame this difficulty and demonstrated the feasibility of a hemoglobin sensing array based on hybrid organic GaAs-based devices, which can remain in biological solutions for more than 24. h. This was achieved by coating devices with a nanometer-thick polymer protective layer with subsequent adsorption of antibodies on its surface. The device is capable of functioning even in harsh physiological fluids, such as urine and bile juice. The surface modification allows a change in electrical potential, created by the interaction, to be efficiently transferred to the surface of the semiconductor device. By utilizing an array configuration, it is possible to obtain high sensitivity and selectivity.

Leading researchers in molecular electronics discuss the motivation behind their work and what they consider to be the grand challenges for the field.

This work explores the electronic states of CdTe semiconductor nanoparticles (NPs) that are immobilized on a polycrystalline Au film through an organic linker (dithiol). The HOMO and LUMO energies of the CdTe NPs were determined by using photoelectron spectroscopy and cyclic voltammetry. The results from these measurements show that the HOMO energy is independent of the nanoparticle size and is pinned to the Fermi level, whereas the LUMO energy changes systematically with the size of the NP. Studies with different capping ligands imply that the dithiol ligand removes surface states and enhances the optoelectronic properties of the NPs.

Highly spin-selective transport of electrons through a helically shaped electrostatic potential is demonstrated in the frame of a minimal model approach. The effect is significant even for weak spin-orbit coupling. Two main factors determine the selectivity: an unconventional Rashba-like spin-orbit interaction, reflecting the helical symmetry of the system, and a weakly dispersive electronic band of the helical system. The weak electronic coupling, associated with the small dispersion, leads to a low mobility of the charges in the system and allows even weak spin-orbit interactions to be effective. The results are expected to be generic for chiral molecular systems displaying low spin-orbit coupling and low conductivity.

We report on a new ultrasensitive and fast technique for the detection and identification of both DNA and RNA with sensitivity of a few molecules. The new method is based on a patterned capillary tube (PCT) in which the internal surface of a glass tube is patterned with rings of different single-stranded DNA probes. A solution containing single-stranded analyte flows through the tube. Upon hybridization of appropriate DNA and RNA from the solution, DNA polymerase and reverse transcriptase (RT) are employed to synthesize the complementary nucleic acids with deoxynucleoside triphosphate (dNTP) labeled with fluorophores. The sample-analyte hybrids are detected by their fluorescence signal. We show that the new method is sensitive, is specific, can detect simultaneously both DNA and RNA from the same sample, and allows detection of analytes in serum.

In this perspective we present several examples of the ability to control electronic and magnetic properties of nano-devices by adsorbing on their surfaces organized self-assembled monolayers (SAM) of organic molecules. The work presented focuses on research in which we were involved and is aimed at demonstrating the ability to control physical properties of metal and semiconductor films by complementing them with the properties of a SAM. The organization of molecules on a surface produces a pseudo two-dimensional dipole layer, owing to the dipolar property of each of the molecules. The field confined in the layer could be enormous, however the molecules are either depolarized or charge is transferred between the substrate and the layer so as to reduce the energy of the dipole layer. This charge transfer process can be exploited for the use of hybrid-organic-inorganic devices as sensors, as wavelength specific light detectors, or for varying the critical temperature in semiconductor ferromagnets. The concept presented here, for combining electronic properties of organic molecules with those of the inorganic substrate, is another venue toward "molecular controlled electronics".

We investigated how isolated are the electronic states of the core in a core-shell (c/s) nanoparticles (NPs) from the surface, when the particles are self-assembled on Au substrates via a dithiol (DT) organic linker. Applying photoemission spectroscopy the electronic states of CdSe core only and CdSe/ZnS c/s NPs were compared. The results indicate that in the c/s NPs the HOMO interacts strongly with electronic states in the Au substrate and is pinned at the same energies, relative to the Fermi level, as the core only NPs. When the capping molecules of the NPs were replaced with thiolated molecules, an interaction between the thiol groups and the electronic states of the NPs was observed that depends on the properties of the NPs studied. Thiols binding to the NPs induce the formation of surface trap states. However, while for the core only CdSe NPs the LUMO states are strongly coupled to the surface traps, independent of their size, this coupling is size dependent in the case of the CdSe/ZnS c/s NPs. For a large core, the LUMO is decoupled from the surface trap states. When the core is small enough, the LUMO is delocalized and interacts with these states.

Self-assembled monolayers (SAMs) of organic dipolar molecules have new electronic and magnetic properties that result from their organization, despite the relatively weak interaction among the molecules themselves. Here we review the origin of this cooperative effect and summarize results obtained on spin selective electron transmission through such monolayers that are made from chiral molecules. We show that SAMs containing chiral dipolar molecules behave like magnetic layers which may serve as spin filters, even without applying an external magnetic field to the layer.

Toward the construction of double stranded DNA-based biosensors, packing of thiolated double-stranded DNA adsorbed on gold nanoparticles was observed to induce DNA denaturation. The denaturation was investigated as a function of DNA density, nanoparticle surface area, and DNA length. Direct correlation was found between DNA surface coverage and the denaturation. Denaturation occurred only at high densities of adsorbed DNA and was dependent on DNA length and therefore stability, providing guidelines for controlled adsorption of dsDNA on GNPs. Our results invoke a model in which the formation of a thiol-gold bond competes with the free energy associated with the denaturation of two DNA strands. Denaturation vacates space for additional molecules to bind through a thiol-gold bond.

Self-assembled monolayers (SAMs) of organic dipolar molecules have new electronic and magnetic properties that result from their organization, despite the relatively weak interaction among the molecules themselves. Here we review the origin of this cooperative effect and summarize work performed on spin selective electron transmission through SAMs. The spin selectivity observed, in some cases, is consistent with a model in which a SAM containing chiral dipolar molecules behaves like a magnetic layer. The magnetic properties result in the SAMs behaving as spin filters, even without applying an external magnetic field to the layer.

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.

We examine the current response of molecularly controlled semiconductor devices to the presence of weakly interacting analytes. We evaluate the response of two types of devices, a silicon oxide coated silicon device and a GaAs/AlGaAs device, both coated with aliphatic chains and exposed to the same set of analytes. By comparing the device electrical response with contact potential difference and surface photovoltage measurements, we show that there are two mechanisms that may affect the underlying substrate, namely, formation of layers with a net dipolar moment and molecular interaction with surface states. We find that whereas the Si device response is mostly correlated to the analyte dipole, the GaAs device response is mostly correlated to interactions with surface states. Existence of a silicon oxide layer, whether native on the Si or deliberately grown on the GaAs, eliminates analyte interaction with the surface states.

Assemblies of CdSe nanoparticles (NPs) on a dithiol-coated Au electrode were created, and their electronic energetics were quantified. This report describes the energy level alignment of the filled and unfilled electronic states of CdSe nanoparticles with respect to the Au Fermi level. Using cyclic voltammetry, it was possible to measure the energy of the filled states of the CdSe NPs with respect to the Au substrate relative to a Ag/AgNO₃ electrode, and by using photoemission spectroscopy, it was possible to independently measure both the filled state energies (via single photon photoemission) and those of the unfilled states (via two photon photoemission) with respect to the vacuum level. Comparison of these two different measures shows good agreement with the IUPAC-accepted value of the absolute electrode potential. In contrast to the common model of energy level alignment, the experimental findings show that the CdSe filled states become 'pinned' to the Fermi level of the Au electrode, even for moderately small NP sizes.

An array of hybrid organic-semi conductor sensors was developed aimed at selectively detecting triace-tone triperoxide (TATP). Each element of the array is a field effect-like transistor based on GaAs. The gate in these devices has been replaced by a self-assembled monolayer of receptor molecules. The current layout features selectivity to TATP and is able to detect less than 100 ppb. A correlation among the responses, the chain length of the grafted molecules, and the surface potential is exhibited, suggesting that the sensing mechanism involves charge transfer between the adsorbed molecules and the substrate. (C) 2009 Elsevier B.V. All rights reserved.

This letter demonstrates the ability to pattern surfaces chemically with submicrometer resolution by applying the simple and inexpensive magnetolithography (NIL) method. This method allows fast patterning of large surfaces without having to face contamination problems or the need to remove the substrate from the solution. With M L it is possible to obtain pattern whose width is narrower than the width of the lines in the mask. By applying the green fluorescent protein (GFP), we were able to probe a 30 rim line of hydrophobic molecules patterned oil a substrate coated with a hydrophilic monolayer.

Our understanding of processes involved in two-photon photoemission (2PPE) from surfaces can be tested when we try to exercise control over the electron emission. In the past, coherently controlled 2PPE has been demonstrated using very short pulses and single crystal surfaces. Here we show that by applying polarization pulse shaping on surfaces, it is possible to vary both the angular distribution of the emitted photoelectrons and the total photoemission yield. The presented 2PPE experimental setup introduces pulse shaping in the visible range, which is a unique property that allows control of polarization. We relate the ability to use polarization as a means of control to the surface corrugation.

8-Oxo-7,8-dihydroguanosine (8-oxoG) is among the most common forms of oxidative DNA damage found in human cells. The question of damage recognition by the repair machinery is a long standing one, and it is intriguing to suggest that the mechanism of efficiently locating damage within the entire genome might be related to modulations in the electronic properties of lesions compared to regular bases. Using laser-based methods combined with organizing various oligomers self-assembled monolayers on gold substrates, we show that indeed 8-oxoG has special electronic properties. By using oligomers containing 8-oxoG and guanine bases which were inserted in an all thymine sequences, we were able to determine the energy of the HOMO and LUMO states and the relative density of electronic states below the vacuum level. Specifically, it was found that when 8-oxoG is placed in the oligomer, the HOMO state is at higher energy than in the other oligomers studied. In contrast, the weakly mutagenic 8-oxo-7,8- dihydroadenosine (8-oxoA) has little or no effect on the electronic properties of DNA.

The adsorption of DNA on surfaces is a widespread procedure and is a common way for fabrication of biosensors, DNA chips, and nanoelectronic devices. Although the biologically relevant and prevailing in vivo structure of DNA is its double-stranded (dsDNA) conformation, the characterization of DNA on surfaces has mainly focused on singlestranded DNA (ssDNA). Studying the structure of dsDNA on surfaces is of invaluable importance to microarray performance since their effectiveness relies on the ability of two DNA molecules to hybridize and remain stable. In addition, many of the enzymatic transactions performed on DNA require dsDNA, rather than ssDNA, as a substrate. However, it is not established that adsorbed dsDN A remains in its structure and does not denature. Here, two methodologies have been developed for distinguishing between surface-adsorbed single- and double-stranded DNA. We demonstrate that, upon formation of a dense monolayer, the nonthiolated strand comprising the dsDNA is released and the monolayer consists of mostly ssDNA. The fraction of dsDNA within the ssDNA monolayer depends on the length of the oligomers. A likely mechanism leading to this rearrangement is discussed.

The predetermined patterned adsorption of two types of nanoparticles on the same substrate may be of considerable importance in various applications, among others, to enhance the absorption of semiconductor nanoparticles by the plasmonic effect of metal NPs. We describe here a simple method for self-assembling 2D lateral patterns in which both gold and semiconductor nanoparticles are adsorbed, each in a predesigned area. Our method is based on a one-step lithographic process and the adsorption of two distinct self-assembled monolayers that can selectively bind only one type of nanoparticle.

A realistic picture of a cell is that of a highly viscous, condensed gel-like substance, crowded with macromolecules that are mostly anchored to membranes and to intricate networks of cytoskeletal elements. Theoretical and experimental approaches to investigating crowding have not considered the role of diffusion through a crowded medium in affecting the selectivity and specificity of reactions. Such diffusion is especially important when one considers interfaces, where at least one reactant must move through the medium and reach the interface. Here, we address this issue by directly investigating how diffusion through a gel medium affects the competition between a single specific reaction and a large number of weak nonspecific interactions, a process that is typical of reactions occurring at interfaces. We present an approach for achieving orientation-controlled interactions based on the configuration-dependent diffusion rate of the reacting molecule through a gel medium. The effectiveness of the method is demonstrated by the high selectivity obtained both in the adsorption of DNA to a surface and in DNA hybridization to preadsorbed single-strand oligomer on a surface.

The ability to place DNA on surfaces with increased and controllable reactivity is of fundamental importance in the development of next-generation DNA and protein biochips. The present work demonstrates the ability to control both the localization of the DNA on a surface and its reactivity by a self-assembly approach that is dependent on two variables: DNA structure and surface environment. Here we utilize a two-step adsorption scheme to control the adsorption and reactivity of DNA embedded within two types of alkyl thiol monolayers (either methyl-terminated or hydroxyl-terminated). In addition, by changing the structure of the chemisorbed DNA from fully single stranded to a 50% double stranded at its side adjacent to the surface, we were able to observe a clear dependence of DNA reactivity on both the DNA structure and the type of alkyl thiol monolayer covering the surface. The adsorption and the reactivity yield of the DNA were monitored either by its ability to hybridize to a complementary target molecule or by an enzymatic reaction involving DNA phosphorylation catalyzed by the enzyme T4 polynucleotide kinase.

We present and experimentally verify a concept for electronic devices based on nanoscale vacuum phototubes. Such devices are expected to be both reliable and amenable to large-scale integration. We further suggest several generalizations of the concept and discuss possible applications and advantages.

The induction of chirality in metallic gold and silver has been demonstrated by the doping of these metals with a chiral organic molecules. Consistent chiral asymmetry was observed in the photoemission of electrons from gold and silver embedded with chiral biomolecules including L-glutathione, L-quinine, and tryptophan. Counter-clockwise circularly polarized light (ccw-CPL) and clockwise CPL (cw-CPL) induce different photoelectron emission yields from the doped metal, thus exhibit different diastereomeric interactions between light and the metal electrons. All possible diastereomeric interactions that were observed between ccw-CPL and cw-CPL and dopants supported that the dopant-nduced chirality was real and significant.

In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolayers. Using this method, we measure the relative work-function of alkanethiols of different length on gold substrate, and we are able to follow the changes occurring when the surface coverage is varied. We also computed the work-function of model alkanethiols using a plane-wave density functional theory approach, in order to demonstrate the correlation between changes in the work-function with the monolayer organization and density.

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 (

The electrical conduction through three short oligomers (26 base pairs, 8 nm long) with differing numbers of GC base pairs was measured. One strand is poly(A)-poly(T), which is entirely devoid of GC base pairs. Of the two additional strands, one contains 8 and the other 14 GC base pairs. The oligomers were adsorbed on a gold substrate on one side and to a gold nanoparticle on the other side. Conducting atomic force microscope was used for obtaining the current versus voltage curves. We found that in all cases the DNA behaves as a wide band-gap semiconductor, with width depending on the number of GC base pairs. As this number increases, the band-gap narrows. For applied voltages exceeding the band-gap, the current density rises dramatically. The rise becomes sharper with increasing number of GC base pairs, reaching more than 1 nA/nm² for the oligomer containing 14 GC pairs.

The aim of this work is the development of a NO sensor for asthma control and medication monitoring. The transducer is a Molecular Controlled Semiconductor Resistor (MOCSER), which is a GaAs based heterostructure. Protoporphyrins IX, containing carboxylic groups to chemisorb on GaAs, were used as sensing molecules. Characterization of the protoporphyrin monolayers was held using Attenuated Total Reflection in Multiple Internal Reflection (ATR/MIR), High Resolution Electron Energy Loss Spectroscopy (HREELS) in the vibrational and electronic domain and X-ray Photoelectron Spectroscopy (XPS). Degreasing and etching of the GaAs substrates were accomplished before adsorption. Interfacial bonding investigated by ATR/MIR shows that protoporphyrin adsorbs to the GaAs (100) through a unidentate complex and remains mostly vertically oriented. The electronic domain of the HREELS spectra exhibits the Q band with α and β components on the same position as in the UV/Vis spectrum. Soret band is blue shifted showing a face to face stacking of the protoporphyrin molecules on the GaAs substrates. XPS spectra reveal the presence of Cobalt in monolayers prepared with 8 × 10 ^{- 5} M CoPP solutions. Kinetics is best fitted by an Elovich equation, showing some hindrance due to the previous adsorbed molecules. Thickness found from XPS data ranges from 1.3 to 1.5 nm, which fits with the molecular dimensions. Using the GaAs preparation methods developed here, an NO sensor prototype was assembled and tested for NO sensitivity and repeatability. Relative to NO, tests reveal a good sensitivity between 1.6 and 200 ppb. NO sensitivity was also measured towards CO, CO₂ and O₂. Pure nitrogen sweeps NO from the porphyrin layer, opening the possibility of the sensor reutilization.

We recently reported electrical transport measurements through double-stranded (ds)DNA molecules that are embedded in a self-assembled monolayer of single-stranded (ss)DNA and attached to a metal substrate and to a gold nanoparticle (GNP) on opposite ends. The measured current flowing through the dsDNA amounts to 220 nA at 2 V. In the present report we compare electrical transport through an ssDNA monolayer and dsDNA monolayers with and without upper thiol end-groups. The measurements are done with a conductive atomic force microscope (AFM) using various techniques. We find that the ssDNA monolayer is unable to transport current. The dsDNA monolayer without thiols in the upper end can transport low current on rare occasions and the dsDNA monolayer with thiols on both ends can transport significant current but with a much lower reliability and reproducibility than the GNP-connected dsDNA. These results reconfirm the ability of dsDNA to transport electrical current under the appropriate conditions, demonstrate the efficiency of an ssDNA monolayer as an insulating layer, and emphasize the crucial role of an efficient charge injection through covalent bonding for electrical transport in single dsDNA molecules.

Professor Naaman replied: The role of the DNA is to bind the nanotube to the substrate in a controlled way. However, as we show we can use the DNA also as an electrical element, when charge is tunnelled between the electrodes and the nanotubes through the short DNA oligomers. By thermal heating we can remove the DNA and thereby form a direct contact between the electrode and the nanotube....

A straightforward method for the self assembly of single walled carbon nanotubes (SWNTs) between gold electrodes was developed. The technique utilizes the hybridization between short complementary DNA sequences located on metal contacts and SWNTs. The new technique enables simple production of hundreds of devices with high yields. The electrical characteristics are shown to depend strongly on the existence of the chemical binding groups at the contacts as well as along the tubes. This technique was used to drive the self assembly of SWNT-based field effect transistors (CNTFETs). In principle, the devices made by this method behave like those made using direct metal-carbon nanotube contacts. The inverse subthreshold slope of the CNTFETs depends on the source-drain voltage applied to the device, confirming that the conductance of CNTFETs is determined by the Schottky barriers at the interfaces between the CNTs and the gold electrodes.

Here we show that self-assembled monolayers on gold of double-stranded DNA oligomers interact with polarized electrons similarly to a strong and oriented magnetic field. The direction of the field for right-handed DNA is away from the substrate. Moreover, the layer shows very high paramagnetic susceptibility. Interestingly, thiolated single-stranded DNA oligomers on gold do not show this effect. The new findings are rationalized based on recent results in which high paramagnetism was measured for diamagnetic films adsorbed on diamagnetic substrates.

Using a dissymmetrically-perturbed particle-in-a-box model, we demonstrate that the induced optical activity of chiral monolayer protected clusters, such as Whetten's Au₂₈(SG)₁₆ glutathione-passivated gold nanoclusters (J. Phys. Chem. B, 2000, 104, 2630-2641), could arise from symmetric metal cores perturbed by a dissymmetric or chiral field originating from the adsorbates. This finding implies that the electronic states of the nanocluster core are chiral, yet the lattice geometries of these cores need not be geometrically distorted by the chiral adsorbates. Based on simple chiral monolayer protected cluster models, we rationalize how the adsorption pattern of the tethering sulfur atoms has a substantial effect on the induced CD in the NIR spectral region, and we show how the chiral image charge produced in the core provides a convenient means of visualizing dissymmetric perturbations to the achiral gold nanocluster core.

The study of the effect of self-assembled organic monolayers on the critical current in thin superconducting Nb films is presented. Correlation is found between the coverage of the adsorbed layer and the critical current. A large change of up to 50% in the critical current by the well-organized monolayers is observed. The phenomenon is explained by the adsorption-induced electric field effect diminishing the surface pinning force.

The adsorption of phenylphosphonic acid (PPA) on GaAs (100) surfaces from solutions in acetonitrue/ water mixtures was studied using Fourier transform infrared spectroscopy in attenuated total reflection in multiple internal reflections (ATR/MIR), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and atomic force microscopy (AFM). ATR/MIR in situ showed that the accumulation of PPA molecules near the GaAs surface increased with the water concentration in the solution. For water contents lower than 4%, ATR/MIR and XPS results are consistent with the formation of a low-density monolayer. A mechanism is proposed for H₂O percentages lower than 4% involving the creation of interfacial bonds through a Brønsted acid-base reaction, which involves the surface hydroxyl groups most probably bound to Ga. It was found that the morphology of the final layer depended strongly on the water concentration in the adsorbing solution. For water concentrations equal to or higher than 5%, the amount of adsorbed molecules drastically increased and was accompanied by modifications in the infrared spectral region corresponding to P-O and P=O. This sudden change indicates a deprotonation of the acid. XPS studies revealed the presence of extra oxygen atoms as well as gallium species in the layer, leading to the conclusion that phosphonate and hydrogenophosphonate ions are present in the PPA layer intercalated with H₃O⁺ and Ga³⁺ ions. This mechanism enables the formation of layers ∼10 times thicker than those obtained with lower H₂Q percentages. HREELS indicated that the surface is composed of regions covered by PPA layers and uncovered regions, but the uncovered regions disappeared for water contents equal to or higher than 5%. XPS results are interpreted using a model consisting of a monolayer partially covering the surface and a thick layer. This model is consistent with AFM images revealing roughness on the order of 7 nm for the thick layer and 0.2-0.5 nm for the thin layer. Sonication proves to be an effective method for reducing layer thickness.

To enable the use of GaAs-based devices as chemical sensors, their surfaces must be chemically modified. Reproducible adsorption of molecules in the liquid phase on the GaAs surfaces requires controlled etching procedures. Several analytical methods were applied, including Fourier transform infrared spectroscopy (FTIRS) in attenuated total reflection and multiple internal reflection mode (ATR/MIR), high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for the analysis of GaAs (100) samples treated with different wet-etching procedures. The assignment of the different features due to surface oxides present in the vibrational and XPS spectra was made by comparison with those of powdered oxides (Ga₂O₃, As₂O₃ and As₂O₅). The etching procedures here described, namely, those using low concentration HF solutions, substantially decrease the amount of arsenic oxides and aliphatic contaminants present in the GaAs (100) surfaces and completely remove gallium oxides. The mean thickness of the surface oxide layer drops from 1.6 nm in the raw sample to 0.1 nm after etching. However, in presence of light, water dissolution of arsenic oxides is enhanced, and oxidized species of gallium cover the surface.

Characterizing the structure and dynamic properties of a single monolayer is a challenge due to the minute amount of material that is probed. Here, EPR spectroscopy is used for investigating the spatial and temporal organization of self-assembled monolayers of 5- and 16-doxyl stearic acid (5 DSA and 16 DSA, respectively) adsorbed on a GaAs substrate. The results are complemented with FTIR and ellipsometery measurements, which provide the evidence for the formation of monolayers. Moreover, a comparison with the FTIR spectrum of a monolayer of stearic acid shows that the monolayers of the spin labeled molecules are less packed due to the hindrance introduced by the labeling group. The EPR spectra provide a new insight on the ordering in the layer and more interestingly, it reveals the time dependence of the organization. For 5DSA, with the spin-label group situated close to the substrate, the EPR spectrum immediately after adsorption is poorly resolved and dominated by the spin-exchange interaction between neighboring molecules. As time increases (up to 1 week) the resolution of the ¹⁴N hyperfine coupling increases, revealing a better organized monolayer where the molecules are more homogenously spaced. Moreover, the spectrum of the layer, after reaching equilibrium, shows that there is no motional freedom near the GaAs surface. Orientation dependence measurements on the equilibrated sample show the presence of a preferred orientation of the molecules, although with a wide distribution. The spectrum of the 16DSA monolayer, where the nitroxide spin label is situated at the end of the chain, far from the surface, also showed a poorly resolved spectrum at short times, but unlike 5DSA, it did not exhibit any time dependence. Through EPR line-shape simulations and by comparison with FTIR results, the differences between 5DSA and 16DSA were attributed to difference in coverage caused by the bulky spin label near the surface in the case of 5DSA.

Complementary, single-strands of DNA (ssDNA), one bound to a gold electrode and the other to a gold nanoparticle were hybridized on the surface to form a self-assembled, dsDNA bridge between the two gold contacts. The adsorption of a ssDNA monolayer at each gold interface eliminates non-specific interactions of the dsDNA with the surface, allowing bridge formation only upon hybridization. The technique used, in addition to providing a good electrical contact, offers topographical contrast between the gold nanoparticles and the non-hybridized surface and enables accurate location of the bridge for the electrical measurements. Reproducible AFM conductivity measurements have been performed and significant qualitative differences were detected between conductivity in single- and double-strand DNA. The ssDNA was found to be insulating over a 4 eV range between ±2 V under the studied conditions, while the dsDNA, bound to the gold nanoparticle, behaves like a wide band gap semiconductor and passes significant current outside of a 3 eV gap.

Cooperative effects generate new electronic and magnetic properties in closed packed organized organic layers. In layers made from chiral molecules, unexpectedly large electronic dichroism is observed, which manifests itself as spin specific electron transmission. For many thiolated molecules self-assembled on gold, a surprisingly large ferromagnetism is observed. All the observations can be rationalized by assuming orbital ferromagnetism of the organic thin layer. This is a new type of magnetism that is caused by the formation of closed packed layers of organic molecules on metal. In particular, charge transfer occurs between the substrate and the adsorbed layer. This charge is responsible for the appearance of magnetism.

Low energy electron photoemission spectroscopy (LEPS) allows the study of the electronic properties of organized organic thin films (OOTF) adsorbed on conducting surfaces by monitoring the energy and angular distribution of electrons emitted from the substrate and transmitted through the film. This technique provides unique information on the electronic properties of the adsorbed layer. The electron transmission properties are explained by electronic band structure in the organic film. This band is an example of an electron resonance that is delocalized in the layer. It results from the two dimensional nature of the layer. Other resonances in the transmission spectra are also discussed, as well as their experimental manifestation. Despite the fact that the molecules in the OOTF are weakly interacting, when not charged, the electron transmission through the film is governed by cooperative effects. These effects must be taken into account when considering electronic properties of adsorbed layers.

Chemisorption of self-assembled monolayers of organic molecules was found to substantially change the magnetic properties of ferromagnetic semiconductors. For ordered adsorbates, T_C falls by at least an order of magnitude relative to that of the untreated heterostructure. This suppression of T_C was attributed to redistribution of charge between the semiconductor and organic-inorganic interface states introduced by binding of the moelcules to the GaAs surface.

Hybrid organic/inorganic devices may find applications as sensors and in futuristic molecular-electronic devices. Here, we demonstrate molecular control of vertical transport in semiconductor superlattices in strong magnetic fields by adsorption of organic molecules onto the sidewalls of a GaAs/AlGaAs device. The molecules have identical attachment groups functionalized by end groups with different electronegativities. For magnetic fields in quantized Hall states, we find that the adsorbate substantially modifies the network of edge states that carries the electrical current. The data indicate that molecules with appropriately chosen end groups can enhance or decrease the vertical conductivity of the edge state system.

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.

Low-energy electron photoemission spectroscopy (LEPS) allows the study of the electronic properties of organized organic thin films (OOTF) adsorbed on conducting surfaces by monitoring the energy and angular distribution of electrons emitted from the substrate and transmitted through the film. The transmission properties are explained by electronic band structure in the organic film. This band is an example of an electron resonance that is delocalized in the layer. It results from the two-dimensional nature of the layer. Other resonances in the transmission spectra are also discussed, as well as their experimental manifestation. The temperature dependence of the electron transmission efficiency is explained in terms of dependence of the transmission probability on the initial momentum of the electron and on the relative orientation of the electron velocity and the molecules in the film. Hence, despite the fact that the molecules in the OOTF are weakly interacting, when not charged, the electron transmission through the film is governed by cooperative effects. These effects must be taken into account when considering electronic properties of adsorbed layers.

Single-walled carbon nanotubes (SWNT) were covalently modified with DNA via carbodlimide-assisted amidation, yielding a highly watersoluble adduct. The specific and nonspecific interactions between DNA and SWNTs were examined by UV-vis spectroscopy and confocal fluorescence microscopy. Fluorescence imaging of individual bundles shows that the SWNT-DNA adducts hybridize selectively with complementary strands with minimal nonspecific interactions with noncomplementary sequences.

We investigated the electronic properties of hybrid inorganic/organic thin films where the inorganic components are monodisperse CdS quantum size particles (diameters 2.5 and 5 nm). For this purpose, low-energy photoelectron spectroscopy was applied in which the energy of photoelectrons, ejected from the metal substrate or from the films themselves, is measured. The electronic properties of hybrid Langmuir-Blodgett films were studied, when the films were deposited on gold or silicon surfaces and contained particles of different sizes arranged in layers at different distances from the substrate. For comparison films containing only the organic matrix were examined, so the effect of the quantum particles (QPs) on the electronic properties of the whole film could be resolved. The present study proves the importance of the organization of hybrid structure on its electronic properties. It is clearly demonstrated that the QPs cannot be considered as insolated species and that their electronic properties are affected by the structure of the film as a whole and by the substrate. It is shown that the QP size affects both the photoelectron emission yield and the scattering efficiency of electrons from the QPs.

α-Helical L- and D-polyalanines have magnetic properties when self -assembled as monolayers on gold surfaces. The properties depend on the chirality of the molecules and on the direction of their dipole moment relative to the substrate. They were investigated by IR spectroscopy, which determined the orientation of the molecules relative to the surface in the presence of a magnetic field, and by measuring the spin selectivity for electron transmission through the layers.

The dependence of the electronic structure of an organic monolayer on its composition was investigated using two-photon photoelectron spectroscopy. It was found that the binding energy of an extra electron to the monolayer depends on its dielectric constant. The dependence could, in large, be fitted assuming the continuum dielectric model, however the electron affinity of the organic molecules required to fit the data is too large. Several explanations for this result are presented.

Calculations are presented on the electrostatic potential modification arising from molecular adsorbates on a molecular monolayer. Analysis at the ab initio level indicates that the overall dipole moment of the bimolecular monplayer-adsorbate complex formed on physisorption is dominated by the dipole moments of the individual molecules, with a small correction due to charge transfer. The induced dipole moment is greatest with more polarizable molecular monolayer constituents such as p-nitroaniline. Additionally, numerical simulation shows that the average electric field across the adsorbing surface increases linearly with the number of adsorbed analyte molecules and exhibits an inverse dependence on the thickness of the monolayer. This thickness dependence indicates a long-range effect of the dipole layer resulting from the two-dimensional nature of the system.

The temperature dependence of electron transmission through various organized organic thin films (OOTF) was investigated. The low energy photoelectron spectroscopy (LEPS) method was used to monitor the energy distribution of photoelectrons that were emitted from a metal substrate covered with OOTF. The structure of the OOTFs and the monitoring of the angular dependence of both the initial and the final velocities of photoelectrons were used to reveal the mechanism behind the temperature effect and the electron transmission mechanism.

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.

The tunneling motion in (HCl)₂ hydrogen bonded dimer and its deuterate was probed by a 2 m long electrostatic hexapole field. The focusing curves of the dimers confirmed the existence of homo and heterodimers in the cluster beam. The homodimer, either H³⁵Cl-H³⁵Cl or H³⁷Cl-H³⁷Cl, undergoes a fast tunneling motion for the two hydrogen atoms in the dimer. The heterodimer, namely H³⁵Cl-H³⁷Cl, on the other hand, does not show such fast tunneling motion in the time scale of the experiment. The electric dipole moments for both (DCl)₂ isotopomers were determined to be 1.5 ± 0.2 D, which is the same value for (HCl)₂. The observed ratio of homo to heterodimer was estimated to be 30 ± 10, and this value differs largely from the natural abundance for the chlorine isotope. An experimental scheme to discern homo and heterodimers is proposed here. By looking at fragments in the (HCl)₂ dimer photodissociation using a Doppler-selected time-of-flight (TOF) technique, internal energy distribution of the [ClHCl] fragment was measured in 121.6 nm photodissociation. The TOF spectrum consists of fast and slow velocity components for the dissociated H atoms. It is found that the slow H component that arises from the hydrogen escapes after many collisions. The fast H component that arises from the direct H escape without any collision, thus this component reflects an internal and/or electronic state of the counter part fragment, i.e. [ClHCl]. The vibrational structure of [ClHCl] was observed for the fast H component of the TOF spectrum. (C) 2000 Elsevier Science B.V.

The angular distribution of laser-induced low-energy photoelectron emission from both gold and gold coated with organized organic thin films (OOTF) is investigated using electron imaging and multiple channel detection schemes. Based on these studies, the effect of the electrons' k̄ vector on their transmission through various organized organic thin films is investigated. The transmission is found to be by far more efficient along the direction of the organic chains than in any other direction. A channeling effect is observed for electrons ejected into the organic films. These results explain the extremely high sensitivity of the transmission efficiency through organized organic films on their structural order.

A new generic transducer has been developed, based on a Molecular Controlled Semiconductor Resistor (MOCSER). It is based on a GaAs/(AI, Ga)As structure, to the surface of which the specially designed bifunctional organic molecules are covalently bound The electrical current through the device is very sensitive to the surface polential. Therefore, it changes when metal ions bind to the receptor site of the organic molecule. The new sensor has high sensitivity over a wide dynamic range, high selectivity, short measurement time and it is inexpensive to produce.

The reaction of O(¹D) with water and water clusters was re-examined. We monitored the nascent product state distributions in the reaction photo-initiated by the dissociation of N₂O at 193 and 212.8 nm, and the corresponding photo-initiated intracluster reaction. The study at two different dissociation wavelengths and the use of D₂O allowed us to obtain direct information on the effect of initial collision kinetic energy on the energy distribution in the product. Based on the new results obtained we conclude that the reaction of Q(¹D) with water occurs through abstraction mechanism with a relatively short lived collision complex. In the case of the intracluster reaction, we have indication that more internal energy is deposited in the N₂ moiety, compared to the dissociation of an isolated N₂O. In addition the results indicate that the reaction between the oxygen atom and the water in the complex involved the formation of a short lived collision complex, with a lifetime of probably only few rotations of OH.

A simple low-vacuum mass spectrometer (LVMS) operating in the milliTorr pressure range was developed. The instrument resolves masses by time-of- flight measurements and employs a high-gain, fast-response detector that can operate at these pressures. This instrument allows simultaneous determination of mass and collision cross sections of the ions with the bath gas. Here we demonstrate the LVMS's abilities to determine total collision cross sections for the collisions of organic ions with three background gases, He, N₂, and SF₆. As a demonstration of the system capabilities, the unimolecular interconversion of photochemically produced C₇H₇⁺ to the tropylium ion structure is investigated.

The focusing of HCl and DCl dimers was observed using a 2-m-long electrostatic hexapole field. The results indicate the existence of two types of species. The first is the homodimers, either the H³⁵Cl-H³⁵Cl or the D³⁵Cl-D³⁵Cl, for which the data indicate a fast tunneling motion. The second is the heterodimers, H³⁵Cl-H³⁷Cl or D³⁵Cl-D³⁷Cl, that do not show evidence for significant tunneling motion on the time scale of the experiment. In the case of HCl dimers, even at relatively high fields, only one species could be focused, the heterodimer. The electric dipole moments for both (DCl)₂ isotopomers were determined to be 1.5±0.2D, which is the same value as observed for (HCl)₂.

This article discusses general issues associated with electron transmission through thin molecular films. On the experimental side, we emphasize recent investigations of photoemission through organized organic films adsorbed on metal surfaces. Theoretical and numerical approaches to transmission and tunneling through such films are discussed. We focus on the relation between the structure of the film and its transmission properties. In the experimental work, these are controlled by varying the organic layer, by changing its thickness and by inducing disorder via thermal heating and by depositing mixtures of two molecular types. In numerical simulations of simple model systems, we consider the dimensionality of the process, effect of molecular ordering, and relation between electronic band structure in the film and its transmission properties. It is shown that electron transmission through thin molecular layers constitutes a sensitive tool for investigating molecular film properties in addition to providing a convenient prototype system for the study of electron transport in molecular electronic devices.

The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated.

In this work we probe the effect of the three dimensional structure of the medium on the efficiency of electron transmission (ET) through it, and demonstrate that all three dimensions are playing a crucial role in the ET through thin films. By producing Langmuir-Blodgett layers from two type of amphiphiles we could vary the order in the plane perpendicular to the direction of electron propagation, It was found that the order in this plane affects the low energy electron transmission efficiency. The results are explained by the long wavelength associated with the low energy electrons. (C) 1997 American Institute of Physics.

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.

The interaction between electrons and organized organic thin films was investigated by measuring the energy distribution of photoelectrons injected from a thin silver film coated with thin organic layers. Electrons with energy above ∼0.8 eV were transmitted ballistically through an organic layer that contains up to five monolayers, each ∼2 nm thick. Elastic scattering processes contribute significantly to the electron energy distribution only for thicker layers. The transmission of low-energy electrons is controlled mainly by an electrostatic barrier perpendicular to the surface. A signature of a band structure in the organic layer was observed when the electrons were transmitted through 13 layers.

This issue of the Israel Journal of Chemistry focuses on studies of chemical processes applying the molecular beam technique. The introduction of free jet expansion in molecular beam generation endowed this technique with unique power in the investigation of atoms, molecules, and clusters in an isolated environment. The adiabatic expansion conditions in the jet allow for extreme cooling in the translational degrees of freedom in the beam, and through coupling, lead to extensive cooling in other degrees of freedom. This property enables the generation of isolated particles close to their ground state and, through the use of electromagnetic fields, the generation of particles in specific, well-defined quantum states. For these reasons, molecular beams have gained wide acceptance in fundamental spectroscopic and scattering studies. In chemistry, it allows for a detailed study of binary reactive collisions. The high and uniform kinetic energy in the beam enables precise collision experiments with gas molecules and surfaces. The contribution of the method to chemistry was widely recognized when the Nobel prize was given to Herschbach, Lee, and Polanyi. In recent years, the molecular beam has been used to study species that are unstable in bulk. These may be clusters condensed in the adiabatic expansion, as well as radicals or other photo-generated species. In this special issue of the Israel Journal of Chemistry we have tried to bring together the flavor of new developments in the field. We wish to thank all of the contributors for accepting our invitation to share with us their latest results. We realize that the research presented here reflects only a sample of the work done applying the technique of molecular beams in diverse scientific communities. Any scientific activity depends mainly on the people pursuing it. We are all lucky to have among us excellent scientists who have inspired us. In addition to those mentioned above, we recognize the pioneering work done by people such as John Fenn, the late Richard Bernstein, and many more. We wish to take this opportunity to salute one of the most influential contributors to the field, Prof. Peter Toennies, on the occasion of his retirement. From our first days with molecular beams we have learned to admire the depth, the breadth, and the thoroughness of his scientific endeavor. We wish him many more years of fruitful activity.

HCl dimers were prepared in a pulsed supersonic beam of neat HCl. The dimers were focused in an electrostatic hexapole field. Using computer simulation, the focusing curve could be fit, assuming contributions from both first and second order Stark effects. The dipole moment, based on simulation, was found to be 1.5 ± 0.1 D. The ability to observe an effective dipole moment in a system where the donor-acceptor tunneling is fast is surprising. It may indicate that the understanding of the interaction of floppy systems with hexapole field is not complete.

Reactions of O(1D) with water and propane monomers and clusters were investigated via a crossed molecular beam experiment and by dissociation of ozone in a water-ozone complex, applying laser-induced fluorescence for the detection of the OH product. The rotational and spin-orbit state populations were analyzed. A strong preference for the 2PI3/2 spin-orbit state of the OH product is observed. The new results presented here demonstrate the effect of the initial rotational temperature of propane on the spin-orbit states distribution in the product OH. The preference for the low 2PI3/2 state is attributed to conservation of electronic angular momentum. It is conserved through the curve crossing which occurs in the entrance channel and during the lifetime of the long-lived collision complexes. A quantitative model is presented which rationalizes all the experimental observations.

The electron transfer through an organized organic monolayer of alkyl chains adsorbed on a silicon wafer has been studied. The silicon was used as an electrode in a three-electrode electrochemical cell, and the current versus voltage response was measured. The results show that when the chains in the monolayer are in the "all trans" configuration, the charge transfer efficiency is higher than when the chains have a "gauche" configuration. A mechanism rationalizing all the observations is suggested.

A description of a system for molecular structure imaging at the Weizmann Institute is presented. A novel method of controlled laser beam photo detachment inside the high voltage terminal of a tandem accelerator, enabling the study of neutral fast molecules by the Coulomb Explosion Imaging technique, is described. Also, a new type of three dimension multiparticle detector is presented.

The combination of laser photodetachment of C₄^- and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C₄ isomers and was correlated with their measured electron affinities.

The effect of cluster formation on the reactivity of cyclohexane was investigated. When single molecules react with O(³P) the products are the OH radical and cyclohexyl radical. In contrast, we found that when small clusters react with O(³P), the OH product is suppressed; furthermore, the "liquid"-like product, cyclohexanol, is observed, although the yield is unknown. It is proposed that blocking of the abstraction reaction occurs in the reaction when clusters are involved. In addition, an efficient insertion process can take place. These results provide a new explanation for the process in the liquid phase.

The temperature dependence of the reactivity of organized organic thin films with an atomic beam of O(³P) was investigated. The films, either single or multilayers, were prepared by the Langmuir-Blodgett method. The reactivity varied linearly with temperature, which is explained by the penetrability of the atoms between the organic chains. Phase transitions in the thin films could be observed by the change in the dependence of reactivity on temperature.

A quantitative analysis of the influence of multiple scattering on fast molecules dissociating in solids is made using a Monte Carlo technique. The simulations allow the computation of asymptotic velocities of all of the fragments after Coulomb explosion including the effects of ion-solid interactions. Examples for deducing moleculat structure using this method are given for two well-known molecular ions.

Aniline molecules were state selectively detected using multiphoton ionization following trapping/desorption from amorphous C₆₀ and C₇₀ films, and from single-crystal C₆₀ surfaces. Vibrational population inversion was observed in all three cases in the inversion mode of the NH₂ group. The vibrational excitation was much less efficient for scattering from a LiF single-crystal surface. A charge-transfer model rationalizes the observations and indicates that the fullerene surfaces strongly interacts with molecules with low ionization potentials.

The destruction of organized organic monolayers by an atomic beam of O(³P) was investigated as a function of the surface temperature. Changes in the infrared absorption and wettability were monitored to measure the extent of reaction of the monolayer. Two different monolayer type were used - one consisting of a methyl-terminated 18-carbon chain and the other of a single methyl group, both bound to the surface through a siloxyl bond. The long chain was found to be much more reactive than the short one, and the reactivity was dependent on the change in structure of the monolayer due to variations in temperature. Clear indications for a phase transition in the long-chain monolayer were found.