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.

The reaction of D-glucose oxidase (GOx) with D- and L-glucose was investigated using confocal fluorescence microscopy and Hall voltage measurements, after the enzyme was adsorbed as a monolayer. By adsorbing the enzyme on a ferromagnetic substrate, we verified that the reaction is spin dependent. This conclusion was supported by monitoring the reaction when the enzyme is adsorbed on a Hall device that does not contain any magnetic elements. The spin dependence is consistent with the chiral-induced spin selectivity (CISS) effect; it can be explained by the improved fidelity of the electron transfer process through the chiral enzyme due to the coupling of the linear momentum of the electrons and their spin. Since the reaction studied often serve as a model system for enzymatic activity, the results may suggest the general importance of the spin-dependent electron transfer in bio-chemical processes.

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.

Knot-like structures were found to have interesting magnetic properties in condensed matter physics. Herein, we report on topologically chiral molecular knots as efficient spintronic chiral material. The discovery of the chiral-induced spin selectivity (CISS) effect opens the possibility of manipulating the spin orientation with soft materials at room temperature and eliminating the need for a ferromagnetic electrode. In the chiral molecular trefoil knot, there are no stereogenic carbon atoms, and chirality results from the spatial arrangements of crossings in the trefoil knot structures. The molecules show a very high spin polarization of nearly 90%, a conductivity that is higher by about 2 orders of magnitude compared with that of other chiral small molecules, and enhanced thermal stability. A plausible explanation for these special properties is provided, combined with model calculations, that supports the role of electron-electron interaction in these systems.

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 3540 % 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 × 102. 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 × 103, 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 materials 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 electronelectron 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 PGKs negatively charged nucleotide substrates modulate CRA substantially, even though they bind far away from the His-tagantibody 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.

The technological advancement of data storage is reliant upon the continuous development of faster and denser memory with low power consumption. Recent progress in flash memory has focused on increasing the number of bits per cell to increase information density. In this work an optical multilevel spin bit, based on the chiral induced spin selectivity (CISS) effect, is developed using nanometer sized chiral quantum dots. A double quantum dot architecture is adsorbed on the active area of a Ni based Hall sensor and a nine-state readout is achieved.

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 oxygen reduction efficiency of a laccase-modified electrode was found to depend on the chirality of the oligopeptide linker used to bind the enzyme to the surface. At the same time, the electron transfer between the cathode electrode and the enzyme is improved by using a copper(II) complex with amino-acid derivative Schiff base ligand with/without azobenzene moiety as a mediator. The increased electrochemical current under both O₂ and N₂ proves that both the mediators are active towards the enzyme.

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.

Organic semiconductors and organic-inorganic hybrids are promising materials for spintronic-based memory devices. Recently, an alternative route to organic spintronic based on chiral-induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical pi-conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin-filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both "up" and "down" orientations of filtered spins can be obtained in a single enantiopure system via the temperature-dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure-property relationship for spintronic applications.

We show that enantioselective reactions can be induced by the electron spin itself and that it is possible to replace a conventional enantiopure chemical reagent by spin-polarized electrons that provide the chiral bias for enantioselective reactions. Three examples of enantioselective chemistry resulting from electron-spin polarization are presented. One demonstrates the enantioselective association of a chiral molecule with an achiral self-assembled monolayer film that is spin-polarized, while the other two show that the chiral bias provided by the electron helicity can drive both reduction and oxidation in enantiospecific electrochemical reactions. In each case, the enantioselectivity does not result from enantiospecific interactions of the molecule with the ferromagnetic electrode but from the polarized spin that crosses the interface between the substrate and the molecule. Furthermore, the direction of the electron-spin polarization defines the handedness of the enantioselectivity. This work demonstrates a new mechanism for realizing enantioselective chemistry.

Chiral symmetry is ubiquitous in Biology, Physics, and Chemistry. The biomolecules essential for life on Earth-such as deoxyribonucleic acid (DNA), sugars, and proteins-display homochirality that affects their function in biological processes. Ten years ago, it was discovered that electron transfer through chiral molecules depends on the direction of the electron spin, and more recently, it was shown that the charge displacement in chiral molecules creates transient spin polarization. Thus, the properties of ferromagnet/chiral molecule interfaces are affected by spin exchange interactions, via the overlap of the chiral molecule with the ferromagnet's spin wave function. This effect offers a mechanism for homochiral bias in Biology, which was previously unappreciated, and an approach to enantioselective chemistry and chiral separations, which is controlled by the electron spin.

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.

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.

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.

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.

CONSPECTUS: The optical and electronic properties of semiconductor quantum dots (QDs) make them attractive candidates for applications in photovoltaics, spintronics, photocatalysis, and optoelectronics. Understanding how to control the flow of charge in QD assemblies is essential for realizing novel applications. This Account explores some unique characteristics of charge transport in QD dyads, triads, and their assemblies. The emerging features of these assemblies that provide new opportunities to manipulate charge flow at the nanoscale are (1) cascading energy landscapes and band offsets to inhibit charge recombination, (2) electrostatic fields that direct charge flow through QD-QD and QD-conjugated polymer junctions, and (3) QD chirality and chiral imprinting that promotes vectorial electron and spin selective transport.Charge flow kinetics is determined by a combination of familiar electron transfer parameters (reaction free energy, reorganization energy, and electronic coupling), donor and acceptor electronic densities of states, and internal electric fields. Electron transfer and electronic structure theory, combined with kinetic modeling, place the measured kinetics of QD electron transfer donor-acceptor assemblies into a unified conceptual context. The experimental transfer rates measured in these systems depend upon structure and the internal electric fields that are present in the assemblies. A negatively charged donor and positively charged acceptor, for example, facilitates (inhibits) electron (hole) transfer, while an electric field of opposite orientation (reversal of charges) inhibits (promotes) electron (hole) transfer. These and other emerging rules that govern charge flow in NP assemblies provide a strategy to design the directionality and yield of interfacial charge transport.Chirality at the nanoscale can induce spin selective charge transport, providing new ways to direct charge (and spin) flow in QD assemblies. Magnetoresistance and magnetic conductive probe atomic force microscopy experiments show spin selective electron transport for chirally imprinted QD assemblies. Photoinduced electron transfer from achiral donor-QDs to chiral acceptor-QDs depends on the electron spin and chiroptical properties of the acceptor-QDs. These assemblies show transport characteristics that correlate with features of the QDs' circular dichroism spectra, presenting intriguing challenges to theory, and indicating that spectroscopic signatures may assist in the design and diagnosis of functional molecular assemblies.Theoretical and experimental studies of charge transport in well-defined QD assemblies are establishing design principles for vectorial charge transport and are also refining questions surrounding the mechanism and control of these processes. These intensified efforts are forging links between fundamental discoveries regarding mechanism and practical applications for these novel assembled nanostructures.

This review describes a new perspective on the role that electron spin plays in the intermolecular forces between two chiral molecules and between chiral molecules and surfaces. This different role of the spin arises from the chiral induced spin selectivity (CISS) effect which is manifest when electrons are moving in chiral molecules. Namely, it has been shown that as chiral molecules are charge polarized, the electron displacement is accompanied by spin polarization. The spin direction associated with each electric pole depends on the specific handedness of the molecule. Thus, the consideration of the dispersive forces between two molecules, or between a molecule and a substrate, must include the spin polarization which adds an enantioselective electronic term to the interaction potential. We review recent experiments that show the relation between charge polarization and spin polarization in chiral molecules. The spin polarization also affects the direction of the ferromagnetic substrate magnetic moment of a surface, upon which the chiral molecules are adsorbed.

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 shown that \u201cspontaneous magnetization\u201d occurs when chiral oligopeptides are attached to ferrocene and are self-assembled on a gold substrate. As a result, the electron transfer, measured by electrochemistry, shows asymmetry in the reduction and oxidation rate constants; this asymmetry is reversed between the two enantiomers. The results can be explained by the chiral induced spin selectivity of the electron transfer. The measured magnetization shows high anisotropy and the \u201ceasy axis\u201d of magnetization is along the molecular axis.

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

The integration of nanostructures in electronic devices utilizes their unique quantum properties for realizing discrete measuring systems. Specifically, self-assembled organic monolayers and nanocrystals (NCs), together with bottom-up production methods, can lead to new types of electronic devices. In this work, we present a wavelength-tunable near-infrared detection device in which PbS NCs are used to create an optical gate for an AlGaAs/GaAs high electron mobility device. By integrating side gates, we were able to enhance light detection sensitivity by optimizing the conductivity of the channel. Both DC and AC modulations of the side gate were tested and compared in order to enhance the detector's signal-to-noise ratio (SNR). Higher harmonic signals of the side gate modulation supply additional information about the detection mechanism. (C) 2017 Elsevier B.V. All rights reserved.

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).

Ferromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 × 10⁶ A·cm^-2, or about 1 × 10²⁵ electrons s^-1 cm^-2. This relatively high current density significantly affects the devices' structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 10¹³ electrons per cm² are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magnetic anisotropy. These results present a simple low-power magnetization mechanism when operating at ambient conditions.

CONSPECTUS: Molecular spintronics (spin + electronics), which aims to exploit both the spin degree of freedom and the electron charge in molecular devices, has recently received massive attention. Our recent experiments on molecular spintronics employ chiral molecules which have the unexpected property of acting as spin filters, by way of an effect we call "chiral-induced spin selectivity" (CISS). In this Account, we discuss new types of spin dependent electrochemistry measurements and their use to probe the spin-dependent charge transport properties of nonmagnetic chiral conductive polymers and biomolecules, such as oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin (bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures. Spin-dependent electrochemical measurements were carried out by employing ferromagnetic electrodes modified with chiral molecules used as the working electrode. Redox probes were used either in solution or when directly attached to the ferromagnetic electrodes. During the electrochemical measurements, the ferromagnetic electrode was magnetized either with its magnetic moment pointing "UP" or "DOWN" using a permanent magnet (H = 0.5 T), placed underneath the chemically modified ferromagnetic electrodes. The spin polarization of the current was found to be in the range of 5-30%, even in the case of small chiral molecules. Chiral films of the L- and D-cysteine tethered with a redox-active dye, toludin blue O, show spin polarizarion that depends on the chirality. Because the nickel electrodes are susceptible to corrosion, we explored the effect of coating them with a thin gold overlayer. The effect of the gold layer on the spin polarization of the electrons ejected from the electrode was investigated. In addition, the role of the structure of the protein on the spin selective transport was also studied as a function of bias voltage and the effect of protein denaturation was revealed. In addition to "dark" measurements, we also describe photoelectrochemical measurements in which light is used to affect the spin selective electron transport through the chiral molecules. We describe how the excitation of a chromophore (such as CdSe nanoparticles), which is attached to a chiral working, electrode, can flip the preferred spin orientation of the photocurrent, when measured under the identical conditions. Thus, chirality-induced spin polarization, when combined with light and magnetic field effects, opens new avenues for the study of the spin transport properties of chiral molecules and biomolecules and for creating new types of spintronic devices in which light and molecular chirality provide new functions and properties.

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.

We developed and investigated the properties of molecularly controlled semiconductor resistors (MOCSERs) based on AIGaN/GaN structure. The response of the sensor for two different analytes was investigated when the sensor was coated with two molecules that differ only in their binding groups. We studied the ability to enhance the specificity of the sensor by adding illumination at various sub-bandgap frequencies. It was verified that the sensor is sensitive to the electronegativity of the analyte and illumination can affect the sensitivity and selectivity when the system does not reach a steady state. Hence, we differ between two operational modes in which orthogonal sensing is made.

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.

Chiral conducting polymer films are selfassembled 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.

This work examines whether electrochemical redox reactions are sensitive to the electron spin orientation by examining the effects of magnetic field and molecular chirality on the charge transfer process. The working electrode is either a ferromagnetic nickel film or a nickel film that is coated with an ultrathin (5-30 nm) gold overlayer. The electrode is coated with a self-Assembled monolayer that immobilizes a redox couple containing chiral molecular units, either the redox active dye toluidine blue O with a chiral cysteine linking unit or cytochrome c. By varying the direction of magnetization of the nickel, toward or away from the adsorbed layer, we demonstrate that the electrochemical current depends on the orientation of the electrons spin. In the case of cytochrome c, the spin selectivity of the reduction is extremely high, namely, the reduction occurs mainly with electrons having their spin-Aligned antiparallel to their velocity.

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.

We have performed a comparison of the radiation damage occurring in DNA adsorbed on gold in two different configurations, when the DNA is thiolated and bound covalently to the substrate and when it is unthiolated and interacts with the substrate through the bases. Both molecules were found to organize so as to protrude from the surface at ∼45 degrees. Changes in the time-dependent C 1s and O 1s X-ray photoelectron (XP) spectra resulting from irradiation were interpreted to arise from cleavage of the phosphodiester bond and possibly COH desorption. By fitting the time-dependent XP spectra to a simple kinetic model, time constants were extracted, which were converted to cross sections and quantum yields for the damage reaction. The radiation induced damage is significantly higher for the thiolated DNA. N 1s X-ray absorption spectrum revealed the N-CN LUMO is more populated in the unthiolated molecule, which is due to a higher degree of charge transfer from the substrate to this LUMO in the unthiolated case. Since the N-CN LUMO of the thiolated molecule is comparatively less populated, it is more effective in capturing low energy electrons resulting in a higher degree of damage. This journal is

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.

A hybrid device made from gold nanoparticles connected by alkyldithiol molecules of different lengths was produced and its conduction properties were investigated for various lengths of the organic linker molecules. It was found that the conductivity increases with the length of the molecules. The surprising dependence of the conductivity on the molecules' length was explained by a model that takes into account the probability for forming continuous conductive paths for the different molecules.

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".

In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

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.

A new method is presented for patterning surfaces with gradient properties. The method is based on magnetolithography in which the surface patterning is performed by applying a gradient of a magnetic field on the substrate, using paramagnetic metal masks in the presence of a constant magnetic field. Superparamagnetic nanoparticles (NPs) are deposited on the substrate, and they assemble according to the field and its gradients induced by the mask. Once they pattern the substrate, they protect their sites on the substrate from interacting with any other species. The areas not protected by the NPs can be covered by molecules that chemically bind to the substrate. After these molecules are bound, the NPs are removed, and other molecules may be adsorbed on the newly exposed area. The new technique is based on a parallel process that can be carried out on a full wafer. It provides high resolution, it creates gradient continuously from submicrometers to milimeters, and it can be performed on surfaces that are not flat and that are even on the inside of a tube. The gradient that is formed is not limited to a specific property or type of substrate.

Magnetolithography (ML), a technology for patterning surfaces, is a single-step process, and is much simpler and less expensive to apply than the common photolithography method. ML does not require application of resists, and therefore, the surfaces remain clean and can be easily modified chemically as needed. ML also not depends on the surface topography and planarity and can be used for patterning non-flat and the inside surfaces of a closed volume. The widths of lines made with ML can actually be smaller than the lines on the mask used. ML uses backside lithography, which can easily produce multilayers with highly accurate alignment and with the same efficiency for all layers. Positive and negative ML can be applied to a wide range of surfaces for patterning with either small or large molecules or biomolecules sensitive to the chemical environment. ML method can be used for chemical patterning and also for common microelectronic processes such as etching, deposition, and ion implant.

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.

In the search for new materials and concepts in materials science, metallo-organic hybrids are attractive candidates; they can combine the rich diversity of organic molecules with the advantages of metals. Transition metals such as palladium are widely applied in catalysis, and small organic molecules such as those in the cinchona alkaloid family can control the stereochemistry of a number of organic reactions. Here, we show that reducing a metal salt in the presence of a cinchona alkaloid dopant gives a chirally imprinted metallo-organic hybrid material that is catalytically active and shows moderate enantioselectivity in hydrogenation. Furthermore, using photoelectron emission spectroscopy, we show that the metal retains some chiral character even after extraction of the dopant. This simple and effective methodology opens exciting opportunities for developing a variety of chiral composite materials.

A recent report of ferromagnetism appearing in silicon after etching in hot KOH (Kopnov et al., Adv. Mater. 2007, 79, 925) is shown to be due to iron from the pyrex glassware, which precipitates on the silicon surface in the form of well-separated ferromagnetic nanoparticles. The reaction is explained in terms of the Pourbaix diagram.

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.

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.

We present results from high-resolution electron energy loss spectroscopy (HREELS) and XPS studies of self-assembled monolayers of DNA. The monolayers are well-organized and display sharp vibrational peaks in the HREEL spectra. The electrons interact mainly with the backbone of the DNA. The XPS results indicate that, in most of the samples studied, the phosphates on the DNA are not charged.

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 a light sensing device based on nearly spherical, defect free colloidal nanocrystals (NCs) of InAs acting as a light activated gate for a GaAsAlGaAs field effect semiconductor transistor. We use self-assembled organic monolayer as linkers that attach the InAs NCs to the surface of the semiconductor device, instead of the gate that exists in common transistors. When the NCs absorb light, at a frequency corresponding to their resonance, a change in the current through the transistor takes place while no current flows through the NCs themselves.

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.

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p -polarized versus s -polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.

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.

Experimental evidences are presented supporting the existence of a new optical absorption band observed when monolayers of alkylthiol are self-assembled on gold substrate. This new absorption is centered at about 800 nm and has a very broad absorption peak. The intensity of the new band correlates with the density of molecules and the quality of the organization of the monolayer.

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....

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.

Professor Naaman commented: * Working with oxidized single wall carbon nanotubes we did observe a change in the Raman spectrum as a result of the reaction. Upon heating, the Raman spectrum returned to its original form.* The frequency of the noise is very low indicating nuclear motion and perhaps a rotation-like motion of the molecule. This motion will involve a change from trans to gauche configuration.* There is a basic difference between electron transfer and conduction. In the first the molecule is neutral; in the second the system includes an additional electron.

InAs/ZnSe core/shell nanoparticles (NP) were self-assembled on GaAs substrates using different organic molecules of varying length and properties as linkers. The molecules provide control over the coupling and tunneling properties between the substrate and the nanocrystals. By coadsorbing of gold NP on the GaAs substrate, enhancement of the photoluminescence from the InAs NP was achieved. The enhancement factor was found to depend on the properties of the organic linkers.

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.

Seemingly contradicting results raised a debate over the ability of DNA to transport charge and the nature of the conduction mechanisms through it. We developed an experimental approach for measuring current through DNA molecules, chemically connected on both ends to a metal substrate and to a gold nanoparticle, by using a conductive atomic force microscope. Many samples could be made because of the experimental approach adopted here, which enabled us to obtain reproducible results with various samples, conditions, and measurement methods. We present multileveled evidence for charge transport through 26-bp-long dsDNA of a complex sequence, characterized by S-shaped current-voltage curves that show currents >220 nA at 2 V. This significant observation implies that a coherent or band transport mechanism takes over for bias potentials leading to high currents (>1 nA).

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.

Gallium arsenide coating by molecular layers is a of increasing interest both for its surface passivation and for its use as a chemical or biochemical sensor. The surface state of GaAs and the nature of the molecular functionality to be bound to the surface are very important to assure good and durable adhesion. This work, using both the vibrational and the electronic energy loss range of high resolution electron energy loss spectra, showed that the water content in the solvent acetonitrile - has a dramatic effect on the amount of phenylphosphonic acid molecules adsorbed on the GaAs substrate. There is a poor molecular adsorption for water contents ranging from 0 to 4% volume: HREELS spectrum is always a combination of the substrate and the adsorbed molecule spectra. For a water content of 5% there is an abrupt jump in the HREELS spectra shape: they become typical of phenyl groups in the electronic region. In the vibrational region, the typical C-H stretching peaks of aliphatic chains disappear showing that the extreme surface is exclusively covered by phenyl functions. Also for the samples, where a large adsorption occurs, surfaces become negatively charged under electron irradiation showing the existence of a large number of traps for incident electrons. Sonication of such well covered substrates destroys intermolecular bonds but keeps molecules that are chemically bound to the substrate.

A model is given that shows that the electronic properties of close packed organized organic layers, adsorbed on conductive substrate, may be very different from the properties of the single adsorbed molecule. The difference arises from a cooperative effect that results in electron transfer between the substrate and the layer. It is induced when molecules having dipole moment and low polarizability are organized so that their dipole moment is perpendicular to the surface. The thermodynamics of the problem is described. The model provides a possible rationalization to recent observed new experimental properties of adsorbed organized organic layers.

Photoelectrons from gold covered with self-assembled monolayers of L-polyalanine were measured as a function of temperature. Spectra and asymmetries to circularly polarized light were recorded while monitoring the electric potential across the monolayer. Upon cooling, this potential changes gradually, passing through zero at 264 K. But, contrary to the gradual potential change, both the shape of spectra and the polarization asymmetry change abruptly at the edges of the temperature range Δ=264±10 K. Above and below Δ, opposite asymmetries are observed and the spectra are broad. Within Δ, a series of several sharp resonances appear with null asymmetry.

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.

We report on the direct observation of collective electronic properties in assembled CdS quantum particles (QPs) arranged in periodic layers. Within each layer the QPs are of the same average size, either 2.5 or 5 nm, and the layers are arranged in a cascade-like pattern. The electronic properties of the QPs were studied using a new method, the attenuated low energy photoelectron spectroscopy (A-LEPS), in which a `pump' laser excites the QPs and a `probe' laser ejects photoelectrons from the QPs and from the metal substrate. The A-LEPS method provides information about the populated electronic states of the QPs (including the splitting between the light/heavy hole and split-off bands) and how these states depend on the interparticle interactions.

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.

Geringe Konzentrationen von NO in physiologischen Pufferlösungen (1 ppm, ca. 3 μM) lassen sich mit dem gezeigten maßgeschneiderten molekularen Zweikomponentensystem ein pinzettenartig von einem organischen Liganden gehaltenes Eisen(III)porphyrin nachweisen, das mit einer Seite an einen Detektor auf GaAsBasis gekoppelt ist. Bindet NO an das Eisen(III)porphyrin, ändert sich die Stromstärke.

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.

In recent experiments on HCl dimers, three surprising observations were found. A "dipole moment" was measured in Stark-shift-related experiments despite the spectroscopic indication of fast tunneling motion. The measured dipole moment varied with the state probed, and the apparent first-order Stark effect was observed only in the case of the heterodimer H³⁵Cl-H³⁷Cl but not for the homodimers (H³⁵Cl)₂ or (H³⁷Cl)₂. We present physical arguments which explain all the observations and indicate that the spectroscopy has to be reinterpreted and that the isotope effect in the tunneling motion can not be inferred simply from the spectroscopy.

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.

A new type of electron multiplier that has recently been developed, the microsphere plate (MSP), does not have the problem of ion feedback, which limits the operation of the known microchannel plates. Due to its structure, the MSP can amplify charge current even at pressures as high as 0.1 mbar.

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 reactions of oxygen atoms in their triple and singlet states with hydrocarbon clusters were investigated. The results indicate that some liquidtype effects can be demonstrated already in the reactions of small clusters. Two examples are discussed: (1) The reaction of O( 3 P ) with cyclohexane in which a single solvent molecule is enough to reproduce the products of the liquid reaction. (2) The reactions of O( 1 D ) with methane clusters for which the translational, vibrational, rotational, spinorbit, and Λdoubling state populations were analyzed, onstatistical distributions are observed even for the reaction of large methane clusters. The results of these studies are discussed in terms of nonadiabatic effects induced by the long lived collision complex.

The structure of small carbon clusters (C_n, n = 3-8) has been studied using the Coulomb explosion imaging technique. The experimental system is based on the 14 MV Pelletron accelerator at the Weizmann Institute. Negative molecules are accelerated and photodetached by a laser beam inside the high voltage terminal of the accelerator. The neutral molecules are then electron stripped by passing through a thin Formvar target and the resulting atomic ions are collected by a new 3D multiparticle imaging detector. Nonlinear structures of C₄ and C₅ clusters were observed. New measurements of laser photodetachment thresholds for C_n (n = 3-8) indicate that new compact isomers exist.

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 reaction between an O(³P) atom and a hydrocarbon molecule weakly bound to an argon atom was studied by classical trajectory simulations. The results are compared to those obtained for the reaction of a free hydrocarbon. A simplistic model system was constructed in which the hydrocarbon was represented as a pseudodiatomic molecule. Although simple, the model reproduced correctly the internal energy distribution in the OH produced in the reaction of the free species. It was found that the OH, produced from the reaction of the van der Waals complex, emerges with less internal energy and less translational energy than the OH from the monomeric process. In the case of the complexed reagents, the collision complex lifetime is longer and the oxygen explores portions of the potential energy surface that are not available in the monomeric reaction.

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.

A specialized ion source for use in the high voltage terminal of an electrostatic accelerator has been developed to produce vibrationally cold beams of molecular ions. A pulsed valve is used to produce a supersonic expansion of the source gas which is ionized by electron impact near the beginning of the free expansion. Because of the harsh electromagnetic environment in the terminal, special care has been taken to protect the electronic components related to the ion source. The effect of vibrational cooling in the supersonic expansion is demonstrated using Coulomb explosion techniques with a He ₂⁺ beam.

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.