Publications

We present a method for enantioselective separation of a racemic mixture of thiolated molecules by applying ferromagnetic surfaces. Depending on the direction of the magnetization on the surface, up or down relative to the surface normal, one enantiomer is adsorbed and the other one is extracted. After an electric field is applied, the adsorbed enantiomer is released and collected. The same ferromagnetic surfaces can be used, in principle, for all thiolated chiral molecules; the method operates at low pressure and allows both enantiomers to be retrieved. Because the thiol group can be easily attached to many molecules, this method can be widely applied.

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

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

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

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

The electron’s spin, its intrinsic angular momentum, is a quantum property that plays a critical role in determining the electronic structure of molecules. Despite its importance, it is not used often for controlling chemical processes, photochemistry excluded. The reason is that many organic molecules have a total spin zero, namely, all the electrons are paired. Even for molecules with high spin multiplicity, the spin orientation is usually only weakly coupled to the molecular frame of nuclei and hence to the molecular orientation. Therefore, controlling the spin orientation usually does not provide a handle on controlling the geometry of the molecular species during a reaction. About two decades ago, however, a new phenomenon was discovered that relates the electron’s spin to the handedness of chiral molecules and is now known as the chiral induced spin selectivity (CISS) effect. It was established that the efficiency of electron transport through chiral molecules depends on the electron spin and that it changes with the enantiomeric form of a molecule and the direction of the electron’s linear momentum. This property means that, for chiral molecules, the electron spin is strongly coupled to the molecular frame. Over the past few years, we and others have shown that this feature can be used to provide spin-control over chemical reactions and to perform enantioseparations with magnetic surfaces.

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.

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The chiral-induced spin selectivity (CISS) effect entails spin-selective electron transmission through chiral molecules. In the present study, the spin filtering ability of chiral, helical oligopeptide monolayers of two different lengths is demonstrated using magnetic conductive probe atomic force microscopy. Spin-specific nanoscale electron transport studies elucidate that the spin polarization is higher for 14-mer oligopeptides than that of the 10-mer. We also show that the spin filtering ability can be tuned by changing the tip-loading force applied on the molecules. The spin selectivity decreases with increasing applied force, an effect attributed to the increased ratio of radius to pitch of the helix upon compression and increased tilt angles between the molecular axis and the surface normal. The method applied here provides new insights into the parameters controlling the CISS effect. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

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

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

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

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

Correction for “Sensing of molecules using quantum dynamics,” by Agostino Migliore, Ron Naaman, and David N. Beratan, which appeared in issue 19, May 12, 2015, of Proc Natl Acad Sci USA (112:E2419–E2428; first published April 24, 2015; 10.1073/pnas.1502000112).The authors note that on page E2428, right column, in the Acknowledgments section, lines 4–5, “We also thank the US Department of Energy for support of this research (Grant SC0010662)” should instead appear as “This research is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award ER46430.”

Recent experiments have demonstrated that the electron transmission yield through chiral molecules depends on the electron spin orientation. This phenomenon has been termed the chiral-induced spin selectivity (CISS) effect, and it provides a challenge to theory and promise for organic molecule–based spintronic devices. This article reviews recent developments in our understanding of CISS. Different theoretical models have been used to describe the effect; however, they all presume an unusually large spin-orbit coupling in chiral molecules for the effect to display the magnitudes seen in experiments. A simplified model for an electron's transport through a chiral potential suggests that these large couplings can be manifested. Techniques for measuring spin-selective electron transport through molecules are overviewed, and some examples of recent experiments are described. Finally, we present results obtained by studying several systems, and we describe the possible application of the CISS effect for memory devices.

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 0 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.

We report on the observation of chirality induced spin selectivity for electrons transmitted through monolayers of oligopeptides, both for energies above the vacuum level as well as for bound electrons and for electrons conducted through a single molecule. The dependence of the spin selectivity on the molecular length is measured in an electrochemical cell for bound electrons and in a photoemission spectrometer for photoelectrons. The length dependence and the absolute spin polarization are similar for both energy regimes. Single molecule conductance studies provide an effective charge transport barrier between the two spin channels and it is found to be on the order of 0.5 eV.

Creation and manipulation of spin current is one of major aspects of memory devices. In conventional devices spin-polarized current is created by permanent magnetic layer. Further miniaturization of the memory is limited by super-paramagnetic behavior of layer. Hence, high density memory requires out-of-plane geometry with perpendicular magnetic anisotropy. Achieving this goal with inorganic magnetic layers is a challenge. We present a new approach in which the permanent magnetic layer has been replaced with inorganic chiral film producing spin polarized current due to Chirality Induced Spin Selectivity (CISS) effect. Chiral Al2O3 film grown by ALD on self-assembled monolayer of chiral molecules acts as a spin filter. Spin polarization is parallel/antiparallel to the electron velocity depending on chirality. Devices show asymmetric magneto-resistance and slopes with opposite sign for left/right handed chirality. Hence, CISS-effect based device shows, for first time, an asymmetric magneto-resistance, which has potential application in magnetic memory and magnetic field sensors.

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 review some of the sensors based on the molecular control semiconductor resistor (MOCSER) technology. The technology can be applied for developing highly sensitive and selective molecular sensors that operate both in gas and in liquid environments. Specifically, we describe here a set of GaAs gas-phase sensors based on an array constructed from several elements, each coated with a different sensing molecule. Coating the same semiconductor device with an ultrathin polymer layer also allows the use of the device in a harsh liquid environment and in performing measurements in physiological liquids. Hence, the MOCSER-based platform opens the way for producing inexpensive, easy to use, and robust sensors for a wide variety of applications.

This study provides insight into the mechanism of capturing low energy electrons by peptide nucleic acid (PNA) and the role of the oligonucleotide backbone in the capture of low energy electrons. We studied by photoemission self-assembled monolayers of two types of oligonucleotides, DNA and PNA. PNA is a synthetic analogue of DNA that has a pseudopeptide backbone and which may have important medical and biotechological applications. We found that in both PNA and DNA, the guanine nucleobases capture the electrons more efficiently than thymines. In PNA, once the electrons are captured, their state is at least partially localized on the nucleobases, and the PNA molecule undergoes structural changes that stabilize the electron. This situation is in contrast to DNA, in which the captured electrons are transferred very efficiently to the backbone, and the final state of captured electron is base independent.

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. (C) 2013 Elsevier B.V. All rights reserved.

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

We studied the photoluminescence and time-resolved photoluminescence from self-assembled bilayers of donor and acceptor nanoparticles (NPs) adsorbed on a quartz substrate through organic linkers. Charge and energy transfer processes within the assemblies were investigated as a function of the length of the dithiolated linker (DT) between the donors and acceptors. We found an unusual linker-length-dependency in the emission of the donors. This dependency may be explained by charge and energy transfer processes in the vertical direction (from the donors to the acceptors) that depend strongly on charge transfer processes occurring in the horizontal plane (within the monolayer of the acceptor), namely, parallel to the substrate.

The chiral-induced spin selectivity (CISS) effect was recently established experimentally and theoretically. Here, we review some of the new findings and discuss applications that can result from special properties of this effect, like the reduction of the elastic backscattering in electron transfer through chiral molecules. The CISS effect opens the possibility of using chiral molecules in spintronics applications and for providing a deeper understanding of spin-selective processes in biology.

The superconducting critical temperature, T-C, of thin Nb films is significantly modified when gold nanoparticles (NPs) are chemically linked to the Nb film, with a consistent enhancement when using 3 nm long disilane linker molecules. The T-C increases by up to 10% for certain linker length and NP size. No change is observed when the nanoparticles are physisorbed with nonlinking molecules. Electron tunneling spectra acquired on the linked NPs below T-C typically exhibit zero-bias peaks. We attribute these results to a pairing mechanism coupling electrons in the Nb and the NPs, mediated by the organic linkers.

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.

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO2 electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band-LUMO offsets, and near "pinning" of the QD LUMO relative to the conduction band of the TiO2 electrode, which is explained by the strong QD-electrode interaction. That interaction is the origin for the highly efficient QD to electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

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.

Chiral molecules, molecules that lack mirror symmetry, have been the focus of attention since it was established that all organisms are built of molecules with specific handedness. The understanding of biological processes that involve intermolecular recognition, including drug interactions with biomolecules, is enhanced by an understanding of the structure and interactions among chiral structures, as well as an ability to synthesize and separate enantiomers and diastereomers. As such, enormous focus has been placed on molecular stereochemistry, beginning with the earliest pioneering studies of van’t Hoff, who established the tetrahedral valency of carbon and the consequent origins of molecular chirality. Recent progress in molecular chirality has been recognized for synthetic breakthroughs through the awarding of the 2001 Nobel Prize in Chemistry to Knowles and Noyori for chirally catalyzed hydrogenation and to Sharpless for discoveries on chirally catalyzed oxidation.

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

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

The effect of the surface treatments on the transport properties of a two-dimensional electron gas was studied at the quantum limit. The surface of the Al(0.36)Ga(0.64)As/GaAs heterostructure was either coated with gold or etched with HCl solution, or etched and then coated by a self-assembled monolayer (SAM) of either phosphonated (ODP-C(18)H(39)PO(3)) or thiolated (ODT-C(18)H(37)S) molecules. The etching process was found to reduce significantly both the mobility and the charge density. This effect was reversed upon sequential adsorption of the phosphonated SAM. We propose fine tuning of the device performance by the flexible chemistry of the assembled molecules, two of them demonstrated here. The results indicate that the surface oxidation does not necessarily play the dominant role in this respect and, in particular, that octadecane phosphonic acid (ODP) can protect the substrate from both oxidation and the formation of a passivating carbon layer. In contrast, octadecanethiol (ODT) is not stable enough and is not effective in eliminating surface states, as a result devices covered with ODT behave like those with etched surfaces.

We examine the current response of molecularly controlled semiconductor devices to the presence of weakly interacting analytes. We evaluate the response of two types of devices, a silicon oxide coated silicon device and a GaAs/AlGaAs device both coated with aliphatic chains and exposed to 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 where the Si device response is mostly correlated to the analyte dipole, the GaAs device response is mostly correlated to interaction 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/AgNO(3) electrode, and by using photoemission spectroscopy, it was possible to independently measure both the filled state energies (via single photon photoemission) and those of the unfilled states (via two photon photoemission) with respect to the vacuum level. Comparison of these two different measures shows good agreement with the IUPAC-accepted value of the absolute electrode potential. In contrast to the common model of energy level alignment, the experimental findings show that the CdSe filled states become 'pinned' to the Fermi level of the Au electrode, even for moderately small NP sizes.

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

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

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

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

The adsorption of DNA on surfaces is a widespread procedure and is a common way for fabrication of biosensors, DNA chips, and nanoelectronic devices. Although the biologically relevant and prevailing in vivo structure of DNA is its double-stranded (dsDNA) conformation, the characterization of DNA on surfaces has mainly focused on single-stranded 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 notestablished that adsorbed dsDNA 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 and experimentally verify a concept for electronic devices based on nanoscale vacuum phototubes. Such devices are expected to be both reliable and amenable to large-scale integration. We further suggest several generalizations of the concept and discuss possible applications and advantages. (c) 2008 American Institute of Physics.

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.

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. (C) 2007 American Institute of Physics.

Silicon/silicon oxide interfaces are found to possess magnetic properties when the surface is etched to a specific morphology. By measuring the surface roughness (see figure) using atomic force microscopy, and monitoring the magnetic response as a function of magnetic field, the relation between the magnetism and the surface structure is established. The observations indicate that the magnetism is related to a cooperative effect on the surface.

The authors investigate effects of chemisorption of polar organic molecules onto ferromagnetic GaAs/GaMnAs heterostructures. The chemisorbed heterostructures exhibit striking anisotropic enhancement of the magnetization, while GaAs substrates that are physisorbed with the same molecules show no change in magnetic properties. Thus the enhanced magnetism of the chemisorbed heterostructures reflects changes in spin alignment that arise from surface bonding of the organic monolayer. (c) 2006 American Institute of Physics.

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/nm(2) for the oligomer containing 14 GC pairs.

The aim of this work is the development of a NO sensor for asthma control and medication monitoring. The transducer is a Molecular Controlled Semiconductor Resistor (MOCSER), which is a GaAs based heterostructure. Protoporphyrins IX, containing carboxylic groups to chemisorb on GaAs, were used as sensing molecules. Characterization of the protoporphyrin monolayers was held using Attenuated Total Reflection in Multiple Internal Reflection (ATR/MIR), High Resolution Electron Energy Loss Spectroscopy (HREELS) in the vibrational and electronic domain and X-ray Photoelectron Spectroscopy (XPS). Degreasing and etching of the GaAs substrates were accomplished before adsorption. Interfacial bonding investigated by ATR/MIR shows that protoporphyrin adsorbs to the GaAs (100) through a unidentate complex and remains mostly vertically oriented. The electronic domain of the HREELS spectra exhibits the Q band with alpha and beta components on the same position as in the UV/Vis spectrum. Soret band is blue shifted showing a face to face stacking of the protoporphyrin molecules on the GaAs substrates. XPS spectra reveal the presence of Cobalt in monolayers prepared with 8 x 10(-5) M CoPP solutions. Kinetics is best fitted by an Elovich equation, showing some hindrance due to the previous adsorbed molecules. Thickness found from XPS data ranges from 1.3 to 1.5 nm, which fits with the molecular dimensions. Using the GaAs preparation methods developed here, an NO sensor prototype was assembled and tested for NO sensitivity and repeatability. Relative to NO, tests reveal a good sensitivity between 1.6 and 200 ppb. NO sensitivity was also measured towards CO, CO2 and O-2. Pure nitrogen sweeps NO from the porphyrin layer, opening the possibility of the sensor relitilization. (c) 2005 Elsevier B.V. All rights reserved.

New electronic and magnetic properties are induced by the adsorption of closed packed monolayers on solid substrates. For many thiolated molecules self-assembled on gold, a surprisingly large paramagnetism is observed. In the case where the layers are made from chiral molecules, in addition an unexpectedly large electronic dichroism is observed, which manifests itself as spin specific electron transmission. This dichroism was observed for monolayers made from polyalanine and from DNA. Self-assembled monolayers of double-stranded DNA oligomers on gold interact with polarized electrons similarly to a strong and oriented magnetic field. The direction of the field for right-handed DNA is away from the substrate. Moreover, the layer shows very high paramagnetic susceptibility. Interestingly, thiolated single-stranded DNA oligomers on gold do not show this effect. All the observations can be rationalized by assuming organization induced charge transfer between the substrate and the organic layer. The charge transfer results in spin alignment of the transferred electrons/holes. While for achiral molecules the spin alignment varies among the domains, in the case of monolayer made from chiral molecules the alignment is the same across the entire sample. When magnetic field is applied, large magnetic moment is observed that results from orbital magnetism.

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

Using a dissymmetrically-perturbed particle-in-a-box model, we demonstrate that the induced optical activity of chiral monolayer protected clusters, such as Whetten's Au-28(SG)(16) glutathione-passivated gold nanoclusters (J. Phys. Chem. B, 2000, 104, 2630 2641), could arise from symmetric metal cores perturbed by a dissymmetric or chiral field originating from the adsorbates. This finding implies that the electronic states of the nanocluster core are chiral, yet the lattice geometries of these cores need not be geometrically distorted by the chiral adsorbates. Based on simple chiral monolayer protected cluster models, we rationalize how the adsorption pattern of the tethering sulfur atoms has a substantial effect on the induced CD in the NIR spectral region, and we show how the chiral image charge produced in the core provides a convenient means of visualizing dissymmetric perturbations to the achiral gold nanocluster core.

The study of the effect of self-assembled organic monolayers on the critical current in thin superconducting Nbfilms 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 acetonitrile/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 Bronsted 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 similar to 10 times thicker than those obtained with lower H2O 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

To enable the use of GaAs-based devices as chemical sensors, their surfaces must be chemically modified. Reproducible adsorption of molecules in the liquid phase on the GaAs surfaces requires controlled etching procedures. Several analytical methods were applied, including Fourier transform infrared spectroscopy (FTIRS) in attenuated total reflection and multiple internal reflection mode (ATR/MIR), high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for the analysis of GaAs (100) samples treated with different wet-etching procedures. The assignment of the different features due to surface oxides present in the vibrational and XPS spectra was made by comparison with those of powdered oxides (Ga2O3, As2O3 and As2O5). The etching procedures here described, namely, those using low concentration HF solutions, substantially decrease the amount of arsenic oxides and aliphatic contaminants present in the GaAs (100) surfaces and completely remove gallium oxides. The mean thickness of the surface oxide layer drops from 1.6 nm in the raw sample to 0.1 nm after etching. However, in presence of light, water dissolution of arsenic oxides is enhanced, and oxidized species of gallium cover the surface. Copyright (c) 2005 John Wiley & Sons, Ltd.

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 N-14 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.

A straightforward, two step method for the self-assembly of single-walled carbon nanotubes (SWNTs) between gold electrodes is presented. The technique utilizes the hybridization between short complementary DNA sequences located on metal contacts and SWNTs. The new technique enables simple production of hundreds of devices with high yields. The electrical characteristics are shown to depend strongly on the existence of the chemical binding group, and are controlled by the transport through these groups. The current measured is larger by two orders of magnitude than the values reported for direct metal-SWNT contacts. (C) 2004 Elsevier B.V. All rights reserved.

Recently, unusual giant magnetic properties were found experimentally in some organized organic monolayers adsorbed on solid substrates. A model is presented which explains the observed phenomenon. The model is based on the special properties of electrons transferred from the substrate to the layer as a result of the adsorption process. Triplet pairing of those electrons is forced by the special 2D properties of the organic layer. Such pairs are confined within domains in the organic layer and their quantum statistics provide a model that explains the unique magnetization as well as all other features of the experimental observations. The model suggests the possible existence of Bose-Einstein condensation at room temperature on the scale of the domains.

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.

We report that adsorption of monolayers of organic molecules onto the surface of ferromagnetic semiconductor heterostructures produces large, robust changes in their magnetic properties. The heterostructures have half a monolayer of MnAs embedded in GaAs, 50 Angstrom beneath the surface. The molecules investigated are alkylphosphonic acids that bind to GaAs via a phosphonate group. The organization of the organic monolayer determines the reduction in the Curie temperature, with ordered monolayers producing nearly complete suppression of ferromagnetism. We attribute this striking chemical modulation of magnetic properties to electronic changes brought about by the binding of the molecules to the semiconductor surface. (C) 2003 American Institute of Physics.

Hybrid organic/inorganic devices may find applications as sensors and in futuristic molecular-electronic devices. Here, we demonstrate molecular control of vertical transport in semiconductor superlattices in strong magnetic fields by adsorption of organic molecules onto the sidewalls of a GaAs/AlGaAs device. The molecules have identical attachment groups functionalized by end groups with different electronegativities. For magnetic fields in quantized Hall states, we find that the adsorbate substantially modifies the network of edge states that carries the electrical current. The data indicate that molecules with appropriately chosen end groups can enhance or decrease the vertical conductivity of the edge state system. (C) 2003 Published by Elsevier Ltd.

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 US 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.

The mechanism by which dicarboxylic acid molecules adsorb onto ambient-exposed GaAs (10 0) surfaces is studied by combining infrared spectroscopy and measurements that are sensitive to changes of the electric potential on the surface. For the latter we used the recently developed molecular controlled semiconductor resistor. By comparing the time dependences of the two measurements we conclude that adsorption proceeds sequentially, with virtually all of the rearrangement of electrical charge in the adsorbates taking place when the first carboxylic group, in each molecule, binds to the surface. This can be understood if charge rearrangement is necessary for forming a close-packed adsorbed layer. Since creation of such a layer, that is made up of molecules with significant molecular dipoles, requires some degree of depolarization of the molecules. (C) 2002 Published by Elsevier Science B.V.

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. (C) 2001 Elsevier Science B.V. All rights reserved.

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 monolayer-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 (OOTFs) was investigated. For most systems a strong dependence of the transmission efficiency on temperature was observed, even for a relatively small temperature range. The well defined 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 further elucidate the transmission mechanism. A simple model, which assumes that the electron transmission yield is much higher along the organic chains than in any other direction, was able to reproduce the experimental observations. We find that the photoelectron transmission yield through OOTFs is extremely sensitive to the structural order in the film. (C) 2000 American Institute of Physics. [S0021-9606(00)71241-4].

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. (C) 2000 Academic Press.

The tunneling motion in (HCl)(2) hydrogen bonded dimer and its deuterate was probed by a 2 m long electrostatic hexapole field. The focusing curves of the dimers confirmed the existence of homo and heterodimers in the cluster beam. The homodimer, either (HCl)-Cl-35-(HCl)-Cl-35 Or (HCl)-Cl-37-(HCl)-Cl-37, undergoes a fast tunneling motion for the two hydrogen atoms in the dimer. The heterodimer, namely (HCl)-Cl-35-(HCl)-Cl-37, On the other hand, does not show such fast tunneling motion in the time scale of the experiment. The electric dipole moments for both (DCl)(2) isotopomers were determined to be 1.5 +/- 0.2 D, which is the same value for (HCl)(2). The observed ratio of homo to heterodimer was estimated to be 30 +/- 10, and this value differs largely from the natural abundance for the chlorine isotope. An experimental scheme to discern homo and heterodimers is proposed here. By looking at fragments in the (HCl)(2) dimer photodissociation using a Doppler-selected time-of-flight (TOF) technique, internal energy distribution of the [ClHCl] fragment was measured in 121.6 nm photodissociation. The TOF spectrum consists of fast and slow velocity components for the dissociated H atoms. It is found that the slow H component that arises from the hydrogen escapes after many collisions. The fast H component that arises from the direct H escape without any collision, thus this component reflects an internal and/or electronic state of the counter part fragment, i.e. [ClHCl]. The vibrational structure of [ClHCl] was observed for the fast H component of the TOF spectrum. (C) 2000 Elsevier Science B.V. All rights reserved.

The interaction between water and organic substances is of extreme importance in physical, biological, and geological chemistries. Understanding the interactions between water and organic interfaces is one of the earliest chemical quandaries. In this research, self-assembled monolayers (SAMs) were used as a tool to investigate the interaction between water molecules and hydrophobic surfaces. Real-time adsorption and desorption kinetics of water on hydrophobic SAM surfaces was monitored using a new type of field effect transistor (FET)-like device called MOCSER (molecular controlled semiconductor resistor) coated with SAMs, A quartz crystal microbalance (QCM) was used as a complementary technique to give an estimate of total water mass adsorbed. It is shown that water adsorption depends on relative humidity and is reversible. The amount of adsorbed water increased with surface corrugation. The measurements suggest that adsorption takes place as small water clusters, originating on irregularities on the surface organic layer. Molecular dynamics simulations were carried out to study the interactions of water and hydrophobic surfaces as well. These simulations also suggest the formation of water microdroplets on hydrophobic surfaces, and indicate a strong correlation between increased surface corrugation and adsorption. This paper examines the possible consequences of these interactions on the properties of organic aerosols in the troposphere.

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

Studies of reactions of van der Waals (vdW) molecules or clusters can provide a window through which we can investigate a variety of effects of weak intermolecular interactions on the chemical process of interest. Here we focus on the reactions of oxygen atoms with vdW complexes. These reactions were chosen since the reactions of oxygen atoms with hydrogen halides, saturated hydrocarbons and water molecules have been studied extensively in both the gas and the condensed phases. In addition, the presence of a low-lying excited electronic state, in the oxygen atom, means that its chemistry may be particularly sensitive to perturbations of the environment, for example those introduced by incorporating one or more of the reactants into a vdW complex. In this review we discuss association reactions of oxygen atoms with CO and WO and the reactions of oxygen atoms, both in their ground O(P-3) state and in the excited O(D-1) state, with saturated hydrocarbons, water and with HCl complexes. The studies of the reactions of oxygen atoms with vdW molecules were used to gain insights into generic effects of complex formation on chemical reaction dynamics, reactivity and product state distributions. The results of these studies were summarized in several general rules that describe the effects of the 'environment' on bimolecular reactions.

Synchrotron radiation (SR) pulses are used to eject electrons from a gold substrate covered with organized organic thin films (OOTF) in order to investigate their transmission probability through the OOTF as a function of the electron initial kinetic energy. By variation of the SR photon energy within a few eV above the Au-4f binding energy levels we controlled the initial kinetic energy of the substrate electrons. The observed oscillations in the transmission probability for porphyrin-based films as a function of the kinetic energy is argued to be due to effects of band structure above the vacuum level in the well-ordered molecular adsorbate. We also present valence photoemission spectra (PES) of different type OOTF and demonstrate how their coverage of the substrate affects the PES.

Electron transmission experiments demonstrate a Large asymmetry in the scattering probability of polarized electrons by thin organized films of chiral molecules. This large asymmetry results from the interaction of the electron's wavefunction with many scatterers (molecules) in the organized monolayer structure and represents a manifestation of quantum interference on the scale of supramolecular lengths.

The electron transmission through thin organic films was investigated in order to reveal the effect of the chemical nature of the molecules composing the thin organic layer and the role of the three-dimensional structure of the film on the electron transmission efficiency. By preparing Langmuir-Blodgett layers from two types of amphiphile and a film which is a mixture of both, we could vary the chemical properties of the film and the order in the plane perpendicular to the direction of electron propagation. It was found that the electron transmission yield depends on the chemical nature of the him and varies with the him thickness in a non-trivial way. In the case of the mixed film, the electron transmission is reduced dramatically compared to the films composed from one type of molecule. The results are explained by the long wavelength associated with the low-energy electrons. (C) 1998 Published by Elsevier Science S.A. All rights reserved.

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.

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.

Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy was used to investigate the surface state properties of CdTe crystals before and after the adsorption of specially designed organic molecules. One photon was used to modify the population of the surface states and a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of surface states, relative to the bands, was found to correlate with the relaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron transfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic molecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfaces, with sensitivity exceeding that of most commonly used techniques.

The reaction of O(P-3) with HCl . M (M=HCl, Ar) complexes has been studied. While the monomer HCl, in its ground vibrational state, reacts extremely slow with O(P-3), it is shown here that the van der Waals complexes react with an efficiency of about 3 orders of magnitude larger than that of the monomer. The reactivity of DCl, on the other hand, is not enhanced by the complex formation. Molecular dynamics simulation indicates that the collision complex lifetime increases by several orders of magnitude due to the existence of the ''third body'' in the cluster. A model for explaining the complex induced enhancement of reactivity is presented and is supported by ab initio calculations. (C) 1997 American Institute of Physics.

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

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

The reactions of oxygen atoms in their triple and singlet states with hydrocarbon clusters were investigated. The results indicate that some liquid‐type 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, spin‐orbit, and Λ‐doubling state populations were analyzed, on‐statistical distributions are observed even for the reaction of large methane clusters. The results of these studies are discussed in terms of non‐adiabatic effects induced by the long lived collision complex.