Publications
2021
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(2021) Chirality. 2, p. 93-102 Abstract
Monolayers of chiral molecules can preferentially transmit electrons with a specific spin orientation, introducing chiral molecules as efficient spin filters. This phenomenon is established as chirality-induced spin selectivity (CISS) and was demonstrated directly for the first time in self-assembled monolayers (SAMs) of double-stranded DNA (dsDNA)1. Here, we discuss SAMs of double-stranded peptide nucleic acid (dsPNA) as a system which allows for systematic investigations of the influence of various molecular properties on CISS. In photoemission studies, SAMs of chiral, γ-modified PNA show significant spin filtering of up to P = (24.4 ± 4.3)% spin polarization. The polarization values found in PNA lacking chiral monomers are considerably lower at about P = 12%. The results confirm that the preferred spin orientation is directly linked to the molecular handedness and indicate that the spin filtering capacity of the dsPNA helices might be enhanced by introduction of chiral centers in the constituting peptide monomers.
(2021) Chemistry - A European Journal. 27, 1, p. 298-306 AbstractSquaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co-assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co-assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin-filtering measurements using mc-AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart.
2020
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(2020) Nano Letters. 20, 12, p. 8675-8681 Abstract
The technological advancement of data storage is reliant upon the continuous development of faster and denser memory with low power consumption. Recent progress in flash memory has focused on increasing the number of bits per cell to increase information density. In this work an optical multilevel spin bit, based on the chiral induced spin selectivity (CISS) effect, is developed using nanometer sized chiral quantum dots. A double quantum dot architecture is adsorbed on the active area of a Ni based Hall sensor and a nine-state readout is achieved.
(2020) Journal of the American Chemical Society. 142, 48, p. 20456-20462 AbstractA 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.
(2020) Physical Chemistry Chemical Physics. 22, 38, p. 21570-21582 AbstractEssential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.
(2020) Accounts of Chemical Research. 11, p. 2659-2667 AbstractThe 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.
(2020) ACS Nano. 12, p. 16624-16633 AbstractRoom-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal–organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above ∼50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.
(2020) Journal of Physical Chemistry C. 124, 38, p. 20974-20980 AbstractThe 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.
(2020) Angewandte Chemie - International Edition. 59, 34, p. 2-8 AbstractSpin dependent conduction and polarization in chiral polymers were studied, when the polymers are organized as self‐assembled monolayers and the conduction is along the polymer backbone, namely along its longer axis. Large spin polarization and magnetoresistance effects were observed, which showed a clear dependence on the secondary structure of the polymer. The results indicate that the spin polarization process does not include spin flipping and hence it results from backscattering probabilities for the two spin states.
(2020) Symmetry. 12, 5, 808. AbstractThe 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.
(2020) Journal of Physical Chemistry C. 124, 19, p. 10776-10782 AbstractThe effect of spin polarization in conduction and electric field-induced polarization was measured for double-stranded DNA oligonucleotides and oligopeptides of different lengths. The measurements were conducted using magnetic contact AFM, spin-dependent electrochemistry, spin-dependent polarization, and magnetoresistance studies. It was established that the spin-dependent conduction through chiral molecules depends on the voltage applied with a power of d, when d is larger than unity, and that there is a different voltage threshold for conducting each of the spin polarizations. In addition, there is no spin flipping during the conduction through the chiral system. The spin polarization depends linearly on the length, within the range of lengths studied, and it seems to scale like the optical activity. These results suggest the importance of the electric polarizability in the chiral-induced spin selectivity process.
(2020) Journal of Physical Chemistry Letters. 11, 9, p. 3660-3666 AbstractThis 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.
(2020) Journal of Physical Chemistry Letters. 11, 4, p. 1550-1557 AbstractKelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet's magnetization direction and the molecules' chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.
(2020) Advanced Materials. 7, 1904965. AbstractOrganic 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.
(2020) Physical Review B. 101, 2, 026403. AbstractIn a model presented in Yang et al. [Phys. Rev. B 99, 024418 (2019)10.1103/PhysRevB.99.024418] it was stated that in the linear regime spin selectivity cannot be measured when using two contacts configuration, and that the results observed when studying the chiral-induced spin-selectivity effect are not consistent with this statement. Here we claim that the results cited clearly indicate nonlinearity and therefore the spin selectivity observed is valid observation.
(2020) Physical Chemistry Chemical Physics. 22, 3, p. 997-1002 AbstractChirality-induced spin selectivity is evidenced by exciting the spin resonance of radicals in an electrochemical cell where the working electrode is covered with a chiral self-assembled monolayer. Because the electron transfer to and from the paramagnetic radical is spin dependent, the electrochemical current changes at resonance. This electrically-detected magnetic resonance (EDMR) is monitored by a lock-in detection based on electrode voltage modulation, at a frequency that optimizes the sensitivity of the differential conductance to the electrode charge transfer process. The method is validated using p-doped GaAs electrodes in which the conduction band electrons are hyperpolarized by a well-known method of optical spin pumping with circularly polarized light. Gold electrodes covered with peptides consisting of 5 alanine groups (Al5) present a relative current change of up to 5 x 10(-5) when the resonance condition is met, corresponding to a spin filtering efficiency between 6 and 19%.
2019
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(2019) Angewandte Chemie - International Edition. 58, 4, p. 1653-1658 Abstract
We show that enantioselective reactions can be induced by the electron spin itself and that it is possible to replace a conventional enantiopure chemical reagent by spin-polarized electrons that provide the chiral bias for enantioselective reactions. Three examples of enantioselective chemistry resulting from electron-spin polarization are presented. One demonstrates the enantioselective association of a chiral molecule with an achiral self-assembled monolayer film that is spin-polarized, while the other two show that the chiral bias provided by the electron helicity can drive both reduction and oxidation in enantiospecific electrochemical reactions. In each case, the enantioselectivity does not result from enantiospecific interactions of the molecule with the ferromagnetic electrode but from the polarized spin that crosses the interface between the substrate and the molecule. Furthermore, the direction of the electron-spin polarization defines the handedness of the enantioselectivity. This work demonstrates a new mechanism for realizing enantioselective chemistry.
(2019) Journal of the American Chemical Society. 141, 49, p. 19198-19202 AbstractMultiheme 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.
(2019) Journal Of Physical Chemistry B. 123, 44, p. 9443-9448 AbstractThe effect of an electric field on the adsorption of oligopeptides and DNA on a ferromagnetic substrate magnetized perpendicular to the surface was investigated. The direction of the magnetic moment of the substrate defines different adsorption rates for different enantiomers, and the direction of the electric field, perpendicular to the surface, defines different adsorption rates depending on the direction of the dipole moment of the adsorbed molecules. ©
(2019) Journal of the American Chemical Society. 141, 37, p. 14707-14711 AbstractSpin 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.
(2019) Applied Physics Letters. 115, 13, 133701. AbstractChiral symmetry is ubiquitous in Biology, Physics, and Chemistry. The biomolecules essential for life on Earth-such as deoxyribonucleic acid (DNA), sugars, and proteins-display homochirality that affects their function in biological processes. Ten years ago, it was discovered that electron transfer through chiral molecules depends on the direction of the electron spin, and more recently, it was shown that the charge displacement in chiral molecules creates transient spin polarization. Thus, the properties of ferromagnet/chiral molecule interfaces are affected by spin exchange interactions, via the overlap of the chiral molecule with the ferromagnet's spin wave function. This effect offers a mechanism for homochiral bias in Biology, which was previously unappreciated, and an approach to enantioselective chemistry and chiral separations, which is controlled by the electron spin.
(2019) Journal of Physical Chemistry C. 123, 27, p. 17043-17048 AbstractThe 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.
(2019) ACS Nano. 13, 5, p. 4928-4946 AbstractOptical control and readout of electron spin and spin currents in thin films and nanostructures have remained attractive yet challenging goals for emerging technologies designed for applications in information processing and storage. Recent advances in room-temperature spin polarization using nanometric chiral molecular assemblies suggest that chemically modified surfaces or interfaces can be used for optical spin conversion by exploiting photoinduced charge separation and injection from well-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by molecules with central or helical chirality enables indirect measurements of spin polarization attributed to the chiral-induced spin selectivity effect and of the efficiency of spin-dependent electron transfer relative to competitive relaxation pathways. Herein, we highlight recent approaches used to detect and to analyze spin selectivity in photoinduced charge transfer including spin-transfer torque for local magnetization, nanoscale charge separation and polarization, and soft ferromagnetic substrate magnetization- and chirality-dependent photoluminescence. Building on these methods through systematic investigation of molecular and environmental parameters that influence spin filtering should elucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.
(2019) Chemical Science. 10, 20, p. 5246-5250 AbstractEnantiospecific 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.
(2019) Nature Reviews Chemistry. 3, 4, p. 250-260 AbstractThe 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.
(2019) Proceedings Of The National Academy Of Sciences Of The United States Of America-Physical Sciences. 116, 13, p. 5931-5936 AbstractBiological structures rely on kinetically tuned charge transfer reactions for energy conversion, biocatalysis, and signaling as well as for oxidative damage repair. Unlike man-made electrical circuitry, which uses metals and semiconductors to direct current flow, charge transfer in living systems proceeds via biomolecules that are nominally insulating. Long-distance charge transport, which is observed routinely in nucleic acids, peptides, and proteins, is believed to arise from a sequence of thermally activated hopping steps. However, a growing number of experiments find limited temperature dependence for electron transfer over tens of nanometers. To account for these observations, we propose a temperature-independent mechanism based on the electric potential difference that builds up along the molecule as a precursor of electron transfer. Specifically, the voltage changes the nature of the electronic states away from being sharply localized so that efficient resonant tunneling across long distances becomes possible without thermal assistance. This mechanism is general and is expected to be operative in molecules where the electronic states densely fill a wide energy window (on the scale of electronvolts) above or below the gap between the highest-occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). We show that this effect can explain the temperature-independent charge transport through DNA and the strongly voltage-dependent currents that are measured through organic semiconductors and peptides.
(2019) Journal of Physical Chemistry Letters. 10, 5, p. 1139-1145 AbstractWe 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.
(2019) Journal of the American Chemical Society. 141, 1, p. 123-126 AbstractA Hall device was used for measuring spin polarization on electrons that are either reorganized within the molecules or transmitted through the self-assembled monolayers of DNA adsorbed on the device surface. We were able to observe spin-dependent charge polarization and charge transport through double stranded DNA of various lengths and through double stranded DNA containing oxidative damage. We found enhancement in the spin-dependent transport through oxidatively damaged DNA. This phenomenon can be rationalized either by assuming that the damaged DNA is characterized by a higher barrier for conduction or by charge transfer through the DNA being conducted through at least two channels, one involves the bases and is highly conductive but less spin selective, while the other pathway is mainly through the ribophosphate backbone and it is the minor one in terms of charge transmission efficiency, but it is highly spin selective.
(2019) Journal of Physical Chemistry C. 123, 5, p. 3024-3031 AbstractThis work demonstrates the chiral-induced spin selectivity effect for inorganic copper oxide films and exploits it to enhance the chemical selectivity in electrocatalytic water splitting. Chiral CuO films are electrodeposited on a polycrystalline Au substrate, and their spin filtering effect on electrons is demonstrated using Mott polarimetry analysis of photoelectrons. CuO is known to act as an electrocatalyst for the oxygen evolution reaction; however, it also generates side products such as H2O2. We show that chiral CuO is selective for O-2; H2O2 generation is strongly suppressed on chiral CuO but is present with achiral CuO. The selectivity is rationalized in terms of the electron spin-filtering properties of the chiral CuO and the spin constraints for the generation of triplet oxygen. These findings represent an important step toward the development of all-inorganic chiral materials for electron spin filtering and the creation of efficient, spin-selective (photo)electrocatalysts for water splitting.
(2019) Small. 15, 1, 1804557. AbstractThe 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.
2018
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(2018) Advanced Materials. 30, 41, 1707390. Abstract
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.
(2018) Accounts of Chemical Research. 51, 10, p. 2565-2573 AbstractCONSPECTUS: The optical and electronic properties of semiconductor quantum dots (QDs) make them attractive candidates for applications in photovoltaics, spintronics, photocatalysis, and optoelectronics. Understanding how to control the flow of charge in QD assemblies is essential for realizing novel applications. This Account explores some unique characteristics of charge transport in QD dyads, triads, and their assemblies. The emerging features of these assemblies that provide new opportunities to manipulate charge flow at the nanoscale are (1) cascading energy landscapes and band offsets to inhibit charge recombination, (2) electrostatic fields that direct charge flow through QD-QD and QD-conjugated polymer junctions, and (3) QD chirality and chiral imprinting that promotes vectorial electron and spin selective transport.Charge flow kinetics is determined by a combination of familiar electron transfer parameters (reaction free energy, reorganization energy, and electronic coupling), donor and acceptor electronic densities of states, and internal electric fields. Electron transfer and electronic structure theory, combined with kinetic modeling, place the measured kinetics of QD electron transfer donor-acceptor assemblies into a unified conceptual context. The experimental transfer rates measured in these systems depend upon structure and the internal electric fields that are present in the assemblies. A negatively charged donor and positively charged acceptor, for example, facilitates (inhibits) electron (hole) transfer, while an electric field of opposite orientation (reversal of charges) inhibits (promotes) electron (hole) transfer. These and other emerging rules that govern charge flow in NP assemblies provide a strategy to design the directionality and yield of interfacial charge transport.Chirality at the nanoscale can induce spin selective charge transport, providing new ways to direct charge (and spin) flow in QD assemblies. Magnetoresistance and magnetic conductive probe atomic force microscopy experiments show spin selective electron transport for chirally imprinted QD assemblies. Photoinduced electron transfer from achiral donor-QDs to chiral acceptor-QDs depends on the electron spin and chiroptical properties of the acceptor-QDs. These assemblies show transport characteristics that correlate with features of the QDs' circular dichroism spectra, presenting intriguing challenges to theory, and indicating that spectroscopic signatures may assist in the design and diagnosis of functional molecular assemblies.Theoretical and experimental studies of charge transport in well-defined QD assemblies are establishing design principles for vectorial charge transport and are also refining questions surrounding the mechanism and control of these processes. These intensified efforts are forging links between fundamental discoveries regarding mechanism and practical applications for these novel assembled nanostructures.
(2018) ACS Energy Letters. 3, 10, p. 2308-2313 AbstractPhotoelectrochemical (PEC) water splitting is a promising approach for generating hydrogen from water. In order to enhance PEC water splitting efficiency, it is essential to inhibit the production of the hydrogen peroxide byproduct and to reduce the overpotential required by an inexpensive catalyst and with high current density. In the past, it was shown that coating TiO2 electrodes by chiral molecules or chiral films enhances the hydrogen production and reduces the production of H2O2 byproduct. This was explained to be a result of the chiral-induced spin selectivity (CISS) effect that induces spin correlation between the electrons transferred to the anode. However, typically the current observed in those studies was in the range of 1-100 mu A/cm(2). Here we report currents in the range of 10 mA/cm(2) obtained by adsorbing chiral molecules on a well-established Fe3O4 nanoparticle (NP) catalyst deposited on the anode. The results indicate a new strategy for designing low-cost earth-abundant catalysts where the advantages of the CISS effect are combined with the large effective area provided by the NPs to promote PEC water splitting with high current density.
(2018) Science. 360, 6395, p. 1331-1334 AbstractIt 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.
(2018) Small. 14, 30, 1801249. AbstractThere 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.
(2018) Chimia. 72, 6, p. 394-398 AbstractThis review describes a new perspective on the role that electron spin plays in the intermolecular forces between two chiral molecules and between chiral molecules and surfaces. This different role of the spin arises from the chiral induced spin selectivity (CISS) effect which is manifest when electrons are moving in chiral molecules. Namely, it has been shown that as chiral molecules are charge polarized, the electron displacement is accompanied by spin polarization. The spin direction associated with each electric pole depends on the specific handedness of the molecule. Thus, the consideration of the dispersive forces between two molecules, or between a molecule and a substrate, must include the spin polarization which adds an enantioselective electronic term to the interaction potential. We review recent experiments that show the relation between charge polarization and spin polarization in chiral molecules. The spin polarization also affects the direction of the ferromagnetic substrate magnetic moment of a surface, upon which the chiral molecules are adsorbed.
(2018) Advanced Materials. 30, 21, 1706423. AbstractIt is shown that “spontaneous magnetization” occurs when chiral oligopeptides are attached to ferrocene and are self-assembled on a gold substrate. As a result, the electron transfer, measured by electrochemistry, shows asymmetry in the reduction and oxidation rate constants; this asymmetry is reversed between the two enantiomers. The results can be explained by the chiral induced spin selectivity of the electron transfer. The measured magnetization shows high anisotropy and the “easy axis” of magnetization is along the molecular axis.
(2018) ACS Nano. 12, 4, p. 3892-3897 AbstractSpin-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.
(2018) World Scientific Reference on Spin in Organics. p. 235-270 (Materials and Energy). AbstractThis chapter reviews the chiral induced spin selectivity (CISS) effect and its utilization for creating biomolecular and molecular scale analogs of spintronic devices. The spin filtering properties of biomolecules and systems, especially the purple membrane that contains the bacteriorhodopsin system and their relation to spintronic device features is emphasized. Among the methods described are a new spindependent electrochemistry and opto-electronic control over spin transfer, opto-spintronics. The work presented indicates that chiral molecules can serve as a new material for spintronics and they can replace ferromagnets as spin injectors. It has been demonstrated that the spin filtering through a single monolayer of the chiral molecules is efficient and produces a ratio of 4:1 in the transmission between the preferred and unpreferred spin.
(2018) Physical Chemistry Chemical Physics. 20, 2, p. 1091-1097 AbstractWe discuss spin injection and spin valves, which are based on organic and biomolecules, that offer the possibility to overcome some of the limitations of solid-state devices, which are based on ferromagnetic metal electrodes. In particular, we discuss spin filtering through bacteriorhodopsin in a solid state biomolecular spin valve that is based on the chirality induced spin selectivity (CISS) effect and shows a magnetoresistance of similar to 2% at room temperature. The device is fabricated using a layer of bacteriorhodopsin (treated with n-octyl-thioglucoside detergent: OTG-bR) that is adsorbed on a cysteamine functionalized gold electrode and capped with a magnesium oxide layer as a tunneling barrier, upon which a Ni top electrode film is placed and used as a spin analyzer. The bR based spin valves show an antisymmetric magnetoresistance response when a magnetic field is applied along the direction of the current flow, whereas they display a positive symmetric magnetoresistance curve when a magnetic field is applied perpendicular to the current direction.
2017
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(2017) ACS Nano. 11, 12, p. 12713-12722 Abstract
We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.
(2017) ACS Omega. 2, 12, p. 8550-8556 AbstractOver 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.
(2017) Sensors And Actuators A-Physical. 267, p. 1-7 AbstractThe integration of nanostructures in electronic devices utilizes their unique quantum properties for realizing discrete measuring systems. Specifically, self-assembled organic monolayers and nanocrystals (NCs), together with bottom-up production methods, can lead to new types of electronic devices. In this work, we present a wavelength-tunable near-infrared detection device in which PbS NCs are used to create an optical gate for an AlGaAs/GaAs high electron mobility device. By integrating side gates, we were able to enhance light detection sensitivity by optimizing the conductivity of the channel. Both DC and AC modulations of the side gate were tested and compared in order to enhance the detector's signal-to-noise ratio (SNR). Higher harmonic signals of the side gate modulation supply additional information about the detection mechanism. (C) 2017 Elsevier B.V. All rights reserved.
(2017) Angewandte Chemie - International Edition. 56, 46, p. 14587-14590 AbstractElectron 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.
(2017) Current Opinion in Electrochemistry. 4, 1, p. 175-181 AbstractRecent developments in our understanding of charge and spin transport through nucleic acid duplexes are discussed. Particular emphasis is placed on recent findings that point to the importance of nucleobase fluctuations, transient but extended length-scale electronic coherence, and chiral-induced spin selectivity.
(2017) Journal of Physical Chemistry C. 121, 29, p. 15777-15783 AbstractEfficient 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.
(2017) Journal Of Physics-Condensed Matter. 29, 10, 103002. AbstractWe review a recently discovered phenomenon, the chiral induced spin selectivity (CISS) effect, that can enable a new technology for the injection of spin polarized current without the need for a permanent magnetic layer. The effect occurs in chiral molecules and systems without parity symmetry, i.e. systems that do not have inversion symmetry. The theoretical foundations for the effect are presented first and then followed by several examples of spin valves that are based on chiral systems. The CISS-based spin valves introduce the possibility to inject spin current without the use of a permanent magnet and to achieve relatively large magnetoresistance at room temperature.
(2017) J. Chem. Phys.. 146, 9, 092302. AbstractThe 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
(2017) Proceedings Of The National Academy Of Sciences Of The United States Of America-Physical Sciences. 114, 10, p. 2474-2478 AbstractNoncovalent interactions between molecules are key for many biological processes. Necessarily, when molecules interact, the electronic charge in each of them is redistributed. Here, we show experimentally that, in chiral molecules, charge redistribution is accompanied by spin polarization. We describe how this spin polarization adds an enantioselective term to the forces, so that homochiral interaction energies differ from heterochiral ones. The spin polarization was measured by using a modified Hall effect device. An electric field that is applied along the molecules causes charge redistribution, and for chiral molecules, a Hall voltage is measured that indicates the spin polarization. Based on this observation, we conjecture that the spin polarization enforces symmetry constraints on the biorecognition process between two chiral molecules, and we describe how these constraints can lead to selectivity in the interaction between enantiomers based on their handedness. Model quantum chemistry calculations that rigorously enforce these constraints show that the interaction energy for methyl groups on homochiral molecules differs signif-icantly from that found for heterochiral molecules at van der Waals contact and shorter ( i. e., similar to 0.5 kcal/mol at 0.26 nm).
(2017) Nature Communications. 8, 14567. AbstractFerromagnets are commonly magnetized by either external magnetic fields or spin polarized currents. The manipulation of magnetization by spin-current occurs through the spin-transfer-torque effect, which is applied, for example, in modern magnetoresistive random access memory. However, the current density required for the spin-transfer torque is of the order of 1 x 10(6) A . cm(-2), or about 1 x 10(25) electrons s(-1) cm(-2). This relatively high current density significantly affects the devices' structure and performance. Here we demonstrate magnetization switching of ferromagnetic thin layers that is induced solely by adsorption of chiral molecules. In this case, about 10(13) electrons per cm(2) are sufficient to induce magnetization reversal. The direction of the magnetization depends on the handedness of the adsorbed chiral molecules. Local magnetization switching is achieved by adsorbing a chiral self-assembled molecular monolayer on a gold-coated ferromagnetic layer with perpendicular magnetic anisotropy. These results present a simple low-power magnetization mechanism when operating at ambient conditions.
(2017) Journal of the American Chemical Society. 139, 7, p. 2794-2798 AbstractThe 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.
(2017) IEEE Sensors Journal. 17, 3, p. 660-666 AbstractIn clinical applications, inspection of the hemorrhagic spots in the gastrointestinal (GI) tract is a challenging task. The hybrid GaAs-based device, developed by us, has been successfully used for sensing hemoglobin (Hb) in biological solutions. In this paper, a new method and apparatus were applied for using the GaAs-based sensor to detect Hb in a mimicked gastrointestinal circumstance. A surface protection layer of polymerized thiolated silanes was deposited on the top of the device to obtain a chemical passivation coating against surface etching and to achieve bio-compatibility. A selective device functionalization was achieved by the subsequent adsorption of Hb antibodies on the top of the protection layer. An integrated sensor containing two different antibodies enabled examining its selectivity to purely Hb. In vitro testing of the sensor indicated that it is capable of discriminating fasted-state simulated intestinal fluids when the concentration of Hb was above 10 mu g/mL. Moreover, the sensor was capable of detecting Hb in swine intestinal fluids with the same sensitivity. This paper verified the feasibility to apply the sensor in clinical use, by embedding it on different types of endoscopes, in order to localize GI bleeding with high precision and sensitivity.
2016
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(2016) Accounts of Chemical Research. 49, 11, p. 2560-2568 Abstract
CONSPECTUS: Molecular spintronics (spin + electronics), which aims to exploit both the spin degree of freedom and the electron charge in molecular devices, has recently received massive attention. Our recent experiments on molecular spintronics employ chiral molecules which have the unexpected property of acting as spin filters, by way of an effect we call "chiral-induced spin selectivity" (CISS). In this Account, we discuss new types of spin dependent electrochemistry measurements and their use to probe the spin-dependent charge transport properties of nonmagnetic chiral conductive polymers and biomolecules, such as oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin (bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures. Spin-dependent electrochemical measurements were carried out by employing ferromagnetic electrodes modified with chiral molecules used as the working electrode. Redox probes were used either in solution or when directly attached to the ferromagnetic electrodes. During the electrochemical measurements, the ferromagnetic electrode was magnetized either with its magnetic moment pointing "UP" or "DOWN" using a permanent magnet (H = 0.5 T), placed underneath the chemically modified ferromagnetic electrodes. The spin polarization of the current was found to be in the range of 5-30%, even in the case of small chiral molecules. Chiral films of the L- and D-cysteine tethered with a redox-active dye, toludin blue O, show spin polarizarion that depends on the chirality. Because the nickel electrodes are susceptible to corrosion, we explored the effect of coating them with a thin gold overlayer. The effect of the gold layer on the spin polarization of the electrons ejected from the electrode was investigated. In addition, the role of the structure of the protein on the spin selective transport was also studied as a function of bias voltage and the effect of protein denaturation was revealed. In addition to "dark" measurements, we also describe photoelectrochemical measurements in which light is used to affect the spin selective electron transport through the chiral molecules. We describe how the excitation of a chromophore (such as CdSe nanoparticles), which is attached to a chiral working, electrode, can flip the preferred spin orientation of the photocurrent, when measured under the identical conditions. Thus, chirality-induced spin polarization, when combined with light and magnetic field effects, opens new avenues for the study of the spin transport properties of chiral molecules and biomolecules and for creating new types of spintronic devices in which light and molecular chirality provide new functions and properties.
(2016) Nano Letters. 16, 7, p. 4583-4589 AbstractThis 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.
(2016) Nano Letters. 16, 4, p. 2806-2811 AbstractThe combination of photonics and spintronics opens new ways to transfer and process information. It is shown here that in systems in which organic molecules and semiconductor nanoparticles are combined, matching these technologies results in interesting new phenomena. We report on light induced and spin-dependent charge transfer process through helical oligopeptide-CdSe nanoparticles' (NPs) architectures deposited on ferromagnetic substrates with small coercive force (100-200 Oe). The spin control is achieved by the application of the chirality-induced spin-dependent electron transfer effect and is probed by two different methods: spin-controlled electrochemichemistry and photoluminescence (PL) at room temperature. The injected spin could be controlled by excitation of the nanoparticles. By switching the direction of the magnetic field of the substrate, the PL intensity could be alternated.
(2016) ACS Nano. 10, 4, p. 4525-4531 AbstractSpin-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.
(2016) Advanced Materials. 28, 10, p. 1957-1962 AbstractChiral helicene, a fully conjugated system without stereogenic carbon, can filter spins effectively at room temperature, a consequence of the chiral-induced spin-selectivity effect. The chirality dictates the spin of the electrons transferred through helicene, and magnetoresistance devices based on these molecules show antisymmetric magnetoresistance versus H plots.
(2016) Nature Communications. 7, 10744. AbstractChirality-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.
(2016) Physical Review B. 93, 7, 75407. AbstractRecent experiments have demonstrated the efficacy of chiral helically shaped molecules in polarizing the scattered electron spin, an effect termed chiral-induced spin selectivity. Here we solve a simple tight-binding model for electron transport through a single helical molecule, with spin-orbit interactions on the bonds along the helix. Quantum interference is introduced via additional electron hopping between neighboring sites in the direction of the helix axis. When the helix is connected to two one-dimensional single-mode leads, time-reversal symmetry prevents spin polarization of the outgoing electrons. One possible way to retrieve such a polarization is to allow leakage of electrons from the helix to the environment, via additional outgoing leads. Technically, the leakage generates complex site self-energies, which break unitarity. As a result, the electron waves in the helix become evanescent, with different decay lengths for different spin polarizations, yielding a net spin polarization of the outgoing electrons, which increases with the length of the helix (as observed experimentally). A maximal polarization can be measured at a finite angle away from the helix axis.
(2016) Sensors. 1, 2, p. 185-189 AbstractWe developed and investigated the properties of molecularly controlled semiconductor resistors (MOCSERs) based on AIGaN/GaN structure. The response of the sensor for two different analytes was investigated when the sensor was coated with two molecules that differ only in their binding groups. We studied the ability to enhance the specificity of the sensor by adding illumination at various sub-bandgap frequencies. It was verified that the sensor is sensitive to the electronegativity of the analyte and illumination can affect the sensitivity and selectivity when the system does not reach a steady state. Hence, we differ between two operational modes in which orthogonal sensing is made.
2015
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(2015) The journal of physical chemistry letters. 6, 24, p. 4916-22 26615833. Abstract
We show that in an electrochemical cell, in which the photoanode is coated with chiral molecules, the overpotential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. The hydrogen evolution efficiency is studied comparing seven different organic molecules, three chiral and four achiral. We propose that the spin specificity of electrons transferred through chiral molecules is the origin of a more efficient oxidation process in which oxygen is formed in its triplet ground state. The new observations are consistent with recent theoretical works pointing to the importance of spin alignment in the water-splitting process.
(2015) Journal of Physical Chemistry C. 119, 28, p. 15839-15845 AbstractThis 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.
(2015) Proceedings of the National Academy of Sciences of the United States of America. 112, 25, p. E3310-E3310 AbstractCorrection 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.”
(2015) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 54, 25, p. 7295-7298 AbstractThe search to understand the origin of homochirality in nature has been ongoing since the time of Pasteur. Previous work has shown that DNA can act as a spin filter for low-energy electrons and that spin-polarized secondary electrons produced by X-ray irradiation of a magnetic substrate can induce chiral selective chemistry. In the present work it is demonstrated that secondary electrons from a substrate that are transmitted through a chiral overlayer cause enantiomeric selective chemistry in an adsorbed adlayer. We determine the quantum yields (QYs) for dissociation of (R)-or (S)-epichlorohydrin adsorbed on a chiral self-assembled layer of DNA on gold and on bare gold (for control). The results show that there is a significant difference in the QYs between the two enantiomers when adsorbed on DNA, but none when they are adsorbed on bare Au. We propose that the effect results from natural spin filtering effects cause by the chiral monolayer.
(2015) Proceedings of the National Academy of Sciences of the United States of America. 112, 19, p. E2419-E2428 AbstractWe design sensors where information is transferred between the sensing event and the actuator via quantum relaxation processes, through distances of a few nanometers. We thus explore the possibility of sensing using intrinsically quantum mechanical phenomena that are also at play in photobiology, bioenergetics, and information processing. Specifically, we analyze schemes for sensing based on charge transfer and polarization (electronic relaxation) processes. These devices can have surprising properties. Their sensitivity can increase with increasing separation between the sites of sensing (the receptor) and the actuator (often a solid-state substrate). This counterintuitive response and other quantum features give these devices favorable characteristics, such as enhanced sensitivity and selectivity. Using coherent phenomena at the core of molecular sensing presents technical challenges but also suggests appealing schemes for molecular sensing and information transfer in supramolecular structures.
(2015) Annual Review of Physical Chemistry. 66, 1, p. 263-281 AbstractRecent 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.
(2015) Langmuir. 31, 11, p. 3546-3552 AbstractThis article reports on a facile and fast strategy for the self-assembled monolayer (SAM) functionalization of nickel surfaces, employing cyclic voltammetry (CV) cycling of a suitable tailored solution containing the species to be adsorbed. Results are presented for ultrathin films formed on Ni by 1-hexadecanethiol (C16), l-cysteine (l-cys), and the poly{methyl (2R)-3-(2,2'-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate} (PCT-L) thiophene-based chiral polymer. The effective formation of high-quality ultrathin organic films on the nickel was verified both electrochemically and by exploiting typical surface characterization techniques such as contact angle, ellipsometry, atomic force microscopy (AFM), polarization modulation-infrared reflection-absorption spectroscopy (PMIRRAS), and X-ray photoelectron spectroscopy (XPS).
(2015) ACS Nano. 9, 3, p. 3377-3384 AbstractThis 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.
(2015) Nano Letters. 15, 2, p. 1052-1056 AbstractThe role of the electron spin in chemistry and biology has received much attention recently owing to to the possible electromagnetic field effects on living organisms and the prospect of using molecules in the emerging field of spintronics. Recently the chiral-induced spin selectivity effect was observed by electron transmission through organic molecules. In the present study, we demonstrated the ability to control the spin filtering of electrons by light transmitted through purple membranes containing bacteriorhodopsin (bR) and its D96N mutant. The spin-dependent electrochemical cyclic voltammetry (CV) and chronoamperometric measurements were performed with the membranes deposited on nickel substrates. High spin-dependent electron transmission through the membranes was observed; however, after the samples were illuminated by 532 nm light, the spin filtering in the D96N mutant was dramatically reduced whereas the light did not have any effect on the wild-type bR. Beyond demonstrating spin-dependent electron transmission, this work also provides an interesting insight into the relationship between the structure of proteins and spin filtering by conducting electrons.
(2015) AbstractCreation 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.
(2015) Advanced Materials. 27, 11, p. 1924-1927 AbstractChiral conducting polymer films are self‐assembled on ferromagnetic materials and exhibit an efficient spin filtering at room temperature. In addition, the present system displays asymmetric magnetoresistance with respect to zero magnetic field.
(2015) The Journal of Physical Chemistry C. 119, 26, p. 14542–14547 AbstractWe 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.
Chiral supramolecular structures as spin filters(2015) RSC Smart Materials. 2015-January, 12, p. 203-225 Abstract2014
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(2014) Applied Physics Letters. 105, 24, Abstract
Spin injection into solid-state devices is commonly performed by use of ferromagnetic metal electrodes. Here, we present a spin injector design without permanent magnet; rather, the spin selectivity is determined by a chiral tunneling barrier. The chiral tunneling barrier is composed of an ultrathin Al2O3 layer that is deposited on top of a chiral self-assembled monolayer (SAM), which consists of cysteine or oligopeptide molecules. The experimentally observed magnetoresistance can be up to 20% at room temperature, and it displays an uncommon asymmetric curve as a function of the applied magnetic field. These findings show that the spin injector transmits only one spin orientation, independent of external magnetic field. The sign of the magnetoresistance depends on the handedness of the molecules in the SAM, which act as a spin filter, and the magnitude of the magnetoresistance depends only weakly on temperature. (C) 2014 AIP Publishing LLC.
(2014) Advanced Functional Materials. 24, 37, p. 5833-5840 AbstractA 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.
(2014) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 53, 34, p. 8953-8958 AbstractPhotosystem I (PSI) is one of the most studied electron transfer (ET) systems in nature; it is found in plants, algae, and bacteria. The effect of the system structure and its electronic properties on the electron transfer rate and yield was investigated for years in details. In this work we show that not only those system properties affect the ET efficiency, but also the electrons' spin. Using a newly developed spintronic device and a technique which enables control over the orientation of the PSI monolayer relative to the device (silver) surface, it was possible to evaluate the degree and direction of the spin polarization in ET in PSI. We find high-spin selectivity throughout the entire ET path and establish that the spins of the electrons being transferred are aligned parallel to their momenta. The spin selectivity peaks at 300 K and vanishes at temperatures below about 150 K. A mechanism is suggested in which the chiral structure of the protein complex plays an important role in determining the high-spin selectivity and its temperature dependence. Our observation of high light induced spin dependent ET in PSI introduces the possibility that spin may play an important role in ET in biology.