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Naaman, Ron

Publications and source records attributed to Naaman, Ron.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Spin-Dependent Electrochemistry

Recent experiments that use spin polarized electron currents to improve the selectivity and enantioselectivity of electrochemical reactions are described. Electrochemical studies with both chiral working electrodes and with magnetic working electrodes are used to generate spin polarized electron currents. Studies with chiral electrodes, which are formed through the adsorption of chiral molecules and through the growth of chiral oxide films, are shown to improve the efficiency and selectivity of the oxygen evolution reaction and oxygen reduction reaction. These improvements are attributed to the exploitation of spin selection rules (triplet vs. singlet pathways) in the reaction mechanism. Studies with magnetic electrodes, which are formed through ferromagnetic/Au thin film electrodes, are reported to initiate enantioselective chemical transformations, and three different cases are discussed here (enantioresolution of a racemic mixture, enantioselective polymer grafting on electrode surfaces, and the generation of achiral carbon centers). The addition of spin control to the conventional control over charge currents provides an added dimension to electrochemistry and electrosynthesis.

Waldeck, David H↗
DOE OSTI2024

Insights into the Mechanism of Chiral‐Induced Spin Selectivity: The Effect of Magnetic Field Direction and Temperature

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

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electron Spin Polarization and Rectification Driven by Chiral Perylene Diimide-Based Nanodonuts

The chirality-induced spin selectivity (CISS) effect allows thin-film layers of chiral conjugated molecules to function as spin filters at ambient temperature. Through solvent-modulated dropcasting of chiral l- and d-perylene diimide (PDI) monomeric building blocks, two types of aggregate morphologies, nanofibers and nanodonuts, may be realized. Spin-diode behavior is evidenced in the nanodonut structures. Stacked PDI units, which form the conjugated core of these nanostructures, dominate the nanodonut–Au electrode contact; in contrast, the AFM tip contacts largely the high-resistance solubilizing alkyl chains of the chiral monomers that form these nanodonuts. Current–voltage responses of the nanodonuts, measured by magnetic conductive AFM (mC-AFM), demonstrate substantial spin polarizations as well as spin current rectification ratios (>10) that exceed the magnitudes of those determined to date for other chiral nanoscale systems. Furthermore, these results underscore the potential for chiral nanostructures, featuring asymmetric molecular junctions, to enable CISS-based nanoscale spin current rectifiers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Spin polarization through axially chiral linkers: Length dependence and correlation with the dissymmetry factor

Abstract The chiral‐induced spin selectivity (CISS) effect relates to the spin‐selective electron transport through chiral molecules; therefore, the chiral molecules act as spin filters. In past studies, correlation was found between the magnitude of the spin filtering and the intensity of the circular dichroism (CD) spectrum (the first Compton peak) of the molecules. Since the intensity of the CD peak relates to both the magnitude of the electric and magnetic dipole transitions, it was not clear which of these properties correlate with the CISS effect. This work aims at addressing this question. By studying the spin‐dependent conduction and the CD spectra of the thiol‐functionalized enantiopure binaphthalene (BINAP) and ternaphthalene (TERNAP), we found that both BINAP and TERNAP exhibit a similar spin polarization of 50%, despite the first Compton peak in TERNAP being almost twice as intense as the peak in BINAP. These results can be explained by the similar values of their anisotropy (or dissymmetry) factor, g abs , which is proportional to the magnetic transition dipole moment. Hence, we concluded that the CISS effect is proportional to the transition dipole moment in chiral molecules, namely, to the dissymmetry factor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

New Perspective on Electron Transfer through Molecules

Motivated by experiments which display unusual length and temperature effects for electron transfer in the nanometer length regime, we propose a new approach for describing long range electron transfer (ET) processes through molecules. We posit that the electron reorganization in the molecules (e.g., the electronic polarization of a macromolecule or organic film by an applied electric potential, or the injected charge generating a dipole moment) should be included in the description. We numerically solve a one-dimensional model for the electron transport, which includes electron-electron interactions explicitly; and we show that it generates a power law distance dependence for electron similar to that observed in experiments. The model does not include vibrations explicitly and should be consistent with the weak temperature dependences observed experimentally. This approach emphasizes the need to treat the electronic changes in the molecule(s) more explicitly to understand the behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spin-induced asymmetry reaction—The formation of asymmetric carbon by electropolymerization

We describe the spin polarization–induced chirogenic electropolymerization of achiral 2-vinylpyridine, which forms a layer of enantioenhanced isotactic polymer on the electrode. The product formed is enantioenriched in asymmetric carbon polymer. To confirm the chirality of the polymer film formed on the electrode, we also measured its electron spin polarization properties as a function of its thickness. Two methods were used: First, spin polarization was measured by applying magnetic contact atomic force microscopy, and second, magnetoresistance was assessed in a sandwich-like four-point contact structure. We observed high spin-selective electron transmission, even for a layer thickness of 120 nm. A correlation exists between the change in the circular dichroism signal and the change in the spin polarization, as a function of thickness. The spin-filtering efficiency increases with temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Chirality enhances oxygen reduction

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

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Spin Selectivity in Photoinduced Charge-Transfer Mediated by Chiral Molecules

Optical control and readout of electron spin and spin currents in thin films andnanostructures haveremained attractive yet challenging goalsfor emerging technologiesdesigned forapplications ininformation processing and storage. Current advances in room-temperature spin polarization using chiral molecular assemblies suggest that chemically modified surfaces or interfaces canbe used for optical spin conversion by exploiting photoinduced charge separation and injection fromwell-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by chiral molecule linkers 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 radiative relaxation pathways. Herein, we highlightrecent 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 substrate magnetization-and chirality-dependentphotoluminescence. Building upon these techniques through systematic investigation of molecular and environmental parameters that influence spin filtering shouldelucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Chiral molecules and the electron spin

The electron’s spin is necessary 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. Our 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.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Voltage-induced long-range coherent electron transfer through organic molecules

Biological 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Electric Field-Controlled Magnetization in GaAs/AlGaAs Heterostructures–Chiral Organic Molecules Hybrids

Here, we study GaAs/AlGaAs devices hostinga 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 injectedinto the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching ratein the MHz range. Therefore, our devices implement an efficient, electric field controlled magnetization, which has long been desired for their technical prospects.

36 MATERIALS SCIENCE↗