Colossal anisotropic absorption of spin currents induced by chirality.

The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.

Halide exchange mediated cation exchange facilitates room temperature co-doping of d-and f-block elements in cesium lead halide perovskite nanoparticles.

This study presents a halide exchange mediated cation exchange reaction to co-dope d- and f-block elements in CsPbX3 NPs at room temperature. Addition of MnCl2 and YbCl3 to CsPbBr3 NPs induces ion exchange reactions generating the corresponding CsPbBr3/MnCl2YbCl3 NPs. In addition to the perovskite emission, the NPs display sensitized Mn2+ and Yb3+ emissions in concert spanning the UV, visible, and NIR spectral region. Structural and spectroscopic characterizations indicate a substitutional displacement of Pb2+ by the Mn2+ and Yb3+. The identity of the host halide in modulating the ion exchange reactions was also tested. An effective perovskite host NP is presented that can be used to incorporate d-f or f-f dopant combinations to realize a gamut of dopant emission lines. A charge trapping based photophysical model is developed that focuses on rational energy alignments to predict dopant emissions semi-empirically and aids the design of optimal perovskite host-multi-dopant combinations.

Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking.

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

Spin polarized electron dynamics enhance water splitting efficiency by yttrium iron garnet photoanodes: a new platform for spin selective photocatalysis.

This work presents a spectroscopic and photocatalytic comparison of water splitting using yttrium iron garnet (Y3Fe5O12, YIG) and hematite (α-Fe2O3) photoanodes. Despite similar electronic structures, YIG significantly outperforms widely studied hematite, displaying more than an order of magnitude increase in photocurrent density. Probing the charge and spin dynamics by ultrafast, surface-sensitive XUV spectroscopy reveals that the enhanced performance arises from (1) reduced polaron formation in YIG compared to hematite and (2) an intrinsic spin polarization of catalytic photocurrents in YIG. Ultrafast XUV measurements show a reduction in the formation of surface electron polarons compared to hematite due to site-dependent electron-phonon coupling. This leads to spin polarized photocurrents in YIG where efficient charge separation occurs on the Td sub-lattice compared to fast trapping and electron/hole pair recombination on the Oh sub-lattice. These lattice-dependent dynamics result in a long-lived spin aligned hole population at the YIG surface, which is directly observed using XUV magnetic circular dichroism. Comparison of the Fe M2,3 and O L1-edges show that spin aligned holes are hybridized between O 2p and Fe 3d valence band states, and these holes are responsible for highly efficient, spin selective water oxidation by YIG. Together, these results point to YIG as a new platform for highly efficient, spin selective photocatalysis.

Chiral Induced Spin Selectivity.

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

Electron-donating functional groups strengthen ligand-induced chiral imprinting on CsPbBr3 quantum dots.

Chiral perovskite nanoparticles and films are promising for integration in emerging spintronic and optoelectronic technologies, yet few design rules exist to guide the development of chiral material properties. The chemical space of potential building blocks for these nanostructures is vast, and the mechanisms through which organic ligands can impart chirality to the inorganic perovskite lattice are not well understood. In this work, we investigate how the properties of chiral ammonium ligands, the most common organic ligand type used with perovskites, affect the circular dichroism of strongly quantum confined CsPbBr3 nanocrystals. We show that aromatic ammonium ligands with stronger electron-donating groups lead to higher-intensity circular dichroism associated with the lowest-energy excitonic transition of the perovskite nanocrystal. We argue that this behavior is best explained by a modulation of the exciton wavefunction overlap between the nanocrystal and the organic ligand, as the functional groups on the ligand can shift electron density toward the organic species-perovskite lattice interface to increase the imprinting.

On the circularly polarized luminescence of individual triplet sublevels.

We discuss the possibility of using circularly polarized luminescence (CPL) as a tool to probe individual triplet spin sublevels that are populated nonadiabatically following photoexcitation. This study is motivated by a mechanism proposed for chirality-induced spin selectivity in which coupled electronic-nuclear dynamics may lead to a non-statistical population of the three triplet sublevels in chiral systems. We find that low-temperature CPL should aid in quantifying the exact spin state/s populated through coupled electronic-nuclear motion in chiral molecules.

Tailored PVDF Graft Copolymers via ATRP as High-Performance NCM811 Cathode Binders.

High-nickel layered oxides, e.g., LiNi0.8Co0.1Mn0.1O2 (NCM811), are promising candidates for cathode materials in high-energy-density lithium-ion batteries (LIBs). Complementing the notable developments of modification of active materials, this study focused on the polymer binder materials, and a new synthetic route was developed to engineer PVDF binders by covalently grafting copolymers from poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) with multiple functionalities using atom transfer radical polymerization (ATRP). The grafted random copolymer binder provided excellent flexibility (319% elongation), adhesion strength (50 times higher than PVDF), transition metal chelation capability, and efficient ionic conductivity pathways. The NCM811 half-cells using the designed binders exhibited a remarkable rate capability of 143.4 mA h g-1 at 4C and cycling stability with 70.1% capacity retention after 230 cycles at 0.5 C, which is much higher than the 52.3% capacity retention of nonmodified PVDF. The well-retained structure of NCM811 with the designed binder was systematically studied and confirmed by post-mortem analysis.

Spin-Based Chiral Separations and the Importance of Molecule-Solvent Interactions.

This work uses magneto-electrochemical quartz crystal microbalance methods to study the enantiospecific adsorption of chiral molecules onto a ferromagnetic substrate. The effects of solution conditions, pH, and solvent isotope composition indicate that the kinetics of the enantiomeric adsorption depend strongly on the charge state and geometry of the adsorbate, whereas no thermodynamic contributions to enantiospecificity are found. Density functional theory calculations reveal that an interplay between the adsorbate and solvent molecules is important for defining the observed enantiospecific preference with an applied magnetic field; however, it remains unclear if intermolecular vibrational couplings contribute to the phenomenon.

Beyond Stereoisomeric Effects: Exploring the Importance of Intermolecular Electron Spin Interactions in Biorecognition.

This work shows that electron spin polarization and stereoisomeric effects make comparable contributions to the enantioselective binding of amino acids. Magneto-electrochemical quartz crystal microbalance methods are used to study the adsorption of chiral amino acids onto a monolayer film of chiral molecules that are spin polarized by an underlying ferromagnetic substrate. The direction of the electron spin polarization affects both the kinetics and thermodynamics of the enantiospecific adsorption of the amino acids. Comparison of these data with the circular dichroism (CD) spectra of the amino acid adsorbates shows that the CD spectrum of the interacting group provides a good figure-of-merit for predicting the contributions of electron spin to the intermolecular interaction. These findings demonstrate the importance of electron spin in enantioselective intermolecular interactions between chiral amino acids and represent a paradigm shift for how selectivity should be viewed in biorecognition.

Examining the Effects of Homochirality for Electron Transfer in Protein Assemblies.

Protein voltammetry studies of cytochrome c, immobilized on chiral tripeptide monolayer films, reveal the importance of the electron spin and the film's homochirality on electron transfer kinetics. Magnetic film electrodes are used to examine how an asymmetry in the standard heterogeneous electron transfer rate constant arises from changes in the electron spin direction and the enantiomer composition of the tripeptide monolayer; rate constant asymmetries as large as 60% are observed. These findings are rationalized in terms of the chiral induced spin selectivity effect and spin-dependent changes in electronic coupling. Lastly, marked differences in the average rate constant are shown between homochiral ensembles, in which the peptide and protein possess the same enantiomeric form, compared to heterochiral ensembles, where the handedness of the peptide layer is opposite to that of the protein or itself comprises heterochiral building blocks. These data demonstrate a compelling rationale for why nature is homochiral; namely, spin alignment in homochiral systems enables more efficient energy transduction.

Spin Selectivity Damage Dependence of Adsorption of dsDNA on Ferromagnets.

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

Chiral electrocatalysts eclipse water splitting metrics through spin control.

Continual progress in technologies that rely on water splitting are often hampered by the slow kinetics associated with the oxygen evolution reaction (OER). Here, we show that the efficiency of top-performing catalysts can be improved, beyond typical thermodynamic considerations, through control over reaction intermediate spin alignment during electrolysis. Spin alignment is achieved using the chiral induced spin selectivity (CISS) effect and the improvement in OER manifests as an increase in Faradaic efficiency, decrease in reaction overpotential, and change in the rate determining step for chiral nanocatalysts over compositionally analogous achiral nanocatalysts. These studies illustrate that a defined spatial orientation of the nanocatalysts is not necessary to exhibit spin selectivity and therefore represent a viable platform for employing the transformative role of chirality in other reaction pathways and processes.

Size-dependent chiro-optical properties of CsPbBr3 nanoparticles.

Chiral metal halide perovskites have garnered substantial interest because of their promising properties for application in optoelectronics and spintronics. Understanding the mechanism of chiral imprinting is paramount for optimizing their utility. To elucidate the nature of the underlying chiral imprinting mechanism, we investigated how the circular dichroism (CD) intensity varies with nanoparticle size for quantum confined sizes of colloidal CsPbBr3 perovskite nanoparticles (NPs) capped by chiral ß-methylphenethylammonium bromide ligands. We find that the CD intensity decreases strongly with increasing NP size, which, along with the shape of the CD spectra, points to electronic interactions between ligand and NP as the dominant mechanism of chiral imprinting in smaller NPs. We observe that as the NP size increases and crosses the quantum confinement threshold, the dominant mechanism of chirality transfer switches and is dominated by surfaces effects, e.g., structural distortions. These findings provide a benchmark for quantitative models of chiral imprinting.

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 transport 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.

Spin-Polarized Photoemission from Chiral CuO Catalyst Thin Films.

The chirality-induced spin selectivity (CISS) effect facilitates a paradigm shift for controlling the outcome and efficiency of spin-dependent chemical reactions, for example, photoinduced water splitting. While the phenomenon is established in organic chiral molecules, its emergence in chiral but inorganic, nonmolecular materials is not yet understood. Nevertheless, inorganic spin-filtering materials offer favorable characteristics, such as thermal and chemical stability, over organic, molecular spin filters. Chiral cupric oxide (CuO) thin films can spin polarize (photo)electron currents, and this capability is linked to the occurrence of the CISS effect. In the present work, chiral CuO films, electrochemically deposited on partially UV-transparent polycrystalline gold substrates, were subjected to deep-UV laser pulses, and the average spin polarization of photoelectrons was measured in a Mott scattering apparatus. By energy resolving the photoelectrons and changing the photoexcitation geometry, the energy distribution and spin polarization of the photoelectrons originating from the Au substrate could be distinguished from those arising from the CuO film. The findings reveal that the spin polarization is energy dependent and, furthermore, indicate that the measured polarization values can be rationalized as a sum of an intrinsic spin polarization in the chiral oxide layer and a contribution via CISS-related spin filtering of electrons from the Au substrate. The results support efforts toward a rational design of further spin-selective catalytic oxide materials.

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.

Electron transfer and spin-orbit coupling: Can nuclear motion lead to spin selective rates?

We investigate a spin-boson inspired model of electron transfer, where the diabatic coupling is given by a position-dependent phase, eiWx. We consider both equilibrium and nonequilibrium initial conditions. We show that, for this model, all equilibrium results are completely invariant to the sign of W (to infinite order). However, the nonequilibrium results do depend on the sign of W, suggesting that photo-induced electron transfer dynamics with spin-orbit coupling can exhibit electronic spin polarization (at least for some time).