Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field

Chiral light-matter interactions in solution-processable semiconductors

Chirality is a fundamental property widely observed in nature, arising in objects without a proper rotation axis, therefore existing as forms with distinct handedness. This characteristic can profoundly impact the properties of materials and can enable new functionality, especially for spin-optoelectronics. Chirality enables asymmetric light and spin interactions in materials, with widespread potential applications ranging from energy-efficient displays, holography, imaging, and spin-selective and enantio-selective chemistry to quantum information technologies. This Review focuses on the emerging material class of solution-processable chiral semiconductors, a broad material class comprising organic, inorganic and hybrid materials. These exciting materials offer the opportunity to design desirable light-matter interactions based on symmetry rules, potentially enabling the simultaneous control of light, charge and spin. We briefly discuss the various types of solution-processible chiral semiconductors, including small molecules, polymers, supramolecular self-assemblies and halide perovskites. We then examine the interplay between chirality and spin in these materials, the various mechanisms of chiral light-matter interactions, and techniques utilized to characterize them. We conclude with current and future applications of chiral semiconductors that take advantage of their chiral light-matter interactions.

Emerging devices based on chiral nanomaterials

As advanced materials, chiral nanomaterials have recently gained vast attention due to their special geometry-based physical and chemical properties. The fast development of the related science and technology means that various devices involving polarization-based information encryption, photoelectronic and spintronic devices, 3D displays, biomedical sensors and measurement, photonic engineering, electronic engineering, solar devices, etc., been explored extensively. These fields are at their beginning, and much effort needs to be made, including improving the optical, electronic, and magnetic properties of advanced chiral nanomaterials, precisely designing materials, and developing more efficient construction methods. This review tries to offer a whole picture of these state-of-the-art conditions in these fields and offers perspectives on future development.

Spin polarization detection via chirality-induced tunnelling currents in indium selenide

Chirality, a basic property of symmetry breaking, is crucial for fields such as biology and physics. Recent advances in the study of chiral systems have stimulated interest in the discovery of symmetry-breaking states that enable exotic phenomena such as spontaneous gyrotropic order and superconductivity. Here we examine the interaction between light chirality and electron spins in indium selenide and study the effect of magnetic field on emerging tunnelling photocurrents at the Van Hove singularity. Although the effect is symmetric under linearly polarized light excitation, a non-symmetric signal emerges when the excitation is circularly polarized, making it possible to electrically detect light's chirality. Our study shows a negligible out-of-plane g-factor for few-layer indium selenide at the valence band edge, resulting in an unbalanced Zeeman splitting in hexagonal boron nitride spin bands. This finding allows us to measure the change in energy barrier height with exceptional resolution (~15 μeV). Furthermore, we confirm the long-standing theoretical prediction of spin-polarized hole accumulation in the flat valence band at increasing laser powers.

Chirality induced spin selectivity in electron transport investigated by scanning probe microscopy

Chirality induced spin selectivity (CISS) effect implies the relationship between chirality and magnetism, attracting extensive attention in the fields of physics, chemistry and biology. Since it was first discovered with photoemission method in 1999, the CISS effect has been investigated and measured by a variety of methods. Among different means of measurements, scanning probe microscopy (SPM) as a powerful tool to explore the CISS effect, can directly measure and present the spin filtering property of chiral molecules in electron transport. In this paper, we summarize the recent experiments on the CISS effect studied with scanning tunneling microscopy and atomic force microscopy, analyzing the experimental setups and results, and delving into the underlying mechanisms. The present review offers a concise introduction to several chiral molecules which are investigated by SPM for the CISS effect, and a detailed exploration of various experimental techniques tailored to the unique adsorption structures of these molecules. The impact of molecular structure on spin selectivity and the profound implications of CISS are also demonstrated together with a concise overview of CISS theory. A conclusive synopsis and forward-looking perspectives on the investigation of the CISS effect in electron transport utilizing SPM techniques are presented.

Effects of Chiral Polypeptides on Skyrmion Stability and Dynamics

Magnetic skyrmions, topologically stabilized chiral spin textures in magnetic thin films, have garnered considerable interest due to their efficient manipulation and resulting potential as efficient nanoscale information carriers. One intriguing approach to address the challenge of tuning skyrmion properties involves using chiral molecules. Chiral molecules can locally manipulate magnetic properties by inducing magnetization through spin exchange interactions and by creating spin currents. Here, Magneto-Optical Kerr Effect (MOKE) microscopy is used to image the impact of chiral polypeptides on chiral magnetic structures. The chiral polypeptides shift the spin reorientation transition temperature, reduce thermal skyrmion motion, and alter the coercive field locally, enhancing skyrmion stability and thus enabling local control. These findings demonstrate the potential of chiral molecules to address challenges for skyrmion based devices, thus paving the way to applications such as the racetrack memory, reservoir computing and others.

Chiral-induced unidirectional spin-to-charge conversion

The observation of spin-dependent transmission of electrons through chiral molecules has led to the discovery of chiral-induced spin selectivity (CISS). The remarkably high efficiency of the spin polarizing effect has recently gained substantial interest due to the high potential for future sustainable hybrid chiral molecule magnetic applications. However, the fundamental mechanisms underlying the chiral-induced phenomena remain to be understood fully. In this work, we explore the impact of chirality on spin angular momentum in hybrid metal/chiral molecule thin-film heterostructures. For this, we inject a pure spin current via spin pumping and investigate the spin-to-charge conversion at the hybrid chiral interface. Notably, we observe a chiral-induced unidirectionality in the conversion. Furthermore, angle-dependent measurements reveal that the spin selectivity is maximum when the spin angular momentum is aligned with the molecular chiral axis. Our findings validate the central role of spin angular momentum for the CISS effect, paving the path toward three-dimensional functionalization of hybrid molecule-metal devices via chirality.

Surface Magnetic Stabilization and the Photoemission Chiral-Induced Spin-Selectivity Effect

The spinterface mechanism was suggested as a possible origin for the chirality-induced spin-selectivity (CISS) effect and was used to explain and reproduce, with remarkable accuracy, experimental data from transport experiments showing the CISS effect. Here, we apply the spinterface mechanism to explain the appearance of magnetization at the interface between nonmagnetic metals and chiral molecules, through the stabilization of otherwise fluctuating magnetic moments. We show that the stabilization of surface magnetic moments occurs for a wide range of realistic parameters and is robust against dephasing. Importantly, we show that the direction of the surface magnetic moments is determined by the chiral axis of the chiral molecules. Armed with the concept of stable surface magnetic moments, we then formulated a theory for the photoemission CISS effect. The theory, based on spin-dependent scattering, leads to direct predictions regarding the relation between the photoemission CISS effect, the chiral axis direction, the spinterface "size", and the tilt angle of the detector with respect to the surface. These predictions are within reach of current experimental capabilities and may shed new light on the origin of the CISS effect.

Magnetic Monopole-Like Behavior in Superparamagnetic Nanoparticle Coated With Chiral Molecules

Superparamagnetic iron oxide nanoparticles (SPIONs) have attracted wide attention due to their promising applications in biomedicine, chemical catalysis, and magnetic memory devices. In this work, the force is measured between a single SPION coated with chiral molecules and a ferromagnetic substrate by atomic force microscopy (AFM), with the substrate magnetized either toward or away from the approaching AFM tip. The force between the coated SPION and the magnetic substrate depends on the handedness of the molecules adsorbed on the SPION and on the direction of the magnetization of the substrate. By inserting nm-scale spacing layers between the coated SPION and the magnetic substrate it is shown that the SPION has a short-range magnetic monopole-like magnetic field. A theoretical framework for the nature of this field is provided.

Chiral Nematic Cellulose Nanocrystal Films for Enhanced Charge Separation and Quantum-Confined Stark Effect

Efficient charge separation is essential in various optoelectronic systems, yet it continues to pose substantial challenges. Building upon the recent evidence that chiral biomolecules can function as electron spin filters, this study aims to extend the application of chirality-driven charge separation from the molecular level to the mesoscale and supramolecular scale. Utilizing cellulose nanocrystals (CNCs) derived from cellulose, the most abundant biomaterial on Earth, this research leverages their self-assembly into chiral nematic structures and their dielectric properties. A device is introduced featuring a chiral nematic hybrid film composed of CNCs and quantum dots (QDs), decorated with iron oxide nanoparticles. Using the quantum-confined Stark effect (QCSE) to probe charge separation, we reveal significant sensitivity to the circular polarization of light and the chiral nematic structure of the film. This approach achieves effective, long-lasting charge separation, both locally and across length scales exceeding 1 μm, enabling potential applications such as self-assembled devices that combine photovoltaic cells with electric capacitance as well as optical electric-field hybrid biosensors.

The promise of chiral electrocatalysis for efficient and sustainable energy conversion and storage: a comprehensive review of the CISS effect and future directions

The integration of chirality, specifically through the chirality-induced spin selectivity (CISS) effect, into electrocatalytic processes represents a pioneering approach for enhancing the efficiency of energy conversion and storage systems. This review delves into the burgeoning field of chiral electrocatalysis, elucidating the fundamental principles, historical development, theoretical underpinnings, and practical applications of the CISS effect across a spectrum of electrocatalytic reactions, including the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). We explore the methodological advancements in inducing the CISS effect through structural and surface engineering and discuss various techniques for its measurement, from magnetic conductive atomic force microscopy (mc-AFM) to hydrogen peroxide titration. Furthermore, this review highlights the transformative potential of the CISS effect in addressing the key challenges of the NRR and CO2RR processes and in mitigating singlet oxygen formation in metal-air batteries, thereby improving their performance and durability. Through this comprehensive overview, we aim to underscore the significant role of incorporating chirality and spin polarization in advancing electrocatalytic technologies for sustainable energy applications.

Nonlinear Optics-Driven Spin Reorientation in Ferromagnetic Materials

Based on nonlinear optics, we propose that light irradiation could induce a nonequilibrium steady state magnetization variation. We formulize a band theory to elucidate its general microscopic mechanisms, which are rooted by the quantum geometric structure and topological nature of electronic Bloch wave functions. The existence is determined by the light polarization and specific material symmetry, based on the magnetic group theory. In general, for a magnetic system, both circularly and linearly polarized light could exert an effective magnetic field and a magnetic "velocity" (magnetization variation rate over time, serving as an effective torque) to reorient the magnetization direction. They are contributed by spin and orbital angular momenta simultaneously. Aided by group theory and first-principles calculations, we illustrate this theory using a showcase example of monolayer NiCl2, showing that light irradiation effectively generates an out-of-plane effective magnetic torque, which lifts its in-plane easy magnetization. According to magnetic dynamic simulations, under light with a modest intensity, this switching could occur on the order of 0.1-1 ns time scale, demonstrating its ultrafast nature that is desirable for quantum manipulation.

A chemical perspective on the chiral induced spin selectivity effect

This review discusses opportunities in chemistry that are enabled by the chiral induced spin selectivity (CISS) effect. First, the review begins with a brief overview of the seminal studies on CISS. Next, we discuss different chiral material systems whose properties can be tailored through chemical means, with a special emphasis on hybrid organic-inorganic layered materials that exhibit some of the largest spin filtering properties to date. Then, we discuss the promise of CISS for chemical reactions and enantioseparation before concluding.

The mechanism of the molecular CISS effect in chiral nano-junctions

The chirality induced spin selectivity (CISS) effect has been up to now measured in a wide variety of systems but its exact mechanism is still under debate. Whether the spin polarization occurs at an interface layer or builds up in the helical molecule is yet not clear. Here we have investigated the current transmission through helical polyalanine molecules as a part of a tunnel junction realized with a scanning tunneling microscope. Depending on whether the molecules were chemisorbed directly on the magnetic Au/Co/Au substrate or at the STM Au-tip, the magnetizations of the Co layer had been oriented in the opposite direction in order to preserve the symmetry of the IV-curves. This is the first time that the CISS effect is demonstrated for a tunneling junction without a direct interface between the helical molecules and the magnetic substrate. Our results can be explained by a spin-polarized or spin-selective interface effect, induced and defined by the helicity and electric dipole orientation of the molecule at the interface. In this sense, the helical molecule does not act as a simple spin-filter or spin-polarizer and the CISS effect is not limited to spinterfaces.

Current Induced Spin-Polarization in Chiral Molecules

The inverse spin-galvanic effect or current-induced spin-polarization is mainly associated with interfaces between different layers in semiconducting heterostructures, surfaces of metals, and bulk semiconducting materials. Here, we theoretically predict that the inverse spin-galvanic effect should also be present in chiral molecules, as a result of the chiral induced spin selectivity effect. As proof-of-principle, we calculate the nonequilibrium properties of a model system that previously has been successfully used to explain a multitude of aspects related to the chiral induced spin selectivity effect. Here we show that current driven spin-polarization in a chiral molecule gives rise to a magnetic moment that is sensitive to external magnet field. The chiral molecule then behaves like a soft ferromagnet. This, in turn, suggests that magnetic permeability measurement in otherwise nonmagnetic systems may be used noninvasively to detect the presence of spin-polarized currents.

Persistent Chirality-Induced Spin-Selectivity Effect in Circular Helix Molecules

The spin-orbit coupling (SOC), the dynamics of the nonequilibrium transport process, and the breaking of time-reversal and space-inversion symmetries have been regarded as key factors for the emergence of chirality-induced spin selectivity (CISS) and chirality-dependent spin currents in helix molecules. In this work, we demonstrated the generation of persistent CISS currents in various circular single-stranded DNAs and 310-helix proteins for the first time, regardless of whether an external magnetic flux is applied or not. This new CISS effect presents only in equilibrium transport processes, distinct from the traditional CISS observed in nonequilibrium transport processes and linear helix molecules; we term it as the PCISS effect. Notably, PCISS manifests irrespective of whether the SOC is chirality-driven or stems from heavy-metal substrates, making it an efficient way to generate chirality-locked pure spin currents. Our research establishes a novel paradigm for examining the underlying physics of the CISS effect.

Quantification of chirality based on electric toroidal monopole

Chirality ubiquitously appears in nature; however, its quantification remains obscure owing to the lack of microscopic description at the quantum-mechanical level. We propose a way of evaluating chirality in terms of the electric toroidal monopole, a practical entity of time-reversal even pseudoscalar (parity-odd) objects reflecting relevant electronic wave functions. For this purpose, we analyze a twisted methane molecule at the quantum-mechanical level, showing that the electric toroidal monopoles become a quantitative indicator for chirality. In the twisted methane, we clarify that the handedness of chirality corresponds to the sign of the expectation value of the electric toroidal monopole and that the most important ingredient is the modulation of the spin-dependent imaginary hopping between the hydrogen atoms, while the relativistic spin-orbit coupling within the carbon atom is irrelevant for chirality.

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.

Phonon-assisted nearly pure spin current in DNA molecular chains: a multifractal analysis

Motivated by the development of molecular spintronics, we studied the phonon-assisted spin transport along a DNA chain in the presence of environmental-induced dephasing using multifractal analysis. The results demonstrate that a nearly pure spin current is generated in the presence of the voltage gate. The pure spin current is enhanced by increasing thermal effects. The vibration modes due to the thermal phonon bath assist in generating the spin current, so the spin state is more delocalized in strong electron-phonon coupling. The phonon chirality can translate to the electron spin to create a nontrivial spin texture, including spin currents. The spin states become more extended by increasing the phonon temperature. On the other hand, the spin states are less localized in longer chains as the spin selectivity is higher in longer chains than in short ones. Therefore, we can engineer a molecular spintronic device by controlling phonon effects on the storage and transport of binary digits.

Comparing Electrical Magnetochiral Anisotropy and Chirality-Induced Spin Selectivity

The combination of chirality and magnetism has steadily grown over the past decennia into an area of intense research that evolves around two distinct manifestations and in two nonoverlapping communities: electrical magnetochiral anisotropy (eMChA) and chirality-induced spin-selectivity (CISS). Here, we discuss the similarities and differences of these two effects. Whereas the original CISS reports suggest an intimate relation with eMChA, magnetoresistance (MR) results on two-terminal chiral devices attributed to CISS have symmetry properties that are different from those of eMChA. At the same time, the magnitudes of CISS MR and eMChA turn out to be similar when normalized to current density and spin polarization, suggesting a common underlying mechanism.

Chirality-induced avalanche magnetization of magnetite by an RNA precursor

Homochirality is a hallmark of life on Earth. To achieve and maintain homochirality within a prebiotic network, the presence of an environmental factor acting as a chiral agent and providing a persistent chiral bias to prebiotic chemistry is highly advantageous. Magnetized surfaces are prebiotically plausible chiral agents due to the chiral-induced spin selectivity (CISS) effect, and they were utilized to attain homochiral ribose-aminooxazoline (RAO), an RNA precursor. However, natural magnetic minerals are typically weakly magnetized, necessitating mechanisms to enhance their magnetization for their use as effective chiral agents. Here, we report the magnetization of magnetic surfaces by crystallizing enantiopure RAO, whereby chiral molecules induce a uniform surface magnetization due to the CISS effect, which spreads across the magnetic surface akin to an avalanche. Chirality-induced avalanche magnetization enables a feedback between chiral molecules and magnetic surfaces, which can amplify a weak magnetization and allow for highly efficient spin-selective processes on magnetic minerals.

Spin dynamics and chirality induced spin selectivity

By now, it is well known that chiral molecules can affect the spin of electrons passing through. In addition, the magnetization of, e.g., nanomagnets covered by chiral molecules can be affected by the presence of molecules. We are studying the mechanisms that explain various observations involving combinations of magnets and chiral molecules. We find that there exists a molecule induced contribution to the magnetic anisotropy of the magnets. Out of equilibrium, when electrons are actually being transported through a nano-magnet covered with chiral molecules, a molecule induced torque acting on the magnetization is emerging. It is of the spin-transfer-torque kind, already discussed in other parts of spintronics. This current induced torque can help explain the observed breaking of the Onsager reciprocity principle in experiments involving magnets and chiral molecules.

Achiral dipoles on a ferromagnet can affect its magnetization direction

We demonstrate the possibility of a coupling between the magnetization direction of a ferromagnet and the tilting angle of adsorbed achiral molecules. To illustrate the mechanism of the coupling, we analyze a minimal Stoner model that includes Rashba spin-orbit coupling due to the electric field on the surface of the ferromagnet. The proposed mechanism allows us to study magnetic anisotropy of the system with an extended Stoner-Wohlfarth model and argue that adsorbed achiral molecules can change magnetocrystalline anisotropy of the substrate. Our research aims to motivate further experimental studies of the current-free chirality induced spin selectivity effect involving both enantiomers.

Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides

Advanced Kelvin probe force microscopy simultaneously detects the quantum capacitance and surface potential of an α-helical peptide monolayer. These indicators shift when either the magnetic polarization or the enantiomer is toggled. A model based on a triangular quantum well in thermal and chemical equilibrium and electron-electron interactions allows for calculating the electrical potential profile from the measured data. The combination of the model and the measurements shows that no global charge transport is required to produce effects attributed to the chirality-induced spin selectivity effect. These experimental findings support the theoretical model of Fransson et al. Nano Letters 2021, 21 (7), 3026-3032. Measurements of the quantum capacitance represent a new way to test and refine theoretical models used to explain strong spin polarization due to chirality-induced spin selectivity.

Temperature activated chiral induced spin selectivity

Recent experiments performed on chiral molecules, comprising transition metal or rare earth elements, indicate temperature reinforced chiral induced spin selectivity. In these compounds, spin selectivity is suppressed in the low temperature regime but grows by one to several orders of magnitude as the temperature is increased to room temperature. By relating temperature to nuclear motion, it is proposed that nuclear displacements acting on the local spin moments, through indirect exchange interactions, generate an anisotropic magnetic environment that is enhanced with temperature. The induced local anisotropy field serves as the origin of a strongly increased spin selectivity at elevated temperature.

Strategies and applications of generating spin polarization in organic semiconductors

The advent of spintronics has undoubtedly revolutionized data storage, processing, and sensing applications. Organic semiconductors (OSCs), characterized by long spin relaxation times (>μs) and abundant spin-dependent properties, have emerged as promising materials for advanced spintronic applications. To successfully implement spin-related functions in organic spintronic devices, the four fundamental processes of spin generation, transport, manipulation, and detection form the main building blocks and are commonly in demand. Thereinto, the effective generation of spin polarization in OSCs is a precondition, but in practice, this has not been an easy task. In this context, considerable efforts have been made on this topic, covering novel materials systems, spin-dependent theories, and device fabrication technologies. In this review, we underline recent advances in external spin injection and organic property-induced spin polarization, according to the distinction between the sources of spin polarization. We focused mainly on summarizing and discussing both the physical mechanism and representative research on spin generation in OSCs, especially for various spin injection methods, organic magnetic materials, the chiral-induced spin selectivity effect, and the spinterface effect. Finally, the challenges and prospects that allow this topic to continue to be dynamic were outlined.

Magneto-optical imaging of magnetic-domain pinning induced by chiral molecules

Chiral molecules have the potential for creating new magnetic devices by locally manipulating the magnetic properties of metallic surfaces. When chiral polypeptides chemisorb onto ferromagnets, they can induce magnetization locally by spin exchange interactions. However, direct imaging of surface magnetization changes induced by chiral molecules was not previously realized. Here, we use magneto-optical Kerr microscopy to image domains in thin films and show that chiral polypeptides strongly pin domains, increasing the coercive field locally. In our study, we also observe a rotation of the easy magnetic axis toward the out-of-plane, depending on the sample's domain size and the adsorption area. These findings show the potential of chiral molecules to control and manipulate magnetization and open new avenues for future research on the relationship between chirality and magnetization.

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.

Origin of biological homochirality by crystallization of an RNA precursor on a magnetic surface

Homochirality is a signature of life on Earth, yet its origins remain an unsolved puzzle. Achieving homochirality is essential for a high-yielding prebiotic network capable of producing functional polymers like RNA and peptides on a persistent basis. Because of the chiral-induced spin selectivity effect, which established a strong coupling between electron spin and molecular chirality, magnetic surfaces can act as chiral agents and be templates for the enantioselective crystallization of chiral molecules. Here, we studied the spin-selective crystallization of racemic ribo-aminooxazoline (RAO), an RNA precursor, on magnetite (Fe3O4) surfaces, achieving an unprecedented enantiomeric excess (ee) of about 60%. Following the initial enrichment, we then obtained homochiral (100% ee) crystals of RAO after a subsequent crystallization. Our results demonstrate a prebiotically plausible way of achieving system-level homochirality from completely racemic starting materials, in a shallow-lake environment on early Earth where sedimentary magnetite deposits are expected to be common.

Chiral-induced spin selectivity in biomolecules, hybrid organic-inorganic perovskites and inorganic materials: a comprehensive review on recent progress

The two spin states of electrons are degenerate in nonmagnetic materials. The chiral-induced spin selectivity (CISS) effect provides a new strategy for manipulating electron's spin and a deeper understanding of spin selective processes in organisms. Here, we summarize the important discoveries and recent experiments performed during the development of the CISS effect, analyze the spin polarized transport in various types of materials and discuss the mechanisms, theoretical calculations, experimental techniques and biological significance of the CISS effect. The first part of this review concisely presents a general overview of the discoveries and importance of the CISS effect, laws and underlying mechanisms of which are discussed in the next section, where several classical experimental methods for detecting the CISS effect are also introduced. Based on the organic and inorganic properties of materials, the CISS effect of organic biomolecules, hybrid organic-inorganic perovskites and inorganic materials are reviewed in the third, fourth and fifth sections, especially the chiral transfer mechanism of hybrid materials and the relationship between the CISS effect and life science. In addition, conclusions and prospective future of the CISS effect are outlined at the end, where the development and applications of the CISS effect in spintronics are directly described, which is helpful for designing promising chiral spintronic devices and understanding the natural status of chirality from a new perspective.

Metal-Free Organic Radical Spin Source

Organic radicals have long been suggested as candidates for organic magnets and components in organic spintronic devices. Herein, we demonstrate spin current emission from an organic radical film via spin pumping at room temperature. We present the synthesis and the thin film preparation of a Blatter-type radical with outstanding stability and low roughness. These features enable the fabrication of a radical/ferromagnet bilayer, in which the spin current emission from the organic radical layer can be reversibly reduced when the ferromagnetic film is brought into simultaneous resonance with the radical. The results provide an experimental demonstration of a metal-free organic radical layer operating as a spin source, opening a new avenue for the development of purely organic spintronic devices and bridging the gap between potential and real applications.

Chirality-Induced Spin Selectivity (CISS) Effect: Magnetocurrent-Voltage Characteristics with Coulomb Interactions I

One of the manifestations of chirality-induced spin selectivity (CISS) is the appearance of a magnetocurrent. Magnetocurrent is the observation that the charge currents at finite bias in a two terminal device for opposite magnetizations of one of the leads differ. Magnetocurrents can only occur in the presence of interactions of the electrons either with vibrational modes or among themselves through the Coulomb interaction. In experiments on chiral molecules assembled in monolayers, the magnetocurrent seems to be dominantly cubic (odd) in bias voltage while theory finds a dominantly even bias voltage dependence. Thus far, theoretical work has predicted a magnetocurrent which is even bias. Here we analyze the bias voltage dependence of the magnetocurrent numerically and analytically involving the spin-orbit and Coulomb interactions (through the Hartree-Fock and Hubbard One approximations). For both approximations it is found that for strong Coulomb interactions the magnetocurrent is dominantly odd in bias voltage, confirming the symmetry observed in experiment.

Spin Polarization of Polyalanine Molecules in 2D and Dimer-Row Assemblies Adsorbed on Magnetic Substrates: The Role of Coupling, Chirality, and Coordination

Propagation of electrons along helical molecules adsorbed on surfaces comes along with a robust spin polarization effect called chirality induced spin selectivity CISS. However, experiments on the molecular scale that allow a true correlation of spin effects with the molecular structure are quite rare. Here we have studied the structure of self-assembled chiral molecules and the electronic transmission and spin polarization of the current through the system by means of ambient scanning tunneling microscopy and spectroscopy in heterostructures of various α-helix polyalanine-based molecules (PA) adsorbed on Al₂O₃/Pt/Au/Co/Au substrates with perpendicular magnetic anisotropy. We have found a phase separation of the molecules into well-ordered enantiopure 2D hexagonal phases and quasi-1D heterochiral-dimer structures, which allows for the analysis of the spin polarization with almost atomic precision of PA in different phases. The spin polarization reaches up to 75% for chemisorbed molecules arranged in a hexagonal phase. On the contrary, for weakly coupled PA molecules without cysteine anchoring groups in a quasi-1D phase, a spin polarization of around 50% was found. Our results show that both the intermolecular interaction as well as the coupling to the substrate are important and point out that collective effects within the molecules and at the interfaces are required to achieve a high chiral induced spin selectivity.

Ultrafast Coherent Delocalization Revealed in Multilayer QDs under a Chiral Potential

In recent years, it was found that current passing through chiral molecules exhibits spin preference, an effect known as Chiral Induced Spin Selectivity (CISS). The effect also enables the reduction of scattering and therefore enhances delocalization. As a result, the delocalization of an exciton generated in the dots is not symmetric and relates to the electronic and hole excited spins. In this work utilizing fast spectroscopy on hybrid multilayered QDs with a chiral polypeptide linker system, we probed the interdot chiral coupling on a short time scale. Surprisingly, we found strong coherent coupling and delocalization despite having long 4-nm chiral linkers. We ascribe the results to asymmetric delocalization that is controlled by the electron spin. The effect is not measured when using shorter nonchiral linkers. As the system mimics light-harvesting antennas, the results may shed light on a mechanism of fast and efficient energy transfer in these systems.

Chiral-Induced Spin Selectivity in Photon-Coupled Achiral Matters

Chiral-induced spin selectivity is a phenomenon in which electron spins are polarized as they are transported through chiral molecules, and the spin polarization depends on the handedness of the chiral molecule. In this study, we show that spin selectivity can be realized in achiral materials by strongly coupling electrons to a circularly polarized mode of an optical cavity or waveguide. Through the investigation of spin-dependent electron transport in a two-terminal setup using the nonequilibrium Green's function approach, it is found that a large spin polarization can be obtained if the rate of dephasing is sufficiently small and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the optical mode. To obtain a wider range of energies for a large spin polarization, chiral molecules can be combined with light-matter interactions. To demonstrate this, the spin polarization of electrons transported through a helical molecule strongly coupled to a circularly polarized optical mode is evaluated.

An artificial synapse based on molecular junctions

Shrinking the size of the electronic synapse to molecular length-scale, for example, an artificial synapse directly fabricated by using individual or monolayer molecules, is important for maximizing the integration density, reducing the energy consumption, and enabling functionalities not easily achieved by other synaptic materials. Here, we show that the conductance of the self-assembled peptide molecule monolayer could be dynamically modulated by placing electrical biases, enabling us to implement basic synaptic functions. Both short-term plasticity (e.g., paired-pulse facilitation) and long-term plasticity (e.g., spike-timing-dependent plasticity) are demonstrated in a single molecular synapse. The dynamic current response is due to a combination of both chemical gating and coordination effects between Ag⁺ and hosting groups within peptides which adjusts the electron hopping rate through the molecular junction. In the end, based on the nonlinearity and short-term synaptic characteristics, the molecular synapses are utilized as reservoirs for waveform recognition with 100% accuracy at a small mask length.

Multifunctional Hybrid Nanocomposite Hydrogel Releasing Different Biomolecular Species Triggered with Different Biochemical Signals Processed by Orthogonal Biocatalytic Reactions

A micro/nanoshaped system composed of alginate microspheres (microgels) decorated with silica oxide nanoparticles functionalized with nitroavidin was used for on-demand biomolecule release stimulated by different input signals. Enzymes preloaded in the microgels processed the applied signals producing either basic pH locally near the microspheres or generating H₂O₂ inside the hydrogel, or both simultaneously. The pH increase resulted in cleavage of the affinity bonds between nitroavidin and biotin, then releasing the latter. The H₂O₂ produced resulted in oxidative cleavage of cross-linking bonds in the alginate matrix, then opening pores and releasing a loaded model protein (bovine serum albumin).

The effect of spin exchange interaction on protein structural stability

Partially charged chiral molecules act as spin filters, with preference for electron transport toward one type of spin ("up" or "down"), depending on their handedness. This effect is named the chiral induced spin selectivity (CISS) effect. A consequence of this phenomenon is spin polarization concomitant with electric polarization in chiral molecules. These findings were shown by adsorbing chiral molecules on magnetic surfaces and investigating the spin-exchange interaction between the surface and the chiral molecule. This field of study was developed using artificial chiral molecules. Here we used such magnetic surfaces to explore the importance of the intrinsic chiral properties of proteins in determining their stability. First, proteins were adsorbed on paramagnetic and ferromagnetic nanoparticles in a solution, and subsequently urea was gradually added to induce unfolding. The structural stability of proteins was assessed using two methods: bioluminescence measurements used to monitor the activity of the Luciferase enzyme, and fast spectroscopy detecting the distance between two chromophores implanted at the termini of a Barnase core. We found that interactions with magnetic materials altered the structural and functional resilience of the natively folded proteins, affecting their behavior under varying mild denaturing conditions. Minor structural disturbances at low urea concentrations were impeded in association with paramagnetic nanoparticles, whereas at higher urea concentrations, major structural deformation was hindered in association with ferromagnetic nanoparticles. These effects were attributed to spin exchange interactions due to differences in the magnetic imprinting properties of each type of nanoparticle. Additional measurements of proteins on macroscopic magnetic surfaces support this conclusion. The results imply a link between internal spin exchange interactions in a folded protein and its structural and functional integrity on magnetic surfaces. Together with the accumulating knowledge on CISS, our findings suggest that chirality and spin exchange interactions should be considered as additional factors governing protein structures.

Unusual Spin Polarization in the Chirality-Induced Spin Selectivity

Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through chiral molecules in a nanoscale transport device or in photoemission experiments. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected; that is, transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same type of spin polarization, which is actually ubiquitous for a two-terminal device. More accurately, chiral molecules play the role of a spin polarizer rather than a spin filter. The direction of spin polarization is determined by the molecule chirality and the electron incident direction. And the magnitude of spin polarization relies on local spin-orbit coupling in the device. Our work brings a deeper understanding on CISS and interprets recent experiments, for example, the CISS-driven anomalous Hall effect.

Deposition of Chiral Heptahelicene Molecules on Ferromagnetic Co and Fe Thin-Film Substrates

The discovery of chirality-induced spin selectivity (CISS), resulting from an interaction between the electron spin and handedness of chiral molecules, has sparked interest in surface-adsorbed chiral molecules due to potential applications in spintronics, enantioseparation, and enantioselective chemical or biological processes. We study the deposition of chiral heptahelicene by sublimation under ultra-high vacuum onto bare Cu(111), Co bilayer nanoislands on Cu(111), and Fe bilayers on W(110) by low-temperature spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS). In all cases, the molecules remain intact and adsorb with the proximal phenanthrene group aligned parallel to the surface. Three degenerate in-plane orientations on Cu(111) and Co(111), reflecting substrate symmetry, and only two on Fe(110), i.e., fewer than symmetry permits, indicate a specific adsorption site for each substrate. Heptahelicene physisorbs on Cu(111) but chemisorbs on Co(111) and Fe(110) bilayers, which nevertheless remain for the sub-monolayer coverage ferromagnetic and magnetized out-of-plane. We are able to determine the handedness of individual molecules chemisorbed on Fe(110) and Co(111), as previously reported for less reactive Cu(111). The demonstrated deposition control and STM/STS imaging capabilities for heptahelicene on Co/Cu(111) and Fe/W(110) substrate systems lay the foundation for studying CISS in ultra-high vacuum and on the microscopic level of single molecules in controlled atomic configurations.

Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al₂O₃/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Response of Spin to Chiral Orbit and Phonon in Organic Chiral Ferrimagnetic Crystals

Achiral organic materials show nearly negligible orbit angular momentum, whereas organic ferrimagnets with chirality and reduced electron-lattice scattering could fundamentally bridge the gap between ferromagnetism and antiferromagnetism in the rapidly emerging field of ferrimagnetic spintronics. In this work, we report enantiomeric organic chiral ferrimagnets, where the chirality results from the molecular torsion by propeller-like arrangement of the donor and acceptor molecules. The ferrimagnetism results from the difference in electron-phonon coupling of the donor and acceptor inside the chiral crystals. Because the spin polarization is significantly dependent on the chirality, the magnetization of right-handed organic chiral ferrimagnetic crystals is larger than that of left-handed ones by 300% at 10 K. In addition, the processes of both excitation and recombination are strongly related to spin, phonon, and chiral orbit in these chiral ferrimagnets. Overall, both the organic chiral ferrimagnetism and spin chiroptical activities may substantially enrich the field of organic spintronics.

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.

Molecular Compounds in Spintronic Devices: An Intricate Marriage of Chemistry and Physics

Spintronics, a flourishing new field of microelectronics, uses the electron spin for reading and writing information in modern computers and other spintronic devices with a low power consumption and high reliability. In a quest to increase the productivity of such devices, the use of molecular materials as a spacer layer allowed them to perform equally well or even better than conventional all-inorganic heterostructures from metals, alloys, or inorganic semiconductors. In this review, we survey various classes of chemical compounds that have already been tested for this purpose─from organic compounds and coordination complexes to organic-inorganic hybrid materials─since the creation of the first molecule-based spintronic device in 2002. Although each class has its advantages, drawbacks, and applications in molecular spintronics, together they allowed nonchemists to gain insights into spin-related effects and to propose new concepts in the design and fabrication of highly efficient spintronic devices. Other molecular compounds that chemistry could offer in great numbers may soon emerge as suitable spacers or even electrodes in flexible magnetic field sensors, nonvolatile memories, and multifunctional spintronic devices.

Chiral-Molecule-Based Spintronic Devices

Spintronics and molecular chemistry have achieved remarkable achievements separately. Their combination can apply the superiority of molecular diversity to intervene or manipulate the spin-related properties. It inevitably brings in a new type of functional devices with a molecular interface, which has become an emerging field in information storage and processing. Normally, spin polarization has to be realized by magnetic materials as manipulated by magnetic fields. Recently, chiral-induced spin selectivity (CISS) was discovered surprisingly that non-magnetic chiral molecules can generate spin polarization through their structural chirality. Here, the recent progress of integrating the strengths of molecular chemistry and spintronics is reviewed by introducing the experimental results, theoretical models, and device performances of the CISS effect. Compared to normal ferromagnetic metals, CISS originating from a chiral structure has great advantages of high spin polarization, excellent interface, simple preparation process, and low cost. It has the potential to obtain high efficiency of spin injection into metals and semiconductors, getting rid of magnetic fields and ferromagnetic electrodes. The physical mechanisms, unique advantages, and device performances of CISS are sequentially clarified, revealing important issues to current scientific research and industrial applications. This mini-review points out a key technology of information storage for future spintronic devices without magnetic components.

Chiral-Induced Spin Selectivity in Capacitively Coupled Molecules

The chiral-induced spin selectivity (CISS) effect is usually associated with electrons passing through chiral molecules. There are a number of experiments where external electrical fields rearrange electrons inside otherwise isolated molecules, allegedly also generating spin polarization of the molecule. There are theoretical suggestions of pairs of molecules generating spin polarizations in each other due to fluctuating electrical dipoles. This paper critically theoretically evaluates such spin-dependent charge reorganization (SDCR) effects. We find that such effects are not ruled out in principle, but they are very small and probably not detectable.

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Spin-orbit coupling and the fine optical structure of chiral helical polymers

With the discovery of the chiral-induced spin selectivity (CISS) effect, it has been recognized that spin and structural spin-orbit coupling (SOC) play important roles in the electro-optical properties of chiral materials. We redefine the spin-dependent current and magnetic moment operators to include chiral-induced SOC in a helical polymer and deduce optical absorption and circular dichroism (CD) formulae. The fine structure in the optical spectra is calculated for a helical polymer described with the tight-binding model. The effects of both the electron orbit and spin on the optical absorption and CD are discussed. Our investigations demonstrate that the synergy between the electron orbit and spin will contribute to higher-sensitivity circularly polarized light (CPL) detection.