Chirality-Induced Spin Selectivity in Composite Materials: A Device Perspective.

ConspectusMagnetism is an area of immense fundamental and technological importance. At the atomic level, magnetism originates from electron "spin". The field of nanospintronics (or nanoscale spin-based electronics) aims to control spins in nanoscale systems, which has resulted in astronomical improvement in data storage and magnetic field sensing technologies over the past few decades, recognized by the 2007 Nobel Prize in Physics. Spins in nanoscale solid-state devices can also act as quantum bits or qubits for emerging quantum technologies, such as quantum computing and quantum sensing.Due to the fundamental connection between magnetism and spins, ferromagnets play a key role in many solid-state spintronic devices. This is because at the Fermi level, electron density of states is spin-polarized, which permits ferromagnets to act as electrical injectors and detectors of spins. Ferromagnets, however, have limitations in terms of low spin polarization at the Fermi level, stray magnetic fields, crosstalk, and thermal instability at the nanoscale. Therefore, new physics and new materials are needed to propel spintronic and quantum device technologies to the true atomic limit. Emerging new phenomena such as chirality induced spin selectivity or CISS, in which an intriguing correlation between carrier spin and medium chirality is observed, could therefore be instrumental in nanospintronics. This effect could allow molecular-scale, chirality controlled spin injection and detection without the need for any ferromagnet, thus opening a fundamentally new direction for device spintronics.While CISS finds a myriad of applications in diverse areas such as chiral separation, recognition, detection, and asymmetric catalysis, in this focused Account, we exclusively review spintronic device results of this effect due to its immense potential for future spintronics. The first generation of CISS-based spintronic devices have primarily used chiral bioorganic molecules; however, many practical limitations of these materials have also been identified. Therefore, our discussion revolves around the family of chiral composite materials, which may emerge as an ideal platform for CISS due to their ability to assimilate various desirable material properties on a single platform. This class of materials has been extensively studied by the organic chemistry community in the past decades, and we discuss the various chirality transfer mechanisms that have been identified, which play a central role in CISS. Next, we discuss CISS device studies performed on some of these chiral composite materials. Emphasis is given to the family of chiral organic-carbon allotrope composites, which have been extensively studied by the authors of this Account over the past several years. Interestingly, due to the presence of multiple materials, CISS signals from hybrid chiral systems sometimes differ from those observed in purely chiral systems. Given the sheer diversity of chiral composite materials, CISS device studies so far have been limited to only a few varieties, and this Account is expected to draw increased attention to the family of chiral composites and motivate further studies of their CISS applications.

Chirality-Induced Spin Selectivity in Supramolecular Chirally Functionalized Graphene.

Chiral graphene hybrid materials have attracted significant attention in recent years due to their various applications in the areas of chiral catalysis, chiral separation and recognition, enantioselective sensing, etc. On the other hand, chiral materials are also known to exhibit chirality-dependent spin transmission, commonly dubbed "chirality induced spin selectivity" or CISS. However, CISS properties of chiral graphene materials are largely unexplored. As such, it is not clear whether graphene is even a promising material for the CISS effect given its weak spin-orbit interaction. Here, we report the CISS effect in chiral graphene sheets, in which a graphene derivative (reduced graphene oxide or rGO) is noncovalently functionalized with chiral Fmoc-FF (Fmoc-diphenylalanine) supramolecular fibers. The graphene flakes acquire a "conformational chirality" postfunctionalization, which, combined with other factors, is presumably responsible for the CISS signal. The CISS signal correlates with the supramolecular chirality of the medium, which depends on the thickness of graphene used. Quite interestingly, the noncovalent supramolecular chiral functionalization of conductive materials offers a simple and straightforward methodology to induce chirality and CISS properties in a multitude of easily accessible advanced conductive materials.

Chirality-induced spin selectivity in functionalized carbon nanotube networks: The role of spin-orbit coupling.

Spin-orbit coupling in a chiral medium is generally assumed to be a necessary ingredient for the observation of the chirality-induced spin selectivity (CISS) effect. However, some recent studies have suggested that CISS may manifest even when the chiral medium has zero spin-orbit coupling. In such systems, CISS may arise due to an orbital polarization effect, which generates an electromagnetochiral anisotropy in two-terminal conductance. Here, we examine these concepts using a chirally functionalized carbon nanotube network as the chiral medium. A transverse measurement geometry is used, which nullifies any electromagnetochiral contribution but still exhibits the tell-tale signs of the CISS effect. This suggests that CISS may not be explained solely by electromagnetochiral effects. The role of nanotube spin-orbit coupling on the observed pure CISS signal is studied by systematically varying nanotube diameter. We find that the magnitude of the CISS signal scales proportionately with the spin-orbit coupling strength of the nanotubes. We also find that nanotube diameter dictates the supramolecular chirality of the medium, which in turn determines the sign of the CISS signal.

Transverse magnetoconductance in two-terminal chiral spin-selective devices.

The phenomenon of chirality induced spin selectivity (CISS) has triggered significant activity in recent years, although many aspects of it remain to be understood. For example, most investigations are focused on spin polarizations collinear to the charge current, and hence longitudinal magnetoconductance (MC) is commonly studied in two-terminal transport experiments. Very little is known about the transverse spin components and transverse MC - their existence, as well as any dependence of this component on chirality. Furthermore, the measurement of the CISS effect via two-terminal MC experiments remains a controversial topic. Detection of this effect in the linear response regime is debated, with contradicting reports in the literature. Finally, the potential influence of the well-known electric magnetochiral effect on CISS remains unclear. To shed light on these issues, in this work we have investigated the bias dependence of the CISS effect using planar carbon nanotube networks functionalized with chiral molecules. We find that (a) transverse MC exists and exhibits tell-tale signs of the CISS effect, (b) transverse CISS MC vanishes in the linear response regime establishing the validity of Onsager's relation in two-terminal CISS systems, and finally (c) the CISS signal remains present even in the absence of electric magneto chiral effects, suggesting the existence of an alternative physical origin of CISS MC.

Bayes factor functions for reporting outcomes of hypothesis tests.

Bayes factors represent a useful alternative to P-values for reporting outcomes of hypothesis tests by providing direct measures of the relative support that data provide to competing hypotheses. Unfortunately, the competing hypotheses have to be specified, and the calculation of Bayes factors in high-dimensional settings can be difficult. To address these problems, we define Bayes factor functions (BFFs) directly from common test statistics. BFFs depend on a single noncentrality parameter that can be expressed as a function of standardized effects, and plots of BFFs versus effect size provide informative summaries of hypothesis tests that can be easily aggregated across studies. Such summaries eliminate the need for arbitrary P-value thresholds to define "statistical significance." Because BFFs are defined using nonlocal alternative prior densities, they provide more rapid accumulation of evidence in favor of true null hypotheses without sacrificing efficiency in supporting true alternative hypotheses. BFFs can be expressed in closed form and can be computed easily from z, t, χ2, and F statistics.

Thermally-driven large current-perpendicular-to-plane magnetoresistance in ultrathin flakes of vanadium diselenide.

Current-perpendicular-to-plane magnetoresistance (CPP MR) in layered heterojunctions is at the heart of modern magnetic field sensing and data storage technologies. van der waals heterostructures and two-dimensional (2D) magnets opened a new playground for exploring this effect, although most 2D magnets exhibit large CPP MR only at very low temperatures due to their very low Curie temperatures. vanadium diselenide (VSe2) is a promising material since its monolayers can potentially act as room temperature ferromagnets. VSe2multilayers have been predicted to exhibit CPP MR effects, although experimental work in this area remains scarce. In this work we investigate CPP MR in 1T-VSe2ultrathin flakes, revealing alarge (∼60%-70%), positive, linear, and non saturating CPP MR, which persists close to room temperature (∼250 K), in a relatively small magnetic field range of ±12 kG. The CPP MR has been found to increase with decreasing flake thickness. The CPP MR originates due to the intrinsic inhomogeneity in the CPP transport path, andexhibits unprecedented immunity against thermal fluctuations, leading to increasingly enhanced MR as temperature is increased, even significantly beyond the charge density wave transition temperature. The observed 'thermally-driven' MR features are remarkably robust and reproducible, and can offer a viable route for developing practical room temperature 2D based magnetic sensor technologies. Our results also suggest that harnessing similar effects in other 2D systems could result in large MR as well, thereby motivating further research on CPP transport in these systems, which has been relatively unexplored so far.

Chirality-Induced Spin Selectivity in Heterochiral Short-Peptide-Carbon-Nanotube Hybrid Networks: Role of Supramolecular Chirality.

Supramolecular short-peptide assemblies have been widely used for the development of biomaterials with potential biomedical applications. These peptides can self-assemble in a multitude of chiral hierarchical structures triggered by the application of different stimuli, such as changes in temperature, pH, solvent, etc. The self-assembly process is sensitive to the chemical composition of the peptides, being affected by specific amino acid sequence, type, and chirality. The resulting supramolecular chirality of these materials has been explored to modulate protein and cell interactions. Recently, significant attention has been focused on the development of chiral materials with potential spintronic applications, as it has been shown that transport of charge carriers through a chiral environment polarizes the carrier spins. This effect, named chirality-induced spin selectivity or CISS, has been studied in different chiral organic molecules and materials, as well as carbon nanotubes functionalized with chiral molecules. Nevertheless, this effect has been primarily explored in homochiral systems in which the chirality of the medium, and hence the resulting spin polarization, is defined by the chirality of the molecule, with limited options for tunability. Herein, we have developed heterochiral carbon-nanotube-short-peptide materials made by the combination of two different chiral sources: that is, homochiral peptides (l/d) + glucono-δ-lactone. We show that the presence of a small amount of glucono-δ-lactone with fixed chirality can alter the supramolecular chirality of the medium, thereby modulating the sign of the spin signal from "up" to "down" and vice versa. In addition, small amounts of glucono-δ-lactone can even induce nonzero spin polarization in an otherwise achiral and spin-inactive peptide-nanotube composite. Such "chiral doping" strategies could allow the development of complementary CISS-based spintronic devices and circuits on a single material platform.

Efficient alternatives for Bayesian hypothesis tests in psychology.

Bayesian hypothesis testing procedures have gained increased acceptance in recent years. A key advantage that Bayesian tests have over classical testing procedures is their potential to quantify information in support of true null hypotheses. Ironically, default implementations of Bayesian tests prevent the accumulation of strong evidence in favor of true null hypotheses because associated default alternative hypotheses assign a high probability to data that are most consistent with a null effect. We propose the use of "nonlocal" alternative hypotheses to resolve this paradox. The resulting class of Bayesian hypothesis tests permits more rapid accumulation of evidence in favor of both true null hypotheses and alternative hypotheses that are compatible with standardized effect sizes of most interest in psychology. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Molecular Functionalization and Emergence of Long-Range Spin-Dependent Phenomena in Two-Dimensional Carbon Nanotube Networks.

Molecular functionalization of CNTs is a routine procedure in the field of nanotechnology. However, whether and how these molecules affect the spin polarization of the charge carriers in CNTs are largely unknown. In this work we demonstrate that spin polarization can indeed be induced in two-dimensional (2D) CNT networks by "certain" molecules and the spin signal routinely survives length scales significantly exceeding 1 µm. This result effectively connects the area of molecular spintronics with that of carbon-based 2D nanoelectronics. By using the versatility of peptide chemistry, we further demonstrate how spin polarization depends on molecular structural features such as chirality as well as molecule-nanotube interactions. A chirality-independent effect was detected in addition to the more common chirality-dependent effect, and the overall spin signal was found to be a combination of both. Finally, the magnetic field dependence of the spin signals has been explored, and the "chirality-dependent" signal has been found to exist only in certain field angles.

Carrier localization and magnetoresistance in DNA-functionalized carbon nanotubes.

Helical functionalization of carbon nanotubes using DNA strands can polarize carrier spins through chirality induced spin selectivity (or CISS) effect. Detection of this effect using transport experiments unravels an underlying magnetoresistance effect, origin of which is not well understood. In the present study, we investigate this effect, a fundamental understanding of which is crucial for the potential use of this system in spintronic devices. The conduction mechanism has been found to be in the strongly localized regime due to DNA functionalization, with the observed magnetoresistance originating from the interference effects between the forward and backward hopping paths. CISS-induced spin polarization has been estimated to increase the carrier localization length by an order of magnitude in the low temperature range and it affects the magnetoresistance effect in a non-trivial way that is not observed in conventional systems.

A Modified Sequential Probability Ratio Test.

We describe a modified sequential probability ratio test that can be used to reduce the average sample size required to perform statistical hypothesis tests at specified levels of significance and power. Examples are provided for z tests, t tests, and tests of binomial success probabilities. A description of a software package to implement the test designs is provided. We compare the sample sizes required in fixed design tests conducted at 5% significance levels to the average sample sizes required in sequential tests conducted at 0.5% significance levels, and we find that the two sample sizes are approximately equal.

Carrier Transport Engineering in Carbon Nanotubes by Chirality-Induced Spin Polarization.

Carbon nanotubes (CNTs), helically wrapped with single-stranded DNA, have recently emerged as a spin-filtering material. The inversion asymmetric helical potential of DNA creates a spin-filtering effect (commonly known as "chirality-induced spin selectivity" or CISS), which polarizes carrier spins in the nanotube. Thus, tuning of the DNA-CNT interaction is expected to affect carrier spins in nanotubes. The CISS effect induces spin polarization, which is coupled with the carrier's momentum direction, and therefore, in one-dimensional systems, such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This spin momentum locking can have a profound impact on charge transport in nanotubes as backscattering due to phonons and disorder will be suppressed as these mechanisms are spin-independent. Here, we report DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of interaction with the CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and interference phenomena arising from carrier backscattering. This work raises the intriguing possibility of engineering charge transport in nanotubes via CISS-induced spin polarization by tailor-made DNA sequences.

High-Temperature Polaritons in Ceramic Nanotube Antennas.

High-temperature thermal photonics presents unique challenges for engineers as the database of materials that can withstand extreme environments are limited. In particular, ceramics with high temperature stability that can support coupled light-matter excitations, that is, polaritons, open new avenues for engineering radiative heat transfer. Hexagonal boron nitride (hBN) is an emerging ceramic 2D material that possesses low-loss polaritons in two spectrally distinct mid-infrared frequency bands. The hyperbolic nature of these frequency bands leads to a large local density of states (LDOS). In 2D form, these polaritonic states are dark modes, bound to the material. In cylindrical form, boron nitride nanotubes (BNNTs) create subwavelength particles capable of coupling these dark modes to radiative ones. In this study, we leverage the high-frequency optical phonons present in BNNTs to create strong mid-IR thermal antenna emitters at high temperatures (938 K). Through direct measurement of thermal emission of a disordered system of BNNTs, we confirm their radiative polaritonic modes and show that the antenna behavior can be observed even in a disordered system. These are among the highest-frequency optical phonon polaritons that exist and could be used as high-temperature mid-IR thermal nanoantenna sources.

Long Carbon Nanotubes Functionalized with DNA and Implications for Spintronics.

Helical molecules such as DNA have recently been found to behave as an efficient source and detector of spin-polarized charge carriers. This phenomenon, often dubbed as chirality-induced spin selectivity or CISS, could be used to significantly improve the performance of spintronic devices, which utilize carrier spins (rather than charge) to realize electronic and sensing functions. Recently, it has been reported that carbon nanotubes, helically wrapped with DNA, can also act as an efficient source and detector of spin-polarized carriers, by virtue of spin-orbit coupling originating from the helical potential. It has been postulated that spin polarization should increase with the length of the wrapped tubes. However, in literature, most fabrication processes yield tubes with submicron lengths, which can produce ∼70% spin polarization. In an effort to enhance this effect further, here, we report a fabrication process that can yield DNA-wrapped nanotubes of length ∼1-4 microns. Detailed characterization of these devices, using atomic force microscopy, Raman, UV-vis, and temperature-dependent transport, has been presented. Initial transport measurements indicate the presence of strong magnetoresistance in these tubes, which could be attributed to spin-dependent effects. Systematic fabrication of long DNA-wrapped nanotubes, which has hitherto not been reported, is expected to enable further investigation into the spin-dependent properties of these ultimate one-dimensional nanoscale hybrids and may have a significant impact on nanoscale spintronics.

Spin independent magnetoresistance effects in vertical graphene spin valves.

Vertical spin valve device configuration with multilayer graphene (MLG) as spacer has drawn significant attention in recent years because of its potential to produce large magnetoresistance (MR) effect due to perfect spin filtering. However, demonstration of this effect has remained elusive so far. Here we consider MLG vertical spin valve structures and show that they exhibit spin independent MR effects, which are orders of magnitude stronger than the spin dependent effects reported to date. These effects manifest within a moderate field range of 10 kG and depend on various factors such as hybridization near the top graphene surface, doping, defects and interlayer coupling. These effects highlight the rich spectrum of physical phenomena that manifest in such systems, which could be exploited in low to moderate magnetic field sensing applications.

Spin filtering with poly-T wrapped single wall carbon nanotubes.

Spin filtering is an essential operation in spintronics that allows the creation and detection of spin polarized carriers. Transition metal ferromagnets are used as spin filters in most cases, though their spin filtering efficiency is only around ∼50%, thereby limiting the efficiency of spintronic devices. Recently, chiral systems such as DNA have been shown to exhibit efficient spin filtering, a phenomenon often dubbed as "chirality induced spin selectivity" (CISS). In this work, we consider single wall carbon nanotubes helically wrapped with single stranded poly-T DNA. By magnetoresistance measurements we show that this system exhibits significant spin polarization of ∼80%, which could be attributed to the Rashba spin-orbit interaction induced by the inversion-asymmetric helical potential of the DNA. The observed spin polarization is larger than that reported before for d(GT)15 strands. Such systems allow tailoring of spin polarization by chemical means and also allow extremely localized creation and detection of spin polarization without any magnetic element and could lead to extreme miniaturization and compact integration of spintronic devices and circuits.

Ultrahigh density array of vertically aligned small-molecular organic nanowires on arbitrary substrates.

In recent years π-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication. A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently π-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important π-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics. Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

Photophysics and energy transfer studies of Alq3 confined in the voids of nanoporous anodic alumina.

We report on a hierarchical nanoarchitecture wherein distinct chromophores are deterministically placed at two different types of sites in a nanoporous metal oxide framework. One chromophore, namely Tris(8-hydroxyquinoline)aluminium(III) (Alq3), is embedded in the 1-2 nm sized nanovoids of anodic aluminum oxide (AAO) and another chromophore (carboxyfluorescein or pyrenebutyric acid) is anchored in the form of a monolayer to the surface of the walls of the cylindrical nanopores (- 20 nm in diameter) of AAO. We found the luminescence maximum to occur at 492 nm, blueshifted by at least 18 nm from the value in solutions and thin films. The excited state decay of Alq3 molecules in nanovoids was found to be biexponential with a fast component of 338 ps and a slower component of 2.26 ns, different from Alq3 thin films and solutions. Using a combination of steady state and time-resolved luminescence studies, we found that efficient Forster-type resonance energy transfer (FRET) from Alq3 in the nanovoids to the carboxyfluorescein monolayer could be used to pump the emission of surface-bound chromophores. Conversely, the emission of nanovoid-confined Alq3 could be pumped by energy transfer from a pyrenebutyric acid monolayer. Such intra-nanoarchitecture interactions between chromophores deterministically placed in different spatial locations are important in applications such as organic light emitting diodes, chemical sensors, energy transfer fluorescent labels, light harvesting antennas and organic spintronics.

Spin transport in self assembled all-metal nanowire spin valves: a study of the pure Elliott-Yafet mechanism.

We report experimental study of spin transport in all metal nanowire spin valve structures. The nanowires have a diameter of 50 nm and consist of three layers--cobalt, copper, and nickel. Based on the experimental observations, we determine that the primary spin relaxation mechanism in the paramagnet layer--copper--is the Elliott-Yafet mode associated with elastic scattering caused by charged states on the surface of the nanowires. This mode is overwhelmingly dominant over all other modes, so that we are able to study the pure Elliott-Yafet mechanism in isolation. We deduce that the spin diffusion length associated with this mechanism is about 16 nm in our nanowires and is fairly temperature independent in the range 1-100 K, which is consistent with the spin relaxation being associated with elastic scattering by surface states. The corresponding spin relaxation time is about 100 femtoseconds. We also find that the spin relaxation rate is fairly independent of the electric field driving the current in the field range 0.3-3 kV/cm.