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.

Paramagnetic Molecular Semiconductors Combining Anisotropic Magnetic Ions with TCNQ Radical Anions.

We report the synthesis, magnetic properties, and transport properties of paramagnetic metal complexes, [Co(DMF)4(TCNQ)2](TCNQ)2 (1), [La(DMF)8(TCNQ)](TCNQ)5 (2), and [Nd(DMF)7(TCNQ)](TCNQ)5 (3) (DMF = N,N-dimethylformamide, TCNQ = 7,7,8,8-tetracyanoquinodimethane). All three compounds contain fractionally charged TCNQδ- anions (0 < δ < 1) and mononuclear complex cations in which the coordination environment of a metal center includes several DMF molecules and one or two terminally coordinated TCNQδ- anions. The coordinated TCNQδ- anions participate in π-π stacking interactions with noncoordinated TCNQδ- anions, forming columnar substructures that provide efficient charge-transporting pathways. As a result, temperature-dependent conductivity measurements demonstrate that all three compounds exhibit semiconducting behavior.

Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine.

A proton-transfer reaction between squaric acid (H2sq) and 2,3-dimethylpyrazine (2,3-Me2pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me2pyzH+)(Hsq-)·H2O (1), which possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me2pyzH+ cations and strips featuring extensive hydrogen bonding between the Hsq- anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me2pyzH+)2(Hsq23-)(H5O2+), whereas the AFE phase can be described as (2,3-Me2pyzH+)(Hsq-)(H2O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.

Chirality-Induced Magnet-Free Spin Generation in a Semiconductor.

Electrical generation and transduction of polarized electron spins in semiconductors (SCs) are of central interest in spintronics and quantum information science. While spin generation in SCs is frequently realized via electrical injection from a ferromagnet (FM), there are significant advantages in nonmagnetic pathways of creating spin polarization. One such pathway exploits the interplay of electron spin with chirality in electronic structures or real space. Here, utilizing chirality-induced spin selectivity (CISS), the efficient creation of spin accumulation in n-doped GaAs via electric current injection from a normal metal (Au) electrode through a self-assembled monolayer (SAM) of chiral molecules (α-helix l-polyalanine, AHPA-L), is demonstrated. The resulting spin polarization is detected as a Hanle effect in the n-GaAs, which is found to obey a distinct universal scaling with temperature and bias current consistent with chirality-induced spin accumulation. The experiment constitutes a definitive observation of CISS in a fully nonmagnetic device structure and demonstration of its ability to generate spin accumulation in a conventional SC. The results thus place key constraints on the physical mechanism of CISS and present a new scheme for magnet-free SC spintronics.

Spin Polarization in Transport Studies of Chirality-Induced Spin Selectivity.

Chirality-induced spin selectivity (CISS) is a recently discovered effect in which structural chirality can result in different conductivities for electrons with opposite spins. In the CISS community, the degree of spin polarization is commonly used to describe the efficiency of the spin filtering/polarizing process, as it represents the fraction of spins aligned along the chiral axis of chiral materials originating from non-spin-polarized currents. However, the methods of defining, calculating, and analyzing spin polarization have been inconsistent across various studies, hindering advances in this field. In this Perspective, we connect the relevant background and the definition of spin polarization, discuss its calculation in different contexts in the CISS, and propose a practical and meaningful figure of merit by quantitative analysis of magnetoresistance in CISS transport studies.

Interplay of structural chirality, electron spin and topological orbital in chiral molecular spin valves.

Chirality has been a property of central importance in physics, chemistry and biology for more than a century. Recently, electrons were found to become spin polarized after transmitting through chiral molecules, crystals, and their hybrids. This phenomenon, called chirality-induced spin selectivity (CISS), presents broad application potentials and far-reaching fundamental implications involving intricate interplays among structural chirality, topological states, and electronic spin and orbitals. However, the microscopic picture of how chiral geometry influences electronic spin remains elusive, given the negligible spin-orbit coupling (SOC) in organic molecules. In this work, we address this issue via a direct comparison of magnetoconductance (MC) measurements on magnetic semiconductor-based chiral molecular spin valves with normal metal electrodes of contrasting SOC strengths. The experiment reveals that a heavy-metal electrode provides SOC to convert the orbital polarization induced by the chiral molecular structure to spin polarization. Our results illustrate the essential role of SOC in the metal electrode for the CISS spin valve effect. A tunneling model with a magnetochiral modulation of the potential barrier is shown to quantitatively account for the unusual transport behavior.

Electrically gated molecular thermal switch.

Controlling heat flow is a key challenge for applications ranging from thermal management in electronics to energy systems, industrial processing, and thermal therapy. However, progress has generally been limited by slow response times and low tunability in thermal conductance. In this work, we demonstrate an electronically gated solid-state thermal switch using self-assembled molecular junctions to achieve excellent performance at room temperature. In this three-terminal device, heat flow is continuously and reversibly modulated by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices have ultrahigh switching speeds above 1 megahertz, have on/off ratios in thermal conductance greater than 1300%, and can be switched more than 1 million times. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design.

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.

Linear and Nonlinear Two-Terminal Spin-Valve Effect from Chirality-Induced Spin Selectivity.

Various mechanisms of electrical generation of spin polarization in nonmagnetic materials have been a subject of broad interest for their underlying physics and device potential in spintronics. One such scheme is chirality-induced spin selectivity (CISS), with which structural chirality leads to different electric conductivities for electrons of opposite spins. The resulting effect of spin filtering has been reported for a number of chiral molecules assembled on different surfaces. However, the microscopic origin and transport mechanisms remain controversial. In particular, the fundamental Onsager relation was argued to preclude linear-response detection of CISS by a ferromagnet. Here, we report definitive observation of CISS-induced magnetoconductance in vertical heterojunctions of (Ga,Mn)As/AHPA-L molecules/Au, directly verifying spin filtering by the AHPA-L molecules via spin detection by the (Ga,Mn)As. The pronounced and robust magnetoconductance signals resulting from the use of a magnetic semiconductor enable a rigorous examination of its bias dependence, which shows both linear- and nonlinear-response components. The definitive identification of the linear-response CISS-induced two-terminal spin-valve effect places an important constraint for a viable theory of CISS and its device manifestations. The results present a promising route to spin injection and detection in semiconductors without using any magnetic material.

Efficient, Stable, and Low-Cost PbS Quantum Dot Solar Cells with Cr-Ag Electrodes.

PbS quantum dots (QDs) are a promising nanostructured material for solar cells. However, limited works have been done to explore the active layer thickness, layer deposition techniques, stability improvement, and cost reduction for PbS QD solar cells. We address those issues of device fabrication herein and suggest their possible solutions. In our work, to get the maximum current density from a PbS QD solar cell, we estimated the optimized active layer thickness using Matlab simulation. After that, we fabricated a high-performance and low-cost QD photovoltaic (PV) device with the simulated optimized active layer thickness. We implemented this low-cost device using a 10 mg/mL PbS concentration. Here, spin coating and drop-cast layer deposition methods were used and compared. We found that the device prepared by the spin coating method was more efficient than that by the drop cast method. The spin-coated PbS QD solar cell provided 6.5% power conversion efficiency (PCE) for the AM1.5 light spectrum. Besides this, we observed that Cr (chromium) interfaced with the Ag (Cr-Ag) electrode can provide a highly air-stable electrode.

Molecular Patterning and Directed Self-Assembly of Gold Nanoparticles on GaAs.

The ability to create micro-/nanopatterns of organic self-assembled monolayers (SAMs) on semiconductor surfaces is crucial for fundamental studies and applications in a number of emerging fields in nanoscience. Here, we demonstrate the direct patterning of thiolate SAMs on oxide-free GaAs surface by dip-pen nanolithography (DPN) and microcontact printing (µCP), facilitated by a process of surface etching and passivation of the GaAs. A quantitative analysis on the molecular diffusion on GaAs was conducted by examining the writing of nanoscale dot and line patterns by DPN, which agrees well with surface diffusion models. The functionality of the patterned thiol molecules was demonstrated by directed self-assembly of gold nanoparticles (Au NPs) onto a template of 4-aminothiophenol (ATP) SAM on GaAs. The highly selective assembly of the Au NPs was made evident with atomic force microscopy (AFM) and scanning electron microscopy (SEM). The ability to precisely control the assembly of Au NPs on oxide-free semiconductor surfaces using molecular templates may lead to an efficient bottom-up method for the fabrication of nanoplasmonic structures.