Emerging Spintronic Materials and Functionalities

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.

Stable organic radicals - a material platform for developing molecular quantum technologies

An electron spin is a natural candidate for a quantum bit - the quantum information storage unit. Stable organic radicals, consisting of unpaired electron spins, can thus be explored for the development of quantum science and technologies, owing to their excellent chemical tunability and their great promise for scalability. The molecular network formed by the stable organic radicals can be used for the design of spin-based quantum computing circuits. Here the state-of-the-art development of stable organic radicals is reviewed from a variety of perspectives. The categories of stable organic radicals are discussed, emphasizing on the π-conjugated radical networks. The applications of the stable organic radicals to quantum communications, quantum computing and quantum sensing are reviewed. The quantum teleportation based on the donor-acceptor-radical molecular system is reviewed. For controllable quantum gate operations, the spin dynamics in a bi-radical molecule driven by a triplet is discussed. Quantum sensing of lithium ions using stable organic radicals is realized for the development of new energy materials. Quantum timing and quantum imaging are still unexplored by using stable organic radicals. In conclusion, stable organic radicals, especially the π-conjugated radical networks, can make a great new contribution to the development of quantum technologies.

Resonance-Assisted Self-Doping in Robust Open-Shell Ladder-Type Oligoaniline Analogues

A novel resonance-assisted self-doping mechanism has been demonstrated in ladder-type oligoaniline-derived organic conductors. The new class of compounds has a unique structure incorporating acidic phenolic hydroxyl groups into the ladder-type cyclohexadiene-1,4-diimine core, enabling efficient resonance-assisted proton transfer and electronic doping without the need for external dopants. Mechanistic and computational studies confirm the open-shell, zwitterionic nature of the self-doped state and the significant role played by the dielectric environment. This new self-doping mechanism allows for higher stability and durability in the material's electronic performance. The self-doped form retains durability under harsh conditions and maintains its properties over extended periods of time.

Halogen-Dependent Circular Dichroism and Magneto-Photoluminescence Effects in Chiral 2D Lead Halide Perovskites

Chiral lead halide perovskites (chiral LHPs) have emerged as one of the best candidates for opto-spintronics due to their large spin-orbit coupling (SOC) and unique chirality-induced spin selectivity (CISS) even in the absence of a magnetic field. Here, we report the impact of halide composition on circular dichroism (CD) and magneto-photoluminescence (PL) effects of chiral 2D LHPs (R/S-MBA)2PbBrxI4-x (MBA = C6H5CH2(CH3)NH3). By tuning the mixing ratio of Br/I halide anions, we find that (R/S-MBA)2PbBrxI4-x thin films exhibit tunable and wide wavelength range CD signals. Simultaneously, the main CD signals near the exciton absorption band gradually blue shift until they disappear. Moreover, the halogen-dependent negative magneto-PL effects of (R/S-MBA)2PbBrxI4-x thin films excited by left/right circularly polarized light can be detected at room temperature. We demonstrated that the halide composition can effectively modulate exciton splitting and chirality transfer in (R/S-MBA)2PbBrxI4-x owing to the chirality-induced SOC and crystalline structure transition, which lead to the adjustable CD signals. The interplay of Rashba-type band spin splitting and spin mixing among bright triplet exciton states is responsible for the halogen-dependent magneto-PL effect of chiral 2D LHPs. This study enables chiral 2D LHPs with CISS to be a new class of promising opto-spintronics materials for exploring high-performance spin-light-emitting diodes by halide engineering.

Gated off-site radical injection: Bidirectional conductance modulation in single-molecule junctions

Uncovering the effects of radical injection into responsive organic molecules is a long-sought goal, and the single-molecule junctions provide a unique way to investigate molecular conductance evolution during the radical injection. We can modulate the main channel conductance by using electronic injection from off-site neutral radicals acting as gating terminals. Two families of cyclopentadienone derivatives were synthesized, featuring the inter-pyridyl main conductance channel and the inter-radical paths that are linear (FCF) or cross conjugated (PCP). Using a scanning tunneling microscope break junction technique, we find that the injection of mono- and diradicals in the PCP system unexpectedly decreases the conductance regarding the closed-shell analog, while that of FCF systems increases. Through-bond and through-space conductance mechanisms are found in the FCF and PCP series, respectively, and jointly modulate the overall charge transmission. This off-site injection concept offers a promising approach for developing molecular devices by manipulating electrical conductance in single-molecule junctions.

Realizing Long Magnon Diffusion in Organic-Inorganic Hybrid Perovskite Film by the Universal Isotope Effect

Organic-inorganic halide perovskite (OIHP) spintronics has become a promising research field, as it provides a new precisely manipulable degree of freedom. Recently, by utilizing the spin Seebeck effect and inverse spin-Hall effect measurements, we have discovered substantial magnon injection and transport in Pt/OIHP/Y3Fe5O12 nonlocalized structure. In theory, hyperfine interaction (HFI) is considered to have an important role in the magnon transport of OIHP, but there is no clear experimental evidence reported so far. We report increased spin Seebeck coefficient and lengthened magnon diffusion length in deuterated- (D-) OIHP films that have weaker HFI strength compared with protonated- (H-) OIHP. Consequently, D-MAPbBr3 film, as a non-ferromagnetic spacer, achieves long magnon diffusion length at room temperature (close to 120.3 nm). Our finding provides valuable insights into understanding magnon transport in OIHP films and paves the way for the use of OIHPs in multifunctional applications.

Achieving 9% EQE in light-emitting electrochemical cells via a TADF-sensitized fluorescence strategy

Light-emitting electrochemical cells (LECs) are appealing for cost-effective, large-area emission applications; however, their luminescence efficiency is significantly limited by exciton annihilation caused by high concentration polarons. Here, we present thermally activated delayed fluorescence (TADF) sensitized fluorescence LECs (TSF-LECs) that achieve a record 9% EQE. The TADF sensitizers with rapid reverse intersystem crossing (RISC) rates can effectively convert triplet excitons to singlet excitons in LECs, thereby establishing a more efficient overall energy transfer pathway. Importantly, magneto-electroluminescence measurements indicate that the additional RISC route in TSF-LECs significantly suppresses the annihilation of triplet excitons and thus reduces exciton loss under high concentration polaron conditions. Compared to LECs without a sensitizer, TSF-LECs exhibit improved EQE and luminance, extended operational lifetimes, and suppressed efficiency roll-off. A flexible display prototype based on TSF-LECs was further fabricated, capable of stably displaying high-brightness preset patterns for extended periods. The exploration of the exciton dynamics in high concentration polaron environments offers valuable insights for future developments in high-efficiency LEC technology.

Effect of metal-ligand interactions on magnetic characteristics of two-dimensional Kagome structured perthiolated coronene (PTC) metal-organic frameworks (MOFs)

In recent years, the potential applications of two-dimensional (2D) metal-organic framework (MOF) materials in fields like spintronics have drawn increasing attention. Inspired by the successful synthesis of a perthiolated coronene (PTC)-Fe MOF structure, this study explores the fine-tuning of its electronic and magnetic structure by substituting Fe elements with various transition metals. Our calculations demonstrate a substantial increase in the Curie temperature (Tc) by a factor of 5 for Co and 10 for Mn when replacing Fe. This enhancement is attributed to the elevated electron density near the Fermi level, facilitating the generation of additional itinerant electrons crucial for the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism. However, substituting Fe with V, Cr, Ni, and Cu leads to a loss of ferromagnetic ground state. Our work enhances the understanding of the electronic and magnetic behavior of the 2D PTC-TM (transition metal) MOF family, and provides a promising avenue for engineering 2D magnetic MOF systems.

Magnetic Structure-Dependent Ultrafast Spin Relaxation in Magnet CrI3: A Time-Domain ab Initio Study

Two-dimensional magnet CrI3 is a promising candidate for spintronic devices. Using nonadiabatic molecular dynamics and noncollinear spin time-dependent density functional theory, we investigated hole spin relaxation in two-dimensional CrI3 and its dependence on magnetic configurations, impacted by spin-orbit and electron-phonon interactions. Driven by in-plane and out-of-plane iodine motions, the relaxation rates vary, extending from over half a picosecond in ferromagnetic systems to tens of femtoseconds in certain antiferromagnetic states due to significant spin fluctuations, associated with the nonadiabatic spin-flip in tuning to the adiabatic flip. Antiferromagnetic CrI3 with staggered layer magnetic order notably accelerates adiabatic spin-flip due to enhanced state degeneracy and additional phonon modes. Ferrimagnetic CrI3 shows a transitional behavior between ferromagnetic and antiferromagnetic types as the magnetic moment changes. These insights into the spin dynamics of CrI3 underscore its potential for rapid-response spintronic applications and advance our understanding of two-dimensional materials for spintronics.

Structural Isomeric Effect on Spin Transport in Molecular Semiconductors

Molecular semiconductor (MSC) is a promising candidate for spintronic applications benefiting from its long spin lifetime caused by light elemental-composition essence and thus weak spin-orbit coupling (SOC). According to current knowledge, the SOC effect, normally dominated by the elemental composition, is the main spin-relaxation causation in MSCs, and thus the molecular structure-induced SOC change is one of the most concerned issues. In theoretical study, molecular isomerism, a most prototype phenomenon, has long been considered to possess little difference on spin transport previously, since elemental compositions of isomers are totally the same. However, here in this study, quite different spin-transport performances are demonstrated in ITIC and its structural isomers BDTIC experimentally, for the first time, though the charge transport and molecular stacking of the two films are very similar. By further experiments of electron-paramagnetic resonance and density-functional-theory calculations, it is revealed that noncovalent-conformational locks (NCLs) formed in BDTIC can lead to enhancement of SOC and thus decrease the spin lifetime. Hence, this study suggests the influences from the structural-isomeric effect must be considered for developing highly efficient spin-transport MSCs, which also provides a reliable theoretical basis for solving the great challenge of quantificational measurement of NCLs in films in the future.

Van der Waals Magnetic Electrode Transfer for Two-Dimensional Spintronic Devices

Two-dimensional (2D) materials are promising candidates for spintronic applications. Maintaining their atomically smooth interfaces during integration of ferromagnetic (FM) electrodes is crucial since conventional metal deposition tends to induce defects at the interfaces. Meanwhile, the difficulties in picking up FM metals with strong adhesion and in achieving conductance match between FM electrodes and spin transport channels make it challenging to fabricate high-quality 2D spintronic devices using metal transfer techniques. Here, we report a solvent-free magnetic electrode transfer technique that employs a graphene layer to assist in the transfer of FM metals. It also serves as part of the FM electrode after transfer for optimizing spin injection, which enables the realization of spin valves with excellent performance based on various 2D materials. In addition to two-terminal devices, we demonstrate that the technique is applicable for four-terminal spin valves with nonlocal geometry. Our results provide a promising future of realizing 2D spintronic applications using the developed magnetic electrode transfer technique.

Emerging Nonlinear Photocurrents in Lead Halide Perovskites for Spintronics

Lead halide perovskites (LHPs) containing organic parts are emerging optoelectronic materials with a wide range of applications thanks to their high optical absorption, carrier mobility, and easy preparation methods. They possess spin-dependent properties, such as strong spin-orbit coupling (SOC), and are promising for spintronics. The Rashba effect in LHPs can be manipulated by a magnetic field and a polarized light field. Considering the surfaces and interfaces of LHPs, light polarization-dependent optoelectronics of LHPs has attracted attention, especially in terms of spin-dependent photocurrents (SDPs). Currently, there are intense efforts being made in the identification and separation of SDPs and spin-to-charge interconversion in LHP. Here, we provide a comprehensive review of second-order nonlinear photocurrents in LHP in regard to spintronics. First, a detailed background on Rashba SOC and its related effects (including the inverse Rashba-Edelstein effect) is given. Subsequently, nonlinear photo-induced effects leading to SDPs are presented. Then, SDPs due to the photo-induced inverse spin Hall effect and the circular photogalvanic effect, together with photocurrent due to the photon drag effect, are compared. This is followed by the main focus of nonlinear photocurrents in LHPs containing organic parts, starting from fundamentals related to spin-dependent optoelectronics. Finally, we conclude with a brief summary and future prospects.

Tuning d-orbitals to control spin-orbit coupling in terminated MXenes

Theory and experiment have revealed that spin-orbit coupling (SOC) strongly depends on the relativistic effect in topological insulators (TIs), while the influence of orbitals is always ignored. Herein, we provide a direct way of controlling effective SOC with the help of orbital effects, reducing the dependence on elements. Taking 5d W2CO2 and 4d Mo2CO2 MXenes as a specific example, we predict that by decreasing the hybridization strength of W atoms with C or O atoms in 2D W2CO2, the nontrivial bandgaps at the Γ-point are directly enhanced. The weak hybridization of W atoms with ligand elements enhances the electron localization of degenerate d-orbitals of three groups under the triangular prism crystal field, inducing stronger on-site Coulomb repulsion that enhances orbital polarization as well as boosts the SOC effect. Meanwhile, similar results have also been observed in 4d Mo2CO2. This implies that the orbital effects are an efficient and straightforward way to control the nontrivial bandgap in 2D MXene TIs. Our work not only provides an alternative perspective on designing large nontrivial bandgaps but also brings a possibility to control the SOC effect for TI devices.