Activating the molecular spinterface

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Radical-Induced Changes in Transition Metal Interfacial Magnetic Properties: A Blatter Derivative on Polycrystalline Cobalt

In this work, we study the interface obtained by depositing a monolayer of a Blatter radical derivative on polycrystalline cobalt. By examining the occupied and unoccupied states at the interface, using soft X-ray techniques, combined with electronic structure calculations, we could simultaneously determine the electronic structure of both the molecular and ferromagnetic sides of the interface, thus obtaining a full understanding of the interfacial magnetic properties. We found that the molecule is strongly hybridized with the surface. Changes in the core level spectra reflect the modification of the molecule and the cobalt electronic structures inducing a decrease in the magnetic moment of the cobalt atoms bonded to the molecules which, in turn, lose their radical character. Our method allowed us to screen, beforehand, organic/ferromagnetic interfaces given their potential applications in spintronics.

Enhancement of Magnetic Stability in Antiferromagnetic CoO Films by Adsorption of Organic Molecules

Antiferromagnets are a class of magnetic materials of great interest in spintronic devices because of their stability and ultrafast dynamics. When interfaced with an organic molecular layer, antiferromagnetic (AF) films are expected to form a spinterface that can allow fine control of specific AF properties. In this paper, we investigate spinterface effects on CoO, an AF oxide. To access the magnetic state of the antiferromagnet, we couple it to a ferromagnetic Co film via an exchange bias (EB) effect. In this way, the formation of a spinterface is detected through changes induced on the CoO/Co EB system. We demonstrate that C60 and Gaq3 adsorption on CoO shifts its blocking temperature; in turn, an increase in both the EB fields and the coercivities is observed on the EB-coupled Co layer. Ab initio calculations for the CoO/C60 interface indicate that the molecular adsorption is responsible for a charge redistribution on the CoO layer that alters the occupation of the d orbitals of Co atoms and, to a smaller extent, the p orbitals of oxygen. As a result, the AF coupling between Co atoms in the CoO is enhanced. Considering the granular nature of CoO, a larger AF stability upon molecular adsorption is then associated with a larger number of AF grains that are stable upon reversal of the Co layer.

Temperature Induced Reversible Switching of the Magnetic Anisotropy in a Neodymium Complex Adsorbed on Graphite

Controlling the magnetic anisotropy of molecular layers assembled on a surface is one of the challenges that needs to be addressed to create the next-generation spintronic devices. Recently, metal complexes that show a reversible solid-state switch of their magnetic anisotropy in response to physical stimuli, such as temperature and magnetic field, have been discovered. The complex Nd(trensal) (H3trensal = 2,2',2''-tris(salicylideneimino)triethylamine) is predicted to exhibit such property. An ultra-thin film of Nd(trensal) is deposited on highly ordered pyrolytic graphite as a proof-of-concept system to show that this property can be retained at the nanoscale on a layered material. By combining single crystal magnetometric measurements and synchrotron X-ray-based absorption techniques, supported by multiplet ligand field simulations based on the trigonal crystal field surrounding the lanthanide centre, it is demonstrated that changing the temperature reverses the magnetic anisotropy of an ordered film of Nd(trensal), thus opening significant perspectives for the realization of a novel family of temperature-controlled molecular spintronic devices.

Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Strain-restricted transfer of ferromagnetic electrodes for constructing reproducibly superior-quality spintronic devices

Spintronic device is the fundamental platform for spin-related academic and practical studies. However, conventional techniques with energetic deposition or boorish transfer of ferromagnetic metal inevitably introduce uncontrollable damage and undesired contamination in various spin-transport-channel materials, leading to partially attenuated and widely distributed spintronic device performances. These issues will eventually confuse the conclusions of academic studies and limit the practical applications of spintronics. Here we propose a polymer-assistant strain-restricted transfer technique that allows perfectly transferring the pre-patterned ferromagnetic electrodes onto channel materials without any damage and change on the properties of magnetism, interface, and channel. This technique is found productive for pursuing superior-quality spintronic devices with high controllability and reproducibility. It can also apply to various-kind (organic, inorganic, organic-inorganic hybrid, or carbon-based) and diverse-morphology (smooth, rough, even discontinuous) channel materials. This technique can be very useful for reliable device construction and will facilitate the technological transition of spintronic study.

Spin-polarized electrical transport properties of organic radicals in presence of zigzag-graphene nanoribbon leads

The present work delves into the spin-polarized transport property of organic radicals sandwiched between two zigzag-graphene nanoribbon (ZGNR) electrodes by employing density functional theory and nonequilibrium Green's function technique. We demonstrated that the magnetic center(s) of the radical can manipulate the localized edge states of the ZGNR in the scattering region, causing ferromagnetic coupling. Such manipulation of the magnetic edges results in a high spin-filter effect in molecular junctions, and even the antiferromagnetic diradicals serve as nearly perfect spin filters. We have confirmed that this is a general phenomenon of ZGNR by analyzing two antiferromagnetic diradicals and a doublet. The spin-polarized density of states, transmission spectra, and current vs voltage curves of the systems provide strong evidence for our findings. This research strongly suggests that ZGNRs attached with organic radicals could be the perfect building blocks for spintronic materials.

Kondo Effect of Co-Porphyrin: Remarkable Sensitivity to Adsorption Sites and Orientations

We investigated the Kondo effect of cobalt(II)-5-15-bis(4'-bromophenyl)-10,20-bis(4'-iodophenyl)porphyrin (CoTPPBr2I2) molecules on Au(111) with low-temperature scanning tunneling microscopy under ultrahigh vacuum conditions. The molecules exhibit four adsorption configurations at the top and bridge sites of the surface with different molecular orientations. The Kondo resonance shows extraordinary sensitivity to the adsorption configuration. By switching the molecule between different configurations, the Kondo temperature is varied over a wide range from ≈8 up to ≈250 K. Density functional theory calculations reveal that changes of the adsorption configuration lead to distinct variations of the hybridization between the molecule and the surface. Furthermore, we show that surface reconstruction plays a significant role for the molecular Kondo effect.

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.

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.

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Enantioselectivity of discretized helical supramolecule consisting of achiral cobalt phthalocyanines via chiral-induced spin selectivity effect

Enantioselectivity of helical aggregation is conventionally directed either by its homochiral ingredients or by introduction of chiral catalysis. The fundamental question, then, is whether helical aggregation that consists only of achiral components can obtain enantioselectivity in the absence of chiral catalysis. Here, by exploiting enantiospecific interaction due to chiral-induced spin selectivity (CISS) that has been known to work to enantio-separate a racemic mixture of chiral molecules, we demonstrate the enantioselectivity in the assembly of mesoscale helical supramolecules consisting of achiral cobalt phthalocyanines. The helical nature in our supramolecules is revealed to be mesoscopically incorporated by dislocation-induced discretized twists, unlike the case of chiral molecules whose chirality are determined microscopically by chemical bond. The relevance of CISS effect in the discretized helical supramolecules is further confirmed by the appearance of spin-polarized current through the system. These observations mean that the application of CISS-based enantioselectivity is no longer limited to systems with microscopic chirality but is expanded to the one with mesoscopic chirality.

Recent Progress for Single-Molecule Magnets Based on Rare Earth Elements

Single-molecule magnets (SMMs) have attracted much attention due to their potential applications in molecular spintronic devices. Rare earth SMMs are considered to be the most promising for application owing to their large magnetic moment and strong magnetic anisotropy. In this review, the recent progress in rare earth SMMs represented by mononuclear and dinuclear complexes is highlighted, especially for the modulation of magnetic anisotropy, effective energy barrier (U_eff) and blocking temperature (T_B). The terbium- and dysprosium-based SMMs have a U_eff of 1541 cm^-1 and an increased T_B of 80 K. They break the boiling point temperature of liquid nitrogen. The development of the preparation technology of rare earth element SMMs is also summarized in an overview. This review has important implications and insights for the design and research of Ln-SMMs.

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

Revealing the key role of molecular packing on interface spin polarization at two-dimensional limit in spintronic devices

Understanding spinterfaces between magnetic metals and organic semiconductors is essential to unlock the great potentials that organic materials host for spintronic applications. Although plenty of efforts have been devoted to studying organic spintronic devices, exploring the role of metal/molecule spinterfaces at two-dimensional limit remains challenging because of excessive disorders and traps at the interfaces. Here, we demonstrate atomically smooth metal/molecule interfaces through nondestructively transferring magnetic electrodes on epitaxial grown single-crystalline layered organic films. Using such high-quality interfaces, we investigate spin injection of spin-valve devices based on organic films of different layers, in which molecules are packed in different manners. We find that the measured magnetoresistance and the estimated spin polarization increase markedly for bilayer devices compared with their monolayer counterparts. These observations reveal the key role of molecular packing on spin polarization, which is supported by density functional theory calculations. Our findings provide promising routes toward designing spinterfaces for organic spintronic devices.

Structural characterization and electronic properties of Ni/rubrene bilayers with alternative stacking sequences

Recent progress in organic electronics has attracted interest due to their excellent characteristics that include photovoltaic, light emission, and semiconducting behaviours. Spin-induced properties play important roles in organic electronics, while introducing spin into an organic layer in which spin responses, such as a weak spin-orbital coupling and long spin-relaxation time, allows a variety of spintronic applications to be achieved. However, such spin responses are rapidly attenuated by misalignment in the electronic structure of hybrid structures. We report herein on the energy level diagrams of Ni/rubrene bilayers that can be tuned by an alternating stacking. The band edges of the highest occupied molecular orbital (HOMO) levels were determined to be 1.24 and 0.48 eV relative to the Fermi level for Ni/rubrene/Si and rubrene/Ni/Si bilayers, respectively. This raises a possibility of accumulating electric dipoles at the ferromagnetic/organic semiconductor (FM/OSC) interface, which would inhibit the transfer of spin in the OSC layer. This phenomenon is caused by the formation of a Schottky-like barrier in the rubrene/Ni heterostructures. According to the information about the band edges of the HOMO levels, schematic plots of the HOMO shift in the electronic structure of the bilayers are presented. Based on the lower value of the effective uniaxial anisotropy for Ni/rubrene/Si, the uniaxial anisotropy was suppressed compared to that of rubrene/Ni/Si. The characteristics of the formation of Schottky barriers at the FM/OSC interface have an impact on the temperature-dependent spin states in the bilayers.

Magnetic and Luminescent Dual Responses of Photochromic Hexaazamacrocyclic Lanthanide Complexes

Herein, hexaazamacrocyclic ligand L^N6 was employed to construct a series of photochromic rare-earth complexes, [Ln(L^N6)(NO₃)₂](BPh₄) [1-Ln, Ln = Dy, Tb, Eu, Gd, Y; L^N6 = (3E,5E,10E,12E)-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-3,5,10,12-tetraene]. The behavior of photogenerated radicals of hexaazamacrocyclic ligands was revealed for the first time. Upon 365 nm light irradiation, complexes 1-Ln exhibit photochromic behavior induced by photogenerated radicals according to EPR and UV-vis analyses. Static and dynamic magnetic studies of 1-Dy and irradiated product 1-Dy* indicate weak ferromagnetic interactions among Dy^III ions and photogenerated L^N6 radicals, as well as slow magnetization relaxation behavior under a 2 kOe applied field. Further fitting analyses show that the magnetization relaxation in 1-Dy* is markedly different from 1-Dy. Time-dependent fluorescence measurements reveal the characteristic luminescence quenching dynamics of lanthanide in the photochromic process. Especially for irradiated product 1-Eu*, the luminescence is almost completely quenched within 5 min with a quenching efficiency of 98.4%. The results reported here provide a prospect for the design of radical-induced photochromic lanthanide single-molecule magnets and will promote the further development of multiresponsive photomagnetic materials.

Enhancing Electron Correlation at a 3d Ferromagnetic Surface

Spin-resolved momentum microscopy and theoretical calculations are combined beyond the one-electron approximation to unveil the spin-dependent electronic structure of the interface formed between iron (Fe) and an ordered oxygen (O) atomic layer, and an adsorbate-induced enhancement of electronic correlations is found. It is demonstrated that this enhancement is responsible for a drastic narrowing of the Fe d-bands close to the Fermi energy (E_F ) and a reduction of the exchange splitting, which is not accounted for in the Stoner picture of ferromagnetism. In addition, correlation leads to a significant spin-dependent broadening of the electronic bands at higher binding energies and their merging with satellite features, which are manifestations of a pure many-electron behavior. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible. The results show that the concepts developed to understand the physics and chemistry of adsorbate-metal interfaces, relevant for a variety of research areas, from spintronics to catalysis, need to be reconsidered with many-particle effects being of utmost importance. These may affect chemisorption energy, spin transport, magnetic order, and even play a key role in the emergence of ferromagnetism at interfaces between non-magnetic systems.

Highly interface-dependent spin transport in an Fe-Mn(DBTAA)-Fe single molecule spintronic device

Understanding the spinterface between magnetic electrodes and molecules, and realizing the controllable spin filtering effect, are crucial for the development of high-performance molecular devices, but both still face big challenges. Here, based on first-principles calculations of an Fe-Mn(DBTAA)-Fe single molecule spintronic device, we unveil that spin filtering efficiency is highly dependent on interface configurations, which can modulate and even reverse the spin polarization of tunnelling electrons. For Fe-Mn(DBTAA)-Fe, a varied spin filtering from -93% to +75% is observed. The underlying mechanism could be attributed to the distinct magnetic and electronic couplings between the Fe electrode and the Mn(DBTAA) molecule in different interface configurations. This work not only highlights the importance of a magnetic electrode-molecule interface, but also implies that through suitable interface design, the performance, e.g., of the spin filtering channel of single molecule spintronic devices, can be flexibly tuned.

On-Surface Synthesis and Spectroscopy of Aluminum Phthalocyanine on Superconducting Lead

Large ordered islands of aluminum phthalocyanine (AlPc) molecules, which are unstable in air, are synthesized from ClAlPc on Pb(100) via dechlorination. Low-temperature scanning tunneling microscopy reveals that isolated AlPc molecules lose their spin moment on superconducting Pb(100). Molecular magnetism, which is detected via Yu-Shiba-Rusinov (YSR) resonances, may be restored by surrounding a molecule with an array of neighbor molecules in artificial arrays or in a self-assembled monolayer. Unlike phthalocyanine (H₂Pc) or lead phthalocyanine (PbPc) monolayers, where the YSR energy was found to depend strongly on the detailed configuration of the neighboring molecules, we find a similar magnetic moment on every second molecule for AlPc. In addition, YSR resonances lead to unusually high conductance peaks that are due to vibrational excitations. Twelve vibrational modes are resolved and discussed with respect to similar results from PbPc. The enhancement of the inelastic transitions is tentatively attributed to the large amplitude of the YSR resonances and the long lifetime of electrons in the molecular bound state. By assembling neighboring molecules into configurations that differ from those of the monolayer, the YSR energy may be fine-tuned, and a simple spin-state switching device is constructed.

Spatially Confined Face-Selective Growth of Large-Area 2D Organic Molecular Crystals in a Supramolecular Gel for Highly Efficient Flexible Photodetection

2D organic molecular crystals (2DOMCs) are promising materials for the fabrication of high-performance optoelectronic devices. However, the growth of organic molecules into 2DOMCs remains a challenge because of the difficulties in controlling their self-assembly with a preferential orientation in solution-process crystallization. Herein, fullerene is chosen as a model molecule to develop a supramolecular gel crystallization approach to grow large-area 2DOMCs by controlling the perfect arrangement on the {220} crystal plane with the assistance of a gelated solvent. In this case, the gel networks provide tuneable confined spaces to control the crystallization kinetics toward the growth of dominant crystal faces by their inhibiting motions of solvent or solute molecules to enable the growth of perfect crystals at appropriate nucleation rates. As a result, a large-area fullerene 2DOMC is produced successfully and its corresponding device on a flexible substrate exhibits excellent bendable properties and ultra-high weak light detection ability (2.9 × 10¹¹ Jones) at a 10 V bias upon irradiation with 450 nm incident light. Moreover, its photoelectric properties remain unchanged after 200 cycles of bending at angles of 45, 90, and 180°. These results can be extended to the growth of other 2DOMCs for potentially fabricating advanced organic (opto)electronics.

Spinterface Effects in Hybrid La0.7Sr0.3MnO3/SrTiO3/C60/Co Magnetic Tunnel Junctions

Orbital hybridization at the Co/C₆₀ interface been has proved to strongly enhance the magnetic anisotropy of the cobalt layer, promoting such hybrid systems as appealing components for sensing and memory devices. Correspondingly, the same hybridization induces substantial variations in the ability of the Co/C₆₀ interface to support spin-polarized currents and can bring out a spin-filtering effect. The knowledge of the effects at both sides allows for a better and more complete understanding of interfacial physics. In this paper we investigate the Co/C₆₀ bilayer in the role of a spin-polarized electrode in the La_0.7Sr_0.3MnO₃/SrTiO₃/C₆₀/Co configuration, thus substituting the bare Co electrode in the well-known La_0.7Sr_0.3MnO₃/SrTiO₃/Co magnetic tunnel junction. The study revealed that the spin polarization (SP) of the tunneling currents escaping from the Co/C₆₀ electrode is generally negative: i.e., inverted with respect to the expected SP of the Co electrode. The observed sign of the spin polarization was confirmed via DFT calculations by considering the hybridization between cobalt and molecular orbitals.

Combined spin filtering actions in hybrid magnetic junctions based on organic chains covalently attached to graphene

We present a bias-controlled spin-filtering mechanism in spin-valves including a hybrid organic chain/graphene interface. Wet growth conditions of oligomeric molecular chains would usually lead, during standard CMOS-compatible fabrication processes, to the quenching of spintronics properties of metallic spin sources due to oxidation. We demonstrate by X-ray photoelectron spectroscopy that the use of a protective graphene layer fully preserves the metallic character of the ferromagnetic surface and thus its capability to deliver spin polarized currents. We focus here on a small aromatic chain of controllable lengths, formed by nitrobenzene monomers and derived from the commercial 4-nitrobenzene diazonium tetrafluoroborate, covalently attached to the graphene passivated spin sources thanks to electroreduction. A unique bias dependent switch of the spin signal is then observed in complete spin valve devices, from minority to majority spin carriers filtering. First-principles calculations are used to highlight the key role played by the spin-dependent hybridization of electronic states present at the different interfaces. Our work is a first step towards the exploration of spin transport using different functional molecular chains. It opens the perspective of atomic tailoring of magnetic junction devices towards spin and quantum transport control, thanks to the flexibility of ambient electrochemical surface functionalization processes.

Quantum anomalous Hall effect in germanene by proximity coupling to a semiconducting ferromagnetic substrate NiI2

Interfaces between materials are ubiquitous in materials science, especially in devices. As device dimensions continue to be reduced, understanding the physical characteristics that appear at interfaces is crucial to exploit them for applications, spintronics in this case. Here, based on first-principles calculations, we propose a general and tunable platform to realize an exotic quantum anomalous Hall effect (QAHE) with the germanene monolayer by proximity coupling to a semiconducting ferromagnetic NiI₂ (Ge/NiI₂). Through analysis of the Berry curvature and band structure with spin-orbit coupling, the QAHE phase with an integer Chern number (C = -1), which is induced by band inversion between Ge-p orbitals, can achieve complete spin polarization for low-dissipation electronic devices. Also, the proximity coupling between germanene and the NiI₂ substrate makes the non-trivial bandgap reach up to 85 meV, and the Curie temperature of the Ge/NiI₂ heterostructure (HTS) is enhanced to 238 K, which is much higher than that of pristine NiI₂. An effective k·p model is proposed to clarify the quantum phenomena in the Ge/NiI₂ HTS. These findings shed light on the possible role of magnetic proximity effects on condensed matter physics in germanene and open new perspectives for multifunctional spin quantum devices in spintronics.

Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle

A new spin- and angle-resolved inverse photoemission setup with a low-energy electron source is presented. The spin-polarized electron source, with a compact design, can decouple the spin polarization vector from the electron beam propagation vector, allowing one to explore any spin orientation at any wavevector in angle-resolved inverse photoemission. The beam polarization can be tuned to any preferred direction with a shielded electron optical system, preserving the parallel beam condition. We demonstrate the performances of the setup by measurements on Cu(001) and Au(111). We estimate the energy resolution of the overall system at room temperature to be ∼170 meV from k_BT_eff of a Cu(001) Fermi level, allowing a direct comparison to photoemission. The spin-resolved operation of the setup has been demonstrated by measuring the Rashba splitting of the Au(111) Shockley surface state. The effective polarization of the electron beam is P = 30% ± 3%, and the wavevector resolution is Δk_F ≲ 0.06 Å^-1. Measurements on the Au(111) surface state demonstrate how the electron beam polarization direction can be tuned in the three spatial dimensions. The maximum of the spin asymmetry is reached when the electron beam polarization is aligned with the in-plane spin polarization of the Au(111) surface state.

Self-assembly of C60 on a ZnTPP/Fe(001)-p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

Fullerene (C₆₀) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)-p(1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C₆₀/ZnTPP/Fe(001)-p(1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C₆₀ nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C₆₀ and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C₆₀/ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C₆₀ is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C₆₀ interfaced with a metallic electrode is required.

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.

Proposal for All-Electrical Spin Manipulation and Detection for a Single Molecule on Boron-Substituted Graphene

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S=1 and S=3/2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fe-d_{z^{2}} and B-p_{z} orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (>10% for typical coverage). Our work paves the way to realize all-electrical spintronics devices using molecular spinterfaces.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Exchange Bias in Molecule/Fe3 GeTe2 van der Waals Heterostructures via Spinterface Effects

The exfoliation of layered magnetic materials generates atomically thin flakes characterized by an ultrahigh surface sensitivity, which makes their magnetic properties tunable via external stimuli, such as electrostatic gating and proximity effects. Another powerful approach to engineer magnetic materials is molecular functionalization, generating hybrid interfaces with tailored magnetic interactions, called spinterfaces. However, spinterface effects have not yet been explored on layered magnetic materials. Here, the emergence of spinterface effects is demonstrated at the interface between flakes of the prototypical layered magnetic metal Fe₃ GeTe₂ and thin films of Co-phthalocyanine. Magnetotransport measurements show that the molecular layer induces a magnetic exchange bias in Fe₃ GeTe₂ , indicating that the unpaired spins in Co-phthalocyanine develop antiferromagnetic ordering and pin the magnetization reversal of Fe₃ GeTe₂ via magnetic proximity. The effect is strongest for a Fe₃ GeTe₂ thickness of 20 nm, for which the exchange bias field reaches -840 Oe at 10 K and is measurable up to ≈110 K. This value compares very favorably with previous exchange bias fields reported for Fe₃ GeTe₂ in all-inorganic van der Waals heterostructures, demonstrating the potential of molecular functionalization to tailor the magnetism of van der Waals layered materials.

Chirality-Induced Magnetoresistance Due to Thermally Driven Spin Polarization

Chirality-induced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from current-induced spin polarization in molecules. The current-induced spin polarization is widely recognized as a fundamental principle of chiral-induced spin selectivity (CISS). In this study, we investigate chirality-induced current-in-plane magnetoresistance (CIP-MR) in a chiral molecule/ferromagnetic metal bilayer at room temperature. In contrast to CPP-MR, CIP-MR observed in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests that thermally driven spontaneous spin polarization in chiral molecules is the key to the observed MR. The novel MR is consistent with recent CISS-related studies, that is, chiral molecules in contact with a metallic surface possess a finite spin polarization.

Magnetic nanoribbons with embedded cobalt grown inside single-walled carbon nanotubes

Molecular magnetism and specifically magnetic molecules have recently gained plenty of attention as key elements for quantum technologies, information processing, and spintronics. Transition to the nanoscale and implementation of ordered structures with defined parameters is crucial for advanced applications. Single-walled carbon nanotubes (SWCNTs) provide natural one-dimensional confinement that can be implemented for encapsulation, nanosynthesis, and polymerization of molecules into nanoribbons. Recently, the formation of atomically precise graphene nanoribbons inside SWCNTs has been reported. However, there have been only a limited amount of approaches to form ordered magnetic structures inside the nanotube channels and the creation of magnetic nanoribbons is still lacking. In this work we synthesize and reveal the properties of cobalt-phthalocyanine based nanoribbons (CoPcNRs) encapsulated in SWCNTs. Raman spectroscopy, transmission electron microscopy, absorption spectroscopy, and density functional theory calculations allowed us to confirm the encapsulation and to reveal the specific fingerprints of CoPcNRs. The magnetic properties were studied by transverse magnetooptical Kerr effect measurements, which indicated a strong difference in comparison with the pristine unfilled SWCNTs due to the impact of Co incorporated atoms. We anticipate that this approach of polymerization of encapsulated magnetic molecules inside SWCNTs will result in a diverse class of protected low-dimensional ordered magnetic materials for various applications.

Rare-earth based tetrapyrrolic sandwiches: chemistry, materials and applications

This review summarises advances in chemistry of tetrapyrrole sandwiches with rare earth elements and highlights the current state of their use in single-molecule magnetism, organic field-effect transistors, conducting materials and nonlinear optics.

Observation of a Metastable Honeycomb Arrangement of C60 on Ni(111) with (7 × 7) Periodicity: Tailoring an Interface for Organic Spintronics

Hybrid nanostructures in which organic molecules are interfaced with metal surfaces hold promise for the discovery of intriguing physical and chemical phenomena, as well as for the development of innovative devices. In this frame, it is crucial to understand the interplay between the structural details of the interface and the electronic properties of the system. Here, an experimental investigation of the C₆₀/Ni(111) interface is performed by means of scanning tunneling microscopy/spectroscopy (STM/STS) and low-energy electron diffraction (LEED). The deposition of C₆₀ at room temperature, followed by high-temperature annealing, promotes the stabilization of two different phases. A hitherto unreported phase forming a (7 × 7) honeycomb overlayer coexists with the well-known (4 × 4) reconstruction. Highly resolved STM images disclose the adsorption geometry of the molecules for both phases. STS reveals that the electronic properties of C₆₀/Ni(111) are strongly influenced by the morphology of the interface, suggesting the possibility of tuning the electronic properties of the organic/inorganic heterostructures by adjusting the structural coupling with the substrate. This achievement can be important for hybrid magnetic interfaces, where the harmonization between the molecular and the magnetic orders can enhance the development of hybrid magnetic states.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Voltage-Induced Bistability of Single Spin-Crossover Molecules in a Two-Dimensional Monolayer

Bistable spin-crossover molecules are particularly interesting for the development of innovative electronic and spintronic devices as they present two spin states that can be controlled by external stimuli. In this paper, we report the voltage-induced switching of the high spin/low spin electronic states of spin-crossover molecules self-assembled in dense 2D networks on Au(111) and Cu(111) by scanning tunneling microscopy at low temperature. On Au(111), voltage pulses lead to the nonlocal switching of the molecules from any─high or low─spin state to the other followed by a spontaneous relaxation toward their initial state within minutes. On the other hand, on Cu(111), single molecules can be addressed at will. They retain their new electronic configuration after a voltage pulse. The memory effect demonstrated on Cu(111) is due to an interplay between long-range intermolecular interaction and molecule/substrate coupling as confirmed by mechanoelastic simulations.

Combined Role of Biaxial Strain and Nonstoichiometry for the Electronic, Magnetic, and Redox Properties of Lithiated Metal-Oxide Films: The LiMn2O4 Case

Understanding the interplay between strain and nonstoichiometry for the electronic, magnetic, and redox properties of LiMn₂O₄ films is essential for their development as Li-ion battery (LIB) cathodes, photoelectrodes, and systems for sustainable spintronics applications as well as for emerging applications that combine these technologies. Here, density functional theory (DFT) simulations suggest that compressive strain increases the reduction drive of (111) LiMn₂O₄ films by inducing >1 eV upshift of the valence band edge. The DFT results indicate that, regardless of the crystallographic orientation for the LiMn₂O₄ film, biaxial expansion increases the magnetic moments of the Mn atoms. Conversely, biaxial compression reduces them. For ferromagnetic films, these changes can be substantial and as large as over 4 Bohr magnetons per unit cell over the simulated range of strain (from -6 to +3%). The DFT simulations also uncover a compensation mechanism whereby strain induces opposite changes in the magnetic moment of the Mn and O atoms, leading to an overall constant magnetic moment for the ferromagnetic films. The calculated strain-induced changes in atomic magnetic moments reflect modifications in the local electronic hybridization of both the Mn and O atoms, which in turn suggests strain-tunable, local chemical, and electrochemical reactivity. Several energy-favored (110) and (111) ferromagnetic surfaces turn out to be half-metallic with minority-spin band gaps as large as 3.2 eV and compatible with spin-dependent electron-transport and possible spin-dependent electrochemical and electrocatalytic properties. The resilience of the ferromagnetic, half-metallic states to surface nonstoichiometry and compositional changes invites exploration of the potential of LiMn₂O₄ thin films for sustainable spintronic applications beyond state-of-the-art, rare-earth metal-based, ferromagnetic half-metallic oxides.

3D Printable concentrated liquid metal composite with high thermal conductivity

Heat dissipation materials in which fillers are dispersed in a polymer matrix typically do not exhibit both high thermal conductivity (k) and processability due to a trade-off. In this paper, we fabricate heat dissipation composites which overcome the trade-off using liquid metal (LM). By exceeding the conventional filler limit, ten times higher k is achieved for a 90 vol% LM composite compared with k of 50 vol% LM composite. Further, an even higher k is achieved by introducing h-BN between the LM droplets, and the highest k in this study was 17.1 W m^-1 K^-1. The LM composite is processable at room temperature and used as inks for 3D printing. This combination of high k and processability not only allows heat dissipation materials to be processed on demand under ambient conditions but it also increases the surface area of the LM composite, which enables rapid heat dissipation.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)₄]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

Observation of a Half-Metallic Interface State for Pyridine-Adsorbed H/Fe3O4(100)

Organic molecule/half-metallic oxide interfaces play essential roles in recent molecular spintronics devices, yet factors governing the spin polarization of the hybridized interface state (HIS) formed there remain unclear because of the lack of direct spectroscopic evidence for it. Here, we present a spin-polarized metastable deexcitation spectroscopy (SPMDS) experiment, which is sensitive to the topmost surface, showing that the adsorption of pyridine on H-terminated Fe₃O₄(100) at 100 K produces a highly spin-polarized HIS, while no such HIS forms on a bare Fe₃O₄(100) surface. A density functional theory calculation has predicted the formation of half-metallic HIS, which is consistent with the SPMDS results. This can be understood on the basis of the interface chemical bonding formed by the coordination of the nitrogen end of pyridine to the surface Fe atom where half-metallic conduction electrons are distributed.

Robust Giant Magnetoresistance in 2D Van der Waals Molecular Magnetic Tunnel Junctions

The spin transport across a zero-dimensional (0D) single-molecule sandwiched by two-dimensional (2D) van der Waals (vdW) ferromagnetic electrodes may open vast opportunities to create novel mixed-dimensional spintronics devices. However, this remains unexplored yet. Inspired by the recent discovery of 2D intrinsic ferromagnets Fe₃GeTe₂, using first-principles spin transport calculations, we show that single-molecule junctions based on Fe₃GeTe₂ can yield perfect spin filtering and a significant magnetoresistance (MR) of up to ∼6075%. This remarkable MR is more than 2 orders of magnitude higher than the MR obtained for the corresponding junctions with conventional ferromagnetic metals (e.g., Ni, Fe, and Co). We demonstrate the results of two representative examples that are feasible in the experiments: (i) A benzene or (ii) bezenedithiol (BDT) connected either through a scanning tunneling microscope or break-junction setups. We find that the conductance of BDT junctions is more than 10 times larger than that of the benzene junction due to a much stronger hybridization effect at the molecule-metal interfaces. The key mechanism of the perfect spin filtering and large MR in single-molecule junctions is mainly determined by the intrinsic properties of Fe₃GeTe₂ electrodes, while the actual conductance is determined by the hybridization strength of the majority spin channel at the molecule-metal interfaces. It is also predicted that the perfect spin filtering and the remarkably huge MR are highly insensitive to structural variations, interface defects, and stacking orders of the electrodes. Our results provide important insights for expanding molecular spintronics platforms from conventional ferromagnetic metals to new 2D vdw magnets.

Synthesis, Structure, and Surface Adsorption Characteristics of a Polynuclear MnII,IV-YbIII Complex

The controlled adsorption of polynuclear coordination compounds with specific structural and electronic characteristics on surfaces is crucial for the prospective implementation of molecule-surface interfaces into practical electronic devices. From this perspective, a neutral 3d,4f-coordination cluster [Mn^II₃Mn^IVYb₃O₃(OH)(L·SMe)₃(OOCMe)₉]·2MeCN·3EtOH (1·2MeCN·3EtOH), where L·SMe^- is a Schiff base, has been synthesized and fully characterized and its adsorption on two different solid substrates, gold and graphite, has been studied. The mixed-valence compound with a bilayered metal core structure and the structurally exposed thioether groups exhibits a substantially different surface bonding to metallic gold and semimetallic graphite substrates. While on graphite the adsorption takes place only on distinguished attraction points with a locally increased number of potential bonding sites such as terrace edges and other surface defects, on gold the molecules were found to adsorb rather weakly on randomly distributed adsorption sites of the surface terraces. This entirely different behavior provides important information for the development of advanced surface materials that may enable well-distributed ordered molecular assemblies.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Challenges for Incorporating Optical Switchability in Organic-Based Electronic Devices

Transistors operate by controlling the current flowing from a source to a drain electrode via a third electrode (gate), thus giving access to a binary treatment (ON/OFF or 0/1) of the signal currently exploited in microelectronics. Introducing a second independent lever to modulate the current would allow for more complex logic functions amenable to a single electronic component and hence to new opportunities for advanced electrical signal processing. One avenue is to add this second dimension with light by incorporating photochromic molecules in current organic-based electronic devices. In this Spotlight, we describe different concepts that have been implemented in organic thin films and in molecular junctions as well as some pitfalls that have been highlighted thanks to theoretical modeling.

Spin-Crossover Molecules on Surfaces: From Isolated Molecules to Ultrathin Films

Molecular spintronics seeks to use single or few molecules as functional building blocks for spintronic applications, directly relying on molecular properties or properties of interfaces between molecules and inorganic electrodes. Spin-crossover molecules (SCMs) are one of the most promising classes of candidates for molecular spintronics due to their bistability deriving from the existence of two spin states that can be reversibly switched by temperature, light, electric fields, etc. Building devices based on single or few molecules would entail connecting the molecule(s) with solid surfaces and understanding the fundamental behavior of the resulting assemblies. Herein, the investigations of SCMs on solid surfaces, ranging from isolated single molecules (submonolayers) to ultrathin films (mainly in the sub-10 nm range) are summarized. The achievements, challenges and prospects in this field are highlighted.