Inducing Motions of Polymers in Liquid Nitrogen with Light.

Polymer materials that show macroscopic deformation in response to external stimuli are feasible for novel soft actuators including microactuators. Incorporation of photochromic moieties, such as azobenzenes, into polymer networks enables macroscopic deformation under irradiation with light through photoisomerization. Under cryogenic conditions, however, it has been difficult to induce macroscopic deformation as polymers lose their soft nature due to the severe restrictions of molecular motions. Here, activation of molecular motions and macroscopic deformation in liquid nitrogen only with light for polymers containing photochromic moieties is reported. Photoinduced bending of polymer networks with normal azobenzenes in liquid nitrogen is enabled by preliminary UV irradiation at room temperature to produce cis-isomers. To realize photoinduced deformation directly in liquid nitrogen, polymer networks are functionalized with bridged azobenzenes, which exist as cis-isomers in thermodynamic equilibrium. The films with bridged azobenzenes exhibit reversible photoisomerization and bending upon irradiation with light in liquid nitrogen without the need of preliminary irradiation, implying that the change in conformation of polymer chains can be isothermally induced even under cryogenic conditions. Achievement of flexible motions under cryogenic conditions through isothermal processes will greatly expand the operating temperature range of soft actuators.

Chiral Van Der Waals Superlattices for Enhanced Spin-Selective Transport and Spin-Dependent Electrocatalytic Performance.

The emergence of the chiral-induced spin-selectivity (CISS) effect offers a new avenue for chiral organic molecules to autonomously manipulate spin configurations, thereby opening up possibilities in spintronics and spin-dependent electrochemical applications. Despite extensive exploration of various chiral systems as spin filters, one often encounters challenges in achieving simultaneously high conductivity and high spin polarization (SP). In this study, a promising chiral van der Waals superlattice, specifically the chiral TiS2 crystal, is synthesized via electrochemical intercalation of chiral molecules into a metallic TiS2 single crystal. Multiple tunneling processes within the highly ordered chiral layered structure of chiral TiS2 superlattices result in an exceptionally high SP exceeding 90%. This remarkable observation of significantly high SP within the linear transport regime is unprecedented. Furthermore, the chiral TiS2 electrode exhibits enhanced catalytic activity for oxygen evolution reaction (OER) due to its remarkable spin-selectivity for triplet oxygen evolution. The OER performance of chiral TiS2 superlattice crystals presented here exhibits superior characteristics to previously reported chiral MoS2 catalysts, with an approximately tenfold increase in current density. The combination of metallic conductivity and high SP sets the stage for the development of a new generation of CISS materials, enabling a wide range of electron spin-based applications.

Highly Durable Spin Filter Switching Based on Self-Assembled Chiral Molecular Motor.

Chiral molecules have recently received renewed interest as highly efficient sources of spin-selective charge emission known as chiral-induced spin selectivity (CISS), which potentially offers a fascinating utilization of organic chiral materials in novel solid-state spintronic devices. However, a practical use of CISS remains far from completion, and rather fundamental obstacles such as (i) external controllability of spin, (ii) function durability, and (iii) improvement of spin-polarization efficiency have not been surmounted to date. In this study, these issues are addressed by developing a self-assembled monolayer (SAM) of overcrowded alkene (OCA)-based molecular motor. With this system, it is successfully demonstrated that the direction of spin polarization can be externally and repeatedly manipulated in an extremely stable manner by switching the molecular chirality, which is achieved by a formation of the covalent bonds between the molecules and electrode. In addition, it is found that a higher stereo-ordering architecture of the SAM of OCAs tailored by mixing them with simple alkanethiols considerably enhances the efficiency of spin polarization per a single OCA molecule. All these findings provide the creditable feasibility study for strongly boosting development of CISS-based spintronic devices that can simultaneously fulfill the controllability, durability, and high spin-polarization efficiency.

Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems.

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole-dipole interactions.

Hybrid Chiral MoS2 Layers for Spin-Polarized Charge Transport and Spin-Dependent Electrocatalytic Applications.

The chiral-induced spin selectivity effect enables the application of chiral organic materials for spintronics and spin-dependent electrochemical applications. It is demonstrated on various chiral monolayers, in which their conversion efficiency is limited. On the other hand, relatively high spin polarization (SP) is observed on bulk chiral materials; however, their poor electronic conductivities limit their application. Here, the design of chiral MoS2 with a high SP and high conductivity is reported. Chirality is introduced to the MoS2 layers through the intercalation of methylbenzylamine molecules. This design approach activates multiple tunneling channels in the chiral layers, which results in an SP as high as 75%. Furthermore, the spin selectivity suppresses the production of H2 O2 by-product and promotes the formation of ground state O2 molecules during the oxygen evolution reaction. These potentially improve the catalytic activity of chiral MoS2 . The synergistic effect is demonstrated as an interplay of the high SP and the high catalytic activity of the MoS2 layer on the performance of the chiral MoS2 for spin-dependent electrocatalysis. This novel approach employed here paves way for the development of other novel chiral systems for spintronics and spin-dependent electrochemical applications.

Ubiquitous organic molecule-based free-standing nanowires with ultra-high aspect ratios.

The critical dimension of semiconductor devices is approaching the single-nm regime, and a variety of practical devices of this scale are targeted for production. Planar structures of nano-devices are still the center of fabrication techniques, which limit further integration of devices into a chip. Extension into 3D space is a promising strategy for future; however, the surface interaction in 3D nanospace make it hard to integrate nanostructures with ultrahigh aspect ratios. Here we report a unique technique using high-energy charged particles to produce free-standing 1D organic nanostructures with high aspect ratios over 100 and controlled number density. Along the straight trajectory of particles penetrating the films of various sublimable organic molecules, 1D nanowires were formed with approximately 10~15 nm thickness and controlled length. An all-dry process was developed to isolate the nanowires, and planar or coaxial heterojunction structures were built into the nanowires. Electrical and structural functions of the developed standing nanowire arrays were investigated, demonstrating the potential of the present ultrathin organic nanowire systems.

Chirality-Induced Spin-Polarized State of a Chiral Crystal CrNb_{3}S_{6}.

Chirality-induced spin transport phenomena are investigated at room temperature without magnetic fields in a monoaxial chiral dichalcogenide CrNb_{3}S_{6}. We found that spin polarization occurs in these chiral bulk crystals under a charge current flowing along the principal c axis. Such phenomena are detected as an inverse spin Hall signal which is induced on the detection electrode that absorbs polarized spin from the chiral crystal. The inverse response is observed when applying the charge current into the detection electrode. The signal sign reverses in the device with the opposite chirality. Furthermore, the spin signals are found over micrometer length scales in a nonlocal configuration. Such a robust generation and protection of the spin-polarized state is discussed based on a one-dimensional model with an antisymmetric spin-orbit coupling.

Light-driven molecular switch for reconfigurable spin filters.

Artificial molecular switches and machines that enable the directional movements of molecular components by external stimuli have undergone rapid advances over the past several decades. Particularly, overcrowded alkene-based artificial molecular motors are highly attractive from the viewpoint of chirality switching during rotational steps. However, the integration of these molecular switches into solid-state devices is still challenging. Herein, we present an example of a solid-state spin-filtering device that can switch the spin polarization direction by light irradiation or thermal treatment. This device utilizes the chirality inversion of molecular motors as a light-driven reconfigurable spin filter owing to the chiral-induced spin selectivity effect. Through this device, we found that the flexibility at the molecular scale is essential for the electrodes in solid-state devices using molecular machines. The present results are beneficial to the development of solid-state functionalities emerging from nanosized motions of molecular switches.

An Ambipolar Superconducting Field-Effect Transistor Operating above Liquid Helium Temperature.

Superconducting (SC) devices are attracting renewed attention as the demands for quantum-information processing, meteorology, and sensing become advanced. The SC field-effect transistor (FET) is one of the elements that can control the SC state, but its variety is still limited. Superconductors at the strong-coupling limit tend to require a higher carrier density when the critical temperature (TC ) becomes higher. Therefore, field-effect control of superconductivity by a solid gate dielectric has been limited only to low temperatures. However, recent efforts have resulted in achieving n-type and p-type SC FETs based on organic superconductors whose TC exceed liquid He temperature (4.2 K). Here, a novel "ambipolar" SC FET operating at normally OFF mode with TC of around 6 K is reported. Although this is the second example of an SC FET with such an operation mode, the operation temperature exceeds that of the first example, or magic-angle twisted-bilayer graphene that operates at around 1 K. Because the superconductivity in this SC FET is of unconventional type, the performance of the present device will contribute not only to fabricating SC circuits, but also to elucidating phase transitions of strongly correlated electron systems.

Field-, strain- and light-induced superconductivity in organic strongly correlated electron systems.

Stimulated by the discovery of high-temperature superconductivity in 1986, band-filling control of strongly correlated electron systems has been a persistent challenge over the past three decades in condensed matter science. In particular, recent efforts have been focused on electrostatic carrier doping of these materials, utilising field-effect transistor (FET) structures to find novel superconductivity. Our group found the first field-induced superconductivity in an organic-based material in 2013 and has been developing various types of superconducting organic FETs. In this perspective, we summarise our recent results on the development of novel superconducting organic FETs. In addition, this perspective describes novel functionality of superconducting FETs, such as strain- and light-responsivity. We believe that the techniques and knowledge described here will contribute to advances in future superconducting electronics as well as the understanding of superconductivity in strongly correlated electron systems.

N-Type Superconductivity in an Organic Mott Insulator Induced by Light-Driven Electron-Doping.

The presence of interface dipoles in self-assembled monolayers (SAMs) gives rise to electric-field effects at the device interfaces. SAMs of spiropyran derivatives can be used as photoactive interface dipole layer in field-effect transistors because the photochromism of spiropyrans involves a large dipole moment switching. Recently, light-induced p-type superconductivity in an organic Mott insulator, κ-(BEDT-TTF)2 Cu[N(CN)2 ]Br (κ-Br: BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) has been realized, thanks to the hole carriers induced by significant interface dipole variation in the spiropyran-SAM. This report explores the converse situation by designing a new type of spiropyran monolayer in which light-induced electron-doping into κ-Br and accompanying n-type superconducting transition have been observed. These results open new possibilities for novel electronics utilizing a photoactive SAMs, which can design not only the magnitude but also the direction of photoinduced electric-fields at the device interfaces.

Critical Behavior in Doping-Driven Metal-Insulator Transition on Single-Crystalline Organic Mott-FET.

We present the carrier transport properties in the vicinity of a doping-driven Mott transition observed at a field-effect transistor (FET) channel using a single crystal of the typical two-dimensional organic Mott insulator κ-(BEDT-TTF)2CuN(CN)2Cl (κ-Cl). The FET shows a continuous metal-insulator transition (MIT) as electrostatic doping proceeds. The phase transition appears to involve two-step crossovers, one in Hall measurement and the other in conductivity measurement. The crossover in conductivity occurs around the conductance quantum e2/h, and hence is not associated with "bad metal" behavior, which is in stark contrast to the MIT in half-filled organic Mott insulators or that in doped inorganic Mott insulators. Through in-depth scaling analysis of the conductivity, it is found that the above carrier transport properties in the vicinity of the MIT can be described by a high-temperature Mott quantum critical crossover, which is theoretically argued to be a ubiquitous feature of various types of Mott transitions.

Superconductivity. Light-induced superconductivity using a photoactive electric double layer.

Electric double layers (EDLs) of ionic liquids have been used in superconducting field-effect transistors as nanogap capacitors. Because of the freezing of the ionic motion below ~200 kelvin, modulations of the carrier density have been limited to the high-temperature regime. Here we observe carrier-doping-induced superconductivity in an organic Mott insulator with a photoinduced EDL based on a photochromic spiropyran monolayer. Because the spiropyran can isomerize reversibly between nonionic and zwitterionic isomers through photochemical processes, two distinct built-in electric fields can modulate the carrier density even at cryogenic conditions.

Strain-tunable superconducting field-effect transistor with an organic strongly-correlated electron system.

A novel type of flexible organic field-effect transistor in which strain effects can be finely tuned continuously has been fabricated. In this novel device structure, electronic phases can be controlled both by "band-filling" and by "band-width" continuously. Finally, co-regulation of "band-filling" and "band-width" in the strongly-correlated organic material realize field-induced emergence of superconducting fractions at low temperature.

A strained organic field-effect transistor with a gate-tunable superconducting channel.

In state-of-the-art silicon devices, mobility of the carrier is enhanced by the lattice strain from the back substrate. Such an extra control of device performance is significant in realizing high-performance computing and should be valid for electric-field-induced superconducting (SC) devices, too. However, so far, the carrier density is the sole parameter for field-induced SC interfaces. Here we show an active organic SC field-effect transistor whose lattice is modulated by the strain from the substrate. The soft organic lattice allows tuning of the strain by a choice of the back substrate to make an induced SC state accessible at low temperature with a paraelectric solid gate. An active three-terminal Josephson junction device thus realized is useful both in advanced computing and in elucidating a direct connection between filling-controlled and bandwidth-controlled SC phases in correlated materials.

Cholesterol biosynthesis pathway intermediates and inhibitors regulate glucose-stimulated insulin secretion and secretory granule formation in pancreatic beta-cells.

Cholesterol is reportedly abundant in the endocrine secretory granule (SG) membrane. In this study, we examined the involvement of cholesterol biosynthesis intermediates and inhibitors in insulin secretion and SG formation mechanisms. There are two routes for the supply of cholesterol to the cells: one via de novo biosynthesis and the other via low-density lipoprotein receptor-mediated endocytosis. We found that insulin secretion and content are diminished by ß-hydroxy-ß-methylglutaryl-coenzyme A inhibitor lovastatin but not by lipoprotein depletion from the culture medium in MIN6 ß-cells. Cholesterol biosynthesis intermediates mevalonate, squalene, and geranylgeranyl pyrophosphate enhanced glucose-stimulated insulin secretion, and the former two increased insulin content. The glucose-stimulated insulin secretion-enhancing effect of geranylgeranyl pyrophosphate was also confirmed in perifusion with rat islets. Morphologically, mevalonate and squalene increased the population of SGs without affecting their size. In contrast, lovastatin increased the SG size with reduction of insulin-accumulating dense cores, leading to a decrease in insulin content. Furthermore, insulin was secreted in a constitutive manner, indicating disruption of regulated insulin secretion. Because secretogranin III, a cholesterol-binding SG-residential granin-family protein, coincides with SG localization based on the cholesterol composition, secretogranin III may be associated with insulin-accumulating mechanisms. Although the SG membrane exhibits a high cholesterol composition, we could not find detergent-resistant membrane regions using a lipid raft-residential protein flotillin and a fluorescent cholesterol-Si-pyrene probe as markers on a sucrose-density gradient fractionation. We suggest that the high cholesterol composition of SG membrane with 40-50 mol% is crucial for insulin secretion and SG formation functions.

Systematic investigation of an array of TCNQ lanthanide complexes: synthesis, structure and magnetic properties.

An array of TCNQ (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) lanthanide complexes, namely, [Ln2(TCNQ)4(H2O)10(EtOH)2][2TCNQ] x xH2O [Ln = La (1, x = 5), Pr (2, x = 3)], [Ln2(TCNQ)4(H2O)10][2TCNQ] x yH2O [Ln = Gd (3, y = 6), Dy (4, y = 5)] and [Ln(TCNQ)2(H2O)6][TCNQ] x H2O x MeOH [Ln = Er (5), Lu (6)], have been synthesized by salt-elimination reactions. X-ray crystallographic analysis indicates that 1-6 are discrete ion complexes. Cations of 1-4 adopt a dimer structure, while cations of 5 and 6 display a monomer structure. The magnetic study proposes that an antiferromagnetic interaction between TCNQ radicals exists in 1-6. The depopulation of the Stark levels for 2 and 5 leads to a continuous decrease in mT when the samples are cooled from 300 K to 2 K. The desolvated 3, namely, 3a exhibits a unique magnetic conversion from the magnetic disordering to the magnetic ordering at 4 K. 4 shows a ferromagnetic coupling between paramagnetic centers.

Sequential assembly of phototunable ferromagnetic ultrathin films with perpendicular magnetic anisotropy.

Getting organized: Assemblies of ferromagnetic FePt nanoparticles were generated with large perpendicular magnetic anisotropy by a magnetic-field-assisted layer-by-layer method, and subsequently layer-by-layer films consisting of L1(0)-FePt nanoparticles and organic polymers were prepared. These films are phototunable when photochromic molecules are used as polymer layers.

Reversible phototuning of the large anisotropic magnetization at the interface between a self-assembled photochromic monolayer and gold.

We have observed the emergence of a large anisotropic magnetization and significant photoinduced changes in the magentization that appear at the interface between a gold film and an azobenzene-containing self-assembled monolayer. The magnetization value was extremely high, up to 50 mu(B) per adsorbed molecule. These photomagnetic effects can be attributed to a photoinduced change in the loss of d charge due to photoisomerization of the azobenzene monolayer, which is accompanied by inversion of the surface dipoles. Furthermore, we have also observed reversible changes in the work function of the gold film by alternating UV and visible light, showing that the value of the surface dipole moment is changed as a result of photoisomerization. This allowed us to control the magnetization by alternating the photoillumination between UV and visible light, and we have clarified the mechanism for these photomagnetic effects. A novel strategy such as this, which enables significant reversible phototuning of the magnetic order, has great potential for applications in future magneto-optical devices.