Sachio Horiuchi

We present novel electro-optic (EO) molecular crystals suitable for future optoelectronic integration that combine high EO activity, thin-film processability, and thermal stability.

We investigate terahertz radiation via an ultrafast polarization change in a typical hydrogen-bonded molecular ferroelectric, croconic acid, induced by a proton vibrational excitation with a mid-in...

Dielectric crystals with switchable electric polarizations represent the key functional materials utilized for a broad range of practical applications. They allow for academically intriguing platforms, where the use of a strong external electric field can potentially unveil hidden crystal phases. Proton–π-electron correlated bistable systems turn out to be promising for exploring such electrically induced crystal polymorphisms, mainly because strong π-electronic polarization can be sensitively switched depending on mobile hydrogen locations. Pseudo-symmetry and hydrogen disorder are utilized as clues for the data mining of the Cambridge Structural Database in the search for molecular candidates with novel switchable dielectrics. The polarization hysteresis, electrostriction, and second harmonic generation of the candidates were experimentally evaluated, together with the re-inspection of crystal structure. This feature article highlights the rich variation and competition of some candidate polarization configurations and switching modes in close relation to high and efficient electrical energy storage/discharge, large electrostriction effects, polarization rotations, and multistage switching phenomena. The experimental findings are well-reproduced by the computational optimization of crystal structure and the simulation of the switchable polarization, piezoelectric coefficients, and relative stability for each of the real or hypothetical hydrogen-ordered crystal phases. Effective prediction and strategic design are thereby guaranteed by systematically understanding the appropriate integration of experimental, computational, and data sciences.

The challenge in developing molecule-based electronic materials lies in the uncontrollable or unpredictable nature of their crystal structures, which are crucial for determining both electrical properties and thin-film formability. This review summarizes the findings of a research project focused on the systematic development of crystalline organic semiconductors (OSCs) and organic ferroelectrics by integrating experimental, computational, and data sciences. The key outcomes are as follows: 1) Data Science: We developed a method to identify promising materials from crystal structure databases, leading to the discovery of unique molecule-based ferroelectrics. 2) Computational Science: The origin of high layered crystallinity in π-core – alkyl-chain-linked molecules was clarified based on intermolecular interaction calculations. We proposed a stepwise structure optimization method tailored for layered OSCs. 3) Material Development: We developed various alkylated layered OSCs, which exhibit high mobility, heat resistance, and solubility. We discovered several unique phenomena, including frozen liquid crystal phases, significant polar/antipolar control, and phase control through mixing, leveraging the variability of alkyl chain length. We also developed molecule-based ferroelectrics showing peculiar ferroelectricity, including multiple polarization reversal, competing ferroelectric/antiferroelectric order, and spinner-type configurations with π-skeletons. 4) Advanced Structural Analysis: By combining cryo-electron microscopy and X-ray-free electron laser (XFEL), we enabled crystal structure analysis for ultrathin crystals that are usually difficult to analyse. 5) Device Development: Utilizing the self-organized growth of layered OSCs through solution processes, we developed a method to produce exceptionally clean semiconductor – insulator interfaces, achieving field-effect transistors that show sharp (near theoretical limit) and stable switching at low voltages.

The convergence of electronics and photonics is attracting attention for its potential to surpass performance limitations of existing information‐processing devices. In particular, the electro‐optic (EO) effect plays a critical role in high‐speed and low‐power conversion between electrical and optical signals, which is demanded for future communication networks. Here, a novel class of EO material is demonstrated, the organic ferroelectric crystal of croconic acid (CRCA). The recently developed birefringence field‐modulation imaging technique enables high‐throughput evaluation of the EO coefficient for as‐grown bulk crystals, unveiling a figure of merit of >400 for CRCA, which exceeds that of 320 in the conventional EO material LiNbO₃ in the visible‐light range. Analyses in conjunction with theoretical calculations clarify that its remarkable EO performance is attributable to deformation of the π‐orbital coupled with the proton displacement. This finding provides a new route for the molecular design of high‐performance EO materials: proton–π‐electron‐coupled ferroelectrics.

In this study, we performed high-magnetic-field magnetization, dielectric, and ultrasound measurements on an organic salt showing a ferroelectric spin-Peierls (FSP) state, which is in close proximity to a quantum critical point. In contrast to the sparsely distributed gaslike spin solitons typically observed in conventional spin-Peierls (SP) states, the FSP state exhibits dense liquidlike spin solitons resulting from strong quantum fluctuations, even at low fields. Nevertheless, akin to conventional SP systems, a magnetic-field-induced transition is observed in the FSP state. In conventional high-field SP states, an emergent wave vector results in the formation of a spin-soliton lattice. However, in the present high-field FSP state, the strong quantum fluctuations preclude the formation of such a soliton lattice, causing the dense solitons to remain in a quantum-mechanically melted state. This observation implies the realization of a quantum liquid-liquid transition of topological particles carrying spin and charge in a ferroelectric insulator.

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π‐conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side‐by‐side unidirectionally to form self‐organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n , when the molecules are designed to involve effectively weakened head‐to‐head affinity. They exhibit remarkable odd–even effect in the interlayer stacking: alternating head‐to‐head and tail‐to‐tail (antipolar) arrangement in odd‐ n crystals, and uniform head‐to‐tail (polar) arrangement in even‐ n crystals. The films show excellent field‐effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second‐order nonlinear optical response along normal to the film plane. These findings are key for creating polarity‐controlled optoelectronic materials and devices.

Optically driven ferroelectric polarization reversal and its dynamics are observed in hydrogen-bonded organic ferroelectrics on a scale of about 10 ps.

Strong coupling of charge, spin, and lattice in solids brings about emergent elementary excitations with their intertwining and, in one dimension, solitons are known as such. The charge-transferred organic ferroelectric, TTF-CA, has been argued to host charge solitons; however, the existence of the charge solitons remains unverified. Here, we demonstrate that the charge-transport gap in the ionic Mott-Peierls insulating phase of TTF-CA is an order of magnitude smaller than expected from quasiparticle excitations, however, being entirely consistent with the charge soliton excitations. We further suggest that charge and spin solitons move with similar diffusion coefficients in accordance with their coexistence. These results provide a basis for the thermal excitations of the emergent solitons.

Switchable π-electron systems are very powerful fragments to emphasize ferroelectric or antiferroelectric polarizations up to record-high levels among organic molecular crystals. According to the Cambridge Structural Database, many azole crystals such as imidazoles and tetrazoles contain polar and bistable hydrogen-bonded molecular sequences suitable for ferroelectricity or antiferroelectricity. Indeed, polarization hysteresis experiments on the 5-phenyl-1H-tetrazole (PHTZ) family combined with single crystal structural analysis have revealed one ferroelectric, two antiferroelectrics, and two hybrid-like dielectrics. Here, the rich variations for the interrelation between the hydrogen-bonding states and the polarization switching modes are interpreted by density functional theory (DFT) calculations with an excellent consistency. Large switchable polarizations are theoretically confirmed, and, as expected, the largest contribution is the switchable π-electron systems. By mapping the energy levels of polar/antipolar states, the disordered hydrogen bonds always appear when the ground state is accompanied by a nearly degenerate state. The straightforward case is the hybrid-like dielectric caused by the competition between the polar and antipolar states. However, contrastive behaviors are observed when the switchable dipoles are involved in competition between the different antipolar arrangement. For example, the PHTZ crystal exhibits typical antiferroelectric switching regardless of the hydrogen disorder, whereas polarization switching is silent in the imidazole derivatives. The latter is explained by the switching field increase with depth of the ground state relative to the energy level of the polar state.