Hidekazu Shimotani

Epitaxial strain in perovskite oxide thin films is conventionally understood through lattice parameter modulation, characterized by the c / a ratio of the pseudocubic cell. However, internal atomic arrangements such as octahedral tilting can relax independently, a phenomenon that has remained largely unexplored. Here, we demonstrate that electrochemical operation induces internal structural relaxation in epitaxial La 0.7 Sr 0.3 CoO 3 films while in-plane lattice parameters remain pinned to the substrate. Synchrotron x-ray diffraction reveals that prolonged electrochemical treatment drives the octahedral tilt mode from the as-grown strained configuration toward the bulk structure without conventional lattice parameter changes. This relaxation is enabled by ionic oxygen migration during electrochemical reactions, which reduces energy barriers between structural configurations. Our findings reveal a relaxation pathway in epitaxial oxides that operates independently of conventional lattice parameter relaxation, and demonstrate the importance of monitoring internal atomic arrangements under operating conditions. This establishes electrochemically driven ionic migration as a versatile tool for controlling internal structure in epitaxial oxides, opening opportunities for designing strain-engineered functional materials.

The surface structure of the La_0.6Sr_0.4CoO₃ film, a typical model water-splitting catalyst, is examined under vacuum and electrochemical conditions using surface X-ray diffraction. The pristine sample has a two-unit-cell-thick strongly polarized SrCoO₃ layer at the surface, and the surface termination is predominantly a CoO₂ layer with apical oxygen atoms. After electrochemical treatment, the surface was covered with an additional edge-shared CoO₆ double layer. The polarization in the SrCoO₃ region was greatly reduced. These structural changes were accompanied by an increase in the working electrode current, suggesting a strong relationship between surface structure modulation and catalytic activity. Reversible structural modulation induced by the film's electric potential was observed and was qualitatively understood as atomic displacements caused by the local electric field and change in the Co ionic radii.

A furan derivative, 2-[1,1'-biphenyl]-4-yl-5-(5'-[1,1'-biphenyl]-4-yl-[2,2'-bithiophen]-5-yl)furan (BPFTT), was designed and synthesized and shown to have excellent optical and electronic characteristics. The synthesized compound exhibits a photoluminescence quantum yield (PLQY) of 28%. In addition, a field-effect transistor fabricated using this material in the active layer exhibits ambipolar mobilities of 0.54 (hole) and 0.03 (electron) cm² V^-1 s^-1. Single crystals with their natural crystal edges acting as Fabry-Pérot resonators exhibit dual gain narrowing phenomena with optical pumping thresholds as low as 15 μJ/cm². Introducing planar distributed feedback resonators reduces the threshold to 2.41 μJ/cm², with the Q-factor reaching 1.5 × 10⁴. The full width at half-maximum is measured to be 0.074 nm, which is an excellent value reported to date for any organic single-crystal laser.

We design and synthesise an organic lasing system, 2,5-bis(5-[1,1′-biphenyl]-4-ylthiophen-2-yl)furan, which has advanced optoelectronic properties. Grown as a single crystal, this monoclinic system exhibits typical H-aggregation and has a photoluminescence quantum yield of 29% and ambipolar field effect mobilities of approximately 0.7 (hole) and 0.1 (electron) cm² V⁻¹ s⁻¹. The system is validated for its suitability towards laser development, and it exhibits excellent gain along with dual gain-narrowing emission in the yellow region. It has superior optical and electrical properties compared to its structural counterpart and similar systems. The system can be further developed by incorporating a planar resonator structure and using contact engineering techniques to realise electrically driven semiconductor lasers.

We conduct comparative research on the density of states of electron- and hole- carrier trap levels (DTR(E)), dispersing inside the energy gap of a rubrene single crystal in a field effect transistor (FET) struction with Ca and Au hetero-electrodes for an ambipolar carrier injection mode, by using polymeric protection-layer materials on a Si substrate. Three different types of polymeric materials, poly(methyl-methacrylate) (PMMA), poly(styrene) (PS) and poly(chloro-styrene) (PCS) are employed. From the temperature (T)-dependent source-drain current and gate voltage (ISD-VG) transfer characteristics, the values of DTR(E) are evaluated. PS exhibits the most efficiently-balanced ambipolar carrier transport, which is superior to PMMA that is most typically used as the standard protection layer on a SiO$_2$/doped-Si substrate. Discussions are made in the framework of a carrier multiple trap and release (CMTR) model.

The conditions for determining the oscillation wavelength of organic distributed-feedback (DFB) lasers were investigated by employing 5,5″-bis(biphenyl-4-yl)-2,2′:5′,2″-terthiophene (BP3T) single crystals with various thicknesses as an active layer. Analysis using experimentally obtained refractive indexes revealed that the oscillation wavelengths of some BP3T DFB lasers are at the edge of a stopband, whereas the others are at an edge of a mini stopband, which satisfies the phase-matching condition among different transverse modes and has not been considered in DFB lasers so far. The coupling of different transverse modes was verified by finite-element optical-waveguide simulations of the BP3T DFB laser.

To develop high-performance organic light-emitting organic field-effect transistors (LE-OFETs), a fundamental problem in organic semiconductors is to compromise light luminescent efficiency for high carrier mobility and vice versa. Therefore, LE-OFETs can avoid this problem by separating the light-emission and carrier-transport functions. Here, a bilayer LE-OFET composed of a tetracene crystal as a carrier transporter (bottom crystal) and a 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM1)-doped tetracene crystal as a light emitter (top crystal) was fabricated. Red light-emission color, which is distinct from the green emission color of tetracene, was detected in the top crystal. Light emission from the top layer was prohibited when an insulating thin film was inserted between the two crystals. These observations indicate that excitons are formed in the bottom crystal and transferred to the top crystal, emitting reddish light. Bilayer LE-OFETs have the advantage of providing both high current density and a bright emission for high-performance light-emitting FETs.

Laser is one of the most important discoveries in the 20th century, and inorganic semiconductor lasers (ISCL) are most frequently used in many applications nowadays. Organic semiconductor lasers (OSCL) have many attractive features when they compared to ISCLs, such as flexibility, human friendliness, feasible and inexpensive production process, light weight, and multicolor emission. However, electrically driven OSCLs (el-OSCL) have not yet been realized, although they are possible in an optically driven mode. Here, we report that an el-OSCL can be realized in field-effect transistor (FET) structure. The FET el-OSCL with distributed feedback (DFB) construction is made using a BP3T single crystal as a lasing medium electrostatically laminated on a silicon substrate modified with periodically patterned polystyrene. An emergent sharp-linewidth emission spectrum to the resolution limit of a detector and a non-linear increase in intensity above the threshold current density of ca. 1 kA cm-2 was observed, being indicative of lasing. Discussions on the possible realization of lasing in el-OSCLs can be made from the comparison between optical and electrical-driven mode.