Microbial Deracemization of α-Substituted Carboxylic Acids: Substrate Specificity and Mechanistic Investigation

A new enzymatic method for the preparation of optically active α-substituted carboxylic acids is reported. This technique is called deracemization reaction, which provides us with a route to obtain the enantiomerically pure compounds, theoretically in 100% yield starting from the racemic mixture. This means that the synthesis of a racemate is almost equal to the synthesis of the optically active compound, and this concept is entirely different from the commonly accepted one in the asymmetric synthesis. Using the growing cell system of Nocardia diaphanozonaria JCM3208, racemates of 2-aryl- and 2-aryloxypropanoic acid are deracemized smoothly and (R)-form-enriched products are recovered in high chemical yield (>50%). In addition, using optically active starting compounds and deuterated derivatives as well as inhibitors, we have disclosed the fact that a new type of enzyme takes part in this biotransformation, and that the reaction proceeds probably via the same mechanism as that in rat liver

Microbial Deracemization of α-Substituted Carboxylic Acids

An enzyme system of Nocardia diaphanozonaria JCM 3208 catalyzes the inversion of the chirality of various α-substituted carboxylic acids, such as 2-phenylpropanoic acid and 2-phenoxypropanoic acid derivatives, via a novel deracemization reaction

Ferroelectricity, High Permittivity, and Tunability in Millimeter-Size Crack-Free Ba_1–<i>x</i>Sr_<i>x</i>TiO₃ Flexible Epitaxial Sheets

Flexible oxide sheets exhibiting ferroelectricity and high permittivity are crucial for the advancement of various emerging technologies. However, achieving large-area crack-free flexible oxide sheets remains difficult because oxides easily crack when their thicknesses are significantly reduced. In this study, we focused on Ba1–xSrxTiO3 (BST), which is an important material owing to its high permittivity and electric-field-induced tunability. By employing an amorphous AlOx protective layer with a thickness greater than 10 nm, we successfully fabricated millimeter-sized crack-free BST epitaxial sheets. In contrast, the sheets fabricated without protective layers exhibited breakage. In addition, we observed that a polycrystalline indium tin oxide layer acted as a suitable bottom electrode. The BST sheet with a composition of x = 0.25 exhibited excellent ferroelectric switching behavior and minimal current leakage, even when used with electrodes with a diameter of 100 μm. Furthermore, the BST sheet with a composition of x = 0.5 simultaneously exhibited high permittivity (εr ∼ 3500 at 10 kHz) and tunability (56%), combining the desirable characteristics of both bulk and thin-film materials. These improved dielectric properties are attributed to the absence of substrate-induced strain, which is a characteristic not observed in thin-film materials

Significant Reduction in the Switching Time of Solid-State Electrochemical Thermal Transistors

Thermal transistors have the remarkable ability to electrochemically switch the thermal conductivity (κ) of an active material. Several thermal transistors have been reported to control heat flow, but they are impractical because they use liquid electrolytes. Recently, we realized a solid-state thermal transistor that electrochemically controls the κ of SrCoOx (2 ≤ x ≤ 3) using 0.5-mm-thick yttria-stabilized zirconia (YSZ) single crystal substrates as a solid electrolyte at 280 °C. The applicable electric current is low (50 μA) due to the high electrical resistivity of YSZ at 280 °C. Consequently, κ switching is slow (∼3 min). Herein, we aim to reduce the switching time by examining several SrCoOx-based thermal transistors using YSZ substrates with varied thicknesses. The x in SrCoOx is controlled between 2 and 3, and the κ switches between 0.97 and 3.86 W m–1 K–1. The overall electrical resistance decreases as the YSZ thickness decreases. For a 0.1-mm-thick YSZ substrate, the applicable current increases to 1 mA and the switching time is significantly reduced to ∼10 s

Significant Suppression of Cracks in Freestanding Perovskite Oxide Flexible Sheets Using a Capping Oxide Layer

Flexible and functional perovskite oxide sheets with high orientation and crystallization are the next step in the development of next-generation devices. One promising synthesis method is the lift-off and transfer method using a water-soluble sacrificial layer. However, the suppression of cracks during lift-off is a crucial problem that remains unsolved. In this study, we demonstrated that this problem can be solved by depositing amorphous Al2O3 capping layers on oxide sheets. Using this simple method, over 20 mm2 of crack-free, deep-ultraviolet transparent electrode La:SrSnO3 and ferroelectric Ba0.75Sr0.25TiO3 flexible sheets were obtained. By contrast, the sheets without any capping layers broke. The obtained sheets showed considerable flexibility and high functionality. The La:SrSnO3 sheet simultaneously exhibited a wide bandgap (4.4 eV) and high electrical conductivity (>103 S/cm). The Ba0.75Sr0.25TiO3 sheet exhibited clear room-temperature ferroelectricity with a remnant polarization of 17 μC/cm2. Our findings provide a simple transfer method for obtaining large, crack-free, high-quality, single-crystalline sheets

Thermopower Modulation Analyses of High-Mobility Transparent Amorphous Oxide Semiconductor Thin-Film Transistors

Transparent amorphous oxide semiconductor InSnZnOx (ITZO)-based thin-film transistors (TFTs) exhibit a high field-effect mobility (μFE). Although ITZO-TFTs have attracted increasing attention as a next-generation backplane of flat panel displays, the origin of the high μFE remains unclear due to the lack of systematic quantitative analyses using thermopower (S) as the measure. Here, we show that the high μFE originates from an extremely light carrier effective mass (m*) and a long carrier relaxation time (τ). The S measurements of several ITZO films with different carrier concentrations clarified that m* of ITZO films is ∼0.11 m0, which is ∼70% of that of a commercial oxide semiconductor, amorphous InGaZnO4 (∼0.16 m0). We then fabricated bottom-gate-top-contact ITZO-TFTs displaying excellent transistor characteristics (μFE ∼ 58 cm2 V–1 s–1) using amorphous AlOx as the gate insulator and demonstrated that the effective thickness increases with the gate voltage. This suggests that the bulk predominantly contributes to the drain current, which results in τ as long as ∼3.6 fs, which is quadruple that of amorphous InGaZnO4-TFTs (∼0.9 fs). The present results are useful to further improve the mobility of ITZO-TFTs

Suppression of Strain Relaxation in VO₂/TiO₂ Multilayered Films

Strained VO2 films grown on (001) rutile TiO2 exhibit an abrupt insulator-to-metal transition (IMT) around room temperature. The transition temperature (Tc) increases when the critical thickness of ∼15 nm is exceeded. The strain relaxation is responsible for crack formation. Here, we show that inserting thin TiO2 layers suppresses the strain relaxation of VO2. We fabricated several VO2/TiO2/VO2 trilayer films on (001) TiO2 substrates. Each film had a VO2 film of 8 nm, but the TiO2 thickness was varied. Strained VO2/TiO2 multilayer films show IMT with clear hysteresis around room temperature when the TiO2 thickness exceeds 4 nm. In addition, we fabricated 8 nm thick VO2/9 nm thick TiO2 multilayered films. Tc is maintained around room temperature even though the total VO2 thickness is ∼50 nm. The present results provide a useful design concept of thin-film materials for thermochromic applications of strained VO2

Plasmon-Assisted Polarity Switching of a Photoelectric Conversion Device by UV and Visible Light Irradiation

The plasmon-induced charge separation between metallic nanoparticles and a semiconductor following an electron transfer process has been extensively studied as one of the mechanisms in plasmonic light energy conversion devices. In this study, we propose that the switching of photocurrent polarity can be realized by changing the rectification properties of plasmonic photoelectric conversion devices and utilizing the difference in carrier mobility between electrons and holes. We fabricated plasmonic photoelectric conversion devices using gold nanoparticles (Au-NPs), nickel oxide (NiO), and mobility-limited TiO₂ (ML-TiO₂) to control the photocurrent polarity according to irradiation wavelengths of visible and UV light. A pulsed laser deposition technique was employed to deposit the ML-TiO₂ and NiO layers. The photoelectric properties were measured, and <i>in situ</i> spectroelectrochemical measurements were performed to investigate the relationship between the rectification properties of the plasmonic photoelectric conversion devices and the change in the Fermi level of the Au-NPs under UV light irradiation condition. Additionally, UV and visible light irradiation selectively induced the current of opposite polarity with the small applied voltage. The electron transfer phenomena from ML-TiO₂ to Au-NPs and from Au-NPs to ML-TiO₂ give us important information to understand plasmon-related charge separation

Magnetic Phase Transition-Induced Modulation of Ferroelectric Properties in Hexagonal <i>R</i>FeO₃ (<i>R</i> = Tb and Ho)

Hexagonal rare-earth iron oxides (h-RFeO3) exhibit spontaneous magnetization and room-temperature ferroelectricity simultaneously. However, achieving a large magnetoelectric coupling necessitates further exploration. Herein, we report the impact of the magnetic phase transition on the ferroelectric properties of epitaxial h-RFeO3 (R = Tb and Ho) films prepared by pulsed laser deposition. The metastable h-RFeO3 phase is successfully stabilized with high crystallinity and low leakage current due to the ITO buffer layer, making it possible to investigate the ferroelectric properties. The h-TbFeO3 film exhibits a magnetic-field-induced transition from antiferromagnetic (AFM) to weak ferromagnetic (wFM) phases below 30 K, while also exhibiting ferroelectricity at 300 K. The dielectric constants change with the magnetic phase transition, demonstrating hysteresis in the magnetocapacitance. In contrast, the h-HoFeO3 film exhibits antiferroelectric-like behavior and an AFM–wFM phase transition. Notably, the h-HoFeO3 film shows a rapid increase in the remnant polarization during the AFM–wFM phase transition accompanied by an increase in the ferroelectric component. Considering the strong connection between the antiferroelectric behavior in the h-RFeO3 system and the ferroelectric domain wall motion, this considerable modification of ferroelectric properties during the magnetic phase transition is probably due to the faster movement of the ferroelectric domain walls in the wFM phase induced by the clamping effect. Our findings indicate the effectiveness of magnetic phase transitions in enhancing the magnetoelectric coupling, particularly when utilizing domain wall clamping properties