Uchida Hiroshi

Abstract This study systematically investigated the role of Ba(OH)₂ concentration on the properties of thin films. Films of (001)-oriented barium titanate (BaTiO₃) were grown on (100) La-doped SrTiO₃ substrates at 200°C by hydrothermal synthesis. X-ray diffraction analysis confirmed the films showed epitaxial and highly (100)-oriented growth with no secondary phases. The as-deposited films exhibited a significant unit cell volume expansion relative to bulk BaTiO₃, which is attributed to the incorporation of hydroxyl (OH^-) groups inherent in the hydrothermal process. Scanning electron microscopy revealed that a high Ba(OH)₂ concentration (2.0 mol dm^⁻3) was essential to achieving a uniform film approximately 150 nm thick, lower concentrations resulted in discontinuous growth. The dense film exhibited a clear ferroelectric domain structure and good dielectric properties. These results demonstrate that precise control over precursor concentration is a key factor in fabricating high-quality epitaxial ferroelectric oxide thin films via low-temperature hydrothermal methods.

Abstract The piezoelectric and ferroelectric applications of heterovalent ternary materials are not well explored. Epitaxial MgSiN₂ films are grown at 600 °C on (111)Pt//(001)Al₂O₃ substrates by the reactive sputtering method using metallic Mg and Si under the N₂ atmosphere. Detailed X‐ray diffraction measurements and transmission electron microscopy observations revealed that the epitaxially grown films on the substrates have a hexagonal wurtzite structure with c‐axis out‐of‐plane orientation. The random occupation of this structure by Mg and Si differs from that of the previously reported structure in which these two cations periodically occupy the cationic sites. However, the lattice spacings closely approximate those that are previously reported, irrespective of the ordering, and they are almost comparable with those of (Al_0.8Sc_0.2)N. The wide bandgap of >5.0 eV in deposited MgSiN₂ is compatible with that of AlN and suggests durability against the application of strong external electric fields, possibly to induce polarization switching. In addition, MgSiN₂ is shown to have piezoelectric properties with an effective d₃₃ value of 2.3 pm V⁻¹ for the first time. This work demonstrates the compositional expansion of hexagonal wurtzite to heterovalent ternary nitrides for novel piezoelectric materials, whose ferroelectricity is expected.

Abstract This study examines the effect of tensile stress on the ferroelectric properties of Pb(Zr0.4Ti0.6)O3 thin film, with a focus on Barkhausen noise, observed for the first time under such conditions. Tensile stress significantly alters domain wall motions, affecting Barkhausen noise more than average polarization. Frequency analysis identifies grain boundaries as primary pinning sites, consistent across stress levels. A non-linear relationship between stress, domain wall mobility, and polarization is found, where increased stress initially enhances pinning and polarization changes, but this effect diminishes at higher stress levels, indicating a shift in behavior.

Abstract To investigate the Ta<sup>5+</sup>-substitution effects on crystal structure and ferroelectric property in HfO<sub>2</sub>-based films, Ta _x Hf_1-x O_2+δ films with various film thicknesses and Ta content were prepared. The ferroelectric orthorhombic phase was formed in a wide film thickness range of 20-100 nm while in a narrow composition range of x = 0.10-0.14. These thickness-insensitive and composition-sensitive characteristics of Ta⁵⁺-substituted HfO₂ film are similar to Y³⁺ rather than Zr⁴⁺. The X-ray photoelectron spectroscopy measurement suggests that the ionic state of Ta is not reduced and Ta _x Hf_1-x O_2+δ film has an excess oxygen state. The excess oxygen may consist of a combination of oxygen vacancies and more interstitial oxygens. These defects facilitate the formation of the ferroelectric phase, while decreasing the breakdown voltage and increasing the leakage current in Ta⁵⁺-substituted HfO₂ films. On the other hand, the generation of excess oxygen indicates the possibility of controlling oxygen vacancies which deteriorate fatigue and retention properties.

Abstract Verbascoside (VB), a phenylpropanoid glycoside found in many medicinal plants, is attracting the attention of researchers due to its significant clinical value. This study, for the first time, attempted the green synthesis of silver nanoparticles (AgNPs) using VB as a reducing and capping agent. The synthesized VB–AgNPs were characterized using ultraviolet–visible, dynamic light-scattering, Fourier-transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analyses. The cytotoxic potency against LX-2 human hepatic stellate cells was also investigated.

Ferroelectric hafnium and zirconium oxides have undergone rapid scientific development over the last decade, pushing them to the forefront of ultralow-power electronic systems. Maximizing the potential application in memory devices or supercapacitors of these materials requires a combined effort by the scientific community to address technical limitations, which still hinder their application. Besides their favorable intrinsic material properties, HfO2–ZrO2 materials face challenges regarding their endurance, retention, wake-up effect, and high switching voltages. In this Roadmap, we intend to combine the expertise of chemistry, physics, material, and device engineers from leading experts in the ferroelectrics research community to set the direction of travel for these binary ferroelectric oxides. Here, we present a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading, what challenges need to be addressed, and possible applications and prospects for further development.

The aqueous extract of the leaves of Odontonema strictum (OSM) is used in folk medicine for its antihypertensive properties, and it contains a wide range of secondary metabolites, mostly polyphenols such as verbascoside and isoverbascoside, which could play a major role in the preparation of silver nanoparticles. In this study, we aimed to prepare AgNPs for the first time using the OSM leaf extract (OSM-AgNPs) to investigate their free radical-scavenging potency against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydrogen peroxide (H2O2). Dynamic light scattering (DLS), UV/Vis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) were used to characterize the OSM-AgNPs. With a size around 100 nm and a ζ-potential of -41.1 mV, OSM-AgNPs showed a good stability and a better colloidal property due to electrostatic repulsion and the dispersity. The strong absorption peak at 3 keV in the EDX spectra indicated that silver was the major constituent. Additionally, the existence of silver atoms was confirmed by the Ag 3d5/2 peak around 367 eV in the XPS spectra. IC50 values of 116 μg/mL and 4.4 μg/mL were obtained for the scavenging activities of DPPH and H2O2, respectively. The synthetic OSM-AgNPs can be further exploited as potential antioxidant agents.

Thin films of BaCe0.8Y0.2O3−δ (BCYO) and SrZr0.8Y0.2O3-δ (SZYO) with a perovskite structure were deposited on (111)Pt//(111)SrTiO3 (STO) single crystal substrates by an RF-magnetron sputtering method. X-ray diffraction revealed that the BCYO and SZYO thin films were polycrystalline but highly (110)-one-axis oriented with local epitaxy on (111)Pt//(111)STO in the 〈110〉-direction. However, these films have six-fold symmetric domains and each domain showed in-plane rotations because of the lattice mismatches between (110)BCYO or (110)SZYO and (111)Pt. Especially, the BCYO film showed larger in-plane rotation than that of SZYO because of its larger lattice mismatch.

Mg<inf>2</inf>Si thin films were deposited at 320 °C on (001)Al<inf>2</inf>O<inf>3</inf>and (100)CaF<inf>2</inf>substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg<inf>2</inf>Si films on (001)Al<inf>2</inf>O<inf>3</inf>substrates were under in-plane tensile strain, while those on (100)CaF<inf>2</inf>substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 °C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature.