Uchida Hiroshi

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.