Kazumi Kato

It has been reported that the flexoelectric effect could be dominant in the nanoscale. The discrepancy between theory and experiments on the frequency dependence of the dielectric constant of an ordered assembly of BaTiO₃ nanocubes is nearly resolved by assuming the coexistence of flexo- and ferro-electric effects. Although flexoelectric polarizations perpendicular to the applied alternating electric field contribute to the dielectric constant, those parallel to the electric field do not contribute because the magnitude of the flexoelectric polarization does not change due to the mismatch of strain at the interface of the nanocubes. On the other hand, some dielectric response is possible for the ferroelectric component of the polarization parallel to the electric field.

Heat treatment is a key process to determine the dielectric properties of nanocrystalline barium titanate (BaTiO₃) ceramics, which are a prospective candidate to realize further miniaturization of dielectric components in electric devices. Here, we use Raman spectroscopy and scanning microwave impedance microscopy to investigate the dependence of the internal stress and the relative permittivity of BaTiO₃ nanocube three-dimensional assemblies on heat treatment temperature. We show that heat treatment in the temperature range from 700 °C to 1000 °C causes internal compressive stress in the nanocube assemblies without grain growth. The internal compressive stress is caused by formation of tight attachments between neighboring BaTiO₃ nanocubes and of Ti-rich phases in the nanocube assemblies in the lower and higher temperature ranges, respectively. We also show that the relative permittivity of the nanocube assemblies at 3 GHz shows a positive correlation with the internal compressive stress. The result indicates that the internal compressive stress enhances the relative permittivity of BaTiO₃ nanocubes.

Statistics worldwide show that women remain underrepresented in science, technology, engineering, and mathematics (STEM)-related fields relative to the total population. In the IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society (UFFC Society), members who identify as women remain below 10% of the total society members, suggesting that their underrepresentation persists in ferroelectrics research and development activities as well. Addressing this issue and shifting this gender imbalance will require multiple approaches and actions. The Guest Editors, as persistent volunteers and participants in UFFC Society administration and activities, proposed this Special Issue on the Contributions of Women in Ferroelectrics Research and Development as one of many initiatives to further promote diversity, equity, and inclusion in science, engineering, and the UFFC Society.

Novel cube-shaped nanocrystals of BaTiO₃ (BT) surrounded by CaTiO₃ (CT) that can be used for piezo-driven energy harvesters were synthesized using a one-step hydrothermal reaction with surfactant and additives at 250°C for 24 h. The size distribution of the nanocubes (NCs) was narrow, with an approximate average size of 50 ~ 60 nm. The nanocubes had a core-shell configuration, such that the Pbnm structured CT shell was grown on a P4mm structured BT core by sharing corresponding [001] directions. STEM-EDX and EELS indicated that Ba ions did not diffuse into the CT shell region. Piezoelectric hysteresis loops of BT/CT NCs were obtained by piezoresponse force microscopy.

Assembly of nanocrystals into ordered two- or three-dimensional arrays is an essential technology to achieve their application in novel functional devices. Among a variety of assembly techniques, evaporation-induced self-assembly (EISA) is one of the prospective approaches because of its simplicity. Although EISA has shown its potential to form highly ordered nanocrystal arrays, the formation of uniform nanocrystal arrays over large areas remains a challenging subject. Here, we introduce a new EISA method and demonstrate the formation of large-scale highly ordered monolayers of barium titanate (BaTiO₃, BT) nanocubes at the air-water interface. In our method, the addition of an extra surfactant to a water surface assists the EISA of BT nanocubes with a size of 15–20 nm into a highly ordered arrangement. We reveal that the compression pressure exerted by the extra surfactant on BT nanocubes during the solvent evaporation is a key factor in the self-assembly in our method. The BT nanocube monolayers transferred to substrates have sizes up to the millimeter scale and a high out-of-plane crystal orientation, containing almost no microcracks and voids.

Single-crystalline barium zirconate titanate nanocubes (BZ_xT NCs; x is Zr contents) with various composition of Zr/Ti ratio were synthesized by hydrothermal method using water-soluble titanium and zirconium complex and surfactants. The highly-ordered assemblies of the BZT NCs were directly fabricated on the substrate by dip-coating method with low withdrawal rate. High dielectric constant above 4200 and relative low loss tangent below 1% with extremely small frequency dependence were achieved in the case of BZ_0.2T NC assembly. The temperature dependence of capacitance of BZ_0.1T NC assembly was quite small from −60 to 150°C. A specific peak in the Raman spectra of BZ_0.2T NC assembly was slightly shifted to lower wavenumbers from 294 to 288 cm⁻¹ after heat-treatment which meant the change of local bonding condition was induced by local strains at the interfaces. Zr-substitution and local strain synergistically affected to the dielectric properties of BZT NC assemblies.

The dielectric constant of an ordered assembly of BaTiO ₃ nanocubes is numerically calculated as a function of temperature assuming a distribution of tilt angles of attached nanocubes. As the phase transition temperature from the tetragonal crystal structure to the cubic crystal structure of a BaTiO ₃ nanocube decreases as the tilt angle increases, the temperature at the peak of the dielectric constant of an ordered assembly is considerably lower than the Curie temperature of a free-standing BaTiO ₃ crystal. The peak of the dielectric constant as a function of temperature for an ordered assembly becomes considerably broader than that for a single crystal owing to the contribution of nanocubes with various tilt angles.

Elaborate two-dimensional (2D) microarrays of tetragonal Mn₃O₄ nanocuboids 10–20 nm in size were constructed with parallel trenches 500 nm wide and 500 nm deep on a silicon substrate. By adjusting the conditions, including the dispersion medium, particle concentration, and evaporation rate, the a-face and c-face 2D arrays were selectively deposited on the upper and lower stages of the trenches, respectively. The crystallographic direction of the tetragonal crystal was alternately switched in the 2D microarrays under these specific conditions at the optimal particle concentration and evaporation rate. Spatial switching of their crystallographic direction was achieved via interaction of the anisotropic nanoblocks and the specifically shaped surfaces.