Papers by Author: Hiroaki Imai

Abstract: Barium titanate (BT) and strontium titanate (ST) nanocubes which have been synthesized by hydrothermal method with surfactants were assembled in order directly on the substrates by using capillary-force-assisted self-assembly method. The ordered structures, crystallinity and orientation of the nanocubes were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning probe microscopy (SPM) and X-ray diffraction (XRD). The piezoresponse properties of the nanocubes ordering structures characterized by Piezoresponse Force Microscopy (PFM) seemed to depend on the constituents and their interface.

Abstract: Highly crystallized BaTiO₃ thin films were fabricated by a nanocrystal sintering process. Boron alkoxide was introduced into a slurry of Mn-doped BaTiO₃ nanocrystals with particle sizes of 5-7 nm. The deposited nanocrystal film on a (111)-oriented Pt/TiO₂/Al₂O₃ substrate was sintered at a low temperature of 800 °C and the obtained film had highly densified and oriented microstructures. We found that the boron additive enhanced the grain growth of nanoparticles and as a result the dielectric constant of the thin film increased to 1100 at 10 kHz, which is much higher than that of undoped BaTiO₃ thin films.

Abstract: The morphology of wurtzite-type zinc oxide (ZnO) grown from an aqueous solution was successfully controlled by addition of phosphate ions and various organic molecules having carboxy groups. Basically, array of hexagonal needles with a diameter of 50–100 nm was grown on a substrate in the absence of the additives. Hexagonal plates were perpendicularly arranged on the substrate by the adsorption of citric, tartaric and maleic acids. The presence of phosphate ions induced an open cellular structures consisting of zinc oxide nanosheets. Densely packed columns and bundles consisting of nanoscale fibrous crystals were produced by the addition of bulky dye molecules, such as phenolphthalein. The influence of the dye molecules depended on pH of the solution. The morphological variation of ZnO films was ascribed to the selective adsorption of the anionic species on the basal and prism planes of the wurtzite structure.

Abstract: We successfully controlled nanoscale morphology of ZnO, SnO2 and TiO2 grown in aqueous solution systems. Nanograins, nanorods and nanosheets of the metal oxides were obtained with variation of the preparation conditions including the concentration of the precursor reagents, pH and addition of various organic molecules and inorganic ions. The morphological evolution of wurtzite-type ZnO crystals from hexagonal nanorods to nanoplates and nanosheets was induced by the coexistence of phosphate anions and specific carboxylates. An increase in the concentration of SnF2 and pH changed the nanoscale morphology of crystalline SnO2 from grains into rods, flakes and sheets. Nanorods of anatase and rutile were deposited in the solutions of TiF4 and TiOSO4 by controlling pH and addition of urea. Nanosheets of titanate were prepared with ammonia anions in agar gel containing TiF4 and easily transformed into nanosheets of anatase.