Akifumi Matsuda

Buffer-enhanced room-temperature growth and characterization of epitaxial ZnO thin films

The room-temperature epitaxial growth of ZnO thin films on NiO buffered sapphire (0001) substrate was achieved by using the laser molecular-beam-epitaxy method. The obtained ZnO films had the ultrasmooth surface reflecting the nanostepped structure of the sapphire substrate. The crystal structure at the surface was investigated in situ by means of coaxial impact-collision ion scattering spectroscopy. It was proved that the buffer-enhanced epitaxial ZnO thin films grown at room temperature had +c polarity, while the polarity of high-temperature grown ZnO thin films on the sapphire was −c. Photoluminescence spectra at room temperature were measured for the epitaxial ZnO films, showing only the strong ultraviolet emission near 380nm.

Nanosecond time-resolved Raman spectroscopy on phase transition of polytetrafluoroethylene under laser-driven shock compression

Nanosecond time-resolved Raman spectroscopy is performed on polytetrafluoroethylene (PTFE) under laser shock compression at approximately 1 GPa. Blueshifts (8 cm−1) of a C–C stretching and redshifts (−8 cm−1) of a CF2 twisting in Raman spectrum due to a high-pressure phase (phase III) are observed. High-pressure phase transition to phase III of PTFE is suggested to occur within 10 ns.

Atomically Stepped Glass Surface Formed by Nanoimprint

We investigated atomic-scale surface modifications of silicate glass by nanoimprint using an atomically stepped sapphire (α-Al2O3 single crystal) plate as nanopattern mold. The sapphire mold had regularly arranged straight atomic steps, with uniform height and terrace width of about 0.2 and 80 nm, respectively. During pressing, vertical positions of the sapphire mold and glass plate significantly affected the morphology of the imprinted glass surface. The nanopattern was transferred to the glass surface when the mold was set on the glass plate, while the nanowave pattern was formed on the glass surface when the glass plate was set on the mold.

High-energy x-ray scattering in grazing incidence from nanometer-scale oxide wires

A method has been developed for analyzing the structure of crystalline nanowires deposited on a crystal surface. It combined in-air grazing-incidence surface scattering with high-energy synchrotron x-ray scattering. The technique has allowed a direct reciprocal-lattice space mapping of the x-ray intensities scattered from ultrathin nanowires. Using this method, the sheet-shape diffraction emanating from ultrathin NiO wires was observed. The average nanowire–nanowire distance of 46 nm and a crystallographic domain size of approximately 7.5 nm across the nanowire were evaluated.

Time-resolved Raman spectra of ring-stretching modes of benzene derivatives under laser-driven shock compression at 1 GPa

Raman spectra were collected from benzene, toluene and nitrobenzene, under laser-driven shock compression at approximately 1 GPa. Raman peaks attributed to the ring-breathing and ring-vibrational mode showed a blue shift. The magnitudes of the frequency shift show a systematic dependence on substituents. The magnitude of the shift of the ring-breathing mode decreased in the order of benzene, nitrobenzene, toluene, while that of the ring-vibrational (distortion along an axis) increased.

Crystallinity of Nio nanowires grown at step edges of sapphire substrate

NiO nanowires were formed along atomic step edges on the ultrasmooth sapphire (0001) substrates by laser molecular beam epitaxy. From atomic force microscopy, the nanowires were found to be ~20 nm in width and ~0.5 nm in height along the straight, 0.2 nm-high step edges of the substrate. The crystal structure of NiO nanowires was examined by in situ coaxial impact collision ion scattering spectroscopy (CAICISS). The CAICISS results on the azimuth dependences of Ni signal for the NiO nanowires as well as NiO (111) epitaxial thin films indicate that the NiO nanowires were epitaxially grown with (111) orientation under in-plane stress.

Anisotropic electric conduction derived from self-organized nanogroove array on Li-doped NiO epitaxial film

The self-assembly formation of straight and periodic nanogroove arrays was carried out on the surface of Li-doped NiO thin films by use of atomic steps on the substrate. The nanostructure was formed by annealing Li-doped NiO (111) epitaxial thin film prepared on an atomically stepped α-Al2O3 (0001) substrate via pulsed laser deposition at room temperature. V-shaped nanogrooves, with a depth of ∼20nm and an open-end width of ∼50nm, were observed over the entire substrate and characterized by cross-sectional transmission-electron microscopy. The separation of the aligned nanogrooves was from 80to100nm, comparable to the size of the atomic steps on the substrate. Anisotropic electric conduction was definitely attained for the Li-doped NiO thin film with the nanogroove array. A resistance ratio up to about 100 was obtained for both directions parallel and perpendicular to the nanogrooves. The resultant anisotropy was considered to be mainly caused by anisotropic grain growth due to self-organized reconstructi...

Layer matching epitaxy of NiO thin films on atomically stepped sapphire (0001) substrates

Thin-film epitaxy is critical for investigating the original properties of materials. To obtain epitaxial films, careful consideration of the external conditions, i.e. single-crystal substrate, temperature, deposition pressure and fabrication method, is significantly important. In particular, selection of the single-crystal substrate is the first step towards fabrication of a high-quality film. Sapphire (single-crystalline α-Al2O3) is commonly used in industry as a thin-film crystal-growth substrate, and functional thin-film materials deposited on sapphire substrates have found industrial applications. However, while sapphire is a single crystal, two types of atomic planes exist in accordance with step height. Here we discuss the need to consider the lattice mismatch for each of the sapphire atomic layers. Furthermore, through cross-sectional transmission electron microscopy analysis, we demonstrate the uniepitaxial growth of cubic crystalline thin films on bistepped sapphire (0001) substrates.

Formation of 0.3-nm-high stepped polymer surface by thermal nanoimprinting

We performed atomic-scale surface patterning with a vertical resolution of approximately 0.3 nm on a poly(methyl methacrylate) (PMMA) polymer sheet (10 × 10 mm2) by thermal nanoimprinting using an atomically stepped sapphire template (α-Al2O3 single crystal). The sapphire mold with () r-plane exhibited regularly arranged straight steps with a uniform height of approximately 0.31 nm. The template nanopattern could be transferred onto the surface of the PMMA sheet under the imprinting conditions of 0.2 MPa load for 300 s at 140 °C. Atomic stairs with approximately 0.26-nm-high straight steps and approximately 600-nm-wide terraces were formed on the PMMA surface.

Influence of Rapid Thermal Annealing and Substrate Terrace Width on Self-Organizing Formation of Periodic Straight Nanogroove Array on NiO(111) Epitaxial Thin Film

Room-temperature-grown NiO(111) epitaxial thin films on atomically stepped sapphire (0001) substrates by pulsed laser deposition have straight atomic steps. For a terrace width of about 50 nm, a periodic straight nanogroove array with a depth of about 6 nm was formed over the film surface after rapid thermal annealing. When using a substrate with a terrace width of about 250 nm, it is observed that two types of 180°-rotated triangular crystalline domain are alternately grown on each film terrace divided by the nanogrooves.