Hiroyo Segawa

An aqueous solution process and subsequent UV treatment for highly transparent conductive ZnO films

High electric conductivity was achieved in ZnO films prepared by a low-temperature (<100 °C) wet-chemical process and subsequent UV treatment by a commercial blacklight-blue lamp with a central wavelength of 360 nm and output power of 2.0 mW cm−2. The UV treatment to the as-deposited film successfully decreased the electric resistivity of the film by three orders of magnitude from 11 to 4.4 × 10−3 Ω cm. The resistivity was not restored to the initial value even after dark storage for 50 days, indicating that the decrease of the resistivity has not mainly originated from conventional persistent photoconductivity. The results of Hall measurement and Fourier transformed infrared measurement showed that an increase of carrier concentration and decomposition of organic impurities of the films occurred simultaneously, which indicates that the phenomenon was caused by introduction of donor impurities such as hydrogen atoms, carbon species and oxygen vacancies into the ZnO lattice by the photocatalytic decomposition of residual organic impurities remaining in the film.

Fabrication of silica glass containing yellow oxynitride phosphor by the sol–gel process

Abstract We have prepared silica glass by the sol–gel method and studied its ability to disperse the Ca-α-SiAlON:Eu2+ phosphor for application in white light emitting diodes (LEDs). The emission color generated by irradiating doped glass with a blue LED at 450 nm depended on the concentration of SiAlON and the glass thickness, resulting in nearly white light. The luminescence efficiency of 1-mm-thick glass depended on the SiAlON concentration, and was highest at 4 wt% SiAlON.

Organic–Inorganic Hybrid Materials for Photonic Applications

Organic-inorganic hybrid materials are derived from the chemical reaction of silane coupling reagents and metal alkoxides, involving O-Si-C bonds in the matrix. They offer superior characters of combined organic groups and inorganic linkages for various optical applications. This paper reviews preparation and characterization of optical devices and materials derived from these hybrid materials. We mainly describe three topics for micrometer-nanometer scale optical components in our laboratories using organic-inorganic hybrid materials: 1) film formation; 2) patterning by the photolithography techniques; and 3) laser emission from dye-doped hybrid spheres and coated glass spheres.

Crystallization behavior in Se90Te10 and Se80Te20 thin films

Isothermal crystal growth kinetics in Se90Te10 and Se80Te20 thin films was studied by microscopy and in situ X-ray diffraction (XRD) measurements. The spherulite-like crystals grew linearly with time. In a narrow temperature range of between 65 and 85 °C, crystal growth rates exhibit simple exponential behavior with activation energies EG = 193 ± 4 kJ mol−1 for Se90Te10 and EG = 195 ± 4 kJ mol−1 for Se80Te20. The crystal growth in both compositions is controlled by liquid-crystal interface kinetics and can be described by a screw dislocation growth model. From the XRD data, the crystallization fraction was estimated. The crystallization data were described by Johnson-Mehl-Avrami (JMA) model with Avrami exponents m = 1.4 ± 0.3 for Se90Te10 and m = 1.6 ± 0.4 for Se80Te20. Activation energies were estimated from the temperature dependence of rate constant evaluated from the JMA model. The activation energies of nucleation-growth process were found to be Ec = 184 ± 21 kJ mol−1 for Se90Te10 and Ec = 179 ± 7 kJ...

Direct Growth of Highly Ordered Crystalline Zirconia Nanowire Arrays with High Aspect Ratios on Glass by a Tailored Anodization

We report a facile approach to fabricate highly ordered zirconia nanowire arrays with controllable dimensions of φ25-40 nm in diameter, 100—550 nm in length, and 3-20 in aspect ratio by a tailored two-step anodization from superimposed At/Zr layers sputter-deposited on glass substrates. A porous-type Al anodization in a constant potential mode was performed on the Al layer on Zr/ glass in strong acidic electrolytes to form highly ordered porous alumina films with different pore sizes and intervals. Successively, a barrier-type Zr anodization in a constant current mode, at a low-critical-current density of 2 A m -2 with open potentials up to 70-500 V in different acidic electrolytes, was carried out on the underlying Zr film via the overlying PAA films, thereby allowing direct growth of the zirconia nanowires within the alumina nanopores on glass. The as-anodized zirconia nanowires were crystalline, comprising a mixture of monoclinic and orthorhombic ZrO 2 . In particular, the orthorhombic phase in the zirconia nanowires becomes more predominant with increasing terminal anodizing potentials and exhibits a preferential crystalline orientation in the (002) facet. The formation of anodic zirconia nanowires could be ascribed to the enhancement of the transport number of Zr cations in crystalline zirconia affected by the low-critical-current density and the impurities in the initial Zr film.

Fabrication of alumina films with laminated structures by ac anodization

Abstract Anodization techniques by alternating current (ac) are introduced in this review. By using ac anodization, laminated alumina films are fabricated. Different types of alumina films consisting of 50–200 nm layers were obtained by varying both the ac power supply and the electrolyte. The total film thickness increased with an increase in the total charge transferred. The thickness of the individual layers increased with the ac voltage; however, the anodization time had little effect on the film thickness. The laminated alumina films resembled the nacre structure of shells, and the different morphologies exhibited by bivalves and spiral shells could be replicated by controlling the rate of increase of the applied potentials.

Pyramidal assemblies of colloidal particles by micromolding underneath top-gathering pillar arrays

Assemblies of colloidal particles are known to have special photonic and optical properties. Periodic pyramidal assemblies of SiO2 particles with diameters of 0.5 and 1 microm were fabricated using top-gathering pillar arrays as a new template. These top-gathering pillar arrays consisted of four pillars gathered at the top, and were fabricated by photolithography of an organic-inorganic hybrid material. The top-gathering units were obtained by controlling both the capillary and restoring forces. When a colloidal water suspension was spread on the template and the water was allowed to evaporate, the SiO2 particles were molded under the top-gathering pillars according to particle size, resulting in pyramidal assembly arrays of the particles. From in situ observation during the evaporation of water, it was found that the particles were molded underneath the top-gathering pillars by flux generated by the evaporation and by the capillary force among the particles.

Fabrication of Periodic Arrays of Top-Gathering Titania-organic Hybrid Pillars Derived from Multi-beam Laser Interference

Multi-beam laser interference (MLI) lithography is known to be one of the fabrication techniques of photonic crystals. In MLI lithography, laser beams are interfered on a scale of the wavelength of light and are irradiated to photosensitive films. We have paid attention to photosensitive TiO2-organic hybrid film, which has high refractive index. TiO2-organic hybrid periodic pillar patterns were fabricated by MLI lithography. The TiO2 hybrid film, which was prepared from Ti-alkoxide and β-diketone by the sol-gel method, was exposed to the interference pattern of femtosecond pulses at 800 nm wavelength. After laser irradiation, the unirradated portion was removed by the development and rinse. Two-dimensional periodic standing alone pillars and 2×2 and 3×3 top-gathering pillars, which were gathered at the top by means of self-organization were formed by changing conditions such as laser irradiation time and film thickness. The pillar patterns depended upon the rinse liquid, the diameter, and the height of pillars. The top-gathering pillars are applicable for diffractive optics.

Control of cohesive-force ordering in organic-inorganic hybrid pillar arrays

Organic-Inorganic hybrid pillar arrays have been controlled by cohesive force during drying in photolithography. Two-dimensional periodic pillars with micrometer repetitions were fabricated from an organic-inorganic hybrid material. The pillars were gathered at the top and gtop-gatheringh pillar patterns were obtained depending on pillar sizes such as height of the pillars and distance between neighboring pillars and types of rinse liquids. The top-gathering pillar patterns could be obtained easily in the pillar arrays with same structural parameters using 1-PrOH as a rinse liquid rather than water. From in situ observation of the drying rinse liquids, it was found that the drying of 1-PrOH differs from that of water in the pillar arrays because of the difference in the contact angles. Top-gathering pillars were partially introduced in a homogeneous periodic pillar array by the different pillar formations between two types of rinse liquids.

Optical properties of green-emitting β-sialon:Eu phosphor-containing silica glasses and their deterioration mechanism

β-sialon:Eu phosphor-containing silica glass was synthesized as a wavelength converter in solid-state lighting. The optical properties and pore density of the sample strongly depended on the sintering temperature. Sintering at a high temperature of 1050 °C led to a severe deterioration of β-sialon:Eu phosphor owing to N2 evaporation, whereas a high conversion efficiency could be achieved by lowering the sintering temperature down to 600 °C. The β-sialon:Eu phosphor-containing silica glass sintered at 600 °C showed a high durability against high-power excitation, making it possible to be used in high-luminosity and high-power solid-state laser lighting.