坂間 弘

有機太陽電池の構成要素であるフラーレンとチタニアを利用した有機太陽電池では，光活性分子から励起電子をフラーレンが受容し，それを電子輸送層として機能させるチタニアに受け渡すことで発電につなげている。本研究では，フラーレンとチタニアの接合界面における光励起キャリアの挙動を時間分解光電子分光により検証し，界面エネルギー準位接続とキャリア移動の関係を議論する。

宇宙機から放出された分子状コンタミネーションが光学機器（太陽光パネル、ミラー等）に付着するとコンタミネーションの光吸収、散乱により性能低下を引き起こす。付着した分子状コンタミネーションの除去法として現在用いられている方法はベーキングのみであり、効果が不十分なため、新たな除去法を構築する必要がある。我々は光触媒に着目し、作製した二酸化チタンによる宇宙機コンタミネーションモデル物質の分解実験を行った。

Synchrotron-radiation angle-resolved photoelectron spectroscopy has been utilized to examine the bulk valence-band structure of anatase TiO2(001) thin films fabricated on LaAlO3(100) by pulsed laser deposition. The energy-momentum dispersion relation of O 2p-derived nonbonding, O 2p-Ti 3d sigma bonding and several pi bonding states is determined experimentally. The nonbonding state at the top of the valence band is located at 4.3 eV at the center of the bulk Brillouin zone, and it shifts towards the shallower energies to 3.8 eV at the zone boundary. No other states with binding energies smaller than 3.8 eV are found on any other high-symmetry axes and points. Thus the valence-band maximum is located at the zone boundary. Our finding proves that anatase is an indirect-band-gap semiconductor.

The initial growth of a Ag three-dimensional island on an atomically resolved Si(001) substrate was investigated in situ by scanning tunneling microscopy (STM) at room temperature. It took similar to 20 min for the island to grow from nucleation to a final dimension of 25 nm x 35 nm x 22 monolayers (ML). Uniquely, the island growth occurred under no Ag deposition. The Ag atoms required for the growth were provided from the two-dimensional Ag layer and diffusing Ag atoms on the layer that was deposited before the observation. Thanks to this unique growth mechanism, it was allowed to observe the island growth under an isotropic supply of Ag atoms without the shadowing effect of metal deposition by a scanning probe. On the other hand, the STM measurement itself affected finite effects on the growth; attractive interaction between the probe and Ag atoms promoted nucleation of the island, and tunnel current injection may have increased the effective temperature of the system. Despite such a measurement effect, some growth processes that are characteristic of typical metal thin-film growth on silicon substrates were clearly visualized, such as anisotropic and nonmonotonic growth rates that were affected by atomic surface defects, and the growth mode transition from area oriented to height oriented due to an accumulation of stress arising from the lattice mismatch.

Anatase TiO2 is well known to exhibit high photocatalytic activity. It is believed that photo-excited carriers create activated species by reacting with water and oxygen at the crystal surface, and such activated species decompose organic pollutants. However, the relaxation and diffuse process of the photo-excited carriers is not understood sufficiently. In this study, we investigated such dynamics of photo-excited carriers by transient absorption spectroscopy in anatase TiO2 epitaxial thin film. Moreover, the transient absorption signal and photoluminescence (PL) results are compared for the first time. © 2011 IEEE.

We measured transient absorption spectra with pump-probe technique to investigate the dynamics of photo-excited carriers at 300 K and 12 K in anatase TiO(2) epitaxial film. Although the photoluminescence (PL) originated from self-trapped excitons (STEs) vanished at 300 K, we observed the change in probe signal. On the other hand, the intensity of the transient absorption signal at 12 K was much weaker than that at 300K. Therefore the carriers at 300 K become trapped at the surface, leading to a pump-probe signal, while at 12 K, the remained carriers inside the bulk arise from STE recombination. These results suggest that the pump-probe signal is derived from the trap state(s) near the surface.

We report the fabrication of BiFeo(3)/BiCrO3(111) artificial superlattices with 1/1 stacking in a layer-by-layer growth mode on atomically flat SrTio(3)(111) surfaces. While BiFeo(3) and BiCro(3) are antiferromagnets having Fe-O-Fe and Cr-O-Cr bonds, these superlattices contain Fe-O-Cr bonds, in which ferromagnetic interaction is expected. Magnetization measurements at 300 K revealed that the magnetic moment per transition metal ion was 1.7 mu(B), suggesting ferromagnetic spin order. Ferroelectric behavior at room temperature was confirmed by an analysis using a scanning non-linear dielectric microscope. (c) 2008 The Japan Society of Applied Physics

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO2 thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed. (c) 2007 Elsevier B.V. All rights reserved.

We fabricated BiFeO3/BiCrO3 (111) artificial superlattice in a layer-by-layer growth mode by using pulsed laser deposition. The stacking period was chosen to be 1/1 so that Fe3+ and Cr 3+ are ordered in a rock-salt fashion in a perovskite structure. To prepare well-defined interfaces we started from obtaining an atomically-flat surface of SrTiO3 (111) substrate, and restricted the growth mode to only layer-by-layer growth by in situ observation of RHEED intensity during all through the superlattice fabrication. Magnetization measurements at 300 K showed ferromagnetic hysteresis with the magnetic moment per transition metal ion was 1.7μB. This behavior was considered to be resulted from the Fe-O-Cr bonds, realized by the (111) orientation and 1/1 layer-by-layer growth, giving rise to ferromagnetic correlation, although the comprising BiFeO3 and BiCrO3 are both antiferromagnets. Ferroelectric behavior at room temperature was observed by scanning non-linear dielectric microscope. These results indicate that BiFeO3/BiCrO3 (111) 1/1 artificial superlattice is a multiferroic material at room temperature. © 2009 Materials Research Society.

The dynamics of photoexcited carriers in standard TiO2 photocatalytic powders has been studied by investigating the time decay and temperature dependence of the photoluminescence. A biexponential decay curve in a time-resolved signal suggests that there are different formation processes of the self-trapped exciton states. The peak position of the luminescence spectrum shifted to a higher energy at room temperature. On the basis of these experimental results, we propose a relaxation model of photoexcited carriers.

We have investigated the relaxation process of photo-excited carriers in anatase TiO2 films, in which the concentration of the oxygen vacancy is controlled by annealing in ultra high vacuum. A broad luminescence band peaked at 2.32 eV is ascribed to a recombination of self-trapped excitons (STEs). The luminescence decay curve was composed of several time constants corresponding to the direct and indirect formation of STEs. We have confirmed that the longer annealing, i.e. higher concentration of the oxygen vacancy, leads to an increase in the indirect STEs.

Nanometer-sized particles of MoS2 were synthesized on mica and MoS2 surfaces and observed using an atomic force microscope. Molybdenum oxide was deposited on each substrate and sulfided with H 2S gas to simulate catalysts used in petroleum refining. The height of the sulfided particles was regulated to be single or double layers of S-Mo-S suggesting basal-bonded, flat-lying MoS2. When Ni metal was simultaneously deposited with molybdenum oxide, the sulfided particles remained basal-bonded and the number of stacked layers increased by one or two. © 2005 The Japan Society of Applied Physics.

TiO<Sub>2</Sub> films were prepared using RF magnetron sputtering with a variety of parameters such as sputtering pressure, substrate temperature, the kind of target, and atmosphere. Crystal structures and photocatalytic activities of these films under UV light irradiation were compared with sputtering conditions. In general, rutile phases were formed at low sputtering pressure and anatase phases became predominant at high pressure. Highest activities were obtained at high pressures irrespective of substrate temperatures in Ar + O atmosphere. Similar high activities were also achieved at high temperatures and high pressures when TiO<Sub>2</Sub> target was sputtered in pure Ar. Although the films were produced under various conditions, it was indicated that decomposition rate of methylene blue (MB) was almost solely determined by anatase content.

Nanometer-sized particles of MoS₂ were synthesized on (100), (001), and (110) surfaces of rutile TiO₂. Molybdenum oxide was deposited on the TiO₂ substrates and then sulfided to simulate the preparation of industrial catalysts. The nanoparticles which appeared on the sulfided surfaces were assigned to MoS₂ particles. The topography of the nanoparticles was observed by atomic force microscopy (AFM) and by transmission electron microscopy (TEM). The number density of the nanoparticles was related to the surface energy of the substrates through the population of nucleation centers. On the TiO₂(110) surface, AFM and TEM images assignable to MoS₂ crystals edge-bonded to the substrate were observed. The coincidental lattice spacings of MoS₂ and this particular surface were related to the observed edge-bonded growth. [DOI: 10.1380/ejssnt.2004.32]

The c(6×2) phase of a silver monolayer on Si(001) substrate was investigated by scanning tunneling microscopy (STM). The c(6×2) structure appeared when silver was deposited at room temperature and then annealed at temperatures between 100<FONT SIZE="-1">^º</FONT>C and 250<FONT SIZE="-1">^º</FONT>C, and always coexisted with a 2×3 structure. In contrast, when silver was deposited onto a substrate at a temperature in this range, only the 2×3 structure grew. In the STM images with negative and positive sample bias voltages, one and two maxima were observed, respectively, in a c(6×2) unit cell. The finding of a lack of mirror symmetry in the unit cell reveals that the previously reported "single-domain" c(6×2) surface actually consists of two crystalographically equivalent domains on a single-domain substrate.

The dependence curve of the resonance frequency shift of a dynamic mode atomic force microscope (AFM) cantilever on the distance between the tip and the sample is examined. For a system sith clean semiconductor sample and a metal-coated tip, the obtained curve exhibited a larger frequency shift compared to one with a uncoated Si tip, and an increasing deviation from van der Waals characteristics as the separation decreased. This is due to an additional attractive force which becomes dominant at a small separation. This force is considered to play a crucial role in high-resolution imaging of semiconductor surfaces with a dynamic made AFM.

Dynamical low-energy electron diffraction analysis reveals that the p(2 x 2)-p4g ordered bimetallic phase formed on aluminum-deposited Pd(100) consists of a clock-reconstructed Pd top layer and a strongly buckled c(2 x 2)-PdAl second layer. A buried-heteroatom model is proposed to interpret the obtained structure. The Pd-Al heteroatomic bonding interaction forces the top-layer Pd atoms to move systematically to make Pd-Al-Pd units. The four-fold hollows of the top layer open up only as a result of the systematic unit formation, in contrast to the squeezed-adatom type p(2 x 2)-p4g C/Ni(100) surface. © 1997 Elsevier Science B.V.