Sakama Hiroshi

In orbit, contamination outgassed from spacecraft materials has been reported to degrade optical systems such as CCD cameras, lenses, mirrors, and optical filters mounted on a spacecraft. We focus on TiO2 photocatalyst to solve this contamination problem. TiO2 has the potential to decompose contaminants and suppress the deterioration of optical systems in orbit. However, the resistance of TiO2 itself to electrically charged particles, i.e., electrons in space, has not been clarified. The TiO2 films were irradiated with an electron beam (EB) with an energy of <480 eV in a vacuum. The irradiation dose was about 2.4 MGy. Chemical properties, surface morphology, and photocatalytic activity were compared before and after EB irradiation. Almost basic properties except surface roughness were unchanged, and the change of surface roughness was very small. These results indicate that TiO2 has sufficient resistance to an EB.

Contamination outgassed from spacecraft’s materials can degrade optical devices in orbit. Therefore, solving the contamination problem is important because it influences the success of spacecraft missions. In this study, methyl red (MR) and oleamide were decomposed in vacuum by TiO2 photocatalyst. Absorbance spectra, mass decrease and GC/MS were measured after and before decomposition. In vacuum, TiO2 could decompose MR to intermediate products, whereas it could not decompose the intermediate products to volatile substances because TiO2 cannot open benzene rings of the intermediate products. On the other hand, TiO2 could decompose oleamide to volatile substances even in vacuum. However, decomposition by TiO2 stopped after a certain period of time in vacuum. The decrease in mass for oleamide by photocatalytic reaction in vacuum was enough compared to standard molecular contamination levels.

In low earth orbit (LEO), atomic oxygen (AO) has shown to cause degradation of organic materials used in spacecrafts. Similar to other metal oxides such as SiO2, Al2O3 and ITO, TiO2 has potential to protect organic materials. In this study, the anatese-type TiO2 thin films were fabricated by a sol-gel method and irradiated with AO. The properties of TiO2 were compared using mass change, scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmittance spectra and photocatalytic activity before and after AO irradiation. The results indicate that TiO2 film was hardly eroded and resistant against AO degradation. AO was shown to affects only the surface of a TiO2 film and not the bulk. Upon AO irradiation, the TiO2 films were slightly oxidized. However, these changes were very small. Photocatalytic activity of TiO2 was still maintained in spite of slight decrease upon AO irradiation, which demonstrated that TiO2 thin films are promising for elimination of contaminations outgassed from a spacecraft's materials.

Nitrogen (N)-doped TiO2 thin films were synthesized by sol-gel methods from precursor solutions with or without urea and post calcination in NH3 gas. Their structural and electronic properties were characterized by X-ray diffraction and X-ray photoelectron spectroscopy utilizing synchrotron radiation. N-doped TiO2 powders were synthesized for the estimation of visible light photocatalytic activities. N-doped TiO2 thin films revealed polycrystalline anatase phases. N was chiefly doped into substitutional sites. The densities of N and defects (oxygen vacancies and reduced Ti species) increased with elevating calcination temperature. Localized states associated with doped N were successfully found from shoulder structures of valence band spectra, which were located at 0.24 eV to 0.34 eV above the valence band maximum in the band gap. Incorporated N enhanced the photocatalytic activity, whereas defects reduced it. The highest photocatalytic activity was obtained by synthesizing N-doped TiO2 powders from a precursor solution with urea and subsequent calcination at 550 degrees C due to high-density N and low-density defects. Consequently, the optimum N/O atomic ratio was shown to be approximately 0.06. It became possible to achieve heavier N doping and better photocatalytic activity under vis light irradiation using urea than any other study only using NH3 gas for nitridation.

Pump-probe time-resolved X-ray photoelectron spectroscopy measurements have been carried out to comparatively assess the relaxation process of the photoexcited states on pristine and Ar+-sputtered TiO2(110) surfaces and a TiO2(011)-2 x 1 surface, on which the accumulation-type space charge layers are developed. Ultraviolet laser irradiation induces a surface photovoltage (SPV) of around 0.1 eV. The SPV relaxation time on pristine TiO2(110) is determined to be approximately 100 ns and is doubled on the sputtered surface. In contrast, a much shorter time of 1 ns is observed on TiO2(011)-2 x 1. The difference in the relaxation time on the two TiO2(110) surfaces is explained by differences in the O vacancy density on the surface as well as the barrier height of the surface potential for the photoexcited holes. A large hole capture cross section of a state characteristic of TiO2(011)-2 x 1 is, on the other hand, responsible for the fast SPV relaxation on this surface.

We have focused on photocatalytic materials to solve contamination problem for spacecraft. Photocatalytic materials can decompose organic matters by absorbing UV ray. Because of strong irradiation of UV ray in space, photocatalytic materials have a great potential to decompose molecular contaminations in orbit. In this study, we have measured decomposition rate of Dioctyl phthalate (DOP) in a vacuum using titanium dioxide (TiO2) photocatalysts, which were fabricated on quartz glass substrates by a sol-gel method. Transmission spectra of contaminated TiO2 films were measured before and after UV irradiation to estimate decomposition rates. The results show that TiO2 thin films have photocatalytic activities in vacuum.

Time-resolved soft X-ray photoelectron spectroscopy is utilized to determine an energy level alignment and the photoexcited carrier dynamics at a C-60/TiO2(110) interface. The interface electronic structure is characterized by a type II junction, which favors an injection of photoexcited electrons from C-60 to TiO2. Ultraviolet (UV) laser pulse irradiation induces transient shifts of both C 1s and Ti 2p core levels towards the higher binding energies. These energy shifts are caused by a laser-induced charge transfer between the C-60 layer and the TiO2(110) surface. Upon UV absorption, valence electrons of C-60 are promoted to unoccupied levels, followed by a resonant transfer to TiO2, leaving C-60 in a cationized state. On the TiO2(110) side, the electrons are injected into the conduction band to raise the carrier density so that downward bending of the TiO2 band is induced. The UV-excited states of C-60 and TiO2 have sufficiently longer lifetime than the lifetime of the electron-hole pairs in solid C-60. The C-60/TiO2(110) interface is, thus, proved to be efficient for separating the electronehole pairs generated within the C-60 layer. (c) 2016 Elsevier B.V. All rights reserved.

We have focused on photocatalytic materials to solve contamination problem for spacecraft. We have fabricated TiO2 thin films and measured decomposition rates of methyl orange (MO) and dioctyl phthalate (DOP) in vacuum by TiO2 thin films as a photocatalyst. From XRD results, fabricated TiO2 thin films have anatase-type crystal structure, which is known to have stronger decomposition activities than rutile-type TiO2. The TiO2 thin films we made were shown to decompose methylene blue (MB) solution, which means that the TiO2 thin films have general photocatalystic activity in atmosphere. In decomposition of MO in atmosphere and vacuum, TiO2 shows photocatalytic activity even in vacuum although the decomposition rate in vacuum is slower than that in atmosphere. In decomposition of DOP in vacuum, DOP was effused from an effusion cell in vacuum chamber and was deposited on a TiO2 thin film using the in-situ measurement apparatus at Tsukuba Space Center, JAXA. Transmission spectra of DOP on TiO2 thin films after UV irradiation were measured to estimate decomposition rate from absorbance of DOP. The results show that TiO2 thin films can decompose DOP even in vacuum. Moreover, H2O can promote the decomposition of DOP. In order to use photocatalyst materials in vacuum for long time, the studies on the durability of photocatalystic activity of TiO2 in vacuum and the effects of O-2 and H2O are necessary in the future.

Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained numerically using PES data, showing a depletion layer which extends to 1 mu m. Temporal evolution of the photovoltage effect is well explained by a recombination process of a thermionic model, giving the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This lifetime agrees with a temporal range reported by the previous time-resolved optical experiments. (C) 2014 AIP Publishing LLC.

Angle-resolved photoelectron spectroscopy utilizing linearly polarized synchrotron radiation was conducted to examine the electronic structure of an anatase TiO2(001) thin film fabricated on LaAlO3(100) by pulsed laser deposition. Polarization-dependent measurements by changing the incidence angle of the light were performed to identify the orbital symmetry. Energy-momentum dispersion curves along the [001] direction are mapped and resolved into four components: O 2p-derived p(pi) non-bonding, O 2p-Ti 3d sigma bonding, and two pi bonding states. The intensities of the pi bonding states are obviously different between both polarizations while the intensities of the peaks due to P-pi and sigma are almost equivalent. Two different pi states with polarizatiion dependence were attributed to Ti t(1u) pi orbital. The result implies the distoriton of TiO6 octahedra resolves degeneracy of T-1u bands and/or breaking T-2g symmetry. (C) 2014 Elsevier Ltd. All rights reserved.

Angle-resolved photoelectron spectroscopy has been applied to investigate the Shockley-type surface state on the (111) surface of a-brass, a Cu-Zn alloy with a face-centered cubic structure. An energy position and an effective mass of the Shockley-state band are determined, respectively, to be 0.51 eV and 0.52m(e) with m(e) being the rest mass of an electron. A decay length of the Shockley-state wave function into the bulk is also estimated to be 0.43 nm. These parameters are compared with those of the Shockley state on Cu(111), and it is concluded that the Cu sp orbitals in a-brass have a more isolated nature than those in Cu. Also examined in this study is the response of the Shockley state to oxygen adsorption. The behavior of the Shockley state upon oxygen adsorption strongly depends on the substrate temperature. At room temperature, the Shockley state is quenched along with the binding-energy shift towards the Fermi level. Contrastingly, oxygen adsorption at low temperatures (&gt;= 180 K) hardly affects the energy position of the Shockley state. The conversion of the surface oxidation process at a certain temperature between 180 K and room temperature is proposed. (C) 2014 Elsevier B.V. All rights reserved.

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.

Titanium dioxide thin films were grown on a lattice-matched LaAlO3(100) surfaces using pulsed laser deposition (PLD) in oxygen atmosphere. The films were characterized using X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The crystal structure of all the films was anatase. Preferred oriented films with a c-axis normal to the substrate surface were obtained. RHEED analysis also revealed that the films had the preferential in-plane orientation, demonstrating that anatase films were epitaxially grown on the substrate. The flatness of the films depended on their growth conditions and thickness. (c) 2005 Elsevier B.V. All rights reserved.

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 structure of the p(2 x 2)-p4g ordered bimetallic phase formed on aluminum-deposited Pd(100) is determined by dynamical low-energy electron diffraction analysis. The best-fitted structure among 29 different model types examined consists of the Pd top layer clock-reconstructed to the hard sphere limit, and a buckled c(2 x 2)-PdAl second layer. The buried-heteroatom scheme, a new mechanism of in-plane atom movements, is proposed to interpret the yielded structure; Al atoms migrate beneath the topmost Pd layer and replace half of the second-layer Pd atoms to demand optimum coordination. Pd-Al bonding interaction forces top-layer Pd atoms to move systematically towards the buried Al. Half of the fourfold hollows of top layer are opened and the other half are deformed as a result of the movements. The top and second layers of the reconstructed surface can be regarded as a surface-limited compound of Pd3Al composition supported on the Pd(100) substrate, because a reduced d-state overlap between the top two layers and the Pd substrate is expected. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

We have investigated the 3X2 structure on a Si(113) clean surface using scanning tunneling microscopy (STM). The results are compared with the theoretical images obtained for two proposed geometrical models. One of these models can almost account for our images. A high density of surface defects is observed on Si(113) and their structures are proposed on the basis of STM images.

The structure of Si(100)2 X 2-In was analyzed using tensor low-energy electron diffraction. The parallel dimer model agrees with the experimental results. The features of atomic arrangement are discussed in terms of the size and chemical nature of In atoms.

The atomic arrangements of semiconductor surfaces, which are important for an understanding of surface properties, are determined by the dynamic low-energy electron diffraction (LEED) method. Real-time control of epitaxial growth is possible via the measurement of intensity variations of LEED spots. The size and distance distribution of islands are clarified by spot profile analysis of LEED (SPA-LEED). Direct imaging of surface processes such as phase transition and epitaxial growth is possible using low-energy electron microscopy (LEEM).

The structures of an indium-adsorbed Si(110) surface are investigated by LEED. Three specific LEED patterns (alpha, beta and gamma) are observed depending on the adsorption condition. The saturation coverages of the corresponding alpha, beta and gamma structures are estimated as 0.2, 0.3 and 1.0 monolayers, respectively. The symmetries of these structures are discussed from I-V curves of LEED patterns. Models of the atomic arrangement are proposed on the basis of the symmetries of the LEED patterns.