Atsumi Miyashita

Sulfur-doping of rutile-titanium dioxide by ion implantation: Photocurrent spectroscopy and first-principles band calculation studies

Sulfur (S)-doped titanium dioxide (TiO2) was synthesized by ion implantation and subsequent thermal annealing. The S ions were implanted into the single crystals of rutile TiO2 at a fluence of 8×1015 ions/cm2. According to the results of Rutherford backscattering spectroscopy and ion channeling analysis, the irradiation damage recovered by annealing at 600 °C in air. In the annealed crystal, the S atoms occupied oxygen sites for form Ti-S bonds, as confirmed by x-ray photoelectron spectroscopy. Compared to the pure TiO2, a photocurrent was observed in the lower-energy regions for the S-doped TiO2. Based on the theoretical analyses by the first-principles band calculations using the full potential linearized augmented plane-wave methods within the generalized gradient approximation, the mixing of the S 3p states with the valence band (VB) was found to contribute to the increasing width of the VB. This leads to the band gap narrowing in the S-doped TiO2. Therefore, the photon-to-carrier conversion was induc...

Preparation of epitaxial TiO2 films by pulsed laser deposition technique

Abstract TiO 2 films with anatase and rutile structure were deposited by pulsed laser deposition (PLD) with a Nd/YAG laser under the controlled O 2 atmosphere. Epitaxial anatase films have been successfully prepared on several kinds of oxide substrates with different lattice parameters. Further the high quality epitaxial rutile films were also grown on α-Al 2 O 3 (0001) substrate. During the deposition, the substrates were kept at 500°C and exposed to 35 mtorr O 2 gas pressure. The typical thicknesses of epitaxial films were in the range from 200 to 880 nm. The epitaxial films were analyzed by Rutherford backscattering spectrometry (RBS) combined with channeling and X-ray diffraction for the crystallographic relationships with the substrates. The optical properties were characterized in the ultraviolet–visible region employing optical absorption and photoconductivity measurements. The optical band gap energies of anatase and rutile TiO 2 epitaxial films were evaluated to be 3.22 and 3.03 eV, respectively.

Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin films

Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile films was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied field. According to the measurements, the quantum efficiency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap.

Molybdenum substitutional doping and its effects on phase transition properties in single crystalline vanadium dioxide thin film

Molybdenum (Mo) doped vanadium dioxide thin films were synthesized using a Mo striped vanadium (V) target during pulsed laser ablation process. The film structure was characterized by high resolution x-ray diffraction, x-ray rocking curve and Rutherford backscattering/channeling measurements. The results show that the full width at half magnitude of the x-ray rocking curve is as narrow as 0.0074°, comparable to that of the (0001) sapphire substrate, 0.0042°, in this study. The ratio of the aligned-to-random backscattered yield reaches 5%, implying that the growth is that of the single crystalline epitaxy. The result of angular scans for both V and Mo atomic channelings reveals that Mo atoms successfully take sites of the V sublattice as a substitutional dopant. It has been noted that the degradation of the phase transition properties of the film upon doping is closely related to the conductivity in the semiconductor phase.

Time-and-space resolved X-ray absorption spectroscopy of laser-ablated Si particles

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the LI \kern-1ptI,I \kern-1ptI \kern-1ptI absorption edges of neutral Si atom ( Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.

Photo-induced surface charge separation in Cr-implanted TiO2 thin film

Abstract Surface charge separation efficiencies of uniformly Cr-distributed rutile grown by pulse laser deposition (PLD) using Cr/TiO 2 multi-targets and rutile with Cr concentration gradient produced by ion implantation with the appropriate post-annealing process were characterized by photo-induced transient charge separation (PITCS) measurement. Cr depth profiles were obtained by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) was used to determine that both surface Cr concentrations were the same. PITCS can be measured without direct contacts and without an externally applied field, and demonstrate the inherent property of charge separation without disturbing spontaneously formed surface band bending. There was a large charge separation for implanted rutile with Cr gradient, even in the visible light region (∼520 nm), which was more remarkable than that of uniform rutile and epitaxial anatase films.

Crystalline and nearly stoichiometric vanadium nitride thin film by PLD

Abstract Pulsed laser deposition (PLD) with a YAG:Nd laser was used to prepare vanadium nitride thin film. Different deposition conditions of different temperatures, substrates and gas pressure were explored. The films were characterized with XRD (X-ray diffractometer), XRD pole figure, AES (Auger electron spectrometry) and RBS (Rutherford backscattering spectrometry) /Channeling. It was found that crystalline and nearly stoichiometric VN x films with an x value of 0.96 on c plane α -Al 2 O 3 had been prepared by YAG:Nd laser at 450°C with the mixed gas of nitrogen and 3% hydrogen of 200 Pa. X-ray diffractometer, XRD pole figure, AES and RBS/Channeling surely confirm epitaxial and near stoichiometric growth.

Structural, optical, and electrical properties of laser deposited FeTiO3 films on C- and A-cut sapphire substrates

FeTiO3 (FTO) thin films were epitaxially grown on two kinds of α-Al2O3 substrates [(0001) and (1120)] through pulsed laser deposition under a mixtured gas ambient of 5% oxygen with argon used as the balance. The x-ray diffraction (XRD) and XRD pole figure measurements show parallel growth relationships of the films on these two kinds of substrates, (0006)∥(0006), (1120)∥(1120), respectively. The interplane XRD pole texture measurement also shows that a parallel relationship exists. Rutherford backscattering spectrometry and Auger electron spectrometry analyses show that the films are stoichiometric and without distinguishable impurities. The optical properties were characterized in the ultraviolet-visible region, and 3.55 eV was obtained for the first time as the band gap of these epitaxial and stoichiometric FTO films. This value in the present high quality films is higher than the band gap reported for bulk material, 2.58 eV. The measurement of the electrical resistance in a quite wide temperature ra...

Formation mechanisms for carbon onions and nanocapsules in C+-ion implanted copper

Copper substrates were implanted with carbon ions at temperature ranging from 570 to 973 K. Implantation microstructures were investigated using transmission electron microscopy and high-resolution electron microscopy. Carbon onions and nanocapsules were observed together with amorphous carbon layers. Most of the nanocapsules were found to be hollow and rarely included copper nanoparticles. The encapsulating of Cu nanoparticles with graphene layers, the gradual shrinkage of the encapsulated clusters, and finally the disappearance of the clusters (leaving behind hollow nanocapsules) were observed under electron irradiation at 783 K. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nanocapsules. One process involves the formation of graphene layers on grain boundaries to encapsulate copper particles. The other process is the nucleation of graphene cages, probably fullerenes, due to high co...

Preparation of TiO2-anatase film on Si(001) substrate with TiN and SrTiO3 as buffer layers

An epitaxial TiO2-anatase thin film was grown on the Si(001) substrate with SrTiO3/TiN as the buffer layers by the pulsed laser deposition technique under an oxygen gas supply. The typical thicknesses of TiO2-anatase, SrTiO3 and TiN layers were 760 nm, 450 nm and 230 nm, respectively. The characterization of the epitaxial TiO2-anatase film was performed using the x-ray diffraction method. The crystallographic relationships between the TiO2-anatase film and SrTiO3/TiN buffer layers were analysed by the θ-2θ scan and pole figure measurement. The growth direction of the films was determined as TiO2001/SrTiO3001/TiN001/Si001 and their in-plane relationship TiO2{110}||SrTiO3{100}||TiN{100}||Si{100}. The crystalline quality of TiO2-anatase was examined by the rocking curve analysis. The composition of the thin film packaging was characterized by Rutherford backscattering spectrometry (RBS) using a 2.0 MeV 4He beam. The simulation of the RBS data points to the possible intermixing of Ti (from TiN) and Si.