Hideyuki Kamisaka

Theoretical study of the structure and optical properties of carbon-doped rutile and anatase titanium oxides

The structure and optical properties of carbon-doped titanium oxides, TiO2, in the rutile and anatase forms have been investigated theoretically from first principles. Two possible doping sites were studied, carbon at an oxygen site (anion doping) and carbon at a titanium site (cation doping). The calculated structures suggest that cation-doped carbon atoms form a carbonate-type structure, whereas anion-doped carbon atoms do not invoke any significant structural change. A density-of-states analysis revealed three in-gap impurity states for anion doping. The optical properties of anion-doped cells qualitatively agree with the experimentally reported visible-light absorbance values. We ascribe part of the absorption to transitions from the valence band to one of the impurity states. These transitions should be able to promote photocatalytic reactions, because electron holes in the valence band are considered to be crucial for this process. Neither in-gap impurity states nor visible-light absorbance were observed in the case of cation doping. The effect of oxygen vacancies was also investigated. Introduction of oxygen vacancies into anion-doped TiO2 populates the impurity states and thus suppresses photocatalysis. The interaction of a doped carbon atom with an oxygen vacancy at a finite spatial separation was also carried out. The possibility of either a carbon-oxygen vacancy pair or higher carbon-oxygen vacancy complex existing is discussed.

Electronic Band Structure of Transparent Conductor: Nb-Doped Anatase TiO2

We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.

Density functional theory based first-principle calculation of Nb-doped anatase TiO2 and its interactions with oxygen vacancies and interstitial oxygen.

The structure and electronic properties of Nb-doped anatase (TNO) were studied from first principles using the density functional theory based band structure method. Four independent types of unit cells were studied; i.e., pure anatase, anatase with Nb dopant at Ti sites (Nb(Ti)), and cells with either interstitial oxygen (O(i)) or oxygen vacancies (V(O)). In addition, a unit cell with a Nb(Ti) and O(i), and a cell with Nb(Ti) and V(O) were investigated to clarify the role of nonstoichiometry in TNO. From the calculated results, the importance of the adjacent Nb(Ti)-V(O) and Nb(Ti)-O(i) structures was pointed out, and the experimental observation of the relationship between nonstoichiometry and electronic conductivity was rationalized. The shape of the impurity states found in these structures was used to comprehend the experimental observation of carrier concentration and the charge state of Nb dopant. The changes in lattice constants supported the existence of these structures as well. On the contrary, the cell with a simple Nb(Ti) did not show significant changes in structure and electronic properties, other than the emission of an electron in the conduction band. A stabilization of the impurity state was observed in the adjacent Nb(Ti)-V(O) structure compared to the V(O). The possibility of an essential role of this state in electric conduction was discussed. The formation of the adjacent Nb(Ti)-O(i) structure by O(2) gas annealing was discussed using statistical mechanics. The Gibbs free energies were calculated for O(i) atoms in TNO and compared to that of O(2) molecules in the gas phase. The analysis was qualitatively consistent with experimental behavior under the assumption of the Nb(Ti)-V(O) structures.

Possible ferroelectricity in perovskite oxynitride SrTaO2N epitaxial thin films

Compressively strained SrTaO2N thin films were epitaxially grown on SrTiO3 substrates using nitrogen plasma-assisted pulsed laser deposition. Piezoresponse force microscopy measurements revealed small domains (101–102 nm) that exhibited classical ferroelectricity, a behaviour not previously observed in perovskite oxynitrides. The surrounding matrix region exhibited relaxor ferroelectric-like behaviour, with remanent polarisation invoked by domain poling. First-principles calculations suggested that the small domains and the surrounding matrix had trans-type and a cis-type anion arrangements, respectively. These experiments demonstrate the promise of tailoring the functionality of perovskite oxynitrides by modifying the anion arrangements by using epitaxial strain.

Topotactic fluorination of strontium iron oxide thin films using polyvinylidene fluoride

We report herein the topotactic fluorination of SrFeO3−δ thin films (δ ∼ 0, 0.5, 1) with polyvinylidene fluoride (PVDF). SrFeO3−xFx epitaxial thin films were obtained by fluorination at 150–270 °C, which is substantially lower than the reaction temperature for polycrystalline bulk samples prepared with PVDF. The fluorine content (x) of the film was widely varied by controlling the PVDF-treatment temperature and/or the amount of oxygen vacancies in the precursor film. The higher reactivity of the SrFeO2 and SrFeO2.5 thin films can be reasonably explained by a fluorine-diffusion mechanism via oxygen vacancies.

Computational study of ZrSiO4 polymorphs

Using the density functional theory in a local density approximation and generalized gradient approximation (GGA) with a plane wave basis set we have revealed eight new polymorphs of ZrSiO4 within the energy range ∼1eV above the most stable zircon which have higher and lower density than experimentally known zircon and reidite. Two structures, which have both silicon and zirconium atoms sixfold coordinated, orthorhombic AlTaO4-like (alumotantite), and monoclinic PbWO4-like (raspite), have similar energies at a GGA level ∼0.35eV above reidite and density intermediate between zircon and reidite. Among two low-density structures, which can be potentially revealed experimentally in the nanocrystalline thin films, the orthorhombic CaSO4-like form has an energy similar to reidite but with much lower density.

Stress stabilization of a new ferroelectric phase incorporated into SrTaO2N thin films

Microstructural analyses of highly stressed SrTaO2N thin films deposited on SrTiO3 substrates by cathodoluminescence spectroscopy revealed coexistence of ferroelectric and relaxor-ferroelectric-like phases in the films. These two phases are, respectively, associated with “trans-type” and “cis-type” anion orders, as supported by the relative difference of the band gap energies calculated by first principles calculations based on the density functional theory. The formation of the new ferroelectric phase is considered to occur upon stabilization by the high compressive residual stress stored into the film structure, with the length/size of the “trans-type” region strongly depending upon the local stress state in the film.

Electron trapping at the lattice Ti atoms adjacent to the Nb dopant in Nb-doped rutile TiO2

The Nb-doped anatase TiO2 is considered one of the most promising alternative transparent conducting oxides to substitute for indium tin oxide. However, studies have found that the conductivity emerges only in the anatase form, not in the rutile form. We applied the first-principle band structure method for the Nb-doped TiO2 in both polymorphs. The calculation was carried out using the spin-restricted and spin-polarized GGA+U level of the theory. Special care was taken in the calibration of +U parameters to satisfy the generalized Koopman’s theorem. A significant difference was found between the spin-polarized and spin-restricted calculations. We noticed that spin polarization was necessary to reproduce the electron trapping in rutile. In addition, electrons are trapped at two lattice Ti atoms adjacent to the NbTi dopant along the [001] direction, as described with the formal charge state of Ti3.5+–Nb5+–Ti3.5+. A careful convergence of the electron trapping character was conducted against the unit cell size based on the Bader population analysis.

Topotactic synthesis of strontium cobalt oxyhydride thin film with perovskite structure

The substitution of hydride anions (H−) into transition metal oxides has recently become possible through topotactic reactions or high-pressure synthesis methods. However, the fabrication of oxyhydrides is still difficult because of their inherently less-stable frameworks. In this study, we successfully fabricated perovskite SrCoOxHy thin films via the topotactic hydride doping of brownmillerite SrCoO2.5 epitaxial thin films with CaH2. The perovskite-type cation framework was maintained during the topotactic treatment owing to epitaxial stabilization. Structural and chemical analyses accompanied by X-ray absorption spectroscopy measurements revealed that the doped hydride ions form a two-dimensional network of Co-H−-Co bonds, in contrast to other reported perovskite oxyhydrides, SrMO3−xHx (M = Cr, Ti, V). The SrCoOxHy thin film exhibited insulating behavior and had a direct band gap of 2.1 eV. Thus, topotactic hydride doping of transition-metal-oxide thin films on suitable substrates is a promising method...

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1-xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.