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.

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.

Ab initio study of vibrational dephasing of electronic excitations in semiconducting carbon nanotubes.

Phonon-induced dephasing of electronic transitions in semiconducting single-wall carbon nanotubes (CNT) is investigated by ab initio molecular dynamics. Pure-dephasing is shown to be the source of the photoluminescence linewidths observed experimentally in isolated CNTs at low and room temperatures. In ideal tubes, the dephasing is found to occur by coupling to optical phonons. The dephasing proceeds notably faster in the presence of some defects due to stronger coupling to local modes, suggesting that the defects can be identified in CNTs by broadened optical bands.

Ultrafast vibrationally-induced dephasing of electronic excitations in PbSe quantum dots.

Vibrationally induced pure-dephasing of electronic states in PbSe quantum dots (QDs) at room temperature is investigated using two independent theoretical approaches based on the optical response function and semiclassical formalisms. Both approaches predict dephasing times of around 10 fs and reproduce the recently measured homogeneous linewidths of optical absorption well. Because dephasing slows down with increasing cluster size, the dephasing times calculated for the small clusters correspond to the lower end of the experimental data. The dephasing is almost independent of the electronic excitation energy and occurs faster for biexcitons than single excitons. The dephasing time is roughly proportional to the square root of the mass of the lighter atom (Se), suggesting that dephasing should be faster in PbS and slower in PbTe relative to PbSe. Core atoms produce stronger dephasing than surface atoms. In the collective description, pure-dephasing occurs via low-frequency acoustic modes, in support of the elastic QD model of dephasing. Because the electron-phonon coupling in PbSe QDs is relatively weak compared to other semiconductor nanocrystals, fast vibrationally induced dephasing can be expected in semiconductor QDs in general.

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.