Ryoji Asahi

Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping

TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Optically tunable amino-functionalized graphene quantum dots.

Amino-functionalized graphene quantum dots (af-GQDs) with discrete molecular weights and specific edges were self-limitedly extracted from oxidized graphene sheet. Their optical properties can be precisely controlled only by the selective and quantitative functionalization at the edge sites. The af-GQDS exhibit bright colorful fluorescence under a single-wavelength excitation.

First-principles study of Cu2ZnSnS4 and the related band offsets for photovoltaic applications

First-principles calculations of the band offsets between Cu(2)ZnSnS(4) (CZTS) and XS (X = Cd, Zn) are performed. While the interface dipole contribution for the band offsets is calculated using the Perdew-Burke-Ernzerhof functional, the Heyd-Scuseria-Ernzerhof hybrid functional is employed to introduce the quasiparticle corrections to the band offsets. The calculated conduction band offset between CZTS and CdS is 0.2 eV, validating CdS for the buffer layer of the CZTS solar cell. The small conduction band offset stems from the band gap narrowing of CdS under the interface strain caused by the lattice misfit with CZTS. A large valence band offset over 0.9 eV between CZTS and ZnS indicates that precipitated ZnS is regarded as an inactive insulator phase in CZTS absorbers.

Improvement of thermoelectric properties for half-Heusler TiNiSn by interstitial Ni defects

We have synthesized off-stoichiometric Ti-Ni-Sn half-Heusler thermoelectrics in order to investigate the relation between randomly distributed defects and thermoelectric properties. A small change in the composition of Ti-Ni-Sn causes a remarkable change in the thermal conductivity. An excess content of Ni realizes a low thermal conductivity of 2.93 W/mK at room temperature while keeping a high power factor. The low thermal conductivity originates in the defects generated by an excess content of Ni. To investigate the detailed defect structure, we have performed first-principles calculations and compared with x ray photoemission spectroscopy measurement. Based on these analyses, we conclude that the excess Ni atoms randomly occupy the vacant sites in the half-Heusler structure, which play as phonon scattering centers, resulting in significant improvement of the figure of merit without any substitutions of expensive heavy elements, such as Zr and Hf.

Electronic and optical properties of anatase TiO 2

First-principles calculations using the full-potential linearized augmented plane-wave method have been performed to investigate detailed electronic and optical properties of

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

{\mathrm{TiO}}_{2}

Atom-probe tomographic study of interfaces of Cu2ZnSnS4 photovoltaic cells

in the anatase structure. The fully optimized structure, obtained by minimizing the total energy and atomic forces, are in good agreement with experiment. Stabilization of the structure by the trade off between a favorable coordination in the

What Makes the Photocatalytic CO2 Reduction on N-Doped Ta2O5 Efficient: Insights from Nonadiabatic Molecular Dynamics

{\mathrm{sp}}^{2}

Highly luminescent flexible amino-functionalized graphene quantum dots@cellulose nanofiber–clay hybrids for white-light emitting diodes

hybridization and the Coulomb repulsion among oxygen atoms is also demonstrated. We calculate band structure, densities of states and charge densities, and interpret their features in terms of the bonding structure in the molecular orbital picture. The optical properties, calculated within the dipole approximation, are found to agree with recent experiments on single crystals of anatase

Field-modulated thermopower in SrTiO3-based field-effect transistors with amorphous 12CaO・7Al2O3 glass gate insulator

{\mathrm{TiO}}_{2}.