Kan Hachiya

Photoluminescence and states in the bandgap of germanium sulfide glasses

Abstract We report the characteristic photoluminescence (PL) behavior in Ge1−xSx ( 0.60 ≦ x ≦ 0.90 ) chalogenide glasses. The compositional and excitation energy dependence of the PL spectra has been investigated. The PL spectra observed near 2.0 eV are strongly suggested to be the result of the recombination process between the photo-excited electrons at the bottom of the conduction band and holes in the defect levels based on their compositional dependence. We also found that the PL intensity is directly related to the concentration of neutral defects. Reduction of the PL through prolonged irradiation by excitation light has also been investigated and is found to recover after relaxation in the dark at room temperature. The states in the bandgap of the Ge–S chalcogenide glasses and their changes due to photo-irradiation are discussed.

Highly Conductive Plastic Crystals Based on Fluorohydrogenate Anions

The new ionic plastic crystals N,N-dimethylpyrrolidinium fluorohydrogenate [DMPyr(FH)(2)F] and N-ethyl-N-methylpyrrolidinium fluorohydrogenate [EMPyr(FH)(2)F] were prepared, and their physicochemical, structural, and electrochemical properties were investigated. The DMPyr(FH)(2)F and EMPyr(FH)(2)F salts exhibited small entropy changes of melting, 4.1 and 2.0 J K(-1) mol(-1), respectively, and had ionic plastic crystal phases in the temperature ranges of 258-325 and 236-303 K, respectively. These phases had NaCl-type structures; the lattice constants were 9.90 Å for DMPyr(FH)(2)F and 10.18 Å for EMPyr(FH)(2)F. The ionic conductivities of the ionic plastic crystal phases ranged from 10(0) to 10(1) mS cm(-1) [e.g., 10.3 mS cm(-1) at 298 K for DMPyr(FH)(2)F and 14.4 mS cm(-1) at 288 K for EMPyr(FH)(2)F]. Pulsed-field gradient spin-echo NMR spectroscopy revealed that only the anion could move in the ionic plastic crystal phase as a charge carrier with a diffusion coefficient of ~10(-7) cm(2) s(-1). The self-diffusion coefficient of the cation in the ionic plastic crystal phase of DMPyr(FH)(2)F was too small to measure, although the cation in EMPyr(FH)(2)F had a slight mobility below 303 K.

Photoluminescence excitation process and optical absorption in Ge-S chalcogenide glasses

Abstract The photoluminescence (PL) excitation mechanisms in Ge–S chalcogenide glasses are investigated based on the comparison between the PL excitation and optical absorption spectra. The spectra show a remarkable dependence on the glass composition and the excitation energy. The excitation mechanism is different in the Ge-rich and S-rich glasses, and a threshold excitation energy (∼2.45–2.50 eV) exists for the PL intensity in the S-rich glasses. Interpretation of the optical absorption spectra revealed that the threshold change the excitation from the defect states in the bandgap to the excitation from the localized states in the valence band. When the excitation energy is greater than the threshold energy, the electronic transition from the top of the valence band, where the photo-excited carriers are located, tends to become trapped by the non-radiative recombination centers. On the other hand, when the excitation energy is lower than the threshold, only the electronic transition from the charged defects to the conduction band is permitted, which leads to a greater PL intensity. Interpretation of the optical absorption spectra also revealed the additional lower excitation energy states of defects in the bandgap.

Plasma Emission and Surface Reduction of Titanium Dioxides by Microwave Irradiation

We present a systematic study of the microwave irradiation effects on TiO2 under vacuum in order to distinguish between the mechanisms of microwave and conventional thermal reductions, while paying particular attention to the changes in structural and optical properties of a sample. We observe the grain surface reduction of a sample through oxygen atomic plasma emission during an intensive absorption of microwave under vacuum without any reducing agent and obtained a dark-colored solid specimen after such emission. Reflectance spectroscopy, resistance measurement and emission spectroscopy confirm that the TiO2 grain surface is directly reduced to a lower-oxygenated condition while emitting oxygen atomic plasma by microwave irradiation under vacuum. This is different from thermal reduction. Possible mechanisms of microwave plasma emission and grain surface reduction are also presented.

Electronic structure of amorphous germanium sulphides

Abstract First-principles calculations results are presented for the electronic structures of amorphous and liquid Ge–S. Model cluster calculation of the density of states using a geometry similar to that of the crystalline state coordination in GeS2 can reproduce the peak structure of the experimental valence band (VB) photoemission spectra. Conduction band densities of states are also calculated and their structure is interpreted together with the VB. The present calculations confirm that the states at the VB top are lone-pair states, and that the VB below them and the conduction band are bonding or anti-bonding states of the Ge–S bond. A definite shape of the molecular orbitals around the bandgap is also revealed.

Density functional electronic structures calculations of GeSe2

Abstract Valence and conduction band electronic structures for amorphous and crystalline GeSe 2 are discussed based on the density functional calculation of the density of states for model clusters. Results are compared with photoemission and inverse photoemission spectra and their peak structures are reproduced very well. We present a full interpretation of these spectra through investigations of partial density of states and molecular orbitals around the bandgap, and reveal that the states around the bandgap are Se 4p lone-pair states and Ge–Se anti-bonding states, and the split band structure of the higher conduction band is made up of the anti-bonding states correspond to the bonding states in the split valence band. The higher conduction band is also revealed to be combined with effects of 4d orbitals and inter-layer interactions.

Molecular dynamics simulations of the self-diffusion phenomena in Ni2Y intermetallic phase

Abstract We present a molecular dynamics study of the diffusion phenomena in Ni 2 Y, transition-metal intermetallic phase via nearly-free-electron–tight-binding-bond interatomic interaction model. Interpretations of the dynamic structures from the calculations of the mean square displacements and spectral densities of velocity show that the rapid growth of this intermetallic phase found in the experimental study is mainly due to the high-rate self-diffusion in and near the grain-boundaries and independent of the vacancy. Therefore, the standard vacancy-based diffusion process is found to be not essential in this phase.

Optical Properties of the Carbon-Modified TiO2 Prepared by Microwave Carbonization Process

The carbon-modified TiO2 were synthesized through microwave carbonization of ethanol by using a domestic microwave oven. This process enabled to form the carbonaceous compounds on the surface of TiO2 and created several new mid-gap bands into the original bandgap within few minutes operation. The sample showed a remarkable visible-light absorption even at the wavelength of around 800 nm. The promotion of photocatalytic activity under visible and ultraviolet (UV) light irradiation were also confirmed by the I3- formation in KI aqueous solution. The I3- formation rate of carbon-modified TiO2 per unit mass under visible light is almost 25 times higher than that of pure TiO2. The mid-gap optical absorption mechanisms were investigated through analysis of absorption edges. It is revealed that surface state change against microwave-treatment time results in different mid-gap optical absorption processes.

Zinc Plasma Emission from Zinc Oxide Ceramics under a Microwave Electric Field

We studied the effects of microwave irradiation on ZnO ceramics under vacuum to clarify the emission of zinc and oxygen plasmas from ZnO, while simultaneously focusing on the materials optical properties. We observed the emission of zinc and oxygen plasmas during intense absorption of microwaves as well as the deposition of zinc and zinc oxide films. Absorption coefficient and photoluminescence spectra suggest that zinc and oxygen plasmas were produced from grain surfaces of ZnO by microwave irradiation under vacuum, a phenomenon which was observed in a previous study on TiO2 [Jpn. J. Appl. Phys. 48 (2009) 116003].

Photoluminescence from localized states in rutile by Ar+-ion laser excitation

Abstract We report on a photoluminescence (PL) band at around 2.2 eV for rutile through an Ar+-ion laser excitation. The PL consists of two sub-bands which are located at 2.22 eV and 2.28 eV , respectively, and have bandwidths of 0.06– 0.07 eV . The emission and absorption band shapes, which are composed of two asymmetrically distorted sub-peaks, are examined and considered in detail. The PL emissions are proved to originate in the localized states and the Stokes shift are estimated to be 0.12–0.16 and 0.15 eV . The radiative centres for these PL are found to be those reported for the interstitial Ti3+ ions.