Kazuhiko Yazawa

Above‐band‐gap photoluminescence from Si fine particles with oxide shell

Strong photoluminescence with sub‐band‐gap photon energies has been observed in fine Si particles prepared by the gas‐evaporation technique. After surface oxidation, the Si particles show above‐band‐gap photoluminescence, the band tail covering the visible light region. The amount of the increased apparent band gap (0.3 eV) estimated from this blueshift can be explained by a quantum‐size effect expected to be observed in Si quantum dots with a diameter of 50 A.

Photoelectrolysis of water with TiO2‐covered solar‐cell electrodes

A TiO2–solar‐cell hybrid structure has been used successfully as the anode electrode in the photoelectrolysis of water. The TiO2 films have been fabricated by chemical vapor deposition. Conversion efficiency of solar energy of about 0.1% has been attained in the preliminary experiment.

Anomalous photoresponse of n‐TiO2 electrode in a photoelectrochemical cell

The current‐potential characteristics of n‐TiO2 electrodes have been investigated in a photoelectrochemical cell. Photoresponse induced by the light of energy lower than the band gap of TiO2 has been observed in the potential range between −1.0 and −1.4 V vs SCE in 1N NaOH. The photoresponse, which is sensitive to the oxygen gas dissolved in solution, has been attributed to some surface states at the TiO2‐electrolyte interface.

Negative Magneto-Resistance in Pyrolytic Carbons

Negative magneto-resistance is observed in poorly graphitized carbons at liquid nitrogen temperature or even higher. This peculiar phenomenon has been studied on pyrolytic graphite, as a model material. It is concluded that the negative magneto-resistance is a quantum phenomenon caused by a change of the carrier concentration induced by the applied magnetic field. The incentive for the change of the carrier concentration, which is unusual in itself, lies in the peculiar energy band structure of this material. Theoretical curves calculated on this conjecture reproduce the experimental results very well.

Iron-Oxide Coated n-Si as a Heterostructure Photoanode for the Photoelectrolysis of Water

The first application of n-Si as an efficient photoelectrode for the photoelectrolysis of water is reported. The Si surface has been coated with a thin layer of iron-oxide to overcome corrosion problems in an electrolyte. By using this heterostructure electrode as a photoanode in a photo-electrochemical cell, O2-evolution currents as high as 50 mA/cm2 have been attained under an illumination of 0.1 W/cm2 (1 sun). The highly efficient conversion of light into photocurrent has been attributed to efficient hole injection from the Si substrate across the interface of the heterostructure.

Photoelectrochemical Properties of Iron-Oxide Films and the Coating Effects onto n-Si as an Efficient Photoanode

The photoelectrochemical properties of iron-oxide thin films prepared by various techniques have been investigated extensively, and it was found that the quantum efficiency of the photoresponse is low for visible light. Iron-oxide films have been used in fabricating heterostructure electrodes composed of Si coated with iron-oxide. This iron-oxide/Si heterostructure has been characterized by X-ray diffraction, Auger electron spectroscopy and capacitance measurements, and Auger analysis has shown that the substances of the heterostructure interdiffuse deep across the interface. Some possible energy diagrams for the heterostructure are proposed.

Electroluminescence at the n‐TiO2/electrolyte interface

We have observed electroluminescence at the n‐TiO2/electrolyte interface under strong polarization of TiO2 electrodes above 8 V versus SCE. The quantum yield, obtained from current and intensity measurements at 10 V versus SCE, was in a range between 10−3 and 10−4. The emission spectrum, measured by optical glass filters, was found to be distributed in a range between 450 and 800 nm, the spectrum peak being approximately 630 nm, which is equivalent to a photon energy of 2.0 eV. This luminescence has been attained to the radiative recombination of electrons in some surface states with holes created in the valence band by electron tunneling.

Electroluminescence study on the semiconducting-oxide anodes in aqueous electrolytes.

We have made photoelectrochemical and luminescence studies on the semiconducting-oxide electrodes in aqueous electrolytes. The luminescence, observed in TiO2, SrTiO3 and WO3 at strong anodic polarization, has been attributed to the radiative recombination via some surface states. The energies corresponding to the maximum distributions of the surface states which have been evaluated from the observed luminescence spectra are found to coincide with the OH-/O2 redox level in the electrolyte. This coincidence is explained by a hypothesis that the efficient charge transfer between the surface states and the redox level pins the energy band of the semiconductor relative to quantum states in the electrolyte and thus the flatband potential.

Oxygen-Sensitive Surface Layer in Pt/TiO2 Composite Film

It is shown that the surface resistance of Pt/TiO2 composite films is sensitive to oxygen gas at temperatures higher than 130°C. The measured potential distribution along the film surface suggests that there is an oxygen-sensitive high resistance layer in the surface region. According to the results of an ESCA analysis, the high resistance is due to the Pt-free surface layer formed on the Pt/TiO2 granular metal layer.

Evaluation of energy band parameters of carbons

Abstract The systematic dependence of the Hall effect of carbons on the electronic band parameters has analytically been studied on the basis of Yazawas band model which takes into account in a simple way the interlayer perturbation. Many experimental results on the magnetic-field dependence of the Hall effect of carbons can be satisfactorily explained by adjusting the band parameters. The change in the Hall voltage with the band parameters is so drastic as to provide a powerful tool for evaluating these parameters of carbons. The evaluation procedures are described in detail and some examples of the application to experimental results presented. It is concluded that as-deposited pyrolytic graphite has a high concentration of acceptors with small activation energy (~ l0 meV), which may probably originate from the structural defects. On the other hand, the activation energy of boron in graphite is found to exceed 100 meV.