Shoichi Kawai

WO3/metal thin-film bilayered structures as optical recording materials

Interface reactions in WO3∕metal thin-film bilayered structures were examined for applications to optical recording materials. Decreases in the reflectance of the structures caused by temperature elevation were observed and were attributed to (i) coloration of the WO3 layers, and (ii) decreases in the reflectance of the metal layers resulting from redox reactions between the WO3 and the metals. The reflectance spectra of the structures before temperature elevation showed moderate wavelength dependence over a wide range, from the visible to the near infrared. Decreases in the reflectance due to temperature elevation occurred over a wide wavelength range. Examination of the WO3∕Al–Ti bilayered structures as potential optical disk memories revealed that the redox reactions occurred within submicrosecond time periods during irradiation with the recording laser. The results indicate that these structures possess promising properties for applications to optical recording materials that can be used over a wide w...

Electrochemical deposition of iron sulfide thin films and heterojunction diodes with zinc oxide

Iron sulfide thin films were fabricated by the electrochemical deposition method from an aqueous solution containing FeSO4 and Na2S2O3. The composition ratio obtained was Fe:S:O = 36:56:8. In the photoelectrochemical measurement, a weak negative photo-current was observed for the iron sulfide films, which indicates that its conduction type is p-type. No peaks were observed in X-ray diffraction pattern, and thus the deposited films were considered to be amorphous. For a heterojunction with ZnO, rectification properties were confirmed in the current-voltage characteristics. Moreover, the current was clearly enhanced under AM1.5 illumination.

GeS2/metal thin film bilayered structures as write-once-type optical recording materials

GeS2/metals thin film bilayered structures were examined for write-once-type optical recording materials. Decreases in the reflectance of GeS2∕Ag and GeS2∕Cu structures at a certain elevated temperature were observed. The phenomena were attributed to the formation of metal sulfides with high optical absorption coefficients by the interface reaction between transparent GeS2 and highly reflective metals. In contrast to GeS2∕Ag or GeS2∕Cu, an increase in the reflectance was observed in GeS2∕Au, which was presumably due to the change of optical interference conditions caused by the decrease in the GeS2 film thickness and the improvements of the crystallinity of Au caused by the decrease of the lattice defects along the grain boundaries of Au. Examination of the GeS2/metals bilayered structures as optical disc memories revealed that the reactions occurred within submicrosecond during the recording laser irradiations. The recording sensitivities of GeS2∕Ag or GeS2∕Cu which corresponded to the reaction temperatu...

Preparation of highly luminescent hybrid gel incorporating NAC-capped CdTe quantum dots through sol–gel processing

Highly photoluminescent gel was prepared by embedding water soluble quantum dots (QDs) in an inorganic–organic hybrid gel matrix using a conventional sol–gel process. Aminopropyltrimethoxysilane and citric acid (CA) were found to be the best combination for the gel preparation. 13C-NMR and FT-IR studies indicated hydrogen bond formation between the amine group of APS and the carboxyl group of CA. IR-light radiation curing was comparable to thermal curing and reduced the gelation time to a considerable extent (71 %). The resulting composite formed a hybrid gel phosphor with excellent transparency by embedding CdTe QDs into the matrix and emitted light of various colors with high photoluminescence efficiency (40 %). The gel phosphor retained the PL properties after storage in air for one year. In addition, the strength of the hybrid phosphor was demonstrated by a coin-flipping test.

Mössbauer study of electrochemically deposited amorphous iron-sulfide-oxide thin films

Iron-sulfide-oxide thin films, which are promising candidates for solar cell materials, were deposited by electrochemical deposition. As-deposited and annealed films were characterized by Mossbauer spectroscopy, X-ray diffraction (XRD), and Raman scattering at room temperature. The as-deposited film is amorphous, and the oxygen content is about 1/4 of the sulfur content (S/Fe ≈ 1.5, O/Fe ≈ 0.4). The Mossbauer spectrum for the as-deposited film is a doublet with a broad line profile having hyperfine parameters similar to those of FeS2 pyrite or marcasite. This indicates that Fe atoms are in the Fe2+ low-spin state, as in FeS2.

Band alignment at the CdS/FeS2 interface based on the first-principles calculation

FeS2 is potentially well-suited for the absorber layer of a thin-film solar cell. Since it usually has p-type conductivity, a pn heterojunction cell can be fabricated by combining it with an n-type material. In this work, the band alignment in the heterostructure based on FeS2 is investigated on the basis of the first-principles calculation. CdS, the most popular buffer-layer material for thin-film solar cells, is selected as the partner in the heterostructure. The results indicate that there is a large conduction band offset (0.65 eV) at the interface, which will hinder the flow of photogenerated electrons from FeS2 to CdS. Thus an n-type material with the conduction band minimum positioned lower than that of CdS will be preferable as the partner in the heterostructure.

Comparison of amorphous Fe–S–O and crystalline FeS2 pyrite for photovoltaic application

Iron sulfide oxide thin films were deposited by electrochemical deposition and annealed with sulfur powder. The composition ratio of the as-deposited film is Fe:S:O = 1:1.5:0.4. X-ray diffraction (XRD) and Raman measurement results showed that the as-deposited film is amorphous. After the annealing at 400 °C with sulfur powder, oxygen content was significantly reduced, and crystallization of FeS2 pyrite was confirmed by XRD. Heterojunction cells were fabricated using both the as-deposited and annealed films with ZnO thin film as the partner, and rectification properties were confirmed for all the samples. However, the leakage current was increased and the forward current was decreased by the annealing. Thus, the as-deposited amorphous film seems to be more suited for photovoltaic application than the annealed ones.