Nobuaki Kitazawa

Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals

Thin films of microcrystalline CH3NH3PbX3 (X = halogen) as well as their mixed-halide crystals were fabricated by the spin-coating technique, and their optical properties were investigated. X-ray diffraction investigation revealed that CH3NH3PbBr3 − x Clx (x = 0–3) were successfully formed on glass substrate self-assembly and oriented with the a-axis. Owing to due to their large exciton binding energy, these materials showed clear exciton absorption and free-exciton emission in the visible region at room temperature. Replacing Br with CI made it possible to control the band structure of these materials. As a result, the peak position of the exciton band shifted continuously towards blue region with increasing the CI content in the films.

Optical Absorption and Photoluminescence Properties of Pb(I, Br)-Based Two-Dimensional Layered Perovskite

The influence of ( C6H5C2H4NH3)2Pb(Brx I4-x ) concentration on the formation of mixed crystals and optical properties of these materials was investigated. Thin films of microcrystalline ( C6H5C2H4NH3)2Pb(Brx I4-x ) were prepared by the spin-coating method using different concentrations of DMF solutions. Macroscopic inhomogeneity of the composition due to the precipitation of undesired phases could be suppressed by controlling the ( C6H5C2H4NH3)2Pb(Brx I4-x ) concentration. These films showed strong exciton absorption with large (250–430 meV) binding energy due to both the quantum and dielectric confinement effect. Replacing I with Br made it possible to shift the exciton band to the blue region; however, photoluminescence from the mixed crystals gradually broadened with increasing the Br content.

Compositional modulation of two-dimensional layered perovskite (RNH3)2Pb(Cl, Br, I)4 and its optical properties

The compositional modulation of two-dimensional layered perovskites, (RNH3)2Pb(X, I)4(R: C6H5C2H4-; X: Cl, Br), and optical properties of these materials were investigated. Thin films of microcrystalline (RNH3)2- Pb(X, I)4 were successfully fabricated by the spin-coating method. X-ray diffraction spectra revealed that these materials could be regarded as inorganic/organic superlattice structures. In the spectra obtained for the (RNH3)2Pb(Br, I)4 films, a strong exciton absorption band was observed even at room temperature, and the peak position shifted continuously towards the blue and violet regions. The exciton absorption bands observed in the spectrum of (RNH3)2Pb(X, I)4 mixed crystal were consistent with the 6s to 6p transition of Pb2+. The peak shift of the exciton absorption bands of these materials could be explained by the valence band structure composed of the Pb(6s) orbital which was hybridized with the I(5p) and X(np) orbitals.

Growth of polycrystalline silicon thin films on glass

Abstract Polycrystalline silicon (poly-Si) thin films were grown by plasma-enhance CVD from fluorinated precursors on glass substrate by two-step growth (TSG), i.e. (1) growth of seeds on glass and (2) epitaxial-like growth on the seeds. The seed crystal with high crystallinity was grown by layer-by-layer technique. An optimum condition to promote epitaxial-like growth on the seeds having a certain texture was also found by varying the mixing ration of source gases, SiF4 and H2, under the real time observation with a spectroscopic ellipsometer (SE). Smooth interface on atomic scale between the seeds and the layer grown epitaxial-like made by TSG was established by measurements with SE and SIMS. High spectral response obtained in a fully poly-Si p-i-n diode proves that high quality poly-Si is able to be grown on glass by TSG.

DC electrical conductivity study of amorphous carbon nitride films prepared by reactive RF magnetron sputtering

The effects of chemical bonding states on the electrical properties of hydrogen-free amorphous carbon nitride (a-CNx) films were reported. a-CNx films were prepared by reactive RF magnetron sputtering at various deposition temperatures. The electrical conductivity of the a-CNx films increased with increasing deposition temperature because of the predominant sp2C–C bonding sites. Their conductivity increased by almost one order of magnitude with a 25% decrease in the fraction of the N-sp3C bonding state. It was found that the fraction of the N-sp2C bonding state strongly contributed to the increase in the electrical conductivity. Nitrogen incorporation led to an increase in the sp3C–C bonding fraction in the films; as a result, the conductivity of the a-CNx films was found to be lower than that of the a-C films deposited under the same conditions.

Stability of (C6H5C2H4NH3)2Pb(BrxI4-x) Mixed Crystals

The stability of (C6H5C2H4NH3)2Pb(Brx I4-x ) mixed crystals was investigated and the degradation process was discussed. A thin film of microcrystalline (RNH3)2PbBr4 was more stable than that of (RNH3)2PbI4, and the stability of (C6H5C2H4NH3)2Pb(Brx I4-x ) mixed crystals was improved by the replacement of I with Br. X-ray diffraction and optical absorption measurements suggested that the (RNH3)2PbI4 crystal changed to β-phenethylamine ( C6H5C2H4NH2) coordinated PbI2 with the elapse of time. In order to determine the origin of degradation, a light irradiation-induced degradation effect was also studied. The photochemical reaction was one of the possible reasons for degradation.

Influence of Chemical Bonding States on Electrical Properties of Amorphous Carbon Nitride Films

The electrical properties of amorphous carbon nitride (a-CNx) films have been investigated in terms of the nitrogen concentration (N/C) and chemical bonding states in the films. The films were deposited by the reactive rf magnetron sputtering method. Nitrogen concentration and chemical bonding states in the films were controlled by regulating the deposition temperature. C–C networks in the films changed to those having a graphite like structure with decreasing N/C, as deduced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, the N–sp2C bonding state becomes more predominant. These results indicate the contributions of the N–sp2C component to the decrease in electrical resistivity and increase in photoconductivity.

Synthesis of mesostructured titanium dioxide films by surfactant-templated sol-gel method

Mesostructured titanium dioxide films have been synthesized by modifying sol-gel methods in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymer surfactants as a structure-directing agent. The synthesized films were analyzed using X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM) and selected area electron diffraction (SAED). XRD investigation revealed that as-deposited and as-dried films showed a hexagonal arrangement. With increasing heat-treatment temperature, the mesostructure of the films degraded. From TEM and SAED studies, the degradation of the mesostructure can be explained by the grain growth of microcrystalline titanium dioxide.

Effect of substrate temperatures on amorphous carbon nitride films prepared by reactive sputtering

Amorphous carbon nitride, a-CNx, films were deposited by reactive radio frequency magnetron sputtering of a graphite target in nitrogen gas. This kind of films could be used as novel electric and optical devices. The authors investigated effects of the substrate temperature up to 873 K on the films in this study. The films were characterized with x-ray photoelectron spectroscopy (XPS), ellipsometry, atomic force microscopy, and nanoindentation tests. XPS studies show that the decreasing tendency in the composition ratio of carbon to nitrogen in a-CNx films with the substrate temperature is observed; however, the bonding fraction of sp3C–N increases depending on the substrate temperature. The nanoindentation tests reveal that the film hardness increases from 2 to 12 GPa as the substrate temperature increases from room temperature to 823 K. These results suggest that the film hardness is closely related to the bonding states between carbon and nitrogen.

Fabrication of Graded Band Gap Amorphous Carbon Nitride Thin Films for New Generation Photovoltaic Applications

We have reported the fabrication of amorphous carbon nitride, a-CNx, films with a graded band gap structure to improve their photoconductivity and their structural, optical, and electrical properties are studied. In this study, two different structured a-CNx films with graded band gap structures were compared with single-layered a-CNx films. One of the graded band gap structure films consists of four stacked layers with different band gaps (multilayered film), and the other film consists of one layer with a gradually changed band gap (graded-layered film). All of the a-CNx films are prepared by the reactive radio frequency magnetron sputtering method using a graphite target in pure nitrogen gas. The ratio of photo- and dark-conductivity, σp/σd, of the multilayered film is quite low. In contrast, the σp/σd value of the graded-layered a-CNx film is about 11 times larger than that of the single-layered film and 80 times larger than that of the multilayered film.