Yoichi Ishiwata

Synthesis of Copper–Antimony-Sulfide Nanocrystals for Solution-Processed Solar Cells

The p-type nanocrystals (NCs) of copper-based chalcogenides, such as CuInSe2 and Cu2ZnSnS4, have attracted increasing attention in photovoltaic applications due to their potential to produce cheap solution-processed solar cells. Herein, we report the synthesis of copper-antimony-sulfide (CAS) NCs with different crystal phases including CuSbS2, Cu3SbS4, and Cu12Sb4S13. In addition, their morphology, crystal phase, and optical properties were characterized using transmission electron microscopy, X-ray diffractometry, UV-vis-near-IR spectroscopy, and photoemission yield spectroscopy. The morphology, crystal phase, and electronic structure were significantly dependent on the chemical composition in the CAS system. Devices were fabricated using particulate films consisting of CAS NCs prepared by spin coating without a high-temperature treatment. The CAS NC-based devices exhibited a diode-like current-voltage characteristic when coupled with an n-type CdS layer. In particular, the CuSbS2 NC devices exhibited photovoltaic responses under simulated sunlight, demonstrating its applicability for use in solution-processed solar cells.

Magnetism and metal-insulator transition in III-V based diluted magnetic semiconductors

Abstract We report experiments on the magnetism and the transport in III–V based diluted magnetic semiconductors (Ga,Mn)As and (In,Mn)As. Heat treatment (annealing) at comparatively low temperatures (slightly above the growth temperature) is unexpectedly found to be effective to improve the ferromagnetism and the metallic conduction. IR optical conductivity measurements and soft X-ray absorption spectroscopy reveal that the double exchange model is a convenient picture to describe the ferromagnetism. The transport in the vicinity of metal–insulator critical point was studied in detail by using the low-temperature annealing method.

High resolution soft X-ray absorption spectroscopy for the chemical state analysis of Mn

Abstract Mn L 2,3 X-ray absorption spectra of various manganese compounds were measured using a synchrotron radiation facility. Very high-resolution spectra were obtained. Sharp lines, which were not found before, were observed, and reproducibility was checked using a different beam line in the synchrotron radiation facility. Difference in spectral line shape was observed as the difference of quantum yield method. An unknown sample was measured and the specimen was concluded to be a mixture of two different oxidation states of Mn. The effects on the change of spectral profiles are discussed.

Resonant Soft X-Ray Emission Spectroscopy of NiO across the Ni L2,3 Thresholds.

We study the electronic structure of NiO using resonant soft X-ray emission spectroscopy across the Ni L 2,3 thresholds. We observe inelastic (Raman) features within 10 eV of the elastic peak energ...

Polarization dependence of the soft X-ray raman scattering at the L edge of TiO2

Abstract Polarization dependence of the soft X-ray Raman scattering was investigated at the Ti 2p absorption edge of TiO2. Strong Raman scattering features appear about 14 eV below elastic peaks with strong polarization dependence. These Raman scattering structures are charge transfer excitations to the antibonding state between 3d 1 L and 3d0 states, because they are enhanced when the incident photon energies are tuned at satellite structures of Ti 2p absorption spectrum. Broad Raman scattering structures are found between 3 eV and 10 eV below elastic peaks. They show weak polarization dependence and are assigned to be nonbonding type charge transfer excitations.

Metal-insulator transition sustained by Cr-doping in V2O3 nanocrystals

We have obtained monodisperse (V1−xCrx)2O3 nanocrystals with crystal sizes of 21.0 ± 4.1 nm using organic-phase synthesis. The (V1−xCrx)2O3 nanocrystals clearly show the transition from a corundum structured paramagnetic metal to a monoclinic structured antiferromagnetic insulator in contrast to non-doped V2O3 nanocrystals, in which the disappearance of the metal-insulator transition has been observed. We have found that Cr doping works effectively in narrowing the a1g band, which tends to be broadened by nanocrystallization. This result suggests that chemical doping is useful for control of material phase transitions at the nanoscale.

Unusual low-temperature phase in VO2 nanoparticles

We present a systematic investigation of the crystal and electronic structures and the magnetic properties above and below the metal-insulator transition of ball-milled

Electronic Transport of a Carbon Nanotube Quantum Dot in Different Coupling Regimes

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Photoluminescence core-level excitation of CdSe quantum dot structures

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Metal–Insulator Transition for V2O3 Powder Observed Using a Soft X-ray Emission Spectrometer

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