Tetsuji Kume

Enhancement of Photoelectric Conversion Efficiency in Copper Phthalocyanine Solar Cell: White Light Excitation of Surface Plasmon Polaritons

Excitation of surface plasmon polaritons (SPP) in copper phthalocyanine (CuPc) solar cells was achieved by means of a wavelength-scan attenuated total reflection (λ-scan ATR) method. The action spectra of the solar cell (wavelength dependence of the photocurrent) were measured simultaneously with the λ-scan ATR spectra for various incident angles. The action spectra were found to exhibit a pronounced peak around 550 nm due to the excitation of SPP. The existence of an optimum incident angle which yields a maximum enhancement of the photoelectric conversion was also found. The enhancement of the conversion efficiency under white light illumination was demonstrated by using a Xe lamp.

Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate

Visual observations through a microscope and in situ Raman scattering measurements of a synthesized nitrogen hydrate have been performed at pressures up to 6 GPa and 296 K. High-pressure transformations have been found at 0.85 and 1.45 GPa. The cubic structure II (sII) nitrogen hydrate initially transforms to the hexagonal structure (sH) at 0.85 GPa and finally forms the orthorhombic dihydrate (sO) above 1.45 GPa. The sO phase of nitrogen hydrate exists up to at least 6 GPa. A variety of Raman spectra composed of three peaks have been sometimes observed in sII phase below 0.50 GPa, which implies that the guest nitrogen molecules doubly occupy the large hexakaidecahedron cages. Two Raman bands of the guest nitrogen vibrations with nearly equal intensities appearing in sH phase suggest that five nitrogen molecules are filling in extra large icosahedron cages.

Enhancement of Photoelectric Conversion Efficiency in Copper Phthalocyanine Solar Cell by Surface Plasmon Excitation

In order to enhance the photoconversion efficiency, surface plasmon polaritons (SPP) in an organic solar cell consisting of Al, copper phthalocyanine (CuPc) and Ag thin films were excited by an attenuated total reflection (ATR) method. The dependence of the enhancement factor for the short-circuit current (Isc) on the wavelength of the incident light (λin) was measured in detail in the visible region. It was found that the enhancement factor depends strongly on λin and takes a maximum (a factor of 8) around λin=550 nm. The good agreement between the calculated λin dependence of light absorption in the CuPc layer and the observed λin dependence of the enhancement factor suggests that the enhancement is caused by increased carrier generation in the CuPc layer due to increased light absorption upon SPP excitation.

Optical microscopy and Raman scattering of a single crystalline argon hydrate at high pressures

Abstract An argon hydrate (AH) single crystal was synthesized in a diamond anvil cell to investigate its intrinsic high-pressure properties by the optical microscopy and in situ Raman scattering measurements. Single crystalline AH at room temperature showed three phase transitions at P =0.43, 0.66, and 1.05 GPa through the changes in crystalline morphology or color and in Raman spectra of lattice vibrational modes and O–H stretching vibrations of H 2 O host cages. Argon multi-occupancies of large cages in the initial sII and higher-pressure sH and sT phases are investigated by Raman bands observed around 135 cm −1 in view of recent MD calculation, and neutron and X-ray diffraction experiments.

Raman and x-ray diffraction studies of Ba doped germanium clathrate Ba8Ge43 at high pressures

High-pressure Raman and x-ray diffraxtion (XRD) measurements of a defect clathrate Ba8Ge43◻3 have been carried out at room temperature up to 40 GPa. Three vibrational modes associated with guest Ba atoms were observed in the low-frequency region, and the structureless spectrum due to Ge vacancies was found in the framework vibrational region. The Raman spectra shows a pressure-induced phase transition at 8 GPa, which is due to the structural distortion through the three-bonded Ge atoms and to the change in the guest-host electronic interaction. Both Raman spectra and XRD patterns present the evidence for the amorphization of Ba8Ge43 around 30–40 GPa. The isostructural phase transition often present in intercalated silicon clathrates and associated with a large volume reduction was not observed for Ba8Ge43 up to 40 GPa. The pressure dependence of the lattice constant (a) normalized by a0 at 1 bar (a∕a0) shows the continuous decrease with pressure until amorphization. From the good coincidence of this curve...

Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Crystal structures of endohedrally Na-doped type II silicon clathrates with variable Na content were refined by Rietveld analysis. Type II Si clathrates have two types of cages: small and large. The large cages were preferentially occupied by Na atoms. Upon occupation by Na in the large cages, the lattice constant of the clathrates decreased slightly. The attractive interaction of the Na atoms in the large cages with a framework that caused displacement of the Na atoms from the center might influence lattice shrinkage. Moreover, in the region where almost large cages contained Na atoms and the small cages were partially occupied by Na, the lattice constant increased with the Na content; however, the relationship between these features was complex. The lattice constant demonstrated a linear relationship with the size of the small cages; however, the enlargement of the small cages was not linear with respect to its Na occupancy, resulting in complex changes in the lattice constant with respect to the Na content. It was inferred that enlargement of the small cages by Na insertion may be dependent on the Na occupancy of neighboring small cages.

A transformation of carbon dioxide to nonmolecular solid at room temperature and high pressure

Raman and infrared absorption spectra of solid carbon dioxide (CO2) have been measured at high pressures up to 80GPa and room temperature. The broadening and weakening of lattice vibrational signals and the disappearance of molecular vibration were detected at 65GPa by means of the Raman and infrared absorption experiments, respectively. The present Raman spectral feature is exactly the same as that of amorphous CO2 synthesized around 600K and 60GPa [M. Santoro et al., Nature (London) 441, 857 (2006)]. It is demonstrated that the polymerization of CO2 proceeds even at room temperature and almost independently of temperature.

High pressure ultraviolet-visible-near infrared study of colored solid hydrogen sulfide

Optical absorption measurements at high pressure were extended up to 5.5 eV in the ultraviolet region by constructing an ultraviolet-visible-near infrared absorption spectrometer adapted for diamond anvil cells and by using impurity free synthetic diamonds. Using the improved spectrometer, absorption spectra of solid hydrogen sulfide (H2S) were measured under various pressures from 0.3 to 29 GPa at room temperature. The band gap estimated from the absorption edge was 4.8 eV at 0.3 GPa and decreased to 2.7 eV at 29 GPa almost linearly except for a small jump at about 11 GPa corresponding to the phase transition to phase IV. At the same pressure, an Urbach tail responsible for the coloring of the solid H2S suddenly appeared. Raman signals excited with a green Ar ion laser (2.41 eV) were found to increase in intensity with pressure, which was explained by preresonance Raman effects induced by the decreasing of the gap energy near to the Raman excitation energy.

Raman and x-ray diffraction studies of cationic type-I clathrate I8Sb8Ge38: Pressure-induced phase transitions and amorphization

Raman scattering and x-ray diffraction (XRD) measurements of iodine-doped cationic type-I clathrate I8Sb8Ge38 have been carried out at 1 bar and high pressures of up to 50 GPa at room temperature, by using a diamond anvil cell. The guest-iodine rattling vibrations were observed in a low-frequency region of 53–58 cm−1 at ambient conditions as well as the host vibrations in a higher-frequency region of 75–273 cm−1. High-pressure Raman spectroscopy showed the frequency softening of iodine rattling vibrations at pressures above 16 GPa, which originates from the weakening of guest-host interactions due to the transfer of electrons from the large-size guest I to the framework. High-pressure Raman and XRD studies revealed that the amorphization proceeds gradually around 36–44 GPa and is completed at about 50 GPa. The isostructural phase transition with a volume reduction was observed at 42 GPa, which is compared with other type-I Ge clathrates.

Raman and optical absorption studies of rare-earth hydrides under high pressure

Raman and visible absorption study were performed for YH3 and ScH3 at high pressures, in order to clarify the structural and electronic phase transitions. The Raman results of YH3 revealed the presence of an intermediate phase at 9-24 GPa between the low-pressure hexagonal and high-pressure fcc phases. The characteristic behaviour was commonly observed for vibrational properties of YH3 and ScH3, suggesting a common mechanism of the structural transformation. The results of the absorption measurements for YH3 demonstrate that the optical band gap starts to close in response to the phase transition.