Ryouhei Nakata

High-Conversion-Efficiency Solar Cell Using Fluorescence of Rare-Earth Ions.

An experiment was performed to obtain a high-conversion-efficiency solar cell by shifting UV-region sunlight to the visible region, in which the optical absorption and emission properties of rare-earth Eu activator ions were exploited. A thin disc of CaF2:Eu single crystal was used as an alternative to the antireflection (AR) film in a solar cell. The effectiveness of rare-earth ions was examined. Their relative efficiencies were found to be higher at Eu concentrations from 0.001 to 2 mol%, with a maximum value of 1.5 at 0.05 mol%, than those of nondoped crystal when subjected to low illuminance by a Xe-lamp-based solar simulator. For the purpose of putting the crystal to practical use as an AR film, rare-earth ions were implanted into CaF2 crystal at a dose of 1014–1015 ions/cm2 and an energy of up to 140 keV.

Electron spin resonance study of laser-annealed (Zn,Mn)O ceramics

Investigations have been made on the effects of pulse laser annealing on the ceramics (Zn,Mn)O by means of electron spin resonance (ESR) measurements. A remarkable change in the ESR spectrum was observed after annealing by a Nd:YAG pulse laser (λ=1.064 μm, 5–7 J/cm2). It suggests that segregated Mn2+ precipitates at grain boundaries in ZnO are diffused into crystal grains by preferential heat conduction through the boundaries. The scanning electron microscope observation verified enough melting features within the ceramics.

Characterization of nitrogen terminated silicon nanoparticles on AFI zeolite with X-ray and ultraviolet photoelectron spectroscopies

Abstract Nitrogen terminated silicon nanoparticles were synthesized on AFI AlPO 4 -5 zeolite by pulsed laser ablation of silicon target in the presence of 1.33×10 −5 Pa of ammonia, and were characterized in situ by X-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Nonstoichiometric silicon nitride and unterminated silicon nanoparticles with size of estimated to be ∼5.4 A in diameter were existed. They were found to migrate and diffuse into the AlPO 4 -5 channels at 373 K and migrate back to the external surface at 503 K. The effect of sample annealing was studied at 373 K. The change of silicon surface state (unterminated silicon nanoparticles) to hydride species was observed concurrently with breaking of the NH x species. This result implies that the NH x species remained on the silicon particles form Si 3 N 4 like bonds and the dissociated H-species are bound to the unterminated silicon surface.

Sm-doped CdWO4 thin films synthesized by pulsed laser deposition

Samarium doped CdWO4 (Sm-CdWO4) thin films were synthesized by pulsed laser deposition (PLD) using pressed powder targets composed of Sm2O3, CdO, and WO3 mixtures. Congruent, stoichiometric, and crystalline thin films were obtained on glass substrates at 873 K using targets with 0.01 at. % Sm. The x-ray activated photoluminescence (x-ray PL) spectra were redshifted by about 40 nm at 298 K. Changes of the PL component at 2.85 eV into those at 2.20 and 2.52 eV were typical in their convolutions. These PL components showed very similar dependencies on excitation wavelengths, indicating that CdWO4 related PL emissions in the Sm-doped PLD films are relaxed from the same midband excited state. The PL peaks characteristic to Sm3+ had two sidebands shifted to both lower and higher energy sides as much as 0.03 eV. Plume emission spectra were analyzed to elucidate effects of Sm on our PLD processes. It was interpreted that energy transfer should occur between excited neutral Cd to the excited state of Sm3+ in laser...

Negatively charged subnanometer-sized silicon clusters and their reversible migration into AFI zeolite pores studied with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy

Abstract Subnanometer sized silicon clusters were deposited on AFI zeolite (AlPO4-5: one-dimensional channel diameter

Electron spin resonance studies on locally additive low symmetry distortions in the mixed-crystal Sr1 − xBaxF2 system monitored with Eu2+ dopants

Abstract Local distortions in mixed crystals or solid solutions Sr 1 − x Ba x F 2 have been investigated by monitoring a doping Eu 2+ ion by means of electron spin resonance measurement. With increasing mixture ratios x near 0, the resolved hyperfine linewidths broaden in sequence from larger | M | spin magnetic quantum numbers to smaller values for the M → M − 1 transition. This tendency also holds on decreasing the mixture ratio x near 1. As a source of these characteristic broadening effects, distortion compensation for lattice misfit has been considered to be made at local sites in the mixed crystal by adding the second-order low symmetry crystal fields of B 2 0 followed by B 2 2 . It was found that the observed line broadening effects would be reasonable if the sum of the B 2 0 term and the second-order contribution term of B 2 2 was distributed over the region from −2.75 to 1.75 G, in which it corresponded to the displacements of fluorine ions to be detectable with the high sensitivity below 0.4% of the lattice parameter of SrF 2 .

Temperature Dependence of Resistivities in TiS2 and Its Cu-Intercalate

The electrical resistivities of a layered dichalcogenide TiS 2 and its Cu-intercalate were investigated in the temperature range 100 K to 350 K. The a -axis resistivity of the Cu-intercalate showed a linear temperature dependence rather than the T 2 -dependence reported in TiS 2 . With increasing x in Ti 1+ x S 2 and Cu x TiS 2 , both resistivities and their slopes decrease with temperature, while the carrier concentrations enlarge up to metallike orders of 10 22 cm -3 . This origin is discussed on a common basis of the slow transition from electron-electron to electron-phonon scatterings which is due to the increase of conduction electrons of the intercalated Cu metalas well as the self-intercalated Ti metal.

Photoluminescence of Eu2+ Ion Doped into Ca1-xSrxF2 Mixed Crystals

The spectral peak, linewidth and quantum efficiency of the luminescence band of Eu2+(4f65d) ions in Ca1-xSFxF2 mixed crystals were studied for different x-values. With increasing x, the absorption peak shifted linearly to that of SrF2:Eu2+, while the luminescence peak varied in a parabolic manner with a minimum around x=0.5. In addition, the half-width broadened remarkably, and the activation energy for the nonradiative relaxation was minimized down to 0.4 eV. The temperature dependences of the half-width and the quantum efficiency agreed well with the calculated results on the basis of electron-phonon interaction.

Studies of high temperature ionic thermocurrents observed in CaF2 and CaF2:Eu

Abstract A strong and thermally stable ionic thermocurrent (ITC) band peaked at 316 K was observed in europium-free CaF 2 crystals. The experimental ITC spectrum can be described by an ionic conduction kinetic expression of the type τ = τ 0 exp ( E a kT ) , with τ 0 = 1.90 × 10 −13 s and E a = 0.93 eV. This new band was very sensitive to europium doping, the ITC peak intensity being drastically reduced with the addition of only 0.002 mol.% europium. At the same time, a high-temperature typical ionic thermocurrent (HT ITC) band due to mobile interstitial fluorine ions appeared at 416 K. The decrease of the new band at 316 K is explained by assuming that mobile fluorine vacancies are compensated with fluorine ions introduced into CaF 2 as charge compensators for the trivalent europium ions. The dependence of the temperature T m on the ITC peak in the higher-temperature ITC band on europium concentration is determined and the kinetic parameters are estimated.

Crystal growth of Bi4Ge3O12 and heat transfer analyses of horizontal bridgman techniques

Nonhygroscopic, high-radiation-absorptivity scintillation crystals of BGO (bismuth germanate: Bi4Ge3O12) have been grown by the horizontal Bridgman (HB) method using very sharply focused gas jets as the heat source. It was determined by means of optical and X-ray characterization that the crystal growth had been performed successfully. On the basis of a one-dimensional time-dependent heat transfer model, simulation analyses have been used to examine the validity of this method under various growth conditions: temperature distribution, locations of the boundaries between the crystal, melt and polycrystal components, and growth rates.