Yasuo Nozue

Bound-Exciton Luminescence of Cu-Doped ZnSe

As an experimental approach to clarify the correlation between doped impurity and native defects, copper was deped at a low temperature to high-purity ZnSe crystals with controlled deviation from stoichiometry by a previous heat treatment under molten Zn or saturation pressure of Se. Photoluminescence was measured and examined as a function of deviation from stoichiometry. The site selection doping of Cu was achieved by controlling native defect concentrations. I1deep lines are known to arise from acceptors of two different origins, VZn and CuZn.

Absorption spectra of selenium clusters and chains incorporated into zeolites

Absorption spectra are reported for Se adsorbed in faujasite (X or Y) as a function of x, where x is the Al-to-(Al+Si) ratio. The amount of adsorbed Se increases with increasing x for small x and is saturated for x>or approximately=0.1. The results are interpreted using the model in which the finite number of Se atoms are physically adsorbed by each permanent dipole which consists of (AlO4)- in a zeolite framework and cation. The energy of the lower absorption edge changes with x and the spectral shapes are classified into three types. Spectral differences are also observed for Se incorporated into Linde type A and Linde type L lattices. From a comparison with the spectrum of the isolated Se chain in mordenite, the structures of Se in A and L are expected to be isolated clusters and double parallel chains, respectively.

Formation and photoluminescence of Ge and Si nanoparticles encapsulated in oxide layers

Abstract Ge and Si semiconductor nanoparticles encapsulated in oxide layers were prepared by the inert gas condensation method. High-resolution electron microscopy and energy dispersive X-ray spectrometry showed the formation of the core-shell structures with a size below 10 nm. The optical absorption spectra of Ge showed the band edges at higher energy compared to the ordinary Ge element. Photoluminescence measurements of them showed the luminescence at 2.2–2.6 eV, which would be due to the formation of core-shell structures. Especially, Ge clusters with a size of approximately 1 nm were formed in the GeOx matrix, which showed strong luminescence at 3.12 eV, which would be due to the quantum size effect of the clusters. The present work indicates that formation of the core-shell structure of semiconductor nanoparticles with oxide layers would be effective for blue shifts of the energy band.

Excitons in PbI2 clusters incorporated into zeolite cages

Abstract The absorption spectra are reported for PbI2 clusters incorporated into zeolite cages. At 3.0 ∼ 3.6 eV, there appear absorption bands whose energies depend on the size of the cage. These energies are considerably blue-shifted compared with the bulk exciton energy of 2.5 eV, and shift to the higher energy with decreasing the size of the cage. The absorption band is attributed to the exciton generated at an isolated cluster stabilized in the cage, where the diameter of cluster is expected to be 1 ∼ 1.3 nm. The remarkable blue-shift is qualitatively interpreted in terms of the quantum size effect on excitons, but does not coincide quantitatively with the value calculated by the effective mass approximation.

Optical studies of nanoscale materials incorporated in the space of zeolite crystals

Abstract Optical spectroscopy is used for the characterization of the quantum electronic state of nanoscale materials incorporated in zeolite crystals. Alkali metal clusters generated in zeolite exhibit interesting phenomena in optical properties as well as magnetic ones. Potassium clusters incorporated in zeolite LTA have shown ferromagnetism at low temperatures. The origin of the ferromagnetism was explained by the itinerant electron model. The infrared spectrum and magnetization curves, however, suggest that the itinerant electron model is not applicable to the ferromagnetism. Magnetic properties are interpreted based on the model of ferrimagnetism. Polarized micro-optical spectroscopy suggests the possibility of the one-dimensional array of potassium clusters in zeolite MOR single crystal

Relaxation due to multiple longitudinal‐optical‐phonon emission and dissociation of exciton in Cd0.3Zn0.7S/ZnS strained‐layer superlattices

The optical properties of Cd0.3Zn0.7S/ZnS strained‐layer superlattices grown on (100) GaAs substrates by low‐pressure metalorganic chemical vapor deposition, were investigated by means of photoluminescence excitation spectroscopy and through the effect of an electric field on exciton emission. A multiple longitudinal optical‐phonon emission process, related to the relaxation of excitons, has been observed for the first time in the excitation spectra. The localization of excitons has been found to be the dominant cause of the linewidth broadening. The effects on an axial electric field on exciton emission intensity and peak position have revealed that the dissociation of the localized excitons on the low‐energy side of the spectrum principally takes place due to impact ionization.

Temperature and Crystal Size Dependence of Excitonic Absorption for Polydiacetylene Microcrystals

Abstract The excitonic absorption of well defined polydiacetylene microcrystals was measured as a function of temperature and crystal size. Poly-DCHD microcrystals were prepared by a recently reported reprecipitation method, and crystal size was controlled by changing the concentration of diacetylene monomer solution injected into water. The excitonic absorption peak position was blue-shifted from 650 nm to 635 nm with decreasing crystal size from 120 nm to 30 nm. On the other hand, the peak position was red-shifted by 15 nm (300 cm−1) upon decreasing the temperature from 273 K to 4 K. Narrowing of the half peak width was also observed to accompany this red- shift. These phenomena are considered to be related to the coupling interaction between the excitonic state and phonons depending on crystal size and temperature.

Loading density dependence of ferromagnetic properties in potassium clusters arrayed in a simple cubic structure in zeolite LTA

Magnetic properties are investigated for potassium clusters arrayed in zeolite LTA. The potassium loading density per cluster, n, is changed up to 7.2 per cluster. The n-dependence of the Curie constant and the magnetization are measured in detail. Possible origins of ferromagnetic properties are discussed. ( 2000 Elsevier Science B.V. All rights reserved.

Luminescence of highly excited ZnSe

Two-photon-resonant Raman scattering due to the excitonic molecule has been observed at LHeT, and its binding energy is found to be 2.2 meV for two triplet excitons. A luminescence band due to the excitonic molecule is confirmed under moderate power excitation (⪅50 kW/cm2) in very pure single crystals.

Optical Absorption Spectra of Sodium Clusters Incorporated into Zeolite Lta

Abstract Absorption spectra are reported for sodium clusters incorporated into cages of zeolite LTA crystals, where the expected cluster diameter is about 11 A at maximum. The spectra exhibit the bands originated from the excitation of the quantum-confined electron-hole pair as well as the surface plasmon excitation.