CdS Microcrystals Research Articles

圧力誘起相転移の研究への放射光の応用 赤外スペクトルによる微粒子の相転移

In this article, the important role of the synchrotron radiation in the field of spectroscopic study under high pressure up to 20 GPa in the infrared region is emphasized. As a typical example of such experiments which became possible to perform, the experiment on the surface phonon of CdS microcrystals under high pressure using a DAC is presented. Through an analysis of the energy shifts of the surface phonon with pressure, we found that the magnitude of the critical pressure indicating a structural phase transition in the microcrystal differs from that for the bulk.

Photoelectrochemical Properties of Size-Quantized CdS Microcrystals Modified with Various Amounts of Viologen-Functionalized Thiol

Size-quantized CdS microcrystals (Q-CdS) modified with various amounts of electroactive 1-decyl-1[prime]-(3-mercaptopropyl)-4,4[prime]-bipyridinium dibromide (VSH) and electroinactive decanethiol were prepared. The surface-modified VSH was photoreduced by irradiation of Q-CdS to give both dimeric and monomeric radical cations of the viologen, and the formation of the former was enhanced by increasing the amount of the surface-bound VSH. Kinetic analyses revealed that the rate constants for the reduction of the surface-modified VSH and for the reoxidation of the produced radical cations were independent of the amount of the VSH. Q-CdS having the higher amount of VSH showed the higher activity for electron transfer to methylene blue in solution, evidencing that the surface-bound viologen mediated electron transfers from Q-CdS to the electron acceptor in solution. 37 refs., 10 figs.

ESR of Photodarkened CdS-Doped and Undoped Glasses

Electron spin resonance (ESR) and luminescence were measured in CdS-doped glass and undoped glass, which does not contain CdS microcrystals. Photoinduced centers responsible for the photodarkening effects were detected by ESR in the CdS-doped glass. This ESR signal is different from that of the host glass. This result indicates that the centers responsible for the photodarkening are not associated with the centers in the glass.

Photothermal deflection and photoluminescence studies of CdS and CdSe quantum dots

We report on the observation of the quantum confinement effect by using photothermal deflection spectroscopy (PDS) experiment and the time dependence of optically induced degradation in CdS and CdSe semiconductor-doped glasses. The observed absorption peaks in the PDS experiment, together with a simple model, were used to evaluate the average radius of semiconductor microcrystals. It is found that the estimated average radii of quantum dots are consistent with that obtained from other methods. This result demonstrates that the PDS technique provides an alternative tool for the study of the optical properties of semiconductor microscrystals. The time dependence of the luminescence degradation of the impurity band, which is attributed to the process of Auger ionization, follows a stretched-exponential function. The inconsistency with the previously proposed exponential relaxation may be due to the size distribution of CdS and CdSe microcrystals.

Effect of Charged Conditions of Stabilizers for Cadmium Sulfide Microcrystalline Photocatalysts on Photoreduction of Carbon Dioxide

Abstract The use of stabilizers having negative charges such as polyacrylic acid for CdS microcrystals in aqueous solutions yielded selectively formate as a photoreduction product of CO2, while positively charged stabilizers such as polyethyleneimine worked well for selective production of CO. Furthermore, it was found that the rate of photo-induced reduction of carbon dioxide was influenced by the amount of charges of the stabilizers used.

Preparation of CdS Microcrystals Covalently Bound with Viologen Groups and Their Photoelectrochemical Properties

Abstract CdS microcrystals (Q-CdS) covalently bound with 1-decyl-1′-(3-mercapto)propyl-4,4′-bipyridinium were prepared. The surface viologen groups picked up photo-generated electrons in the conduction band of Q-CdS particles, causing a decrease of their emission. Irradiation of these particles resulted in new absorption peaks which are assigned to viologen radical cation monomers and their dimers. The photo-reduced viologen groups can mediate photo-generated electron transfer from Q-CdS to methylene blue in solution.

Optical spectra of free and collected CdS microcrystals

The time evolution of extinction spectra of the CdS vapor zone and a CdS microcrystal layer on silica have been measured during thermal evaporation in helium gas. The structure of the spectra and their time dependence show the quantum confinement effect of carriers in CdS.

Electro-optic properties of CdS embedded in a polymer.

CdS microcrystals under weak confinement conditions embedded in a polyvinyl alcohol polymer film have been optically characterized by linear absorption and time-resolved luminescence spectroscopy. They are studied in the presence of an external electrical field at low and room temperature and different densities of photoexcited carriers. By applying a voltage of 400 V corresponding to an external electric-field strength of 5\ifmmode\times\else\texttimes\fi{}${10}^{4}$ V/cm, the observed absorption change in this material is \ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}\ensuremath{\approxeq}0.07. The electrical field produces a redshift of the absorption band which is explained in terms of the quantum-confined Stark effect. In addition, a restoring of the oscillator strength is observed and explained by screening effects of internal fields in the interface region by the photogenerated electron-hole pairs. At high laser excitation an electric-field-induced change of the nonlinear absorption spectrum of \ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}=0.25 is achieved, giving a considerable modulation of the absorption edge. The electrical field separates the laser excited carriers and the change in the absorption is attributed to a compensation of the many-particle interaction by the external electrical field.

Spectral sensitization of a TiO 2 semiconductor electrode by CdS microcrystals and its photoelectrochemical properties

CdS microcrystals were deposited on TiO 2 and ZnO semiconductors and photoelectrochemical properties were studied from the viewpoint of semiconductor sensitization. The photocurrent of a CdSTiO 2 photoelectrode was caused by photoinduced electron transfer in the direction from CdS to TiO 2. The photoelectrochemical properties of CdSTiO 2, such as a flat band potential, were found to be strongly influenced by those of CdS as sensitizer. The quantum yield of the photocurrent amounted to 30%. A photoelectrochemical solar cell composed of CdSZnO and polysulphide solution was studied.

Low-frequency Raman scattering from CdS microcrystals embedded in a germanium dioxide glass matrix.

We report here results from experiments on low-frequency Raman scattering from CdS microcrystals of various sizes embedded in a germanium dioxide glass matrix. We observed peaks in the low-frequency region in the tail parts of the Rayleigh lines and we found that the frequencies of these peaks were proportional to the inverse microcrystal diameters and that the size dependences of the peak shifts agreed fairly well with the calculated results based on Lamb's theory. Furthermore, we found that these Raman-scattering spectra had the characteristic polarization properties. Our results show that the observed low-frequency Raman scattering originates from confined acoustic vibrations of a spherical CdS microcrystal.

Pressure Effects on CdS Microcrystals Embedded in Germanate Glasses

We measured the absorption edge, photo-luminescence and Raman scattering spectra of CdS microcrystals embedded in germanate glasses under high hydrostatic pressure. We found that the one L0 phonon frequency shifts to the higher energy side with the increase of the pressure. The Raman shift with pressure variation is rather insensitive to the size of the CdS microcrystal and the change of its value with pressure is approximately equal to that of the bulk material. This strongly suggests that the stress induced by the glass medium is small. We also found that the absorption edge energy increases with the increase of the pressure. The bulk CdS shows a phase transition at 2.7 GPa pressure, however, the microcrystals exhibit no drastic change in the measured pressure range. The dependence of the luminescence on the pressure was also measured. The surface emission, which is in the lower energy part of the edge emission luminescence spectra, shifts to the higher energy side and disappears gradually with the Increase of the pressure.

One Phonon Raman Scattering of CdS Microcrystals Embedded in a Germanium Dioxide Glass Matrix

Raman scattering spectra of LO phonon modes in CdS microcrystals having various sizes in a germanium dioxide glass matrix are measured. The bandwidth of the LO phonon broadens with decreasing the microcrystal size. The size dependence of the bandwidth agrees with the calculated results based on the phonon confinement theory. This result shows that this size dependence originates from the relaxation of the q -vector selection rule. However, for the microcrystal sizes smaller than d =5.0 nm, the higher-frequency shift of the LO phonon peak with decreasing the microcrystal size were observed in contrast to the prediction by the phonon confinement theory. The origin of this shift might be due to the defects contained in the CdS microcrystals.

Photochemical Properties of CdS Microcrystals Prepared in Poly (ethylene oxide)

Photochemical Properties of CdS Microcrystals Prepared in Poly (ethylene oxide)

Transient effects in photoexcited CdS microcrystals

The time evolution of nonlinear absorption of CdS microcrystals in glass matrix was studied by means of the pump-and-probe technique with 5 ps time resolution. The electron-hole density dependent competition between concurrent band filling, causing the bleaching of the sample, and the darkening effect due to band gap renormalization has been observed in time-resolved experiments, as well as in those where the excitation intensity at a fixed time delay is changed between pumping and probing beams.

Oxidation of small-sized CdS crystals in sol-gel-derived glasses

Small-sized CdS crystals were prepared in SiO 2 and Na 2O ZrO 2 SiO 2 glasses by the sol-gel process and the oxidation of CdS crystals was investigated. Gels synthesized by the hydrolysis of a complex of metal alkoxides and Cd(CH 3COO) 2·2H 2O were heated at 500°C and allowed to react with H 2S gas to form hexagonal CdS microcrystals. CdS crystals in SiO 2 glass were sensitive to oxidation by heating in air. CdS crystals in Na 2O ZrO 2 SiO 2 glass were smaller than those in SiO 2 glass and stable against oxidation in air. The effect of glass composition on the oxidation of CdS crystals was apparent in the emission spectra of glasses soaked in methyl viologen.

ＣｄＳ微結晶ドープガラスのＣｄＳ酸化におよぼすガラス組成の影響

SiO2 and 1.4Na2O20.8ZrO277.8SiO2 glasses doped with small-sized CdS crystals were prepared by the sol-gel process. Gels synthesized by the hydrolysis of Si(OC2H5)4, Zr(OC3H7)4, NaOCH3 and Cd(CH3CO2)22H2O were heated at 500°C, and then allowed to react with H2S gas to form the glasses doped with hexagonal CdS microcrystals. The degradation of the glasses was investigated with respect to the oxidation of CdS crystals. CdS crystals were incompletely embedded in the porous silica glass with high specific surface area. When heated, CdS crystals were oxidized in air at temperatures from 300 to 500°C and disappeared from glass at temperatures from 500 to 700°C in nitrogen atmosphere. On the other hand, 1.4Na2O20.8ZrO277.8SiO2 gel shrank at lower temperatures, in which CdS crystals were embedded. CdS crystals doped in the glass were stable against oxidation even though heated 500°C in air.

Formation of CdS Crystals in Small-Sized CdS-Doped Glasses Prepared by the Sol-Gel Process

Glasses doped with small sized-CdS crystals with a significant quantum size effect were prepared by the sol-gel process. Gels synthesized through the hydrolysis of a complex solution of Si(OC2H5)4 and Cd(CH3COO)2⋅2H2O were heated at 500° to 700°C, then allowed to react with H2S gas to form silica glasses doped with fine CdS microcrystals. The formation of CdS crystals was controlled by diffusion of H2S gas into the porous glass and the diffusion coefficient was from 10-18 to 10-16cm2/s at 100° to 200°C as determined from the measurement of weight gain of glass. The size of CdS microcrystals increased with heat-treatment time, reaching 2 to 6nm in diameter, which was dependent on the heat-treatment temperature and CdO content in glass. The optical absorption edge exhibited a blue shift compared to that of a bulk CdS crystal and its energy shift was reciprocally proportional to the square of the crystal size. In the photoluminescence spectra, two peaks were observed at 2.0 and 2.5eV, the intensity of which at 2.0eV decreased as the heat-treatment time was increased. It was considered that the emission at 2.0eV, pronounced at low temperature of 77K, was due to the carrier recombination via the deep level due to oxygen impurity in CdS crystals.

Photoluminescence spectra of gas-evaporated CdS microcrystals

The temperature dependence of photoluminescence spectra of gas-evaporated CdS microcrystals ranging from 200 to 1500Å in average size were measured. At low temperatures, new peaks which were not previously observed at room temperature were found. The low-temperature spectra consist of two green (G1 and G2), a yellow (Y) and a red (R) band. On the basis of the observed temperature dependencies of the peak positions, intensities and widths, possible origins of the four emission bands are discussed. Our results strongly suggest that the R band is due to the surface states.

Auger ionization of semiconductor quantum drops in a glass matrix

A microscopic model of degradation of both the nonlinear optical and luminescent properties of semiconductor-doped glasses is proposed. In the framework of the model the degradation is due to the process of Auger ionization of microcrystals when some electron-hole pairs are excited. The theoretical dependences of Auger ionization rate on the microcrystal size and energy band parameters of the glass/semiconductor heterostructures are obtained. The luminescence degradation kinetics have been studied in CdS microcrystals doped glasses with different average size at various pumping light intensities. The experimental dependences of the ionization rate on the microcrystal size and intensity are found to be in good agreement with the theoretical calculations. It is shown that an abrupt heterointerface leads to a strong increase of the Auger process rate. Oscillations of the Auger ionization probability and their dependences on the microcrystal size are predicted.

Spectra and decay kinetics of radiative recombination in CdS microcrystals

Low-temperature luminescence spectra of CdS microcrystals grown in a transparent silicate glass matrix have been studied. The microcrystal size could be varied during growth from a new tens to a few thousands of an angstrom. The spectra of fairly large microcrystals (≈400Å) reveal lines originating from free and bound exciton annihilation. The exciton luminescence intensity drops with decreasing microcrystal size, the emission spectra of microcrystals of size less than a few hundred angstrom being dominated by transitions involving impurities and lattice defects. Effects associated with surface band bending in large microcrystals have been revealed and studied. A study has been made of the position and kinetics of the interimpurity luminescence bands as a function of microcrystal size. A theory has been developed to describe the energy spectrum of shallow donor levels and of the interimpurity recombination kinetics in semiconductor microcrystals.