CdS Microcrystals Research Articles

Growth kinetics of CdS in germanium oxide glassy matrix

In the present work we revisit the size data of CdS microcrystals previously collected in the glassy matrix of Germanium oxide. The CdS clusters analyzed using electron microscopy images have shown a wurtzite structure. The mean average radius, dispersion and volume evaluated from the histograms showed good agreement for t1/3, t2/3 and t laws, respectively. We observed that the amount of microcrystals remains constant throughout the heat treatment process, as well as that the radii distribution has a lower limit and increases with heat treatment. The distribution of radii follows a distribution similar to the Lifshitz–Slyozov–Wagner distribution limited in the origin. Discussions led to the conclusion that the growth of CdS is a process that occurs after the fluctuating nucleation and coalescence phases. We then analyze the growth process, assuming that the evaporation is overcome by the precipitation rate, stabilizing all clusters with respect to dissolution back into the matrix. The problem was simplified neglecting anisotropy and the assuming a spherical shape for clusters and particles. The low interface tension was described in terms of an empirical potential barrier in the surface of the cluster. The growth dynamics developed considering that the number of clusters remains constant, and that the minimum size of these clusters grow with time, as the first order approximation showed a good agreement with the t law.

A New Postprocessing Strategy for Secondary Pollution: Synthesis of CdS Crystals

Eichhornia crassipes (E.C.) is a special aquatic plant, which exhibits excellent tolerance to heavy metals and can be used for the large scale removal of heavy metals from waste water. Here the E.C was used to capture Cd2+, then as an in situ bio-template to successfully synthesize photoelectric CdS microcrystals. The products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and fluorescence spectrometer. The results indicated that the peanut-like microcrystals with a length of 1.5–1.8 µm were oriented along (002) plane. The main organic groups in E.C including hydroxyl, primary amide and carboxylate played inducing and controlling roles during the formation of CdS microcrystals in the solution. The PL spectrum showed an emission peak at 450 nm and indicated that the product may have applications in an electronic light device. This approach provided an inspiration on the post-processing of the secondary pollution.

Mineralizer-Assisted Shape-Controlled Synthesis, Characterization, and Photocatalytic Evaluation of CdS Microcrystals

High-quality, well-crystallized, single-crystal, wurtzite CdS hexagonal microtowers (with and without tubular structure) and CdS with octahedral geometry were successfully synthesized by tuning the solvent ratio of water and ethanol for the first time on a large scale without using any catalysts or templates. The products were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectrometry, transmission electron microscopy, and field emission scanning electron microscopy. CdS with microtowers, octahedral geometry, and hexagonal plates was formed by thermal decomposition of cadmium thiocyanate at 400 °C under open atmospheric conditions. The influence of solvent, initial precursor concentration, decomposition temperature, and time on the morphology of CdS was investigated, and the results were discussed. The XRD results showed that the cadmium thiocyanate complex is completely decomposed into CdS at 300 °C for 2 h and the wurtzite phase was formed under all experimental conditions, which opens a door for well-crystallized, single-phase CdS synthesis on a large scale. The diffuse reflectance spectra showed that CdS microtowers have strong visible light absorption compared with the octahedral geometry, revealing the existence of a profound shape−property relationship of CdS. The CdS microtowers with a Pt cocatalyst showed a high rate of photocatalytic hydrogen production from water containing 0.1 M Na2S and 0.1 M Na2SO3 as sacrificial reagents under visible light irradiation (λ ≥400 nm). The possible growing mechanism of various types of the CdS morphology was also discussed.

Characterization of chemical-bath-deposited CdS thin films doped with methylene blue

The optical and electrical properties of methylene blue-doped CdS films, grown by chemical bath deposition, have been studied. Doping was achieved by adding specific volumes of methylene blue (MB) diluted in distilled water to the chemical bath. X-ray diffraction patterns display the zincblende crystalline structure for all the CdS samples, with a remarked preferred orientation along the (111) direction. The interplanar distance among the (111) planes indicates a minimum value when the relative volume (VR) of MB in the growing solution is about 7%. Photoacoustic absorption spectra show an evolution of the CdS band-gap energy (Eg) when VR increases, indicating adsorption of MB on the CdS microcrystals. Starting from an Eg of 2.42 eV for the undoped sample, an Eg value of nearly 2.49 eV is reached for the sample doped with VR=7% of MB. For higher concentrations, no increase is observed, showing rather a saturation effect. Dark conductivity (σ) measurements at room temperature (RT) display an increase of the conductivity from 1×10−4 Ω−1 cm−1, for the undoped film, up to 3.0×10−2 Ω−1 cm−1 for the highest doping value reached, that is when VR=7%. Thermoelectrical power measurements carried out at RT indicate that the layers are n-type doped, with a density of carriers (n) whose data basically oscillate in the range (5.3±1.3)×1016 cm−3. An exception is found for the sample with VR=7%, for which a value of n=1.02×1017 cm−3 is measured.

Highly Efficient CdS/CdTe Solar Cells Investigated by Cathodoluminescence Spectroscopy

Thin film solar cells consisting of a glass/ITO/CVD-CdS/CSS-CdTe/ Cu:C/Ag layer system with conversion efficiencies up to 14.4% and (5–8) μm thick CdTe layers as well as CdS(90 nm)/CdTe(6 μm) layers deposited on Si have been investigated by cathodoluminescence (CL) spectroscopy at 5 K. The spectral position of the CdTe exciton line was used to probe the interface region between the n-type CdS layer and CdTe grains at a cleaved edge of the solar cell and neighboring grains at the top of uncovered CdTe layers. For substrate temperatures (Tsub) smaller than 610 C during the CdTe deposition, the width of the intermixed region between the CdS layer and CdTe grains was found to be smaller than the lateral resolution of the CL (≈300 nm). For Tsub as large as (630−650) C, a decrease of the CdTe band gap appears to be present up to 0.6 μm from the CdS/CdTe interface. For the grain boundaries, however, a remarkable red shift of the exciton CL line is observed throughout the whole CdTe layer. A CdCl2 treatment homogenizes the distribution of acceptor-like defects or impurities leading to an optimized p-conversion of the CdTe layer. CL spectra of the CdS window layer exhibit two broad bands centered at 1.72 (red) and 2.04 eV (yellow). Other authors found similar bands in CdS microcrystals and in single crystal CdS bombarded with electrons indicating that the yellow band can be assigned to Cd interstitials. In the solar cells, the yellow band is suppressed by the CdCl2 treatment indicating a passivation or out-diffusion of Cd interstitials.

Second harmonic generation from thermally poled CdS microcrystal-containing glasses

The alkali borosilicate glasses containing CdS as a constituent were heat-treated or thermally poled to prepare CdS microcrystal-containing glasses. Second harmonic generation (SHG) from these glasses was measured, and the effects of the applied voltage on the crystallization behavior and SHG intensity were discussed. In poled glass samples, wurtzite type CdS microcrystals were precipitated at lower temperature than in unpoled ones. It was considered that the applied electric field affected the crystallization behavior of CdS crystals through the increase of temperature of the glass sample and migration of S 2− ions. The poled sample showed larger SHG intensity than the sample merely heat-treated, which was explained in terms of modification structure and orientation of precipitated CdS microcrystals.

Photoelectrochemical behavior of nanostructural composites based on polycrystalline cadmium sulfide

Nanostructural composites can be used as suspension electrodes; they are made by substitution cadmium(II) at the surfaces of CdS microcrystals for Cu(I, II), Fe(III), Bi(III), and Sb(III). Features are considered in the photoelectrochemical action mechanism of the various nanocomposites.

Structural Phase Transition of CdS Microcrystals under High Pressure

CdS microcrystals embedded in two kinds of matrices, SiO films and GeO2 glasses, with various sizes have been studied by Raman scattering and optical absorption under high pressure. A structural phase transition of the CdS microcrystals from wurtzite to rock salt phase was observed at a pressure higher than the bulk CdS transition pressure of 2.7 GPa. For CdS microcrystals embedded in SiO films, the phase transition pressure increased with decreasing microcrystal size. However, the phase transition pressure of CdS microcrystals embedded in GeO2 glasses does not depend strongly on the microcrystal size. These observations suggest that the matrix plays an important role on the phase stability of CdS microcrystals.

Structural Phase Transition of CdS Microcrystals under High Pressure

Structural Phase Transition of CdS Microcrystals under High Pressure

Structural phase transition of CdS microcrystals embedded in glassy matrix under high pressure

CdS microcrystals of various sizes embedded in a germanium dioxide glass matrix have been studied by Raman scattering and optical absorption under high pressure up to about 9 GPa. Structural phase transition of the CdS microcrystals from wurtzite to rock salt phase was observed at a pressure higher than 6 GPa, which is far above the bulk CdS transition pressure of 2.7 GPa. Under a pressure higher than 6 GPa, the 1-LO Raman intensity of the wurtzite phase of CdS microcrystals decreases gradually with time. After the pressure is released from about 9 GPa to atmospheric pressure, the high pressure rock salt phase is preserved at room temperature. We found that CdS microcrystals of the rock salt phase recovered to the wurtzite phase by temperature annealing at atmospheric pressure. Our observations suggest that the matrix plays an important role.

Phase transitions of CdS microcrystals under high pressure.

The experiment under high pressure using a diamond anvil cell (DAC) requires the utilization of synchrotron radiation. High-pressure experiments were performed using a DAC on CdS microcrystals, both in the far-IR and in the X-ray regions, in order to study the lattice dynamics and lattice stability concerned with the phase transitions. From these experiments, experimental evidence is presented indicating that a CdS microcrystal of smaller diameter shows a higher transition pressure for lattice transformation under pressure. The origin of such an increase in the transition pressure in the microcrystals is discussed in relation to the surface tension.

Erratum: Low-frequency Raman scattering from CdS microcrystals embedded in a germanium dioxide glass matrix [Phys. Rev. B47, 1237 (1993)

Erratum: Low-frequency Raman scattering from CdS microcrystals embedded in a germanium dioxide glass matrix [Phys. Rev. B<b>47</b>, 1237 (1993)

Size-Dependence of Phase Transition of CdS Microcrystals

We present experimental evidence which shows that the critical pressure (P c ) of the structural phase transition in CdS microcrystals under pressure differs from that of the bulk state and also that its magnitude depends on the microcrystal size. Far-infrared transmission measurements on CdS microcrystals of various sizes were carried out at pressure up to 8.3 GPa at room temperature by using synchrotron light source. The surface phonon at around 268 cm -1 at P=0 (atmosphere pressure) showed a blue shift with pressure. With further increase in pressure, the surface phonon showed a sudden decrease in energy. The very broad absorption band was found at the high pressure phase after the structural phase transition. Particle with smaller diameters was found to show higher P c values. The origin of the increase in the P c value is discussed related to thermodynamics and a surface tension of microcrystal.

Photonic band gaps in 3D ordered fcc silica matrices

The photonic band structure of 3D ordered silica matrices was studied by optical transmission measurements. A decrease of the light attenuation length within the forbidden gap was observed after filling of the opal pores with CdS microcrystals. The parameters of a photonic lattice required for a depletion of the photonic density of states were estimated.

Observation of Second-Generation Nonequilibrium Phonons in Highly Photoexcited CdS Microcrystals

Observation of Second-Generation Nonequilibrium Phonons in Highly Photoexcited CdS Microcrystals

Studies of grain size effects in rf sputtered CdS thin films

CdS thin films with various grain sizes were produced by rf sputtering and in situ postannealing processes. For a series of annealing time intervals, the average grain sizes of an as-deposited thin film with thickness ∼4000 Å were found to vary from ∼280 to ∼1500 Å. Three bands located in the green, yellow, and red regions were observed in photoluminescence (PL) spectra of these as-deposited films. The temperature dependence of (PL) spectra of CdS microcrystals was investigated. The variations of the (PL) spectra with average grain size and thickness of CdS thin films were also studied. It was found that the intensity of the yellow band was increased as increasing the film thickness while the intensity of the red band was decreased as the grain size of the film became larger. The peak positions of the yellow and red bands were found to be blue-shifted as the films became thinner.

Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects

The spectra of transmission and reflection of synthetic opal which has 3-dimensional periodic structure were measured at different orientations of incident beam relative to the sample facets. It is shown that opal behaves as «semi-metallic» photonic band gap (PBG) material in the vicinity of photon energy 2.3 eV. The synthesis of CdS microcrystals embedded in the pores of opal was made for the first time in an attempt to form a system of quantum dots. Optical spectra (reflection and transmission, photoluminescence and Raman scattering) were studied. The results demonstrate good crystallinity of microcrystals embedded in opal matrix and exhibit well-pronounced quantum confinement effects in fundamental edge absorption spectra. The spectral overlap of the PBG of opal with electronic band gap of many of II–VI semiconductors seems to make opal/semiconductor system a promising media for experimental studies of such PBG-related effects as inhibition of spontaneous emission, microcavity polariton, etc.

Influence of Microcrystallite Size on Photodarkening Effects in CdS-Doped Glasses

Size dependence of photodarkening effects in CdS-doped glasses was investigated using luminescence and electron spin resonance (ESR). As the microcrystallite size decreases, the ESR signal associated with the photodarkening effects increases. This result indicates that the photoinduced centers responsible for the photodarkening effects are created in the interface region between CdS microcrystals and glass matrix.

Pressure-induced structural phase transition of CdS microcrystals studied by raman scattering

The effects of finite size on the pressure-induced structural phase transition of CdS microcrystals have been studied by Raman scattering. Both 1-LO and 2-LO phonon modes increased slowly in intensity and then diminished rapidly with an increase in pressure, indicating the occurrence of the structural phase transition from a wurtzite to a rock salt phase. The pressure at which the structural phase transition occurred was found to be elevated gradually from 2.7 GPa (the phase transition pressure of bulk) as the particle size decreases. However, the elevation of the phase transition pressure for the CdS microcrystals prepared by a break-down method was observed at much larger size than that for the CdS microcrystals produced by evaporation in Ar gas and embedded in SiO films. The observation is explained by the presence of remarkable inhomogeneous strain in the former samples. It was also found that the samples with smaller particle size showed poorer recovery after being retrieved at 1 atm. We discuss the importance of inhomogeneous strains as well as defects and surface condition in understanding the pressure-induced structural phase transition in a finite system.

Photoreduction of carbon dioxide using chalcogenide semiconductor microcrystals

The photoreduction behaviour of CO 2 on cadmium-loaded ZnS microcrystals and ZnSCdS solid solution microcrystals was investigated. The photoreduction product on Cd-loaded ZnS was formate, and the highest quantum efficiency of production was 32.5%, which was twice as large as that obtained on bare ZnS microcrystals. However, solid solutions of ZnSCdS microcrystals did not exhibit high activities for the photoreduction of CO 2. With increasing mole fraction of CdS, the activities for formate production decreased, and the production of CO was observed for a CdS mole fraction of 0.5–0.67.