CdS Single Crystals Research Articles

Solid-CdS and hollow-ZnS porous submicrospheres: Direct preparation by C–S decomposition of thiol via solvothermal processing

Porous solid-CdS or hollow-ZnS submicrospheres (SMSs) were prepared by reaction of 2-mercaptoethanol as S-source with Cd 2+ or Zn 2+ in ethanol using solvothermal route. Without any alkaline, solid-CdS SMSs with an average diameter of 440 nm were obtained and their size distribution in the range of 390–490 nm is above 80%; With the addition of alkaline, single crystal CdS nanorods with high-temperature-stable hexagonal phase were synthesized. With or without poly( N-vinyl-2-pyrrolidone) (PVP), solid-CdS SMSs can be prepared, but the addition of the PVP leads to the preparation of relative uniform CdS-SMSs. Hollow-ZnS SMSs with an average diameter of 340 nm were obtained under the existence of alkaline. The longer reaction time hardly influences on the size of the hollow-ZnS SMSs. More or less amount of alkaline directly influences the morphology of final product. Close-ring-shaped ZnS was prepared under the existence of less alkaline. With or without PVP under preparation reaction, hollow-ZnS SMSs can also be synthesized. Thus, during preparation of either CdS or ZnS, the PVP function is not as template, but as surfactant. The BET measurement results show that the special surface areas of solid-CdS and hollow-ZnS SMSs are 25.56 and 83.55 m 2/g, respectively. The adsorption of methylene blue shows that the hollow-ZnS SMSs have very strong adsorption and decomposition ability.

Photoconductive Characteristics of Single-Crystal CdS Nanoribbons

The photoconductive characteristics of CdS single nanoribbons were investigated. The device characteristics, including spectral response, light intensity response, and time response, were studied systematically. It is found that CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size of nanoribbons has a significant influence on the response speed with smaller CdS nanoribbons showing higher response speed. The high photosensitivity and high photoresponse speed are attributable to the large surface-to-volume ratio and high single-crystal quality of CdS nanoribbons and the reduction of recombination barrier in nanostructures. Measurements in a different atmosphere demonstrate that the absorption of ambient gas (mainly oxygen) can significantly change the photosensitivity of CdS nanoribbons through trapping electrons from the nanoribbons.

A hydrothermal approach to flake-shaped CdS single crystals

The morphological transformation process from CdS nanorods to hexagonal CdS flakes was investigated in detail by transmission electron microscope (TEM). The CdS flakes were liable to form with the alkalinity being increased and the reaction time being prolonged. Using this transformation, hexagonal CdS flakes with diameters of 0.3 cm were grown via a recrystallization process in 6 mol/L sodium hydroxide solution at 250 °C. And the formation mechanism of CdS flakes is suggested based on the growth habits of polar crystals under proper basic hydrothermal conditions.

05/02580 Village electrification technologies — an evaluation of photovoltaic cells and compact fluorescent lamps and their applicability in rural villages based on a Tanzanian case study

Photovoltaics in Latin America are in their infancy. The research and development work is mainly directed to device physics and is done by groups in Mexico, Brazil and Colombia. Photovoltaics systems are now being used in some countries in the area. The Mexican case is special, not only in this region but amongst all the developing countries.Photovoltaic research and development in Mexico is done in three institutions and there are projects relating to single-crystal silicon, polycrystalline silicon, CdS, GaAs semiconductor-insulator-semiconductor and amorphous silicon solar cells. A special feature is a pilot manufacturing plant working with locally developed technology for 314 in siliconsolar cells.Photovoltaic applications include stand-alone systems such as meteorological equipment, rural telephone stations, maritime and railway-crossing signals, rural television receivers and seismological detectors on the commercial side, and educational and information television, rural telephone stations, water pumping and rural indoor and outdoor lighting on the experimental side. Reference is made to the anarchy in the Mexican market and prices. An excessively high price, around U.S. $8 W−1 per warranted year, prevents the expansion of the market. Commercial and industrial activities since the first trade representation in 1972 – 1973 are reviewed.

Synthesis of Single-Crystal CdS Microbelts Using a Modified Thermal Evaporation Method and Their Photoluminescence

Large-scale synthesis of single-crystal CdS microbelts was achieved by a modified thermal evaporation method, in which an adiabatic layer was used to provide abrupt temperature decrease and high gas concentration for the microbelt growth. The synthesized CdS microbelts were usually several micrometers in width, 70 nm in thickness, and tens to several hundred micrometers in length. Studies found that the adiabatic layer has great influence on the formation of CdS microbelts. Room-temperature photoluminescence indicated the disappearance of the emission at 1.77 eV, the appearance of an emission at 2.07 eV, and also the enhancement of the emission band at 2.38 eV, whose position shifted toward the blue region compared with conventional narrow CdS nanobelts. A possible growth mechanism of the CdS microbelts was briefly discussed.

Determination of the Concentration of Deep Levels in Semi-insulating CdS Single Crystals by Photoinduced-Current Transient Spectroscopy

Deep levels in semi-insulating CdS single crystals grown with a variation in the stoichiometric composition are investigated by photoinduced-current transient spectroscopy (PICTS). A series of deep levels with a thermal activation energy ranging from 0.066–0.54 eV is revealed. It is found that the signal ratio in the set of spectra disagrees with the basic PICTS model. A procedure for evaluation of the concentration of deep levels is developed for these conditions.

A critical analysis of investigation of deep levels in high-resistivity CdS single crystals by photoelectric transient spectroscopy

The investigation of high-resistivity CdS single crystals by photoelectric deep-level transient spectroscopy (PEDLTS) is discussed. Computer simulation of the experiment is carried out. This simulation enables us to compare the results obtained by the PEDLTS and thermally stimulated conductivity methods. The experimental potentials of these methods are compared.

Assessment of subsurface damage in polished II–VI semiconductors by ion channeling

Surfaces of bulk single crystal CdS, ZnSe and ZnO were prepared by mechanical polishing with 1 and 1/4 μm diamond abrasive slurries and by chemomechanical polishing with sodium hypochlorite:colloidal silica. Etched surfaces, indicative of original crystalline quality were also prepared. Near surface damage was investigated by ion channeling with He ions using incident beam energies of 2 and 5 MeV and detector positions of 75° and 13°. Damage depths were found to be significantly higher for ZnSe when compared to CdS or ZnO. The chemomechanical polishing process was seen to introduce small but measurable subsurface damage in CdS and ZnSe. However, channeling was unable to detect any damage for the chemomechanically polished ZnO surfaces, a result which was supported by cross-section transmission electron microscopy (XTEM). The presence of damage beyond that identified by the surface peak for the mechanically polished ZnSe surfaces was indicated by the dechanneling behavior below the surface.

Virus-based toolkit for the directed synthesis of magnetic and semiconducting nanowires.

We report a virus-based scaffold for the synthesis of single-crystal ZnS, CdS, and freestanding chemically ordered CoPt and FePt nanowires, with the means of modifying substrate specificity through standard biological methods. Peptides (selected through an evolutionary screening process) that exhibit control of composition, size, and phase during nanoparticle nucleation have been expressed on the highly ordered filamentous capsid of the M13 bacteriophage. The incorporation of specific, nucleating peptides into the generic scaffold of the M13 coat structure provides a viable template for the directed synthesis of semiconducting and magnetic materials. Removal of the viral template by means of annealing promoted oriented aggregation-based crystal growth, forming individual crystalline nanowires. The unique ability to interchange substrate-specific peptides into the linear self-assembled filamentous construct of the M13 virus introduces a material tunability that has not been seen in previous synthetic routes. Therefore, this system provides a genetic toolkit for growing and organizing nanowires from semiconducting and magnetic materials.

Ion‐Selective Adsorption/Desorption Processes at Inorganic Materials/Solution Interfaces as a Novel Mode for Ion Sensing†

Several studies on response mechanisms of solid membrane ion‐selective electrodes (ISEs) and developments of ion sensors mainly from the authors' laboratory were reviewed. Face‐specific responses of CdS single crystals showed that ion‐selective response of solid membrane ISEs based on scarcely soluble salts is due to adsorption of component ions, and resultant charge separation at the membrane/solution interfaces. Potentiometric response mechanisms of CuS‐based ISEs in metal buffer solutions were discussed. Crystal surfaces of scarcely soluble inorganic salts were formed to provide remarkably selective molecular recognition sites at which adsorbed component ions were detected by an underlying quartz crystal microbalance (QCM). Not only single layer adsorption of component ions on the respective inorganic salt, but also multilayer crystal formations at QCM surface also showed ion‐selective response to the respective component ions. Ion selectivities of ion sensors based on scarcely soluble salts, which include ISE and QCM, are basically limited by solubility products. Several studies on ISEs based on inorganic materials having a three‐dimensional (3D) network structure with interstitial ions demonstrated that the formation of vacant sites by dissolution of the interstitial ions from the membrane surfaces is prerequisite for ion‐selective responses. For these ISEs, potentiometric selectivities are essentially governed by size and dehydration energies of analyte ions. Taken these together, approaches for ion‐sensor developments based on inorganic materials were critically evaluated and updated. †In memory of Professor Harry B. Mark Jr., who greatly contributed to the progress of modern electrochemistry and electroanalytical chemistry.

Sonochemical synthesis of large-scale single crystal CdS nanorods

Large-scale single crystal CdS nanorods were successfully achieved by ultrasound irradiation of Cd(EDTA) and Na 2S 2O 3 in deionized water solution. The obtained CdS nanorods have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electronic diffraction (SAED) and X-ray photoelectron spectrometry (XPS), and appear to be structurally uniform with diameter of about 80 nm and length of up to 1.3 μm, and with single crystalline, where the growth mechanism has been tentatively discussed.

Photoreflectance spectroscopy of wurtzite CdS

Photoreflectance (PR) spectra have been measured to determine the lowest direct-band edge E0α (α=A, B, and C) of wurtzite CdS single crystal in the temperature range T=13–300 K for both E⊥c and E∥c polarizations. The measured PR spectra can be interpreted by the three-dimensional (3D) excitonic plus one-electron line shapes over the entire temperature range. The temperature dependence of the excitonic and critical-point parameters (energy, amplitude, and broadening parameter) have been determined and analyzed using the Varshni [Physica (Amsterdam) 34, 149 (1967)] equation and an analytical four-parameter expression recently developed for the explanation of the band-gap shrinkage effect in semiconductors. The 3D- exciton binding energies have also been determined to be 27 (A), 31 (B), and 30 meV (C), respectively.

Growth and characterization of CdS and doped CdS single crystals

CdS and phosphorus, silicon and selenium doped CdS have been grown by physical vapour technique. The quartz ampoule used for the growth is designed according to Schieber's (Technology of crystal growth, ULCA, Extension course Engineering, 1974, p. 881) conditions. The growth temperature is 915°C with undercooling of 25°C. For better transport the starting material is prepared as needle-like polycrystalline crystal under high temperature gradient. The crystals of cm size are formed like boule. The growth pattern of different boule is analysed by taking AFM photograph and growth observed in both polar directions. The Hall mobility is measured for the pure and doped CdS. At room temperature mobility is found to be 335 (cm 2/V s) and the dislocation density to be 5×10 6 cm −2. From photocurrent spectra the optical band gap is found for CdS, CdS:P, CdS:Se, and CdS:Si using Ex( A) exciton peak.

Characterisation of thin films of CdS deposited on Ag(111) by ECALE. A morphological and photoelectrochemical investigation

The electrochemical atomic layer epitaxy (ECALE) methodology is a valid and low-cost approach for thin film formation on a metallic substrate. This paper reports on a morphological and photoelectrochemical investigation carried out on CdS samples formed with various numbers of deposition cycles. Ex-situ atomic force microscopy (AFM) measurements showed the attainment of a well-defined morphology, with an almost constant size and density of clusters. The layer-by-layer growth mechanism that is the aim of ECALE methodology, and that is strongly suggested by the electrochemical characterisation, seems to be confirmed. The photoresponse of the material, evaluated in alkaline polysulphide medium, shows a spectral dependence in very good agreement with literature reports for CdS single crystals.

Catalytic Growth of Large-Scale Single-Crystal CdS Nanowires by Physical Evaporation and Their Photoluminescence

Large-scale CdS nanowires were achieved by a new, simple, and low cost process based on thermal evaporation of CdS powders under controlled conditions with the presence of Au catalyst. The synthesized CdS nanowires have lengths up to several tens of micrometers and diameters about 60−80 nm. The growth of CdS nanowires is controlled by the conventional vapor−liquid−solid (VLS) mechanism. A strong red emission with a maximum around 750 nm was observed from the synthesized CdS nanowires at room temperature, which was attributed to their surface states. The technique we used is, in principle, generalizable as a means of fabricating other group II−VI semiconductor nanowires.

Electrical, optical, and photoelectric properties of electron-irradiated indium-doped cadmium sulfide single crystals

The effect of irradiation by 1.2-MeV electrons to a dose Φ=2×1017 cm−2 on the electrical, optical, and photoelectric properties of In-doped CdS single crystals was studied. The experimental data obtained permit one to conclude that irradiation initiates decomposition of the supersaturated In solution in CdS, with the indium atoms at the sites of the cation sublattice being expelled by cadmium interstitial atoms. New slow-recombination centers were observed to exist in the irradiated CdS: In samples, with the maxima of optical quenching of the photoconductivity lying in the region of \(\lambda _{M_1 } = 0.75\mu m\) and \(\lambda _{M_2 } = 1.03\mu m\). It is suggested that the new recombination centers are related to complexes containing cadmium vacancies and indium atoms.

Numerical simulation of dynamics of GHz acoustic phonon fluxes amplified from thermal background phonons in a n-type CdS single crystal.

Numerical simulation of dynamics of GHz acoustic phonon fluxes amplified from thermal background phonons in a n-type CdS single crystal.

Dye sensitization of antimony-doped CdS photoelectrochemical solar cell

Sb-doped CdS single crystal was used as a photoanode to fabricate a photoelectrochemical solar (PECS) cell. The three organic dyes; eosin, thymol blue and rhodamin 6G were used as sensitizers in (PECS) cell. In the absence of the dye, the results showed that with Sb-doped CdS single crystal electrode, a higher power conversion efficiency ∼9.27% has been achieved compared to ∼5.7–7.4% for pure crystal. Application of the dye in PECS cell increases the efficiency to about 13%. The efficiency reaches its maximum value when the dye concentration is (2.5×10 −5)M, sufficient to cover the surface of the semiconductor electrode with a continuous monolayer of the dye. Exceeding this value resulted in a gradual decrease of the efficiency from its maximum value. Mott–Schottky plots gave a doping density of 3.14×10 17 cm −3 and a space charge width of ∼4.95×10 −6 cm for the sample used. A flat-band potential equal to −0.84 V, independent of both frequency and pH, was also predicted. Cyclic voltammetry (c.v.) measurements showed an anodic current peak at 0.4 V vs. SCE. The disappearance of this peak after excess addition of the reducing agent Na 2S, indicates that this peak is due to the PEC corrosion of the semiconductor electrode.

Influence of substrate structure on the growth of CdTe thin films

We report on a high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy study of the microstructure of CdTe thin films epitaxially grown on single-crystal hexagonal CdS and cubic CdTe substrates. We find that the different structures of the substrates do not make a great structural difference on the grown CdTe films; i.e., on both substrates, the grown CdTe films have a cubic structure and high density of planar defects near the interface regions. At the CdTe/CdS interface, interdiffusion occurs, forming CdTe1−xSx and CdS1−xTex alloys. These alloys lead to significantly reduced mismatch at the interface.

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.