CdS Single Crystals Research Articles

Laser Technologies in Spintronics and Nanoelectronics as the Method of Changing the Structure and Magnetic Characteristics of Thin Films

In the present article we want to consider some features of not thermal influence of laser pulses on multilayer heterogeneous nanofilms to present the results of our experimental researches of change of the roughness of a surface and magnetic characteristics of permalloy films after their irradiation nanosecond laser pulses and the results of measurement of dynamics of magnetic reversal of magnetic tunnel nanostructures with one and two magnetic nanolayers. It is shown that the photon drag effect of electrons can not only generate an electric potential difference between the input and output surfaces in a semiconductor, but may also lead to a drift of the impurities. The results of our research show that in the thin CdS single crystals can be obtained stimulated emission of electromagnetic radiation in the terahertz frequency range.

Catalyst- and template-free low-temperature in situ growth of n-type CdS nanowire on p-type CdTe film and p-n heterojunction properties.

CdS is an important semiconductor used in optoelectronic devices. Simple techniques for growing CdS nanostructures are thus essential at a low cost. This study presents a novel method for growing single-crystal n-type CdS nanowires on p-type CdTe films by thermal annealing in an H2S/N2 mixed gas flow, which does not require the help of a catalyst or template. The formation process and growth mechanism of the nanowires are investigated. Well-dispersed whiskerlike CdS nanostructures are obtained at an appropriate annealing temperature and duration. We suggest that the stress-driving mechanism of nanowire formation may contribute to the growth of CdS nanowires, and that the evaporation of Te through the boundaries of the CdS grain seeds plays an important role in the sustainable growth of nanowire. In addition, CdS/CdTe heterojunction device is fabricated on Mo glass. The I-V characteristic of the heterojunction in dark shows typical rectifying diode behavior. The turn-on voltage can be regulated by annealing conditions. Meanwhile, the obvious photovoltaic effect is obtained on the in situ growth heterojunction prepared at low annealing temperature. Hence, this is a new fabricated method for CdTe-based materials in the field of energy conversion.

Solubility of oxygen in CdS single crystals and their physicochemical properties

The specific features of oxygen dissolution in CdS using the example of single crystals grown by the gas-transport method with deviations from stoichiometry at 1100°C are considered. The effect of various types of intrinsic point defects in crystals of different composition on the form in the presence of oxygen is analyzed. It is shown that the most stable composition thermodynamically is that corresponding to nonstoichiometric “self-activated cadmium sulphide” stabilized with oxygen.

Electronic structure study of the CdS buffer layer in CIGS solar cells by X-ray absorption spectroscopy: Experiment and theory

A systematic investigation of the electronic structure of the CdS buffer layer of CIGS solar cells has been undertaken using S K-edge X-ray absorption spectroscopy (XAS), both experimentally and theoretically. We found from XAS that growing CdS films by chemical bath deposition (CBD) exhibits more long-range disorder when compared to single crystal CdS, CdS grown by atomic layer deposition (ALD) and theory. We investigated the significance of a variety of point defects and potential atomic substitutions in first-principles estimates of the differential S K-edge XAS sensitivity. We find that substituting some sulfur atoms with e.g. oxygen or selenium does not introduce significant changes to the partial S(3p) density of states of the conduction band in CdS and we infer that the electronic structure modifications of these defects are spatially localized and do not hybridize strongly with bands with significant sulfur p character. Moreover, by comparison with experimental XAS should be sensitive to CdSO4; however this is not in the CdS layers of CIGS devices studied here. We also find that the CBD of CdS on FTO-glass and separately on CIGS does not produce any significant changes in the local atomic structure of CdS indicating no CIGS-influenced growth.

Optical properties of impurity Co(3d) and Er(4f)-doped CdS single crystals

Optical properties of impurity Co(3d) and Er(4f)-doped CdS single crystals

Spectroscopy of quasi-atomic nanostructures

A theory is developed for a superatom composed of a spatially separated electron and hole. The electron in this case is localized above a spherical interface. The hole moves in the volume of a semiconductor quantum dot (a quantum-dot–dielectric matrix). An effect is detected consisting of a substantial increase (by a factor of 4.1–72.6) of the binding energy of a superatom containing a cadmium sulfide quantum dot by comparison with the binding energy of an exciton in single-crystal CdS. The possibility is discussed of experimentally studying superatoms and their role in various regions of physical phenomena.

Anomalous Series of Bands in the Edge Emission Spectra of CdS(О)

The region of the edge emission spectrum of CdS(O) single crystals with cadmium excess is examined. An anomalous series of equidistant bands with leading line at 514 nm and phonon replicas has been revealed. These bands grow in intensity with increase of the excitation density up to 1026–1027 cm–3⋅s–1 at 80 K, and the leading line of the series is observed even at 300 K. It is shown that luminescence is conditioned by the exciton spectrum in perfect bulk single-crystals of CdO. Some characteristics of this spectrum are presented, in particular, the dependence on temperature, excitation intensity, composition and size of the crystals, and the LO interaction. The results experimentally confirm the theoretically calculated magnitude of the direct band gap of CdO.

Nanocrystalline Oxides: CdS nanowires synthesized by solvothermal method

Recent advances in the field of nanotechnology produced an assortment of one-dimensional (1D) structures, such as nanowires and nanorods. These fascinating materials are the potential building blocks for a wide range of nanoscale electronics, optoelectronics, magnetoelectronics, or sensing devices [1]. Parallel to the success with group IV and groups III–V compounds semiconductor nanostructures, semiconducting metal oxide materials with wide band gaps are attracting attention [2-3]. The main aim of this communication is to report our results on the application of several new techniques, particularly the use of hydrothermal synthesis, to fabricate single crystal one-dimensional nanostructured materials, study their growth processes, understand the growth mechanisms and investigate their physical properties. A wide range of remarkable features are then presented, to cover a number of metal oxides, such as ZnO, Sb2O3, CdS, MgO, α-Fe2O3, or TiO2, describing their structures, optical, magnetic, mechanical and chemical sensing properties. These studies constitute the basis for developing versatile applications based on metal oxide 1D systems as well as highlighting the current progress in device development. To exemplify, the as-prepared CdS nanowires have average 28 nm in diameter and length up to several micrometres. The direct band gap of the CdS nanowires is 2.56 eV calculated by the UV-vis absorption spectra. The PL spectrum has two distinct emission bands at 502 nm and 695 nm, which are associated with the near-band-edge emission and defect emission, respectively. These synthesized single-crystal CdS nanowires have a high potential in the optoelectronic applications of nanolasers, solar cells, lighting-emitting diodes or photodetectors. Acknowledgments: Erasmus Mundus MEDASTAR (Mediterranean Area for Science, Technology and Research) Programme, 2011–4051/002–001-EMA2, Spanish MINECO (MAT2010-15094, Factoría de Cristalización – Consolider Ingenio 2010) and ERDF.

Solvothermal synthesis and characterization of one-dimensional CdS nanowires

One-dimensional single crystal wurtzite phase CdS nanowires were successfully synthesized through a facile solvothermal method. The obtained CdS nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis absorption spectroscopy, Raman spectrum, N2 adsorption–desorption isotherm experiment and photoluminescence spectrum (PL). The SEM and TEM results show that the diameter of the obtained CdS nanowires is about 50nm and the length is several micrometers, growing along [100] direction. UV–vis spectrum exhibits a strong absorption peak centered at 486nm. Raman spectrum shows there are three vibration modes at 300, 600 and 900cm−1. N2 adsorption–desorption isotherm experiment shows that the BET specific surface area of CdS nanowires is 21.97m2g−1. Room temperature photoluminescence spectrum shows the CdS nanowires has one emission band at 524nm and a shoulder band at 700nm.

Facile fabrication of CdS nanorod arrays on the transparent conducting substrates and their photoelectrochemical properties

Oriented and high-density CdS nanorod arrays were successfully grown on transparent conductive substrates by facile chemical bath deposition and hydrothermal method free from organic solvents and surfactants. The morphology, structure and phase composition of these samples were examined, and the results demonstrated that the high purity and uniform single crystal hexagonal CdS nanorod array was obtained. Further investigation revealed that chemical bath deposition time played an important role in the formation of high quality CdS nanorod array. In addition, the optical absorption spectra and photoelectrochemical properties were also studied. It was found that the CdS nanorod film demonstrated superior photoelectrochemical properties when the chemical bath deposition and hydrothermal time were 30min and 14h respectively, which was ascribed to the large absorption coefficient of the material and the excellent charge collection efficiency of the nanorods. These results demonstrated that the CdS nanorod film had a potential application in various nanostructured optoelectronic devices.

Single crystal CdS nanowires with high visible-light photocatalytic H2-production performance

Single crystalline hexagonal CdS nanowires were prepared by a solvothermal method using ethylenediamine as a solvent. Pt/CdS nanocomposites were produced by different reduction methods and characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, UV-visible spectroscopy, nitrogen absorption, pulse H2 chemisorption and X-ray photoelectron spectroscopy. The photocatalytic performance of the as-synthesized Pt/CdS nanocomposites was evaluated towards H2 generation from a lactic acid aqueous solution under visible light (λ ≥ 420 nm). The sample prepared by the NaBH4 reduction method showed a higher photocatalytic H2-production activity than the sample prepared by the photo-reduction deposition method. A remarkable H2-production rate of 1.49 mmol h−1 with a quantum efficiency (QE) of 61.7% was first achieved on the former with a small amount of Pt (0.3 wt%) as a co-catalyst. The photocatalytic activity difference of the samples is because reduction methods influence the size and dispersion of Pt nanoparticles (NPs), which further influence the photocatalytic H2-production activity. The sample from the NaBH4 reduction method contains smaller Pt NPs (ca. 1–2 nm) and has higher Pt dispersion (72%), thus resulting in the higher H2-production activity. Further experiments showed that the one-dimensional nanostructure markedly enhanced the photocatalytic H2-production activity of CdS. After ball milling of the nanowires, which leads to destruction of their structure, the photocatalytic H2-production activity of CdS decreases by more than 30%. The underlying mechanism for the observed photocatalytic H2-production performance of Pt/CdS nanocomposites was discussed.

Features of the exciton spectra of CdS(O) single crystals

The exciton cathodoluminescence spectra of CdS(O) single crystals are investigated depending on the depth (0.3–125 μm). It is found that the CdS(O) solid solution is stable in the bulk, where the crystal lattice provides the compensation of strains due to the isoelectronic impurity OS. Near the surface (to 0.5 μm), the CdS(O) solid solution can lose oxygen when forming CdO or SO2. The stimulated radiation wavelength of the CdS(O) crystals depends on the OS concentration in the bulk, which corresponds to the data of analyses.

Synthesis and characterization of single-crystal CdS nanosheet for high-speed photodetection

One-dimensional nanostructures have several unique advantages over bulk material and thin films, which can be exploited for high-speed photodetection. Furthermore, as bulk CdS has a high photosensitivity and quantum efficiency, there is considerable potential for the use of CdS nanostructures in advanced devices. In this study, single-crystal CdS nanosheets were grown by thermal evaporation and fully characterized to determine their potential for application in high-speed photodetectors. A high-quality nanosheet was confirmed to have a smooth surface with no extraneous particles and a strong orientation to the (110) plane of the wurtzite (hexagonal) phase of CdS. The Cd/S ratio was found to be nearly stoichiometric at 1.09. Photoluminescence measurement of a single-crystal CdS nanosheet showed a high emission intensity at a wavelength of 493nm. The current–voltage characteristics of the CdS nanosheet on Al thin film indicated an Ohmic contact in dark and under illumination by ambient, 365-nm, 405-nm, and 460-nm light. The light responsivity showed a peak at 460nm. Under 365-nm, 405-nm, and 460-nm chopped light, at a bias voltage of 1, 3, and 5V, the photocurrent rise and decay times were investigated. The device showed faster response times for 460-nm light. This fast response was attributed to the high quality of the single crystal, the absence of defect states, and the high surface/volume ratio. The device showed a high quantum efficiency of 22.3×103% when it was illuminated by 365-nm light under a bias of 5V; this efficiency increased to 36.3×103% and 40.5×103% when the device was illuminated by 405-nm and 460-nm light, respectively.

Radiation from a semiconductor target excited by an electron beam in a gas diode

Radiation from a semiconductor target excited by an electron beam in a gas-filled diode was investigated at different gas (air) pressures. Subnanosecond high-voltage pulses (up to 200 kV) were applied to the pointed cathode of the diode. The targets in the form of a 15 — 20-μm-thick single-crystal CdS film with reflecting coatings forming an optical cavity and 0.7 — 1-mm-thick ZnSe plates were used. As the air pressure increased from 0.1 to 5 Torr, a decrease in the amplitude and duration of laser radiation pulses from the targets was observed. Lasing (λ = 520 nm) of CdS targets terminated at pressures greater than 2.2 Torr. The laser pulse duration varied from 125 to 20 ps. The study of the dynamics of radiation from ZnSe targets (λ = 460 nm) at atmospheric pressure showed that when the gap between the target and the electrodes was 0.2 — 1 mm thick, an intense near-surface glow was observed that consisted of a few pulses with the duration from 20 to 100 ps, caused by runaway electrons. Investigations showed that the runaway electrons may play an essential role in the excitation of semiconductors by subnanosecond high-voltage pulses.

Controlling crystal growth orientation and crystallinity of cadmium sulfide nanocrystals in aqueous phase by using cationic surfactant

Dispersed CdS single crystals were obtained via a hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as a surfactant. The strong interaction between the cationic head group of CTAB and the (001) plane of CdS hexagonal crystals results in a single-crystalline growth along the [100] direction.

Low-temperature synthesis of ZnO/CdS hierarchical nanostructure for photovoltaic application

Hierarchical nanostructures involving primary wide-band-gap nanowires and secondary narrow-band-gap branches are promising for photovoltaic application due to their excellent properties for light harvesting and fast carrier transport. In the present work, we developed a low-temperature process for facile synthesis of ZnO/CdS hierarchical nanowires, where the primary ZnO nanowires were first prepared via a hydrothermal route and then the secondary single-crystal CdS tips were grown on the ZnO nanowires by electrochemical deposition. The as-grown hierarchical ZnO/CdS nanowires are superior in charge separation as well as carrier transport, thus achieving higher open circuit voltage, short circuit current and final conversion efficiency than the common coaxial nanocables with CdS nanocrystal shell on core ZnO nanowires.

Template synthesis of nanocomposite CdS with nonlinear optical properties

Nanoparticles of CdS were isolated by the treatment of thereaction mixture followed by the precipitation of CdS ina centrifuge, and the precipitate was washed with ether.Nanoparticles of CdS in pyridine were mixed in an ultrasonic stirrer with poly(methyl methacrylate) (PMMA) inan amount of 7% of the polymer weight. Then the suspension obtained was poured on a quartz support and dried at60 °C. The resultant sample was a film 0.3 mm thick. Theabsorption spectrum of the nanocomposite СdS—PMMAexhibits a shift to the blue region compared to that ofa single crystal of CdS. This shift is due to the quantumdimensional effect characteristic of nanoparticles of semiconductor crystals.

Influence of defects formed by fast reactor neutrons on exciton luminescence spectra of cadmium sulfide single crystals

The influence of clusters of defects formed by irradiation with neutrons in CdS single crystals on parameters of the spectra of exciton photoluminescence at T ∼ 4.2 K has been studied. It is experimentally established that irradiation of the samples with fast reactor neutrons with energies E ∼ 1 MeV and dose Φ = 3 × 1018 cm−2 brings about a decrease (by approximately 50 times) in the intensity of lines of exciton photoluminescence I1 (λm = 488.7 nm), I2 (λm = 486.9 nm), and I3 (λm = 486.3 nm) with redistribution of intensities in favor of the lines I1 and I3 and an increase in their half-width from 2 to 5–6 the positions of their peaks in the emission spectrum. Observed experimental facts are explained on the basis of a model of a two-phase system composed of slightly damaged region in the CdS single crystal in which clusters of defects (formed as a result of irradiation with neutrons) are embedded; these clusters are nanosized grains with a highly disordered structure.

Features of subthreshold defect formation in CdS and CdS:Cu single crystals subjected to irradiation with X-ray photons

Experimental results of the study of the effect of irradiation with 230-keV electrons and with X-ray photons with energies of 8.06 and 17.5 keV and the effect of quenching on the formation and reconstruction of the centers of slow recombination of nonequilibrium charge carriers (the so-called r centers) in nominally undoped and Cu doped (N Cu ∼ 1018 cm−3) CdS single crystals are reported. It is shown that defects in cadmium sublattice in the crystal (specifically, the V Cd vacancies and the CuCd defects with parameters close to those of the above vacancies) are responsible for the r centers. In the case of the X-ray irradiation of both undoped and Cu-doped CdS single crystals, subthreshold defect formation of cadmium vacancies and CuCd defects takes place; this occurs at sites with distorted and weakened interatomic bonds, i.e., at “weak sites” near large structural imperfections of the lattice, of technological or other origin. Starting with a quenching temperature 170°C, the spectrum of slow-recombination centers is appreciably affected by thermally formed vacancies V Cd and secondary defects CuCd. At quenching temperatures higher than 250°C, a significant contribution to the spectrum of optical quenching of photoconductivity is made by thermally introduced free (away from structural imperfections) r centers, i.e., the V Cd and CuCd defects.

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass μ = μ(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy Eex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii aex, the exciton binding energy Eex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.