Bulk ZnS Research Articles

Role of growth parameters on structural and optical properties of ZnS nanocluster thin films grown by solution growth technique

In this study, the nanosized ZnS thin films that can be used for fabrication of blue light-emitting diodes, electro-optic modulators, and n-window layers of solar cells were grown by the solution growth technique (SGT), and their structural and optical properties were examined. Based on the results obtained, the quantum size effects of ZnS were systematically investigated. Governing factors related to the growth condition were the concentration of precursor solution, growth temperature, concentration of aq. ammonia and growth duration. X-ray diffraction patterns showed that the ZnS thin film obtained in this study had the cubic structure (β-ZnS). With decreasing growth temperature and decreasing concentration of precursor solution, the surface morphology of film was found to improve. In particular, this is the first time that the surface morphology dependence of ZnS film grown by SGT on the ammonia concentration is reported. The energy band gaps of samples were determined from the optical transmittance values, and were shown to vary from 3.69 to 3.91 eV. These values were substantially higher than 3.65 eV of bulk ZnS demonstrating that the quantum size effect of SGT grown ZnS is remarkable.

Island Formation at the Initial Stages of Epitaxial ZnS Films Grown by Single Source Chemical Vapor Deposition

Chemical vapor deposition of single-source precursor zinc diethyldithiocarbamate Zn[S 2 CN(C 2 H 5 ) 2 ] 2 and subsequent decomposition on heated Si( 111) has been shown to produce epitaxial ZnS films. During the initial growth studies, X-ray photoemission spectroscopy indicated a relatively high concentration of carbon at the interface, which decreased with increasing film thickness. The interfacial carbon is attributed to chemisorption of byproducts during precursor decomposition. The higher than expected binding energy of Zn 2p 3 / 2 for the ultrathin films (∼5 A) approached the bulk ZnS value as film thickness increased (∼2000 A). This was ascribed to changes in crystallite size, which resulted in different core-hole screenings. The combined results are related to a kinetic process in which various carbon-terminated sites on Si(111) surface inhibited the two-dimensional coalescence of ZnS clusters. The films were initially grown via formation of epitaxial three-dimensional islands. Our results suggest that while the precursor chemistry and associated byproduct concentration can significantly influence the growth mechanism, the epitaxial driving force is sufficiently strong to overcome such chemical defects at the interface.

Monte Carlo simulation of low‐energy electron trajectories and energy loss in ZnS phosphor powders

AbstractDuring electron beam irradiation of ZnS phosphor powders, a non‐luminescent ZnO layer is formed on the powder due to electron‐beam‐stimulated surface reactions. As the thickness of the oxide layer increases, the energy loss in the ZnS bulk decreases with a subsequent degradation in cathodoluminescence. Using the Monte Carlo technique, the trajectories of low‐energy electrons were simulated in a ZnS phosphor powder with a ZnO overlayer of varying thickness based on recent models describing the energy loss and scattering angles of low‐energy electrons in a solid. A diffusion interface between the ZnO layer and ZnS bulk was simulated by varying the concentration of O and S atoms in the interface. Modelling the interface in this way describes the electron trajectories and energy loss in the interface region, because a sharp interface between two dissimilar layers very seldom exists. In the energy‐loss profiles the transition between ZnO and ZnS corresponds to a sharp increase in energy loss due to the increased rate of energy loss of electrons in ZnS. The diffusion interface has a smoothing effect on this sudden increase. From the electron trajectory data and corresponding energy loss, energy loss profiles were determined indicating the cumulative distribution of all the electron energy losses as a function of the interaction volume depth and thickness of the ZnO layer. When a distribution of incident angles is used, the profile differs from the typical energy‐loss profile seen at normal incident angles. As the thickness of the ZnO layer increases, the total energy loss in the solid decreases due to the increase in the backscattering coefficient of electrons in ZnO. Copyright © 2001 John Wiley & Sons, Ltd.

Factors Influencing the Luminescence Quantum Efficiency of Nanocrystalline ZnS:Mn2+

The influence of the Mn2+ concentration, the nature of the passivating polymer and UV irradiation on the luminescence efficiency of nanocrystalline ZnS:Mn2+ is investigated. With increasing Mn2+ concentration the quantum efficiency increases, until it reaches a stable value. No drastic decrease in quantum efficiency is observed at high Mn2+ concentrations, as was found earlier for both bulk and nanocrystalline ZnS:Mn2+. By altering the passivating polymer many other factors influencing the quantum efficiency are changed, such as the particle diameter and Mn content. This makes it hard to study the influence of the passivating behaviour of the various polymers on the quantum efficiency. The UV enhancement of the quantum efficiency of the various samples could be explained by two mechanisms: UV curing of the passivating polymer and passivation by a photo-oxidation of the surface of the nanoparticles during the irradiation. The highest quantum efficiency obtained was about 15%.

Pressure Tuning Spectroscopy of Mn2+ in Bulk and Nanocrystalline Sulfide Semiconductors

ABSTRACTWe report the results of high pressure luminescence studies of the emission of Mn2+ in bulk Zn0.55Ga0.30S up to ∼214 kbar, bulk ZnS up to ∼184 kbar, and nanocrystalline ZnS (∼8 nm particle sizes) up to ∼300 kbar. We observed a strong redshift with pressure (-30(3) cm−1/kbar) for the 4T1→6A1 emission transition of Mn2+ in all three systems. We also observed emission quenching at high pressure in all three systems and attribute the quenching to a pressure induced phase change of the ZnS host lattice to a rocksalt (NaCl) phase.

Sonoluminescence effects in ZnS materials

A detailed study of sonoluminescence (SL) in ZnS bulk and powdered materials is presented. It is argued that high-frequency vibrations of dislocations in acoustic fields constitute the specific mechanism of a light production related to point defects and complexes of the defects generated by an acoustic wave. The SL spectra in bulk ZnS samples are found to originate from the defect complex (VZn−,Ac) of a zinc vacancy and a single-charged acceptor in a cation site. The computed energy levels are in excellent agreement with the experimentally observed SL peaks at about 512, 526, 540 and 560±10nm. The SL peak at 500nm is suggested to arise from the radiative transitions of free electrons to the energy band formed by moving dislocations. The defect related emissions are partially responsible for the SL bands in powdered ZnS materials whereas the broadness of the SL spectra can be explained in the framework of radiative transitions of free electrons within the conduction band. Possible applications of the phenomenon of crystal SL are outlined.

Energy structure and fluorescence ofEu2+in ZnS:Eu nanoparticles

Eu2+-doped ZnS nanoparticles with an average size of around 3 nm were prepared, and an emission band around 530 nm was observed. By heating in air at 150 degrees C, this emission decreased, while the typical sharp line emission of Eu3+ increased. This suggests that the emission around 530 nm is from intraion transition of Eu2+: In bulk ZnS:Eu2+, no intraion transition of Eu2+ was observed because the excited states of Eu2+ are degenerate with the continuum of the ZnS conduction band. We show that the band gap in ZnS:Eu2+ nanoparticles opens up due to quantum confinement, such that the conduction band of ZnS is higher than the first excited state of Eu2+, thus enabling the intraion transition of Eu2+ to occur.

Monte Carlo simulation on the electron beam incident angle with spherical particles applied to the energy loss in ZnS phosphor powders

During electron beam irradiation of ZnS phosphor powders, a non-luminescent ZnO layer is formed on the powder due to electron-beam-stimulated surface reactions. As the thickness of the oxide layer increases, the energy loss in the ZnS bulk decreases with a subsequent degradation in cathodoluminescence (CL). Owing to the morphology of the phosphor powder, growth of the oxide layer leads to varying effective ZnO thicknesses. In this paper the effect of the interaction between the electron beam and the ZnO/ZnS powder particles on the energy loss in the ZnS is studied using Monte Carlo simulation techniques. In the simulation, an electron beam with a certain profile is spread according to a Gaussian distribution function over the phosphor particles. These particles consists of both flat and spherical-shaped grains. The incident angle between the electron beam and the particle's surface is determined by using simple vector analysis. If the beam contains a sufficient number of electrons, a distribution of incident angles can be obtained. It was found that this distribution is independent upon the profile of the electron beam as long as the full width at half-maximum (FWHM) beam diameter is larger than the size of the irradiated particles. Using these results a comparison was made between the energy loss in the ZnS particles as a function of the ZnO thickness with and without the angular distribution. A publicly available Monte Carlo code for electron trajectory simulations in solids, called CASINO, was used for this purpose. When the distribution is considered, there is a decreased energy loss in the ZnS bulk. Because the light generation during electron beam irradiation is proportional to this energy loss, the morphology of the phosphor powder also contributes to the CL degradation. Copyright © 2000 John Wiley & Sons, Ltd.

Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride

Ensemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.

Mn-doped nanometer-size ZnS clusters in chitosan film matrix prepared by ion-coordination reaction

A novel method, the so called ion-coordination reaction, was introduced to prepare nanometer-sized Mn-doped ZnS clusters stuck to a chitosan medium. A possible growth mechanism of the Mn-doped ZnS clusters in a chitosan film has been proposed. The X-ray diffraction showed that the ZnS clusters have a cubic structure like bulk crystals and the cluster sizes were estimated from the diffraction linewidth to be 2–4 nm based on Scherre's equation. The emission of Mn ion was observed through band-to-band excitation of ZnS. The excitation spectrum showed a large blue shift relative to that of ZnS bulk material. The effect of Zn ion concentration on the luminescence intensity reveals that the emission efficiency of Mn ion increases with the decrease of the ZnS cluster size.

Strain dependent optical phonon frequencies of cubic ZnS

Bulk ZnS crystals have been examined by Raman spectroscopy at room temperature under uniaxial stress. Both transverse optical and longitudinal optical frequencies exhibit shifts which are linear in the stress. From the observed slopes we obtain the complete set of phonon deformation potentials which, in turn, are used to predict the LO phonon frequency shifts in strained ZnTe/ZnS and ZnSe/ZnS superlattices as a function of the layer thickness. The results are in agreement with previously reported values obtained from Raman measurements in such superlattices.

Electronic structure of Mn-doped ZnS nanocrystals

The effects of the inclusion of manganese in small ZnS nanocrystals are investigated. Our calculation is based on a tight-binding model with renormalized parameters similar to those of the defect-molecule approach for substitutional transition-metal ions in semiconductors. The nanocrystals of Mn-doped ZnS were 1.1\char21{}3.6 nm. Their electronic and optical properties differed from bulk ZnS:Mn and ZnS nanocrystals. It is shown that in addition to confinement effects, doping enhances the energy gap.

Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride

AbstractEnsemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.

Optical absorption of ZnS nanocrystals inside pores of silica

The optical absorption of ZnS semiconductor nanocrystals inside pores of silica has been investigated. These ZnS nanoparticles were synthesized through sol–gel processing, followed by firing. X-ray diffraction and nitrogen adsorption/desorption results show that the ZnS nanocrystals are highly defective. Based on the analysis of x-ray and nitrogen adsorption–desorption results, the surface layers of ZnS nanocrystals can be regarded as being amorphous. It was found that, compared with that of the bulk ZnS, the optical absorption edge of ZnS nanoparticles shifts to a longer wavelength. We believe that the amorphous surface layers of ZnS nanocrystals can mainly be responsible for the observed redshift of the optical absorption edge.

Electron Spin Resonance Study of Radicals Produced by Photoirradiation on Quantized and Bulk ZnS Particles

Quantized ZnS particles were prepared and isolated from aqueous solution with a capping agent, thioglycerol. The absorption peak of the particles was located at 259−260 nm, and the diameter was est...

Impact ionization rate and high-field transport in ZnS with nonlocal band structure

The impact ionization rate in ZnS is calculated using a nonlocal empirical pseudopotential band structure and compared to previous results using a local calculation. The two resulting rates are then compared and simple fit formulas are presented. These are included in an ensemble Monte Carlo simulation of electron transport in bulk ZnS. The calculated impact ionization rate is then compared to experimental impact ionization coefficient data; reasonable agreement between the experimental data and the calculated impact ionization rate is obtained with an appropriate choice of optical deformation potentials.

Band Offset and Optical Property of (ZnS)n/(Si2)n (110) Superlattice

Supercell self-consistent calculation of band structures of (ZnS)n/(Si2)n (110) (n = 2, 3, 4, 5) has been performed by means of linear muffin-tin orbitals method. The valence-band offset of such systems is calculated by frozen optential method and their Joint density of states has been computed by tetrahedron method. Our results show that the (ZnS)n/(Si2)n (110) superlattices have a large valence-band offset and their optical properties integrate exellent optical properties of bulk ZnS and that of Si.

Luminescence of Mn 2+ doped ZnS nanocrystallites

Optical properties of Mn 2+-doped ZnS colloids are reported. The band to band excitation energy transfer to Mn 2+ is more efficient compared to Mn 2+ direct excitation, which is different from the case for bulk ZnS: Mn. Aging effects and the radiation-induced luminescence enhancement (RILE) effect are reported and an explanation for this behavior is presented.

Symmetry and electronic structure of the Mn impurity in ZnS nanocrystals.

The symmetry and electronic structure of the Mn impurity in ZnS nanocrystals have been studied with electron paramagnetic resonance (EPR). Experiments were performed at 9 and 35 GHz on crystals with an average diameter of 35 \AA{}. Changes in the spectra over a period of months indicate a gradual degeneration of the passivating layer that separates individual nanocrystals. These changes allow for the deconvolution of the total EPR spectra into two independent spectra. The first has g=2.003 and \ensuremath{\Vert}A\ensuremath{\Vert}=64.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This spectrum is dominated by Mn-Mn dipolar interactions and is similar to the spectrum of heavily Mn-doped, bulk ZnS. The second spectrum has g=2.001 and \ensuremath{\Vert}A\ensuremath{\Vert}=89\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with axial-field splittings (D) from 0.05 to 0.10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This spectrum is associated with isolated Mn sites near the surface of a nanocrystal. The EPR results are discussed in relation to the previously published optical results.

Dielectric properties of nano-particles of zinc sulphide

The dielectric properties of nano-particles of ZnS have been studied over a temperature range from 300 to 525 K. The dielectric constant, dielectric loss and ac conductivity of the samples are larger than those of bulk ZnS crystals. Dielectric properties of composites consisting of nano-particles of Ag of different concentrations dispersed in nano-particles of ZnS have also been studied.