Doped ZnS Research Articles

Laser-induced down-conversion parameters of singly and doubly doped ZnS phosphors

Singly and doubly doped ZnS phosphors have been synthesized using flux method. Laser-induced photoluminescence has been observed in ZnS-doped phosphors when these were excited by the pulsed UV N2 laser radiation. Due to down-conversion phenomenon, fast phosphorescence emission in the visible region is recorded in milliseconds time domain for ZnS:Mn while in the case of ZnS:Mn:killer (Fe, Co and Ni) the lifetime reduces to microseconds time domain. Experimentally observed luminescent emission parameters of excited states such as, lifetimes, trap-depth values and decay constants have been reported here at room temperature. The high efficiency and fast recombination times observed in doped ZnS phosphors make these materials very attractive for optoelectronic applications.

Microwave-assisted synthesis and luminescent properties of pure and doped ZnS nanoparticles

Pure and doped ZnS nanoparticles have been successfully synthesized via microwave irradiation. X-ray diffraction (XRD) analysis shows that the crystal size of the pure ZnS particles is about 6.5 nm. The ultraviolet–visible (UV–vis) absorption spectra of the samples indicate that the irradiation time shows no significant influence on the size of ZnS nanoparticles. It is also found that the luminescent properties of ZnS nanoparticles are greatly affected by either the microwave irradiation time or dopants of various metallic ions (Ag +, Cu 2+, Ce 3+ and Sn 4+). The intensity of photoluminescence (PL) emission reaches its maximum and then decreases with prolonging the microwave irradiation time. The luminescence intensity of ZnS nanoparticles doped with 0.2% cerium, which is stronger than that of other dopants, is about two times that of sample doped with 0.2% Ag + and 1.6 times that of pure ZnS nanoparticles. But when the amount of doped cerium reaches 0.4%, the emission efficiency is lower than that of pure ZnS. All peak emission wavelength is 450 nm, indicating that the luminescent centers of various metallic ions are not formed within the doped ZnS nanoparticles.

Synthesis and Characterization of Monodisperse Doped ZnS Nanospheres with Enhanced Thermal Stability

The monodisperse ZnS nanospheres, with an average diameter of 100 nm, were synthesized through a solvothermal method. Strong photoluminescence emission from blue to green and orange can be observed by doping these ZnS nanospheres with various cations (Mn2+, Cu2+, Al3+). An investigation is reported on the coating of ZnS nanospheres by a thin shell of SiO2 or TiO2 using a sol−gel process. X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used for the characterization of the coated ZnS nanospheres. The photoluminescence properties demonstrated that not only the brightness of the coated nanospheres has been enhanced but also the thermal stability has been improved greatly.

Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centers in ZnS nanocrystals

Multi-colour emitting doped ZnS nanocrystals surface capped with pyridine (P-ZnS) or polyvinyl pyrrolidone (PVP-ZnS) have been synthesized by wet chemical methods. The photoluminescence studies show that the dopant related emission from P-ZnS nanocrystals are caused by the energy transfer from band-to-band excitation of the host lattice. However, in the case of PVP capped ZnS, considerable enhancement in the emission intensity was observed and the corresponding excitation spectra appeared dramatically broadened due to the presence of multiple excitation bands with peak maxima at 235, 253, 260, 275, and 310 nm. The bands from 235 to 275 nm are assigned to the electronic transitions in the chemisorbed PVP molecules whereas the excitation maximum around 310 nm corresponds to the band-to-band transition within the nanocrystalline ZnS host. The presence of PVP related energy bands in the excitation spectrum indicates the process of energy transfer from the surface adsorbed PVP molecules to dopant centers in ZnS nanocrystals. This study brings out a heterogeneous sensitizer-activator relation between organic surface adsorbate and doped semiconducting nanocrystals.

Chemical synthesis and characterization of Cu doped ZnS nano-powder

In the past few years, II–VI compounds semiconductor nanocrystal have received much interest because their optical properties are different from the corresponding bulk crystals [1, 2]. Among II–VI compounds, ZnS is one of the most promising host materials for the production of commercial thin film phosphors for electroluminescent (EL) device application [3–7]. Since its absorption and emission lie in the ultraviolet regime, the studies of its small size particles did not evoke much interest until the 1980s. In 1994, Bhargava and Gallagher first synthesized ZnS semiconductor microcrystallites doped with Mn2+ ions; they found that the photoluminescence of Mn2+doped in ZnS nanoparticles had much higher quantum efficiency than bulk crystals [8]. Hereafter, the metal doped ZnS nanoparticles have received increasing interest. Recently, many methods have been developed in preparation of Cu doped ZnS nanopowder [9, 10]. In this paper, we report a new chemical synthesis method for preparation of Cu doped ZnS nanopowder as shown in Fig. 1. This process concluded two steps: First, selection to synthesize different lengths of the zinc methacrylate polymer by adjusting the molar ratio of mercaptoacetic acid to monomer of the methacrylic acid; second, the polymer reaction with sodium sulfide and cupric sulfate, giving the Cu doped ZnS nanopowder. During formation of the nanoparticles, the nanoparticles were capped by the methacrylic polymer, which passivated the surface atoms, eliminated the energy levels inside of gap, and increased the luminescence intensity. The size of nanoparticles can be well controlled from 1.8 nm to 3 nm by adjusting the molar ratio of mercaptoacetic acid to monomer of methacrylic acid. The production of Cu doped ZnS nanopowder was also characterized by TEM, XRD and PL. Methacrylic acid (99%) monomers were purified by distillation under reduced pressure prior to use. The cupric sulfate (99%), zinc acetate dihydrate (99%), sodium sulfide (99%), potassium persulfate (99.5%) and mercaptoacetic acid (HSCH2COOH) (85%) were used without further purification. All of the chemicals were purchased from Beijing chemical reagent corporation. To synthesize Cu doped ZnS nanopowder, 40 × 10−6 m3 of water, 0.03 mol of zinc acetate, and 5.5 × 10−6 m3 (6.49 × 10−4 molar) of methacrylic acid were added to a three-neck flask equipped with condenser, then heated and stirred. When the temperature increased about 60 ◦C, 40 × 10−6 m3 of 0.005 M potassium persulfate aqueous solution and 10−5 m3 of 0.032 M (3.2 × 10−6 molar) mercaptoacetic acid aqueous solution were added, respectively. The temperature was kept at 80 ◦C for 1 h. 20 × 10−6 m3 of 0.008 M CuSO4 aqueous solution was added into reactor vessel and stirred for 5 min. 40 × 10−6 m3 of 0.8 M sodium sulfide aqueous solution was dropped into the reactor at a constant rate. When the Na2S aqueous solution was complete, the reaction was continued for an additional 30 min at 80 ◦C. The powder was well separated from the solution by centrifuging, rinsed with methanol and dried in vacuum. The Cu doped ZnS nano-powder was obtained. The PL spectra of Cu doped ZnS nanopowder were recorded with a Hitachi F-4500 fluorescence spectrophotometer. The spectra were obtained by exciting the sample with 368 nm wavelength at room temperature. X-ray powder diffraction was performed on the Cu doped ZnS powder on a Rigaku RU-200B rotaflex diffractometer using Cu Kα λ = 0.15406 nm. TEM

Control of Valence States for Zno And Zns with a Wide-Band Gap by a Codoping Method

AbstractWe have investigated the electronic structures of p- or n-type doped ZnO and ZnS based on the basis of the analysis of the results obtained by ab initio electronic band structure calculations. We find the doping problems of the two materials. They at highly N-doping levels have the narrow impurity bands at the top of the valence band, where holes are very localized by repulsion effects. As a solution to the doping problems described above, we proposed a codoping method: The deliberate codoping of the donors with N acceptors is essential for the enhancement of N-acceptor incorporation and delocalization of the N-impurity states with the electrostatic energy gain in p-type highly doped ZnO and ZnS.

Synthesis and photoluminescence characteristics of doped ZnS nanoparticles

Free-standing powders of doped ZnS nanoparticles have been synthesized by using a chemical co-precipitation of Zn2+, Mn2+, Cu2+ and Cd2+ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ∼2–3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles co-doped with Cu2+ and Mn2+. The nanoparticles could be doped with copper and manganese during the synthesis without altering the X-ray diffraction pattern. However, doping shifts the luminescence to 520–540 nm in the case of co-doping with Cu2+ and Mn2+. Doping also results in a blue shift on the excitation wavelength. In Cd2+-doped ZnS nanometer-scale particles, the fluorescence spectra show a red shift in the emission wavelength (ranging from 450 nm to 620 nm). Also a relatively broad emission (ranging from blue to yellow) has been observed. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of transition metal-activated ZnS nanoparticles, form a new class of luminescent materials.

Photoluminescence properties of ZnS nanoparticles co-doped with Pb 2+ and Cu 2+

Nanometer-scale ZnS, ZnS:Cu, ZnS:Pb, and ZnS co-doped with Cu 2+ and Pb 2+ have been synthesized using a chemical precipitation method. X-ray diffraction analysis shows that the diameter of the particles is 2–4 nm. These nanocrystals can be doped with copper and lead during the synthesis without altering the X-ray diffraction pattern. However, doping has shifted the luminescence to 530 nm (Cu 2+-doped) and 500–550 nm (co-doped with Cu 2+ and Pb 2+). In the case of ZnS:Pb nanocrystals, a relatively broad emission band (color range from blue to yellow) has been observed and its excitation wavelength shows a red shift. The photoluminescence intensity increases as the ZnS nanoparticles co-doped with Pb 2+ and Cu 2+. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of metals activated ZnS nanocrystals, form a new class of luminescent materials.

Optical characteristics of undoped and Mn doped ZnS films

Undoped and Mn doped ZnS films were prepared in the temperature range 250–350°C by the spray pyrolysis technique. As the deposition temperature increased, the optical transmittance and the intensity of blue emission in the ZnS film were enhanced due to the decrease in the Zn/S ratio. By adding the manganese ion up to 7 at.% in the mixed solution, the orange-yellow emission of the films was enhanced. But the concentration quenching effect resulted in the decrease of the emission at higher concentration of the manganese ion.

Long-term conduction behavior of white-light emitting ZnS-based phosphor films

To investigate systematically the causes of the aging of thin film electroluminescent devices, time-dependent current–voltage characteristics of doubly doped ZnS:Pr, Ce, Mn layer has been investigated under a direct current high field as one of the accelerated aging methods. The surface roughness along to the direction perpendicular to the indium-tin-oxide–glass substrate and the high peak-to-valley roughness are assumed to be the main sources for current fluctuations during the measurement. It was observed that the leakage current level after long-term stressing increased or decreased depending on post-treatment indicating that the bulk-controlled conduction was the dominant mechanism determining long-term behavior and this mechanism is not sensitive to the variations in the deposition parameters. The experimental results indicate the fact that the long-term conduction behavior of ZnS-based film may be related to defect redistribution after lowering barrier height during initial stressing. Finally, we suggest that an enhancement of contact adhesions via surface smoothing of the upper interface can contribute to the long-term stability.

Synthesis and Characterization of Mn Doped ZnS Quantum Dots from a Single Source Precursor

AbstractNanoparticles of ZnS and Mn-doped ZnS capped with TOPO (tri-n-octylphosphine oxide) and close to mono-dispersed have been prepared by a single source route using bis(diethyldithiocarbamato) zinc(II) as a precursor. The nanoparticles obtained show quantum size effects in their optical spectra and ZnS nanoparticles exhibit near band-edge luminescence. Clear difference in photoluminescence results between ZnS and ZnS:Mn samples. Changes in Mn-doping levels are shown by the changes in photoluminescence intensity. The most intense photoluminescence was observed for 1% and the least intense for 5% doping level. ESR spectra and ICP results confirm the presence of Mn in ZnS quantum dots and also correspond to the amount of Mn in each ZnS:Mn sample.The Selected Area Electron Diffraction (SAED), X-ray diffraction (XRD) pattern and Transmission Electron Microscopy (TEM) show the material to be of the hexagonal phase. The crystallinity of the material was also evident from High Resolution Transmission Electron Microscopy (HRTEM) which gave well-defined images of nanosize particles with clear lattice fringes.

Field-induced ionization of excited luminescent centers at high electric field strengths

In this paper we discuss the mechanism of field-induced ionization of the excited luminescent centers in rare-earth doped ZnS TFEL devices and estimate the upper-level positions of the centers according to Franz-Keldysh effect. Effects of impact and room temperature on ionization of the centers are also considered.

Effect of an interlayer on the emission characteristics of a white-light-emitting electroluminescent device with a Pr and Ce doubly doped ZnS phosphor layer

We have investigated light-emission characteristics of a white-light-emitting electroluminescent device with a doubly doped ZnS:Pr,Ce,F phosphor layer. We found that optimum codoping of Ce enhances the emission characteristics compared to the electroluminescent device with a singly doped ZnS:Pr,F layer. We also found that introducing an additional thin-insulating SixNy interlayer between the lower insulating layer and the phosphor layer significantly stabilizes the aging characteristics and improves the luminous efficiency.

The Effects of Sintering on the Microstructure and the Luminescent Characteristics of Polycrystalline ZnS

AbstractPositron annihilation spectroscopy and cathodoluminescence were employed to study changes in the microstructural and luminescent properties of pure and doped polycrystalline ZnS. All samples show evidence of defect clustering, when sintered at temperatures up to 400 °C, indicated by an increase in the average positron lifetime. The average lifetime of the undoped material is constant (at about 290 ps) from 300 to 700 °C, and then decreases beyond this temperature, due to the out diffusion of vacancy clusters. In comparison, the average lifetime for MnS doped material decreases beyond 600 °C. This decrease in the average lifetime is correlated with an increase in the luminescent intensity of the 585 nm Mn band, indicating that Mn compensation of zinc vacancies contributes to the decrease in the average lifetime. At sintering temperatures above 400 °C, the average lifetime of TbF3 doped material decreases by ∼3 ps / 100 °C, from a peak value of 285 ps, and then at 700 °C drops an additional 15 ps to 260 ps. At 650 °C a change from the distinct Tb spectrum to a broad band emission is observed It is believed that these effects can be linked to the incorporation of Tb3+ ions on Zn2+ lattice sites.

Energy transfer processes in rare earth doped ZnS under non-linear excitation

ZnS: Sm 3+ single crystals are studied by site selective, time resolved and decay time measurements. It will be shown that beside the direct 4f-4f excitation a slow energy transfer component occurs that can be attributed to the blue self-activated emission in ZnS. Analysing the decay behaviour of various Sm 3+ sites in detail a fast transfer component could be selected which is active for only some of the Sm 3+ sites. The nature of the energy transfer processes will be discussed basing on decay fits and conclusions will be drawn on the differences between Sm 3+ centers, which provides to a better understanding of the luminescence data.

Tem investigation of Ga and in doped ZnS, and in doped ZnSe

Crystals of In and Ga doped ZnS and In doped ZnSe have been grown from the vapour phase. As-grown and vacuum annealed samples exhibited a well formed defect microstructure consisting of dislocations and stacking faults, whereas samples annealed in a zinc ambient were characterised by precipitates decorating and disrupting the native defect structure, and dislocation loops.

Role of Mn impurity in the deterioration of ZnS:Cu, Br electroluminescent phosphors

ZnS:Cu, Br powder EL phosphors showed 6-line EPR signal at 25°C whose intensity increases with Cu content and on annealing in Zn-vapour. The signal arises from native Mn impurity. The starting material does not show any EPR signal since Mn 2+ acts as an affinity potential well for a hole in ZnS, forming Mn 3+ - a chemically uncommon situation in sulfides. In doped ZnS, holes are trapped at Cu such that Mn 2+ persists. Deterioration of EL brightness is accompanied by the decrease in EPR signal intensity due to field assisted hole transference to Mn 2+. Intentional addition of Mn in ZnS:Cu, Br decreases the brightness and shortens life time. Stable phosphors require ZnS with Mn content less than 10 14 cm −3.

Electro- and photoluminescence properties of Mn 2+ in ZnS and ZnCdS

Some aspects of the nature of the Mn center in ZnS and ZnCdS and of the influence of electric fields on the luminescence mechanism are reviewed. The photoluminescence properties of isolated Mn ions on the four main sites of polymorphic crystals are considered. The influence of more distant stacking faults and the combined action of several stacking faults on the Mn excitation and emission are discussed tentatively. A survey is given on the effects of an increasing amount of Mn: (1) The shift of the zero-phonon lines which is due to inhomogenous line broadening; (2) the appearance of additional zero-phonon lines both in emission and excitation due to Mn-Mn pairs; (3) the observation of additional low-energy emission bands for highly doped ZnS:Mn crystals. The a.c. EL of ZnS:Cu, Mn, X powders where X stands for Fe, Ni, Co is discussed with respect to an energy transfer between deep centers during the low-field period of the EL cycle. It turns out that a strong energy transfer exists between these centers due to thermally controlled hole liberation. The influence of Auger quenching on the Mn emission is considered. It turns out that some effects already reported earlier in a.c. EL can be explained in that way. Finally the field enhancement of the Mn emission (“out-of-phase enhancement”) is considered as due to the control of the Auger quenching by electric a.c. field and/or additional red or infrared irradiation control of the density of electrons in the conduction band, where any reduction of this density involves a reduction of the Auger quenching.

Reversible Creation and Annihilation of a Violet Fluorescence Band in Doped ZnS Crystals

Annealing of doped (Cu, Ag, Au) ZnS crystals at 1200 °C and subsequent quenching or slow cooling, respectively, create or annihilate a violet fluorescence band, reversibly. The experimental results indicate that the emission must be due to isolated native defects in the ZnS lattice

Excitation and temperature dependence of the blue luminescence of ZnS crystals

The blue luminescence of pure and Al-, Au-, Ag-, Cu-, and Mn-doped ZnS crystals has been studied in the temperature range between LNT and RT and for different wavelengths of the exciting UV-light. All investigated crystals emit two blue bands. The energies of the maxima of these emission bands are independent of the temperature. The temperature dependence of the intensity ratio of the two blue luminescence bands can simulate a temperature shift of the blue emission. The experimental results support the assumption of an essentially uniform structure of the luminescence complex responsible for the visible luminescence of pure and doped ZnS phosphors.