Defects In ZnS Research Articles

Red Emission from Copper-Vacancy Color Centers in Zinc Sulfide Colloidal Nanocrystals

Copper-doped zinc sulfide (ZnS:Cu) exhibits down-conversion luminescence in the UV, visible, and IR regions of the electromagnetic spectrum; the visible red, green, and blue emission is referred to as R-Cu, G-Cu, and B-Cu, respectively. The sub-bandgap emission arises from optical transitions between localized electronic states created by point defects, making ZnS:Cu a prolific phosphor material and an intriguing candidate material for quantum information science, where point defects excel as single-photon sources and spin qubits. Colloidal nanocrystals (NCs) of ZnS:Cu are particularly interesting as hosts for the creation, isolation, and measurement of quantum defects, since their size, composition, and surface chemistry can be precisely tailored for biosensing and optoelectronic applications. Here, we present a method for synthesizing colloidal ZnS:Cu NCs that emit primarily R-Cu, which has been proposed to arise from the CuZn-VS complex, an impurity-vacancy point defect structure analogous to well-known quantum defects in other materials that produce favorable optical and spin dynamics. First-principles calculations confirm the thermodynamic stability and electronic structure of CuZn-VS. Temperature- and time-dependent optical properties of ZnS:Cu NCs show blueshifting luminescence and an anomalous plateau in the intensity dependence as temperature is increased from 19 K to 290 K, for which we propose an empirical dynamical model based on thermally activated coupling between two manifolds of states inside the ZnS bandgap. Understanding of R-Cu emission dynamics, combined with a controlled synthesis method for obtaining R-Cu centers in colloidal NC hosts, will greatly facilitate the development of CuZn-VS and related complexes as quantum point defects in ZnS.

CuO loaded ZnS nanoflower entrapped on PVA-chitosan matrix for boosted visible light photocatalysis for tetracycline degradation and anti-bacterial application

Novel photocatalyst CuO loaded ZnS nanoflower supported on carbon frame work PVA/Chitosan was synthesized by co-precipitation and ultrasonic assisted method. The co-existence of ZnS and CuO and its crystallinity in nanohybrid was verified by XRD, SAED and HR-TEM analysis. The availability of defects in ZnS was identified by EPR. FTIR and TGA verified the presence of PVA and Chitosan. Defects mediated ZnS–CuO/PVA/chitosan heterojunction promote synergistic charge separation with type II interface. Zn-vacancy facilitates two-photon excitation that improves visible-light harvesting. The photocatalytic activity of ZnS–CuO/PVA/Chitosan was 94.7% which is higher when compared to ZnS (40%) and CuO (60%). The photocatalytic mechanism was elucidated using scavenger test and both ·O2− and ·OH were found to play key role in tetracycline degradation. In addition, ZnS–CuO/PVA/Chitosan demonstrated efficient anti-microbial effect against the both gram strains on comparing with individual ZnS and CuO. Thus, the multifunctional ZnS–CuO/PVA/Chitosan is promising for the photocatalytic degradation of tetracycline and as an antimicrobial agent.

Tuning the properties of ZnxS1-x nanoparticles by controlling reaction conditions

In this work, a modified synthesis method was deployed to obtain nanocrystalline zinc sulfide from zinc acetate. By utilizing the hydrothermal and sol gel method, the influence of: reaction time, solvent and temperature control were used to tune the properties of zinc sulfide. Our results showed that ZnS(B) (which was obtained by sol gel in water + hydrothermal) typified formation of increased sulfur vacancies while an increase in reaction time resulted in decreased sulfur vacancies. The introduction of chemical defects in ZnS(A) (which was obtained by sol gel in methanol + KOH) resulted in lower crystallite size. We observed that crystallinity improved with increased reaction time and utilization of water as solvent improved the crystallinity of the material as confirmed in ZnS(C) and ZnS(B). Furthermore, our result showed that reaction time influenced dislocation density of the material to a greater extent than type of solvent used. The crystallite size estimated by Scherer formula was in the range 1.35 nm – 18.64 nm while the band gap energy of the ZnS samples were calculated in the range 3.8 eV- 4.6 eV. Utilizing these novel syntheses methods can stimulate new directions in synthesizing ZnS crystals with options of choosing appropriate method for specific applications depending on properties to be traded off.

Formation of dislocation loops and voids in electron irradiated zinc selenide single crystals

The formation of small dislocation loops in size of 2.5 - 45 nm and a density of 1.4·1011 cm−2, as well as voids in size ≤10 nm in ZnS crystals were found by the transmission electron microscopy method (TEM). Samples were examined and irradiated in situ in a JEOL 4000EX-II electron microscope operated at energy of 400 keV and intensity of (1 - 4)·1019 e/cm2·s. Fine particles of a new phase in size ≤10 nm are formed also. These features can be identified from an analysis of moiré fringe contrast as phase of ZnO2. Similar defects in single crystals of ZnS formed in situ after irradiation in a transmission electron microscope JEM-100CX operated at energy of 100 keV and intensity of 3,5·1017 e/cm2·s. It was found that the formation of structural defects in ZnS under electron irradiation with above-threshold energy of defect formation (400 keV) is similar to the formation of structural defects in these crystals under electron irradiation with sub-threshold defect formation energy (100 keV).

Impact of hydrogen and oxygen defects on the lattice parameter of chemical vapor deposited zinc sulfide

The lattice parameter of cubic chemical vapor deposited (CVD) ZnS with measured oxygen concentrations <0.6 at. % and hydrogen impurities of <0.015 at. % has been measured and found to vary between −0.10% and +0.09% relative to the reference lattice parameter (5.4093 Å) of oxygen-free cubic ZnS as reported in the literature. Defects other than substitutional O must be invoked to explain these observed volume changes. The structure and thermodynamic stability of a wide range of native and impurity induced defects in ZnS have been determined by ab initio calculations. Lattice contraction is caused by S-vacancies, substitutional O on S sites, Zn vacancies, H in S vacancies, peroxy defects, and dissociated water in S-vacancies. The lattice is expanded by interstitial H, H in Zn vacancies, dihydroxy defects, interstitial oxygen, Zn and [ZnHn] complexes (n = 1,…,4), interstitial Zn, and S2 dumbbells. Oxygen, though present, likely forms substitutional defects for sulfur resulting in lattice contraction rather than as interstitial oxygen resulting in lattice expansion. It is concluded based on measurement and calculations that excess zinc atoms either at anti-sites (i.e., Zn atoms on S-sites) or possibly as interstitial Zn are responsible for the relative increase of the lattice parameter of commercially produced CVD ZnS.

Native point defects in ZnS: First-principles studies based on LDA, LDA + U and an extrapolation scheme

First-principles calculations based on LDA, LDA+U and an extrapolation scheme are performed to study the native point defects in zincblende ZnS, and the three methods give similar results. Zinc vacancies in the 2− charge state have extremely low formation energy at the conduction band minimum (CBM), thus they will heavily compensate the n-type doping of ZnS. In Zn-rich conditions, sulfur antisites and zinc interstitials in the zinc cage sites are compensating centers in p-type ZnS. Sulfur vacancies and sulfur intersitials in the zinc cage sites are negative-U centers. We compare our results with previous reports and obtain some new findings.

Time-resolved photoluminescence measurements of InP/ZnS quantum dots

This paper reports the results on the time-resolved photoluminescence study of InP/ZnS core/shell quantum dots. The ZnS shell played a decisive role to passivate imperfections on the surface of InP quantum dots, consequently giving rise to a strong enhancement of the photoluminescence from the InP core. Under appropriate excitation conditions, not only the emission from the InP core but also that from the ZnS shell was observed. The emission peak in InP core quantum dots varied as a function of quantum dots size, ranging in the 600 – 700 nm region; while the ZnS shell showed emission in the blue region around 470 nm, which is interpreted as resulting from defects in ZnS.

Cr‐related energy levels and mechanism of Cr2+ ion photorecharge in ZnS:Cr

AbstractIt is shown that published data and interpretation concerning the energy and origin of Cr‐related defects in ZnS:Cr as well as the mechanism of the Cr2+ → Cr+ photorecharge are ambiguous and contradictory. New data concerning these questions obtained by means of photodepolarization (PDP) spectroscopy methods for the first time are given. Several peaks of the impurity photocurrent are revealed in the PDP spectrum of ZnS:Cr thin‐film structures, which was measured from 0.7 eV to 3.9 eV. Two peaks are due to the photoionization of Cr2+ and Cr+ ions (2.8–3 eV and 1.3–1.5 eV, respectively). Three peaks are attributed to Cr‐related lattice defects, namely, a $ [{\rm Cr}^{2+}_{\rm Zn}]^0 $ isovalent trap (∼3.5 eV) and a $ [{\rm Cr}^{+}_{\rm Zn}]^- $ acceptor isolated (∼2.7 eV) and associated with a donor (1.8–2.2 eV). A new interpretation of the mechanism of the photorecharge of Cr2+ ions is also proposed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High temperature electrical conductivity in ZnSe:In and in CdSe:In under selenium vapor pressure

AbstractHigh temperature electrical conductivity (HTEC) isotherms and isobars of ZnSe:In and of CdSe:In are compared. There are differencies in In‐doping mechanisms of II–VI compounds. When HTEC isotherms and isobars of ZnSe:In and of CdSe:In, measured under metal component vapour pressure give both n‐type conductivity then differences appear in the results of measurements under the selenium vapor pressure (p). ZnSe:In isotherms in the last case are characterized by the conductivity type conversion but no such drastic change of HTEC type is observed on CdSe:In isotherms. Under the conditions of p, the activation energy of HTEC isobars for ZnSe:In is ΔE ≈ 1.3–1.6 eV and for CdSe:In is ΔE ≈ 1.2 eV. The onefold ionized substitutional In at Zn place is proposed to be compensated by native defects in ZnS:In and in CdSe:In under high p. This native defect may be onefold ionized zinc vacancy for ZnSe:In and twofold ionized cadmium vacancy for CdSe:In. Association of defects occur at lower temperatures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the preparation and excitation conditions of ZnS:Er,F TFELS on the radiative characteristics in the visible and near‐infrared spectral regions

Abstract— The voltage (V) dependences of the luminance and transferred charge of ZnS:Er,F TFELS prepared by varying the annealing temperature (Tann) are given at different frequencies (f). The change in the relative intensity of the green band (523 nm) and the near‐infrared bands (983 and 1530 nm) in the emission spectrum when V, f, and Tann are varied is also considered. The influence of the annealing temperature on the energy spectrum of defects in ZnS:Er,F film was studied by photodepolarization spectroscopy. The revealed effect of the above excitation and preparation conditions on the radiative characteristics is discussed.

Effect of electron‐beam treatment of zinc sulfide on electroluminescent phosphor properties

Abstract— Electron‐beam processing of starting‐material ZnS increased the luminance derived from electroluminescent phosphors. As is shown in the paper, irradiation caused the formation of defects in ZnS that resulted in increased copper content in the phosphor due to the promotion of diffusion and the formation of additional “green” luminescence centers. A change in surface properties was also investigated, and it was found that processing resulted in increased the amount of both surface zinc and sulfur vacancies as well as the amount of surface centers related to copper.

High-temperature electrical conductivity of codoped ZnS and CdSe

Systems of binary compounds containing two types of foreign atoms are complicated. In this work we are interested in high-temperature electrical conductivity (HTEC) behaviour of ZnS and CdSe crystals codoped with two impurities (Cu and Cl or In). We tried to find common behaviour and common features in HTEC isobars of these systems. We were also interested in the temperature regions of forming associates between these dopants. At a temperature T=500°C, thermodynamic equilibrium between defects in ZnS:Cu:Cl is achieved. For HTEC isobars, three temperature regions with different dominating defects for ZnS:Cu:Cl, CdSe:Cu:In and CdSe:Cu:Cl can be observed. The low-temperature region characterizes the reactions between associates of dopants and single defects with an activation energy ΔE=1.5eV for ZnS:Cu:Cl and ΔE=1.05eV for CdSe:Cu:In. In the HTEC isobar region, the donor action of both dopants is observed with close to zero or even negative activation energy of HTEC isobars.

Chemical self-diffusion in undoped ZnS and in undoped CdSe

Chemical self-diffusion coefficients D(Δ) as a function of temperature T and metal (Zn or Cd) vapor pressure p Zn or p Cd has been studied in undoped ZnS and in undoped CdSe single crystals at high temperature. In the temperature range from 750 to 850 °C, D(Δ) in undoped ZnS can be described as D(Δ)=4.5×10 −3 exp(−0.69 eV/kT) and in the temperature range from 1050 to 1150 °C, D(Δ) can be described as D(Δ)=1.2×10 −4 exp(−0.43 eV/kT). The ZnS phase transition region is characterized by confused values of D(Δ) because of change due to mixture of phases. D(Δ) in ZnS and in CdSe is about three orders of magnitude faster than self-diffusion in these crystals in the same conditions. It was shown that the doubly ionized interstitial metal atoms are the dominanting diffusible defects in ZnS and in CdSe at high metal vapor pressure. D(Δ) is found to be almost independent on metal vapor pressure for undoped ZnS and for undoped CdSe at high p Zn or p Cd. In the temperature range from 565 to 700 °C, D(Δ) for CdSe can be expressed as D(Δ)=1.6×10 −2 exp(−0.41 eV/kT).

Induced defects in ZnS by electron and proton irradiation and defect-annealing behavior

In this contribution the positron annihilation technique is used, because of its specific sensitivity to vacancies, to investigate the effects of electron and proton irradiation on the lattice structure of ZnS and possible clustering mechanisms during isochronal annealing. Therefore, ZnS crystals were irradiated either with 3 MeV protons to a fluence of 1.2×1018 p/cm2 or with 1 MeV electrons to a fluence of 1×1018 e/cm2 or 5×1018 e/cm2, respectively. It was found that electron and proton irradiation causes different changes in the positron annihilation characteristics. During isochronal annealing these defects agglomerate to larger vacancy complexes or even small voids. In both electron and proton irradiation several annealing stages can be observed, related to the annealing of variously sized vacancy complexes. Electron and proton irradiation leads to discoloration.

Vacancy-Type Defects in Electron and Proton Irradiated II-VI Compounds

In this contribution, the authors present a study aimed at investigating the basic properties of radiation induced defects in ZnS and ZnO and the influence of the atmosphere on the annealing characteristics of the defects. Positron annihilation experiments (both lifetime and Doppler-broadening measurements) were performed on both single- and polycrystalline samples, irradiated with 3 MeV protons or 1 MeV electrons. For ZnS it was found that both electron and proton irradiation caused significant changes in the positron annihilation characteristics. The annealing of proton irradiated ZnS in air leads to significant oxidation and eventual transformation into ZnO.

Vacancy-Type Defects in Electron and Proton Irradiated II- VI Compounds

AbstractIn this contribution, we present a study aimed at investigating the basic properties of radiation induced defects in ZnS and ZnO and the influence of the atmosphere on the annealing characteristics of the defects. Positron annihilation experiments (both lifetime and Dopplerbroadening measurements) were performed on both single- and polycrystalline samples, irradiated with 3 MeV protons or 1 MeV electrons. For ZnS it was found that both electron and proton irradiation caused significant changes in the positron annihilation characteristics. The annealing of proton irradiated ZnS in air leads to significant oxidation and eventual transformation into ZnO.

Vacancy-Type Defects in Electron and Proton Irradiated II- VI Compounds

AbstractIn this contribution, we present a study aimed at investigating the basic properties of radiation induced defects in ZnS and ZnO and the influence of the atmosphere on the annealing characteristics of the defects. Positron annihilation experiments (both lifetime and Dopplerbroadening measurements) were performed on both single- and polycrystalline samples, irradiated with 3 MeV protons or 1 MeV electrons. For ZnS it was found that both electron and proton irradiation caused significant changes in the positron annihilation characteristics. The annealing of proton irradiated ZnS in air leads to significant oxidation and eventual transformation into ZnO.

Defects in ZnS and ZnSe investigated by positron annihilation spectroscopy

Positron lifetime measurements have been performed on as-grown and electron-irradiated single crystals of the wide-gap compound semiconductors ZnS and ZnSe. The temperature dependence of the positron lifetime in as-grown samples reveals the presence of certain grown-in defects which act as shallow positron traps. No evidence for positron trapping at room temperature is found in as-grown crystals. The values of 230+or-3 ps and 240+or-5 ps are proposed for the bulk positron lifetime in ZnS and ZnSe, respectively. The positron lifetime in samples electron irradiated at 320, 77 and 20 K is interpreted in terms of positron trapping at Zn vacancy-related defects and negatively charged Zn antisites. The low-temperature inhibition of the positron trapping at open-volume defects is attributed to the presence of negatively charged Zn antisites. The positron lifetime in the samples irradiated at 20 K reveals the formation of voids in ZnS and ZnSe after annealing above approximately 600 K and approximately 500 K, respectively. These voids in ZnSe appear to anneal for temperatures above 600 K.

An ODMR study of silver- and gold-related defects in ZnS

The results of ODMR experiments carried out whilst monitoring the luminescence bands of silver- and gold-doped ZnS are presented. Doping of ZnS with silver results in the appearance of three emission bands peaking at 440, 520 and 650 nm. The ODMR results show that the 440 and 650 nm bands are the results of recombination between shallow donors and deep-lying centres. The deep centre, (Ag-1), associated with the 650 nm luminescence is nearly axial about the bond direction, and the proposed structure of the defect is (AgZn-Ds), D being unidentified. On doping ZnS with gold, emission bands appear, peaking at 450, 510 and 680 nm. The 450 and 510 nm bands are shown to be the result of shallow-donor-deep-centre pair recombination. A deep-lying donor complex, axial about the bond direction, is shown to act as a quenching centre to both these luminescence bands. The proposed structure for this donor complex is (AuZn-Vs).

An HREM study of the defects in ZnS

Abstract High-resolution electron microscopy (HREM) has been used to investigate the defect structures in ZnS crystals of zincblende type with 〈110〉 beam incidence. It is concluded that there is a high density of 180° rotation twins. Under different imaging conditions the bright dots in the HREM micrographs may either correspond to the atomic column pairs or the structural channels surrounded by atoms, and these make the twin boundary appear in different forms by changing experimental parameters. End-on extended dislocations which are formed by the dissociation of 60° mixed dislocation and 0° screw dislocation with Burgers vectors of 1/2〈110〉 type were analysed and characterized Using anisotropic elasticity theory of dislocations and the width of dissociation measured from HREM images, the stacking fault energy on the {111} plane in ZnS has been determined to be 2·7±0·1 mJm−2. Frank dislocations located at the twin boundary and a Hirth dislocation lock having a Burgers vector of the 1/6〈002〉 type are also...