ZnS Lattice Research Articles

Metal-doped ZnS(O) thin films on glass substrates using chemical bath deposition

Zinc sulfide (ZnS(O)) thin films doped with Mn, Ni, and Co ions are synthesized by chemical bath deposition technique onto glass substrates. X-ray diffraction study reveals that the undoped and metal-doped ZnS(O) films possess a hexagonal wurtzite crystal structure. The morphological change, upon metal-ions doping, from nanorod structures to cluster (Mn doping), compact (Ni doping), and granular shapes (Co doping) is observed. X-ray photoelectron spectroscopy reveals the presence and incorporation of metal ions into ZnS(O) lattice sites and the formation of a metal–ZnS combined structure. The band gap energy of the undoped ZnS(O) film is found to be larger than 4.0eV, while it is 3.8, 3.7, and 3.6eV for the Mn–ZnS(O), Ni–ZnS(O), and Co–ZnS(O) films, respectively. All the undoped and metal-doped ZnS(O) samples exhibit blue luminescence, which originates from the surface defects and trap centers. Thus, the photoluminescence (PL) (blue light emission) is due to the radiative recombination from various trap levels (shallow donor levels) to the valence band. The decrease in the PL peak intensity for the doped samples indicates the reduction of surface defects suggesting the incorporation of metal ions into the host lattice of ZnS(O). Based on the PL results, the PL energy-level diagram for the undoped and metal-doped ZnS(O) samples is proposed.

When and Why Like-Sized, Oppositely Charged Particles Assemble into Diamond-like Crystals

Like-sized, oppositely charged nanoparticles are known to assemble into large crystals with diamond-like (ZnS) ordering, in sharp contrast to analogous molecular ions and micrometer-scale colloids, which invariably favor more closely packed structures (NaCl or CsCl). Here, we show that these experimental observations can be understood as a consequence of ionic screening and the slight asymmetry in surface charge present on the assembling particles. With this asymmetry taken into account, free-energy calculations predict that the diamond-like ZnS lattice is more favorable than other 1:1 ionic structures, namely, NaCl or CsCl, when the Debye screening length is considerably larger than the particle size. A thermodynamic model describes how the presence of neutralizing counterions within the interstitial regions of the crystal acts to bias the formation of low-volume-fraction structures. The results provide general insights into the self-assembly of non-close-packed structures via electrostatic interactions.

Synthesis and Optical Properties of Annealed Cr Doped ZnS Nanoparticles

Undoped and Cr doped ZnS nanoparticles with Cr concentrations of 3.0 at.% were prepared by a chemical co-precipitation method for the fist time, using 2-Mercaptoethanol as the capping agent and annealed the synthesized particles at 600°C for 3h in air. The effect of annealing on morphological, structural and optical properties of ZnS and ZnS:Cr have been studied and compared with as prepared samples. EDAX measurements confirmed the presence of Cr in the ZnS lattice and it also confirms the conversion of ZnS into ZnO after annealed at 600 0C/3h. Surface morphologies of all samples were characterized using scanning electron microscopy (SEM). XRD spectra of as synthesized nanoparticles of ZnS and ZnS:Cr exhibited cubic phase. After annealing, the cubic phase is transformed into hexagonal phase. The particle sizes of the ZnS:Cr powders were increased from 5 to 30 nm when the powders were annealed at 600°C. A stable blue emission peak at 445 nm is observed from the as prepared samples (pure ZnS and Cr doped ZnS) but annealed at 600 0C the PL peaked at 500 nm for pure ZnS and Cr doped ZnS nanoparticles exhibited PL peak at 500 nm as well as 654 nm. The emission intensity decreased in annealed particles compared to as synthesized samples.

The structure and room temperature ferromagnetism property of the ZnS:Cu2+ nanoparticles

ZnS:Cu2+ nanoparticles were synthesized by the solvothermal method. The results showed that the nanoparticles with the diameters of 10–20nm were of cubic zinc blende structure. The Cu2+ ions were substitutionally incorporated into the ZnS lattice and the maximum concentration of the Cu2+ ions in the ZnS nanoparticles can reach to 2.84%. The ferromagnetism property of the ZnS:Cu2+ nanoparticles was observed around room temperature, which was explained by the super-exchange mechanism.

Absence of room temperature ferromagnetism in Mn-doped ZnS nanocrystalline thin film

Mn-doped ZnS nanocrystalline thin films having composition Zn1−xMnxS (x=0.02, 0.05, 0.1 and 0.15) were prepared by the electron beam evaporation technique. Their structural and magnetic properties had been investigated. X-ray diffraction results show single phase of hexagonal type structure samples without any detectable secondary phase providing the evidence that Mn2+ ions are incorporated into the ZnS lattice. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using atomic force microscopy. The magnetic measurements at 5K reveal superparamagnetic behavior. The field dependent magnetization measurements confirm the absence of the room temperature ferromagnetism in the single phase electron beam evaporated Zn1−xMnxS (x=0.02, 0.05, 0.1 and 0.15) nanocrystalline films. Furthermore, the inverse susceptibility versus T confirms paramagnetic behavior for the Mn-doped films in the temperature range 100K≤T≤300K. The absence of significant impurities and/or defects could be attributed to explanation for the absence of free carriers and consequently the absence of room temperature ferromagnetism in all nanocrystalline Zn1−xMnxS films. The absence of room temperature ferromagnetism is evidence by marked concave nature of Arrott's plot without any spontaneous magnetization.

Synthesis of stable and highly luminescent beryllium and magnesium doped ZnS quantum dots suitable for design of photonic and sensor material

Highly luminescent ZnS quantum dots (QDs) and their Be2+ and Mg2+ derivatives having nominal compositions Zn1−xBexS and Zn1−xMgxS (where x=0.0–0.6 and Δx=0.1) have been synthesized using the co-precipitation technique. The synthesized QDs are stable for a period of up to 7 months. They have a cubic closed packed (zinc blende) structure and average particle size of 2–3nm as measured by TEM, XRD and UV–vis spectral analysis. Incorporation of Be2+ and Mg2+ into the ZnS lattice has shown noteworthy effects on the intensity of defect-states emission that enhances the luminescence intensity of the QDs. The absorption bands in the visible spectral range for ions of Be and Mg elements are observed to overlap with the fluorescence transitions of ZnS QDs. These highly luminescent and stable QDs could be utilized for the development of new quantum dot-based sensors, light emitting diodes, phosphor and coating for long term applications.

Synthesis and optical characterization of single phased ZnS:Mn2+/CdS core–shell nanoparticles

Uncoated ZnS, ZnS:Mn2+, CdS and different thickness of CdS coated ZnS:Mn2+ core–shell nanoparticles were successfully synthesized by a simple chemical method in an air atmosphere. X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were used to characterize the uncoated and the novel ZnS:Mn2+/CdS core–shell nanoparticles. The results show that the size of the ZnS:Mn2+/CdS core–shell nanoparticles is less than the bare ZnS:Mn2+. The PL study of ZnS:Mn2+/CdS core–shell nanoparticles shows an enhanced intensity than ZnS:Mn2+. The coating of CdS layer over ZnS:Mn2+ tuned the PL emission in the visible region. Addition of cadmium acetate (Cd 4 and 5M) in ZnS:Mn2+ shows a distinct PL peak centered at 542nm. The presence of Mn2+ ions in ZnS lattice and the growth of the CdS on ZnS:Mn2+ nanoparticles were confirmed by the ESR spectra.

Synthesis and luminescence properties of ZnS and metal (Mn, Cu)-doped-ZnS ceramic powder

ZnS and metal (Mn, Cu)-doped-ZnS were successfully prepared by wet chemical synthetic route. The understanding of substituted metal ions (Mn, Cu) into ZnS leads to transfer the luminescent centre by small amount of metal dopant (Mn, Cu). Fourier transform infrared and X-ray diffraction were used to determine chemical bonding and crystal structure, respectively. It showed that small amount of metal (Mn, Cu) can be completely substituted into ZnS lattice. X-ray fluorescence was used to confirm the existence of metal-doped ZnS. Scanning electron microscope revealed that their particles exhibits blocky particle with irregular sharp. Laser confocal microscope and photoluminescence spectroscopy showed that ZnS and metal-doped-ZnS exhibited intense, stable, and tunable emission covering the blue to red end of the visible spectrum. ZnS, Mn-doped-ZnS and Cu-doped-ZnS generated blue, yellow and green color, respectively.

Magnetic and optical properties of Fe doped ZnS nanoparticles synthesized by microemulsion method

Fe doped ZnS nanoparticles, synthesized in microemulsion, consist of several nanocrystals with size of about 5 nm. The evolvement of the structure and morphology with the increase of Fe concentration indicates that Fe doping reduces crystallinity. The magnetic measurement shows that Zn 0.9Fe 0.1S is ferromagnetic at room temperature and the coercivity decreases with the increase of Fe concentration. Significant blue shift of absorption edge in the UV–vis spectra induced by Fe doping is observed. The X-ray absorption fine structure (XAFS) spectra of Fe K-edge indicate the Fe ions in ZnS lattice are trivalent and the local structure around dopant is analyzed.

Hydrothermal synthesis and optical properties of Zn 1− xMn xS nanorods

Zn 1− x Mn x S ( x = 0–0.05) nanorods were successfully synthesized through a hydrothermal route. The morphology, composition and microstructure of Zn 1− x Mn x S nanorods were characterized respectively by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometer. The optical properties of Zn 1− x Mn x S nanorods were determineded by UV–Vis absorption spectroscopy and photoluminescence (PL) emission spectroscopy. The results show that the introduction of Mn 2+ on interstitial sites in ZnS lattice has significant influence on the Raman spectra, UV–Vis absorption spectra and PL emission spectra. With the increase of Mn 2+, the lengths of the sampled nanorods become shorter and the morphologies of the products show disorder accompanied by some nanoparticles. The absorption band edge shifts to longer wavelength. The intensity of the ZnS-related emission gradually becomes weaker, whereas, the Mn 2+-related broad emission spectra located at 580 nm gradually red-shifts and increases. Occupation of Zn 2+ sites in the lattice by Mn 2+ ions results in lattice distortion and influences the energy level structure of ZnS. The Mn doping is found responsible for the changes in the defect-related emission of the ZnS nanorods.

Zinc sulfide nanoparticles: a mechanism of formation in aqueous solutions and optical properties

The formation of sols and precipitates of zinc sulfide as a result of the exchange reaction in an aqueous solution was studied. The precipitates consist of aggregates of primary particles about 3 nm in size. The primary ZnS particle size in aqueous sols increases with an increase in the concentration of zinc sulfate and sodium sulfide, with the accumulation of the final reaction product, and with temperature. This effect does not exceed an 1.5-fold increase. At the first step, the particles with a considerable fraction of the amorphous phase are formed and undergo intragrain crystallization. The photoluminescence properties of aqueous sols of zinc sulfide were studied. They are caused by defects in the ZnS lattice and by the presence of the lattice oxygen.

Effect of Cr doping on the structural and optical properties of ZnS nanoparticles

AbstractZn1−xCrx S nanoparticles with x = 0.00, 0.005, 0.01, 0.02 and 0.03 were synthesized by chemical co‐precipitation method at room temperature for the first time. Ethylene diamine tetraacetic acid was used as stabilizer. Energy dispersive analysis of X‐rays confirmed the presence of Cr in the samples. The samples were characterized by scanning electron microscopy, X‐ray diffraction (XRD), transmission electron microscopy and optical absorption studies. XRD and selected area electron diffraction results showed that the samples of all compositions crystallized in cubic structure and the lattice parameters decreased linearly with increase in Cr content following Vegard's law indicating that the Cr ions have substituted for Zn in the ZnS lattice. The particle size estimated from XRD was in the range 6–10 nm.The optical absorption studies on the doped samples indicated that the absorption edge blue shifted with respect to those of bulk and nanocrystalline ZnS. The bandgap increased with increasing Cr concentration in a narrow range 3.81–4.03 eV. Photolumonesence studies showed blue emission with appreciable luminescence quenching with increasing Cr concentration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and optical properties of Cr doped ZnS nanoparticles capped by 2-mercaptoethanol

Nanoparticles of Zn 1− x Cr x S ( x=0.00, 0.005, 0.01, 0.02 and 0.03) were prepared by a chemical co-precipitation reaction from homogenous solutions of zinc and chromium salts. These nanoparticles were sterically stabilized using 2-mercaptoethanol. Here a study of the effect of Cr doping on structural, morphological and optical properties of nanoparticles was undertaken. Elemental analysis, morphological, structural and optical properties have been investigated by energy dispersive analysis of X-rays (EDAX), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible spectroscopy .EDAX measurements confirmed the presence of Cr in the ZnS lattice. XRD showed that ZnS:Cr nanoparticles crystallized in zincblende structure with preferential orientation along (1 1 1) plane. The average sizes of the nanoparticles lay in the range of 3–6 nm and lattice parameters were in the range of 5.2–5.4 Å. Lattice contraction was observed with an increase of Cr concentration. The particle size and lattice parameters obtained from TEM and SAED images were in agreement with the XRD results. The absorption edge shifted to lower wavelengths with an increase in Cr concentration as per UV–Vis spectroscopy. The band gap energy values were in the range of 3.85–4.05 eV. This blueshift is attributed to the quantum confinement effect.

Microwave-assisted preparation of Zn1−xCuxS nanoparticles by a fast, green, and template-free method and photocatalytic activity

A series of Zn1−xCuxS nanoparticles with dopant content ranging from x=0.00 to 0.15 was prepared in water using microwave irradiation. The nanoparticles were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–Vis diffuse reflectance spectra (DRS) techniques. It was found that Cu2+ ions are homogeneously substituted for Zn2+ ions in ZnS lattice. The absorption edges for Zn1−xCuxS nanoparticles are red-shifted as content of Cu2+ ions increases. The SEM images demonstrate that the nanoparticles are nearly spherical and the size decreases with Cu2+ content. Photocatalytic degradation of methylene blue (MB) on the prepared nanoparticles was investigated under visible light irradiation. The results indicate that Zn0.97Cu0.03S nanoparticles exhibit highest photocatalytic activity among the prepared samples. Increasing photocatalytic activity of Zn0.97Cu0.03S nanoparticles has been attributed to its high visible light absorption and decrease of electron-hole recombination due to presence of Cu2+ ions in the lattice. Moreover, the reaction rate constant decreases with microwave irradiation time applied for preparation of the nanoparticles and calcinations temperature.

Origin of the Enhanced Visible Photocatalytic Activity in (N, C)-Codoped ZnS Studied from Density Functional Theory

Density functional theory calculations are used to investigate the origin of the experimentally observed changes in visible photoactivity of hexagonal wurtzite ZnS induced by (N, C) codoping. The accurate comparative analyses of geometric and electronic structures for the different doping models have been discussed. For the mono N- or C-doped ZnS systems, the substituted doping just induces a slight band gap narrowing (less than 0.2 eV), while the interstitial doping leads to more significant optical absorption red shift. However, the calculated energy indicates the formation of the interstitial doping is difficult with the lower impurity concentration. For the N+C-codoped systems, the electron transition from the impurity states in the gap to the conduction band will induce a large red shift, suggesting the visible-light absorption. However, the impurity states are partially occupied character, which may act as recombination center and reduce the photoinduced current density. The calculated results of 2N+C-codoped ZnS indicate the N−C−N trimer structure was formed in the ZnS lattice, and the trimer doping induces a larger red shift of photoelectron transition and passivates the partially occupied states by the charge compensation effect in acceptor−donor−acceptor pair. Our work provides a solid basis for the rationalization of experimentally observed red shift of optical absorption in wurtzite ZnS as a consequence of (N, C) codoping and shows that 2N+C codoping will be a promising way for improving the visible-light activity of semiconductor photocatalysts.

Analysis of the effect of annealing on the photoluminescence spectra of Cu+ ion implanted ZnS nanoparticles

In the present study, we report the photoluminescence (PL) study of nanoparticles of ZnS implanted with Cu+ ions at the doses of 5×1014, 1×1015 and 5×1015ions/cm2 and annealed at 200 and 300°C. The photoluminescence spectra of the samples implanted at lower doses of 5×1014 and 1×1015ions/cm2 and annealed at 200 and 300°C showed peaks at around 406, 418 and 485nm. The PL emission peak at 485nm was attributed to the transition of electrons from conduction band of ZnS to the impurity level formed by the implanted Cu+ ions. In the PL spectrum of the sample implanted at the highest dose of 5×1015ions/cm2, in addition to the emission peaks observed in the PL spectra of the samples implanted at lower doses, a peak at around 525nm, the intensity of which decreased with increase in the annealing temperature, was observed. The emission peak at 525nm was attributed to the transitions between sulfur and zinc vacancy levels. The full width at half maximum (FWHM) of the emission peak at 406nm was observed to decrease with increase in annealing temperature, indicating lattice reconstruction. The observation of copper ion impurity related peak at 485nm in the PL spectra of samples of the present study indicated that the doping of copper ions into the ZnS lattice is achievable by implanting Cu+ ions followed by annealing.

Diffusional relaxation in ZnS after thermal doping at 800°C

Photoluminescence spectra of powdered ZnS thermally doped with MnS are studied. Correlations are demonstrated between variations in the luminescence characteristics of ZnS:Mn, on one hand, and some features of radiation center formation and the diffusion of Mn in ZnS after processing, on the other. It is found that after manganese doping at a temperature (T = 800°C) lower than the phase transition temperature of ZnS, relaxation processes owing to diffusion of Mn in ZnS take place in the material over times as long as 6.10 3 h. It is shown that 6.10 3 h after doping the α-MnS phase is essentially completely dissolved in the volume of the ZnS. Diffusion of Mn in powdered ZnS is found to occur via several channels, rapid diffusion along interior boundaries and slow diffusion via interstitial space, which indicates the existence of different activation energies for diffusion of Mn depending on its localization within the ZnS lattice.

XAFS analysis and luminescent properties of ZnS:Mn2+ nanoparticles and nanorods with cubic and hexagonal structure

Abstract ZnS:Mn 2+ nanocrystals (NCs) were synthesized by a simple solvothermal method at 180 °C in different solvents. The NCs prepared in ethanol yielded cubic nanoparticles (NPs) with diameters of 10–15 nm. The NCs prepared in ethylenediamine and water (1:1 in volume ratio) yielded hexagonal nanorods (NRs) with diameters of 8–10 nm. X-ray absorption fine structure (XAFS) measurements were carried out to probe the local environment surrounding the Mn ions in the NPs and the NRs. The results showed that Mn ions were incorporated into the ZnS lattice substituting the Zn sites. The yellow–orange emission from the Mn 2+ 4 T 1 – 6 A 1 transition was observed, its intensity relative to the blue–green emission increased from NPs to NRs. The surface effect was the main factor affecting the thermal and optical stability of NCs. The fluorescence lifetime of the Mn 2+ emission for the NPs and NRs was 0.662 and 0.224 ms, respectively.

Preparation and photoluminescent properties of doped nanoparticles of ZnS by solid-state reaction

Nanometer-sized Eu 3+-doped ZnS and Mn 2+-doped ZnS particles were prepared by solid-state method at low temperature. The structures and properties of those materials were characterized by X-ray diffraction (XRD) and photoluminescent spectroscopy techniques. The XRD patterns reveal that the doped ZnS nanoparticles belong to zinc-blende structure. The concentration of doping ions has little effect on the sizes of the doped ZnS nanoparticles, which mainly depends on the temperature of preparation. The emission peaks from the 5D 0→ 7F J ( J=1, 2, and 4) electronic energy transitions of Eu 3+ were observed in the emission spectra of the ZnS:Eu 3+ nanoparticles. The intensity ratio of the two peaks from the 5D 0→ 7F 1 and 5D 0→ 7F 2 transitions indicates that more Eu 3+ ions occupy the sites with no inversion symmetry. For the ZnS:Mn 2+ nanoparticles, an orange emission from the 4T 1→ 6A 1 transition of Mn 2+ is present, indicating that the doping ions occupy the positions of the ZnS lattices. Meanwhile, UV-induced luminescence enhancement was observed for the ZnS:Mn 2+ nanoparticles, the possible reason of which is discussed primarily.

Soft x-ray emission spectra and ferromagnetism in wide-gap doped semiconductors

A study is made of the resonant and nonresonant L x-ray emission spectra of impurities in the semiconducting compounds ZnS:Mn, ZnO:Mn, ZnO:Co, and Co2O:Mn. An analysis of the Mn L2,3 x-ray emission spectra of Zn1−xMnxS (x=0.1–0.3) reveals that the Mn impurities do not form clusters in the ZnS lattice. Studies of the Mn L2,3 spectra and electronic structure of epitaxial films of Zn0.8Mn0.2O annealed at different temperatures show that the cause of the observed suppression of ferromagnetism at T>600°C is segregation of Mn atoms. In this case the Mn atoms occupy both Zn sites and interstitial positions. For Zn1−xCoxO (x=0.02, 0.06, and 0.10) the absence of free carriers that could mediate an exchange interaction between Co ions is established. Mn L2,3 x-ray emission measurements show that in Mn-doped oxides Cu2O synthesized at 650 and 800°C the Mn atoms are found both in interstitial positions and occupy Cu sites, but the configurations of these defects depend on the synthesis temperature. A decrease of the Curie temperature with increasing synthesis temperature may be explained as a manifestation of antiferromagnetic superexchange between substituent Mn atoms via oxygen.