ZnS Lattice Research Articles

Tb 3+ ions in presence of ZnS:Mn 2+ nanocrystals immobilized on silica: Energy transfer ZnS→Tb 3+ and coordination state of Mn 2+ ions

Three-component luminescent material consisting of silica xerogel as a support with immobilized ZnS:Mn 2+ nanocrystals and Tb 3+ ions was compared with such two-component materials as the silica support with ZnS:Mn 2+ as well as the support with Tb 3+. In each case the nanocrystals and the lanthanide ions were immobilized at silica surface by impregnation procedure. Size of the ZnS quantum dots doped with Mn 2+ were estimated by Scherrer method from the X-ray diffraction (XRD) pattern. The materials have been characterized by EPR and optical spectroscopy techniques. EPR spectra allow to distinguish two different Mn 2+ sites: the first is assigned to isolated Mn 2+ substitutionally and incorporated into cubic ZnS lattice and the second is ascribed to the Mn 2+ situated near the nanocrystal surface. From the optical spectra we have found that in the three-component material, energy transfer from excited ZnS:Mn 2+ nanocrystals to Tb 3+ ions takes place. The different mechanisms of such transfer are discussed.

Effect of Precursor Molar Ratio of [S2−]/[Zn2+] onParticle Size and Photoluminescence of ZnS:Mn2+ Nanocrystals

We investigate the influence of precursor molar ratio of[S2−]/[Zn2+] on particle size and photoluminescence (PL)of ZnS:Mn2+ nanocrystals. By changing the[S2−]/[Zn2+] ratio from 0.6 (Zn-rich) to 2.0 (S-rich),the particle size increases from nearly 2.7 nm to about 4.0 nm. Theincrease in the ratio of [S2−]/[Zn2+] causes a decreaseof PL emission intensity of ZnS host while a distinct increase ofMn2+ emission. The maximum intensity for the luminescence ofMn2+ emission is observed at the ratio of[S2−]/[Zn2+]≈1.5. The possible mechanism for the resultsis discussed by filling of S2− vacancies and the increase ofMn2+ ions incorporated into ZnS lattices.

Electroluminescence materials ZnS:Cu,Cl and ZnS:Cu,Mn,Cl studied by EXAFS spectroscopy

When a high-frequency ac voltage is applied to ZnS:Cu,Cl and ZnS:Cu,Mn,Cl devices, optical fluorescence, known as ac electroluminescence (EL), is observed; it depends on both the Cu (Mn) and Cl dopants. The local structure of these compounds was studied using the extended x-ray-absorption fine-structure (EXAFS) technique to understand the role of Cu and Mn. Data were taken at the $K$ edge of Zn, Cu, and Mn, for powder material and for both new and aged (degraded, low EL) devices. The EXAFS data show that Mn substitutes for Zn in the ZnS lattice, whereas Cu has a different local structure (it cannot be fit well with the ZnS structure). For all the Cu edge data, the first shell of neighbors is best fit using an experimental standard obtained from CuS, suggesting that almost all of the Cu resides in tiny CuS-like clusters. Since these clusters are dominant in both the new and degraded samples and do not change with aging, they likely do not contribute directly to the luminescence. Consequently, our results indicate that a very small fraction of the Cu atoms are EL-active; this is consistent with previous models for which the emission centers involve isolated Cu defects or Cu pairs. Thus, a possible explanation for the rapid degradation of a device is that the isolated Cu ions electrodiffuse to the CuS-like clusters, at which point they no longer produce EL. Based on these results, a low-temperature annealing experiment (200C) was carried out that shows that a degraded device can be nearly completely rejuvenated by heating.

Optimizing the Synthesis of Red- to Near-IR-Emitting CdS-Capped CdTexSe1-x Alloyed Quantum Dots for Biomedical Imaging

Advancements in biomedical imaging require the development of optical contrast agents at an emission region of low biological tissue absorbance, fluorescence, and scattering. This region occurs in the red to near-IR (>600 nm) wavelength window. Quantum dots (Qdots) are excellent candidates for such applications. However, there are major challenges with developing high optical quality far-red- to near-IR-emitting Qdots (i.e., poor reproducibility, low quantum yield, and lack of photostability). Our aim is to systematically study how to prepare alloyed CdTexSe1-x with these properties. We discovered that the precursor concentrations of Te-to-Se and growth time had major impacts on the Qdot's optical properties. We also learned that the capping of these alloyed Qdots were difficult with ZnS but feasible with CdS because of the ZnS's lattice mismatch with the CdTexSe1-x. These systematic and basic studies led to the optimization of synthetic parameters for preparing Qdots with high quantum yield (>30%), narro...

Optical and Magnetic Properties of Mn‐Incorporated ZnS Nanorods

Mn‐doped ZnS (MnxZn1‐xS) nanorods were synthesized by a simple solvothermal process. Synthesized nanorods were single crystalline. Mn incorporation in the ZnS lattice induces a phase transformation from hexagonal wurtzite to cubic zincblende structure. Intense orange luminescence at ∼585 nm was observed for the doped ZnS nanorods. Six‐hyperfine splitting was observed in the EPR spectra for lower Mn concentrations and broad Lorentzian‐shaped EPR spectra were obtained for higher Mn concentrations.

Synthesis and Luminescence of ZnMgS:Mn<SUP>2+</SUP> Nanoparticles

Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.

Optical and Magnetic Properties of Manganese-Incorporated Zinc Sulfide Nanorods Synthesized by a Solvothermal Process

Manganese-incorporated ZnS (MnxZn1-xS) nanorods were synthesized by a simple solvothermal process. Synthesized nanorods were single crystalline. Manganese incorporation in the ZnS lattice induces a phase transformation from hexagonal wurtzite to cubic zinc blende structure. The diameter of the nanorods increased with the increase of Mn concentration. Intense orange luminescence at approximately 585 nm was observed for the nanorods. Six-line hyperfine splitting was observed in the EPR spectra for lower Mn concentrations, whereas broad Lorentzian-shaped EPR spectra were obtained for higher Mn concentrations because of the Mn-Mn cluster formation at higher Mn concentrations.

Influence of Fe content on the structural and optical properties of ZnFeS thin films

ZnFeS thin films have been grown on c-plane sapphire substrate using low-pressure metalorganic chemical vapor deposition (LP-MOCVD) equipment. The X-ray diffraction (XRD) patterns indicated that the diffraction peaks positions of ZnFeS thin films shifts gradually towards the lower angle side due to substitution of Zn by Fe atoms in the crystal lattice with increasing Fe content. When gas flow rates of Fe (CO) 5 varied from 1 to 6 ml min −1, crystalline phase of the ZnFeS thin film changed from the hexagonal to cubic structure. And when gas flow rates of Fe (CO) 5 was less than 6 ml min −1, ZnFeS single-crystal structure was obtained. From the measurements of optical absorption spectrum, the energy gaps of the samples were evidently narrowed with increasing Fe component. The electronic states of ZnFeS were investigated by X-ray photoelectron spectroscopy (XPS). It was found that the S 2p shifted towards higher binding energies side due to the larger bond strength of Fe–S than that of Zn–S, but the Fe incorporated into the ZnS lattice did not affect the Zn 2p core levels.

Certain features of Ga diffusion in ZnS powders

Spectra of photoluminescence and electron spin resonance were investigated for ZnS powders annealed in the presence of metallic Ga with limited access of atmospheric air. Analysis of these spectra showed that there was no Ga impurity in the annealed ZnS powders. It was established that subsequent free access of air to annealed ZnS:Ga promotes active Ga introduction into the ZnS lattice. A mechanism of Ga diffusion in ZnS is suggested.

Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu + and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is ∼2 nm. The effects of change in stoichiometry and doping with Cu + and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (V S) related emission with peak maximum at 434 nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result V Zn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu + activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as co-activator helps to increase the solubility of Cu + ions in ZnS lattice and also enhances the donor–acceptor type emission efficiency. With increase in Cu + doping, Cu-Blue centers (Cu Zn −Cu i +), which were dominant at low Cu + concentrations, has been transformed into Cu-Green (Cu Zn −) centers and the later is found to be situated near the surface regions of nanoparticles. From these studies we have shown that, by controlling the defect chemistry and suitable doping, photoluminescence emission tunability over a wide wavelength range, i.e., from 434 to 514 nm, can be achieved in ZnS nanophosphors.

Comparison of electronic structures of doped ZnS and ZnO calculated by a first-principle pseudopotential method

Electronic band calculations of doped and undoped ZnO and ZnS have been done using density functional theory under the local density approximation so as to clarify the reason of the difference in behaviors of doped ZnO and ZnS. The reason why the electrical conductivity of ZnS is difficult to be increased by doping was discussed. In the case of doped ZnS, an impurity level was generated at deep position below the bottom of the conduction band of the host ZnS lattice and Fermi level was located at this impurity level. On the contrary, the shape of the density of states curve and the band structures of doped and undoped ZnO are alike with each other and the donor band is hybridized with the conduction band of the ZnO host material. This seems to result in contribution of doped electrons to electrical current in the case of doped ZnO.

Structure and luminescence of annealed nanoparticles of ZnS:Mn

Structural and light-emitting properties of nanoparticles of ZnS:Mn annealed in vacuum at temperatures up to 525 °C are presented. Annealing the 3.5 nm particles at temperatures up to 350 °C caused growth of some particles without substantial change in the luminescence or ZnS lattice. After annealing at 400–525 °C, the high-temperature wurtzite phase of ZnS appeared, accompanied by an increase of the average particle diameter to approximately 100 nm and a rearrangement of the Mn ions. Dramatic increase in cathodoluminescence emission was also observed and is compared to the structural information obtained from electron microscopy, x-ray diffraction, x-ray absorption fine structure, and electron paramagnetic resonance measurements.

Photoluminescence of ZnS: Sm phosphor prepared in a reductive atmosphere

The characteristic emission of Sm+3 in Zn phosphors, fired in inert atmosphere (N2 gas) at various temperatures, was not detected. However, fired in reductive atmosphere at 1050°C, the characteristic Sm+3 emission appears, which exhibits intra-4f transitions at 570, 606 and 653 nm, as predicted from the 4G5/2 level to the 6H5/2, 6H7/2 and 6H9/2 levels. Sm doping favors the formation of hexagonal phase in host lattice of ZnS:Sm. The increase in hexagonal phase content will boost the overall photoluminescence emission intensity. The self-activated luminescence intensity increases with the increase in the amount of Sm doping up to a maximum at 0.2 mol% of Sm dopant. According to the phase identification by X-ray diffraction, the Sm2O2S phase favors the Sm+3 characteristic luminescence but it deteriorates in Sm2O3.

X-ray spectra and electronic structure of boron nitride in different crystallographic modifications

The electronic energy structure of boron nitride with ZnS (c-BN) and wurtzite (w-BN) type crystal lattices is calculated by the local coherent potential (LCPA) method in a multiple scattering approximation. The local partial 2p states of boron with c-BN and w-BN are compared with the boron K emission spectra in the corresponding compounds. Fine structure is first obtained in the region of the top of the valence band.

Effect of ultraviolet irradiation on the luminescence and optical properties of ZnS: Mn films

Changes have been observed and investigated in the electrooptic properties of ZnS: Mn films used in thin-film electroluminescent structures as a result of irradiation by ultraviolet pulses with energies per pulse much smaller than the threshold energy of laser annealing. It is found that in disordered ZnS: Mn films processes of defect generation are important even for below-threshold energies of the ultraviolet radiation pulses, and can facilitate the effective diffusion and activation of Mn atoms in the ZnS lattice. It is shown that short-time ultraviolet processing of thin-film electroluminescent structures improves their characteristics and, by making the preparation technology simpler and cheaper, allows structures with detector characteristics to be fabricated on low melting-point substrates.

Structural analysis of ZnS/GaAs heterostructures grown by hydrogen transport vapor-phase epitaxy

The structural characterization of ZnS epilayers grown by hydrogen transport vapor-phase epitaxy on (100)-oriented GaAs substrates is reported. X-ray-diffraction measurements in both double-axis (DA) and single-axis modes were performed to determine the residual strain tensor and the strain temperature dependence of the epilayer in the range between 25 and 650 °C. From the analysis of the data obtained by DA measurements of several symmetric Bragg reflections at different azimuth angles and several asymmetric reflections recorded in different geometries, an orthorhombic distortion of the ZnS lattice was found. The crystallographic symmetry could be explained by an asymmetric distribution of the misfit dislocation density in the interface plane along the [011] and [01̄1] directions. The temperature dependence measurements of the strain tensor components between room temperature and the growth temperature (650 °C) allowed determination of the thermal misfit between ZnS and GaAs and the linear thermal-expansion coefficient of ZnS. Finally, triple-crystal diffractometry and secondary-ion-mass spectrometry were used to investigate the chemical nature of the ZnS-GaAs interface. The results indicate the presence of an interdiffused ZnS-GaAs interface region, whose occurrence turns out to be associated with the initial defect structure of the substrate surface before the growth.

Lattice strain relaxation of ZnS layers grown by vapour-phase epitaxy on (100) GaAs

The structural characterization of ZnS epilayers grown on (100) GaAs by H 2 transport vapour-phase epitaxy is reported. High-resolution X-ray diffraction and ion channelling Rutherford backscattering spectrometric measurements were used to evaluate the overall crystalline perfection of the epilayers and the distortion and reciprocal orientation of the ZnS and GaAs lattices and to measure the epilayer built-in lattice strain. High crystalline quality can be achieved at the surface of relatively thick (above ca. 1 μm) ZnS layers, although a dense distribution of extended defects was observed close to the ZnS/GaAs interface. It turns out that the ZnS unit cell is orthorombically distorted, although its deviation from a purely tetragonal distortion is very small. Systematic measurements of the ZnS lattice strain were performed on the epilayers and the results were compared with the thermal strain value obtained by temperature-dependent X-ray diffraction measurements. The data indicate that the ZnS built-in lattice strain can be entirely ascribed to a thickness-dependent thermal strain contribution, the initial lattice misfit at the growth temperature being almost totally relaxed even for relatively thin epilayers. Finally, no appreciable strain was found for epilayer thicknesses above about 3 μm.

XPS studies of the Zn1-xCoxS electronic structure

We have investigated electronic states of diluted magnetic semiconducting Zn1-xCoxS compounds, using X-ray photoelectron spectroscopy (XPS). The amount of Cox substituting Zn in the ZnS lattice was in the range 0<x<0.25. In order to evaluate the influence of Co on the electronic structure of these compounds, XPS spectra of both the core states and the valence band were analysed. We found that the addition of Co into the ZnS lattice did not affect the Zn 2p core levels but that in these alloys the S 2p doublets were split into two components, with a binding energy difference of 0.3 eV. The intensity ratio of the two S 2p components followed the changes in the Zn-Co ratio of the alloys. The binding energy of Co 2p core levels in the alloys was similar to that in CoO, indicating the similarity between the ionic strength (+2) of the Co-S and Co-O bonds. The valence band spectra revealed a significant contribution of Co 3d electrons to the density of states (DOS). The valence band DOS distribution was also calculated from theory, using a tight-binding version of the coherent potential approximation. According to the calculations, an increase in the Co content of the alloy should cause changes in the valence band DOS similar to those observed from the X-ray photoelectron spectra.

The influence of II–VI lattice damage on rare earth sites as determined by site selective spectroscopy and cathodoluminescence: argon implanted ZnS:Sm 3+

Using site selective and time resolved spectroscopy more than 10 different Sm sites have been discriminated. Beside the direct 4f-intra-shell excitation of the rare earth (RE) impurities a second indirect excitation channel via the host lattice is active with increasing laser power due to two-photon processes. The RE sites differ mainly in their ability to gain energy from this second channel. This knowledge is used to study the influence of host lattice damage — caused by Ar-implantation — on the Sm 3+ luminescence. Using cathodoluminescence (CL) and photoluminescence (PL) it is shown that the RE emission is less affected by the damage than the near-band-edge (NBE), self-activated (SA) and donor-acceptor (DA) emission. No new Sm sites have been created by the implantation and the direct 4f-4f excitation channel is still efficient, whereas the energy transfer (ET) via the ZnS lattice is disturbed leading to an intensity decrease. These results are taken as an evidence for the complex nature and the relative stability of RE centers in a II–VI lattice.

Deep europium-bound exciton in a ZnS lattice.

Eu-related recombination processes in ZnS are discussed on the basis of electron-spin-resonance (ESR) and optical studies. The absence of any ${\mathrm{Eu}}^{2+}$ and/or ${\mathrm{Eu}}^{3+}$ intra-ion emissions is explained as a consequence of the midgap position of ${\mathrm{Eu}}^{2+}$ in ZnS. A new Eu-related infrared emission is observed and attributed to a bound-exciton (BE) recombination. In the Eu-bound exciton the hole is strongly localized on the 4f shell of Eu, whereas the electron is either delocalized on the 12 nearest-neighbor Zn-cation sites (for isolated Eu) or trapped at a compensating ion (for Eu complexes). The BE dissociation energy is determined to be about 10 meV.