Thermally Controlled Cyclic Insertion/Ejection of Dopant Ions and Reversible Zinc Blende/Wurtzite Phase Changes in ZnS Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn2+ concentration in the nanocrystals.

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 similar to2 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 (Vs) 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 (CuZn-Cui+), 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. (C) 2003 Elsevier B.V. All rights reserved.

Observation of Combined Effect of Temperature and Pressure on Cubic to Hexagonal Phase Transformation in ZnS at the Nanoscale

The temperature of allotropic phase transformation in ZnS (cubic to wurtzite) changes with pressure and particle size. In this paper we have explored the interrelation among these through a detailed study of ZnS powders obtained by a temperature-controlled high energy milling process. By employing the combined effect of temperature and pressure in an indigenously built cryomill, we have demonstrated a large-scale, low-temperature synthesis of wurtzite ZnS nanoparticles. The synthesized products have been characterized for their phase and microstructure by the use of X-ray diffraction and transmission electron microscopic techniques. Further, it has been demonstrated that the synthesized materials exhibit photoluminescence emissions in the UV-visible region with an unusual doublet pattern due to the presence of both cubic and hexagonal wurtzite domains in the same particles. By further fine-tuning the processing conditions, it may be possible to achieve controlled defect related photoluminescence emissions from the ZnS nanoparticles.

Optical transmission larger than 1 (T > 1) through ZnS-SiO2/AgOx/ZnS-SiO2 sandwiched thin films

Optical transmission through flat media should be smaller than 1. However, we have observed optical transmission up to T=1.18. The samples were ZnS-SiO2/AgOx/ZnS-SiO2 sandwiched thin films on glass substrate. The supertransmission could only be observed in the near field. We attribute the supertransmission to the lateral propagation relayed by the laser activated and decomposed Ag nanoparticles. (c) 2006 American Institute of Physics.

Structural and optical investigation of Mn-doped ZnS nanocrystals

Using a solution-based chemical method, we have prepared ZnS nanocrystals doped with high concentration of Mn2+. The X-ray diffraction analysis confirmed a zinc blende structure. The average size was about 3 nm. Photoluminescence spectrum showed room temperature emission in the visible spectrum, which consisted of the defect-related emission and the T-4(1)-(6)A(1) emission of Mn2+ ions. Compared with the undoped sample, the luminescence of the ZnS:Mn sample is enhanced by more than an order of magnitude, which indicated that the Mn2+ ions can efficiently boost the luminescence of ZnS nanocrystals.

Photoluminescence properties of Eu3+-doped ZnS nanocrystals prepared in a water/methanol solution

Monodispersed ZnS and Eu3+-doped ZnS nanocrystals have been prepared through the co-precipitation reaction of inorganic precursors ZnCl2, EuCl3, and Na2S in a water/methanol binary solution. The mean particle sizes are about 3-5 nm. The structures of the as-prepared ZnS nanoparticles are cubic (zinc blende) as demonstrated by an x-ray powder diffraction. Photoluminescence studies showed a stable room temperature emission in the visible spectrum region for all the samples, with a broadening in the emission band and, in particular, a partially overlapped twin peak in the Eu3+-doped ZnS nanocrystals. The experimental results also indicated that Eu3+-doped ZnS nanocrystals, prepared by controlling synthetic conditions, were stable. (C) 2002 American Institute of Physics.

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Shape-Controlled Synthesis of Wurtzite ZnS Microstructures Under Mild Solvothermal Condition

Through a facile solvothermal route using zinc chloride and thiourea as reactants, wurtzite ZnS and its precursor ZnS center dot (en)(0.5) (en = ethylenediamine) with various morphologies and sizes were grown, which were characterized by XRD, SEM, TEM and N-2 adsorption and so on. The phase evolution, composition and morphologies of the products are highly dependent on the concentration of en. By keeping the en-water volume ratio at 1/2 to 1, the nanostripes-flower or nanorod-spheric wurtzite ZnS were easily obtained under 120 degrees C for 6-24 h, which possess relatively higher specific surface area and larger total pore volume.

Electrospinning Synthesis and Luminescence Properties of One-Dimensional Zn2SiO4:Mn2+ Microtibers and Microbelts

One-dimensional Mn2+-doped Zn2SiO4 rnicrobelts and microfibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The XRD and DTA results show that the Zn2SiO4 phase begins to crystallize at 800 degrees C and crystallizes completely around 1000 degrees C. SEM results indicate that the as-prepared microbelts/fibers are smooth, whose diameters decrease with increasing the annealing temperature. The average diameter of the Zn2SiO4:Mn2+ microfibers annealed at 1000 degrees C is 0.32 mu m, and their lengths reach up to several millimeters. The average width and thickness of the Zn2SiO4:Mn2+ microbelts fired at 1000 degrees C are around 0.48 and 0.24 mu m, respectively.

Well-organized structures of ZnS semiconductor nanocrystals using amphiphilic triblock copolymer aggregates as templates

Ring- and rod-shaped P4VP-b-PS-b-P4VP ( PS, polystyrene; P4VP, poly( 4-vinylpyridine)) triblock copolymer aggregates are used as templates to synthesize ZnS nanocrystals. Herein, PVP serves as both a stabilizing agent and a structure- directing agent. The resulting ZnS nanocrystals could be aligned along the corona of the copolymer aggregates in near-perfect structures through control of both the molar ratio of Zn2+ to P4VP and the reaction time. The diameter of the as-synthesized ZnS layer on the surface of polymer template is approximate 2 - 3 nm. High-resolution transmission electron microscopy images reveal that the ZnS particles are single crystal in a zinc blende structure. This method provides a simple, reproducible route at room temperature to prepare assembled hybrid polymer - semiconductor nanocrystal nanocomposites.

Langmuir-Blodgett film formation from surfactant-capped zinc sulfide nanoparticles spread on water surface

The surfactant-capped ZnS nanoparticulate multilayer film has been fabricated by Langmuir-Blodgett(LB) technique. ZnS LB firm was investigated by the small-angle x-ray diffraction(XRD), atomic force microscopy(AFM) and transmission electron microscopy(TEM). The results indicate that ZnS nanoparticulate LB film is one-dimensional superlattice.

Investigations on novel polymer nano composites and bio-compatible manganese doped ZnS nanocrystals for photonic and bio-imaging applications

Fluorescence is a powerful tool in biological research, the relevance of which relies greatly on the availability of sensitive and selective fluorescent probes. Nanometer sized fluorescent semiconductor materials have attracted considerable attention in recent years due to the high luminescence intensity, low photobleaching, large Stokes��� shift and high photochemical stability. The optical and spectroscopic features of nanoparticles make them very convincing alternatives to traditional fluorophores in a range of applications. Efficient surface capping agents make these nanocrystals bio-compatible. They can provide a novel platform on which many biomolecules such as DNA, RNA and proteins can be covalently linked. In the second phase of the present work, bio-compatible, fluorescent, manganese doped ZnS (ZnS:Mn) nanocrystals suitable for bioimaging applications have been developed and their cytocompatibility has been assessed. Functionalization of ZnS:Mn nanocrystals by safe materials results in considerable reduction of toxicity and allows conjugation with specific biomolecules. The highly fluorescent, bio-compatible and water- dispersible ZnS:Mn nanocrystals are found to be ideal fluorescent probes for biological labeling

Strutural and optical approach of CdS@ZnS core-shell system

Coordena����o de Aperfei��oamento de Pessoal de N��vel Superior (CAPES)

Addition of Zinc Sulfide Shells to Semiconducting Nanoparticles and its Effect on the Nanoparticle Fluorescence Emission Properties

The addition of a ZnS shell to CdSe and CdS quantum dot cores was explored using various methods. Spectrophotometry was used to assess the success of ZnS overcoating, which produces both an increase in overall fluorescence and decrease in particle size distribution. A new method was developed, involving preheating of the zinc and sulfide precursor solutions, resulting in CdSe(ZnS) particles with improved fluorescence and a more uniform shell coating from oleylamine-capped CdSe core particles.