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 ∼2nm. 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 434nm 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 514nm, can be achieved in ZnS nanophosphors.