ZnS nanoparticles doped with Cu(I) by controlling coordination and precipitation in aqueous solution

A novel method for the synthesis of ZnS:Cu colloids has been studied in aqueous solution, in which the Cu+ ions were incorporated into the ZnS matrix by the assistance of a sulfur-containing ligand, i.e. thiourea or thiosulfate. The optical properties of the colloids (absorption, emission and excitation spectra) were studied. Compared with the bulk materials, the absorption band-edge is blue-shifted, which is attributed to a quantum size effect. The average ZnS nanoparticle size, deduced from the absorption spectra by an effective mass approximation, is about 4 nm. Upon excitation at 325 nm, a broad green emission band is observed, while for samples doped with 0.2% Cu, a blue emission band at 450 nm is also observed, which is quenched at higher dopant concentrations. It is concluded that Cu+ ions occupy Zn2+ sites in the lattice of ZnS nanocrystals and act as both green and blue luminescent centers at different dopant concentrations. The sulfur-containing ligand (thiourea or thiosulfate) plays an important role in the synthesis as both ligand and surfactant; it stabilizes the Cu+ ions, reduces the solubility difference between ZnS and Cu2S, and protects the particles surfaces.