Well-defined fluorescent particles of ZnS:Cu. 1. Preparation and effects of annealing

As an application of the gel-sol procedure for the synthesis of monodispersed particles in large quantities, uniform particles of ZnS:Cu were prepared by different methods from concentrated Zn-chelate solutions of nitrilotriatic acid (NTA) with thioacetamide (TAA), in which the dopant copper ion was introduced as a coexisting mixed chelate with Zn-NTA (Method A), a Cu-NTA complex in a separate solution continuously added to a system of Zn-NTA and TAA (Method B), or a Cu(NO3)2 solution infiltrated into a dry powder of ZnS prepared by Method A (Method C). All powders thus prepared were annealed before the test of fluorescent properties. The main roles of annealing were to remove the grain boundaries of such particles as nonradiative recombination centers of excited electrons and holes and to achieve uniform distribution of dopants. However, the initial intraparticle distribution of the dopant, controlled by the different methods, decisively affected the fluorescent particles, even after the conversion of the polycrystalline particles to single-crystal particles by annealing.