Microstructure and electroluminescence of ZnS:Cu,Cl phosphor powders prepared by firing with CuS nanocrystallites

This paper presents the microstructure and electroluminescent performance of ZnS:Cu,Cl phosphor powders prepared by firing micrometer-sized ZnS with NaCl and CuS nanocrystallites at 900°C in the reducing atmosphere. A series of samples with Cu addition ranging from 40 to 5000 ppm were studied. XRD analysis showed that ZnS:Cu,Cl samples with Cu addition of ∼400 ppm exhibited a transformation from hexagonal to cubic structure. The whole series of ZnS:Cu,Cl samples showed significant photoluminescent (PL) intensity; however, only samples with Cu addition of ∼400 ppm revealed measurable electroluminescent (EL) intensity. This difference was supposed to be a result of nano-sized Cu_xS precipitation in ZnS during firing treatment, where Cu _xS acted as electron emission source to enhance the EL intensity. Furthermore, ZnS:Cu,Cl powder sections were analyzed using X-ray mapping (XRM) of a scanning transmission electron microscope, revealing that Cu_xS precipitates of 50-80 nm in size were observed only in the samples with EL emission, i.e. Cu addition ∼400 ppm. The variation of EL intensity was interpreted in terms of the concentration of Cu activators as well as the phase and the amount of Cu_xS in ZnS.