Copper concentration dependence of structure, morphology and optical properties of ZnS:Cu,Cl phosphor powder

ZnS:Cu,Cl phosphors were prepared by firing ZnS powder with 1 wt% NaCl and CuS nanocrystallites in the range of 40–5000 ppm at 900 °C for 2 h in an atmosphere of 3% H2/Ar and sulfur vapor. The particle size and structure of the ZnS:Cu,Cl powders were found to depend on the Cu concentration. The powders with Cu⩾400 ppm exhibited a cubic structure and were approximately 5 μm in size as compared to the hexagonal structure of those with less Cu additions (∼2 μm). The difference, resulting from the precipitation of CuxS in the powders as determined by X-ray photoelectron and diffuse reflectivity analyses, revealed that the powders with Cu>400 ppm showed the Cu 2p main peaks and higher absorbance in the UV–vis region due to the wide range of the bandgap energy of CuxS (1.2–1.75 eV). Electroluminescence (EL) consisting of blue, self-activated (SA) and green emission peaking at 440, 470 and 510 nm, was observed only in the powders with Cu⩾400 ppm. Besides the activator concentration quenching effect, the variation of EL intensity with the Cu concentration was elucidated to result the from amount of CuxS precipitates as well as charge carriers in the powders.