Effects of coatings on temporal cathodoluminescence quenching in ZnS:Ag,Cl phosphors

Powder phosphors of ZnS:Ag,Cl coated with SiO2 (22 or 130 nm nanoparticles), SnO2 or Al2O3 showed different cathodoluminescent (CL) brightness versus time (temporal CL quenching) behavior as compared to noncoated phosphors. At high current density (e.g., 300–800 μA/cm2), the CL emission intensity of coated ZnS:Ag,Cl decayed over the first ∼15 s of electron beam irradiation, which was postulated to result from a large concentration of nonradiative surface centers generated during surface modification of the phosphor, and from localization of generated electrons at the surface due to primary beam-induced internal electric fields. During the first ∼15 s of excitation, generated electrons are postulated to be redistributed by this induced internal electric fields, resulting in increased nonradiative surface recombination between electrons and holes. The formation of a nonradiative surface layer either from electron-stimulated surface chemical reactions on coated or from heat treatment of noncoated ZnS:Ag,Cl powder phosphors were shown to affect temporal CL quenching.