Photoinduced transient absorption within zinc sulphide phosphors in powder form detected by diffuse reflectance laser flash photolysis

Transient absorptions have been obtained in powdered samples of ZnS : Ag, ZnS : Cu and undoped ZnS at room temperature by the technique of diffuse reflectance laser flash photolysis. For the undoped material an absorption was detected that maximises at 400 nm and decays non-exponentially with an initial half-life of (6.8±1.0)×10^?6s. The absorption is assigned to a transition within a donor impurity. In the cases of ZnS : Ag and ZnS : Cu, broad featureless absorptions in the 500–850 nm region were recorded. These decays are non-exponential and have initial half-lives of (3.1±0.2) ms and (0.48±0.05) ms for the Ag- and Cu-doped phosphors, respectively. These transient absorptions are considered to occur from electron traps introduced by the metal dopant. Laser-induced emission was also studied, and only in the case of undoped ZnS was a correlation observed between the decay of the weak emission at 520 nm and the decay of the transient absorption.