Effects of coating and thermal annealing on the photoluminescence properties of ZnS/ZnO one-dimensional radial heterostructures

Influence of the thermal annealing atmosphere and the ZnO layer thickness on the photoluminescence properties of ZnS-core/ZnO-shell one-dimensional (1D) radial heterostructures was investigated. ZnS nanowires were synthesized by thermal evaporation of ZnS powders and then coated with ZnO by using the sputter-deposition technique to form the nanostructures. TEM and XRD analysis results reveal that a crystalline ZnS phase with a zinc blende structure and a crystalline ZnS phase with an wurtzite structure coexist in the cores whereas amorphous ZnO exists in the shells. Photoluminescence (PL) measurements at room temperature show that the green emission of the ZnS/ZnO 1D radial heterostructures is enhanced by annealing in an oxygen atmosphere due to the decrease in the concentration of oxygen vacancies in the ZnO shells, which normally absorbs the light emitted from the core, during the annealing process. In contrast, the green emission is degraded by annealing in an argon atmosphere since the light emitted from the core is absorbed more efficiently by the defects in the shell due to the crystallization of the ZnO shells during annealing. The PL emission intensity of the ZnS/ZnO 1D radial heterostructures is decreased by increasing the thickness of the ZnO shell layers due to the absorption of the light emitted from the ZnS cores by the ZnO shells or some interfacial effects.