Room temperature visible photoluminescence from undoped ZnS nanoparticles embedded in SiO2 matrices

Abstract

Thin films of ZnS nanoparticles embedded in SiO2 matrices were prepared by the pulsed laser deposition method using a rotating target. The as-prepared samples contained dispersed ZnS nanocrystallites mostly of less than 8 nm in diameter embedded in the SiO2 host. The nanoparticle size of the film increased with increasing sample annealing temperature and/or duration in the vacuum. Room temperature visible photoluminescence (PL) was found from the films without the need for any dopant. Contrary to the usual redshift following annealing, there was, instead, a blueshift of the PL which increased with increasing sample annealing. The mechanism of the PL and its blueshift is attributed to the S vacancies and the formation of ZnS(O2−) in the surface layers of the nanoparticles adjacent to the SiO2. The S vacancies arose partly from the laser ablation during the film preparation stage, and partly from the S-bond disruption at the surface of the ZnS particles during annealing. The ZnS(O2−) could be formed by the ZnS particles thermally chemisorbing oxygen from the SiO2 host in the interfacial regions between the nanoparticles and their host matrix. The effects on the PL due to S vacancies and ZnS(O2−) formation are similar to the effect due to impurities in the films which results in an effective size reduction of the ZnS nanocrystallites.