Self-assembly of ZnS-encapsulated SiO2 nanospheres for enhanced photoluminescence

ZnS-encapsulated spherical SiO2 nanostructures were prepared by a one-step solvothermal method. The samples were characterized by x-ray diffraction, transmission electron microscopy, selected-area electron diffraction, infrared spectra, UV-visible spectra, and photoluminescence. It is found that 27 nm ZnS in a wurtzite phase was encapsulated by a low-porosity SiO2 layer, which compares to a particle size of 8.5 nm for bare ZnS as prepared under similar conditions. Contrary to the theoretical predictions of the quantum size effect, the encapsulated ZnS showed a band gap energy surprisingly larger than that for the bare one. Moreover, the encapsulated ZnS/ SiO2 nanostructures exhibited 'self-activated centers', as characterized by a blue photoluminescence emission band at 430 nm, while the bare ZnS nanoparticles gave a surface sulfur species-mediated green emission at 520 nm. These observations are explained in terms of the encapsulated nanostructure and the relevant defect nature in which the SiO2 layer reduces the nonradiative recombination centers and enhances the photoluminescence intensity.