Structural and optical properties of silica single-layer films doped with ZnS quantum dots: Photoluminescence monitoring of annealing-induced defects

Zinc Sulfide (ZnS) quantum dots (QDs) embedded in sol-gel silica single-layer films were synthesized by dip-coating and thermal treatment. Nucleation and growth of these QDs occurred during annealing. The effect of annealing temperature on morphological, structural and optical properties of the films was considered in the range (200-500 °C). Scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing incident angle X-ray diffraction (GIXD), UV-Visible spectroscopy (UV-Vis) and photoluminescence (PL) were used to characterize the films. The band gap values of the ZnS QDs ranged from 3.83 to 4.03 eV corresponding to average radii in the range 1.5-2.1 nm. PL revealed different annealing-induced defects in the ZnS QDs. Zn interstices dominate at 400 and 500 °C annealing temperatures by the appearance of two excitation bands at 395 nm and 404 nm respectively. Corresponding emission bands were recorded at 402 nm and 408 nm. An emission peak (457 nm) and an excitation peak (437 nm) were also observed at 500 °C. These peaks were attributed to S vacancies which might be created by the transformation of ZnS to ZnO. Finally, an overall energy-level diagram showing all the defect states in the gap of ZnS was proposed.