Photoluminescence of ZnS:Cu quantum dots embedded in silica thin films

Copper-doped zinc sulfide quantum dots embedded in silica films (ZnS:Cu/SiO₂) have been synthesized by sol-gel dip-coating with subsequent thermal annealing. The considered Cu/Zn molar ratios ranged from 0.2% to 2%. Atomic force microscopy analysis shows mostly a granular nanostructure of the deposited films with a roughness less than 30 nm depending on Cu content; i.e. the lowest value of 14 nm is obtained for 0.5% Cu doping. The mean particle size of ZnS:Cu quantum dots (QDs) has been found to be around 2.5 nm as measured by UV–Vis spectroscopy, in agreement with the value of 3 nm estimated by Scherrer equation using X-ray diffraction patterns. Photoluminescence (PL) spectra reveal new features regarding the intensity and width of the peaks. Two well-resolved emission bands are essentially observed in the blue and green regions centered respectively at 438 nm and from 510 to 514 nm. The blue band shows a variation of width with Cu content and a quenching of PL at 2% Cu doping. At 0.5% Cu, a PL line-width of approximately 3 nm is measured for this blue band. Contrarily to the blue emission, the green band is not quenched at 2% Cu content. Finally, the origin of the PL bands has been discussed.