Photoluminescence enhancement of ZnS:Mn nanoparticles by SiO₂ coating

ZnS:Mn nanoparticles (∼3.1 nm) of a cubic structure were prepared by a coprecipitation method and then dispersed with various weights (0.5-8 g) into a solution containing ammonium hydroxide, ethanol and tetraethyl orthosilicate (TEOS) for surface coating. X-ray diffraction (XRD) analysis of these coated ZnS:Mn powders revealed the presence of amorphous materials after the coating process. Characterization of the coated powders using a transmission electron microscope (TEM) and an electron energy loss spectrometer (EELS) showed that ZnS:Mn nanoparticles were randomly embedded in the larger particles of amorphous SiO₂ (50-100 nm). The optical properties of photoluminescence (PL) and diffuse reflection of the SiO₂-coated ZnS:Mn powders were found to depend on the quantity of ZnS:Mn nanoparticles in the coating solution. The increased PL intensity of ZnS:Mn at 590 nm after SiO₂ coating was proposed to result from the inhibition of excitation energy transfer to quenching centers or surface states based on the observation of the reduced absorption in the region of 370-450 nm.