Synthesis of europium-doped ZnS nano-crystalline thin films with strong blue photoluminescence

Eu2+-Doped ZnS (ZnS:Eu2+) thin (similar to 550 nm) films with strong and stable blue photoluminescence have been successfully synthesized by a simple and fast ultrasonic spray pyrolysis method. According to X-ray diffraction (XRD) and high resolution scanning electron microscopy (SEM) analysis, the as grown ZnS:Eu2+ films are composed of hexagonal wurtzite nanocrystals with average size of similar to 25 nm, which are preferentially oriented along the (002) direction and agglomerate to form hexagonal facet nanobars with diameters from 50 to 200 nm. These films show a strong blue emission centered at 454 nm, which can be observed with the naked eye under ambient illumination. The intensity of this peak is similar to 185 times higher than the maximum photoluminescence peak observed from the undoped ZnS films. The presence of the Eu dopant in the valence state Eu2+ was confirmed by electron spin resonance (ESR) measurements. The comparison of the high-resolution UV-vis absorption spectra and the PL characteristics of the ZnS:Eu2+ films with the pure ZnS films, indicate that the strong blue emission of the ZnS:Eu2+ films comes from Eu2+ 4f(6)5d -> 4f(7) intra-ion transitions.