Abnormal co-doping effect on the red persistent luminescence SrS:Eu2+,RE3+ materials
Persistent luminescence materials are a reality in several applications. However, efficient red-emitting materials are still lacking. SrS:Eu2+ phosphor keeps as a potential candidate since its strong nephelauxetic effect shifts Eu2+ 4f65d1→4f7 to red and its weak bond between strontium and sulphide, due to the soft base-hard acid character, generate a high number of intrisic defects. The SrS:Eu2+,RE3+ materials were efficiently prepared by two rounds of 22 min microwave-assisted solid-state synthesis. The highly crystalline purity and the material organization at the micro-scale were observed with X-ray powder diffraction and Scanning Electron Microscopy, respectively. X-ray absorption spectroscopy pointed out a low ammount of Eu2+ compared to Eu3+ due to the efficient Eu2+ photo-oxidation by X-ray irradiation in the high storage capability SrS host matrix. Electron Paramagnetic Ressonance spectra confirmed that at least 50 % of Eu2+ ions in the material are photo-oxidized during excitation, reinforcing previously estabilished mechanisms. The RE2+ energy level positioned very close or into the conduction band led to an abnormal co-doping effect, with similar effect independent on the co-dopant. The high concentration of intrinsic defects in SrS indicates that soft-hard pair hosts is an excellent approach to develop efficient persistent luminescence materials.