Abnormal co-doping effect on the red persistent luminescence SrS:Eu2+,RE3+ materials

Abstract

Persistent luminescence materials are a reality in several applications. However, there is still a lack of efficient red-emitting materials. SrS:Eu²⁺ phosphor is 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, generates a high number of intrinsic defects. 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 revealed a low amount 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 resonance spectra confirmed that at least 50% of Eu²⁺ ions in the material are photo-oxidized during excitation, reinforcing the previously established mechanisms. The RE²⁺ energy level positioned very close to or into the conduction band led to an abnormal co-doping effect, with similar effects independent of the co-dopant. The high concentration of intrinsic defects in SrS indicates that the soft–hard pair host is an excellent approach to develop efficient persistent luminescence materials.