Germanium substitution endowing Cr3+-doped zinc aluminate phosphors with bright and super-long near-infrared persistent luminescence

We present novel near-infrared (NIR) Cr³⁺-doped non-gallate super-long-persistence phosphors (Zn_1+xAl_2-2xGe_xO₄:Cr³⁺) by Germanium substitution in the original ZnAl₂O₄:Cr³⁺. Unlike the negligible NIR persistent luminescence of ZnAl₂O₄:Cr³⁺ upon UV or visible light excitation, the Ge substituted phosphors feature strong and super-long-persistent luminescence at approximately 650–750 nm for more than 120 h. The relation between the Ge substitution and the defect trapping states is investigated systematically. The experimental results combined with the first-principles calculations reveal that Ge_Zn^○○ and Ge_Al^○ in the spinel structure would introduce shallow and deep defect states in the band gap to serve as new efficient traps, which are mainly responsible for the strong and super-long-persistent luminescence upon UV or visible light excitation. The present advanced phosphors is an alternative candidate for applications in biomedical imaging and night-vision surveillance.