Regulating the trap distribution of ZnGa2O4:Cr3+ by Li+ /Ga3+ doping for upconversion-like trap energy transfer NIR persistent luminescence

ZnGa2O4:Cr3+ persistent luminescent phosphors (PLPs) have been widely applied in bioimaging and photonics due to their ultra-long near-infrared (NIR) afterglow. However, UV and visible excitation currently in use have shallow penetration depth or harmful effect on organism, which limits the long-term bioimaging. Therefore, developing NIR PLPs excited by NIR light is urgent for bioimaging. Here, Zn1-x(Li/Ga)xGa2O4:Cr3+ (x=0-1) NIR PLPs were synthesized. All the newly introduced Ga3+ ions occupy the tetrahedral sites. However, with increasing Li+/Ga3+ content, Li+ ions firstly occupy the tetrahedral position, then partially enter octahedral sites, and completely occupy the octahedral sites at x=1. Incorporation of Li+
/Ga3+ contributes to the weakened crystal field strength, which leads to a deeper trap depth and a wider
trap energy level. A complete replacement of Zn2+ by Li+/Ga3+ ions leads to the splitting of the trap energy level into two-divided ones, which reduces the electron transfer between deep/shallow traps and makes the deep trap energy level be near to the 2E energy level of Cr3+. Therefore, an enhanced NIR afterglow excited by the lowenergy NIR light is found for the Li+ /Ga3+ doped sample. This work provides a new category for the NIR-absorptive-NIR-emissive PLPs and proposes a new application for long-term bioimaging.