Insights into the microscopic mechanism of persistent luminescence in Cr3+-doped ZnGa2O4 phosphors

Abstract

Persistent luminescence (PersL) phenomenon is the result of the dynamic distribution of microscopic charges caused by an external field stimulation. A long-standing unresolved question is why red afterglow phosphors activated by Cr³⁺ can mainly be obtained in Ga-based compounds. Here, we systematically investigate the ZnGa₂O₄:0.5%Cr³⁺,0.5%M³⁺ (M = Bi and Tb) and ZnGa₂O₄:0.5%Cr³⁺ phosphors by a combination of experimental characterizations and first-principles calculations. An enhanced Cr³⁺ PersL mechanism is proposed based on the regulation of the band gap structure and the redistribution of the trap states by doping. These changes localize the defect energy levels near the Fermi level, thereby transforming them into effective traps. A quasi-continuous broadband trap state is confirmed by combining the trap state rate equations with the observed monotonically temperature-dependent decrease in the afterglow lifetime in the co-doped samples. Furthermore, the types of traps are systematically analysed through a combination of the thermoluminescence curves, photoluminescence kinetics, X-ray photoelectron spectroscopy (XPS), and rate equations for the trap-state carriers and fluorescent energy levels, coupled with first-principles calculations. Additionally, Cr³⁺ doping maintains the rigidity of ZnGa₂O₄ and enhances the anisotropy of physical parameters. These changes facilitate carrier migration and suppress lattice scattering and non-radiative relaxation. Based on their excellent features, the phosphors are further used for high-level anti-counterfeiting. This study firstly reveals that the large anisotropy of physical quantities induced by Cr³⁺ substitution for Ga³⁺ is responsible for the strong afterglow effect in Cr-activated Ga-based materials.