Vacuum-Referred Binding Energies of Bismuth and Lanthanide Levels in ARE(Si,Ge)O₄ (A = Li, Na; RE = Y, Lu)

Abstract

Developing a feasible design principle for solid-state materials for persistent luminescence and storage phosphors with high charge carrier storage capacity remains a crucial challenge. Here we report a methodology for such rational design via vacuum referred binding energy (VRBE) diagram aided band structure engineering and crystal synthesis optimization. The ARE(Si,Ge)O₄ (A = Li, Na; RE = Y, Lu) crystal system was selected as a model example. Low-temperature (10 K) photoluminescence excitation and emission spectra of bismuth- and lanthanide-doped ARE(Si,Ge)O₄ system were first systematically studied, and the corresponding VRBE schemes were then established. Guided by these VRBE schemes, Bi³⁺ afterglow and storage phosphor properties were explored in NaLu_1-xY_xGeO₄. By combining Bi³⁺ with Bi³⁺ itself or Eu³⁺, Bi³⁺ appears to act as a deep hole-trapping center, while Bi³⁺ and Eu³⁺ act as less-deep electron traps. Trap depth tunable afterglow and storage were realized in NaLu_1-xY_xGeO₄:0.01Bi³⁺ and NaLu_1-xY_xGeO₄:0.01Bi³⁺,0.001Eu³⁺ by adjusting x, leading to conduction band engineering. More than 28 h of persistent luminescence of Bi³⁺ was measurable in NaYGeO₄:0.01Bi³⁺ due to electron release from Bi²⁺ and recombination with a hole at Bi⁴⁺. The charge carrier storage capacity in NaYGeO₄:0.01Bi³⁺ was discovered to increase ∼7 times via optimizing synthesis condition at 1200 °C during 24 h. The thermoluminescence (TL) intensity of the optimized NaYGeO₄:0.001Bi³⁺ and NaYGeO₄:0.01Bi³⁺,0.001Eu³⁺ is ∼3, and ∼7 times higher than the TL of the state-of-the-art X-ray storage phosphor BaFBr(I):Eu. Proof-of-concept color tuning for anti-counterfeiting application was demonstrated by combining the discovered and optimized NaYGeO₄:0.01Bi³⁺ afterglow phosphor with perovskite CsPbBr₃ and CdSe quantum dots. Information storage application was demonstrated by UV-light- or X-ray-charged NaYGeO₄:0.01Bi³⁺,0.001Eu³⁺ phosphor dispersed in a silicone gel imaging film. This work not only reports excellent storage phosphors but more importantly provides a design principle that can initiate more exploration of afterglow and storage phosphors in a designed way through combining VRBE-scheme-guided band structure engineering and crystal synthesis optimization.