Element-Resolved Correlation of Defects and Persistent Luminescence in Stoichiometry-Tuned Zinc Germanate

Abstract

Persistent luminescence (PersL) in inorganic phosphors is typically achieved by incorporating metal-ion activators into a host lattice. Zinc germanate, Zn2GeO4 (ZGO), which has been widely used as a host for activator-doped PersL phosphors, has recently been found to exhibit intrinsic, self-activated PersL without any dopant. Despite its intriguing nature, the underlying mechanism for this activator-free PersL remains poorly understood. Previous studies have suggested a strong dependence of PersL on stoichiometry-driven defects, but investigations have been limited to asymmetric compositional tuning (either by adding excess Zn to make it Zn-rich, or by adding excess Ge to make it Ge-rich). Direct, element-specific correlations between defects and emission have remained elusive. In this paper, we present a systematic study of self-activated PersL in ZGO by spanning both Zn-rich and Zn-deficient regimes relative to the stoichiometric compositions. We show that even slight deviations in Zn/Ge ratio generates distinct defect configurations, leading to drastic differences in photoluminescence (PL) and PersL, despite nearly identical crystal structures. Notably, we identify a previously unrecognized red PersL band in stoichiometric and Zn-deficient ZGO, which is absent under photoexcitation and emerges only at delayed stages of the afterglow. By combining element-selective X-ray excited optical luminescence (XEOL) and Extended X-ray absorption fine structure (EXAFS) spectroscopy, we establish direct correlation between emission features with their elemental origins. These results provide the first element-resolved insight into defect-mediated PersL in ZGO, revealing how stoichiometry governs competing recombination pathways and enabling a more complete understanding of self-activated persistent luminescence.