Ultra-Long Green Persistent Luminescence from Cu2+ in Ba_0.4Sr_1.6Ga₄O₈

Abstract

Green emission is of particular importance in persistent luminescence (PersL) materials due to its high sensitivity to the human eye and broad application potential. Since the 1990s, the commercial material SrAl₂O₄:Eu²⁺, Dy³⁺ has dominated this field for nearly three decades, owing to its high brightness and long afterglow duration. However, given the extraction difficulties and low recyclability of rare-earth ions, the development of efficient rare-earth-free alternatives, especially in the green spectral region, remains a critical challenge. In this work, we present a green-emitting PersL material, Ba_0.4Sr_1.6Ga₄O₈:Cu²⁺, shows strong emission at 533 nm and an exceptional PersL duration of 78 h under ultraviolet excitation, comparable to the performance of a SrAl₂O₄:Eu²⁺,Dy³⁺ reference sample synthesized following an optimized literature protocol of 51 h. A combination of experimental characterization and theoretical calculations indicates that the ultralong PersL arises from the synergy between O^2–→Cu²⁺ charge transfer luminescence and intrinsic hole traps largely related to gallium vacancies (V_Ga), continuous distribution of trap states within the forbidden band with depth from about 0.6 to 1.1 eV. This study demonstrates the comparable potential of Cu²⁺ ions to rare-earth ions for long-persistence luminescence, paving the way for their various applications in fields such as information storage, anticounterfeiting, and so on. Furthermore, it provides a fresh perspective on the design principles for Cu²⁺-activated phosphors.