Local, Temperature-Dependent Trapping and Detrapping in the LiGa₅O₈:Cr Infrared Emitting Persistent Phosphor

The near-infrared emitting persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) has promising applications in bioimaging. In order to improve the persistent luminescence of LGO:Cr and other Cr-doped persistent phosphors, a better understanding of trapping and detrapping mechanisms is necessary. In this work, we study the afterglow and thermoluminescence via a thermal fading experiment. The results show that there is a broad trap distribution present in LGO:Cr. The emission spectrum of chromium changes during the afterglow, which indicates that different Cr ions experience a varying crystal field in the LGO host, due to different defect configurations, and that the detrapping process occurs locally. The results of thermoluminescence and spectral decay measurements show that chromium ions residing near deep traps are subject to a smaller crystal field. Vacancies formed during the synthesis are most probably causing this effect. Codoping LGO with Si4+ or Ge4+ significantly improves the persistent luminescence and increases the number of deep-lying traps in the phosphor.