Origin of the visible light induced persistent luminescence of Cr3+ -doped zinc gallate

Abstract

ZnGa2O4:Cr3+ (ZGO:Cr) is a very bright persistent phosphor able to emit a near infrared light for hours following a UV (band to band excitation) or visible (Cr3 excitation) illumination. As such it serves as an outstanding biomarker for in vivo imaging. Persistent luminescence, due to trapping of electrons/holes at point defects, is studied here on a series of ZGO:Cr spinel compounds where the introduction of defects is controlled by varying the Zn/(Ga+Cr) nominal ratio during synthesis. Simulation of Electron Paramagnetic Resonance spectra revealed up to six types of Cr3+ ions with different neighboring defects and correlated to four emission lines in low temperature photoluminescence spectroscopy. Of particular importance, three EPR signals were attributed to Cr3+ with a pair of neighboring Zn'Ga and GaZn degrees antisite defects. They were identified to the emission line N2 that plays a key role in the persistent luminescence mechanism for both storage of visible excitation and persistent luminescence emission. A model is proposed whereby the local electric field at Cr3+ created by the two neighboring antisite defects triggers the electron-hole separation and trapping upon excitation of Cr3+. The process is equivalent to a photoinduced electron transfer from a donor (here Zn'Ga) to an acceptor (here GaZn degrees) observed in some molecular systems.