Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate

ZnGa2O4:Cr3+ presents near-infrared long-lasting phosphorescence (LLP) suitable for in vivo bioimaging. It is a bright LLP material showing a main thermally stimulated luminescence (TSL) peak around 318 K. The TSL peak can be excited virtually by all visible wavelengths from 1.8 eV (680 nm) via d-d excitation of Cr3+ to above ZnGa2O4 band gap (4.5 eV-275 nm). The mechanism of LLP induced by visible light excitation is entirely localized around Cr-N2 ion that is a Cr3+ ion with an antisite defect as first cationic neighbor. The charging process involves trapping of an electron-hole pair at antisite defects of opposite charges, one of them being first cationic neighbor to Cr-N2. We propose that the driving force for charge separation in the excited states of chromium is the local electric field created by the neighboring pair of antisite defects. The cluster of defects formed by Cr-N2 ion and the complementary antisite defects is therefore able to store visible light. This unique property enables repeated excitation of LLP through living tissues in ZnGa2O4:Cr3+ biomarkers used for in vivo imaging. Upon excitation of ZnGa2O4:Cr3+ above 3.1 eV, LLP efficiency is amplified by band-assistance because of the position of Cr3+4T1 (F-4) state inside ZnGa2O4 conduction band. Additional TSL peaks emitted by all types of Cr3+ including defect-free Cr-R then appear at low temperature, showing that shallower trapping at defects located far away from Cr3+ occurs through band excitation.