Local Structure Regulation in Near-Infrared Persistent Phosphor of ZnGa2O4:Cr3+ to Fabricate Natural-Light Rechargeable Optical Thermometer

Modern engineering fields put forward requirements for optical temperature sensors, which need natural-light excitation/storage and near-infrared (NIR) afterglow emission for some special conditions. Here, NIR persistent luminescent phosphors of ZnGa2–x(Mg/Ge)xO4:Cr3+ (x = 0–1.25) have been synthesized. The incorporation of Mg2+/Ge4+ ions in ZnGa2O4:Cr3+ resulted in more defect clusters of "MgGa′–GeGa•" and "ZnGa′–GeGa•" and interstitial oxygens (OInt). Increasing the calcination temperature and Mg2+/Ge4+ doping both contributed to the generation of OInt. Higher efficiency of visible light excitation was observed, mainly due to the defect clusters and OInt. The samples exhibited a bright NIR emission at 695 nm by exposure to UV or visible light, and the NIR signal can last longer than 1 h after the stoppage of excitation. Incorporation of Mg2+/Ge4+ and increasing the calcination temperature both resulted in a deeper trap depth. However, the density of trapped charge carriers takes the dominant role in the persistent luminescence. Therefore, the x = 0.25 sample, having the most trapped charge carriers, exhibits the best afterglow performance. The prepared phosphor exhibited a temperature-dependent persistent luminescence behavior, which can charge natural light and release NIR light repeatedly many times, indicating that they are the potential natural-light rechargeable materials for temperature sensing.