Lithium Substitution Endowing Cr3+-Doped Gallium Germanate Phosphors with Super-Broad-Band and Long Persistent Near-Infrared Luminescence

A way to improve the properties of near-infrared long persistent luminescence (NIR LPL) phosphor has been proposed, which is introducing the cation Li+ which acts as both a cosolvent and a producer of additional defect. In this work, the afterglow time of Mg2.85Li0.03Ga1.99GeO8:0.01Cr3+ lasts more than 15 h by introducing excessive Li+ to Mg3Ga2GeO8:Cr3+ (4 h). Moreover, the internal relationship between traps and afterglow has been comprehensively and systematically investigated by analyzing the thermoluminescence (TL) properties of Mg3–x/2LixGa1.99GeO8:0.01Cr3+ carefully from three different aspects, such as different Li+ concentrations (x = 0–0.3), irradiation times (30 s–8 min), and afterglow decay times (30 s–10 h). The results reveals that the shallow traps preferentially act on the afterglow process to produce an intense and short luminescence, while deep traps act on the afterglow process by the tunneling effect to produce a weak and long-lasting luminescence. It is confirmed that the traps in the materials are continuously distributed, and the trap depths are calculated by using the initial rise method; the afterglow at room temperature is attributed to the traps with energy of 0.55–1.25 eV. Moreover, the motion trajectory of the carriers in the LPL decay process was further analyzed by establishing the persistent luminescence mechanism model. In addition, the material irradiated for 7 min at 0.3 Li+ shows the optimum TL and afterglow properties and has a broad-band emission from 680 to 1100 nm. Therefore, the NIR LPL phosphor Mg2.85Li0.03Ga1.99GeO8:0.01Cr3+ has good potential application prospects in the field of in vivo imaging.