A new persistent luminescence phosphor of ZnGa2O4:Ni2+ for the second near-infrared transparency window

Recently, near-infrared (NIR) persistent phosphors become a research hotspot in biomedical application, because of their rather low absorption coefficients of biological tissues for NIR light. Most investigated phosphors emit NIR lights, whose wavelengths are no more than 1000 nm and located in NIR-I biological window. However, the phosphors emitting NIR lights in second (NIR-II, 1000–1350 nm) and third (NIR-III, 1500–1800 nm) biological window have advantages over that in NIR-I one. Here, persistent luminescent phosphors of ZnGa2O4:xNi2+ (x = 0–0.01) were synthesized via a traditional high-temperature solid-state reaction, which feature a broad emission band in the second near-infrared (NIR-II) window. Ni2+ tends to occupy Ga3+ site in ZnGa2O4, which results in the formation of oxygen vacancy. Upon ultraviolet (UV) or orange-red lights excitation, the phosphors exhibit a broad NIR emission at about 1300 nm, arising from the 3T2(3F)→3A2(3F) transition of Ni2+. Removing the light source yields intense NIR afterglow, with the duration longer than 500 s. Strongest NIR emission and best persistent luminescence are found for the x = 0.005 sample, which is excited by 254-nm UV light. The ZnGa2O4:Ni2+ persistent phosphors have potential application in vivo imaging, because of their charming emissions and afterglows with the wavelength locating in NIR-II window.