Incorporation of Eu³⁺ in ZnGa₂O₄:Ni²⁺ for improved NIR persistent luminescence located in second transparency window

It is well known that near-infrared (NIR) persistent phosphors have rather low absorption coefficients of biological tissues for NIR light. However, recent research shows that the phosphors emitting NIR lights in second NIR (NIR-II, 1000-1350 nm) and third (NIR-III, 1500-1800 nm) biological window have advantages over that in NIR-I (650-900 nm). Although ZnGa2O4:Ni2+ outputs NIR emission and afterglow locatedin NIR-II, the weak signal significant limits its application. In this work, persistent luminescent phosphors of ZnGa2O4:Ni2+, yEu(3+) (x = 0-0.013, y = 0.01-0.06) (termed as ZEGN) were synthesized via a traditional high-temperature solid-state reaction, which feature a broad emission band in the NIR-II window. The phosphors exhibit a broad NIR emission at about 1300 nm after ultraviolet (UV) or orange-red lights excitation, arising from the T-3(2)(F-3) ? (3)A(2)(F-3) transition of Ni2+. However, incorporation of Eu3+ ions, the NIR emission intensity significantly increases with the increase of Ni2+ ion concentration, reaching the maximum by 18 times at x = 0.005. Removing the light source, the sample still outputs intense NIR afterglow and red afterglow that can last over 500 s. It is noteworthy that the red afterglow of Eu3+ shows a dramatically decrease but the NIR afterglow increases with increasing the Ni2+ ion concentration, because of energy transfer. Under the excitation of 282-nm UV light, the ZGEN sample exhibits a good thermal stability. The phosphor offers a promising application in biological imaging due to broadband NIR-II light and afterglow.