Ti4+/Cr3+ co-doped zinc gallogermanates with persistent NIR emission with modifiable intensity and enhanced luminescence mechanism†

Near-infrared (NIR) long-persistent phosphor emissions are invisible to the naked eye but are well detectable by infrared instruments, making them attractive for applications in high-level secret surveillance. In this study, enhanced luminescence intensity and prolonged afterglow lifetime were achieved by incorporating a definite amount of Ti⁴⁺ and Cr³⁺ into Zn₃Ga₂Ge₂O₁₀ through two-step solid-phase heat treatment. The absorption, excitation and emission spectra of the resulting series of Zn₃Ga_1.9Ge₂O₁₀:(Cr_x,Ti_y)_0.1 with different doping mole ratios were studied to optimize the emission performance and clarify the enhanced emission mechanism. The absorption bands of Zn₃Ga_1.9Ge₂O₁₀:(Cr_x,Ti_y)_0.1 are at around 245 nm, 410 nm and 564 nm, and the emission peak is at around 699 nm. The optimum luminescence intensity and afterglow lifetime were obtained with Zn₃Ga_1.9Ge₂O₁₀:(Cr_x,Ti_y)_0.1 with a Cr³⁺ : Ti⁴⁺ ratio of 1 : 1.5. The mechanism analyses demonstrate that Ti⁴⁺/Cr³⁺ co-doping leads to the formation of more inverse defects, which effectively increases the trap density and regulates the trap depth, thereby promoting the NIR emission intensity. This work highlights the potential of the resulting Zn₃Ga_1.9Ge₂O₁₀:(Cr_x,Ti_y)_0.1 for applications in high-security-level surveillance and information encryption.