Afterglow Properties and Trap-Depth Control in ZrO₂:Ti, M (M = Ca²⁺, Y³⁺, Nb⁵⁺, W⁶⁺)

Ti-doped ZrO₂ is a chemically stable and persistent luminescence material. Doping and co-doping is an effective approach for improving the afterglow properties of phosphors, but few studies have investigated the co-doping of ZrO₂:Ti systems. This study aimed to synthesize ZrO₂:Ti, M (M = Ca²⁺, Y³⁺, Ti single-doped, Nb⁵⁺, W⁶⁺) and evaluate the luminescent properties of the resulting materials, with a specific focus on the relationship between trap depth and the valence state of the co-doped cation. The ratio of the luminescent center to co-doped ion was optimized using the combinatorial approach, where 0.09 mol % Ti led to the best afterglow duration. The emission decay curves of each co-doped sample differed significantly, where a change in curvature was observed in the Ti single-doped and W⁶⁺ co-doped samples due to the presence of multiple traps. From the thermoluminescence glow curves, the trap originating in an oxygen vacancy with a peak at around 270 K was observed. The trap depth was dependent on electrostatic interactions between the trapped electrons and their surrounding cations, and thus related to the valence of the co-dopant. Overall, co-doping with high-valent cations led to improved afterglow duration.