Implantation of Cr3+ in ZnAl2O4:Ti4+ to fabricate thermal-/photo-chromic materials for dynamic anti-counterfeiting

In the field of information encryption, multi-mode luminescent materials that can emit light in response to different forms of excitation are very important. Here, ZnAl 2 O 4 : x Ti 4+ ( x  = 0, 0.25%, 0.5%, 0.75%, 1%, 1.25%) and ZnAl 2 O 4 :1%Ti 4+ , y Cr 3+ ( y  = 0.1%, 0.2%, 0.3%) optical materials were synthesized by solid-state reaction method. The crystal structure does not significantly change after doping Ti 4+ and Cr 3+ . The single-doped Ti 4+ samples output blue light at 440 nm. Further doping Cr 3+ with y  = 0.1%, 0.2% and 0.3%, the emission color shifts from blue to red under 254 nm excitation. There is energy transfer of Ti 4+ →Cr 3+ , and the largest efficiency is 54.0% at y  = 0.3%. Because the thermal stabilities of emissions for Ti 4+ and Cr 3+ are quite different, the emissions at 440 nm (Ti 4+ ) and 700 nm (Cr 3+ ) exhibit significantly different responses to temperature. Furthermore, varying the excitation lights from 254 nm to 365 nm also yields different light outputs. Finally, three typical samples were designed for letters “CHN”. The “CHN” logo outputted lights of different colors at different temperatures, and the optical signals varied under different excitations. The colorful emission with different modes can be coded to achieve the application of multi-mode anti-counterfeiting, indicating that they have broad application prospects in this area.