Calcium Doped Self-activated Zinc Germanate Long Afterglow Materials: Multicolor Afterglow and Application in Dynamic Anti-counterfeiting

Long afterglow materials have wide application in the fields of safety indication, anti-counterfeiting and biological imaging, whereas their defect regulation and afterglow mechanism remain unclear. Self-activated long afterglow materials attracted attention due to their simple composition and structure, which is convenient for revealing the mechanism of long afterglow. A series of calcium doped self-activated zinc germanate long afterglow materials Zn2-xCaxGeO4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by high temperature solid reaction. The results showed that yellow luminescence peak of calcium doped zinc germanate sample was observed when excited by 376 nm UV light. Upon 267 nm excitation, the fluorescence intensity of Zn1.5Ca0.5GeO4 increased by 6.2 times. According to the afterglow spectra and their decay curves, the initial afterglow intensity is enhanced by 25.1 times. In addition, Zn1.5Ca0.5GeO4 maintains an afterglow intensity nearly 5 times that of zinc germanate during the first 300 s afterglow duration. Further characterization of TL curves show that the depth of the main trap is about 0.7 eV. After adding proper concentration of Ca2+, the TL peaks become significantly stronger, indicating that the trap density increases significantly after calcium doping. Finally, taking advantage of the difference of the afterglow decay rate of different emission wavelengths, the dynamic change of the "flower" color is realized, which indicates that the obtained materials have the potential application in the field of dynamic anti-counterfeiting.