Excitation-wavelength-dependent photochromic Bi3+/Ln3+(Eu3+, Er3+)-codoped phosphors for anticounterfeiting applications

Fluorescent powders exhibiting excitation dependence and luminescent discoloration are profoundly relevant for concealing information and enhancing optical antiforgery protection. In this study, Bi3+/Ln3+ (Eu3+, Er3+)-doped phosphors exhibiting multicolor luminescence and excitation wavelength dependence were synthesized. The Bi3+/Eu3+-codoped phosphors emitted blue, pink, lavender, and red lights under excitation from conventional ultraviolet (UV) sources. In addition to emitting blue light, the Bi3+/Er3+-codoped phosphors also transmitted upconversion-emission light from green to yellow under 980-nm laser excitation. Additionally, the luminous color of the Ca3Y2Ge3O12:0.01Bi3+, 0.2Er3+ phosphor changed from yellow to orange as the temperature increased. Combining the above luminous-color-change phenomena, the word-hiding experiment exhibited excellent realistic information hiding, with complex color transformation in the stamen and petal parts of the flower pattern. Furthermore, rapid multimodal and complex discoloration was achieved using easy-to-obtain light sources, confirming the potential of Bi3+/Ln3+ phosphors for anticounterfeiting applications.