Tailor Traps in Bi³⁺-Doped NaGdGeO₄ Phosphors by Introducing Eu³⁺ Ions to Switch Multimodal Phosphorescence Emission

Featured with a tunable excitation/emission wavelength and excellent physicochemical stability, inorganic fluorescent materials are widely used in the fields of anti-counterfeiting. Here, we design a multi-stimuli-responsive dynamic fluorescence and phosphorescence anti-counterfeiting material by introducing Eu³⁺ ions in NaGdGeO₄: Bi³⁺ to tailor the trap structure. The photoluminescence (PL), long persistent luminescence (LPL), and photo-stimulated luminescence (PSL) colors of NaGdGeO₄: Bi³⁺, Eu³⁺ can be switched by varying the excitation modes (ultraviolet, near infrared, and X-ray light). Especially, the LPL and PSL colors of NaGdGeO₄: Bi³⁺, Eu³⁺ vary with increasing decay and stimulation times. In addition, X-ray excitation ensures the specificity of the luminescence of NaGdGeO₄: Bi³⁺, Eu³⁺ compared with ultraviolet excitation. This rapidly-changing-color fluorescent material offers the possibility of sophisticated anti-counterfeiting applications in the future.