Multiple encrypted dynamic fluorescent anti-counterfeiting and optical temperature detection of Ca₂Ge₇O₁₆:Cr³⁺, Mn^{2 +}

In this study, Ca₂Ge₇O₁₆:Mn²⁺, Cr³⁺ is successfully synthesized using the solid phase method. Mn²⁺ ions occupy the Ca²⁺ cationic site, while Cr³⁺ ions occupy the octahedral Ge⁴⁺ cationic site within the Ca₂Ge₇O₁₆ lattice. The crystal field strength of the Cr³⁺ ion is calculated, yielding a Dq/B value of 2.46. Spectral and fluorescence lifetime indicate efficient energy transfer from Mn²⁺ to Cr³⁺ ions with the efficiency to 33.5% as the concentration of Cr³⁺ ions increases. The energy transfer mechanism is determined to result from nearest-neighbor ion interactions. Temperature-dependent spectra reveal that the maximum absolute sensitivity (S_a) is 1.045% K⁻¹ at 373 K, and the maximum relative sensitivity (S_r) is 0.75% K⁻¹ at 303 K with a temperature resolution of 0.0315 K as an optical thermometer. Furthermore, Ca₂Ge₇O₁₆:Mn²⁺, Cr³⁺ exhibit a prominent afterglow. As the Cr³⁺ ion increases, the emitted color of Ca₂Ge₇O₁₆:Mn²⁺, Cr³⁺ changes from orange to deep red. The emissions and afterglow change with the temperature, which enables multiple encrypted dynamic fluorescence anti-counterfeiting under 222 nm excitation.