Sr₂BiF₇: A New Bismuth-Based Host Material for Lanthanide Ions Doping: Synthesis, Downshifting, and Upconversion Luminescence Properties for Multimode Anticounterfeiting

A new bismuth-based host material, i.e., Sr₂BiF₇, is explored in this work. Undoped and lanthanide ion-doped Sr₂BiF₇ nanomaterials are prepared using a simple coprecipitation technique at 120 °C. The undoped nanomaterials exhibit a blue color under 365 nm excitation. The downshifting and upconversion photoluminescent properties of Er and Yb codoped Sr₂BiF₇ nanomaterials are investigated. The optimum up-conversion luminescence is produced by nanomaterials doped with 5% Yb³⁺ and 0.2% Er³⁺. These nanomaterials show blue and magenta colors upon excitation at 365 and 395 nm wavelengths, respectively. Sr₂BiF₇ material doped with Er³⁺ shows green emission, while the codoped Er³⁺, Yb³⁺ nanomaterials exhibit an orange-red color under 980 nm light. A specific amount of polyvinyl chloride (PVC) is used for producing luminescent ink with these nanoparticles for multimode anticounterfeiting applications. The letters and patterns written with luminescent ink based on Er³⁺, Yb³⁺ doped nanomaterials show blue, magenta, and orange-red colors under 365, 395, and 980 nm light, respectively. These results establish that this material can be effectively used as a multimode photoluminescent covert tag to combat counterfeiting.