Sr<sub>2</sub>BiF<sub>7</sub>: A New Bismuth-Based\nHost Material for Lanthanide Ions Doping: Synthesis, Downshifting,\nand Upconversion Luminescence Properties for Multimode Anticounterfeiting

A new\nbismuth-based host material, i.e., Sr<sub>2</sub>BiF<sub>7</sub>,\nis explored in this work. Undoped and lanthanide ion-doped\nSr<sub>2</sub>BiF<sub>7</sub> nanomaterials are prepared using a simple\ncoprecipitation technique at 120 °C. The undoped nanomaterials\nexhibit a blue color under 365 nm excitation. The downshifting and\nupconversion photoluminescent properties of Er and Yb codoped Sr<sub>2</sub>BiF<sub>7</sub> nanomaterials are investigated. The optimum\nup-conversion luminescence is produced by nanomaterials doped with\n5% Yb<sup>3+</sup> and 0.2% Er<sup>3+</sup>. These nanomaterials show\nblue and magenta colors upon excitation at 365 and 395 nm wavelengths,\nrespectively. Sr<sub>2</sub>BiF<sub>7</sub> material doped with Er<sup>3+</sup> shows green emission, while the codoped Er<sup>3+</sup>,\nYb<sup>3+</sup> nanomaterials exhibit an orange-red color under 980\nnm light. A specific amount of polyvinyl chloride (PVC) is used for\nproducing luminescent ink with these nanoparticles for multimode anticounterfeiting\napplications. The letters and patterns written with luminescent ink\nbased on Er<sup>3+</sup>, Yb<sup>3+</sup> doped nanomaterials show\nblue, magenta, and orange-red colors under 365, 395, and 980 nm light,\nrespectively. These results establish that this material can be effectively\nused as a multimode photoluminescent covert tag to combat counterfeiting.