Up-conversion of lanthanide ions and down-conversion defect luminescence in BaGdF5:Yb,Er/Tm for application in anti-counterfeiting

BaGdF5:Yb,Er/Tm materials with dual-mode luminescence were synthesized using oleic acid and a simple one-pot hydrothermal method. The as-obtained materials show different colors under different excitation light wavelengths, showing red, green and blue up-conversion luminescence under irradiation with a 980 nm laser, and blue down-conversion luminescence when excited at 324 nm. The blue down-conversion color might originate from the defects or electron centers produced by the oleic acid molecules adsorbed on the material's surface. BaGdF5:Yb,Er/Tm materials are used to prepare fluorescent ink, and a series of patterns obtained by screen printing can display different colors under different ambient light wavelengths. This work provides simple and promising fluorescent materials for anti-counterfeiting applications.