Tuning multicolour emission of Zn2GeO4:Mn phosphors by Li+ doping for information encryption and anti-counterfeiting applications

Traditional fluorescent materials used in the anti-counterfeiting field usually exhibit monochromatic luminescence at a single-wavelength excitation, which is easily forged by sophisticated counterfeiters. In this work, Zn2GeO4:Mn,x%Li (x = 0 and 20), Zn2GeO4-NaLiGe4O9:Mn,x%Li (x = 50 and 70) and NaLiGe4O9:Mn micro-phosphors with multi-chromatic and multi-mode luminescence have been successfully synthesized via a hydrothermal approach followed by an annealing treatment. As expected these Li+ doped Zn2GeO4:Mn and Zn2GeO4-NaLiGe4O9:Mn phosphors exhibit a double peak emission including a long green afterglow (similar to 540 nm) and red photoluminescence (similar to 668 nm). By tuning Li+ doping concentrations, a gradual colour output and a tuneable afterglow duration are achieved. In particular, the Zn2GeO4:Mn,Li and NaLiGe4O9:Mn phosphors exhibit excellent performance as security inks for printing luminescent numbers and anti-counterfeiting patterns, which show an afterglow time-dependent or excitation wavelength-dependent luminescence colour evolution. This work proves the feasibility of the Li+ doping strategy in emission tuning, which can stimulate further studies on multi-mode luminescent materials for anti-counterfeiting applications.