Tuning multicolour emission of ZnGeO: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, ZnGeO:Mn,%Li ( = 0 and 20), ZnGeO-NaLiGeO:Mn,%Li ( = 50 and 70) and NaLiGeO:Mn micro-phosphors with multi-chromatic and multi-mode luminescence have been successfully synthesized a hydrothermal approach followed by an annealing treatment. As expected these Li doped ZnGeO:Mn and ZnGeO-NaLiGeO:Mn phosphors exhibit a double peak emission including a long green afterglow (∼540 nm) and red photoluminescence (∼668 nm). By tuning Li doping concentrations, a gradual colour output and a tuneable afterglow duration are achieved. In particular, the ZnGeO:Mn,Li and NaLiGeO: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.