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

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 (∼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 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.