Extending the color response range of Yb³⁺ concentration-dependent multimodal luminescence in Yb/Er doped fluoride microrods by annealing treatment

For anti-counterfeiting and full-color display applications, adjusting the Yb³⁺ concentrations or excitation conditions is the basic and simple way to control luminescence color and intensity. However, this approach is significantly limited because the luminescence color is not very sensitive to the response of doped Yb³⁺ concentrations or excitation conditions. Herein, we have demonstrated that the as-synthesized NaLuF₄:Yb/Er and their annealed counterpart LuOF:Yb/Er microrods exhibit upconversion/downshifting double-mode luminescence with complementary and rich colors from green, yellow to red under 980/365/488 nm excitation. The anti-counterfeiting patterns printed with these microrods inks have the ability of the color response to the excitation wavelength, excitation power and laser scanning speed, which make them be hard to duplicate. This study indicates that Yb/Er doping NaLuF₄ and their complementary LuOF microrods have great potential in multilevel anti-counterfeiting application and display. In addition, heat treatment way also brings novel ideas to the synthesis of high efficient luminescence materials.