Multicolor luminescence of hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals with tunable morphology under 940 nm excitation for temperature-responsive anti-counterfeiting

In this study, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals were synthesized controllably, and upconversion luminescence excited at 940 nm and its application in temperature-responsive anti-counterfeiting are reported. It is clarified that the Ln3+ (Ln = Y + Yb + Ho + Ce) density ratio of bottom plane to side plane in the unit cell can be regulated by Ce3+ doping. It is also proved that the energy transfer of Yb3+ to Ho3+ is responsible for the activation of Ho3+ under 940 nm excitation, while the cross relaxation between Ho3+ and Ce3+ participates in the redistribution of electron population of 5S2/5F4 and 5F5 levels. Both theory and experiment confirm that the intensity ratio of red to green emission (IR/IG) as a function of temperature as an independent variable has good linear characteristics in the temperature range of 300–500 K. Due to the good responsiveness of multicolor luminescence to temperature, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ with tunable morphology is a promising candidate for advanced temperature-responsive upconversion anti-counterfeiting. Our results provide a new pathway for the controllable synthesis of hexagonal NaYF4 microcrystals as well as the regulation of upconversion luminescence that is excited by wavelengths other than 980 nm and its application in anti-counterfeiting.