Flux growth of honeycomb-like β-NaYF4:Yb3+, Er3+/Tm3+ crystals with multicolor upconversion luminescence

Highly crystalline NaYF4:Ln3+ (Ln=Yb, Er, Tm) crystals with honeycomb structure were successfully synthesized for the first time by using a mixed NaCl–KCl flux cooling method at a holding temperature of 800 °C for 2 h in air. The results indicated that the crystals of NaYF4:Ln3+ belonged to hexagonal system and three-dimensional open pores had an average diameter of 1 μm interconnected with each other. Upon 980 nm laser diode excitation, yellow green and sky blue upconversion fluorescences of β-NaYF4:20%Yb3+, 2%Er3+ crystals and β-NaYF4:20%Yb3+, 0.5%Tm3+ crystals, which was caused by two- or three-photon upconversion processes, was observed clearly.

► Honeycomb-like β-NaYF4 crystals were successfully synthesized for the first time.
► The mixed NaCl–KCl flux cooling method was adopted.
► The yellow green and sky blue upconversion fluorescences can be observed clearly.