Environmentally friendly growth of functional nanocrystals with upconversion fluorescence

Abstract

Highly crystalline, NaYF₄ and NaYF₄:Ln (Ln=Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the cooling of the solo NaF at a growth temperature of 1100 ℃. The basic forms of NaYF₄ and NaYF₄:Ln crystals were a sphere and hexagonal cube, respectively. The crystal system, form and size were affected by cooling rate. In addition, the upconversion fluorescence property of the grown NaYF₄:Ln crystals was also dependent on the cooling rate, that is, the crystal system, and type of dopant. The red, green and blue upconversion fluorescence of NaYF₄:Ln crystals were clearly observed under 980 nm laser irradiation by a two- or three-photon upconversion process. The upconversion fluorescence of NaYF₄:Ln crystals was successfully controlled by changing the cooling rate and type of dopant. Furthermore, the NaYF₄:Yb,Er crystals were successfully grown using a mixed NaF-KF flux cooling method at a growth temperature of 800 ℃.