Low-Temperature Flux Growth and Upconversion Fluorescence of the Idiomorphic Hexagonal-System NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) Crystals

Abstract

Idiomorphic NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the NaNO3 flux cooling method at a relatively low holding temperature. The grown NaYF4 and NaYF4:Ln crystals had a hexagonal prismatic form, and their well-developed surfaces were relatively flat. TEM images indicated that the NaYF4 crystals were of good crystallinity. Their size and shape were relatively uniform, and they were poorly aggregated. The crystal phase, form, and size depended on the growth temperature and the solute concentration. In contrast, the addition of dopant ions (Yb3+, Er3+, and Tm3+) did not affect the shape, morphology, or crystallinity of the flux-grown NaYF4:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF4:Ln crystals were also dependent on the type and mixture ratio (i.e., starting composition) of the dopants. The green, orange, and blue upconversion fluorescences of NaYF4:10%Yb,1%Er, NaYF4:50%Yb,1%Er, and NaYF4:10%Yb,1%Tm crystals, resp...