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

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, respectively, were observed under 980 nm laser irradiation via two- or three-photon upconversion processes