Controllable Synthesis and Peculiar Optical Properties of Lanthanide-Doped Fluoride Nanocrystals

Yb³⁺ /Tm³⁺ -codoped NaYF₄ nanocrystals with different phases (cubic α and hexagonal β) and morphologies (nanoparticles, nanorods, nanoplates) have been controllably fabricated by a convenient hydrothermal synthesis technique. Up-conversion emission peaks were observed at λ=450, 475, 645, and 700 nm, which were assigned to the ¹ D₂ -³ F₄ , ¹ G₄ -³ H₆ , ¹ G₄ -³ F₄ , and ³ F₂ -³ H₆ transitions of Tm³⁺ ions, respectively. By using a homemade detection system, the anisotropic polarized emission properties of individual NaYF₄ :Yb, Tm nanorods and nanoplates were investigated. The results indicated that the polarization emission ratio, ρ, of the ¹ G₄ -³ F₄ emission of individual NaYF₄ :Yb,Tm nanorods was 0.18, whereas the anisotropic polarization emission of individual NaYF₄ :Yb,Tm nanoplates could be neglected; this indicated that the size and morphology of the nanocrystal played an important role in the polarized emission properties. Investigation into the mechanism indicated that the dielectric mismatch was not the predominant mechanism for the polarized emission of individual NaYF₄ :Yb,Tm nanorods. The as-prepared NaYF₄ :Yb,Tm nanocrystals, with controllable morphology and anisotropic polarized emission properties, are expected to be widely applied as polarized light resources, bio-labels, photodetectors, and so forth.