Polyol-mediated synthesis and luminescence of lanthanide-doped NaYF4 nanocrystal upconversion phosphors

Polyol method has been successfully used to prepare Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 nanocrystal upconversion phosphors. Three kinds of polyols (glycol, diethylene glycol and glycerol) were used not only as reaction medium but also as size-controlled capping ligands to limit particle growth and stabilize the particles. Nanoparticles crystallized in pure cubic phase were obtained via the polyol method, then cubic-to-hexagonal phase transition of the nanoparticles occurred during the follow-up solvothermal treatment. Factors that had impact on the structure and morphology of the nanoparticles and affected the fluorescent performance of the nanocrystal upconversion phosphors were discussed. The obtained Yb3+/Er3+ and Yb3+/Tm3+ codoped NaYF4 nanoparticles could be well dispersed in water, and they could emit multicolor upconversion fluorescence under 980 nm laser excitation; these features were favorable for the multiple labeling in biological detection.