Synthesis and upconversion luminescence of NaYF4:Yb3+,Tm3+/TiO2 nanocrystal colloidal solution.

NaYF4:Yb3+,Tm3+ upconversion nanocrystals were synthesized with oleic acid as a capping ligand. The as-prepared samples were redispersed in cyclohexane, forming transparent and stable solutions which perform bright blue upconversion. Inducing TiO2 as the outer layer of the fluoride cores, we investigated the effect of TiO2 on the upconversion process.