Abstract
Hexagonal phase NaYF4:Yb3+, Tm3+ upconversion nanoparticles with Ba2+ co-doping ions have been prepared by a solvothermal method. A significant enhancement of ultraviolet and blue upconversion emissions of about 30-fold has been achieved with
15 mol% Ba2+ co-doping, which is ascribed to the enlarged particle size and the expanded lattice of the hexagonal phase NaYF4 nanoparticles due to Ba2+ co-doping. 15 mol% is the optimal Ba2+ content in this work. A new cubic phase fluoride with Ba2+,
Tm3+, Yb3+ and Y3+ occupying the cation sites was obtained at a high Ba2+ dopant concentration (20 mol% or above). As expected, the ultraviolet and blue upconversion emissions of Tm3+ weakened obviously since the high symmetric cubic lattice
is not conducive to 4f-4f transitions of Tm3+. The bright blue emission from Ba2+ co-doped β-NaYF4:Yb3+, Tm3+ upconversion nanoparticles is attractive for the potential applications in the fields such as photocatalysis and
blue phosphor.