Enhancement of Blue Emission via Upconversion in Solid-State Synthesized Hexagonal NaYF4:Ln3

Hexagonal NaYF4:Ln3+ (Ln3+=Yb3+ and Tm3+) was prepared via solid-state synthesis route. The synthesized powders were identified with X-ray diffraction patterns. Hexagonal phase was obtained by heating the powders at 550°C in reducing atmosphere, and the ions of Yb3+ and Tm3+ were completely incorporated into the lattice of hexagonal NaYF4 in the doping concentration range. Microstructure of the phosphor was observed by scanning electron microscopy (SEM). Intense blue (1G4→3H6, 1D2→3F4) and weak red (1G4→3F4) upconversion emission of Tm3+ ions with the energy transferred from Yb3+ ions were observed in the phosphor excited with a 980nm continuous wave laser diode. The intensity of blue emission was effectively enhanced by suitable doping of Tm3+ and Yb3+ ions, and strongest blue emission was obtained with the concentration of 0.2mol% Tm3+ and 40mol% Yb3+ doped.