Effects of the reaction time and size on the up conversion luminescence of NaYF4:Yb(20%),Er(1%) microcrystals

NaYF4:Yb(20%),Er(1%) microcrystals were synthesized using a hydrothermal method, and the effects of the reaction time and the particle size on the up conversion (UC) emission properties were systematically studied. The UC emission intensities of the samples are directly related to the particle size and phase structure. The increase in the reaction time resulted in the phase transformation from cubic nanoparticles to hexagonal microprisms, leading to the enhancement of the UC emission. Moreover, the emission intensities and the intensity ratios of the green and red emissions (Fg/r) of the microprisms, in which a normal two-photon UC process was displayed under excitation with a 980 nm laser, remarkably increased with the increase of grain sizes, resulted from the increase of Yb3+ concentrations in crystals.