Enhancement of the up-conversion luminescence from NaYF4:Yb3+,Tb3+

The synthesis conditions of the Yb3+ and Tb3+ co doped NaYF4 were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post annealing of the as prepared materials resulted in the enhancement of the otherwise rather weak up conversion from Tb3+ by 1-2 orders of magnitude. At the same Lime, the temperature of formation of the hexagonal NaRF4 phase with high up conversion could be lowered by 100 degrees C down to 350 degrees C. This improvement in up conversion was concluded to result from the better stoichiometry of the material without washing with wafer. The deficit of Na+ would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up conversion intensify. A further enhancement in the up conversion luminescence was observed to be due to the Er3+ ion impurity frequently associated with high concentration Yb3+ materials. The mechanism involving the unintentional Er3+ sensitizer and the resonance energy transfer in the Yb3+-Er3+-Tb3+ co-doped NaYF4 were discussed based on the energy level schemes of the Yb3+, Er3+, and Tb3+ ions in NaYF4. (C) 2013 Elsevier B.V. All rights reserved.