Rapid microwave reflux process for the synthesis of pure hexagonal NaYF4:Yb3+,Ln3+,Bi3+ (Ln3+ = Er3+, Tm3+, Ho3+) and its enhanced UC luminescence

Pure hexagonal NaYF4:Yb3+,Ln(3+) (Ln(3+) = Er3+, Tm3+, Ho3+) crystals were obtained for the first time through a facile microwave (MW) reflux method at relatively low temperature (160 degrees C) and atmospheric pressure within only 50 min. By controllably increasing the NH4F content in the ethylene glycol (EG) solvent, the phase of as-prepared NaYF4:Yb3+,Ln(3+) gradually transforms from cubic to hexagonal. Correspondingly, the up-conversion (UC) emission intensities of hexagonal NaYF4:Yb3+,Ln(3+) (Ln(3+) = Er3+, Tm3+, Ho3+) are increased by 10-12 times compared to those of the cubic phase. A possible growth mechanism for the phase transformation under these MW conditions has been proposed. Moreover, for the first time, we introduced Bi3+ ion into beta-NaYF4:20%Yb3+,2% Ln(3+) crystals. As expected, the UC emission of beta-NaYF4:Yb3+,Ln(3+),Bi3+ are about 10-40 times higher than those of Bi3+ free samples. It is found that tri-doping of Bi3+ doesn't change the basic emission of Ln(3+) ions. XRD results gives evidence that tri-doping of Bi3+ ions can tailor the local crystal field and dissociate the Yb3+ and Ln(3+) ion clusters, which is the main reason for the UC enhancement. This designed MW reflux method for the synthesis of beta-NaYF4:20%Yb3+,2%Ln(3+) can be applied to prepare other rare earth fluorides. The markedly enhanced UC luminescence through Bi3+ doping also provides an effective way to gain very bright UC emission.