Rapid microwave reflux process for the synthesis of pure hexagonal NaYF4:Yb~(3+),Ln~(3+),Bi~(3+) (Ln~(3+) = Er~(3+), Tm~(3+), Ho~(3+)) and its enhanced UC luminescence

摘要

Pure hexagonal NaYF4:Yb~(3+),Ln~(3+) (Ln~(3+) = Er~(3+), Tm~(3+), Ho~(3+)) crystals were obtained for the first time through a facile microwave (MW) reflux method at relatively low temperature (160 °C) and atmospheric pressure within only 50 mm. By controllably increasing the NH4F content in the ethylene glycol (EG) solvent, the phase of as-prepared NaYF4:Yb~(3+),Ln~(3+) gradually transforms from cubic to hexagonal. Correspondingly, the up-conversion (UC) emission intensities of hexagonal NaYF4:Yb~(3+),Ln~(3+) (Ln~(3+) = Er~(3+), Tm~(3+), Ho~(3+)) are increased by 10~(-1)2 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 Bi~(3+) ion into β-NaYF4:20%Yb~(3+),2% Ln~(3+) crystals. As expected, the UC emission of P-NaYF4:Yb~(3+),Ln~(3+),Bi~(3+) are about 10-40 times higher than those of Bi~(3+) free samples. It is found that tn-doping of Bi~(3+) doesn't change the basic emission of Ln~(3+) ions. XRD results gives evidence that tri-doping of Bi~(3+) ions can tailor the local crystal field and dissociate the Yb~(3+) and Ln~(3+) ion clusters, which is the mam reason for the UC enhancement. This designed MW reflux method for the synthesis of β-NaYF4:20%Yb~(3+),2%Ln~(3+) can be applied to prepare other rare earth fluorides. The markedly enhanced UC luminescence through Bi~(3+) doping also provides an effective way to gam very bright UC emission.