A novel anion doping strategy to enhance upconversion luminescence in NaGd(MoO4)2:Yb(3+)/Er(3+) nanophosphors.

Abstract

We propose a novel and efficient F(-) anion doping strategy for enhancing upconversion luminescence in upconversion nanophosphors. NaGd(MoO4)2:Yb(3+)/Er(3+) nanophosphors doped with different F(-) contents are synthesized hydrothermally. Rietveld refinement results obtained from X-ray diffraction data indicate that the Gd-O bond length decreases and the O-Gd-O bond angle varies with increasing F(-) content, resulting in augmented local crystal field strength and distorted local site symmetry of the dopant lanthanide sites. Judd-Ofelt analysis suggests that the calculated radiative quantum efficiency of the (4)S3/2 level and the radiative branching ratio of (4)S3/2 → (4)I15/2 transition in F(-)-doped NaGd(MoO4)2:Yb(3+)/Er(3+) nanophosphors are much greater than those in F(-) anion-free samples. It is inferred that F(-) anion doping helps to reduce the nonradiative transition probabilities based on the luminescence dynamics. Rietveld refinement results and Judd-Ofelt analysis confirm jointly that doping of interstitial F(-) anions could enhance local crystal field strength with odd parity and modify site symmetry of the lanthanide activator ions, leading to enhanced radiative transitions and inhibited nonradiative transitions. A maximum of 17-fold enhancement of total emission intensity is found in NaGd(MoO4)2:Yb(3+)/Er(3+)/F(-) nanophosphors compared with F(-) anion-free counterparts. The proposed F(-) anion doping strategy provides an alternative approach for enhancing upconversion luminescence efficiency and could be extended to other inorganic upconversion nanomaterials.