A novel anion doping strategy to enhance upconversion luminescence in NaGd(MoO₄)₂:Yb³⁺/Er³⁺ nanophosphors

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This work was selected as journal cover highlight by the editors. https://doi.org/10.1039/C7CP90135F

We propose a novel and efficient F⁻ anion doping strategy for enhancing upconversion luminescence in upconversion nanophosphors. NaGd(MoO₄)₂:Yb³⁺/Er³⁺ 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 ⁴S_3/2 level and the radiative branching ratio of ⁴S_3/2 → ⁴I_15/2 transition in F⁻-doped NaGd(MoO₄)₂:Yb³⁺/Er³⁺ 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:Yb³⁺/Er³⁺/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.