One-pot synthesis of ultrasmall β-NaGdF4 nanoparticles with enhanced upconversion luminescence

Abstract

Rare earth ion (RE³⁺) doped fluoride upconversion nanoparticles (UCNPs) have been proven to have chemical stability, low phonon energy and good optical properties, and therefore they have potential application value in bioimaging areas. However, the luminescence efficiency of the reported ultrasmall NPs is still limited by their low quantum yield (QY) or surface defects. Herein, sub-10 nm β-NaGdF₄:Yb/Er (Gd) NPs were synthesized via one-pot thermal decomposition in hot surfactant solutions (oleic acid and 1-octadecene). In addition, growth of uniform 11.70 nm β-NaGdYF₄:Yb/Er (YGd) and sub-17 nm β-NaGdF₄:Yb/Er@β-NaYF₄ (Gd@Y) NPs was induced by a Gd³⁺ seed-mediated technique to promote luminescence efficiency. Impressively, the obtained Gd@Y NPs can achieve a high QY of 0.22% at a power density of 0.30 W cm⁻² and a long decay lifetime (Er: ⁴S_3/2) of 448.6 μs. The experimental results suggest that the size tunable Gd³⁺ seed-mediated technique demonstrated here in Y NPs could be extended to other RE³⁺ doped NPs.