Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles

The tetragonal-phased LiYF4:Yb3+/Er3+ nanoparticles with different core-shell nanostructures have been successfully synthesized by a modified solution-based method. The upconversion emissions of LiYF4:Yb3+/Er3+ nanoparticles were precisely tuned from green to red with the increase in Yb3+ concentrations under 980 nm excitation. The corresponding intensity ratios of the red and green emissions (R/G ratios) were enhanced from 0.06 to 5.33. However, the overall integral upconversion intensity of LiYF4:Yb3+/Er3+ nanoparticles clearly decreased when the Yb3+ doping concentrations were increased. For the enhancement of the red upconversion intensity and expansion of their application, two kinds of core-shell nanostructures, namely, LiYbF4:Er3+ @LiYF4 and LiYbF4:Er3+ @LiYbF4, were constructed through epitaxial growth at similar experimental conditions. The LiYbF4:Er3+ @LiYF4 core-shell nanostructures exhibited stronger upconversion emission and higher R/G ratios than the LiYbF4:Er3+ and LiYbF4:Er3+ @LiYbF4 nanoparticles. The highest R/G ratios increased to 6.27 in LiYbF4:Er3+ @LiYF4 core-shell nanostructures, increasing 104-fold. The upconversion regulation and enhancing mechanism were also discussed according to the emission spectra and fluorescence lifetimes.