Li+ ion doping: an approach for improving the crystallinity and upconversion emissions of NaYF4:Yb3+, Tm3+ nanoparticles

Abstract

The application of upconversion nanoparticles (UCNPs), especially in vivo, has so far been hampered by their relatively low upconversion efficiency. In this work, a strategy of Li⁺ doping was revisited with the aim of enhancing UV to blue UC luminescence of NaYF₄:Yb³⁺, Tm³⁺ nanocrystals. We have demonstrated that the short wavelength UC emission bands were indeed significantly enhanced. Compared to lithium-free NaYF₄:Yb³⁺, Tm³⁺, the UC emission intensities of 452 nm and 479 nm of the NPs co-doped with 7 mol% Li⁺ ions were increased by 8 and 5 times, respectively. The mechanism of the enhancement was discussed and the improvement of the nanoparticles' crystallinity and the distortion of the local symmetry around the Tm³⁺ ions, when the Li⁺ ions were introduced, were confirmed to be the origin of the improvement.