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

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.