A General Strategy to Enhance Upconversion luminescence in Rare-Earth-Ion-Doped Oxide Nanocrystals

We report a general strategy to enhance upconversion luminescence from rare-earth-doped oxide nanocrystals through tailoring the local crystal filed around rare earth ions. We show that upconversion luminescence from oxide nanocrystals of various host lattice (Y₂O₃, Gd₂O₃ and ZnO) and of diverse rare-earth-ions (such as Er³⁺, Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺) was enhanced by orders of magnitude through an introduction of non-luminescent Li⁺ ions into the lattice. These oxide nanocrsytals with tailored upconversion luminescence are able to evoke higher photoluminescence from organic dye molecules, posing their potential use in photodynamic therapy of deep tumors.