Abstract
The luminescence efficiency of lanthanide-doped upconversion nanoparticles is of particular importance for their embodiment in biophotonic and photonic applications. Here, we show that the upconversion luminescence of typically used NaYF&sub4;:Yb3+30%/Tm3+0.5% nanoparticles can be enhanced by ~240 times through a hierarchical active core/active shell/inert shell (NaYF&sub4;:Yb3+30%/Tm3+0.5%)/NaYbF&sub4;/NaYF&sub4; design, which involves the use of directed energy migration in the second active shell layer. The resulting active core/active shell/inert shell nanoparticles are determined to be about 11 times brighter than that of well-investigated (NaYF&sub4;:Yb3+30%/Tm3+0.5%)/NaYF&sub4; active core/inert shell nanoparticles when excited at ~980 nm. The strategy for enhanced upconversion in Yb3+/Tm3+-codoped NaYF&sub4; nanoparticles through directed energy migration might have implications for other types of lanthanide-doped upconversion nanoparticles.