Enhanced Upconversion Luminescence in Yb(3+)/Tm(3+)-Codoped Fluoride Active Core/Active Shell/Inert Shell Nanoparticles through Directed Energy Migration

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(4):Yb(3+)30%/Tm(3+)0.5% nanoparticles can be enhanced by ~240 times through a hierarchical active core/active shell/inert shell (NaYF(4):Yb(3+)30%/Tm(3+)0.5%)/NaYbF(4)/NaYF(4) 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(4):Yb(3+)30%/Tm(3+)0.5%)/NaYF(4) active core/inert shell nanoparticles when excited at ~980 nm. The strategy for enhanced upconversion in Yb(3+)/Tm(3+)-codoped NaYF(4) nanoparticles through directed energy migration might have implications for other types of lanthanide-doped upconversion nanoparticles.