In-Depth Analysis of Excitation Dynamics in Dye-Sensitized Upconversion Core and Core/Active Shell Nanoparticles

Upconversion nanoparticles (UCNPs) combining both dye sensitization and core/shell enhancement are of great interest for their ability to boost the excitation efficiency of upconversion systems. Here, we report and investigate a 20-fold upconversion luminescence enhancement in dye-sensitized core/active shell UCNPs compared to that in nonsensitized core-only UCNPs. We observe a two-component luminescence rise dynamics in the upconversion kinetics of dye-sensitized UCNPs, distinctly different from the one-component rise dynamics of the nonsensitized UCNPs. For dye-sensitized UCNPs, the fast sub-microsecond component of the upconversion luminescence rise time is attributed to the radiative pumping of Er<sup>3+</sup> ions from the dye, whereas the slow sub-millisecond component is due to the nonradiative energy transfer from the dye predominantly to Yb<sup>3+</sup> ions, followed by the energy migration and the nonradiative energy transfer from Yb<sup>3+</sup> to Er<sup>3+</sup> ions. Our studies provide an insight into the interplay between radiative and nonradiative energy transfer as well as into the role of energy migration across the active shell of dye-sensitized core/active shell UCNPs.