Luminescence Enhancement Mechanism of Lanthanide-Doped Hybrid Nanostructures Decorated by Silver Nanocrystals

Hybrid nanostructures composed of rare earth ion-doped lanthanum trifluoride nanocrystals deposited on silica nanospheres (LaF₃:Yb³⁺, Er³⁺@SiO₂) and decorated with varying quantities of silver nanoparticles (Ag NPs) were synthesized using a simple strategy. Down and upconversion luminescence spectra were recorded. The luminescence dynamics were also recorded following excitation with a 532-nm pulse. Silver loading was found to have a significant effect both on the luminescence intensity and the luminescence decay rate, with the samples with the lowest silver content showing reduced luminescence intensity over silver-free samples, while the samples with large levels of silver loading showed significant luminescence enhancement. The results were successfully (and quantitatively) interpreted in terms of the competition between surface plasmon-induced field enhancement mediated by the Ag nanoparticles and nonradiative energy transfer from the luminescent ions to Ag nanoparticles. By combining the measured luminescence intensity with the luminescence decay rate determined from the dynamics measurements, and with the measured surface plasmon absorption spectra, one could obtain a quantitative and self-consistent understanding of the observed dependence of the green ⁴S_3/2→⁴I_15/2 (~540 nm) and red ⁴F_9/2→⁴I_15/2 (~650) emission bands on the Ag NP metal loading.