Plasmonic Dual-Enhancement and Precise Color Tuning of Gold Nanorod@SiO₂ Coupled Core–Shell–Shell Upconversion Nanocrystals

The last decade has witnessed the remarkable research progress of lanthanide-doped upconversion nanocrystals (UCNCs) at the forefront of promising applications. However, the future development and application of UCNCs are constrained greatly by their underlying shortcomings such as significant nonradiative processes, low quantum efficiency, and single emission colors. Here a hybrid plasmonic upconversion nanostructure consisting of a GNR@SiO₂ coupled with NaGdF_4:Yb³+,Nd³+@NaGdF₄:Yb³+,Er³+@NaGdF₄ core–shell–shell UCNCs is rationally designed and fabricated, which exhibits strongly enhanced UC fluorescence (up to 20 folds) and flexibly tunable UC colors. The experimental findings show that controlling the SiO₂ spacer thickness enables readily manipulating the intensity ratio of the Er³+ red, green, and blue emissions, thereby allowing us to achieve the emission color tuning from pale yellow to green upon excitation at 808 nm. Electrodynamic simulations reveal that the tunable UC colors are due to the interplay of plasmon-mediated simultaneous excitation and emission enhancements in the Er³+ green emission yet only excitation enhancement in the blue and red emissions. The results not only provide an upfront experimental design for constructing hybrid plasmonic UC nanostructures with high efficiency and color tunability, but also deepen the understanding of the interaction mechanism between the Er³+ emissions and plasmon resonances in such complex hybrid nanostructure.