Distance-engineered Au-NaYF4 composite nanostructures for multicolor-tunable upconversion enhancement

Localized surface plasmon resonance (LSPR) in noble-metal nanostructures is a powerful strategy for enhancing upconversion (UC) emission from rare-earth-dopants. Herein, we present a distance-engineered Au–NaYF 4 core shell (CS) platform that selectively amplifies multicolor UC emission by jointly tuning the diameter of Au nanoparticles (NPs) and the thickness of an inert NaYF 4 spacer. Under 980 nm excitation, 56 nm Au NPs generate optimal near-field enhancement, yielding 6.82-fold (Er 3 + , 654 nm) and 5.61-fold (Tm 3+ , 450 nm) increases, respectively, in comparison to NaYF 4 : 20 %Yb 3+ /2 %Ho 3+ @NaYbF 4 : 2 %Er 3+ and NaYF 4 : 20 %Yb 3+ /2 %Ho 3+ @NaYbF 4 : 2 %Er 3+ @NaYF 4 : 20 %Yb 3+ /2 %Tm 3+ CS NPs. Power- dependent and time-resolved measurements attribute the gain to excitation-rate enhancement–an increased Yb 3+ absorption cross section–with negligible modulation of radiative rates. Finite-element simulations reveal intense gap “hot spots” for 56 nm Au NPs that account for the optimum. These results establish a simple distance–size design rule for spectrally selective, intensity-tunable UC emission, enabling compact plasmonic platforms for bioimaging, anti-counterfeiting and nanoscale optoelectronics.