Tip-Enhanced Upconversion Luminescence of Single Sub-10 nm Rare-Earth Ion-Doped Nanoparticle

Rare-earth ion-doped upconversion nanoparticles are attracting considerable attention for their unique luminescent properties and spectroscopic characteristics. Due to the excellent photostability and minimal safety risk, ultrasmall (sub-10 nm) nanoparticles are particularly attractive for their application in single-photon sources and nanomedicine. However, the surface defects and the interaction of rare-earth ions with organic ligands pose challenges for investigating the luminescent characteristics of rare-earth ions doped in an ultrasmall nanoparticle in situ. By employing the surface plasmon effect, one can precisely regulate the luminescent properties of rare-earth ions and better explore the luminescent dynamics and energy transfer mechanisms. Here, the tip-enhanced upconversion luminescence was conducted with a single sub-10 nm NaYF4:Yb3+/Er3+ nanoparticle, which was located between an Ag-coated tip and Au(111) microplate substrate. Strong upconversion emission in the green and red spectroscopic ranges was obtained under excitation at 980 nm. The influence of the localized surface plasmon resonance on the energy transfer process was observed, and enhancement factors of 13 and 9 for emissions at 553 and 650 nm were achieved, respectively. Utilizing atomic force microscopy for distance modulation, the substantial impact of varying gaps on the fluorescence emission of the single upconversion nanoparticle was studied, and effective plasmonic regulation of upconversion luminescence was achieved, resulting in a super-resolution luminescence of the single upconversion nanoparticle (UCNP) at 96 nm for the 650 nm transitions.