Enhancing Upconversion from NaYF₄:Yb,Er@NaYF₄ Core-Shell Nanoparticles Assembled on Metallic Nanostructures

We report a simple method for the fabrication of a three-layered plasmonic structure of silicon substrate-Au nanospheres-upconversion particles (UCNPs) that displays up to 101-fold fluorescence enhancement. Monodispersed pure hexagonal-phase NaYF4:Yb,Er core and NaYF4:Yb,Er@NaYF4 core@shell nanocrystals were prepared by a solvothermal method. Two dimensional (2D) assembled Au spheres were prepared on a Si substrate, and then, 2D arrays of UCNPs were deposited on the grown 2D monolayered Au spheres by a self-organizing process. The distance between plasmonic Au NPs and rare-earth (RE) core was finely adjusted by changing the undoped NaYF4 shell thickness. The UC emission enhancement shows a pronounced shell thickness dependence. For the non-plasmonic structured samples, a single peak in upconversion luminescence (UCL) enhancement was observed as the undoped NaYF4 shell thickness increases from 0 nm to 23.0 nm. In contrast, for the plasmonic structured samples, multi-oscillations in UCL enhancement were observed in the undoped NaYF4 shell thickness range of 0-23.0 nm, where the UCL enhancement factors of three bands (521 nm, 540 nm and 654 nm) are high up to 65, 101 and 61, respectively, at 19.6 nm-thick NaYF4 shell. The multi-oscillations in UCL enhancement in the plasmonic samples can be associated with plasmonic coupling between arrays of core-shell UCNPs with various sizes and the underlying 2D Au spheres. The related mechanisms of the UCL enhancements are discussed.