Statistical Analysis of NIR to Visible Upconversion Luminescence from Single NaYF₄:Yb³⁺,Tm³⁺ Nanoparticles on Nanocavity Arrays and Nanowire Substrates

Tm-doped energy transfer upconversion (ETU) NaYF₄: Yb³⁺:Tm³⁺ nanoparticles have potential applications in deep tissue imaging and energy conversion devices due to their visible (450nm) and infra-red (800nm) upconversion emission, but suffer from low quantum efficiency. We use single particle spectroscopic imaging and statistical analysis to assess the plasmonic enhancement of NIR-to-visible upconversion luminescence (UCL) from single β-NaYF₄:Yb³⁺:Tm³⁺upconverting nanoparticles (UCNPs) supported on substrates consisting of random arrangements of Ag nanowire composites (NWCs) and Au nano-cavity arrays (NCAs). By examining the effects at the single particle level, and accumulating a statistical sampling of single particle emitters, both on and off the plasmonic substrates studied, we obtain a statistical description of UCL emission enhancement in the Tm-doped UCNPs and map out the statistical distribution of excitation and luminescence enhancement on the plasmonic substrates. The distributions obtained are compared to Finite Difference Time Domain (FDTD) calculations of the fields near the plasmonic substrate, assuming the variations are due to variations in UCNP coupling to the plasmonic substrate based on an exponential decay with distance. We use both wide field and confocal spectroscopic imaging of single UCNPs on and off the plasmonic substrates in combination with energy and time resolved spectroscopy and compare these results to a coupled rate equation analysis to elucidate the energy transfer upconversion enhancement mechanisms for these substrates.