Enhancement of single particle rare earth doped NaYF$_{4}$: Yb, Er emission with gold shell

We report on the enhancement in emission of NaYF$_{4}$: Yb, Er upconverting nanoparticles (UCNPs) coated with a gold nanoshell. We have synthesized a doped NaYF$_{4}$ core of 350 nm, with a sufficiently thick undoped NaYF$_{4}$ shell of 65 nm thickness to minimize contact with the gold surface plasmons, and effectively minimizing luminescence quenching. Absorption and fluorescence emission measurements of single NaYF$_{4}$ particles show enhanced absorption in the near infrared and a 1.5 times overall enhanced emission intensity. A relative increase in green/red emission was observed for both gold seed attachment and shell growth, of approximately 1.9 and 2.3 times respectively. Both Au seed attachment and shell growth has been shown to double the green/red emission ratio. The surface plasmon resonance of the UCNP core/Au shell composite is shown to be dependent on the gold shell thickness. Our experimental results are corroborated by finite element calculations.