Crucial Role of the Adhesion Layer on the Plasmonic Fluorescence

A nanoscale layer of chromium or titanium is commonly used in plasmonic nanoantennas to firmly adhere a goldfilm to a glass substrate, yet the influence of this layer on the antenna performance is often ignored. As a result, the need for the use of potentially better materials is not widely recognized. Using a single aperture milled in a goldfilm with 120 nm diameter as a nanobench for these investigations, we present thefirst experimental report of the strong dependence of the plasmonic enhancement of single-moleculefluorescence on the nature of the adhesion layer. By combiningfluorescence correlation spectroscopy andfluorescence lifetime measurements,weshowthatthisstructureisverysensitivetothepropertiesoftheadhesionlayer,andwedetail therespectivecontributionsofexcitationandemissiongainstotheobservedenhancedfluorescence.Anyincrease in the absorption losses due to the adhesion layer permittivity or thickness is shown to lower the gains in both excitation and emission, which we relate to a damping of the energy coupling at the nanoaperture. With this nanobench, we demonstrate the largest enhancement factor reported to date (25) by using a TiO2adhesion layer. The experimental data are supported by numerical simulations and argue for a careful consideration of the adhesion layer while designing nanoantennas for high-efficiency single-molecule analysis.