Distance-dependent fluorescence of tris(bipyridine)ruthenium(II) on supported plasmonic gold nanoparticle ensembles.

Metal surfaces and nanostructures interact with fluorescent materials, enhancing or quenching the fluorescence intensity, modifying the fluorescent lifetime, and changing the emission frequency and linewidth. These interactions occur via several mechanisms, including radiationless energy transfer, electric field enhancement, and photonic mode density modification. The interactions display a strong dependence on the distance between the fluorophore and the metal structures. Here we study the distance-dependent effects of two types of plasmonic gold nano-island films on the emission intensity, wavelength, linewidth and lifetime of a fluorophore layer, separated from the film by a dielectric spacer 2-348 nm thick. The distance dependence is found to be unrelated to the plasmonic field decay lengths. In some cases fluorescence intensity enhancement is seen even at 200 nm metal-fluorophore separation, indicating far-field effects. We report, for the first time, a distance-dependent oscillation in the emission peak wavelength and linewidth, attributed to interference-based oscillations in the intensity of the electric field. We find that the studied nanoparticle (NP) films do not display the previously reported distance profile of single NPs, but rather behave in a collective fashion similar to continuous metal surfaces.