Enhanced Molecular Fluorescence near Thick Ag Island Film of Large Pseudotabular Nanoparticles

Abstract

A highly reflective thick Ag island film (TAIF) sputter-grown on mica, consisting of unique large pseudotabular nanoislands, 60−200 nm across and 30−60 nm thick, produced an unusually strong surface enhanced fluorescence (SEF) for rhodamine dyes situated very close to (only ∼10 Å away from) the metal surface. A significantly greater part of the enhanced fluorescence was emitted into the back half space through TAIF and the mica substrate. The detailed fluorescence angular distribution was very similar to that of the light scattering by TAIF, suggesting that the enhanced emission originated from some large induced dipoles in TAIF. For reference, we also present a quantitative analysis of the fluorescent behavior of the same dye but directly coated on a reference glass surface. TAIF showed no distinct dipolar surface plasmon-like bands for excitation at normal incidence, and the light absorption by dye-coated TAIF could be described by simple superposition of the contribution of TAIF and that of the surface-bound dyes. However, the net dye absorptance was increased by 4−5 times due to the strong interactions of the dye transition dipoles with the TAIF-scattered fields. The estimated SEF quantum yield in the low dye coverage limit suggests a markedly high radiative yield of the induced dipole in TAIF around ∼0.5.