  The photoluminescence (PL) quantum yield of single-walled carbon nanotubes
(SWNTs) is relatively low, with various quenching effects by metallic species
reported in the literature. Here, we report the first case of metal enhanced
fluorescence (MEF) of surfactant-coated carbon nanotubes on nanostructured gold
substrates. The photoluminescence quantum yield of SWNTs is observed to be
enhanced more than 10-fold. The dependence of fluorescence enhancement on
metal-nanotube distance and on surface plasmon resonance (SPR) of the gold
substrate for various SWNT chiralities is measured to reveal the mechanism of
enhancement. Surfactant-coated SWNTs in direct contact with metal exhibit
strong MEF without quenching, suggesting a small quenching distance for SWNTs
on the order of the van der Waals distance, beyond which the intrinsically fast
non-radiative decay rate in nanotubes is little enhanced by metal. The metal
enhanced fluorescence of SWNTs is attributed to radiative lifetime shortening
through resonance coupling of SWNT emission to the re-radiating dipolar
plasmonic modes in the metal.
