Fluorescence Enhancement and Spectral Shaping of Silicon Quantum Dot Monolayer by Plasmonic Gap Resonances

A monolayer of silicon quantum dots (Si-QDs) 2.8 and 3.9 nm in diameter is placed in a gap between a gold (Au) thin film and a Au nanoparticle, and the photoluminescence (PL) properties are studied. By the metal nanoparticle over mirror (MNPoM) structure, the PL spectra of Si-QDs are strongly modified; the full width at half maximum is reduced to similar to 170 meV, which is less than half of that of Si-QDs on a silica Substrate. The spectral shape coincides almost perfectly with that of the scattering spectrum of the MNPoM structure, indicating efficient coupling of the luminescence of Si-QDs with the gap surface plasmon modes. The luminescence intensity of Si-QDs in the gap is estimated to be enhanced about 700-fold compared to those on a Au film.