Sum-Frequency Spectroscopy Amplified by Plasmonics: The Small Particle Case

Abstract

The infrared–visible sum-frequency generation (SFG) spectroscopy response of a composite interface comprising molecules, spherical nanoparticles, and a substrate is modeled in the dipolar approximation. The spheres modify the local electric fields felt by the molecules grafted on either the surfaces of the particles or the substrate below the spheres. In the case of plasmonic spheres, the excitation of their surface plasmons leads to amplifications of the molecular SFG signals at the incoming visible and SFG frequencies for both types of molecules. The spectral evolutions of these amplifications are described as a function of the natures of the metal, the molecules, and the substrate; the chemical groups involved; and the surface density of nanoparticles on the substrate and their surface coverage. The latter parameter is shown to be the main source of SFG from molecules adsorbed on these highly centrosymmetric objects. Models are compared to experimental data, and excellent agreement is found for the amp...