Plasmonic Effects of Au Nanoparticles on the Vibrational Sum Frequency Spectrum of 4-Nitrothiophenol

Vibrational sum frequency spectra (vSFS) of 4-nitrothiophenol (4-NTP) have been utilized to study plasmonic effects arising from its interaction with gold nanoparticles (Au NPs). To this end three systems have been studied: 4-NTP adsorbed on deposited Au nanoparticles, a self-assembled monolayer of 4-NTP on flat Au with dispersed Au NPs atop and, as reference, a self-assembled monolayer of 4-NTP. For 4-NTP on 50 and 80 nm Au NPs an enhancement of the SF intensity due to plasmonic effects by less than 1 order of magnitude is inferred in comparison to the monolayer system after taking the particle density into account. The addition of Au NPs to the monolayer system on flat Au also selectively enhances the response to an in-plane field component of the 532 nm upconversion light.