The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy

TL;DR

This study systematically investigates how plasmonic hotspots influence the enhancement of vibrational sum frequency generation spectroscopy on gold nanoparticles, revealing that hotspots in multimers significantly amplify signals, especially in the orange-red spectrum.

Contribution

It demonstrates that hotspots in gold nanoparticle multimers play a dominant role in plasmon-enhanced SFG, challenging the expectation based on dipolar surface plasmon resonance models.

Findings

Maximal enhancement occurs in the orange-red spectral region.

Hotspots in particle multimers are the main contributors to amplification.

Enhancement varies with incident visible wavelength, correlating with hotspot formation.

Abstract

The plasmonic amplification of non-linear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for twenty visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmonics. Since the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities.