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

Area of Science:

• Surface science
• Nanotechnology
• Spectroscopy

Background:

• Vibrational sum frequency spectroscopy (SFG) is a powerful surface-sensitive nonlinear optical technique.
• Plasmonic nanoparticles, such as gold nanoparticles, can enhance optical signals through localized surface plasmon resonances (LSPRs).
• Understanding plasmonic enhancement mechanisms is crucial for improving SFG sensitivity at interfaces.

Purpose of the Study:

• To systematically investigate the plasmonic amplification of SFG signals at gold nanoparticle surfaces.
• To explore the influence of incident visible wavelength on SFG signal enhancement.
• To elucidate the role of particle aggregation and hotspots in plasmon-enhanced SFG.

Main Methods:

• Synthesis of dodecanethiol-coated gold nanoparticles chemically deposited on silicon substrates.
• Recording SFG spectra across 20 different visible incident wavelengths.
• Analysis of vibrational intensities, specifically thiol methyl stretches, as a function of wavelength.

Main Results:

• SFG vibrational intensities showed significant amplification due to coupling with plasmon excitation.
• Maximum enhancement occurred in the orange-red region of the visible spectrum, deviating from predictions based on simple dipolar LSPRs.
• The observed spectral dispersion was attributed primarily to plasmonic hotspots generated in nanoparticle multimers.

Conclusions:

• Plasmonic amplification of SFG is strongly dependent on the incident wavelength and nanoparticle morphology.
• Hotspots in nanoparticle multimers play a dominant role in enhancing SFG signals, even at low surface densities.
• A simple model incorporating longitudinal surface plasmons of multimers successfully explains the experimental spectral dispersion.