The critical importance of gap modes in surface enhanced Raman scattering

Area of Science:

• Nanophotonics
• Surface Chemistry
• Spectroscopy

Background:

• Surface-enhanced Raman scattering (SERS) relies on surface plasmons for signal amplification.
• Gap modes between metal nanoparticles (MNPs) and substrates are crucial for efficient SERS.
• Understanding nanoparticle interactions and geometry is key to optimizing SERS performance.

Purpose of the Study:

• To investigate the role of gap modes in flocculated MNPs and MNP-substrate interfaces for SERS.
• To elucidate the adsorbed states of molecules and ions using flocculation-SERS.
• To enhance SERS signals through optimized nanoparticle size and geometry under different optical configurations.

Main Methods:

• Utilized flocculation-SERS to form closely adjacent metal nanoparticles (MNPs).
• Employed external and attenuated total reflection (ATR) geometries for plasmon excitation.
• Investigated the effect of nanoparticle size (radius r=15 nm vs. r=50 nm) on SERS enhancement.

Main Results:

• Achieved significant SERS enhancement factors of 10^5-10^8 at nanogaps between gold nanoparticles (AuNPs) and metal substrates.
• Demonstrated a notable increase in enhancement with larger AuNPs (factor of 10^3 for r=50 nm vs. r=15 nm).
• Observed additional signal enhancement under ATR geometry due to coupling of propagating surface plasmons with gap modes.

Conclusions:

• Flocculation-SERS effectively probes molecular adsorption and enables gap mode formation.
• Nanoparticle size and geometry significantly influence SERS enhancement, with larger particles yielding greater signals.
• Combining gap modes with ATR geometry offers a pathway for further SERS signal amplification.