Single-Molecule Surface-Enhanced Raman Scattering: Can STEM/EELS Image Electromagnetic Hot Spots?

Area of Science:

• Plasmonics
• Nanophotonics
• Spectroscopy

Background:

• Single-molecule surface-enhanced Raman scattering (SMSERS) relies on electromagnetic "hot spots."
• The precise nature and location of these hot spots remain debated.
• Understanding hot spots is crucial for SMSERS applications.

Purpose of the Study:

• To spatially and energetically map localized surface plasmon modes in SMSERS-active nanostructures.
• To investigate the role of electromagnetic hot spots in SMSERS.
• To correlate experimental observations with theoretical predictions.

Main Methods:

• Utilized scanning transmission electron microscopy coupled with electron-energy-loss spectroscopy (STEM/EELS).
• Employed a bianalyte approach with Rhodamine 6G isotopologues to confirm single-molecule detection.
• Performed 3D electrodynamics simulations of electron-energy-loss probability and near-electric field enhancements.

Main Results:

• Generated high-resolution spatial and energy maps of localized surface plasmon modes.
• Observed no direct signature of electromagnetic hot spots within nanoparticle gaps using STEM/EELS.
• Simulations indicated hot spot excitation is possible when the electron beam is outside the junction.

Conclusions:

• Experimental plasmon maps do not directly reveal hot spots in nanoparticle gaps.
• Electron beam excitation of hot spots is dependent on the beam's position relative to the junction.
• This work provides new insights into the excitation mechanisms of SMSERS hot spots.