Toward Raman fingerprints of single dye molecules at atomically smooth Au(111)

Area of Science:

• Surface Science
• Spectroscopy
• Nanotechnology

Background:

• Studying organic adsorbates at the submonolayer level on atomically smooth surfaces presents significant challenges.
• Scanning tunneling microscopy (STM) provides atomic resolution but limited chemical information.
• Raman spectroscopy offers chemical identification but typically lacks the sensitivity for single-molecule analysis.

Purpose of the Study:

• To investigate the sensitivity and applicability of tip-enhanced resonance Raman (TERR) spectroscopy for analyzing organic submonolayer adsorbates.
• To determine the detection limits for unambiguous identification and semiquantitative surface coverage determination of organic dyes.
• To assess the influence of well-defined adsorption sites on spectral features at the single- and few-molecule level.

Main Methods:

• Utilized a scanning tunneling microscope (STM) tip to generate a highly enhanced electromagnetic (EM) field.
• Performed tip-enhanced resonance Raman (TERR) spectroscopy on the dye malachite green isothiocyanate adsorbed on an Au(111) surface.
• Correlated TERR spectra with high-resolution STM images of the probed surface regions.

Main Results:

• Achieved a detection limit of less than or equal to 0.7 pmol/cm², corresponding to approximately five molecules within the enhanced field.
• Demonstrated unambiguous identification of the dye and semiquantitative determination of surface coverage.
• Observed no significant variation in band positions or relative intensities at the single- or few-molecule detection level due to well-defined adsorption sites.

Conclusions:

• TERR spectroscopy, combined with STM, is a highly sensitive technique for studying organic adsorbates at the single-molecule level.
• The method allows for precise chemical identification and surface coverage analysis of molecular layers.
• The findings highlight the potential of TERR for detailed investigations of molecular interactions and arrangements on surfaces.