Tip-enhanced Raman spectroscopic imaging of patterned thiol monolayers

Area of Science:

• Surface Science
• Spectroscopy
• Nanotechnology

Background:

• Self-assembled monolayers (SAMs) are crucial for surface functionalization.
• Distinguishing between closely related molecular isomers on surfaces presents analytical challenges.
• Tip-enhanced Raman spectroscopy (TERS) offers high-resolution chemical imaging capabilities.

Purpose of the Study:

• To investigate the spatial distribution of two isomeric thiols (2-mercaptopyridine and 4-mercaptopyridine) within a SAM on a gold surface.
• To demonstrate the capability of TERS for analyzing weakly scattering molecules at monolayer coverage.
• To validate TERS imaging against known sample patterning techniques.

Main Methods:

• Utilized tip-enhanced Raman spectroscopy (TERS) for full spectroscopic imaging.
• Employed microcontact printing to create a patterned SAM of isomeric thiols on a gold substrate.
• Analyzed specific marker bands (1000 cm⁻¹ for 2-PySH, 1100 cm⁻¹ for 4-PySH) for isomer localization.

Main Results:

• Acquired high-resolution images with spectral information in every pixel.
• Spectroscopic data confirmed the expected molecular distribution based on microcontact printing.
• Successfully localized both 2-PySH and 4-PySH isomers on the surface.
• Deduced semi-quantitative information from Raman band intensities.
• Detected monolayer coverage of weakly scattering analytes, which were undetectable by normal Raman spectroscopy.

Conclusions:

• TERS is a powerful technique for mapping the distribution of isomeric molecules in SAMs.
• The signal enhancement of TERS allows for the detection of analytes with low Raman scattering cross-sections.
• TERS provides valuable insights into surface chemistry and molecular organization, even when nanometer resolution is not strictly required.