Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering

Area of Science:

• Surface science
• Chemical physics
• Nanotechnology

Background:

• Tip-enhanced Raman scattering (TERS) offers high spatial resolution for molecular identification.
• Nonlinear TERS, particularly within scanning tunneling microscopy, has achieved sub-nanometer resolution.
• Distinguishing closely packed, chemically similar molecules remains a challenge due to signal mixing.

Purpose of the Study:

• To demonstrate the capability of nonlinear scanning tunneling microscope-controlled TERS for differentiating adjacent, chemically similar molecules.
• To investigate the adsorption configurations and orientations of these molecules on a silver surface.
• To overcome the limitations of signal mixing in conventional TERS.

Main Methods:

• Utilizing a nonlinear scanning tunneling microscope-controlled TERS setup.
• Analyzing vibrational fingerprints of individual molecules.
• Employing density functional theory (DFT) simulations for configuration analysis.

Main Results:

• Successfully distinguished between a metal-centered porphyrin and a free-base porphyrin in van der Waals contact.
• Achieved real-space differentiation of molecules with very similar chemical structures.
• Determined adsorption configurations and orientations on silver surfaces using DFT.

Conclusions:

• Nonlinear TERS microscopy can resolve individual molecules even when closely packed and chemically similar.
• The technique provides insights into molecular adsorption at the nanoscale.
• This advancement is crucial for single-molecule chemical analysis and surface behavior monitoring.