How Precisely Can Individual Molecules Be Analyzed? A Case Study on Locally Quantifying Forces and Energies Using...

Area of Science:

• Surface Science
• Chemical Physics
• Materials Science

Background:

• Scanning probe microscopy (SPM) advances enable picometer-accurate, 3D measurements of tip-sample interactions.
• Understanding local probe-molecule interactions is crucial for designing functional materials and catalysts.

Purpose of the Study:

• To explore the achievable detail and accuracy in quantifying local probe-molecule interaction forces and energies.
• To investigate the interaction of a CO2 reduction catalyst, cobalt phthalocyanine (CoPc), with a scanning probe tip on a silver surface.

Main Methods:

• Utilized low-temperature, ultrahigh vacuum noncontact atomic force microscopy (NC-AFM).
• Recorded force and potential data as a function of tip-surface distance.
• Employed a CO molecule at the tip apex for atomic resolution imaging and interaction analysis.

Main Results:

• Constructed detailed 3D maps of normal and lateral forces, and tip-molecule interaction potential.
• Isolated molecule-tip interactions by analyzing tip-substrate interactions.
• Determined atomically resolved equilibrium interaction energies using a Lennard-Jones-type potential model.
• Observed physisorption interactions with energies below 160 meV.
• Interaction strength varied across the CoPc molecule, being weakest at peripheral hydrogens and strongest around the cobalt center, specifically in surrounding pockets.

Conclusions:

• Picometer-accurate SPM can quantitatively map site-specific, distance-dependent probe-molecule interactions.
• The study successfully characterized the physisorption interaction of CoPc with a CO-terminated tip, providing insights into catalyst-surface interactions.
• The findings demonstrate the potential of NC-AFM for detailed atomic-level characterization of molecular interactions relevant to catalysis and surface chemistry.