Exploring in-plane interactions beside an adsorbed molecule with lateral force microscopy.

Area of Science:

• Surface Science
• Chemical Physics
• Atomic Force Microscopy

Background:

• Atomic force microscopy (AFM) with a CO-functionalized tip images molecular structures and chemical bonds.
• Direct observation of hydrogen atoms is challenging due to their small size, yet they are crucial for directing surface reactions.
• Previous methods struggled to resolve hydrogen atoms and their interactions in molecular systems.

Purpose of the Study:

• To directly identify hydrogen atoms in planar molecules using lateral force microscopy.
• To investigate the influence of tip-apex dipole on atomic force microscopy measurements.
• To understand interactions governing high-resolution imaging of surface adsorbates.

Main Methods:

• Lateral force microscopy (LFM) to measure in-plane interactions at the sides of PTCDA molecules.
• Development and application of a model incorporating radially symmetric atomic interactions.
• Density functional theory (DFT) calculations to verify electrostatic interactions and estimate tip-apex dipole strength.
• Investigation of a single CO molecule as a model surface adsorbate.

Main Results:

• Direct identification of hydrogen atoms via their repulsive signature in PTCDA molecules.
• Observed force data anomalies attributed to the electrostatic interaction of the metal tip-apex dipole, not H-bonding.
• DFT calculations confirmed the significant role of the tip-apex dipole, even in weakly polarized systems like CO.
• The radially symmetric atomic interaction model was found valid over a significant solid angle (82°).

Conclusions:

• Lateral force microscopy can directly image hydrogen atoms through their repulsive signature.
• The metal tip-apex dipole significantly influences AFM measurements, especially at close proximity, masking direct H-bonding observations.
• Understanding and accounting for tip-apex dipole effects are crucial for accurate high-resolution imaging of surface adsorbates.