Atomistic molecular dynamics simulations of chemical force microscopy

Area of Science:

• Surface Science
• Nanotechnology
• Computational Chemistry

Background:

• Chemical force microscopy (CFM) probes molecular-scale adhesion and tribology.
• Interpreting CFM requires understanding unobservable tip-sample junction processes.
• Atomic motions and force distribution are key but experimentally inaccessible details.

Purpose of the Study:

• To simulate atomistic molecular dynamics of a CFM tip interacting with a solid wall.
• To investigate the behavior of a C12 alkylthiolate self-assembled monolayer (SAM) during loading-unloading.
• To compare simulation results with established contact mechanics models.

Main Methods:

• Atomistic molecular dynamics simulations.
• Modeling a CFM stylus with a C12 alkylthiolate SAM in contact with a solid wall.
• Simulating a complete loading-unloading sequence under near-equilibrium conditions.

Main Results:

• The SAM film transitions from a fluid to a solidified state upon compression.
• Contact composition changes during loading, exposing the film's interior.
• Force distribution within the contact is uneven with significant local fluctuations.
• Standard models (JKR, DMT, Hertz) show deviations from simulation data.

Conclusions:

• Molecular dynamics simulations provide insights into CFM tip-sample junction dynamics.
• Experimental interpretations may be flawed due to unobserved phenomena and model limitations.
• Improved models accounting for finite thickness and nonlinear elasticity are needed.