Molecular dynamics study of scanning force microscopy on self-assembled monolayers

Abstract To understand scanning force microscope (SFM) images with molecular resolution on organic carpet-like films, we have performed isothermal molecular dynamics simulations of CH 3 (CH 2 ) 10 S self-assembled monolayers (SAMs) on Au(111) within the “unitedatoms” model. The SFM is represented by a deformable pyramidal cluster connected by orthogonal springs to a rigid support. Most of the relevant time scales (scanning, instrument response, SAM evolution) have been separated. With increasing penetration, continuous sliding is followed by stick-slip, leading to net friction, either extrinsic, i.e. governed by the soft spring(s) of the instrument, or intrinsic. Molecules close to the tip are strongly deformed, and the simulated SAM domain can be collectively tilted towards the scan direction. Defects in the chemisorbed S-layer are formed and are dragged beyond a critical load. Our results provide insight into the molecular origin of friction and viscoplastic response of model lubricant films actively investigated from both fundamental and applied viewpoints.