Molecular-dynamics simulation of lateral friction in contact-mode atomic force microscopy of alkane films: The role of molecular flexibility

¹ Facultad de Física, Pontificia Universidad Católica de Chile - Casilla 306, Santiago, Chile
² Department of Chemistry, Technical University of Denmark - IK 207 DTU, DK-2800 Lyngby, Denmark, EU
³ Department of Physics and Astronomy and University of Missouri Research Reactor, University of Missouri Columbia, MO 65211, USA
⁴ Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA - Avda. Ecuador 3493, Santiago, Chile

^a flemming@kemi.dtu.dk

Received: 11 October 2010
Accepted: 15 June 2011

Abstract

Molecular-dynamics simulations are used to investigate lateral friction in contact-mode atomic force microscopy of tetracosane (n-C₂₄H₅₀) films. We find larger friction coefficients on the surface of monolayer and bilayer films in which the long axis of the molecules is parallel to the interface than on a surface of molecules with the long axis perpendicular to the surface, in agreement with experimental results. A major dissipation mechanism is the molecular flexibility as manifested in the torsional motion about the molecules’ C-C bonds. The generation of gauche defects as a result of this motion does not appear to be in itself a major channel of energy dissipation. As previously reported in the literature, the layer density and thereby the strength of the attractive film-tip interaction is also an important factor in energy dissipation.