AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM9
Self-Assembled Monolayers: The Origin of Molecular Level Friction

Thursday, November 18, 2004, 11:00 am, Room 210C

Session: Tribology, Adhesion, and Friction
Presenter: R.C. Major, University of Minnesota
Authors: R.C. Major, University of Minnesota
X.-Y. Zhu, University of Minnesota
J.E. Houston, Sandia National Laboratories
Correspondent: Click to Email
Functionalized self-assembled monolayers (SAMs) are being used in a broad spectrum of disciplines to control the chemical and physical properties of surfaces at the molecular level. Despite a large number of studies, current understanding of the mechanical properties of these functional SAMs is rather incomplete. This arises in part because of the difficulty in quantitatively measuring adhesion and friction forces on the molecular level, and due to the non-trivial task of preparing well-ordered, functionalized thiol monolayers containing reactive endgroups (e.g. HS-(CH@sub 2@)@sub n@-COOH vs HS-(CH@sub 2@)@sub n@-CH@sub 3@) on Au(111) substrates; the latter has often been overlooked in the past literature. Here we report a quantitative study on the tribological properties of functionalized SAMs on Au(111) using Interfacial Force Microscopy (IFM). We show the intimate relationship of tribological properties with the chemical state of the SAM. For example, in the case of -COOH terminated SAMs, we observed a dramatic reversible change in friction response corresponding to the protonated and deprotonated state of the -COOH groups. This frictional response showed an interesting dependence on the conformation and length of the alkyl chains. Microscopic models correlating chemical interactions to tribological properties at the interface will be discussed. This work was supported by Sandia National Laboratories a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin company, for the DOE under Contract DE-AC04-94AL85000.