Atomic contributions to friction and load for tip-self-assembled monolayers interactions

Scanning force microscopies (SFM) are being routinely used to examine the mechanical and tribological properties of materials with the goal of obtaining information, such as Young's Moduli and shear strengths from the experimental data [Unertl, J. Vac. Sci. Technol. A 17, 1779 (1999)]. Analysis of data obtained from an SFM experiment typically requires the use of continuum mechanics models to extract materials properties. When applying these models care must be taken to ensure that the experimental conditions meet the requirements of the model being applied. For example, despite many successful applications of the Johnson-Kendall-Roberts (JKR) model to SFM data, it does not take into account the presence of a compliant layer on the sample surface. Recent AFM experiments that examined the friction of self-assembled monolayers (SAMs) have confirmed that friction versus load data cannot be fit by the JKR model. The authors suggest that the penetration of the SAM by the tip gives rise to an additional contribution to friction due to "plowing" [Flater , Langmuir 23, 9242 (2007)]. Herein, molecular-dynamics simulations are used to study atomic contact forces between a spherical tip in sliding contact with a SAM. These simulations show that different regions around the tip contribute in unanticipated ways to the total friction between the tip and the monolayer and allow for the number and location of monolayer atoms contributing friction to be determined. The use of atomic contact forces within the monolayer, instead of forces on the rigid tip layers, allows for the contributions to friction force (and load) to be deconvoluted into forces that resist (repel) and assist (attract) tip motion. The findings presented here yield insight into the AFM experiments of SAMs and may have important consequences for the adaptation of continuum contact models for the contact between a sphere and surface where penetration into the sample is possible.