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-KendallRoberts 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 et al., 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.