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

Abstract

Scanning force microscopies ͑SFM͒ 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 ͑1999͔͒. 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 ͑JKR͒ 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 ͑SAMs͒ 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 ͑2007͔͒. 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 load͒ to be deconvoluted into forces that resist ͑repel͒ and assist ͑attract͒ 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.

Atomistic Insights into the Running-in, Lubrication, and Failure of Hydrogenated Diamond-Like Carbon Coatings

“…Although this potential has also been employed in previous studies on hydrocarbon tribology [13][14][15], one should be extremely cautious to use it in its original formulation for mechano-chemical simulations. Bond-order potentials, like the REBO, are typically fitted to ground-state energies and force constants, such as solid-state structures and their respective elastic constants as well as molecules and their respective vibration frequencies.…”

“…Previous theoretical investigations in the tribology of hydrocarbons have focused on systems in which the surfaces were completely saturated with hydrogen and no chemical reaction or just single hydrogen abstraction events occurred (for reviews see the recent articles by Harrison et al [11,12]). Examples include the friction between self-assembled monolayers [13,14] as well as amorphous carbon films [15]. The latter work considered hydrogen-free DLC films sliding with 100 m s -1 against a hydrogen-terminated diamond (111) surface.…”

Dry Sliding Contact Between Rough Surfaces at the Atomistic Scale

“…The system that attracted most attention is the sliding of a small tip over relatively flat surfaces, in an attempt to model the AFM experimental observations [8][9][10][11][12][13][14][15][16][17][18][19]. A second system studied involves the normal loading of rough surfaces, either with deformable flat and rigid rough surfaces [20,21], with one of the surfaces being rigid and flat [22,23], or two flat surfaces [24,25].…”

The Effects of Interface Structure and Polymerization on the Friction of Model Self-Assembled Monolayers

“…This allows for the effect of changes in interfacial structure on friction to be isolated from effects that arise due to indentation of the monolayer. The effects of using a finitesized tip on the friction of monolayers has been reported elsewhere [29][30][31][32].…”

“…A simple feedback control algorithm similar to one used in previous studies maintains approximately a constant load [24,29,33]. Here, the amount by which tip height is adjusted is dependent upon how far the system is away from the target load as opposed to the fixed adjustment previously used.…”