Effect of load on structural and frictional properties of alkanethiol self-assembled monolayers on gold: some odd-even effects.

We have conducted molecular dynamics simulations to study the frictional properties of alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 12 ≤ n ≤ 15) self-assembled monolayers (SAMs) on Au(111) surfaces, under various loading and shearing conditions. For the examined alkanethiols, we found some evidence of the friction coefficient being dependent on the number of carbon atoms in the molecule being odd or even. Alkanethiols with n = odd show consistently higher friction coefficients than those with n = even. Such odd-even effect seems to be independent of the sliding velocity. However, the effect is significant only at lower loads (<700 MPa). The structural origin of this odd-even effect has been discussed. The effect of loading on the structure is also studied. For dodecanethiol (n = 12) we find the film responds to increased loading initially by increasing the tilt and then by deformation of individual molecules. SAM-Au contacts under shear show periodic storage and release of energy and a clear stick-slip pattern in the shear stress, film thickness, and the tilt and tilt orientation angles.