Atomic scale friction of self-assembled monolayers by hybrid molecular simulations

We developed a hybrid molecular, simulation method to extend simulation time scale to that in atomic force microscopy (AFM) experiments. The method combines a 'dynamic element' model for the tip-cantilever system in AFM and a molecular dynamics relaxation approach for the sample. We applied the hybrid simulation method to investigate atomic-scale frictional and mechanical properties of C-8 and C-15 self-assembled monolayers (SAMs) as a function of chain length. A quite different behavior of atomic-scale friction was found from that by conventional molecular dynamics simulation. Stick-slip phenomena were observed when a soft spring was used. The coefficient of friction calculated is chain-length dependent, which is consistent with what was observed in AFM experiments. Our studies further show that elastic modulus is chain-length independent, whereas effective shear modulus is chain-length dependent. Thus, effective shear modulus is responsible for the chain-length dependence of friction.