A Hybrid Continuum-Molecular Analysis of Interfacial Force Microscope Experiments on a Self-Assembled Monolayer

Abstract

Nanoindentation experiments were performed on a defect-free, molecular self-assembled monolayer of octadecyltrichlorosilane (OTS) on silicon using an interfacial force microscope (IFM). The IFM provided repeatable and elastic force profiles corresponding to the adhesive and compressive response of these 2.5nm thick monolayers. As a first step in the analysis of the force profiles, the OTS was assumed to be linearly elastic and isotropic, and adhesive interactions were accounted for via a cohesive zone model. However, the assumption of linearity gave rise to force profiles that did not match the measurements. As a result, the mechanical behavior of the OTS was extracted from molecular-dynamics simulations and represented as a hypoelastic material, which, when used in finite element analyses of the IFM experiments, was able to fully reproduce the force profiles. This suggests that the continuum representation of the mechanical and adhesive behavior of self-assembled monolayers may be directly obtained from molecular analyses.