Nano Mechanical Analysis of IFM Force Profiles on Self-Assembled Monolayers

Abstract

In this study, a defect-free, self-assembled monolayer of octadecyltrichlorosilane(OTS) was deposited on a silicon substrate. Nanoindentation experimentswere performed with an interfacial force microscope (IFM) on these2.5 nm monolayers. As a first step in continuum finite element analyses,the OTS was assumed to be linearly elastic and isotropic. Adhesiveinteractions were also accounted for via a cohesive zone model. However,the assumption of linearity gave rise to force profiles that didnot match the measurements. Molecular dynamics simulations were thereforeemployed in order to provide further insight into the behavior ofOTS. These simulations indicated that the OTS had a highly non-linearand nearly incompressible response. Based on these results, a hypo-elasticmaterial model was developed as a convenient continuum representationof the mechanical behavior of OTS. This was then used in finite elementanalyses, which were able to fully reproduce the IFM force profiles.As a result, molecular and microscopic scales were linked in a relativelysimple but very effective manner. This suggests that there is a classof problems where the continuum representation of the material behaviormay be directly obtained from molecular analyses.