Correlation between the electrical and mechanical behaviours of a nanocontact with an alkanethiol monolayer

Protection against corrosion and wear of gold surfaces used for electrical contacts remains a current problem. Various types of organic layers can be used to overcome it, self assembled monolayers (SAMs) can be good candidates.We present here our first results on the microscopic mechanical and electrical behaviours of such layers. By means of a conducting probe atomic force microscope (CP-AFM), we investigated simultaneously the mechanical and electrical interactions between diamond tips and evaporated gold surfaces covered with SAMs of alkanethiolates. The characteristics of these SAMs were probed as a function of the alkyl-chain length.We show here that the mechanical contact and the electrical contact do not occur simultaneously when the surface is covered by an organic layer: the value of this shift increases as the thickness of the alkanethiolate SAMs increases.We propose a very simple model, allowing to roughly determine the layer thickness from the experimental curves obtained with CPAFM. We also observe that when repeated approach/retract cycles are performed at the same location, the value of this shift tends to decrease and a constant deflection phase progressively appears. This particular regime is attributed to the gradual damaging of the SAM at the location of the contact, and the accumulation of non-bound molecules over the remaining still bound layer. # 2003 Elsevier B.V. All rights reserved.