Mechanical and electrical properties of alkanethiol self-assembled monolayers: A conducting-probe atomic force microscopy study

The structure-property relationships for methyl-terminated alkanethiol [CH3(CH2)n-1SH, where n is the number of carbons in the molecular chain] self-assembled monolayers (SAMs) on gold substrates are considered, with a particular emphasis on using conducting-probe atomic force microscopy (CPAFM) to assess the mechanical and electrical properties and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to determine the molecular structure. CPAFM measurements are analyzed with an elastic adhesive contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal contact in which the insulator is extremely thin. NEXAFS carbon K-edge spectra are used to compute the dichroic ratio for each film, which provides a quantitative measure of the molecular structure. The combination of the two measurement methods provides clear property trends for this system over a wide range of n and a general methodology for the optimization of SAMs for micro- and nano-electromechanical systems, magnetic storage devices, and other applications.