Probing the Structure and Orientation of Carboxylic Acid-Terminated Self-Assembled Monolayers

Self-assembled monolayers (SAMs), such as alkanethiols (AT), are widely used as functional coatings or interfaces between different materials. There is an assumption that the arrangement and alignment of the hydrocarbon chains in films made from carboxyl-terminated alkanethiols are similar to those made from alkanethiols. Here, the structure of the outermost layer and near-surface region of SAMs formed from carboxyl-terminated alkanethiols of various lengths has been analyzed. The chemical composition of the samples was measured using X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (AR-XPS), allowing the film thickness. Metastable induced photoelectron spectroscopy (MIES) as a surface analytical tool sensitive only for the outermost layer in conjunction with density functional theory (DFT) calculations provided insights into the composition of the topmost layer, showing that it consists mainly of the backbone of the SAM-forming molecules. Through combining AR-XPS concentration depth profiles and the measurement of the composition of the outermost layer, it can be shown that SAMs tend to favor a gauche orientation, enabling interactions between the functional groups.