Comparative investigations of the packing and ambient stability of self-assembled monolayers of alkanethiols on gold and silver by friction force microscopy

The frictional behavior of self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols onto silver surfaces has been investigated by friction force microscopy (FFM) and compared with the behavior observed for SAMs on gold. Monolayers have been formed from adsorbates with different terminal group functionalities and alkyl chain lengths. Significant differences in frictional behavior are observed for monolayers on the two different substrates, and these may be interpreted in terms of variations in the packing density of the adsorbate molecules. The collapse of monolayer order following ambient oxidation may conveniently be studied using FFM, revealing that methyl terminated SAMs on silver exhibit substantially enhanced stability as a consequence of their increased packing density. Behavior is different for carboxylic acid terminated SAMs, indicating that the constraints on the packing of the two types of adsorbate are different.