Alkanethiol Monolayers Contain Gold Adatoms, and Adatom Coverage Is Independent of Chain Length

The nature of the sulfur−gold bond in alkanethiol films self-assembled on Au(111) surfaces is widely contested, despite significant interest in these systems over the last 25 years. Recent theoretical and experimental studies have suggested gold adatoms are incorporated into the alkanethiol−gold interface. We have exposed alkanethiol self-assembled monolayers (SAMs) to gas-phase hydrogen atoms to remove the monolayer; the gold adatoms remain on the surface and form features that we observe using scanning tunneling microscopy (STM). The features include the formation of single-atom-thick gold islands, decreasing size of surface vacancy pits, and faceting of terrace step edges; compared to the alkanethiol-terminated surface, these features indicate a net increase in the amount of gold present on the surface, compared to the alkanethiol-terminated bulk structure. Varying the length of the alkane chain does not affect the total adatom coverage, as the adatom coverages for ethanethiol (0.172 ± 0.039), octanethiol (0.143 ± 0.033), and dodecanethiol (0.154 ± 0.024) SAMs are within experimental error of one another. This corresponds to one gold adatom for every six atoms in the bulk-terminated surface, and thus one gold adatom for every two alkanethiol molecules.