Adsorption, Assembly, and Dynamics of Dibutyl Sulfide on Au{111}

Alkanethiol (RSH) monolayers are by far the most extensively studied surface self-assembly systems due to their robustness and the opportunity to control their assembly in the dimension perpendicular to the surface. Thioethers (RSR′), a similar class of molecule, remain largely unstudied to date but may offer a similar level of assembly control parallel to the surface. Here we report the self-assembly of dibutyl sulfide, a symmetric thioether species, on a Au{111} surface using scanning tunneling microscopy. As with thiols, dibutyl sulfide forms well-ordered monolayers, but due to the slightly weaker molecule−metal bond, the coverage and temperature-dependent behavior is very different than that of alkanethiols. Adsorption is sensitive to the different regions of the Au{111} herringbone reconstruction. Dibutyl sulfide lies parallel to the surface and forms well-ordered chains in domains that preferentially bind first in fcc regions, then hcp, and finally on soliton walls. The sulfide−Au interaction is strong enough to disrupt the native herringbone reconstruction of Au; however, unlike thiols, dibutyl sulfide adsorption does not result in etch pit formation. A monolayer of dibutyl sulfide has a very low defect density as compared to thiol−SAM based systems. This low defect density hints at the possibility of using a thioether moiety as a basis for a self-assembled system free of typical defects like etch pits, which allow attack and degradation of the monolayer. Upon annealing the surface with a high molecular coverage to 575 K, dibutyl sulfide desorbs molecularly from the less energetically favorable regions of the surface first. At this reduced coverage the system returns to an intermediate-coverage structure, thereby demonstrating the reversibility of the assembly. Elevating the temperature further causes the entire monolayer to desorb, and the original herringbone structure of Au returns. We postulate that this reversibility, coupled with the high rate of concerted rearrangements, allows this system to reach a very high level of order.