Molecularly resolved surface superstructures of self-assembled butanethiol monolayers on gold

Self-assembled monolayers of butanethiol adsorbed onto highly uniform Au/mica films have been studied using in-air scanning tunneling microscopy (STM) and infrared spectroscopy. STM images taken of fully equilibrated films show molecularly resolved features that are believed to correspond to the methyl terminations of the organic surface. Large domains of organized adsorbates and a variant of the previously reported px root 3 superlattice structure are observed on these surfaces; using this nomenclature, the basic repeat unit of the structure reported herein corresponds to 7x root 3. Lateral variations in the elevations of striped structures are tentatively assigned to thiolates bound into fee sites of the Au(111) substrate. We have also observed molecular structure in regions of the superlattice that were structureless in UHV-STM studies. These observations suggest that the filling factor of the fully equilibrated surfaces is highly sensitive to the manner of sample preparation and storage. Large-scale desorption of thiolates does not appear to be a necessary step in the formation of highly organized films of these short-chain systems when equilibrated in thiol/methanol solutions, and the sample processing history may have significant effects on the ultimate surface topology.