Commensurability and Mobility in Two-Dimensional Molecular Patterns on Graphite

Two-dimensional molecular patterns were obtained by the adsorption of long-chain alkanes, alcohols, fatty acids, and a dialkylbenzene from organic solutions onto the basal plane of graphite. In situ scanning tunneling microscopy (STM) studies revealed that these molecules organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The planes of the carbon skeletons, however, can be oriented either predominantly perpendicular to or predominantly parallel with the substrate surface, causing the lamellar lattice to be either in or near registry with the substrate (alkanes and alcohols) or not in registry (fatty acids and dialkylbenzenes). In the case of the alcohols and the dialkylbenzene the molecular axes are tilted by +30 degrees or -30 degrees with respect to an axis normal to the lamella boundaries, giving rise to molecularly well-defined domain boundaries. Fast STM image recording allowed the spontaneous switch between the two tilt angles to be observed in the alcohol monolayers on a time scale of a few milliseconds.