Cooperative rotation of alkylated sulfide molecules at the liquid-graphite interface

Self-assembled monolayers of hexadecyl sulfide adsorbed on a graphite surface were studied using scanning tunneling microscopy (STM). STM images of the molecule indicate a bright spot and two relatively dim thin bands that correspond to the sulfur atom positioned at the center of the molecule and two alkyl chains that extend linearly from the sulfur atom, respectively. The contrast of the bands changes reversibly between the zigzag and aligned bright spots, which correspond to the zigzag methylene units of the alkyl chains; accordingly, the methylene units alternate between parallel and perpendicular orientations to the surface, respectively, on a time scale of minutes. The variation in contrast indicates cooperative rotation along the long axis of the molecules in the monolayer at the liquid-graphite interface. The reversibility of the contrast suggests that the solvent influences the difference in the free energies of the parallel and perpendicular configurations of the molecules with respect to the graphite surface.