Coexistence of Alkylated Sulfide Molecules along Two Orthogonal Directions of Graphite Lattice

Two coexisting types of molecular arrangements in self-assembled alkylated sulfide monolayers adsorbed on a graphite surface have been studied by scanning tunneling microscopy (STM). Longer (hexadecyl and octadecyl) sulfide molecules showed two types of arrangements along the (112̅0) zigzag and (101̅0) armchair directions of the graphite lattice, while the shorter dodecyl sulfide molecules lay along the zigzag direction adopted by previously studied n-alkanes and their functionalized derivatives. STM observations showed that the molecules along the zigzag direction participate in more favorable interactions with the graphite surface when compared to those along the armchair direction. The coexistence of two types of arrangements enables us to roughly estimate the free energy for adsorption and the interaction energy between the different arrangements. In both types of arrangements, the free energy changes for molecular adsorption need to exceed a threshold value for monolayer formation on the graphite surface.