Structure and Stability of Self-Assembled Monolayers

With unprecedented detail and accuracy, scanning tunneling microscopy with high gap impedance reveals nanoscale molecular domains having several stable molecular conformations of alkanethiol monolayers self-assembled from alkanethiols on gold. Thermal annealing of the monolayers in air causes a small loss of molecules due to evaporation, but the surface remains defect-free and stable because molecular chains can compensate for missing rows of molecules. The molecular packing of the unfunctionalized alkanethiols is dominated by the spacing of sulfur on Au(111). In contrast, monolayers made from azobenzene functionalized thiols show a packing that is dominated by the end group. The molecules expose the upper edges of the phenyls to the surface and allow detection of pairwise aromatic interaction between end groups. Azobenzene as well as hydroxyl functionalized molecules have been identified by STM in monolayers of unfunctionalized thiols, which allowed studies of the stability of molecular patterns in such films. Our results contribute to a long-standing scientific debate on the structure and stability of self-assembled monolayers and provide the basis for an improved understanding of organic interfaces.