AFM Study on Lattice Orientation and of SAMS of Fluorinated and on Au(111): The of the Molecular Structure

The molecular packing and the tribological properties of fluorinated thiols and disulfides in Self-assembled monolayers (SAMs) on Au(111) were studied. Atomic force microscopy (AFM) images with molecular resolution of the tail groups unveiled the lattice structure. The relative orientation of the tail group lattices with respect to the underlying Au(111) could be determined by the comparison of the lattice orientation with the orientation of the triangular Au(111) terraces. Depending on the molecular structure, a transition from a p (2 prime 2) structure for short chain molecules to a c (7 prime 7) structure for long chain molecules was observed for three homologous series of perfluoro organosulfur compounds. The friction force measured with silicon nitride tips was found to depend on the chain length of the molecules. Pull-off forces and adhesion hysteresis in AFM experiments did not correlate with the magnitude of friction. The observed qualitative correlation of disorder and increase in friction force supports the friction mechanism of energy dissipation in SAMs.