Atomic force microscopy study of the mechanical and electrical properties of monolayer films of molecules with aromatic end groups

The effect of intermolecular pi-pi stacking on the electrical and mechanical properties of monolayer film molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA), were used as model systems with different pi-pi stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than I order of magnitude, with MPAA being the most conductive and MPPA being the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films, distinct molecular conductivity changes are observed upon mechanical perturbation. This effect, however, was not observed on the MPPA film, where intermolecular pi-pi interactions are likely weaker.