Low Energy Electron Interactions with Aliphatic and Aromatic Self-assembled Monolayers on Au(111)

Electron scattering experiments have been performed on SAMs of the aliphatic molecule decanethiol and the aromatic molecule biphenylthiol grown on Au(111) via vapor phase deposition. The molecules take on either a lying down (LD) or standing up (SU) geometry, depending on coverage. From LEED, the crystal structure of both the LD phase and SU phase SAMs grown from both types of molecules is easily disrupted upon electron irradiation. Changes in peak intensities and positions of the TPD and XPS spectra indicate that the C-S bonds are cleaved for both molecules upon electron irradiation. In the SU phase of the aliphatic molecule, a large chemical shift in the C-1s and large reduction in the hydrocarbon fragments of the TPD spectra are observed. The LD phase of the aliphatic molecule and both phases of the aromatic molecule show only subtle changes to the spectra. The relative insensitivity of these SAMs to electron beam damage is attributed to charge delocalization effects. For the LD phase of the aliphatic molecule, excess charge is easily quenched since it is in direct contact with the metallic substrate. For the aromatic molecule, conjugated double bonds within the aromatic rings allow for excess charge delocalization, resulting in a lower probability for C-C bond breakage.