The Role of Intermolecular and Molecule−Substrate Interactions in the Stability of Alkanethiol Nonsaturated Phases on Au(111)

The structure and stability of alkanethiols self-assembled on Au(111) have been studied as a function of the molecular chain length by means of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). Below saturation, phases consisting of molecules with different tilt angles and periodicities are formed. Differences in the mechanical stability of these phases are revealed by AFM experiments and discussed in terms of the competition between intermolecular and molecule−substrate interactions as a function of chain length. For long molecules, intermolecular interactions play a dominant role which stabilizes the formation of closed packed 30° tilted (√3×√3)R30° structures. For short molecules, the van der Waals interaction with the gold substrate favors the formation of a 50° tilted phase in which the molecules are arranged in a rectangular configuration.