Repulsion between molecules on a metal: Monolayers and submonolayers of hexa-peri-hexabenzocoronene on Au(111)

We investigate the growth of hexa-peri-hexabenzocoronene (HBC) on Au(111) for monolayer (ML) and sub-ML coverage by scanning tunneling microscopy and low-energy electron diffraction. A transition from a disordered isotropic phase at low coverage to a highly ordered phase with a coverage-dependent lattice constant at higher coverage is found and attributed to a repulsive intermolecular force. To deduce the origin of this repulsion a model is set up, containing the Coulomb interaction between molecules and between localized dipoles from the push-back effect, as well as the intermolecular van der Waals potential. The modeling of the van der Waals interaction is done on a force field level. We find that the observed repulsion can only be explained when assuming a certain screening of the attractive London forces by the presence of the metal substrate.