Densely Packed Perylene Layers on the Rutile TiO2(110)-(1 x 1) Surface

We have studied the interaction of perylene, a prototypical organic molecule, on the TiO2(110)-(1 x 1) surface at different surface coverages by an adequate combination of ultrahigh vacuum STM and UPS techniques, together with accurate many-body corrected ab initio calculations. We show that the molecular coupling and adsorption configuration strongly depend on the coverage. At the submonolayer coverage, the interaction is dominated by the van der Waals substrate attraction, which keeps the molecules in the troughs between adjacent oxygen rows. At this low-coverage regime no additional interaction is detected among the molecules, which rather behave like a 2D gas. When the coverage approaches the saturation of the first layer, a side-to-side molecular attraction sets in, which is shown to be mediated by a lateral intermolecular hybridization.