When do hydrocarbons dewet metal surfaces?

Isolated, large hydrocarbon molecules optimize van der Waals bonding by lying flat on substrates. Likewise, in three-dimensional hydrocarbon crystals, the van der Waals bonding is optimized between the flat molecular planes. In the intermediate case, can a densified monolayer tilt before forming 3D crystals that dewet the substrate? We used scanning tunneling microscopy and density functional theory calculations to study molecular layers of coronene on Cu(111) to determine balance between coronene-substrate and coronene-coronene interactions. Our findings disagree with some of the microscopy observations that reported tilted coronene at the monolayer, and we proposed that configurations of tilted molecules are metastable. Our calculations also show that coronene double layers are composed of tilted molecules, providing an alternative explanation for tilted coronene. However, double layers are metastable with respect to the formation of bulk coronene crystals that dewet the substrate, so their occurrence would suggest nucleation barriers to dewetting into three-dimensional islands. Similar metastability has been observed in metal-on-metal epitaxy.