Role of adatom defects in the adsorption of polyaromatic hydrocarbons on metallic substrates

The adsorption of polyaromatic hydrocarbons (PAHs) on metallic substrates is of interest in the field of optoelectronics, due to the possibility of designing complex materials with tunable properties through surface functionalization with organic molecules. Much of the modelling research in this field has focused on perfectly symmetrical (pristine) substrates. There is limited information on the effect of substrate irregularities, such as adatoms, on the binding of PAHs onto substrates. Here, we examine how the presence of substrate-bound adatoms affects the binding of coronene and hexahelicene monomers and dimers on Au(111) and Cu(111) substrates, using a density functional theory approach. We found that helicene monomers were more effectively able to adapt to the presence of the adatoms than coronene, by coiling around the adatoms. Whereas upon adsorption on a pristine (111) surface, coronene can establish significantly stronger dispersive interactions than helicenes, adatom defects reverse the trend. For helicenes, the extent of flattening near the surface and molecular coiling are strongly influenced by the size of the defect, as a result of the interplay between the molecule’s drive to maximize overlap with the underlying surface and the enhanced reactivity of the low-coordinated adatoms.