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 modeling 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.