Adhesive Forces Between Aromatic Molecules and Graphene

Adhesive forces were investigated
in a computational study for the complexes of graphene with each of
the four adsorbates, benzene, anthracene, pyrene, and tetracene, by
using the DFT method B3LYP and the semiempirical method DFTB, together
with empirical dispersion corrections. The four aromatic molecules
differ in size and flexibility. Whereas the adhesion energy (for complete
separation) is independent of the way an adsorbed molecule is separated
from graphene, the maximum adhesive forces depend strongly on the
way the adsorbate is lifted. We investigated, therefore, four lifting
modes: in rigid vertical lifting, all atoms of the adsorbate are pull-off
points; in flexible central lifting, the pull-off points are in the
middle of the adsorbate; in both rigid and flexible terminal lifting,
the pull-off points are at one end and the adsorbate is regarded as
being either rigid or flexible. For the systems investigated, we find
that, for both the adhesion energies and the maximum adhesive forces,
the DFTB results are at most 10% lower than the corresponding DFT
results. For small or rigid molecules like benzene and pyrene, there
is no significant difference between rigid vertical and flexible central
lifting. For a flexible molecule like tetracene, the difference of
the maximum adhesive force can be up to 70%. Terminal lifting consists
of two subprocesses. For the second subprocess, both the adhesion
energy and the maximum adhesive force are essentially unchanged across
all four adsorbates; for the first subprocess, however, we find a
pronounced size dependence of the adhesion energy.