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.