On the interaction of polycyclic aromatic compounds with graphene

The adsorption and diffusion of benzene, hexafluorinated benzene, perylene, perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on graphene has been studied by density functional means on a generalized gradient approximation level, including a semi-empirical correction to account for dispersive forces. It is shown that for all considered molecules the adsorption strength is mainly due to the latter, with the electronic interaction being relatively small and repulsive. As a rule-of-thumb, the strength of the adsorption interaction is 11-13 kJ mol−1 per C atom. The adsorption energies are large enough to avoid desorption at room temperature. The estimated diffusion and rotation barriers are remarkably small, thus allowing a rapid diffusion and self-arrangement even at cryogenic temperatures. Finally, the adsorption of benzene or perylene derivatives may act, depending on the molecule and nature of the substituents, as a source for n- or p-doping, achieving up to 0.2 electrons(holes)/molecule. The lowest unoccupied molecular orbitals of PTCDI and PTCDA are close in energy to the Dirac point of graphene and induce a conduction gap of ∼210-240 meV in the graphene band structure. Thus, they can be used for graphene band gap engineering and doping by the non-aggressive method of molecular adsorption.