Aluminum adsorption on graphene: Theoretical study of dispersion effects

Abstract

The interaction between a single atom and graphene is an example in which the density functional theory (DFT) presents serious difficulties in giving an appropriate description of the adsorbate-substrate interaction, giving also different predictions according to the chosen approximation. The present calculations sustain that the inclusion of dispersion interactions in the framework of DFT for the Al/graphene system lead to potential energy curves of different nature according to the theoretical approach employed. The adsorption of an Al atom on the graphene surface was studied using both cluster and slab models. Cluster DFT-PBE calculations show the presence of a minimum at hollow site at an Al-graphene distance of about 2.1-2.3 Å corresponding to an exothermic state. Conversely, under B3LYP the same adsorption mode is endothermic. In comparison, our MP2 reference calculations predict the formation of two minima, both of exothermic nature, separated by an important energy barrier (about 0.2-0.4eV). The incorporation of empirical van der Walls (vdW) corrections to B3LYP changes the original behavior, giving an exothermic adsorption; furthermore, it produces a second, more external minimum. Slab calculations with PBE, and specially using the vdW-DF2 functional, predict also the formation of a minimum of very low depth at about 3.1 Å. The analysis of results obtained with cluster and slab models sustains that the bonding of the inner minima is of ionic character while that of the external ones is of dispersion character.