Ab initio study of K adsorption on graphene and carbon nanotubes:: Role of long-range ionic forces

We present a first-principles study of potassium atom adsorption on graphene and carbon nanotubes. Our calculations are carried out using density functional theory combined with the pseudopotential approximation. This study is, in part, inspired by inconsistent results reported for this system in literature. The adsorption energy of potassium atom on graphene obtained in previous studies ranges from 0.44 to 2.0 eV. We find that the reported disagreements can be explained by long-range electrostatic interactions between potassium and graphene. To illustrate this point, we present a simple classical model describing the interaction between the potassium atom and the graphene surface. The adsorption energies predicted by the model appear to be in good agreement with our first-principles calculations. In contrast, we find that the energy of potassium adsorption on carbon nanotubes depends on the nanotube surface curvature and chirality. This result suggests that the mechanism of interaction between potassium and carbon nanotubes may be different from that for the potassium-graphene system.