Metal adsorbate interactions and the convergence of density functional calculations
Christoph Rohmann, Maicol A. Ochoa, Michael P. Zwolak
The adsorption of metal atoms on nanostructures, such as graphene and nanotubes, plays an important role in catalysis, electronic doping, and tuning material properties. Quantum chemical calculations permit the investigation of this process to discover desirable interactions and obtain mechanistic insights into adsorbate behavior, of which the binding strength is a central quantity. However, binding strengths vary widely in the literature, even when using almost identical computational methods. To address this issue, we investigate the adsorption of a variety of metals onto graphene, carbon nanotubes, and boron nitride nanotubes. As is well-known, calculations on periodic structures require a sufficiently large unit cell to remove interactions between periodic images. Our results indicate that there are both direct and indirect mechanisms for this interaction, where the latter can require even larger system sizes than typically employed. The magnitude and distance of the effect depends on the electronic state of the substrate and the open- or closed-shell structure of the adsorbate. For instance, insulating substrates (e.g., boron nitride nanotubes) show essentially no dependence on unit cell size, whereas metallic or semi-metallic systems can have a substantial effect due to the delocalized nature of electronic states interacting with the adsorbate. We derive a scaling relation for the length dependence with a representative tight-binding model. These results enable the binding energies to be found in the isolated-impurity limit.