on a Single Layer Graphene: Diffusion Monte Carlo Study*

Diffusion Monte Carlo (DMC) calculations were performed for an accurate description of the nature of the O₂ adsorption on a single layer graphene. We investigated the stable orientation of O₂ as well as its equilibrium adsorption distance and energy for a specific adsorption site. At equilibrium adsorption distances, an O₂ was found to prefer a horizontal orientation, where the O-O bond is parallel to the graphene surface, to the vertical orientation. However, the vertical orientation is favored at the O₂-graphene distance shorter than the equilibrium distance, which could be understood by their steric repulsion. Contrary to previous DFT calculations, our DMC calculations show that the most energetically-stable adsorption site for O₂ is not the center of a hexagonal ring but the midpoint of a C-C bond. The O₂ adsorption energy at a bridge site was estimated to be 0.135(4) eV, which is in good agreement with the recently-reported experimental value [1]. Finally, we have found that O₂ is very diffusive on the surface of graphene with the diffusion barrier along a bridge-hollow-bridge path being as small as ~ 10 meV.
[1] F. R. Bagsican et al., Sci. Rep. 7, 1774 (2017).