Stability and magnetism of hydrogen dimers on graphene

We report on the magnetism and stability of H dimers on a graphene sheet. Graphene is used as a simple model to grasp the basics of the H interaction with graphitic systems including graphite, graphene, polyaromatic hydrocarbons, and nanotubes. The dimers investigated here are found to be in ferromagnetic, antiferromagnetic, or nonmagnetic states. Results obtained from DFT calculations on the H dimer adsorption are analyzed with the help of spin-density maps. We thereby show that the dimer stability results from the magnetic properties of the pair of H atoms on graphene. The stability of dimers adsorbed in ortho and para positions is particularly emphasized. In order to rationalize the single and double H atom adsorption mechanisms, the whole pair formation process is divided into theoretical elementary steps to which energetic values are assigned.