Vibrational properties and diffusion of hydrogen on graphene

Hydrogen and deuterium chemisorption on a single layer of graphene has been studied by path-integral molecular-dynamics simulations. Finite-temperature properties of these point defects were analyzed in the range from 200 to 1500 K by using a tight-binding potential fitted to density-functional calculations. On one hand, vibrational properties of the adatoms are studied at their equilibrium positions linked to C atoms. The vibrations display an appreciable anharmonicity, as derived from the comparison between kinetic and potential energies, as well as between vibrational energy for hydrogen and deuterium. On the other hand, the adatom motion has been studied by quantum transition-state theory. At room temperature, quantum effects are found to enhance the hydrogen diffusivity on the graphene sheet by a factor of 20.