Hydrogenation effects on the structure and morphology of graphene and single-walled carbon nanotubes

A systematic computational study is presented of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene layers and single-walled carbon nanotubes (SWCNTs). The study is based on a combination of classical molecular-dynamics (MD) and Monte Carlo simulations of structural and compositional relaxation of the hydrogenated surfaces, employing hydrogen distributions consistent with experimental observations and first-principles calculations. Results are reported for the strains induced on the graphene and the SWCNTs, as a result of $sp2-sp3$ bonding transitions due to atomic H chemisorption, and their dependence on the H surface coverage, $Θ$⁠, over the entire range $0≤Θ≤1$ and on nanotube diameters and chiralities. Detailed structural analysis of the relaxed hydrogenated surfaces demonstrates a tendency for clustering of hydrogenated and of nonhydrogenated sites; this leads to surface morphologies characterized by ripples, which consist of hills that form due to clustering of hydrogenated sites and are surrounded by valleys appearing through the formation of long chains of nonhydrogenated sites. These features introduce surface roughness that depends on the degree of hydrogenation and reaches its maximum levels at intermediate levels of H coverage.