Crystalline hydrogenation of graphene by scanning tunneling microscope tip-induced field dissociation of H₂

Because of the sensitivity of 2D material surfaces, chemical functionalization can be exploited tune the electronic structure of these materials. For example, hydrogen bonding to carbon atoms in graphene tunes the material from a semi-metal to a wide-gap insulator. We developed a novel method for crystalline hydrogenation of graphene on the nanoscale. Molecular hydrogen was physisorbed at 5 K onto pristine graphene islands grown on Cu(111) in ultra-high vacuum. Field emission local to the tip of the scanning tunneling microscope dissociates H₂ and results in hydrogenated graphene. At lower coverage, isolated point defects are found on the graphene and are attributed to chemisorbed H on top and bottom surfaces. Repeated H₂ exposure and field emission yielded patches and then complete coverage of a crystalline √3 × √3 R30° phase, as well as less densely packed 3 × 3 and 4 × 4 structures. The hydrogenation can be reversed by imaging with higher bias. Scanning tunneling spectroscopy did not show the predicted band gap which we attributed to the interaction of the hydrogenated graphene with the conducting Cu substrate.

Tjung, et al., Carbon 124 (2017) 97-104