Direct Observation of Ordered Configurations of HydrogenAdatoms on Graphene.

Ordered configurations of hydrogenadatoms on graphene have longbeen proposed, calculated, and searched for. Here, we report directobservation of several ordered configurations of H adatoms on grapheneby scanning tunneling microscopy. On the top side of the grapheneplane, H atoms in the configurations appear to stick to carbon atomsin the same sublattice. Scanning tunneling spectroscopy measurementsrevealed a substantial gap in the local density of states in H-containedregions as well as in-gap states below the conduction band due tothe incompleteness of H ordering. These findings can be well explainedby density functional theory calculations based on double-sided Hconfigurations. In addition, factors that may influence H orderingare discussed. [ABSTRACT FROM AUTHOR]