Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many emerging areas such as graphene-based electronic devices. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice, but experimental evidence for this is so far lacking. Here, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the Moiré superlattice positions of graphene grown on an Ir(111) substrate.

Graphene is easily produced by thermally reducing graphene oxide. However, defect formation in the C network during deoxygenation compromises the charge carrier mobility in the reduced material. Understanding the mechanisms of the thermal reactions is essential for defining alternative routes able to limit the density of defects generated by carbon evolution. Here, we identify a dual path mechanism in the thermal reduction of graphene oxide driven by the oxygen coverage: at low surface density, the O atoms adsorbed as epoxy groups evolve as O(2) leaving the C network unmodified. At higher coverage, the formation of other O-containing species opens competing reaction channels, which consume the C backbone. We combined spectroscopic tools and ab initio calculations to probe the species residing on the surface and those released in the gas phase during heating and to identify reaction pathways and rate-limiting steps. Our results illuminate the current puzzling scenario of the low temperature gasification of graphene oxide.

Using photoemission spectroscopy techniques, we show that oxygen intercalation is achieved on an extended layer of epitaxial graphene on Ir(111), which results in the "lifting" of the graphene layer and in its decoupling from the metal substrate. The oxygen adsorption below graphene proceeds as on clean Ir(111), giving only a slightly higher oxygen coverage. Upon lifting, the C 1s signal shows a downshift in binding energy, due to the charge transfer to graphene from the oxygen-covered metal surface. Moreover, the characteristic spectral signatures of the graphene-substrate interaction in the valence band are removed, and the spectrum of strongly hole-doped, quasi free-standing graphene with a single Dirac cone around the K point is observed. The oxygen can be deintercalated by annealing, and this process takes place at around T = 600 K, in a rather abrupt way. A small amount of carbon atoms is lost, implying that graphene has been etched. After deintercalation graphene restores its interaction with the Ir(111) substrate. Additional intercalation/deintercalation cycles readily occur at lower oxygen doses and temperatures, consistently with an increasingly defective lattice. Our findings demonstrate that oxygen intercalation is an efficient method for fully decoupling an extended layer of graphene from a metal substrate, such as Ir(111). They pave the way for the fundamental research on graphene, where extended, ordered layers of free-standing graphene are important and, due to the stability of the intercalated system in a wide temperature range, also for the advancement of next-generation graphene-based electronics.