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.

By using ab initio molecular dynamics calculations, we show that even where the graphene lattice constant contracts, as previously reported for freestanding graphene below room temperature, the average carbon-carbon distance increases with temperature, in both free and supported graphene. This results in a larger corrugation at higher temperature, which can affect the interaction between graphene and the supporting substrate. For a weakly interacting system as graphene=Irð111Þ, we confirm the results using an experimental approach which gives direct access to interatomic distances. DOI: 10.1103/PhysRevLett.106.135501 PACS numbers: 81.05.ue, 68.65.Àk, 65.80.Ck Graphene's (GR) mechanical [1] and thermal [2,3] properties are crucial for applications such as the cooling of electronic devices [4,5], but their microscopic mechanism is often not well understood [6], in contrast to that of many electronic phenomena [7][8][9][10]. A particular challenge lies in the fact that graphene, while being two dimensional, exists in a three-dimensional world, permitting low-lying vibrational excitations and the formation of large-scale ripples perpendicular to the plane [11,12]. This is also important because it introduces a difference between freestanding GR and the technologically more important supported material.In particular, the thermal expansion coefficient of GR and the underlying microscopic mechanism is currently being debated. For freestanding GR and below % 500-700 K, a state-of-the-art atomistic simulation [13], a nonequilibrium Green's function approach [14], and a harmonic density functional theory (HDFT) calculation [15,16] all show a decreasing in-plane lattice parameter a, i.e., a thermal contraction. Above 900 K or so, the two former techniques show a trend reversal with a thermal expansion, while in HDFT the contraction persists over the entire temperature range studied (up to 2500 K). Experiments clearly confirm the thermal contraction of a below room temperature [3,17], but no high temperature measurements have been reported.In order to clarify this issue and to gain insight into its microscopic origin, we present a study based on ab initio simulations and core level photoelectron spectroscopy. Temperature-dependent ab initio molecular dynamics (AIMD) calculations were performed on both freestanding and supported GR. We have used the VASP code [18] with the projector-augmented wave method [19,20], the Perdew-Burke-Ernzerhof exchange-correlation energy [21], and an efficient extrapolation for the charge density [22]. Single particle orbitals were expanded in plane waves with a cutoff of 400 eV. We used the NPT ensemble (constant particles number N, pressure P, and temperature T), as recently implemented in VASP [23,24]. For the present slab calculations, we only applied the constant pressure algorithm to the two lattice vectors parallel to the surface, leaving the third unchanged during the simulation.In the case of freestanding GR the calculations were done using unit cells of different sizes (8 Â 8, 10 Â...

By combining high-resolution photoelectron spectroscopy and ab initio calculations, we show that carbon nanoislands formed during the growth of a long-range ordered graphene layer on Ir(111) assume a peculiar domelike shape. The understanding of the unusual growth mechanism of these C clusters, which represent an intermediate phase between the strongly coupled carbidic carbon and a quasi-free-standing graphene layer, can provide information for a rational design of graphenelike systems at the nanoscale.