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.

The initial oxidation stages of perfect and defective graphitic surfaces exposed to atomic oxygen have been studied with a combined high-resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT) computational approach. The resulting oxygen-containing surface functional groups are identified by analyzing the multicomponent C 1s and O 1s core level spectra that are then interpreted on the basis of DFT calculations. In the initial oxidation stage, epoxy groups are formed on perfect graphene, whereas the preferential adsorption of the O atoms on the vacancies of the defective surfaces results in structures containing pairs of oxygen atoms in ether and carbonyl (semiquinone) configurations. The formation of these functional groups is preceded by metastable structures consisting of single O atoms occupying single C vacancies.

No abstract

We describe gold nanocages as a new class of potential contrast agent for spectroscopic optical coherence tomography (OCT). Monodispersed gold nanocages of an approximately 35 nm edge length exhibit strong optical resonance, with the peak wavelength tunable in the near-infrared range. We characterized the optical properties of the nanocage by using OCT experiments along with numerical calculations, revealing an absorption cross section approximately 5 orders of magnitude larger than conventional dyes. Experiments with tissue phantoms demonstrated that the nanocages provide enhanced contrast for spectroscopic as well as conventional intensity-based OCT imaging.

We identify mechanisms and surface precursors for the nucleation and growth of extended defects on oxidized graphene. Density functional theory calculations show that the formation of surface structures capable to initiate the unzipping and cracking of the oxidized C network is strongly influenced by the constraint of the graphitic lattice on the surface functional groups. Accounting for this effect on the preferential spatial patterning of O adsorbates allows us to revise and extend the current models of graphene oxidative unzipping and cutting. We find that these processes are rate limited by O diffusion and driven by the local strain induced by the O adspecies. Adsorbate mobility is ultimately recognized as a key factor to control and to prevent the C-network breakdown during thermal processing of oxidized graphene.

The dynamics of a growing interface with conservation of total volume under the interface is investigated using both dynamic renormalization-group and computer simulation. The conservation law leads to a new universality class from that discussed by Kardar, Parisi, and Zhang [Phys. Rev. Lett. 56, 889 (1986)]. The growth exponents are calculated and compared with those from the simulation of a conserved restricted solid-on-solid model. Excellent agreement between theory and simulation is found

Narrow atomically precise graphene nanoribbons hold great promise for electronic and optoelectronic applications, but the previously demonstrated nanoribbon-based devices typically suffer from low currents and mobilities. In this study, we explored the idea of lateral extension of graphene nanoribbons for improving their electrical conductivity. We started with a conventional chevron graphene nanoribbon, and designed its laterally extended variant. We synthesized these new graphene nanoribbons in solution and found that the lateral extension results in decrease of their electronic bandgap and improvement in the electrical conductivity of nanoribbon-based thin films. These films were employed in gas sensors and an electronic nose system, which showed improved responsivities to low molecular weight alcohols compared to similar sensors based on benchmark graphitic materials, such as graphene and reduced graphene oxide, and a reliable analyte recognition. This study shows the methodology for designing new atomically precise graphene nanoribbons with improved properties, their bottom-up synthesis, characterization, processing and implementation in electronic devices.

The reduction of graphene oxide surfaces yielding molecular CO/CO 2 is studied from first principles using density functional theory. We find that this reaction can proceed exothermically only from surface precursors containing more oxygen atoms than strictly needed to produce CO/CO 2 in the gas phase. The calculations show that the lowest-energy configurations of multiple O adsorbates do not involve clustering of epoxy groups (the stable form of O adatoms on graphitic surfaces) but always contain lactone groups either in lactone-ether or in ether-lactone-ether form. We identify these lowest-energy structures as the main reaction precursors. The O adatoms near the lactone group catalyze its gasification to CO/CO 2 by reducing the activation energy from above 1.8 eV (from an isolated lactone) to below 0.6 eV (from a lactone-ether). In addition, the residual O adatoms left behind after the lactone gasification minimize the energy of the graphitic products by saturating the dangling bonds of the resulting defective surface. By analyzing defect-free as well as defective surfaces, we identify a common set of concerted reaction mechanisms in which the formation of the gas products and the saturation of the newly formed C vacancies happen simultaneously. The calculated activation energies are in good agreement with the available experimental data.