Angle-resolved photoemission and X-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films cause adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds to an individual macro-scale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.

In order to engineer a band gap into graphene, covalent bond-forming reactions can be used to change the hybridization of the graphitic atoms from sp 2 to sp 3 , thereby modifying the conjugation length of the delocalized carbon lattice; similar side-wall chemistry has been shown to introduce a band gap into metallic single-walled carbon nanotubes. Here we demonstrate that the application of such covalent bond-forming chemistry modifies the periodicity of the graphene network thereby introducing a band gap (∼0.4 eV), which is observable in the angle-resolved photoelectron spectroscopy of aryl-functionalized graphene. We further show that the chemically-induced changes can be detected by Raman spectroscopy; the in-plane vibrations of the conjugated π-bonds exhibit characteristic Raman spectra and we find that the changes in D, G, and 2D-bands as a result of chemical functionalization of the graphene basal plane are quite distinct from that due to localized, physical defects in sp 2 -conjugated carbon.

A blueprint for producing scalable digital graphene electronics has remained elusive. Current methods to produce semiconducting-metallic graphene networks all suffer from either stringent lithographic demands that prevent reproducibility, process-induced disorder in the graphene, or scalability issues. Using angle resolved photoemission, we have discovered a unique one-dimensional metallic-semiconducting-metallic junction made entirely from graphene, and produced without chemical functionalization or finite size patterning. The junction is produced by taking advantage of the inherent, atomically ordered, substrate-graphene interaction when it is grown on SiC, in this case when graphene is forced to grow over patterned SiC steps. This scalable bottomup approach allows us to produce a semiconducting graphene strip whose width is precisely defined within a few graphene lattice constants, a level of precision entirely outside modern lithographic limits. The architecture demonstrated in this work is so robust that variations in the average electronic band structure of thousands of these patterned ribbons have little variation over length scales tens of microns long. The semiconducting graphene has a topologically defined few nanometer wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet. This work demonstrates how the graphene-substrate interaction can be used as a powerful tool to scalably modify graphene's electronic structure and opens a new direction in graphene electronics research.Patterning a flat graphene sheet to alter its electronic structure was envisaged to be the foundation of graphene electronics. 1 The early focus was to open a finite-size gap in lithographically patterned nanoribbons, a necessary step for digital electronics. 1-5 While early transport measurements supported this possibility, 6 it soon became apparent that these "transport gaps" originated from a series of mismatched-level quantum dots caused by the inability of current lithographically to produce sufficiently narrow, well ordered, and crystallography define graphene edges. 7-10 A working solution to the graphene "gap problem" has yet to be formulated, let alone demonstrated. We show that in fact such a solution exists, not by patterning graphene, but instead by controlling the graphene-substrate geometry.We have been able to construct a unique, reproducible, and scalable semiconducting graphene ribbon with a gap larger than 0.5 eV. Using pre-patterned SiC trenches to force graphene to bend between a high symmetry (0001) face to a low symmetry (112n) facet, we produce a narrow curved graphene bend with localized strain. This "topologically-defined" ribbon is a wide-gap graphene semiconductor strip a few lattice constants wide that extends hundreds of microns long. The strip is connected seamlessly to metallic graphene sheets on both of its sides. One metallic sheet is n-doped and the other pdoped. From this simple morphology, we have not only produced a gap suitable for room temperature...

Graphene stacked in a Bernal configuration (60• relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material.

Using a tunneling-percolation model and Monte Carlo simulations, we study the resistivity of graphene-based nanocomposites as a function of both graphene sheet and device parameters. We observe an inverse power law dependence of resistivity on device dimensions and volume fraction near the percolation threshold, and find that high aspect ratio graphene sheets result in a much lower resistivity, particularly at low sheet densities. Furthermore, we find that graphene sheet area affects nanocomposite resistivity more strongly than sheet density does. These results impart important fundamental insights for future experimental investigations and applications of graphene-based conductive nanocomposites.