The practical realization of nanoscale electronics faces two major challenges: the precise engineering of the building blocks and their assembly into functional circuits. In spite of the exceptional electronic properties of carbon nanotubes, only basic demonstration devices have been realized that require time-consuming processes. This is mainly due to a lack of selective growth and reliable assembly processes for nanotubes. However, graphene offers an attractive alternative. Here we report the patterning of graphene nanoribbons and bent junctions with nanometre-precision, well-defined widths and predetermined crystallographic orientations, allowing us to fully engineer their electronic structure using scanning tunnelling microscope lithography. The atomic structure and electronic properties of the ribbons have been investigated by scanning tunnelling microscopy and tunnelling spectroscopy measurements. Opening of confinement gaps up to 0.5 eV, enabling room-temperature operation of graphene nanoribbon-based devices, is reported. This method avoids the difficulties of assembling nanoscale components and may prove useful in the realization of complete integrated circuits, operating as room-temperature ballistic electronic devices.

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PACK 81.10 -Methods of crystal growth and purification. methods).

Scanning tunneling microscopy ͑STM͒ is one of the most appropriate techniques to investigate the atomic structure of carbon nanomaterials. However, the experimental identification of topological and nontopological modifications of the hexagonal network of sp 2 carbon nanostructures remains a great challenge. The goal of the present theoretical work is to predict the typical electronic features of a few defects that are likely to occur in sp 2 carbon nanostructures, such as atomic vacancy, divacancy, adatom, and Stone-Wales defect. The modifications induced by those defects in the electronic properties of the graphene sheet are investigated using first-principles calculations. In addition, computed constant-current STM images of these defects are calculated within a tight-binding approach in order to facilitate the interpretation of STM images of defected carbon nanostructures.

Being a true two-dimensional crystal, graphene has special properties. In particular, a point-like defect in graphene may induce perturbations in the long range. This characteristic questions the validity of using a supercell geometry in an attempt to explore the properties of an isolated defect. Still, this approach is often used in ab-initio electronic structure calculations, for instance. How does this approach converge with the size of the supercell is generally not tackled for the obvious reason of keeping the computational load to an affordable level. The present paper addresses the problem of substitutional nitrogen doping of graphene. DFT calculations have been performed for 9 × 9 and 10 × 10 supercells. Although these calculations correspond to N concentrations that differ by ∼ 10%, the local densities of states on and around the defects are found to depend significantly on the supercell size. Fitting the DFT results by a tight-binding Hamiltonian makes it possible to explore the effects of a random distribution of the substitutional N atoms, in the case of finite concentrations, and to approach the case of an isolated impurity when the concentration vanishes. The tight-binding Hamiltonian is used to calculate the STM image of graphene around an isolated N atom. STM images are also calculated for graphene doped with 0.5 at% concentration of nitrogen. The results are discussed in the light of recent experimental data and the conclusions of the calculations are extended to other point defects in graphene.

Carbon nanotubes were discovered by electron microscopy in the carbon soot produced in an electric arc between graphite electrodes, as used in the production of fullerenes. Details of this microstructure have been studied mainly by the combined use of electron microscopic imaging and electron diffraction. Due to the small size of the tubes, diffraction patterns of single tubes, which are the most informative ones, can only be obtained by electron diffraction. For a complete interpretation of the observed diffraction effects a detailed theory is required. Successively more refined approximations of the theory allow us to understand the origin of the different features of the diffraction patterns. The most complete kinematical theory for the diffraction by single shell chiral straight tubes is obtained by the direct summation of the complex amplitudes of the waves scattered by the carbon atoms arranged on a helically wound graphene network.The closed form analytical expressions deduced in this way make it possible to compute the geometry and the intensity distribution of diffraction space. Diffraction patterns are computed as planar sections of this diffraction space.High-resolution electron microscopic images reveal the geometry of individual graphene sheets and their defects in multishell tubes. As well as the characteristic features of straight nanotubes those of helix shaped tubes are also discussed.It is shown how the combined use of electron diffraction and electron microscopy makes it possible to completely characterize the geometry of carbon nanotubes.