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Many potential applications of graphene require either the possibility of tuning its electronic structure or the addition of reactive sites on its chemically inert basal plane. Among the various strategies proposed to reach these objectives, nitrogen doping, i.e., the incorporation of nitrogen atoms in the carbon lattice, leads in most cases to a globally n-doped material and to the presence of various types of point defects. In this context, the interactions between chemical dopants in graphene have important consequences on the electronic properties of the systems and cannot be neglected when interpreting spectroscopic data or setting up devices. In this report, the structural and electronic properties of complex doping sites in nitrogen-doped graphene have been investigated by means of scanning tunneling microscopy and spectroscopy, supported by density functional theory and tight-binding calculations. In particular, based on combined experimental and simulation works, we have systematically studied the electronic fingerprints of complex doping configurations made of pairs of substitutional nitrogen atoms. Localized bonding states are observed between the Dirac point and the Fermi level in contrast with the unoccupied state associated with single substitutional N atoms. For pyridinic nitrogen sites (i.e., the combination of N atoms with vacancies), a resonant state is observed close to the Dirac energy. This insight into the modifications of electronic structure induced by nitrogen doping in graphene provides us with a fair understanding of complex doping configurations in graphene, as it appears in real samples.

Grain boundaries in epitaxial graphene on the SiC(0001̅) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition θc = 19 ± 2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ = 33 ± 2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.

Using scanning tunnelling microscopy and spectroscopy, we investigated the atomic and electronic structure of nitrogen-doped single walled carbon nanotubes synthesized by chemical vapor deposition. The insertion of nitrogen in the carbon lattice induces several types of point defects involving different atomic configurations. Spectroscopic measurements on semiconducting nanotubes reveal that these local structures can induce either extended shallow levels or more localized deep levels. In a metallic tube, a single doping site associated with a donor state was observed in the gap at an energy close to that of the first van Hove singularity. Density functional theory calculations reveal that this feature corresponds to a substitutional nitrogen atom in the carbon network.

Prior to the implementation of multi-walled carbon nanotubes in microelectronic devices, investigating their electronic structure down to the nanometer scale is necessary. In that prospect, we used scanning tunneling microscopy (STM) to study the detailed atomic scale structure of double-walled carbon nanotubes, each comprising two rolled monolayers of graphene. Atomically resolved STM images usually displayed a motif and periodicity similar to that found in graphite but, on selected regions, atomically resolved motifs with a clearly defined superstructure were observed. This phenomenon has been reported previously but without a suitable explanation. We discuss the origin of this behavior in terms of modified stacking sequences due to the mismatch in registry between the chiral angles of the inner and the outer shells, associated with the interaction between the two carbon monolayers. These phenomena must be taken into account for the realization of lateral interference devices based on carbon nanotubes or graphene layers.

The atomic and electronic structure of a twisted and collapsed double-walled carbon nanotube was characterized using scanning tunneling microscopy and spectroscopy. It was found that the deformation opens an electronic band gap in an otherwise metallic nanotube, which has major ramifications on the use of carbon nanotubes for electronic applications. Fundamentally, the importance of the intershell interaction in this double-walled carbon nanotube points to the potential of a reversible metal-semiconductor junction, which can have device applications, as well as a caution in the design of semiconductor components based on carbon nanotubes. Lattice registry effects between the two neighboring walls evidenced by atomically resolved images confirm earlier first principle calculations indicating that the helicity influences the collapsed structure and show excellent agreement with the predicted twisted-collapse mode.

Evidence for modified electronic structure in double-walled carbon nanotubes with respect to their individual inner and outer constituent single-walled nanotubes is provided by scanning tunneling microscopy and spectroscopy experiments. The contribution originating from the inner tube to the local density of states of the double-walled system was identified in agreement with previous theoretical calculations. Consequently, the chiral index for the inner tube was extracted based on the additional van Hove singularities present in the experimental tunneling spectra.

The growth of carbon nanotubes from Ni catalysts is reversed and observed in real time in a transmission electron microscope, at room temperature. The Ni catalyst is found to be Ni3C and remains attached to the nanotube throughout the irradiation sequence, indicating that C most likely diffuses on the surface of the catalyst to form nanotubes. We calculate the energy barrier for saturating the Ni3C (2-13) surface with C to be 0.14 eV, thus providing a low-energy surface for the formation of graphene planes.