Mario S. C. Mazzoni

Stability, geometry, and electronic structure of the boron nitride B36N36 fullerene

We apply first-principles calculations to study the structural and electronic properties of a boron nitride fullerene-like cluster, B36N36. This cluster has shape and dimensions comparable to those of a single-shelled BN fullerene recently produced in an electron-beam irradiation experiment. The calculations show that B36N36 is energetically less favorable than C60, when both are compared to nanotube structures. This is consistent with the experimental difficulty to obtain BN fullerenes. On the other hand, B36N36 presents a large energy gap, larger in fact than that of a BN nanotube of the same diameter. This is an indication that the molecule is a stable one, once it is formed.

Group-theory analysis of electrons and phonons in N-layer graphene systems

In this work we study the symmetry properties of electrons and phonons in graphene systems as a function of the number of layers. We derive the selection rules for the electron-radiation interactions and for the electron-phonon interactions at all points in the Brillouin zone. By considering these selection rules, we address the double-resonance Raman-scattering process. The monolayer and bilayer graphenes in the presence of an applied electric field are also discussed.

Bandgap closure of a flattened semiconductor carbon nanotube: A first-principles study

We investigate, through first-principles calculations, the effects of a flattening distortion on the electronic properties of a semiconductor carbon nanotube. The flattening causes a progressive reduction of the band gap from 0.92 eV to zero. The band-overlap insulator-metal transition occurs for an interlayer distance of 4.6 A. Supposing that the flattening of the nanotube can be produced by a force applied by a scanning microscope tip, we estimate that the force per unit length of the nanotube that is necessary to reach the insulator-metal transition is 7.4 N/m.

Stability of antiphase line defects in nanometer-sized boron nitride cones

Departamento de F´isica, Universidade Estadual de Feira de Santana,Km 3 BR-116, 44031-460, Feira de Santana, BA, Brazil.(Dated: February 2, 2008)We investigate the stability of boron nitride conical sheets of nanometer size, using first-principlescalculations. Our results indicate that cones with an antiphase boundary (a line defect that containseither B-B or N-N bonds) can be more stable than those without one. We also find that dopingthe antiphase boundaries with carbon can enhance their stability, leading also to the appearanceof localized states in the bandgap. Among the structures we considered, the one with the smallestformation energy is a cone with a carbon-modified antiphase boundary that presents a spin splittingof ∼0.5 eV at the Fermi level.

Modulating the electronic properties along carbon nanotubes via tube-substrate interaction.

We study single wall carbon nanotubes (SWNTs) deposited on quartz. Their Raman spectrum depends on the tube-substrate morphology, and in some cases, it shows that the same SWNT-on-quartz system exhibits a mixture of semiconductor and metal behavior, depending on the orientation between the tube and the substrate. We also address the problem using electric force microscopy and ab initio calculations, both showing that the electronic properties along a single SWNT are being modulated via tube-substrate interaction.

Two-Dimensional Molecular Crystals of Phosphonic Acids on Graphene

The synthesis and characterization of two-dimensional (2D) molecular crystals composed of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along the graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.

Energetics of the oxidation and opening of a carbon nanotube

We apply first principles calculations to study the opening of single-wall carbon nanotubes (SWNTs) by oxidation. We show that an oxygen rim can stabilize the edge of the open tube. The sublimation of CO

Electron states in boron nitride nanocones

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Asymmetric effect of oxygen adsorption on electron and hole mobilities in bilayer graphene: long- and short-range scattering mechanisms.

molecules from the rim with the subsequent closing of the tube changes from endothermic to exothermic as the tube radius increases, within the range of experimental feasible radii. We also obtain the energies for opening the tube at the cap and at the wall, the latter being significantly less favorable.

Graphene–boron nitride superlattices: the role of point defects at the BN layer

We apply first-principles calculations to study the electronic structure of boron nitride nanocones with disclinations of different angles θ=nπ/3. Nanocones with odd values of n present antiphase boundaries that cause a reduction of the work function of the nanocones, relative to the bulk BN value, by as much as 2 eV. In contrast, nanocones with even values of n do not have such defects and present work functions that are very similar to the BN bulk value. These results should have strong consequences for the field emission properties of boron nitride nanocones and nanotubes.