H. Chacham

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.

Disorder and Segregation in B−C−N Graphene-Type Layers and Nanotubes: Tuning the Band Gap

We investigate structural and electronic properties of B-C-N (boron-carbon-nitrogen) layers and nanotubes considering the positional disorder of the B, C, and N atoms, using a combination of first principles and simulated annealing calculations. During the annealing process, we find that the atoms segregate into isolated, irregularly shaped graphene islands immersed in BN. We also find that the formation of the carbon islands strongly affects the electronic properties of the materials. For instance, in the case of layers and nanotubes with the same number of B and N atoms, we find that the band gap increases during the simulated annealing. This indicates that, for a given stoichiometry, the electronic and optical properties of B-C-N layers and nanotubes can be tuned by growth kinetics. We also find that the excess of B or N atoms results in large variations in the band gap and work function.

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.

Structures of Si and Ge Nanowires in the Subnanometer Range

We report an ab initio investigation of several structures of pristine Si and Ge nanowires with diameters between 0.5 and 2.0 nm. We consider nanowires based on the diamond structure, high-density bulk structures, and fullerenelike structures. Our calculations indicate a transition from sp3 geometries to structures with higher coordination, for diameters below 1.4 nm. We find that diamond-structure nanowires are unstable for diameters smaller than 1 nm, undergoing considerable structural transformations towards amorphouslike wires. For diameters between 0.8 and 1 nm, filled-fullerene wires are the most stable. For even smaller diameters (approximately 0.5 nm), we find that a simple hexagonal structure is particularly stable for both Si and Ge.

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.

Electromechanical Effects in Carbon Nanotubes

We perform ab initio calculations of charged graphene and single-wall carbon nanotubes (CNTs). A wealth of electromechanical behaviors is obtained. (1) Both nanotubes and graphene expand upon electron injection. (2) Upon hole injection, metallic nanotubes and graphene display a nonmonotonic behavior. Upon increasing hole densities, the lattice constant initially contracts, reaches a minimum, and then starts to expand. The hole densities at minimum lattice constants are 0.3 ‖e‖/atom for graphene and between 0.1 and 0.3‖e‖/atom for the metallic nanotubes studied. (3) Semiconducting CNTs with small diameters (d<∼20 A) always expand upon hole injection. (4) Semiconducting CNTs with large diameters (d<∼20 A) display a behavior intermediate between those of metallic and large-gap CNTs. (5) The strain versus extra charge displays a linear plus power-law behavior, with characteristic exponents for graphene, metallic, and semiconducting CNTs. All these features are physically understood within a simple tight-binding total-energy model.

Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS2

We report a novel mechanical response of few-layer graphene, h-BN, and MoS(2) to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS(2)), but it is absent in single-layer graphene and in few-layer mica and Bi(2)Se(3). We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.

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

ATOMIC GEOMETRY AND ENERGETICS OF VACANCIES AND ANTISITES IN CUBIC BORON NITRIDE

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