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We present a theoretical study on the structural and electronic modifications caused by random nitrogen substitution in carbon tubular and branched nanostructures. Finite cluster calculations with hydrogen saturation of the tube ends were performed. Geometry optimizations were carried out through semiempirical quantum chemical calculations. Densities of states (DOS) were calculated by the density functional theory. The energy associated with nitrogen incorporation was obtained. Some tubular structures undergo a length shortening as a consequence of N substitution. DOS analysis is consistent with the shift of the electronic spectrum to lower energies and a more metallic character of the tubes upon nitrogen doping due to the emergence of nitrogen-induced states close to the conduction band. The defective regions of junctions and bends were built including five-, seven-, and eight-membered rings in the otherwise hexagonal network of carbon bonds. In order to reduce the stress caused by the curvature, a chemical doping through nitrogen substitution is proposed. Results are consistent with the shortening of bonds within the junctions and bends and an increased chemical stability of the defects.

Motivated by the existence of helical wrapping patterns in composite nanotube systems, in this work we study the effects of the helical incorporation of carbon atoms in boron nitride nanotubes. We consider the substitutional carbon atoms distributed in stripes forming helical patterns along the nanotube axis. The density of states and energy band gap were calculated adopting Green function formalism by using the Rubio-Sancho technique in order to solve the matrix Dyson equation. We report the effects of the helical atomic distribution of carbon atoms on the behaviour of the density of states and the energy band gap. In particular, we show that the electronic energy band gap displays a non-monotonical dependence on the helical pattern, oscillating as a function of the helical angle θ.

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