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Carbon nanotubes with different nitrogen contents were produced by the arc-discharge technique. The samples were first submitted to a concentration process ͑purification͒ and analyzed by x-ray photoelectron spectroscopy, electron-energy-loss spectroscopy, electron transmission, and scanning electron microscopy to study the materials structure and morphology. Measured values of nitrogen concentration were below 5 at. % and varied with the nitrogen partial pressure inside the arc-discharge chamber. Using an optical microscope, highly localized regions of the samples (ϳ1 mm 2 ) were irradiated by an Ar ion laser. Controlling the laser intensity, further local purification was induced and information about the evolution of the structural order of the nanotube samples with different contents of nitrogen was obtained.

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.

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