Formation, stabilities, and electronic properties of nitrogen defects in carbon nanotubes: A density-functional study

The chemical doping into carbon nanotubes often enhances their physical and chemical properties such as electrical conductivity and chemical reactivity, and therefore functionalized carbon nanotubes by doping are applied to diverse potential applications such as field-effect transistors and gas sensors. Nitrogen is one of the most efficient dopants for carbon-based materials. Nitrogen-doped carbon nanotubes have been experimentally synthesized so far, and x-ray photoelectron spectroscopy and scanning tunneling microscopy measurements suggested the existence of several nitrogen-defect configurations in carbon nanotubes. Such structural variety of nitrogen defects in carbon nanotubes is expected to provide novel electronic and opt-electronic properties. Here, we report on effects of nitrogen doping into carbon nanotubes, based on the first-principles electronic-structure calculations within the density-functional theory. We calculate the total energies of the several nitrogen defects in carbon nanotubes, and discuss the energetics that reveals the plausible nitrogen defect formations. The energy-band structure calculations suggest that substitutional nitrogen defect gives rise to the donor state below the Fermi level, while the pyridine-type defects produce the acceptor states above the Fermi level. In addition, we discuss the origin of the impurity states related to the pyridine-type N-doping defects in the carbon nanotubes.