By employing polarization- and temperature-dependent near-edge x-ray-absorption fine structure (NEXAFS) studies, we show that vertically aligned carbon nanotubes when subjected to low-pressure radio-frequency (rf) glow discharge can easily be doped with nitrogen atoms at predominantly graphitelike substitutional sites and at the same time preserve their vertical alignment in contrast to commonly used wet chemical functionalization methods. The O $K$ edge NEXAFS spectra and angle-dependent x-ray phoelectron spectroscopy measurements established the presence of a significant amount of oxygen containing functional groups localized at the outer part of the walls. The amount of oxygen adsorbed on the carbon nanotubes is dependent on the introduction of nitrogen sites and exposed edge plane defects generated by plasma N treatment. Thermal gravimetric analysis measurements revealed a lower decomposition temperature for the plasma treated carbon nanotubes, which has its origin in the presence of a large fraction of active sites induced by rf glow discharge plasma treatment that accelerates the oxidation rate of N-doped carbon nanotubes. Our results indicate that rf low-pressure glow-discharge-mediated doping of nanotubes is a promising route to control the electronic structure of nanotubes and their reactivity due to introduction of active site defects.

• Received 19 February 2007

DOI:https://doi.org/10.1103/PhysRevB.75.195429