Tuning electronic properties of carbon nanotubes by nitrogen grafting: Chemistry and chemical stability

Plasma-based methods were used to graft nitrogen atoms to the hexagonal lattice of vertically aligned carbon nanotubes (v-CNTs). The nitrogen grafting (as pyridinic, pyrrolic and graphitic) was mediated by the creation of defects induced by energetic species present in the nitrogen plasma. We investigated the effect of adding nitrogen atoms via plasma treatment on the electronic properties of both v-CNT tips and sidewalls using ultraviolet and X-ray photoemission spectroscopy and spectromicroscopy. Site selective nitrogen grafting near the tips, up to a depth of 4 μm, was evaluated, beyond which the properties of the v-CNTs remain unperturbed. The N 1s XPS spectra recorded on the v-CNT tips showed three components related to nitrogen grafted as pyridinic, pyrrolic or graphitic. During thermal heating, we observed variations in the intensity ratio of these components due to the different thermal stability of the nitrogen grafting configurations; the most stable were the sp² pyridinic and graphitic nitrogen. The area ratio variation of these components was accompanied by a change in the density of states at the Fermi energy level, thus suggesting that the nitrogen functionalization strategy employed can be used to activate the v-CNT tips allowing the tuning of electronic properties by controlling the grafting of different nitrogen species.