Plasma-based nitrogen functionalization of vertically aligned carbon nanotubes: tips and side walls photoemission response

The electronic properties of sp2 carbon nanostructures are very sensitive to local perturbations, such as surface charges and adsorbed gas molecules, so that the grafting of functional groups in a controllable way has been proposed as a feasible reproducible solution for band gap engineering and controllable doping, in order to exploit and tailor the extraordinary properties of these materials [1].
Plasma-based functionalization methods have the advantage to be solvent-free, time efficient and flexible; in particular nitrogen is a natural choice of dopant for carbon nanostructures since its atomic radius is similar to that of carbon [2].
Within this context, we present core level X-ray photoelectron spectroscopy (XPS) and scanning X-ray photoelectron spectromicroscopy (SPEM) measurements on nitrogen functionalized vertically aligned carbon nanotubes (v-CNTs). The creation of defects induced by ions, drives the grafting of nitrogen species (pyridinic, pyrrolic and graphitic) at the CNT surface. A depth of functionalization of about 4 µm was evaluated by SPEM, beyond which the properties of the v-CNTs remain unperturbed. Furthermore, an intriguing different behavior of the grafting at the CNT tips with respect to the sidewall, was observed. These differences indicate a different reactivity of the CNT tip, directly exposed to plasma, where the presence of natural defects may be involved in different bonding formations between carbon and nitrogen.