Growth and Doping of Carbon Nanotubes and Graphene

Abstract

Single walled carbon nanotubes (SWCNTs) have been doped with nitrogen (N) by two ion-mediated approaches: directly through irradiation with N+ ions and by a novel indirect technique, creating defects through Ar+ ion irradiation which then react with nitrogen upon annealing in a N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was then employed to determine the chemical environment of the nitrogen within the resulting SWCNT material. Depending upon the exact preparation conditions, nitrogen in graphitic (substitutional) pyridinic and pyrrolic configurations could be identified. Nitrogen doping through the novel method was found to introduce the largest concentration of chemisorbed nitrogen within the SWCNT films, dominated by thermodynamically unstable pyrrolic species at low process temperatures (500ºC). The maximum concentration of nitrogen in graphitic sites was achieved by direct ion bombardment, although both XPS and Raman spectroscopy indicated that this approach to doping led to the greatest damage. The ability to vary both bsolute and relative composition of chemisorbed nitrogen species is expected to be valuable for a range of fundamental studies, particularly of the catalytic behaviour of these materials.

The growth of graphene on copper under atmospheric pressure using a soft solid source (nonadecane) is reported. It is found that the growth rate is best described by a model which involves the continuous supply of reactive species during the entire growth period. This observation is explained in terms of the formation of decomposition produces which reside on an otherwise clean surface after nonadecane desorption and provide a series of ‘mini carbon sources’ for graphene growth. XPS analysis indicates that, as expected, increased growth temperature leads to greater graphitisation at the surface (and hence graphene ‘quality’) which is not accompanied by any substantial change in island size and coverage. It is found that although graphene islands can be produced it is not possible to form continuous films, demonstrating the limitations of this technique.

Although limited in some ways, the use of soft solid precursors for graphene growth allows the ready introduction of potential dopant materials. XPS, Raman and SEM data provide strong evidence that a PDMS precursor can be employed in atmospheric pressure solid-phase CVD to produce graphene heavily doped with silicon, which has not been previously achieved. Since silicon-doped graphene is predicted to possess a band gap related to the Si concentration, this may provide a route to produce a graphene-based material of use in digital electronics.