Carbon nanotubes (CNT) are prepared by thermal as well as plasma-enhanced chemical vapour deposition from metal catalyst nanoparticles (e.g. FeNi, FePt). The growth of the CNT is commonly understood in the way that a carbon containing gas is decomposed at the catalyst surface. The carbon atoms then diffuse through the catalyst particles and get released at the particle substrate interface. The so created internal interface between the catalyst particle and the CNT is of interest in terms of better understanding the mechanisms that are governing the growth of the CNT. For instance, for FePt terminated CNT the (111) facet is energetically favoured for the release of carbon so that graphene layers are emerging predominantly from this facet.
Further reading: D. Pohl et al., Phys. Rev. Lett. 107, 185501 (2011) URL