Nanocatalyst shape and composition during nucleation of single-walled carbon nanotubes.

The dynamic evolution of nanocatalyst particle shape and carbon composition during the initial stages of single-walled carbon nanotube growth by chemical vapor deposition synthesis is investigated. Classical reactive and ab initio molecular dynamics simulations are used, along with environmental transmission electron microscope video imaging analyses. A clear migration of carbon is detected from the nanocatalyst/substrate interface, leading to a carbon gradient showing enrichment of the nanocatalyst layers in the immediate vicinity of the contact layer. However, as the metal nanocatalyst particle becomes saturated with carbon, a dynamic equilibrium is established, with carbon precipitating on the surface and nucleating a carbon cap that is the precursor of nanotube growth. A carbon composition profile decreasing towards the nanoparticle top is clearly revealed by the computational and experimental results that show a negligible amount of carbon in the nanoparticle region in contact with the nucleating cap. The carbon composition profile inside the nanoparticle is accompanied by a well-defined shape evolution of the nanocatalyst driven by the various opposing forces acting upon it both from the substrate and from the nascent carbon nanostructure. This new understanding suggests that tuning the nanoparticle/substrate interaction would provide unique ways of controlling the nanotube synthesis.