There is widespread agreement how a nanotube lengthens, once it has nucleated from a metal particle. The prevailing view is that the metal particle catalytically dehydrogenates a hydrocarbon molecule that impinges on it from the vapor. The carbon then diffuses either on the surface or through the bulk to the chemisorbed open edges of the nanotube, at which time these new carbon atoms insert into the nanotube network. Agreement ends over whether all layers grow together, or one atop the other. We favor early nucleation of all layers followed by their simultaneous growth for the following reasons.
Surface energy plays a dominant role for a nanoscale particle.
A metal particle only a few nm in diameter contains a very high percentage of surface atoms, creating a tremendous surface energy problem on a per atom basis. An excess of carbon can help to solve this problem by assembling a graphene cap on the particle surface, with its edges strongly chemisorbed to the metal. Newly arriving carbon will continue to assemble on the surface of the catalytic particle. The original surface shell can continue to grow around the particle, which if continued would result in overcoating and deactivation of the catalytic particle. Overcoating surely occurs at some rate, but termination of cap growth at the hemisphere seems likely since then the strain of additional pentagons to complete the shell is avoided.