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. A second cap can form underneath the first, spaced by roughly the interlayer spacing of graphite. As additional caps form, one beneath the other, older caps are forced to lift up by forming a cylindrical tube whose open end remains chemisorbed to the catalytic particle.
Carbon can add to the cylindrical section of a growing layer.
Once the smallest yarmulke has formed, insertion of new carbon between the tube edge and the catalytic particle is the best solution, as long as complete overcoating is avoided. We believe that this yarmulke mechanism is responsible for the SWNT produced using the Mo catalyst with CO, as well as catalytic growth of multiwall nanotubes. First, this catalyst preparation has produced, for reasons not fully understood, particles with a size distribution more strongly skewed toward very small sizes than is typical in other catalyst preparations. Second, the larger particles always appear to be onionated, and so are inactive for catalysis of nanotubes. This is likely to result from relative rates for feedstock decomposition and migration on or through the catalyst that depend sensitively on both temperature and particle size.