= Solution
A carbon-containing feed adsorbs and decomposes on a catalytic metal particle, releasing mobile carbon. Carbon can migrate over the surface and/or through the particle, depending on <catalyst>, <temperature> and feedstock. <Supersaturation> and graphitic <nucleation> generate a curved cap; nonhexagonal rings provide the curvature needed to close its end. Continued carbon incorporation at the cap rim lifts or extends the cap into a tube while leaving a growth interface at the <catalyst>.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2019/paper-2-catalytic-growth.png]
{title=Adsorption, carbon transport, cap nucleation and continued graphitic-wall growth at a catalyst}
{source=paper-2-catalytic-growth.py}
With strong catalyst-support adhesion the particle remains at the base; with weaker adhesion it can lift with the growing tube. Several shells can nucleate and extend to form a multiwall tube. Continued growth requires access to active <catalyst> sites: encapsulation by an impermeable carbon coating can terminate it. The <catalyst> need not be liquid in every <chemical vapour deposition> growth mechanism, and bulk dissolution is not the only possible carbon-transport route.
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