 Solution

In catalytic chemical vapor deposition, a metal nanoparticle activates a carbon-containing feedstock and supplies mobile carbon to the growing rim. Carbon can diffuse over the particle surface and, depending on the catalyst state, through its bulk. Once a curved carbon cap nucleates, continued incorporation extends the tube. Strong catalyst-support adhesion favors base growth; weaker adhesion can allow the particle to lift with the tube tip. A liquid catalyst is not universally required.

Particle size constrains tube diameter, but semiconducting character requires a particular wrapping family: $n-m\not\equiv 0(\mathrm{mod}3)$. Merely making particles smaller does not select that congruence class. Routes toward predominantly semiconducting carbon nanotubes include controlling catalyst composition, exposed facets and size distribution to bias cap/rim nucleation and growth, or adjusting carbon chemical potential and growth/etching conditions so that metallic tubes are less likely to survive.

These are mechanistic strategies rather than a universal recipe. Experiments with catalyst conditioning found that high semiconducting selectivity depended sensitively on oxidizing/reducing gas conditions and came with a strong yield penalty, motivating a catalyst-deactivation model.^[ref]

The useful distinction is between selecting a structure at nucleation, selecting its growth rate, and selectively terminating or removing unwanted tubes. Each can enrich the semiconducting fraction, but they do not establish the same growth mechanism.