  Chemical vapor deposition synthesis of single-walled carbon nanotubes
experiments, using Fe catalyst, and alternating methane and carbon monoxide as
carbon feedstocks, lead to the reversible formation of junctions between tubes
of different diameters. Combined with an atomistic modeling of the tube /
catalyst interface, this shows that the ratio of diameters of the tube and its
seeding particle, denoting the growth mode, depends on the carbon fraction
inside the catalyst. With carbon monoxide, nanoparticles are strongly carbon
enriched, and tend to dewet the tube, in a perpendicular growth mode.
Cross-checking our results with available reports from the literature of the
last decade strongly suggests that these latter conditions should favor the
near armchair chiral selectivity observed empirically.
