  Single-walled carbon nanotubes are hollow cylinders, that can grow
centimeters long by carbon incorporation at the interface with a catalyst. They
display semi-conducting or metallic characteristics, depending on their
helicity, that is determined during their growth. To support the quest for a
selective synthesis, we develop a thermodynamic model, that relates the
tube-catalyst interfacial energies, temperature, and the resulting tube
chirality. We show that nanotubes can grow chiral because of the
configurational entropy of their nanometer-sized edge, thus explaining
experimentally observed temperature evolutions of chiral distributions. Taking
the chemical nature of the catalyst into account through interfacial energies,
structural maps and phase diagrams are derived, that will guide a rational
choice of a catalyst and growth parameters towards a better selectivity.
