Producing single-wall carbon nanotubes with a given structure, in terms of diameter and chirality, is still a difficult issue. In this context, understanding the physical and chemical properties of the catalyst nanoparticle during the chemical vapor deposition synthesis is fundamental. By using tight-binding calculations at atomic scale, we show that carbon solubility is larger for smaller nanoparticles (NPs), with solubility limits depending on the size, but also on the physical state (solid, partially molten liquid) of the catalyst. Moreover, optimal thermodynamic conditions have been found to nucleate carbon cap after saturation of the metal catalyst with carbon atoms.
Density profiles along the radius of the nanoparticle calculated at 1000 K (black: C, orange: Ni). The inset presents the structure of C-rich NPs with a crystalline core surrounded by a molten layer.