Nanotube nucleation versus carbon-catalyst adhesion--probed by molecular dynamics simulations.

Catalytic nucleation of carbon nanotubes (CNTs) remains a challenge for the theory: Which factors and forces decide if the gathering sp(2)-network of atoms will adhere to the catalyst particle and fully cover it or the graphitic cap will liberate itself to extend into a hollow filament? This intimate mechanism cannot be seen in experiment, yet it can be investigated through comprehensive molecular dynamics. We systematically vary the adhesion strength (W(ad)) of the graphitic cap to the catalyst and temperature T (and C diffusion rate). Observations allow us to build a statistically representative map of CNT nucleation and define the conditions for growth or metal encapsulation in a fullerene-shell (catalyst poisoning). It shows clearly that weak W(ad), sufficient thermal kinetic energy (high T) or fast C diffusion favor the CNT nucleation. In particular, below 600 K carbon-diffusion on the catalyst surface limits the growth, but at higher T it fully depends on cap lift-off. Informed choice of parameters allowed us to obtain the longest simulated nanotube structures. The study reveals a means of designing the catalyst for better CNT synthesis, potentially at desirably low temperatures.