Multiwall carbon nanotubes: Self-organization and inhibition of step-flow growth kinetics

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Abstract
In this article the results of a numerical study on the self-organization and inhibition of step-flow growth of carbon nanotubes viewed within the framework of the continuum surface diffusion equation are given. Incorporation constants of C atoms, which differ depending on what side of the step the atom has been chemisorbed prior to incorporation, are considered. These differences can lead to the onset of surface multi-island nucleation in front of a propagating step with decrease in the growth temperature. This effect is able to cause formation of defects in the growing layer and even to inhibit stable step-flow modes of nanotube growth, leading to the formation of misoriented surface nuclei which may be likened to amorphous matter. A diagram distinguishing three characteristic temperature regions for nanotube formation is given: (i) the region where there is no secondary layer nucleation because the surface concentration of adsorbate on the surface of the first layer is not sufficiently high, (ii) the region of successive nucleation and propagation of one layer after another, i.e., stable step-flow growth, and (iii) the region where the nanotube surface is prone to multi-island nucleation, which inhibits stable step-flow growth and causes “amorphization” of the external surface of the nanotube. The simultaneous propagation of multilayer steps coupled by lip–lip interaction is shown to be feasible only if a microkinetic mechanism exists, which effectively redistributes to the edges of internal layers the atoms arriving initially by surface diffusion at the edge of the external layer

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