Insights in the synthesis of carbon nanotubes from computer simulation
A characteristic of the novel carbon-based materials (nanotubes, graphene …) is that their properties very strongly depend on their structure: chirality for SWNTs, number of layers and stacking for graphene. However, the materials actually produced are far from being ideal and the direct synthesis of nanotubes with designed structure and properties is not yet achieved. The tremendous progress made is still limited by our poor understanding of the nucleation and growth mechanisms, at an atomic scale not easily accessed experimentally. Atomistic computer simulation is ideally suited for investigations at this level, but the complexity of the synthesis methods, such as the most commonly used Catalytic Chemical Vapour Deposition, makes these studies quite challenging.
Here, we use a carefully assessed tight binding model for nickel and carbon [1, 2] to numerically investigate different aspects of the CCVD synthesis process. Owing to significant technical improvements of our grand canonical Monte Carlo code , we can extend our previous calculations  of carbon adsorption isotherms to nanoparticles (NPs) up to 807 Ni atoms, in a broad temperature range. We thereby study the carbon solubility and physical state of the metal catalyst as a function of size, temperature and carbon chemical potential conditions corresponding to nucleation and growth of SWNTs. Combining experimental information from Transmission Electron Microscopy and atomistic computer simulation, we try and understand the relation between the diameters of the tube and the metallic NP from which it grows . We then study the wetting of the NPs with respect to sp2 carbon walls, that strongly depends on carbon concentration, and emphasize its role in the growth of tubes. This enables us to identify conditions leading to experimentally observed situations: aborted growth by encapsulation of the metal NP with carbon, growth termination by detachment of the tube from the NP and continuous growth under mild carbon chemical potential, temperature and feeding rate conditions.
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