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Modelling the growth of Single Wall Carbon Nanotubes

Bichara, Christophe (CINaM, Aix-Marseille University and CNRS)

In spite of recent progress in the synthesis of single wall carbon nanotubes (SWNT) by catalytic chemical vapor deposition (CCVD), and the rise of commercially viable applications, a full control and understanding of property-selective growth still appears elusive. High synthesis temperatures, nano-metric sizes and the large number of correlated growth parameters involved, make the experimental investigation of growth mechanisms specially challenging. Theoretical approaches are not any easier, but I will show that dedicated computer simulations, including tight binding models [1], as well as DFT-based calculations, can provide useful insights. Different aspects have been investigated, in the case of Ni, taken as a prototypical catalyst, as well as other metals. Our discussion starts with the complex stability pattern of atomic carbon dissolved in subsurface layers of crystalline Ni that depends on the presence of a graphene surface layer [2]. For catalyst nanoparticles below 3 nm, relevant in the context of CCVD growth, the presence of carbon dissolved in surface layers induces a gradual melting at temperatures well below the melting temperature of pure nanoparticles of the same size. Calculated size dependent phase diagrams for Ni-C nanoparticles [3] indicate that facetted crystalline nanoparticles are unlikely to be observed in this size range under growth conditions.This raises the question of the role of the carbon dissolved in the catalyst during growth that is found to have a strong influence on the wetting properties of the metal-SWNT interface [4]. Through careful Transmission Electron Microscopy observations [5], so called tangential and perpendicular growth modes have been identified. Computer simulations are used to analyze these growth modes at the atomic scale, demonstrating that the tangential mode corresponds to a weak carbon supply and slow growth, while the perpendicular mode is observed when the carbon fraction in the nanoparticle is larger [6]. Growth experiments designed to tune the carbon fraction in the nanoparticle by changing the carbon feedstock (CO or CH4) confirm this analysis. Finally we discuss the role of the different contributions to the stability and dynamics of the nanotube/nanoparticle interface on the possibility of a chiral selectivity.  

[1] Amara, H.; Ducastelle, F.; Roussel, J.-M.; Bichara, C.; Gaspard, J.-P. Phys. Rev. B 2009, 79, 014109.

[2] Weatherup, R. S. et al. J. Am. Chem. Soc. 2014, 136, 13698–13708.

[3] Magnin, Y.; Zappelli, A.; Amara, H.; Ducastelle, F.; Bichara, C. Phys. Rev. Lett. 2015, 205502, 1–5.

[4] Diarra, M.; Zappelli, A.; Amara, H.; Ducastelle, F.; Bichara, C. Phys. Rev. Lett. 2012, 109, 185501.

[5] Fiawoo, M.-F. C.; Bonnot, A.-M.; Amara, H.; Bichara, C.; Thibault-Pénisson, J.; Loiseau, A. Phys. Rev. Lett. 2012, 108, 195503.

[6] He, M.; Magnin Y.; Amara, H.; Jiang, H.; Fossard, L.; Castan, A.; Kauppinen, E. I.; Loiseau, A.; Bichara C. submitted.

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