During catalytic chemical vapor deposition, the chirality of single wall carbon nanotubes is determined when the growing graphene nucleus wraps around the catalyst and converts into a tubular structure. Elucidating this critical process is required to develop deterministic bottom-up strategies aiming at better chiral distribution control. Direct observations of carbon nanotube growth, and theoretical modeling and simulations of the nucleation have been published but experimental atomic-resolution evidence of single-walled carbon nanotube nucleation has, until now, eluded us.

The main obstacle is that nucleation involves a few atoms only and a short time scale, thus requiring a combination of high spatial and temporal resolution for direct observation. Here, we overcome the temporal resolution constraint by reducing the growth rate in order to match the temporal resolution of our recording medium. We employ an environmental scanning transmission electron (ESTEM), equipped with an image corrector and a digital video recording system, to follow SWCNT growth using Co-Mo/MgO catalyst and acetylene (C₂H₂) as a carbon source (see Methods). We present atomic-resolution movies that reveal the nucleation of graphene on cobalt carbide nanoparticles followed by its transformation to a single-walled carbon nanotube. We find that the surface termination of the faceted catalyst nanoparticles regulates the nucleation of the graphene sheet and its conversion into a nanotube. Additional density functional theory calculations show that the disparity in adhesion energies for graphene to different catalyst surfaces is critical for nanotube formation: strong work of adhesion provides anchoring planes for the tube rim to attach, while weak work of adhesion promotes the lift-off of the nanotube cap (Fig. 1). [1]

[1] Nucleation of Graphene and Its Conversion to Single-Walled Carbon Nanotubes. Nano Letters. 2014, 14, 6104−6108

Figures:

Fig. 1. Atomic-scale environmental transmission electron movie (individual frames extracted) showing how carbon atoms assemble together on a catalyst nanoparticle during a nanotube nucleation process. Carbon atoms first form a graphene sheet, which progressively lifts-off leading to the conversion into a nanotube. These time-resolved observations (left) combined with theoretical calculations (right) confirm that some nanoparticle facets act like a vice-grip for graphene, thus offering anchoring sites, while other facets allow the graphene to lift-off.

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EMC Abstracts - https://emc-proceedings.com/abstract/nucleation-of-graphene-and-its-conversion-to-single-walled-carbon-nanotube-revealed/