(Invited) Toward Selective Synthesis of Carbon Nanotubes: Paradoxes and Perspectives
Authors: Harutyunyan Avetik R 
Published: February 2020

Synthesis of single wall carbon nanotubes (SWCNTs) with specific helicity, and thereby with particular properties, still remains one of the main challenges for scientific and engineering communities. Accumulated experimental results for the last two decades on preferential or helicity specific growth of nanotubes continue to fuel the field and suggest possible paths to overcome this obstacle [1-3].The common base that everybody agrees on is that the catalyst plays a key role, therefore a properly made catalyst can lead to the helicity controlled growth of nanotubes. Indeed, all efforts in recent years have been focused on manipulating catalyst properties such as size, composition, and structure [4]. Today, the role of a catalyst is more decisive, e.g. as a template that could not only control the diameter of grown tube, but also amend the structure of grown carbon layer via planar or perpendicular interactions between carbon sp2 network. More specifically, the dominant models rely on the epitaxial relationship between the structure of a nanotube and the corresponding facet of a catalyst. However, there are two paradoxes that need to be addressed: I) the structure of a carbon nanotube and circumference of its rims assume that its growth requires more than one facet i.e. 3D geometry. Moreover involvement of various facets (specifically with low Miller indexes) assumes the presence of steps/kinks with various structures on the catalyst surface, which are the most preferable carbon adsorption sites. Therefore, it is hard to picture van der Waals planar epitaxial relationship between one specific facet and carbon layer (nanotube wall) or between one specific catalyst step and carbon edge (nanotube rim); II) It is well established that a catalyst particle undergoes dynamic morphological reconstructions during nanotube nucleation due to the variation of surface energies of facets driven by carbon atoms adsorption. Therefore, it is hard to picture the mechanism that fulfills the correlation between the symmetry of growing carbon layer and the structure of the specific facet of catalyst that is continuously evolving.
To address these problems we have carefully studied the nucleation of carbon nanotubes in very early stages by exploiting in situ environmental TEM technique [5]. We also used catalyst nanoparticles composition with a very high melting point that presumably remains stable during growth of carbon nanotubes. All the attempts, including variations of synthesis conditions, were futile to achieve helicial selectivity and resulted only in variation of the diameter range and of the yield of grown nanotubes. However, our studies [6] reveal that, as a function of morphology of a catalyst nanoparticle such as aspect ratio (L/D, where L-is the length and D-is the diameter) and interfacial angular distribution, there are two compatible scenarios for origination of helicity of a nanotube: 1) in the case of L>>D, the nucleation, the symmetry, and the growth direction of carbon layer along the tubular axis lead to a merging of grown flakes into a nanotube wall with perfect honeycomb lattice in the curved geometry. In this case, the structures of catalyst facets (along the tubular axis) amend the symmetry of grown carbon wall, which in turn defines the helicity of the grown tube. In the meantime, graphene embryo formation on the surface of catalyst nanoparticle with geometry L~D is accompanied by the introduction of the defects (polygons) into the honeycomb structure depending on the interfacial angular distributions. The configurations and the number of defects (pentagons) define the curvature and the helicity of the formed carbon cap and thereby of the grown nanotube.
Hence, depending on catalyst morphology (aspect ratio and interfacial angular distribution), the nanotube helicity originates either by carbon wall symmetry or by carbon cap. Our ability to control these scenarios could lead to the helicity selective growth of SWNTs.

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