Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth and Chirality-Control: Insights from QM/MD Simulations

Abstract

The experimental characterisations of carbon nanotubes (CNTs) (Iijima, 1991) and in particular single-walled CNTs (SWNTs) (Iijima & Ichihashi, 1993) in the early 1990s were landmark moments in 20th century science. The potential uses of these remarkable nanostructures are now becoming realised, as their synthesis is now routinely performed on the industrial scale. The initial successes in this respect were generally experimental techniques that were previously well established in other fields. This is particularly true of the chemical vapor deposition (CVD) and arc-discharge processes. The original experimental characterisation of SWNTs was in fact accomplished using nanotubes synthesised with the former method (Iijima & Ichihashi, 1993). The understanding of the way in which CNTs nucleate and grow was therefore synergic with the evolution and refinement of these synthetic methods. Indeed, the original mechanisms of CNT nucleation and growth were conceived from experimental observations. The most prevalent of these today is the vapor-liquid-solid (VLS) mechanism (Saito, 1995). According to this mechanism, SWNT nucleation growth is postulated to consist of three distinct stages. The first of these features a mixed carbon/catalyst vapor phase, from which co-condensation yields liquid catalyst-carbide nanoparticles. Typical catalysts in the growth of SWNTs are traditionally transition metals such as Fe, Ni, Co, Mo, and alloys thereof (see (Journet et al., 1997; Moisala et al., 2003; Harris, 2007), and references therein). The precipitation of atomic carbon from this liquid carbide phase takes place once the carbide phase is saturated with carbon. This precipitation yields the formation of solid phase CNTs. Due to the inherent limits in spatial and temporal resolutions that are furnished by experimental techniques and instrumentation, there inevitably remain questions regarding the VLS mechanism and CNT growth that, for now, cannot be answered from an experimental standpoint. There are several infamous examples in this respect. For instance, the mechanism of so-called ‘catalyst-free’ SWNT nucleation growth remains unknown, following the recent