Catalytic Growth Mechanisms for Single-Wall NanotubesJ.-C. Charlier
The discovery of single-wall carbon nanotubes (SWNT) in the arc discharge apparatus by co-evaporating a metal catalyst (Ni, Co, ...) stimulated extensive theoretical and experimental research into these utlimate carbon fibers. Such unidemensional systems, which are much more free of defects the multi-wall nanotubes (MWNT), are exhibiting unique electronic, mechanical, and magnetic properties.To exploit these properties, it is necessary to optimise their yield and quality.In spite of the enormous progress in the synthesis, theoretical understanding of the catalytic growth of SWNTs lags behind.In contrast to the MWNTs, SWNTs can only be produced in the presence of metal catalysts. The role played by the metal atoms in determining the growth is, however, unacessible to direct observation and remains controversial. Two distinct situations arise from experiment, the catalytic particle being either much smaller or larger than the nanotube diameter. Here, both growth scenarios are investigated by means of first-principles molecular dynamics simulations, which points to a unique catalytic mechanism. We find that the cobalt-carbon chemical bonds keep breaking and reforming at the experimental temperatures. This provides a pathway for carbon incorporation, necessary for growth. The incorporation process is either direct or precursor-state mediated. In the case of small catalytic particles, this leads to a close-end mechanism for single-wall nanotube growth. Low incorporation barrier and small atomic-rearrangement energy cost were calculated within ab initio framework. If a large metal particle is present, a root growth mechanism is favoured in which carbon atoms are added to the growing tube by a diffusion-segregation process. The simulations provide quantitative insigth at the atomic scale which supportsthe macroscopic models of growth proposed long ago for carbon filaments.
This document at the URL
has been visited
Last modification: 1999.07.15 (Thursday) 11:27:31 EDT