Growth Mechanisms of Carbon Nanotubes

Jean-Christophe Charlier1 and Sumio Iijima2'3

1 Université Catholique de Louvain

Unite de Physico-Chimie et de Physique des Matériaux Place Croix du Sud, 1, Bâtiment Boltzmann, 1348 Louvain-la-Neuve, Belgium [email protected]

2 NEC Corporation, R&D Group

34 Miyukigaoka, Tsukuba, Ibaraki 305, Japan

3 Meijo University, Faculty of Science and Engineering

1-501, Shiogamaguchi Tenpaku, Nagoya, Aichi 4688502, Japan [email protected]

Abstract. This review considers the present state of understanding of the growth of carbon nanotubes based on TEM observations and numerical simulations. In the highlights of these experimental and theoretical approaches, various mechanisms, already proposed in the literature, for single and multi-shell nanotubes nucleation and growth are reviewed. A good understanding of nanotube growth at the atomic level is probably one of the main issues either to develop a nanotube mass production process or to control growth in order to obtain well-designed nanotube structures.

Nearly ten years after their discovery [1], carbon nanotubes (CNs) are still attracting much interest for their potential applications, which largely derives from their unusual structural and electronic properties. Since all these properties are directly related to the atomic structure of the tube, it is essential to understand what controls nanotube size, the number of shells, the helicity, and the structure during synthesis. A thorough understanding of the formation mechanisms for these nanotubular carbon systems is crucial to design procedures for controlling the growth conditions to obtain more practical structures, which might be directly available for nanotechnology.

In order to optimize the single- and multi-walled nanotube yield and quality, three main production methods have been used up to now for their synthesis. Multi-wall carbon nanotubes [1,2] typically grow on the cathode during an arc discharge between two graphitic electrodes (temperature ~3000K). Single-wall carbon nanotubes were first observed in the arc discharge apparatus by co-evaporating iron [3] or cobalt [4] (as metal catalysts) in a methane atmosphere. The discovery of a single-shell tube stimulated intense research to find an efficient way to produce bundles of ordered single-wall nanotubes (called ropes).

The laser ablation technique uses two lasers to vaporize a graphite target mixed with a small amount of Co and/or Ni in order to condense the

M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes, Topics Appl. Phys. 80, 55-81 (2001) © Springer-Verlag Berlin Heidelberg 2001

carbon into single-wall tubes. Through this technique [5], the growth conditions are well controlled and maintained over for a long time, leading to a more uniform vaporization. Another technique [6], based on the carbon-arc method, also provides similar arrays of single-wall nanotubes, produced from an ionized carbon plasma which is generated by joule heating during the discharge. Both techniques synthesize, with yields of more than 70-90 percent, single-wall nanotubes (~1.4nm in diameter) self-organized into bundles. These ropes, which consist of up to a hundred parallel nanotubes packed in a perfect triangular lattice, are typically more than one-tenth of a millimeter long and look very promising for engineering applications. Another attractive synthesis technique, based on a vapor phase growth mechanism which utilizes the decomposition of hydrocarbons at a temperature of ~1500K, has also been recently used to produce large quantities of rope-like bundles of high-purity single-wall nanotubes at a very low cost [7]. However, methods for large-scale production have not really been developed, probably because so little is understood about the synthesis process at the microscopic level.

In the following, we will address from both an experimental and a theoretical point of view, the growth of carbon nanotubes. One of our objectives will be to show that presently available simulation techniques (semi-empirical and ab initio) can provide quantitative understanding not only of the stability, but also of the dynamics of the growth of carbon nanotube systems. We will try to summarize the microscopic insight obtained from these theoretical simulations, which will allow us to isolate the essential physics and to propose good models for multi-shell and single-shell nanotube growth, and to analyze a possible consensus for certain models based on experimental data.

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