Fact and fiction. (a) Simple models consider CNTs as atomically ‘perfect’ structures. (b) In contrast, as-grown CNTs exhibit various diameters, lengths, chiralities, and byproducts such as amorphous carbon and metal catalyst particles.Carbon nanotubes (CNTs) were first identified by Morinobu Endo et al. [1] in the late 1970s and caused tremendous interest when Sumio Iijima published his paper in Nature in 1991 [2]. A number of synthesis techniques have been established and it is possible to produce quantities of CNTs sufficient to conduct further research and use in the first applications.
Virtually every research article on CNTs refers to their extraordinary properties and potential for future applications. Some measurements suggest that CNTs can be up to 100 times stronger than stainless steel but six times lighter. CNTs are stable at high temperatures and their predicted heat transmission is roughly double that of pure diamond at room temperature. The current carrying capacity of CNTs is estimated at 1 × 109 A/cm2, while Cu wires disintegrate at about 1 × 106 A/cm2. Further, CNTs can either be metallic or semiconducting, depending on their atomic structure.
As a result of these outstanding properties, many researchers expect CNTs to prove useful in a wide range of areas, including construction; mechanical, automotive, and aerospace engineering; electrochemical, biomedical, and electrical applications; etc. It has even been suggested that nanotubes could be used to construct a space elevator [3]. Companies are beginning to advertise products containing CNTs, for example vehicle fenders, golf clubs, and X-ray sources.
So far, commercial applications make use of the bulk properties of CNTs and not individual structures. The application of individual CNTs in electronic devices is strongly dependent upon the reproducibility of individual CNTs on the large scale. However, precise control of nanotube morphology has yet to be realized – a fact that has delayed the industrial exploitation of individual nanotubes. The importance of uniform nanotube materials for technological applications was recently pointed out by a leading scientist in the area of nanotube electronics who said that he has seen many start-up companies base their devices on nonuniform CNTs, causing them to fail sooner or later. He even suggested going as far as stopping all research into CNT-based devices until we can control their growth.
An essential step toward nanotube electronics is an understanding of nanotube growth and the role of the metal catalyst often involved in the synthesis process. Despite substantial experimental progress, no definitive model for the growth of CNTs has evolved. Proposed growth mechanisms for CNTs are still mostly based on scenarios originally postulated for carbon fibers in the 1970s [1].
The state-of-the-art in nanotube synthesis and growth may be summarized as follows:
• The quality of catalytically grown CNTs has improved steadily in recent years and large quantities of fairly clean CNTs can be produced, but structural control has yet to be achieved.
• Methods that are not catalyst based, such as arc discharge, can generate CNTs of high crystallinity, but suffer from carbonaceous byproducts that make the material unsuitable for many technological applications.
• No established procedure exists to quantify the properties of CNT material or classify the quality of the material. Inspection of commercially available CNTs shows that the quality of the material can often be poor and unsuitable for further processing.
• Claims that detailed control of nanotube growth and morphology had been established have, so far, turned out to be exaggerations. Consistent atomic-scale data on nanotube formation, a prerequisite for the clarification of growth mechanisms, is still scarce. For example, it is still not clear whether carbon dissolves and diffuses through the metal catalyst particle and then precipitates as carbon filament, whether carbon diffuses on the surface of the catalyst particle, or whether bulk and surface diffusion compete.
CNTs will have significant impact only when we are able to produce them with uniform properties in large quantities, but uniform CNTs are currently not available. Synergetic experimental-theoretical studies of growth must become a focus of CNT research. Unspecific CNT sample descriptions and nomenclature need to be replaced by standardized characterization protocols for CNT materials.
Further reading
[1] A. Oberlin et al. J. Cryst. Growth, 32 (1976), p. 335
[2] S. Iijima, Nature, 354 (1991), p. 56
[3] L.X. Zheng et al. Nat. Mater., 3 (2004), p. 673