Rümmeli et al. Nanoscale Research Letters 2011, 6:303 http://www.nanoscalereslett.com/content/6/1/303 Page 2 of 9 Figure 1 Transmission electron micrographs of SWNT bundles (left panel) and an individual SWNT (right panel) synthesized from cobalt by Bethune et al. Reprinted with permission from Bethune et al. [5]. carbon growth by Baker et al. [13] (Figure 2) which is derived from the vapour-liquid-solid (VLS) theory devel- oped by Wagner and Ellis to describe Si whisker forma- tion [14]. The model proposed that hydrocarbons adsorb on the metal particles and are catalytically decomposed. This results in carbon dissolving into the particle forming a liquid eutectic. Upon supersaturation, carbon precipitates in a tubular, crystalline form. How- ever, various alternative models exist and it is likely that the appropriate description of growth depends on the synthesis route and conditions used. For example, it is argued that at low temperature CNT growth can occur through surface diffusion [15]. In addition, most models assume thermal equilibrium conditions, although in practice, this is not so. In the case of noble metal cata- lyst particles, at temperatures where the VLS model is expected to be valid, they exhibit very low carbon solu- bility and negligible carbide formation. Zhou et al. [16] argue that low carbon solubility results in an increased precipitation rate. To grow carbon nanotubes, Lu and Liu [17] argue one needs to match the carbon supply rate to the tube formation rate. Figure 2 Schematic showing base growth and tip growth of carbon fibres according to the VLS mode described by Baker [13].