The structure of CNTs

The structure of CNTs can be understood as sheets of graphene (i.e. monolayers of sp2 hybridized carbon, see Chapter 2) rolled-up into concentric cylinders. This results in the saturation of part of the dangling bonds of graphene and thus in a decrease of potential energy, which counterbalances strain energy induced by curvature and thus stabilizes the CNTs. Further stabilization can be achieved by saturating the dangling bonds at the tips of the tubes so that in most cases CNTs are terminated by fullerene caps. Consequently, the smallest stable fullerene, i.e. C60, which is - 0.7 nm in diameter, thus determines the diameter of the smallest CNT. The fullerene caps can be opened by chemical and heat treatment, as described in Section 1.5. 

The resolution of SEMs is now suitable for nano-materials characterization. High resolution SEM is a powerful instrument for imaging fine structures of materials and nanoparticles fabricated by nanotechnology. In lens SE, BSE modes, and STEM mode are often performed to check the structure of CNT growths or CNT as delivered by commercial producers, and sometimes coupled with TEM. Even the single-walled carbon nanotubes can easily be observed by HR-SEM (see Figure 3.13). The STEM mode can also be used for free CNT observation (75). 

The structure of CNTs filled by FesC nanoparticles (nanocomposite) was investigated by TEM. The TEM analysis of the images revealed significant dimensional dispersion of the outer diameters of CNTs (20-100 nm) what confirms SEM data. The typical view of such CNTs is shown in Fig. 2. One can observe that CNTs are multi-walled, most of them are filled by nanoparticles allocated in the walls (arrow 1) and in the channels of CNTs. As it was shown by X-ray analysis these particles represent mostly... 

Fig. 2. Structure and active site of clostridial neurotoxins. The upper panel shows the structure of CNTs, and the segments that show significant homology between the different serotypes are in black (Minton, 1995). The highest homology is shown by a short segment corresponding to the amino acid residues 216-244 in TeTx. This segment contains the zinc-binding motif of metallo-proteinases (zincins) and it is dissimilar to the consensus sequence of the metzincin metallo-proteinase family (Jiang and Bond, 1992)...

CNTs were discovered in the early 1990s [312]. CNTs have a tubular structure of closed topology and consist of hexagonal honeycomb lattices made up of sp carbon units. A schematic of the structure of CNTs is shown in Figure 1.15. The diameters of CNTs are typically several nanometers. The length of CNTs can be up to several micrometers. Two basic forms are distinguished, single-walled... 

The structure of CNTs consists of one, two or more, graphene sheets rolled up to make the tubes leading to variations from single (S), double (D) and multi-wall (MW) carbon nanotubes, respectively. Nanotubes naturally align themselves into ropes held together by van der Waals forces, more specifically, pi-stacking. The nature of the dispersion problem for CNTs is rather different from other... 

To enhance the properties of carbon nanostructures, several strategies have been developed. For example, carbon surfaces are functionalized, modified, and customized to selectively detect molecules, chemicals, and biological compounds in liquid or gas phases. Noncovalent attachment can be utilized to preserve the structure of CNTs by adsorbing the material onto their surface. However, covalent attachment needs the surface of CNTs to have defect sites, often requiring the surface to be chemically activated to bind molecules to their surface. Alternatively, CNTs can be embedded or filled with material, as we will discuss later in this section. 

The structures of CNTs are defined by a variety of parameters such as number of carbon layers, length, diameter and ahgnment of hexagons. Broadly, CNTs are classified into the following three types according to the number of carbon layers SWNTs, DWNTs and MWNTs. DWNTs and MWNTs were discovered by lijima at the negative end of the electrode of the arc discharge for the fullerene synthesis in 1991 [35]. SWNTs were first prepared independently by lijima et al. [36] and Bethune et al. [37]. However, there is controversy... 

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