Chiral-type CNTs

Some explanations could be possible for these contradictory results. One is that a various types of CNTs may be obtained by different methods, since SWCNTs as much as 50 % are chiral and nonmetallic [42]. The other is that the result may be attributable to the contact condition of SWCNT bundles. When the bundles closely contact each other, the SWCNT system will likely become a three-dimensional one just as in the case of contacted MWCNTs. 

There are three general types of CNT structure (Figure 12.10). The zigzag nanotubes correspond to ( ,0) or (0,m) and have a chiral angle of 0°. The carbon-carbon position is parallel to the tube axis. Armchair nanotubes have (n,n) with a chiral angle of 30°. The carbon-carbon positions are perpendicular to the tube axis. Chiral nanotubes have general (n,m) values and a chiral angle of between 0° and 30°, and as the name implies, they are chiral. 

Applications of CNTs based on their electrical properties strongly depend on the diameter and helicity as well as parity.2 3 Doping of CNTs by boron and nitrogen renders them p-type and retype, respectively. MWNTs and SWNTs doped with nitrogen 4 17 and boron"1 9 have been reported. Boron-doped carbon nanotubes appear to exhibit enhanced electron field emission due to the presence of the boron atom at the nanotube edges.20 2 N-doped CNTs show retype behavior regardless of tube chirality.22... 

These properties are mainly originated from the molecular structures of CNTs, which consist of graphene sheets rolled to form hollow cylinders with an extremely high aspect ratio [5]. Moreover, CNTs can be ether metallic or semiconducting tubes, depending on their diameter and/or chirality [5]. There are two types of CNTs SWNTs and MWNTs. Details, such as structure, synthesis methods, application and properties, are well documented [4], [5], [6]. 

Figure 5.1 The three types of CNT structures (a) armchair, (b) zigzag, (c) chiral. (Reprinted with permission from International Materials Review, Carbon Nanotube Composites by P. J. F. Harris, 49, 1, 31 3. Copyright (2004) Maney Publishing www.maney.co.uk/journals/lmr)...

SWCNTs can be considered as rectangular strips of hexagonal graphite monolayers rolling up to cylinder tubes. Two types of SWCNTs with high symmetry are normally selected by researchers, which are zigzag SWCNTs and armchair SWCNTs. When some of the atomic bonds are parallel to the tube axis, the CNT is called a zigzag CNT, while if the bonds are perpendicular to the axis, it is called an armchair CNT, and for any other structures, they are called chiral CNTs, as shown in Fig. 16.6 [72]. 

Moreover, for a given type of surfactant molecule, it has been both experimentally and theoretically demonstrated that the density of some types of surfactants, adsorbed on the surface of the CNTs, is a function of the CNT chirality, its electronic structure and diameterT This discovery opens new ways of CNT separation by centrifugation based on different diameter and chirality, since the CNT-surfactant entity is highly sensitive to the number of adsorbed molecules. 

Exhaustive studies of the electronic properties of CNTs indicated that they behave like two types depending on the temperature at high temperature their electrical conductivity may be described by semiclassical models already used for graphite, whereas at low temperature they reveal 2D quantum transport features. Their electrical properties also depend on the rolled-up direction of the graphene layers (armchair, zigzag, or chiral) and the presence of defects at the surface of the tubes [12],... 

All MWCNTs are of metallic in nature but nature of SWCNTs varies with chiral vectors n and m. SWCNTs are very important type of CNTs, because their properties vary significantly with chiral vectors. The band gap of SWCNTs can vary from 0 to 2eV, thus electrical properties can be metallic as well as semiconducting. A major problem in synthesis of SWCNTs is the lack of... 

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