Armchair-type CNTs

Changes in the bandgap values depending on these patterns are summarised in Table 1 [16], where it is shown that only armchair-type CNT can have zero bandgap at a certain bond-alternation pattern even if they have not isodistant bond patterns. It should be emphasised that actual bond pattern is decided only by the viewpoints of energetical stabilisation, which cannot be predicted by the Hiickel-type tight-binding calculation. 

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. 

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]. 

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],... 

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