Carbon nanotubes zigzag tubes

The filling control approach has even been applied to some nanophase materials. For example, the onset of metallicity has been observed in individual alkali metal-doped single-walled zigzag carbon nanotubes. Zigzag nanotubes are semiconductors with a band gap around 0.6 eV. Using tubes that are (presumably) open on each end, it has been observed that upon vapor phase intercalation of potassium into the interior of the nanotube, electrons are donated to the empty conduction band, thereby raising the Fermi level and inducing metallic behavior (Bockrath, 1999). 

Three examples of particular structures of SWCNTs, depending on the orientation of the hexagons related to the tube axis, (a) armchair-type tube (0 = 30°), (b) zigzag type tube (0 - 0°), and chiral tube (0 < 0 < 30°). Reprint from Carbon, vol. 33, No. 7, Dresselhaus M.S., Dresselhaus G., Saito R., Physics of carbon nanotubes, pages 883-891, Copyright (1995) with permission from Elsevier. 

There are multiwalled carbon nanotubes (MWNTs), each consisting of ten inner tubes or more. In a carbon MWNT, the spacing between two adjacent coaxial zigzag tubes (m, 0) and ( 2, 0) is Ad/2 = (0.123/tt)( 2 - i). However, this cannot be made to be close to c/2 = 0.335 nm (the interlayer separation... 

Vm Fig. 28.22 (a) Vectors on a graphene sheet that define achiral zigzag and armchair carbon nanotubes. The angle 0 is defined as being 0° for the zigzag structure. The bold lines define the shape of the open ends of the tube, (b) An example of an armchair carbon nanotube, (c) An example of a zigzag carbon nanotube. 

Examples of capping units that covalently bond to open carbon nanotubes to yield closed tubes. The examples shown are portions of a C o molecule and are compatible with (a) an armchair and (b) a zigzag carbon nanotube. 

A Figure 12.48 Atomic models of carbon nanotubes. Left Armchair nanotube, which shows metallic behavior. Right Zigzag nanotube, which can be either semiconducting or metallic, depending on tube diameter. 

Most theoretical studies of inorganic and carbon nanotubes deal with achiral structures, i.e. zigzag and armchair nanotubes. In terms of their integer denominators these are (n,n) and (w,0) nanotubes. There is, however, a plethora of other tubular structures with denominators (n,m) (and n m) that are chiral. Using periodic-boundary conditions, the calculational unit cells of chiral nanotubes contain a lot more atoms than those of the achiral tubes. Thus, calculations of chiral tubes with the generally used techniques are computationally demanding. 

In a batch of newly synthesized carbcai nanotubes, a significant proportion of carbon nanotubes are capped, i.e. the open ends of the tubes shown in Fig. 28.34 are capped by hemispherical units, formally derived from fuUerenes. By removing atoms from Cgo (Fig. 14.5), end-capping units compatible with an armchair SWNT (Fig. 28.35a) or zigzag SWNT (Fig. 28.35b) can be generated. As noted earlier, arc discharge methods of synthesis tend to favour the formation of MWNTs. These consist of concentric tubes, packed one inside another. 

---