Nanotube zigzag

Fig. 20. Electronic 1D density of states per unit cell of a 2D graphene sheet for two (n, 0) zigzag nanotubes (a) the (10,0) nanotube which has semiconducting behavior, (b) the (9, 0) nanotube which has metallic behavior. Also shown in the figure is the density of states for the 2D graphene sheet (dotted line) [178].

The electronic structure of GaN nanotubes was calculated as well (71) and was essentially in accordance with the band structure calculations of the other inorganic nanotubes. The band gap of nanotubes with a diameter >2 nm is >4 eV and shrinks with the nanotube diameter. Zigzag nanotubes are found to have a direct transition, which suggests that they could serve as an ultrasmall blue light-emitting source. The structure and stability of CaSi2 nanotubes have been investigated but a few details are currently available (88b,c). 

If m = 0, the nanotubes are called zigzag nanotubes, if n = m, the nanotubes are defined as armchair nanotubes, and all other orientations are called chiral . The deviation of Cn from a is expressed by the inclination angle 0 and ranges from 0° ( armchair ) to 30° ( zigzag ) [17]. 

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. 

Nanotubes fall into three groups, depending on the chiral angle, 6, between the < 2110 > direction of the hexagons and the tube axis (Figure 17.9). If 0 = 0, a zigzag nanotube results. If 0 = 30°, the nanotube is called an armchair. Chiral nanotubes are those for which 0 < 9 < 30°. These develop twists. Nanotubes can have metallic conduction others are semiconductors or insulators. 

The SWNT systems chosen in the present studies include 3 armchair nanotubes and 3 zigzag nanotubes with diameters ranging from 4 A to 12 A, and 1 chiral nanotube with a diameter of 8.28 A. The nanotubes were carefully chosen to address the fundamental issues of curvature and chirality and the effect of each on the adsorption capacity. First, to understand the curvature effect on hydrogen uptake, we selected nanotubes with diameters varying from about 4 A to 12 A. Next, to investigate the effect of nanotube chirality, we intentionally chose the nanotubes of different chiral architectures with similar diameters. Finally, to study the capacity of a given nanotube, we included three different H2 loadings at 0.4 wt. %, 3.0 wt. % and 6.5 wt. %, respectively, in our MD simulations. 

We studied the cohesive energy of the BCN-NT and found that zigzag nanotubes are more stable than armchair nanotubes. Moreover, we found that tubes of type-5 and type-6 have lowest energy so they should be synthesized by methods described above. 

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). 

Figure 4 (A) Atomically resolved scanning tunneling microscopy (STM) of a SWNT in the surface of a rope revealing chiral-twist. (Reprinted with permission from Ref 15. 1998 Macmillan Magazines Ltd (www.nature.com).) (B) STM images of SWNTs produced by arc-discharge method (a) chiral nanotube with angle 7°, (b) zigzag nanotube, and (c) armchair nanotube. The tube axis is shown with dashed arrows. (Reprinted with permission Ref. 16. 1998 Macmillan Magazines Ltd)...

From Eq. (2) it follows that 9 = 30° for the (n, n) armchair nanotube and that the n, 0) zigzag nanotube would have 9 = 60°. From Fig. 2A it follows that if we limit 9 to between 0° and 30°, then by symmetry 0=0° for a zigzag nanotube. Both armchair and zigzag nanotubes have a mirror plane and thus are considered as chiral. Differences in the nanotube diameter and chiral angle 9 give rise to differences in the properties of the various CNTs. The symmetry vector R = of the symmetry group for the... 

Prepare a sheet showing an extended graphene structure, approximately 12 by 15 fused carbon rings or larger. Use this sheet to show how the graphene structure could be rolled up to form (a) a zigzag nanotube, (b) an armchair nanotube, and (c) a chiral nanotube. Is more than one chiral structure possible (See M. S. Dresselhaus, G. Dresselhaus, and R. Saito, Carbon, 1995, 33, 883.)... 

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

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