Structure and Properties of Carbon Nanotubes

Carbon is unique among the column IV elements of the periodic table with sp2 hybridised bonding, hexagonal ground state graphite in the condensed phase, close to being two-dimensional (2-D) semi-metal, and anisotropic. Si, Ge, Sn and Pb are sp3 hybridised in cubic solid ground states. Diamond, another form of carbon is a three-dimensional material and isotropic, while the fullerene and carbon nanotubes discovered more than a decade ago have zero- and one-dimensional forms of carbon, respectively. The nanotube may be capped by the hemisphere (^-section) of the fullerene, C60 structure during production. Since the discovery of fullerenes a great deal of investigations both theoretically and experimentally, have been focused into these interesting and unique nanostructures (Fig. 10.3), and tremendous applications have been identified and others proposed for the future of nanotechnology.

Fig. 10.3. Schematics of typical grapheme layers and other carbon nanostructured carbon materials (Graphite, Carbon nanotube, Fullerence, Diamond)

Fig. 10.3. Schematics of typical grapheme layers and other carbon nanostructured carbon materials (Graphite, Carbon nanotube, Fullerence, Diamond)

Carbon being the number 6 element in the periodic table has its electronic configurations arranged in the quantised atomic orbitals as 1s2 2s2 2p2 at the ground state. Graphite crystal structure forms strong interplanar bonds with its nearest neighbours using the occupied 2s, 2px and 2py orbitals normally denoted as sp2 hybridisation, while the remaining 2pz orbital forms weaker interplanar bonding used to hold the planes above and below, that enable the planes to slide, thereby results into the semimetallic characteristics of graphite. Diamond structure on the other hand possesses carbon atom that is bonded to four nearest neighbours with 2s, 2px, 2py and 2pz orbitals in tetrahegdral sp3 configuration that makes diamond hard and strong. Hence while diamond is isotropic, graphite is anisotropic. Thus in similar vein, the structure of carbon nanotube is analogous to that of graphite, but the only difference is when the graphene sheets are rolled to produce tubes. As mentioned earlier, carbon nanotube consisting of one cylindrical graphite sheet is the single walled nanotube, but if several cylinders are nested, it is called multi walled nanotube, with interlayer space range of 0.34-0.36 nm, which is almost similar to the typical atomic spacing in graphite. C-C bonds length in carbon nanotube have been observed by Bonard et al. (Bonard et al. 2001) to be 0.14 nm, which is shorter than the C-C bonds in diamond, implying that carbon nanotube would be stronger than diamond.

Carbon nanotube structures are obtained by rolling up the carbon lattice in one of the symmetry axes or along any other direction that is different from the symmetric axis to produce a zigzag, armchair or chiral tubes, respectively, as shown in Fig. 10.4(b) (http://www.nas.nasa.gov/Groups/SciTech/nano/images/ images.html).

As depicted in Fig. 10.4(a), the nanotube can be seen as a two-dimensional honeycomb lattice of a single layer of graphene, which was likened by Iijima in 1991 to the Bravais lattice vectors ofthe graphite sheet to map the circumference around a cylinder. A lattice vector, B may be defined with two primitive lattice vectors R1 and R2 (Fig. 10.4(a)) and a pair of indices (m, n), which are integers that denote the position of carbon atom on the two-dimensional hexagonal lattice, and the chiral vector of the nanotube (Mintmire 1997; Saito 2003) that is normal to the tube axis as,

Thus rolling up of the sheet as the placement of the atom at (m=0 ,n=0) on the atom at (m,n) would give nanotubes of different structures as,

{(m=n) Armchair or serpentine structure

(m, n=0) Zigzag or saw tooth structure

Due to space limitation, this work would not be able to discuss details of the subject, thus the reader is advised to consult more subject specific literatures (Saito et al. 2001; Mintmire and White 1997) for a better coverage. As a result of the C-C bond strength, extremely small diameter of the carbon atom and the n-electrons available from the sp2 configuration of graphite, carbon nanotubes provide quite a number of remarkable mechanical and electronic properties as summarised by Hoenlein et al. presented in Table 10.2. The diameter, d of the nanotube can be determined from the indices (m,n) from Dresselhaus & Avouris (2003),

Where, lC-C is the C-C bond length (1.42A), and B the length of the chiral vector B (Yamg, 2003). It is obvious therefore that the study on the helicity of nanotube has indicated a great deal of its application in microelectronics. Coupled with other potential applications, such as in gas storage, biomedicine etc, have spurred divergent researches, looking into scaleable and efficient production techniques as discussed in the next sections.

Fig. 10.4. (a) Carbon nanotube structures are obtained by rolling up carbon lattice to form a zigzag, armchair or chiral tube; atom at position (10,5) is projected on (0,0), while others are shown with dotted lines to project armchair and zigzag tubes respectively. (b) graphical representations of morphologies projected from the graphene sheet (Bonard 2001).

Fig. 10.4. (a) Carbon nanotube structures are obtained by rolling up carbon lattice to form a zigzag, armchair or chiral tube; atom at position (10,5) is projected on (0,0), while others are shown with dotted lines to project armchair and zigzag tubes respectively. (b) graphical representations of morphologies projected from the graphene sheet (Bonard 2001).

Was this article helpful?

0 0
Brain Blaster

Brain Blaster

Have you ever been envious of people who seem to have no end of clever ideas, who are able to think quickly in any situation, or who seem to have flawless memories? Could it be that they're just born smarter or quicker than the rest of us? Or are there some secrets that they might know that we don't?

Get My Free Ebook


Post a comment