Fullerenes and related nanostructures

10.5.3 Fullerenes and related nanostructures

In the mid-1980s, research on vaporized carbon led to the discovery of the C 60 molecule, the third and

a novel form of pure carbon. The C 60 molecule has 60 carbon atoms forming a truncated icosahedron shell structure known as buckminsterfullerene (Figure 10.18). Fullerenes larger than C 60 , such as C 70 and molecules containing more than 200 C atoms, have also been found to exist. A related class of

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Figure 10.18 Various fullerene-based structures ( from Inagaki, 2000).

important nanomaterials, known as carbon nanotubes (CNTs), is illustrated in Figure 10.19. CNTs can be imagined to be formed by rolling up a graphene sheet of carbon atoms arranged in a 2-D hexagonal honeycomb lattice. To form a CNT, a hoop vector, called the chiral vector, is first chosen to connect (0,0) and (m,n) in Figure 10.19d, and the tube is formed by rolling up the graphene sheet with this chiral vector forming a closed circle on the circumference of the tube. The chiral vector (m,n) thus defines the diameter as well as the structure of the tube; for example, an armchair configuration (Figure 10.19a) has a chiral vector (m,m) and a zigzag configuration (Figure 10.19b) (m,0) or (0,n).

Another commonly used measure for the chirality is the chiral angle θ, which is the angle the chiral vector makes with the zigzag orientation on the graphene sheet. Thus, a zigzag tube has θ =0 ◦ and an armchair tube has θ = 30 ◦ , and a general chiral structure (Figure 10.19c) has a θ between 0 ◦ and

30 ◦ . Apart from the single-wall architecture in Figure 10.19, multi-walled CNTs also exist in which multiple graphene layers are rolled up to form an assembly of concentric tubes. CNTs can be fabricated using a range of methods. In laser ablation, an intense pulsed laser beam is incident on a target of graphite doped with cobalt or nickel, which act as a catalyst. The target is heated to ∼1200 ◦

C and the laser evaporates carbon from the target to form CNTs, which are carried away by a stream of argon gas and collected by a cooled substrate outside the furnace. In the electric-arc method, a voltage is applied between two graphite electrodes situated in a helium atmosphere. An electric arc forms and carbon atoms are ejected from the anode to the cathode, forming CNTs on the latter. If pure graphite electrodes are used, multi-walled CNTs will be formed, and to produce single- walled CNT, a metallic catalyst needs to be introduced onto the anode. Finally, in the chemical vapor deposition method, a hydrocarbon gas such as methane is decomposed at an elevated temperature. The resultant carbon atoms then form CNTs on a cooled surface containing a metallic catalyst. In all the processes mentioned above, a metallic catalyst in the form of nanopowder is involved. Figure 10.20 shows an in situ experiment in which a CNT was observed to grow by the catalytic action of a trapped

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(1,1) (2,1) (3,1) (4,1) (5,1) (6,1) (7,1) (8,1) (9,1) (10,1) (2,2) (3,2) (4,2) (5,2) (6,2) (7,2) (8,2) (9,2) (10,2) a – 1 (3,3) (4,3) (5,3) (6,3) (7,3) (8,3) (9,3) (4,4) (5,4) (6,4) (7,4) (8,4) (9,4) a – 2 (5,5) (6,5) (7,5) (8,5) (5,6) (7,6) (8,6) Armchair

(d)

Figure 10.19 Different forms of single-walled carbon nanotubes: (a) armchair structure, (b) zigzag structure, (c) chiral structure. (d) graphene sheet of carbons.

Ni nanoparticle at its head, during decomposition of methane inside the TEM. The propulsion of the Ni nanoparticle drew out the CNT, as shown in Figure 10.20h. Fullerene-based structures and in particular CNTs are an important class of building block materials for nanotechnology, and more details are given in Chapter 12.

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(h) Figure 10.20 In situ growth of CNT in a TEM. Methane gas is decomposed inside the TEM over a

catalyst consisting of Ni nanoclusters supported on MgAl 2 O 4 . (Helveg and co-workers, 2004).