Info

Cluster size n.

Fig. 6.5. The cohesive energy (eV/C atom) for fullerenes, plotted as a function of cluster size, where EcoH denotes the cohesive energy relative to bulk graphite [6.16]. Not shown in this figure is the large potential barrier between stable C„c fullerene, where nc, the number of carbon atoms, is even.

In a hot carbon plasma (operating above ~3500°C), carbon monomers, dimers, clusters and rings are expected to be present, and the larger carbon structures that form at these high temperatures are rather flexible [6.19], Each corannulene cluster contains five V-shaped edge sites (see Fig. 3.2(a)) and five flat edges with one carbon atom at each end of the edges. We can argue that the V-shaped sites are the more active sites because a single C2 dimer can form a pentagon at that site (see Fig. 6.6). Once a new row of pentagons is formed at the five adjacent V-shaped sites, the addition of a single C2 dimer is sufficient to form a new hexagonal face. After five C2 dimers are absorbed, five hexagons are added, thus completing the second ring of polygons about the corannulene cluster and forming a hemisphere of the C60 molecule. The next ring of hexagonal faces can then be formed by subsequent addition of C2 dimers following the approach just described. The following ring of pentagons can be formed by the addition of a single C monomer per new pentagon. The final completion of the whole fullerene can then be accomplished simply by five C2 additions, resulting in five new hexagons (see Fig. 6.6).

Some authors have suggested [6.20] that fullerenes are assembled from ring networks in the plasma. The construction suggested above can also be carried out with ring networks of appropriate size, i.e., rings of five pentagons and rings of five hexagons. This is called the "ring road" to fullerene formation, in contrast to the "pentagon road" discussed above. In a sense, the correlated addition of multiple C2 dimers and isolated C atoms is strongly related to the addition of rings of carbon atoms, as is further discussed in §6.1.4. A totally different method for the synthesis of fullerenes by a purely chemical route has been suggested by R. Taylor [6.21] and G. H. Taylor et al. [6.22].

Fig. 6.6. A corannulene cluster grows with the addition of C2 clusters. For the initial corannulene cluster, there are 15 carbon atoms at the periphery. Five of these (a sites) are at V-shaped sites and ten are at edge sites denoted by (b). The addition of two C2 units (denoted by C2 and C'2) forms pentagons, while the subsequent addition c hexagon.

Fig. 6.6. A corannulene cluster grows with the addition of C2 clusters. For the initial corannulene cluster, there are 15 carbon atoms at the periphery. Five of these (a sites) are at V-shaped sites and ten are at edge sites denoted by (b). The addition of two C2 units (denoted by C2 and C'2) forms pentagons, while the subsequent addition c hexagon.

Closely related to the C2 dimer absorption discussed above is the carbon dimer (C2), tetramer (C4), and hexamer (C6) emission shown in Fig. 6.7 [6.4]. Kroto and Iijima et al. [6.23,24] have suggested that the formation of fullerene balls on the surface of a carbon tubule (see §19.2.5) is due to the curling up of a fragment of a graphene sheet near a defect in the surface layer of the tubule.

Was this article helpful?

0 0

Post a comment