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Nanomaterials Fabrication 81

the yield is high because clustering continues in a hot enough region
to permit the growing clusters to anneal to the minimum energy path:
one where the graphene sheet (a) is made up solely of pentagons and
hexagons, (b) has as many pentagons as possible, while (c) avoiding
structures where two pentagons are adjacent. If the pentagon rule
structures really are the lowest energy forms for any open carbon net-
work, then one can readily imagine that high-yield synthesis of C 60
may be possible. In principal, all one needs to do is adjust the condi-
tions of the carbon cluster growth such that each open cluster has
ample time to anneal into its favored pentagon rule structure before it
grows further.
In the Krätschmer-Huffman (KH) experiment [154], carbon radi-
cals are produced by the slow evaporation of the surface of a resistively
heated carbon rod. After the KH method was introduced, it was found
at Rice University that a simple AC or DC arc would produce C 60
and the other fullerenes in good yield as well, and this is now the
method used commercially [155]. Even though the mechanism of a
carbon arc differs from that of a resistively heated carbon rod (because
it involves a plasma) the He pressure for optimum C formation is very
60
similar. Thus, it is not so much the vaporization method that matters,
but rather the conditions prevailing while the carbon vapor condenses.
Adjusting the helium gas pressure, the rate of migration of the carbon
vapor away from the hot graphite rod is controlled and thereby the
carbon radical density in the region where clusters in the size range
near C 60 are formed.
A ratio between the mass of fullerenes and the total mass of carbon
soot defines fullerene yield. The yields determined by UV-Vis absorp-
tion are approximately 40 percent, 10–15 percent, and 15 percent in
laser, electric arc, and solar processes. Productivity of a production
process can be defined as a product between fullerene yield and flow
rate of carbon soot. Interestingly, laser ablation technique has both
the highest yield and low productivity and, therefore, a scale-up to a
higher power is costly. Thus, fullerene commercial production is a
challenging task. The world’s first computer controlled fullerene pro-
duction plant is now operational at the MER Corporation, which pio-
neered the first commercial production of fullerene and fullerene
products.
, interest in the “rational” synthesis
Despite the commercialization of C 60
has continued. Scott and coworkers have reported a 12-step synthesis of
C [156]. Despite the low overall yield the important step is that C is
60
60
the only fullerene produced. A molecular polycyclic aromatic precursor
bearing chlorine substituents at key positions forms C when subjected
60
to flash vacuum pyrolysis at 1100 C (Figure 3.37).