CNTs had been discovered 30 years earlier, but had not been fully appreciated at that time. In the late 1950s, Roger Bacon found a strange new carbon fiber while studying carbon under conditions near its triple point. He observed straight, hollow tubes of carbon that appeared to consist of graphitic layers of carbon separated by the same spacing as the planar layers of graphite.
It is interesting to note that the observation of hollow fibers was documented again in 1952 by Radushkevich and Lukyanovich, and then by Swedish scientists Hillert and Lange as early as 1958.
After transmission electron microscopy (TEM) had been invented, similar results were also reported by Oberlin and Endo et al. in 1976.
In 1991, after the discovery and verification of the fullerenes, Sumio Iijima observed multiwall nanotubes formed in a carbon arc discharge. The first observation of the MWCNTs was credited to Iijima.
It was in 1993 that Iijima and Donald Bethune found SWCNTs known as buckytubes.
Carbon is an interesting chemical element because it is the 4th most abundant element in the universe by mass, after hydrogen, helium, and oxygen. It has four valence electrons. Carbon can form different structures with entirely different properties using these valence electrons. An unbonded carbon atom has the electronic structure 1s2 2s2 2p2. Pure carbon can have several allotropes. This is because the four valence electrons can make different types of bonds with other carbon atoms. The versatility of carbon can be obtained from its ability of hybridization among sp1, sp2 and sp3. Three-well known carbon allotropes are diamond, graphite, and amorphous carbon.
To form covalent bonds, one of the 2s electrons is promoted to 2p, and the orbitals hybridize in one of three ways. The first is a hybridization of the 2s electron with one of the 2p electrons, producing two sp1 orbitals that are separated by an angle of 180○. This bond is linear and is the one in acetylene, C2H2. The second is a hybridization of the 2s electron with two of the 2p electrons, forming three sp2 orbitals which are separated by 120○ and which are coplanar. This is the structure of graphite, which is comprised of s bonds between the in-plane carbon atoms, which are arranged hexagonally. The in-plane bonding allows graphite to conduct electricity effectively along the planar axes. In the third hybridization, sp3, which results in the diamond structure, one 2s electron hybridizes with the three 2p orbitals and yields the characteristically tetrahedral sp3 bond.
The zero-dimensional fullerenes buckyball was discovered in spectroscopy data in 1985. Later, in 1991, Sumio Iijima discovered a one-dimensional nanotube. These structures are recognized as a different phase from graphite, even though such structures maintain the architecture of sp2.


Five different carbon allotropes: (a) diamond, (b) graphite, (c) amorphous carbon, (d) C60 fullerene and (e) single walled carbon nanotube.


By: MSc Halla Thamer Mohammed