In this section, you will learn the atomic structures and properties of different forms of solid carbon, including diamond, graphite, carbon nanotube and fullerenes.
Solids formed by the same single element can exist in two or more different forms. These forms are called allotropes. Examples are the different forms of solid carbon. The well-known carbon allotropes are diamond and graphite. Even though they are both made of carbon atoms, diamond and graphite display very different properties due to the way the atoms are bonded to each other.
Diamond is the hardest known material on earth. Fig. 2 shows how the carbon atoms in diamond are bonded to each other. In this structure, each carbon atom is bonded to four other carbon atoms, with the nearest neighbouring atoms separated by 0.154 nm. This length is called the bond length. Neighbouring bonds make an angle of 109.5o. This angle is called the bond angle. All the bonds are equal in strength and the structure looks like a giant molecule.
The local configuration of the diamond structure is shown in Fig. 3. Each carbon atom is bonded to four other carbon atoms to form a tetrahedral structure, with specific bond length and bond angle.
Silicon, which is the most important semiconductor material that forms the basis of the electronic industry, also takes on the diamond structure.
In graphite, the carbon atoms form a different structure (Fig. 5). Each carbon is bonded strongly to three carbon atoms and bonded weakly to one carbon atom. The strongly bonded atoms form layers, called graphene, with atoms arranged in a hexagonal structure. The layers coupled through the weaker bonds. This gives the property that the layers can easily slide past each other. As a result, graphite is used as a lubricant and in pencil ¡¥lead¡¦ because of the ease of graphene layers sliding past each other. The weak bonds between layers allow some electrons to move freely. This makes graphite a good conductor of electricity.
There are also other allotropes of carbon that have gained intensive attention recently due to their usefulness in nanotechnology. For example, carbon nanotubes are very small cylinders. The average diameter of a nanotube is 1.2 nm, but its length can reach up to 1 mm! Nanotubes look like a graphene sheet rolled up into a cylinder (Fig. 6). Carbon nanotube was discovered by Sumio Iijima of NEC in 1991.
In 1985, Robert Curl, Harold Kroto and Richard Smalley discovered a new form of carbon molecule called C60. C60, which is a member of a class of carbon molecules called fullerenes, is a molecule consisting of 60 carbons atoms bonded in the form of a hollow football (Fig. 7). The diameter of C60 is of the order of 1 nm. Looking at a football, you will see that some pentagons are joined together with some hexagons in order to achieve the spherical shape. Nature makes use of this idea as well. C60 contains a sheet of linked hexagonal and pentagonal rings folded into a spherical surface. These scientists who discovered C60 shared the Nobel Prize in Chemistry in 1996. Why Chemistry? It is because organic chemistry, which is the chemistry of carbon compounds, forms a large part of Chemistry. Thus the discovery of new forms of big carbon molecules opens new research avenues in Chemistry. After the discovery of C60 , many other fullerenes such as the higher fullerenes from C76 to C84 were also found.