3.28 Sspd_chapter 1_part 11_solid state of matter_crystalline nature  (Page 3/5)

1.11.2.2._GRAPHITE_AND_GRAPHENE

[ ‘Graphene: Status and Prospects’, A.K. Gelen, Science, Vol 324, 19 ^th June 2009, 1530-1534;

‘Carbon Woderland’, Andre K. Geim and Philby Kim, Scientific American, April 2008, 90-97]

The two other alltropic forms of Carbon are Graphite and Buckminsterfullerene[Appendix XXXXV]. The crystal structure of these two allotropes are shown in Figure 1.57. The comparative study of these three alltropic forms of carbon is shown in Table 1.22.

In Graphite each layer has hexagonal arrangement of carbon atoms. As seen in Figure 1.57, each carbon atom has three neighbouring carbon atoms covalently bonded. This arrangement leaves 1 electron unbounded hence free to be released through out the lattice. This makes Graphite a good conductor and it has a metallic luster. There is a weak Van der Waals induced bonding between the layers as already discussed in Bonding chapter. Hence layers can move past one another and flak easily. They find applications in lubricants and pencil lead.

Graphite is a 3D bulk material which consists of stacks of 2D one atom hexagonal rings of Carbon sheets. In recent years with the advent of nanotechnology , it has been possible to separate out individual sheets and experiment with it. The flat sheet of hexagonal rings of Carbon is known as GRAPHENE. Wrapped up graphene is known as fullerenes. When it is wrapped up in honey comb cylinder like structure it is called carbon nano-tubes and when it is wrapped as soccer ball shaped as in Figure 1.57 then it is called bucky ball.

This 1 atom thick 2D grapheme is a wonder material. It is the strongest material- stronger than steel. The orderly arrangement of hexagonal rings is repeated through out the sheet with remarkable regularity as a result even at room temperature of 300K electron’s mean free path is in micrometer. This implies very few and far between scattering. Hence under the influence of electric field, electrons behave like mass-less Dirac quasi-particles on which relativistic quantum mechanics or Quantum Electrodynamics applies. Electron travels at (1/300) of speed of light. This is in contrast to electrons in 3D semiconductor where mass is near that in free space and mobility and drift velocity is much lower. These useful properties has opened immense possibilities for grapheme based electronics. Ballistic FETs are emerging which are suitable for 100GHz and above application. Graphene has given a new lease of life to Moore’s Law of microminiaturization. It will enable the scaling down right up to 10 nm.

Under normal conditions graphite crystallizes out but under excessive pressure condition diamond is favoured. Once the diamond has crystallized it remains stable under normal temperature and pressure. Deep in Earth’s vegetation remains are transformed into diamonds and coal over eons. This is the source of natural diamond.

Synthetic diamond can be obtained in the following manner. Graphite is dissolved in molten cobalt or nickel and the mixture is compressed by 60,000 bar pressure at 1600K. Less than 1 mm wide diamonds can be synthesized in this way. It is widely used in industrial applications for cutting and grinding tools.