Nanostructured Forms of Carbon : An Overview

Abstract

Carbon is a remarkable element showing a variety of stable forms ranging from 3D semiconducting diamond to 2D semi-metallic graphite to 1D conducting and semiconducting carbon nanotubes to 0D fullerenes [1]. One distinction between these forms of carbon relates to the many possible configurations of the electronic states of a carbon atom, which is known as the hybridization of atomic orbitals and relates to the bonding of a carbon atom to its nearest neighbors. Carbon is the sixth element of the periodic table and has the lowest atomic number of any element in column IV of the periodic table. Each carbon atom has six electrons which occupy 1s² , 2s², and 2p² atomic orbitals. The 1s² orbital contains two strongly bound core electrons. Four more weakly bound electrons occupy the 2s²2p⁶ valence orbitals. In the crystalline phase, the valence electrons give rise to 2s, 2p_x, 2p_y, and 2p_z orbitals which are important in forming covalent bonds in carbon materials. Since the energy difference between the upper 2p energy levels and the lower 2s level in carbon is small compared with the binding energy of the chemical bonds, the electronic wave functions for these four electrons can readily mix with each other, thereby changing the occupation of the 2s and three 2p atomic orbitals so as to enhance the binding energy of the C atom with its neighboring atoms. The general mixing of 2s and 2p atomic orbitals is called hybridization, whereas the mixing of a single 2s electron with one, two, or three 2p electrons is called spⁿ hybridization with n= 1,2,3. Thus three possible hybridizations occur in carbon: sp, sp² and sp³, while other group IV elements such as Si and Ge exhibit primarily sp³ hybridization. Carbon differs from Si and Ge insofar as carbon does not have inner atomic orbitals, except for the spherical Is orbitals, and the absence of nearby inner orbitals facilitates hybridizations involving only valence s and p orbitals for carbon. The various bonding states are connected with certain structural arrangements, so that sp bonding gives rise to chain structures, sp² bonding to planar structures and sp³ bonding to tetrahedral structures (Fig. 1).