Knowledge Bank

Studies of electric resistivities, thermoelectric properties, and infrared absorption lead to a consistent picture for the electronic processes in condensed ring systems. Small molecules (several rings) have a large gap between the occupied and the first excited electronic level (-1-2 eV) and are practically insulators at room temperature. As the size of the molecules grows the absorption band corresponding to the lowest electronic transition moves progressively into the infrared and a weak conductivity becomes detectable. This is caused by the decrease in the numbers of intermolecular barriers and by the increasing heat activation of electrons from the lower into the conduction band; thus larger condensed molecules are essentially intrinsic semiconductors. In treating the organic substances to temperatures above $600\circ C$, the molecules are further condensed; but on top of that they gradually lose more and more peripheral atoms (hydrogen, etc.). Increasing numbers of free electrons are detected in such materials by thermoelectric studies; the electric resistance decreases sharply; furthermore, an increasing continous absorption all through the infrared is observed. Excess electrons responsible for these effects probably are being supplied by the peripheral carbon atoms, their valence electrons becoming mobile after removal of the hydrogens. Consequently, calcined cokes are extrinsic semiconductors with an energy gap of the order of 0.1 eV (for crystallites size 30-40 \AA). Further increase in the size of molecular planes in process of graphitization leads to a further decrease of the gap. An analysis of the dependence of resistivity on temperature gives an estimated gap of about 0.01 eV for crystallites 1000-5000-{\AA} size. The existence of a measurable energy gap for crystals even that large is probably due to deviations of the lattice from hexagonal symmetry. For a perfect hexagonal system, the energy gap should disappear, thus giving the semi-metallic properties to large graphite crystals.