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The Electronic Band -Gap of Polycrystalline Diamond and Tetrahedral Amorphous Carbon by EELS in a STEM.

Good quality polycrystalline diamond is transparent and has an optical band-gap very close to that of single crystal diamond, 5.5 eV. Tetrahedral amorphous carbon (ta-C) is a form of amorphous carbon which can have up to 90 % sp3 type bonds. It is the form of amorphous carbon, which is closest to having a true diamond-like structure. The optical band-gap of ta-C is however very different from that of polycrystalline diamond, typically in the range 2-3 eV. This difference in the optical bandgap has been ascribed to the electronic states formed by the residual graphitic bonding. The π − π* states at the graphitic sites effectively form tail states on the valance and conduction band edges created by the diamond-like bonding. These tail states extend deep into the band-gap, which would exist if there were only (100 %) diamond-like bonds. The poor electronic properties obtained thus far for ta-C are now accepted as being associated with localised conduction in the π − π* tail states.