Medium-range order in pyrolyzed carbon films: Structural evidence related to metal-insulator transition

Structural properties of carbon films prepared by the pyrolysis of tetra chloro phthalic anhydride are investigated in an attempt to correlate it with the metal-insulator transition observed earlier as a function of deposition temperature, from electrical transport studies. Anomalous features are observed which evolve as the pyrolysis temperature is varied from 700 to 900°C. This includes prepeaks in the x-ray diffraction pattern in the region less than 1 ${\AA}^{-1}$ providing evidence for medium range order, and a broad shoulder around 1200 $cm^{-1}$ in the Raman spectra, which are seen in addition to the usually observed peaks in amorphous carbon films. A simple model is proposed according to which presence of "polymeric domains" containing chains of carbon atoms with single and double bond alternation (-C=C-), in a predominantly aromatic ring structure, could give rise to the anomalous features