Calculation of Band Structure of Doped Polyacetylene

The purpose of this paper is to calculate the conduction and valence band structures of pure and Li-doped (CH)_x by using LCAO-SCF ab initio Crystal Orbital (CO) method, to discuss the effect of Li-doping on the band structures, and the changes of charge transfer in carbon chain, so as to exulain the insulator-conductor transition and the conducting mechanism of (CH)_x.
The two unit cells C₆H₆Li and C₄H₄Li are used to represent doping levels of 16.7% and 25%. The calculated results indicate that the undoped (CH)_x is an insulator near to semi-conductors. For the doped C₆H₆Li and C₄H₄Li systems, the conduction bands rise while the widths decrease, which cause the decreasing of the gaps between conduction and valence bands (reduce to 0.597 eV and 0.460 eV respectively). The results show that the two doped systems being calculated are all conductors. It is evident that doping brought about the transformation from insulator to conductor. After being doped with Li, the energies of valence bands of (CH)_x increase, while those of conduction bands keep almost unchanged, and which cause the decrease of the energy gaps. Several bands which near the conduction and valence bands are to some degree affected by dopants, while the others are much less affected.
The results show that the charge distribution in carbon atoms is well-distributed and there is no charge transfer in carbon chain in undoped systems. While in Li-doped ones, unequal charge distribution in carbon atoms brings about the charged domain. And the charged domain on carbon chain of (CH)_x caused by doping with Li produce charged soliton and increase the concentration of the charged soliton which results in the high conductivity of doped polyacetylene.