Electronic structure of polyacetylene: Optical and infrared studies of undoped semiconducting ${\left(\mathrm{CH}\right)}_x$ and heavily doped metallic ${\left(\mathrm{CH}\right)}_x$

The band structure and electronic properties of pure and heavily doped polyacetylene (both as grown and stretch oriented) have been investigated by a combination of optical-absorption and -reflection measurements in the frequency range from the middle ir (0.1 eV) through the visible (4.0 eV). The absorption data are consistent with a direct gap of approximately 1.4 eV in the trans-${\left(\mathrm{CH}\right)}_x$. A Kramers-Kronig analysis of the reflection data has been carried out to obtain $\sigma \left(\omega \right)$ and $\epsilon \left(\omega \right)$. We find that for the undoped semiconducting polymer, the strong transition observed in the visible exhausts the oscillator strength sum rule for $\pi$ electrons consistent with an interband transition. The frequency-dependent conductivity obtained from Kramers-Kronig analysis of the metallic polymer reflection data suggests "interrupted-strand" behavior. Application of effective-medium theory implies an intrinsic dc conductivity for metallic ${\left[\mathrm{CH}{\left(\mathrm{As}{\mathrm{F}}_5\right)}_{0.15}\right]}_x$ of $\sigma >2\mathrm{}\times {10}^4$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The measured dc values have thus far been limited by the low-density fibril morphology.