Electronic structure of polyacetylene: Optical and infrared studies of undoped semiconducting(CH)xand heavily doped metallic(CH)x

Abstract

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-${(\mathrm{CH})}_{x}$. A Kramers-Kronig analysis of the reflection data has been carried out to obtain $\ensuremath{\sigma}(\ensuremath{\omega})$ and $\ensuremath{\epsilon}(\ensuremath{\omega})$. We find that for the undoped semiconducting polymer, the strong transition observed in the visible exhausts the oscillator strength sum rule for $\ensuremath{\pi}$ 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 ${[\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{0.15}]}_{x}$ of $\ensuremath{\sigma}>2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The measured dc values have thus far been limited by the low-density fibril morphology.