Electronic structure of polyacetylene: Optical and infrared studies of undoped semiconducting<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">CH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>and heavily doped metallic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">CH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

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 $\ensuremath{\sigma}&gt;2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The measured dc values have thus far been limited by the low-density fibril morphology.