Donor and acceptor states in lightly doped polyacetylene,(CH)x

Abstract

The results of an experimental study of the infrared absorption of the ${(\mathrm{CH})}_{x}$ system, lightly doped ( 0.1%) with acceptors and donors, are presented. Additional absorptions are observed near 1370 ${\mathrm{cm}}^{\ensuremath{-}1}$ (width \ensuremath{\sim} 50 ${\mathrm{cm}}^{\ensuremath{-}1}$) and 900 ${\mathrm{cm}}^{\ensuremath{-}1}$ (width \ensuremath{\sim} 400 ${\mathrm{cm}}^{\ensuremath{-}1}$) upon doping with iodine, As${\mathrm{F}}_{5}$, and sodium. The two additional absorption maxima appear to be general features of lightly doped ${(\mathrm{CH})}_{x}$ independent of specific dopant or of cis (trans) content. Measurements on stretch-oriented films demonstrate that these absorption maxima are polarized primarily along the polymer chains. The narrow mode at 1370 ${\mathrm{cm}}^{\ensuremath{-}1}$ is attributed to a molecular vibration made ir active by the doping. The broader absorption centered at 900 ${\mathrm{cm}}^{\ensuremath{-}1}$ is discussed in terms of quasi-one-dimensional donor and acceptor bound states along the ${(\mathrm{CH})}_{x}$ chain. We include an experimental determination of the room-temperature dielectric constant in undoped ${(\mathrm{CH})}_{x}$; ${\ensuremath{\epsilon}}_{\ensuremath{\parallel}}\ensuremath{\simeq}10\ensuremath{-}12$, with uncertainty arising from the incomplete orientation of the ${(\mathrm{CH})}_{x}$ films. The bound-state energy is thus consistent with a one-dimensional hydrogenic model in which the Coulomb potential along the chain is reduced by ${\ensuremath{\epsilon}}_{\ensuremath{\parallel}}$. Alternatively viewing semiconducting ${(\mathrm{CH})}_{x}$ as a Peierls distorted one-dimensional metal, we discuss localized domain-wall-like charged donor (acceptor) states induced by charge-transfer doping.