Morphology of iodine-doped polyacetylene

Abstract

Although the synthesis of polyacetylene powders was first carried out many years ago, Ito et al.¹ have only recently obtained films which can be doped with various chemical species, such as I₂, SbF₅ or Na, K, giving p or n type semiconductors. These and other properties, such as its optical bandgap² (E_g ≈ 1.5 eV) have made polyacetylene an interesting material for study^3–6. Its mechanical stretching ability, for example, leads to partially oriented fibrils and an electrical conductivity anisotropy γ_;¶/γ_⊥≃10 has been measured⁷ with γ_¶ reaching 3×10³ Ω⁻¹ cm⁻¹ when highly doped with AsF₅; roughly 12 orders of magnitude higher than the value measured on undoped cis (CH)_x. Polyacetylene is available in two isomeric configurations (cis and trans, Fig. 1). More than 98% cis isomer is obtained if the synthesis is performed at low temperatures (−78 °C) but it is not thermodynamically stable and becomes trans on increasing the temperature, the total transformation occurring when T > 100°C. Any doping process induces a partial cis/trans isomerization⁸. The exact conduction mechanism is poorly known: the localization and nature of the doping species have been widely discussed. Here we investigate, using scanning electron microscopy, the behaviour of the morphology of (CH)_x films when the dopant concentration varies. Starting from the fibrillar structure we have observed the formation of aggregates and a uniform melt of various globular features at high doping levels. Our results yield information on how the electrical conductivity takes place in these non-uniform materials.