Heiko Rommelmann

Frequency Dependent Conductivity of Polyacetylene

Abstract We report on an extensive study of the frequency and temperature dependent conductivity of largely cis- (CH)x trans- (CH)x, and NH3 compensated and iodine doped materials. The results reveal a strongly temperature (T) dependent dc conductivity and weakly T-dependent ac conductivity for largely cis- (CH)x and for NH3 compensated samples, similar to the behavior that is observed in many crystalline and amorphous semiconductors. The trans- (CH)x and lightly iodine doped trans- (CH)x have larger dc and ac conductivity and a strongly T-dependent ac conductivity. These latter results contrast with the usual expectations of variable range hopping and polaronic hopping, but are in good agreement with the predictions of Kivelsons Theory of charge transport via intersoliton electron hopping.

Morphology of polyacetylene and doped polyacetylene

Abstract A detailed study of the morphology of polyacetylene and iodine and arsenic pentafluoride doped polyacetylene has been carried out using scanning electron microscopy and transmission electron microscopy techniques. The results reveal a variety of fibrillar and rod-like morphologies for trans-polyacetylene. The samples retain this morphology upon doping, with, in many cases, a significant increase in the diameter of these structures. Larger increases in diameter were observed for the AsF5 doped samples than in iodine doped samples. Energy dispersive X-ray analysis and back-scattered electron imaging were used to determine dopant distribution. Both iodine and arsenic distributions were uniform across the film surfaces to a resolution of 5000 A. In addition, both dull and shiny sides of the films are shown to have approximately the same dopant concentration. This demonstrates that nonuniformity of the doping occurs, if at all, at a much finer scale. Finally, we have determined that small crystals, presumably arsenic trioxide, form on the surface of AsF5 doped polyacetylene upon even moderate exposure to air.

Frequency dependent conductivity of lightly doped cis-polyacetylene: Role of multiply connected doped trans-polyacetylene regions

Abstract The evolution of the frequency and temperature dependence of the conductivity of cis -polyacetylene [ cis -(CH) x ] lightly doped with iodine is reported. As the dopant concentration is increased, the frequency-dependent contribution to the conductivity remains unchanged while the frequency-independent contribution increases dramatically. These results are inconsistent with the formation of small isolated metallic islands embedded in undoped cis -(CH) x . The data support the idea that even at the lightest dopant levels studied (d.c. conductivity increased by only one order of magnitude), the doped regions are multiply connected, not isolated. Comparison with the temperature- and frequency-dependent conductivities of cis and trans -(CH) x implies that the doped regions are isomerized to the trans configuration and that there may be some structural evolution in the undoped fibril cores, in agreement with earlier spectroscopic and structural studies.

Morphology and Dopant Distribution in Polyacetylene

Abstract We report the results of scanning electron microscopic stadies of the morpology and dopant distribution in polyacetylene, (CH)x. A variety of fibrillar and rod-like morphologies were observed for trans-(CH)x and (CH)x doped with iodine and AsF5. Results show that iodine and atrsenic distributions are uniform across the surface of the films to a resolution of 5000 A, with no significatn difference in the concentration observed on the dull and shiny sides of the film. Hence any inhomogenieties in the dopant distribution must be on a finer scale. The fiberillar morphology was observed through the bulk of a film even after doping with iodine. Swelling of the fibrils is seen upon doping, with larger effects observed in the AsF5 doped samples.

The evolution of polyacetylene morphology upon doping

Abstract Knowledge of the evolution of the fibrillar morphology of polyacetylene with doping is critical for an understanding of the chemistry of the doping process, the mechanisms for charge transport in the polymer and the insulator-metal transition. Earlier scanning electron microscopic (SEM) studies reported conflicting results of either swelling of the fibrils or a gross morphological change. In order to resolve these differences, SEM experiments were carried out to compare the changes in apparent morphology due to the use of various coatings and coating methods used to reduce charging effects, the effects of tearing on the cross sectional morphology and the effects of contrast in imaging the samples. Results definitively show that the fibrillar morphology is retained upon doping.

A new free-standing dopable electrically conducting polyene film

The synthesis and initial characterization of a new free-standing dopable electrically conducting polyene film, poly(hepta-1,6-diyne), are reported.