Optical studies of conducting polymers in metallic and semiconducting doped states

A study of electromagnetic response was performed at direct current (DC), microwave (6.5 GHz) and optical (15–40,000 cm−1) frequencies on poly (3,4-alkylenedioxythiophenes) (PADOTs) doped with various dopants near the insulator-metal transition together with structural studies (X-ray diffraction). We find these materials to be inhomogeneous, i.e. to have well defined crystalline regions within amorphous media. The transport measurements indicate that the properties of PADOTs vary from insulating to metallic. The metallic state of PADOTs follows trends found previously for the metallic polyaniline and polypyrrole thus confirming the universality of the metallic state for conducting polymers. The features of this state are the positive temperature coefficient of reduced activation energy, negative dielectric response at mw and far infrared frequencies, unique frequency dependence of the dielectric function and optical conductivity. A chain-linked network of metallic grains model is used in the discussion of experimental results. Within this model the transport in metallic conducting polymers is described by three basic electronic processes: intragrain scattering, diffusive spreading within the grains and the resonance hopping between the grains. The latter process is responsible for the negative dielectric responce and very long relaxation times characteristic for the metallic state of the conducting polymers.; To further probe the metallic state of conducting polymers the polarized optical together with microwave and DC conductivity studies on stretch oriented films of polyaniline were performed in directions parallel and perpendicular to the stretch orientation. The low energy (far infrared and microwave) dielectric response was found to be negative for both directions. Together with the positive temperature coefficient of the reduced activation energy observed for parallel and perpendicular directions this proves the metallic state in conducting polymers is three-dimensional.; Also included in this thesis is the study of LiCl treated emeraldine base (EB) form of polyaniline. We find that Li+ dopes EB based on the observation of new electronic states in the bandgap and associated with them spins. However, even at the maximum doping level LiCl doped polyaniline is a strongly localized charged insulator, a conducting polymer in a doped semiconducting state. This conclusion is based on the low DC conductivity, low dielectric constant, rapid decrease of the oscillator strength to zero in the infrared, low magnetic susceptibility dominated by a weak Curie-like component, broad EPR linewidth and predominantly Gaussian lineshape of the EPR signal found for LiCl processed EB. The results for LiCl doped polyaniline are compared to other conjugated polymers in the semiconducting doped state. The strongly localized properties of LiCl doped EB are attributed to the formation of spinless defects such as bipolarons.