T.-C. Chung

Synthesis and properties of chemically coupled poly (thiophene)

We describe the synthesis and physical properties of poly(thiophene) with a molecular weight of approximately 4000 consisting of 46 – 47 thiophene rings (184 – 188 carbons along the backbone). The pristine polymer exhibits an ESR line similar to that of cis-(CH)x; the small number of unpaired spins is consistent with relatively high-quality material. The electrical conductivity can be increased by nearly ten orders of magnitude to values in excess of 10 Ω−1cm−1 upon partial oxidation by AsF5. The magnitude and temperature dependence of the thermopower of the doped polymer (24 mol%) are consistent with metallic behavior. The decrease in conductivity as the temperature is lowered and the magnitude of π at room temperature apparently result from the macroscopic transport being limited by interparticle contacts.

First-order transition to a novel metallic state in [Nay+(CH)−y]x: In situ electron spin resonance during chemical and electrochemical doping

Abstract The onset of the temperature-independent Pauli spin susceptibility takes place in [Na y + (CH) − y ] x as a sharp first-order phase transition at a dopant concentration of approximately 6 mole%. Using either chemical or electrochemical doping techniques, a single symmetric Lorentzian ESR line was observed for the full range of doping and at all temperatures, thereby ruling out the formation of metallic islands. Although the abrupt transition is easily smeared by disorder and/or non-uniform doping, the sharp onset of Pauli susceptibility is an intrinsic feature of the ordered system. The results are interpreted in terms of a first-order phase transition (at y c ∼= 0.06) from a soliton lattice (or a highly correlated soliton liquid) to a novel metallic state in which the finite density of states arises from a lattice of polaron-like distortions with half-filled polaron sub-band in the Peierls gap.

Soliton lattice to metal: A first order phase transition

In-situ measurements of the electron spin resonance of [Na+y (CH)−y]x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.

Doped poly(thiophene): Electron spin resonance determination of the magnetic susceptibility

Abstract E.S.R. studies of poly(thiophene) doped with AsF 5 and iodine are reported. At high AsF 5 concentration (∼ 24 mole % AsF 5 ), we find a temperature-independent Pauli spin susceptibility indicative of a metallic density of states. Measurements on iodine-doped samples in the intermediate regime (below 10 mole % I 3 − ) indicate Curie spin concentration independent of the doping level, but significantly larger than in pristine poly(thiophene). The relaxation of these Curie spins is relatively fast (faster by several orders of magnitude with respect to that of pure poly(thiophene)) and temperature dependent. These features are discussed in terms of charge storage in polarons and bipolarons, and the possibility of formation of a finite density of electronic states in the gap.

Charge Storage In Doped Poly(Thiophene): In-Situ Opto-Electrochemical Spectroscopy

Abstract An in situ study of the absorption spectrum during the electrochemical doping process has been carried out. In the dilute regime, the results are in detailed agreement with charge storage via bipolarons; weakly confined soliton pairs with confinement parameter γ = 0.1-0.2. At the highest doping levels, the data are characteristic of the free carrier absorption expected for a metal.

Solitons or not in polyacetylene (and does it matter

Abstract The results of a series of experiments are described which demonstrate that solitons are the important excitations in trans-(CH)x and that the properties of these non-linear excitations can be directly studied through measurements on trans-(CH)x samples, either during photoexcitation of after doping. The importance of these concepts in the more general context of conducting polymers is addressed. Although the two-fold degenerate ground state of trans-(CH)x is quite special, the relevant concepts can be generalized to confined soliton pairs (bipolarons) and applied to a wide variety of conjugated polymers in which the ground state degeneracy is not present.