P.M. Grant

Polypyrrole: An electrochemically synthesized conducting organic polymer

Abstract The simple, one-step electrooxidation of pyrrole from an organic solvent on a platinum electrode results in the formation of a flexible, metallic polymer. The black films stripped from the electrode are found to be unchanged under ambient storage and thermally stable to temperatures exceeding 300 °C. They are space filling, unlike the fibrillar (CH)x and show considerable disorder. The room temperature conductivity ranges from 10–100 (ohm cm)−1. The thermopower is small and positive, decreasing linearly with decreasing temperature, consistent with p-type metallic conduction. The very small Hall constant is anomalous in both sign and magnitude.

Phototransport effects in polyacetylene, (CH)x

Abstract We have made the first comprehensive measurements of the photovoltaic and photoconductivity effects in polyacetylene in that we have extended the spectral range originally covered by Matsui and Nakamura [17] to include the visible region as well. The photovoltaic experiments were done on Schottky barrier junctions formed between AsF5 lightly-doped p-type trans-(CH)x and a low work function metal, the first junctions of this type to be produced in semiconducting polyacetylene. The observation of a photovoltaic response threshold at 1.48 eV provides the first definitive measurement of the single-particle band gap in trans-(CH)x. In addition, we have found the existence of a peak at 1.35 eV in the photoconductivity spectrum of undoped trans-(CH)x which may be due either to extrinsic sources or to thermal dissociation of a weakly bound Wannier exciton.

Electronic structure of conducting π-electron systems

Abstract We will discuss the application of one-electron band theory to several systems where π-electron delocalization has been experimentally established. We will use polyacetylene as our prototype and extend results on it to related polymers such as (SN)x, polypyrrole and poly(p-phenylene). We will particularly concentrate on those aspects of chemical bonding and symmetry which govern whether a given system will be semiconducting or metallic, and which scale its transport properties.

The preparation and characterization of crystals of the superconducting polymer, (SN)x

Abstract A technique is described for growing (SN) x crystals which have superior electrical properties to those reported previously. The room temperature electrical conductivity has increased, so has the ratio of the room temperature and 4°K resistivities. There is no conductivity maximum in the region 20–30°K and the superconducting transition temperature is higher.

Compositional properties and thermoelectric power of the superconducting ceramic Nd2−xCexCuO4−y

Abstract We report studies on the compositional properties and subsequent thermoelectric power of ceramic samples of superconducting reduced, Ce-doped Nd 2 CuO 4−y and for the undoped parent compound as well. The synthesis of ceramic material with uniform cerium concentration is difficult, but samples can be obtained which are 20–25% superconducting by volume. For such material we find the sign of the Seebeck coefficient to be negative at all temperatures above its superconducting transition at T = 22 K and zero below, consistent with, but not proving purely “n-type” normal state transport and pairing. We also obtain a negative room temperature thermopower for the parent Nd 2 CuO 4−y compound, but with a change to positive sign at lower temperature. We review these findings against thermopower data reported for the “p-type” high-T c compounds where a negative Seebeck coefficient is also observed under certain conditions, and conclude that separation of carriers into specific negative or positive entities in the spirit of the effective mass approximation is in general not possible in copper oxide perovskite superconductors.

The mechanism of arsenic pentafluoride doping of polyacetylene

Abstract Doping of polyacetylene with arsenic pentafluoride is found to involve oxidation of the polymer according to the mechanism: 2(CH)x + 3AsF5 → 2(CH)+x + 2AsF−6 + AsF3. The AsF−6 ions intercalate beteeen the polymer chains in a nonrandom fashion. Oxidation of the polymer is accompanied by replacement of the original interband transition in the visible by a featureless transition extending into the infrared. Treatment of the doped polyacetylene with ammonia destroys the enhanced conductivity and regenerates a visible spectrum similar to that of the undoped polymer but somewhat blue-shifted. Possible explanations are offered for these results in terms of the proposed mechanism of doping.

Photoconductivity and function properties of polyacetylene films

Abstract Photoconductivity and photovoltaic effects of AsF 5 -doped and undoped trans -(CH) x films have been measured at room temperature in the wavelength region from 0.3 to 3.5 μm. The photovoltaic response threshold at 1.48 eV, measured on Schottky barrier junctions with a low work function metal, is interpreted as the single particle band-gap of trans -(CH) x . I-V and C-V characteristics of the junctions indicate that good Schottky barriers are formed between lightly doped p-type (CH) x and low work function metals. Evidence for ∼ 2 × 10 18 cm −3 deep traps in both doped and undoped trans -(CH) x is obtained from analysis of these characteristics.

The preparation and processing of bulk superconducting ceramic Nd 1.85 Ce 0.15 CuO 4−y

We report the development of a reaction technique which yields ceramic Nd 1.85 Ce 0.15 CuO 4−y with macroscopically uniform concentrations of Ce and O. Under appropriate argon annealing conditions, samples exhibiting bulk superconductivity as gauged by resistive transition width and diamagnetic shielding are obtained. In addition. metallic behavior, indicated by positive dp/dT above T c , is obtained for the normal state.

X-ray absorption in polymers☆

Abstract We report on the results of a series of X-ray absorption experiments utilizing synchrotron radiation on the polymeric conductors, brominated (SN) x , and (CH) x doped with AsF 5 . These experiments employ the K-edges of bromine and arsenic at 13.47 keV and 11.89 keV, respectively, for the study of the short range order in the vicinity of the source atom. This information is supplied by the backscattering effect of the surrounding coordination shells on the ejected K-shell electron (EXAFS). In addition, the edge region fine structure is studied to deduce information on the molecular species present (Br 2 orBr 3 − in (SN) x , AsF 5 orAsF 6 − in (CH) x ). Experiments were performed over a range of temperatures between 5 K and room temperature to check the relevant distance parameters and charge transfer for their temperature dependence. Nearest and second nearest neighbor distances are given for the bromine species in (SN) x , while an average Asue5f8F distance for the arsenic species in (CH) x is found to be somewhat larger than for AsF 5 gas molecules. The advantages of X-ray absorption spectroscopy over other methods such as XPS and UPS for the resolution of chemical questions in partially disordered systems are stressed, particularly, the bulk sensitivity of X-ray absorption vis-a-vis the surface escape depth problems of the other techniques.

Comparison of the Physical Properties of Polysulfur Nitride, (SN)x, to Related Organic Polymer Systems and (TTF) (TCNQ)

Abstract Much attention is currently being directed toward the experimental and theoretical study of conducting quasi-one-dimensional systems, of which (TTF) (TCNQ) and KPC (K2Pt(CN)4, Br0.3.3.2H2O)1 are the principal prototypes. All of the known materials derived from these prototypes either undergo a metal-insulator transition at some finite temperature, or become several times more resistive at low temperatures than at room temperature; furthermore, all known organic polymer systems are insulators at all temperatures. By contrast, the highly anisotropic conducting polymer polysulfur nitride, (SN)x, not only becomes continuously less resistive with decreasing temperature, but in fact is a superconductor below 0.3°K.2 It is important to understand why quasi-one-dimensional instabilities are suppressed in (SN)x thus allowing superconductivity to occur. Such know-ledge may serve as a guide in synthesizing new organic metals stable to much lower temperatures.