Metallic State of Conducting Polymers

Background

The electrical conductivities of the intrinsically conducting-poymer systems now range from those typical of insulators (<10-10 S/cm (10-10W-1 cm-1)) to those typical of semiconductors such as silicon (~10-5 S/cm) to those greater than 10+4 S/cm (nearly that of a good metal such as copper, 5 x 105 S/cm). Applications of these polymers, especially polyanilines, have begun to emerge. These include coatings and blends for electrostatic dissipation and electromagnetic-interfernce (EMI) shielding, electromagnetic-radiation absorbers for welding (joining) of plastics, conductive layers for light-emitting polymer devices, and anticorrosion coatings for iron and steel. The common electronic feature of pristine (undoped) conducting polymers is the p-conjugated system, which is formed by the overlap of carbon pz orbitals and alternating carbon-carbon bond lengths. (In some systems, notably polyaniline, nitrogen pz orbitals and C6 rings also are part of the conjugation path.) Figure 1 shows the chemical repeat units of the pristine forms of several families of conducting polymers - that is, trans- and cis-polyacetylene [(CH)x]; polythiophene (PT); polypyrrole (PPy); and the leuco-emeraldine-base (LEB), emeraldine-base (EB), and pernigraniline-base (PNB) forms of polyaniline (PAN).

The conductivities of the pristine electronic polymers are transformed from insulating to conducting through the process of doping, with the conductivity increasing as the doping level increases. Both n-type (electron donating-e.g., Na, K, Li, Ca, tetrabutylammonium) and p-type (electron accepting-e.g., I2, PF6, BF6, Cl, AsF6) dopants have been utilized to induce an insulator-to-conductor transition in electronic polymers. The doping procedures differ from conventional ion implantation used for three-dimensional semiconductors, typically being carried out by exposing the polymer films or powders to vapors or solutions of the dopant, or by electrochemical means. The polymer backbone and dopant ions from new three dimensional structures. There is a rich variety of these structures, with differing structures occuring for different dopant levels and variations in the processing routes, along with a wide range of degrees of local order. For an isolated, one-dimensional metallic chain, localization of charge carriers arises for even weak disorder because of quatum interference due to backscattering of electrons. Localization effects in inhomogeneously disordered (partially crystalline) conducting polymers originates from one-dimensional localization in the disordered regions that connect the relatively ordered regions (or "crystaline islands"). Within this model, conduction electrons are three-dimensionally delocalized in the "crystalline" ordered regions and the conduction electrons must diffuse along electronically isolated chains through the disordered regions where the electrons readily become localized. New concepts of mesoscopic physics and fractal behavior are being applied to understand the anamolous transport and optical properties of these materials.

Experimental Techniques

Recent Advances

Supported in part by NSF

Hopping conductivity of a nearly 1D fractal: A model for conducting polymers
A. N. Samukhin
A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia
Institute of Physics AS CR, Na Slovance 2, 180 40 Prague 8, Czech Republic
V. N. Prigodin
A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
L. Jastrabík
Institute of Physics AS CR, Na Slovance 2, 180 40 Prague 8, Czech Republic
A. J. Epstein
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
(Received 19 May 1998)

We suggest treating a conducting network of oriented polymer chains as an anisotropic fractal whose dimensionality D = 1 +  is close to 1. Percolation on such a fractal is studied within the real space renormalization group of Migdal and Kadanoff. We find that the threshold value and all the critical exponents are strongly nonanalytic functions of  as 0, e.g., the critical exponent of conductivity is –2exp(–1 – 1/). The distribution function for conductivity of finite samples at the percolation threshold is established. It is shown that the central body of the distribution is given by a universal scaling function and only the low-conductivity tail of distribution remains  dependent. Variable range hopping conductivity in the polymer network is studied: both dc conductivity and ac conductivity in the multiple hopping regime are found to obey a quasi-one-dimensional Mott law. The present results are consistent with electrical properties of poorly conducting polymers. ©1998 The American Physical Society
 

Limits for Metallic Conductivity in Conducting Polymers
R. S. Kohlman,1 A. Zibold,2 D. B. Tanner,2 G. G. Ihas,3 T. Ishiguro,4 Y. G. Min,5 A. G. MacDiarmid,5 and A. J. Epstein1,6
1Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
2Department of Physics, University of Florida, Gainesville, Florida 32611
3Center for Ultra Low Temperature Research, University of Florida, Gainesville, Florida 32611
4Department of Physics, Kyoto University, Kyoto 606-01, Japan
5Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323
6Department of Chemistry, The Ohio State University, Columbus, Ohio 43210-1106
(Received 25 July 1996)

The temperature (T) dependent dc conductivity (DC) (down to 20 mK) and dielectric function at optical frequencies (0.002–6 eV) and 6.5 GHz are used to probe the inhomogeneous disorder-driven insulator-metal transition in conducting polymers. A correlation between large low T DC and the presence to low T of a small fraction of the carrier density delocalized with long transport times (> 10-13s) indicates that metallic DC is due to only a small fraction of the charge carriers. The achievable DC for these systems when the entire charge carrier density participates is estimated to surpass that of copper.
 

Charge transport of the mesoscopic metallic state in partially crystalline polyanilines
J. Joo
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
Department of Physics, Korea University, Seoul 136-701, Korea
S. M. Long
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
J. P. Pouget
Laboratoire de Physique des Solides, CNRS URA 02, Université Paris-Sud, 91405 Orsay, France
E. J. Oh and A. G. MacDiarmid
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323
A. J. Epstein
Department of Physics and Department of Chemistry, The Ohio State University, Columbus, Ohio 43210-1106
(Received 21 March 1997; revised 18 November 1997)

Charge transport properties, including temperature-dependent dc conductivity, thermoelectric power, electron paramagnetic resonance,
microwave frequency dielectric constant and conductivity, and electric-field-dependent conductance of partially crystalline ("physically"
cross-linked) HCl-doped polyaniline correlated with x-ray structure studies, demonstrate that charge delocalization in physically cross-linked
polyaniline systems is structurally controlled. Further, we observe a positive dielectric constant at room temperature which increases (to
values ³ 104) with increasing percent crystallinity, the size of crystalline regions, and polymer chain alignment in the disordered regions,
supporting the establishment of mesoscopic metallic regions. We propose an inhomogeneous disorder model for this system in which ordered (crystalline) regions, described by three-dimensional metallic states, are connected through amorphous regions of polymer chains where one-dimensional disorder-induced localization is dominant. We utilize the metallic box, interrupted metallic strands, and Nakhmedov's
phonon-induced delocalization models to account for the temperature dependence of charge transport properties of the various partially
crystalline polyanilines. Analyses for the sample and temperature-dependent electron paramagnetic resonance linewidth and thermoelectric
power are presented.
 

Unusual semimetallic behavior of carbonized ion-implanted polymers
G. Du
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
V. N. Prigodin
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia
A. Burns and J. Joo
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
C. S. Wang
University of Dayton Research Institute, Dayton, Ohio 45469
A. J. Epstein
Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106
(Received 16 December 1997)

We report a comprehensive charge transport study of ion-implanted rigid rod and ladder polymers p-phenylenebenzobisoxazole,
p-phenylenebenzobisthiazole, and benzimidazobenzophenanthroline. The three pristine materials are strong and stable polymers with a room
temperature conductivity sRT ~ 10–12 S/cm. After high dosage ion implantation using Kr+, a carbonized and conducting layer forms on the
surface of the film samples with sRT > 102 S/cm. The experimental results suggest that this carbonized layer is semimetallic with unusual
properties. The observed dc conductivity follows s(T) = s0 + Ds(T), where Ds(T) is weakly temperature dependent and interpreted
within the model of weak localization and electron-electron interaction effects. The model reveals that the interaction effect is three
dimensional for the experimental temperature range (3–300 K), whereas the weak localization effect undergoes a dimensional crossover at ~
60 K from three to two-dimensions with decreasing temperature. The magnetoconductance, thermoelectric power, and microwave dielectric
constant results are all in agreement with this semimetallic model. In addition, all these results consistently point to an enhanced interaction
effect at low temperatures due to the reduced dimensionality of the localization effect. It is concluded that a sp2 rich and three-dimensional
interconnected carbon network reformed upon ion implantation of the densely packed pristine polymers is responsible for the semimetallic
behavior.

Publications

J. Joo, S.M. Long, J.P. Pouget, E.J. Oh, A.G. MacDiarmid, and A.J. Epstein, Charge Transport of the Mesoscopic Metallic State in Partially Crystalline Polyanilines, Physical Review B 57, 9567-9580 (1998).

R.S. Kohlman and A.J. Epstein, Insulator-Metal Transition and Metallic State of Conducting Polymers, in Handbook of Conducting Polymers, Second Edition, edited by T.A. Skotheim, R.L. Elsenbaumer, and J.R. Reynolds (Marcel Dekker, Inc.), Chapter 3, 85-121 (1997).

R.S. Kohlman, A. Zibold, D.B. Tanner, G.G. Ihas, T. Ishiguro, Y.G. Min, A.G. MacDiarmid, and A.J. Epstein, Limits for Metallic Conductivity in Conducting Polymers, Physical Review Letters 78, 3915-3918 (1997).

Created by Darren Gebler.  Maintained by John Rohrbacher. Last updated 5/23/00.