High conductivity of defect doped polymers in metal–polymer–metal systems

Abstract

We have observed that amorphous films of polyimide PI (R-Oph 2 O) and-co-polymer exhibit high conductivity in Metal-Polymer-Metal (M-P-M) structures and supercurrents in Superconductor-PolymerSuperconductor (S-P-S) structures if the polymer films were prepared at T < 150 °C. In the crystalline state, films of PI (R-Oph 2 O) also become highly conductive in M-P-M structures and have supercurrents in S-P-S structures. We consider that the conductivity effect is connected in some way with the effect of polymer electrification.1 Introduction It is well known that doped π-conjugated bulk polymers such as Polyacetylene show an electrical conductivity like that in a disordered solid [1]. The contributions to the electrical transport are intramolecular transport, intermolecular transport or between polymer fibers. At a small doping level the temperature dependence of conductivity corresponds to hopping transport where the activation energy depends on the doping concentration. At an intermediate doping level the transport obeys the variable range hopping laws at low temperatures similar to the behavior of doped semiconductors with an impurity concentration corresponding to the dielectric side of the metal-insulator transition, whereas at high doping levels the conductivity may be of metallic type with a maximum absolute value of conductivity up to 10 5 S/cm. In regard to non-conjugated polymers there is the common opinion that all of them have good dielectric properties only at electric fields less than the breakdown field (E c ). However that is true for bulk polymers only. The first observation to our knowledge that undoped, non-conjugated thin polymer films have a high conductivity was published in [2]. In this paper polypropylene films of very high molecular weight and a small thickness of about 80 nm were studied. They were placed between a planar electrode and a needle under pressure, required for a good electrical contact. The effect of high conductivity at E << E c was observed at separate points of the surface. Later, the high-conductivity effect was also observed in thin films of atactic polypropylene [3,4], in polyimide films prepared by Langmuir-Blodgett technique [5] and in poly(3,3′-phthalidylidene-4,4′-biphenylene)} [5]. There, it was also confirmed that the conductivity occurs through many channels orientated perpendicular to the polymer surfaces and connecting the two metallic electrodes. This fact explained the strong anisotropy of conductivity and it was the essential difference from doped conjugated polymers. It was also shown that the cross-section of the conductive channels grows with increasing current. The smallest observed value of the cross-section was less than 10 -10 cm 2 . The estimated value of the polymer conductivity was higher than 10 12 S/cm at liquid helium temperatures. Moreover, if the electrodes undergo a transition into the superconducting state, a supercurrent flows through the polymer film [4,6,7]. In a small magnetic field, when H was

Spreading resistance microscopy study of polyamidine thin films

“…In our former experiments with polymers such as imid-siloxane and semicrystalline PI(R-BAPB) at the same film thickness of about one micrometer we did not observe conductivity by AFM if the contact area was only about 100 nm 2 [11]. However, in sandwich constructions where electrodes had macroscopic contact areas the high conductivity was observed.…”

The dependence of polymer conductivity on the work function of metallic electrodes

“…If the electrodes undergo a transition to the superconducting state, a supercurrent flows through the polymer film. In this case, Josephson voltage oscillations have been observed, which behave like typical Josephson contacts [1]. The Josephson effect is a direct proof that indeed a supercurrent flows through the polymer film.…”

“…In this connection, a certain interest arises in the high conductivity which is observed in some polymer films placed between metallic electrodes at film thicknesses below 1-2 μm ( [1] and references therein). If the electrodes undergo a transition to the superconducting state, a supercurrent flows through the polymer film.…”

Field emission from metal/polymer construction

“…Previously (Ref. 7 and references therein) we have reported on an anomalously high channel conductivity of this copolymer when placed between metal electrodes. The structure of the copolymer is shown in Fig.…”