We review and compare electronic transport in different types of conducting polymer: conjugated organic polymers, the inorganic polymer polysulphur nitride, alkali-metal fulleride polymers, and carbon nanotubes. In each case, the transport properties show some unusual features compared to conventional metals. In conjugated organic conducting polymers, electronic transport shows a systematic pattern involving both metallic and non-metallic character. We discuss the physical conduction processes that can account for this behaviour. Key roles are played by the metal-semiconductor transition as the doping level is varied, and by the limited size of crystalline regions in the polymers, which gives rise to heterogeneous conduction. Transport data provide indirect evidence that the intrinsic conductivity of doped crystalline polyacetylene, in the absence of disordered regions, is higher than that of copper at room temperature; this high conductivity is consistent with the expected suppression of backscattering in highly anisotropic ('quasi-one-dimensional') metallic conduction. Bundles of single-wall carbon nanotubes have also been found to exhibit metallic behaviour. The temperature dependence of the conductivity of bulk samples is remarkably similar to the pattern characteristic of organic conducting polymers, typically showing a crossover from metallic to non-metallic behaviour as temperature decreases. Quantized one-dimensional conductance and other quantum effects are seen in individual nanotubes.

https://iopscience.iop.org/article/10.1088/0034-4885/64/1/201/pdf

Electronic transport properties of conducting polymers and carbon nanotubes

The design, synthesis, characterization, and understanding of new molecular and macro-molecular substances with “metal-like” electrical properties represents an active research area at the interface of chemistry, physics, and materials science. An important, long-range goal in this field of “materials by design” is to construct supermolecular assemblies which exhibit preordained collective phenomena by virtue of “engineered” interactions between molecular building blocks. In this review, such a class of designed materials is discussed which, in addition, bridges the gap between molecular and polymeric conductors: assemblies of electrically conductive metallomacrocycles. It is seen that efforts to rationally construct stacked metal-like molecular arrays lead logically to structure-enforced macromolecular assemblies of covalently linked molecular subunits. Typical building blocks are robust, chemically versatile metallophthalocyanines. The electrical optical, and magnetic properties of these metallomacrocyclic assemblies and the fragments thereof, provide fundamental information on the connections between local atomic-scale architecture, electronic structure, and the macroscopic collective properties of the bulk solid.

Interfaces between Molecular and Polymeric “Metals”: Electrically Conductive, Structure‐Enforced Assemblies of Metallomacrocycles

We consider the possibility for a pairing in a two-dimensional (2D) repulsive Fermi liquid due to the singularity in the scattering amplitude \ensuremath{\Gamma}(q) at the momentum transfer q\ensuremath{\le}2${\mathit{p}}_{\mathit{F}}$ (Kohn-Luttinger effect). A common belief based on perturbative calculations up to second order in the s-wave scattering amplitude is that this effect is absent in two dimensions. I show that this is not the case. For an arbitrary Fermi liquid, \ensuremath{\Gamma}(q) is found to have a singular part, ${\mathrm{\ensuremath{\Gamma}}}^{\mathrm{sing}}$(q)\ensuremath{\sim} \ensuremath{\surd}1-${\mathit{q}}^{2}$/(2${\mathit{p}}_{\mathit{F}}$${)}^{2}$ , for q\ensuremath{\le}2${\mathit{p}}_{\mathit{F}}$. For large 2D orbital momentum l, this term gives a dominant attractive contribution to the scattering amplitude and leads to a pairing instability in a 2D Fermi liquid with arbitrary short-range repulsion. In the dilute limit, numerical studies show that the effect survives down to l=1 and gives rise to a p-wave pairing. The relevance of these results to experiments on $^{3}\mathrm{He}$ adsorbed on the free surface of $^{4}\mathrm{He}$ is discussed.

Kohn-Luttinger effect and the instability of a two-dimensional repulsive Fermi liquid at T =0

Nuclear-magnetic-resonance measurements of the magnetization of $^{3}\mathrm{He}$ on thin $^{3}\mathrm{\ensuremath{-}}^{4}$He mixture films show discrete structure as a function of $^{3}\mathrm{He}$ coverage over the range 0.0055\ensuremath{\le}${d}_{3}$\ensuremath{\le}4 layers at a $^{4}\mathrm{He}$ coverage of 44 \ensuremath{\mu}mol/${\mathrm{m}}^{2}$ for 30\ensuremath{\le}T\ensuremath{\le}250 mK in a weakly polarizing 2-T field. A steplike doubling of the magnetization at ${d}_{3}$\ensuremath{\approxeq}0.8 layers and a second less pronounced step at ${d}_{3}$\ensuremath{\approxeq}1.5 layers are ascribed to the population of higher energy levels which evolve as the $^{3}\mathrm{He}$ thickens. The magnetization as a function of temperature near the first step is fit by a two-level model with an energy gap ${e}_{12}$\ensuremath{\approxeq}1.8 K at the step.

NMR observation of steps in the magnetization of 3He in thin 3He-4He mixture films.

3He binding energy in thin helium-mixture films.

Nuclear magnetic susceptibility measurements of 3He

The electrical conductivity \ensuremath{\sigma}(T) in poly(p-phenlyene vinylene) (PPV) heavily doped with ${\mathrm{AsF}}_{5}$ has been measured as a function of temperature for 4.2lTl300 K. Films which can be oriented were prepared by a soluble-polymer precursor route and were investigated in the as-doped state and after aging by exposure to air. PPV with \ensuremath{\sigma}(300 K)\ensuremath{\sim}100 (\ensuremath{\Omega} cm${)}^{\mathrm{\ensuremath{-}}1}$ has a temperature dependence given by \ensuremath{\sigma}(T)\ensuremath{\sim}${\ensuremath{\sigma}}_{0}$ exp[-(${T}_{0}$/T${)}^{\ensuremath{\alpha}}$], (1/4\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le} 1) / 2 , independent of the sample history. For samples with higher room-temperature conductivity, the temperature dependence for T100 K is well represented by a power law of the form \ensuremath{\sigma}=${a}_{0}$+${\mathrm{bT}}^{1/2}$; ${a}_{0}$ generally decreases with decreasing \ensuremath{\sigma}(300 K) and becomes negative for \ensuremath{\sigma}(300 K)\ensuremath{\sim}300 (\ensuremath{\Omega} cm${)}^{\mathrm{\ensuremath{-}}1}$. The latter results are consistent with a metal-insulator transition.

Temperature dependence of the electrical conductivity of AsF5-doped poly(p-phenylene vinylene).

We present measurements of the change in the square of the third sound speed, δC3 2, following the addition of small amounts of 3He, d3, to a 4He film of thickness d4 adsorbed on Nuclepore. For d4=2.8 layers and 0≤d3≤.09 layers δC3 2 is a linear function of d3. For d3=.09 layers and 2.8≤d4≤5.3 layers δC3 2/d3 has an unexpected minimum as a function of d4. Comparisons to δC3 2 on glass will be made.

Third Sound in Dilute3He-4He Mixture Films Adsorbed on Nuclepore

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Nuclear magnetic susceptibility measurements of 3He

Temperature dependence of the electrical conductivity of AsF5-doped poly(p-phenylene vinylene).

Third Sound in Dilute3He-4He Mixture Films Adsorbed on Nuclepore