Doped Polyacetylene Research Articles

ESR study of intermediately doped polyacetylene

Motivated by the conjecture of superconductivity by Kivelson and Emery ( Synth. Met. 65 (1994) 249), we have carried out the low-field ESR measurements for intermediately either FeCl 4 − or ClO 4 − doped polyacetylenes. A trace of superconductivity was absent, and as reasons effects of doping phase separation and of disorder are remarked.

Low temperature singularity of the electrical conductivity in potassium doped polyacetylene

We present the temperature dependence of the electrical conductivity of highly potassium doped polyacetylene. While the conductivity σ of these systems usually decreases when T decreases, we show that it is possible to observe an increase of a for decreasing temperatures when T < 7 K. The Sheng model is able to explain the behaviour of the conductivity for T > 10 K but it diverges from our data for T < 10 K. Highly potassium doped polyacetylene behaves like a metal for 4.2 K < T < 10 K and we demonstrate that a model of disordered metal correctly fits our results. This study gives an interpretation for the electrical conductivity behaviour in the whole temperature range 4.2 K – 300 K and shows the importance of the intrinsic disorder of the polymer in the observation of the singularity at low temperature.

Fluctuation-assisted transport in polymers

We consider for the first time the explicit role that multi-mode fluctuations play in determining the transport characteristics of inhomogeneous conductors and insulators. Basing ourselves on rigorous analytical and thermodynamic calculations and recent Monte Carlo simulations, we have been able to identify experimentally the novel exp( aT k ), k>0, behaviour in the low-temperature transport regime of pyrrole-based polymers. The variable range hopping and fluctuation-assisted granular transport regimes can be clearly differentiated from each other at very low temperatures using different pyrrole compounds. The apparent universality of exp[ - ( T o/ T) s ]-type laws can be related to the fact that variable range hopping and variable range tunnelling give rise to similar laws in intermediate temperature regimes. The excitation modes which assist the carriers to cross the barriers at low temperatures can be identified as acoustic phonon modes rather than the charge fluctuation modes invoked in doped polyacetylene.

Positive thermoelectric power of alkali-metal-doped polyacetlene

The experimental results of electrical conductivity and thermoelectric power (TEP) of alkali-metal-doped polyacetylene are presented. The conductivities of (K 0.14CH) x and (Rb 0.17CH) x follow a power law of temperature. The temperature-independent conductivity anistroscopy ratios σ ∥ σ ⊥ for the stretch-oriented samples indicate a quasi-one dimensional charge transport characteristic in heavily doped polyacetylene. Also, we report the positve TEP of alkali-metal-doped polyacetylene. The temperature dependent TEP of K-doped polyacetylene is quasi-linear, which is characteristics to that of diffusive metals. Taking the slope of the linear temperature-dependent TEP, the density of states at the Fermi level for (K 0.14CH) x is estimated to be η(R F)=0.19 states/eV C atom. The positive sign of TEPs for alkali-metal-doped polyacetylene can be understood by the quasi-one dimensional soliton condensed state model. The soliton band is nearly filled, which is possibly due to some compensation from impurities in doped polyacetylene, so that a hole-like charge transport can occur resulting in the positive TEP.

Microwave conductivity of alkali-metal doped polyacetylene

The temperature dependence of microwave conductivity of potassium doped polyacetylene film is reported. The measurement has been performed with 4.5 GHz cylindrical cavity made of copper using the cavity perturbation method. The high density polyacetylene films are synthethized following the modified Shirakawa method. Doping of polyacetylene with potassium has been accomplished by vapor phase method. The temperature dependence is similar to the results of dc conductivity of the p-type doped polyacetylene films.

Hole, electron and energy transfer through bridged systems. VIII. Soliton molecular switching in symmetry-broken Brooker (polymethinecyanine) cations

Brooker ions NH 2-(CH) 2 n+1 -NH + 2 and their derivatives are extremely important dye and photosensitizer materials, and, if the inner odd-polyene chain length can be made sufficiently large, they will display the interesting conductivity properties of polyacetylene derivatives. They could be useful as molecular wires, with the conductivity actually exceeding that of finite-length doped polyacetylene until Peierls distortion sets in and solitons form. We explore the possibility, predicted by AM1 calculations, that, at sufficiently large chain lengths, solitons form and are attracted to localized structures near the chain ends rather than forming a symmetric (C 2v) structure. While non-linear optical responses of the symmetrical Brooker ions are known to be already very large, calculations predict order of magnitude improvements in these properties upon localization. Also, the appearance of a new far-infrared intervalence-type transition is predicted. The isomerization reaction which interconverts the localized soliton structures is a hole transfer process, the passage of the soliton carrying with it the net ionic charge. Details of the kinetics are considered, including their modulation by external electric fields and neighbouring charges, and estimates of the electronic coupling of the diabatic surfaces and of the reorganization energy are made. This suggests that, for Brooker ions with optimal properties, a molecular switch could be constructed with subpicosecond write times and at least microsecond data retention times. The kinetics of this type of switch are fundamentally different from those of other systems which have been considered as possible molecular switches.

Some density-functionalLMTO studies of electronic properties of quasi-one-dimensional systems

Results of first-principles, density-functional, LMTO calculations on quasi-one-dimensional systems are presented. Special emphasis is put on the problems related with studying the properties of real materials by means of electronic structure calculations. As examples we discuss undoped and doped trans polyacetylene, an MX chain compound, CaNiN, and a comparison between polycarbonitrile and hydrogen cyanide. It is argued that single-chain calculations on undoped polyacetylene predict a bond-length alternation, whereas this may be suppressed in multichain calculations. For an MX chain we demonstrate the importance of including the full ligand structure, whereas many properties of CaNiN can be understood as intrinsic to single chains. A comparison between the covalently bonded polycarbonitrile and the hydrogen-bonded hydrogen cyanide reveals differences between delocalized and localized electrons but also inaccuracies in the relative total energies. Finally, results for doped trans polyacetylene indicate relatively large electronic dopant-polymer hopping integrals as well as problems in describing electron transfers for weakly interacting systems when using a local-density approximation. In total it is demonstrated that reliable and useful informations are obtained in most cases, but also that one has to be careful in choosing the idealized system that is to represent the real material and that certain aspects of the local-density approximation should be improved. © 1993 John Wiley & Sons, Inc.

Comparative high resolution NMR studies of n- and p-type doped polyacetylene films

High resolution NMR provides unique abilities to study either properties of the polymer chains or behavior of the doping agent in the conducting polymers. Magic angle spinning proton decoupled 13C NMR was applied to n-type sodium doped, (CHNa y) x (y ≤ 0.3), and p-type iodine doped, [CH(I 3) y] x (y ≤ 0.19), polyacetylene films prepared by Shirakawa's method. Also 2-dimensional nutation spectra of half-integer spin ( I = 3 2 ) quadrupolar 23Na nuclei were measured. All the spectra were studied as a function of the dopant content, y, determined by weight uptake after doping. The films were controlled by measurement of the conductivity which was about 10 and 50 Ω −1 cm −1 for heavily iodine and sodium doped material, respectively. We discuss our results on: (i) formation of the extended polycarbonium ions (charged solitons), (ii) doping induced cis/trans isomerization, (iii) partial compensation of (CHNa) x films by oxygen and moisture, (iv) intersite electron-electron correlations on the polymer chain, and (v) ordering of the sodium ions and occurrence of at least two different crystal phases of the polyacetylene-sodium system.

THE IRAV MODES AROUND CHARGED-SOLITON IN DOPED POLYACETYLENE

With a local potential of site-impurity in the frame work of the SSH model, the Coulomb effect of the dopant ions on the vibrational modes around the charged-soliton in doped poly-acetylene has been studied. The calculated results showed that the Goldstone theorem will be destroyed and the translation mode g1 will be pined by the dopant potential. At the same time, a new even-parity mode g′ 1 has been found. The appearance of g1 and g′1 interpret well the broad peak at 930 cm-1. In addition, other localized modes have also been discussed.

The Metallic State of Conducting Polymers: Microwave Dielectric Response and Optical Conductivity

ABSTRACTRecent advances in processing of polyaniline and polyacetylene have resulted in a new generation of conducting polymers with higher dc conductivities. We present the temperature (T) dependent microwave frequency dielectric constant, dc conductivity, and Kramers-Kronig analysis of conducting polyaniline and polyacetylene. The low temperature dielectric constant, ε, increases with the square of the x-ray crystalline domain length for preparations of HCl protonated polyaniline. For the highest crystalline polyaniline samples, ε increases dramatically with increasing T, supporting formation of three-dimensional (3-D) coupled “mesoscopic” Metallic regions. A “metallic” negative ε is observed for d,1-camphor sulfonie acid doped polyaniline prepared in m-cresol. Optical studies show a linear increase in reflectivity below 7000 cm-1. Below 600 cm-1 the reflectance increases rapidly. Kramers-Kronig analysis of the ir-visible results are presented. Highly conducting polyaniline is shown to have two plasma frequencies, one at ∼ 1.1 eV involving all the conduction band electrons, and one at ∼0.015 eV (120 cm-1) that is suggested to arise from the most delocalized electrons. The concept of inhomogeneous disorder is introduced. The results for polyaniline are compared to those of highly doped polyacetylene which also show metallic negative ε demonstrating the intrinsic metallic nature of the new generation of conducting polymers.

Extended states in disordered doped polyacetylene chains

In this work we study the electronic structure of ordered and disordered distributions of soliton-type defects along large, finite chains of trans-polyacetylene ( trans-PA), as a function of the concentration of defects. The Negative Factor Counting method was used in a tight-binding parameterization with the geometrical data from Austin Method 1 calculations. Our results show the presence of extended (conducting) states at the Fermi level that could explain the semiconductor-metal transition for highly doped trans-polyacetylene, in accordance with the experimentally observed infrared-active vibrational states.

Fluctuation-induced tunneling characteristics of one-dimensional tight-binding models with applications to heavily doped polyacetylene.

We present techniques for obtaining the temperature dependence of the conductance through microscopic defect structures expected to be present in heavily doped, highly conducting samples of polyacetylene. We adopt Sheng's idea of thermally induced voltage fluctuations but replace the phenomenological barrier models used to date with tight-binding models for the defect structures considered. Specifically, we investigate models for chain ends, ${\mathit{sp}}^{3}$ defects, and finite-sized semiconducting regions and obtain both the I-V characteristics of such structures as well as the temperature dependence of the conductance. Results show that chain ends and ${\mathit{sp}}^{3}$ defects are unlikely to be the cause of the activated behavior of the conductivity seen in experiment. Results for finite-sized semiconducting regions show a rich variety of behavior depending on the parameters used including behavior similar to that seen in experiment. Present knowledge of the nature of the heavily doped state is insufficient to favor one type of behavior over another, leaving open the role of semiconducting regions in understanding the activated behavior of the conductance in heavily doped polyacetylene.

Pressure effect on the electrical conductivity of conductor polymer

Abstract Conductivity measurements of Undoped Polyacetylene (CH), and Iodine Doped Polyacetylene (CHIn)x, were carried out as a function of hydrostatic pressure from 1 bar to 3.5 Kbar. Experimental results permit to confm that the conduction in Undoped Polyacetylene (PA), was governed only by the electronic conduction mechanism, while in Iodine Doped Polyacetylene (DPA) two types of carriers (ions and electrons) were observed. This was carried out with the existence of a critical pressure Pc which can be defined as follows: for P < Pc, there is the predominance of ionic conduction (decreasing of the conductivity with P) and for P > Pc, the electronic conduction process becomes more important (increasing conductivity with P).

A density-functional study of undoped and doped trans polyacetylene

The first-principles, density-functional, full-potential, LMTO method for helical polymers is applied to trans polyacetylene, yielding an optimized structure with non-vanishing CC bond-length alternation in good agreement with experiments and in disagreement with another recent density-functional study. It is further demonstrated that the method gives realistic photo-electron spectra without any adjustment, and that information about the energy-loss function can be extracted. Moreover, results of Compton scattering are predicted, and, finally, preliminary results on highly doped polyacetylene are presented.

Conducting films of C60 and C70 by alkali-metal doping

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

Conductivity of doped polyacetylenes and their morphology

Two methods of preparation of mechanically oriented polyacetylene have been applied. The samples were heavily doped with iodine and the temperature dependence of their electrical conductivity was measured. It was concluded that the factors limiting the conductivity measured parallel and perpendicular to the stretching direction were the same. This observation suggests that these factors are related to intrafibrillar properties of polyacetylene samples. Some results of measurements of standard isotropic polyacetylene foils are included for comparison.

Anisotropic conductivity in stretch-oriented polymers measured with coherent microwave transient spectroscopy

Stretch-oriented and doped polyacetylene and polyaniline are characterized using the coherent microwave transient spectroscopy technique. Conductivities parallel and perpendicular to the direction of elongation are determined. The measured orientation dependence of the sample transmissions are observed to follow the predictions of theory.

The one‐dimensional intersoliton electron tunneling conduction in doped polyacetylene: Semiconductor‐metal transition

AbstractThe one‐dimensional intersoliton electron tunneling conduction model in doped polyacetylene, which is a semiconductor‐metal transition mechanism, is proposed. With this model, the observed two transition concentrations (y = 0.001 and y = 0.05) can be estimated correctly. In the lightly doped regime (y &lt; 0.001), the three‐dimensional intersoliton electron hopping conduction is dominant. At the intermediate concentration regime (0.001 &lt; y &lt; 0.05), the electrons (or holes dopending on the dopants) can tunnel through the pinned solitons within a single chain. These tunneling charge carriers contribute to the high conductivity. But these tunneling charge carriers can give no Pauli susceptibility since they are not band‐type carriers. Instead, they are transient carriers tunneling between the two soliton sites. Finally, in the high concentration regime (y &gt; 0.05), the pinned soliton wave function can overlap each other forming a soliton liquid. In this stage, the bond alternation of the trans‐polyacetylene chain vanishes being equivalent to the uniform bond length chain. Therefore, the metallic conductivity and finite Pauli susceptibility are expected consistent with the observed experimental results. However, since this metallic state is formed by the soliton liquid it is not a single particle like metal. The internal vibrational modes and other signals characteristic to the soliton can persist in the metallic polyacetylene.

Air effect on electrical conductivity of oriented doped polyacetylene

Meilleure stabilite a l'air du polyacetylene dope a l'acide sulfurique par rapport a celui dope a l'iode

Mean-field theory for interchain orientational ordering of conjugated polymers.

A generalized anisotropic planar-rotor model on a triangular lattice for interchain orientational ordering of undoped and doped polyacetylene is considered; the effects of various terms on the symmetry and the range of stability of the observed herringbone phases are studied