Showing papers on "Bipolaron published in 1991"

Optical spectroscopy of field-induced charge in poly(3-hexyl thienylene) metal-insulator-semiconductor structures : evidence for polarons

Abstract: We have fabricated metal-insulator-semiconductor structures with poly(3-hexyl thienylene) as the active semiconductor. Optical-absorption bands due to injected carriers seen near 0.4, 1.80, and 2.16 eV are assigned to the optical transitions of the singly charged polaron, which is expected to be the thermodynamically favored excitation under these experimental conditions. Additional absorption bands at 0.5 and 1.18 eV are assigned to optical transitions of doubly charged bipolarons. We contrast the strong confinement deduced for the polaron (gap states separated by 1.80 eV) with the weak confinement observed for bipolarons (gap states separated by 0.7 eV).

Preparation and spectroscopic characterization of polarons and bipolarons of thiophene oligomers within the channels of pentasil zeolites : the evolution of organic radical ions into conducting polymers

Abstract: Pentasil zeolites such as ZSM-5 and Na-β can be used as supporting matrices in which short-chain oligomers of polythiophene can be prepared, oxidatively doped to the conducting state, stabilized, and finally spectroscopically characterized. For the first time the evolution of the electronic structure of doped polythiophene from monomer to polymer has been observed directly for chain lengths between two and nine. Plots of the electronic absorption band energies for the polaron and bipolaron are found to be linear functions of inverse chain length. These results are extrapolated to infinite chain length to predict the positions of heretofore unobserved electronic transitions of bulk polythiophene. These extrapolations suggest that the lowest energy polaron and bipolaron levels of doped polythiophene are remarkably close in energy, implying that transient formation of polarons from bipolarons in energetically feasible and that this process could play a role in interchain charge hopping in this material.

Large bipolarons in two and three dimensions.

Abstract: Feynman path-integral techniques are used to study the large bipolaron system. A new trial action is introduced, which takes into account a nonzero average distance between the electrons. With this trial action, variational expressions are derived for the free energy for arbitrary electron-phonon interactions and spatial dimensions. For large interelectronic repulsions (twice) the Feynman upper bound for a single polaron is reobtained. Therefore it is possible to discuss the single-polaron--bipolaron transition within the same physical picture for both bipolarons and single polarons. Numerical results are presented for the case of LO-phonon interaction and a Coulombic repulsion between the electrons. For this system a scaling relation between the free energies in two (2D) and three dimensions (3D) is obtained. Bipolaron formation is only possible above a critical value for the coupling constant {alpha}{sub {ital c}}, which is lower in two than in three dimensions (in 2D: {alpha}{sub {ital c}}{approx}2.9 and in 3D: {alpha}{sub {ital c}}{approx}6.8). This indicates more favorable conditions for bipolaron formation in two dimensions, which might be of relevance for the bipolaron model of high-{ital T}{sub {ital c}} superconductivity. The single-polaron--bipolaron transition behaves much like a first-order phase transition.

Characterization of the intrachain charge‐generation mechanism of electronically conductive poly (3‐methylthiophene)

Abstract: The charge carriers and band structure that evolve upon doping of electrochemically grown poly (3‐methylthiophene) (PMeT) films are characterized as a function of film preparation conditions by optical absorption, infrared, and electron spin resonance measurements. Regardless of the solvent (acetonitrile or propylene carbonate) or dopant anion (ClO−4, PF−6, BF−4, or CF3SO−3 ) used in preparing these films, bipolarons are found to play a prominent role in the charge‐generation mechanism of PMeT at doping levels above 2 mol %. At low doping concentrations (<2 mol %), structural disorder effects in the polymer are evident. The amount of structural disorder depends on the nature of the solvent and dopant anion. The binding energy of bipolarons to dopant anions was evaluated and found to be independent of the identity of the dopant anion. Solvent‐dependent shifts in the bipolaron gap states and the binding energy are observed. These results are interpreted in terms of current theories of charge‐carrier product...

Variational calculations of bipolaron binding energies.

Abstract: The energies of bipolaron states in three- and two-dimensional systems are calculated variationally, treating the electron-phonon interaction in the Fr\"ohlich approximation, and separating the relative motion from the center-of-mass motion. The bipolaron is bound if the electron-phonon coupling constant \ensuremath{\alpha} is larger than 6 in three dimensions and 2 in two dimensions, provided the ratio \ensuremath{\eta}=${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$/${\mathrm{\ensuremath{\epsilon}}}_{0}$ is smaller than a critical value which depends on \ensuremath{\alpha}. The theory yields the free-polaron energies as given in the Lee, Low, and Pines approximation for arbitrary values of the electron-phonon coupling constant \ensuremath{\alpha}. For \ensuremath{\alpha} larger than \ensuremath{\sim}10 and \ensuremath{\sim}4 in the three- and two-dimensional cases, respectively, the lowest free-polaron self-energies are obtained in the strong-coupling approach; it is shown that a trial harmonic envelope wave function, localized both in the center-of-mass and relative coordinates, with a coherent phonon state gives a bound localized bipolaron state of lower energy. It is interesting for application to high-${\mathit{T}}_{\mathit{c}}$ superconductivity that the bipolaron bounds more easily in two dimensions than in three, and that the mean value of the pair radius is a few angstroms. Furthermore, bipolaron states obtained when the linear total momentum is conserved have intrinsically high mobility, which is also an important condition to make the bipolaron mechanism consistent with high-temperature superconductivity.

Spin and spinless conductivity in polypyrrole. Evidence for mixed-valence conduction

Abstract: In-situ conductivity of polypyrrole (as tosylate) as a function of oxidative doping level attains a maximum at 3/4 the total oxidation charge and the relevant in-situ ESR signal corresponds to an equal concentration of spin-carrying (polaron) and spinless (bipolaron) species. Results are explained on the basis of mixed-valence conduction.

Electronic properties of conjugated polymers

Abstract: Conjugated polymers such as polyacetylene and polyparaphenylene are semiconductors which can be chemically doped to give very high values of electrical conductivity (up to 1000 Ω cm-1). There has been a considerable theoretical and experimental effort directed towards understanding the way in which dopants add charges to the polymer chains, and the nature of the transport processes that can then occur. The addition of charges to a conjugated chain causes the formation of localised defect states; these may be soliton states for trans-polacetylene, or more generally polaron or bipolaron states. These defect states are considered to control the transport, magnetic and optical properties of these materials.

Contribution of polarons and bipolarons to low-temperature conductivity in doped polypyrrole

Abstract: The resistivity of polypyrrole films, from 300 to 1.3 K, doped with $Cl0_4$, ions at various concentrations to have a different number of polarons and bipolarons, can be better fitted by $T^{-1/3}$ than $T^{-1/4}$. The conduction process in these polyconjugated systems involves ionization from deep trappcd states, having a $T^{-1/2}$ dependence, hopping from localized states, having a $T^{-1/4}$ dependence, and tunneling at very low temperatures, having a $T^{-1}$ dependence. The tendency for the saturation of conductivity at low temperatures is due to the possibility of intersite tunnel percolation in disordered polaronic systems. In highly doped samples the concentration of bipolarons increases.Bipolarons require more activation energy than polarons for hopping. But at higher temperatures the thermal energy can dissociate the bipolarons to single polarons, which leads to bipolaron hopping transport. The temperature dependence of conductivity depends on the effective conjugation length, the ionization energy of the carriers, the concentration of the dopant ions, and the number of polarons and bipolarons present in the system.

Analyses of Spectroelectrochemical Behavior of Polypyrrole Films Using the Nernst Equation “Monomer Unit Model” and Polaron/Bipolaron Model

Abstract: A series of the absorption spectrum changes in polypyrrole films were analyzed using the Nernst equation by two models. One is the "monomer unit model" and the other is the "polaron/bipolaron model." In the first model, formal electrode potentials and the n-values were obtained and usec~ to fit calculated hAbs vs. E curves to the experimental plots at three wavelengths. In the second model, the "apparent" excess chemical potentials were introduced to correct for the large deviations between the Nernst equation and the Nernst plots obtained from the absorption spectra. The calculated hAbs vs. E curves were then fitted to the experimental plots. The "apparent" excess chemical potentials were assigned as corrections for species concentrations. Advantages of the monomer unit model over the polaron/bipolaron model are pointed out in the precise analyses of the spectroelectrochemical behavior of the films.

Preparation and oxidative doping of α,ω-dithienyl polyenes

Abstract: α,ω-Dithienyl polyenes can be prepared from appropriately substituted 2-thienyl or 3-thienyl carbaldehydes or propenals by condensation with either bis-Wittig reagents or bis-phosphonate esters (Horner–Emmons–Wadsworth modifications) containing one or two double bonds. These polyenes, containing either 3,4,5 or 6 conjugated double bonds, can be oxidatively doped with SbCl5, in solution, forming polaronic (radical cation) and bipolaronic (dication) charge states by successive one-electron transfers. In all cases, the bipolaron is the more stable charge state.

Metallic state of polymers with nondegenerate ground states.

Abstract: Recent photoemission experiments that followed the motion of the Fermi energy, and the changes in the density of states, with doping have led to the contention that the metallic state in polypyrrole and poly(3-hexylthiophene) is a polaron lattice. We show that these experiments, and others on these materials, are in agreement with expectations for a bipolaron lattice for the holes added to the chains or, more specifically, a bipolaron band merging with the valence band. Such a situation is stable, in contrast to the polaron lattice.

Electrochemical preparation of conducting poly(trans-1,2-di(2-thienyl)ethylene) and poly(1,4-di(2-thienyl)-1,3-butadiene) and their properties: comparison with polythiophene

Abstract: The electrochemical and optical properties in poly(trans-1,2-di(2-thienyl)ethylene), (trans-PTE), and poly(1,4-di(2-thienyl)-1,3-butadiene) (PTB), in comparison with those of polythiophene (PT), during electrochemical p-type doping have been investigated by cyclic voltammetry and in situ optical-absorption-spectrum measurements. The evolution of localized states is evidenced by the spectral change associated with doping. These results are discussed in terms of polaron and/or bipolaron models. The band gap of trans-PTE and PTB was evaluated to be both about 1.9 eV and smaller than that of PT by about 0.2 eV. This result can be interpreted in terms of a conformation change in the molecular structure, such as a change in the torsion angle between neighbouring thiophene rings. The polaron states in trans-PTE appear in the band gap, about 0.65 eV above the valence band but about 0.6 eV below the conduction band. The polaron states in PTB appear at about 0.7 and 0.6 eV below and above the conduction and valence bands respectively. The top of the valence band of both trans-PTE and PTB is located at a higher-energy state than that of PT by about 0.2 eV. This result satisfactorily explains the stability of p-type dopants in trans-PTE and PTB compared with PT. The bottom of the conduction band of both trans-PTE and PTB is in almost the same position as that of PT and successful n-type doping in trans-PTE and PTB is expected.

Possible (bi)polaron effects in the high-Tc superconductors

Abstract: In the present paper, the theory of the large bipolaron is reviewed and the possibility of bipolaron formation in the high-Tc superconductors is indicated. Operator and path-integral formulations of the large bipolaron problem are compared. In the strong-coupling limit, the effect of non-optimal upperbounds to the single-polaron groundstate energy is emphasized. The fact that the interaction with multiple phonon branches enhances the electron–photon interaction and might result in a larger stability region for bipolaron formation is indicated. Experimental values for the static and high-frequency dielectric constants are used to discuss the relevance of bipolaron formation as a pair-forming mechanism in the high-Tc superconductors.

Stability of the Fröhlich bipolaron in two and three dimensions.

Abstract: The stability of the Fr\"ohlich bipolaron in the strong-coupling limit is studied in both two and three dimensions using the Feynman-Haken path-integral method and the Landau-Pekar formalism. The ground-state energy, the effective mass, and the size of the bipolaron are obtained, and its stability criteria are predicted. The role of Coulomb correlation is also studied.

The bipolaron theory of high-temperature superconductors and its relation to the properties of superfluid 4He

Abstract: Starting from the model of spin-dielectric polarons proposed by the author and others, it is suggested that all the carriers (oxygen 2p holes) form bipolarons, and that they are responsible for the current above and below T c. A comparison is made with superfluid 4He in which only 10% of the helium atoms are believed to condense into the zero-momentum state. A similar assumption for the superconductors can account for the small values of the change of entropy through the transition. The model breaks down for concentrations (x) of carriers so great that Tc decreases with x, for which overlap between the polarons may lead to BCS behaviour with breakup of the bosons above Tc , but not necessarily phonon-induced pairing. It is suggested also that the model is compatible with observations of a Fermi surface.

Inter-impurity interaction and structure of a trapped bipolaron in doped conjugated polymers

Abstract: The effects of an inter-impurity interaction are studied by the extended Peierls-Hubbard model. The Coulomb interactions in the chain are treated by the Hartree-Fock approximation. There are two acceptor-type impurities expressed by long-ranged Coulomb potentials with anisotropic dielectric constants. Stationary positions of the impurities as well as stationary lattice configurations and electronic wave functions are numerically determined self-consistently. Trapping of a bipolaron around impurities is found. The distance between impurities is of the order of the coherence length. Two mid-gap levels, which would exist for a free bipolaron, move to higher energies due to the strong dopant potential. The Coulomb interactions in the chain effectively weaken the strength of the dopant potential. The energy levels shift to lower energies with increasing interaction strengths. Dynamical conductivity (optical absorption) is calculated. The isolated peak in the lower frequencies is associated with the position of the localized level in the energy gap. The consequences for experimental observation of the impurity clustering are discussed.

Phenomenological description of the longitudinal vibrations of the quasi-one-dimensional solid PtCl: calculation of the valence defect frequencies

Abstract: The longitudinal vibrations of the PtCl linear chain and those of various valence defects are described by a one-dimensional model in a first-nearest-neighbour interaction approximation. The analysis is performed as a function of the K2/K1 ratio (K1 and K2 representing the PtClIV-Cl and PtII-Cl force constants, respectively), which is shown to play an important role in the vibrational characteristics of the defects while the infrared frequencies are not strongly dependent on a particular ratio. A coherent description of the frequencies of the modes of the perfect chain and those of the polaronic defects is obtained for K2/K1=0.3. For this ratio the electron polaron and the electron bipolaron give rise to one Raman-active mode while at least two are predicted for the other investigated defects. The shapes of the Raman bands associated with the electron and hole polarons are explained by the chlorine isotopic effect. The vibration associated with the electron bipolaron is predicted to be at about 210 cm-1 where a broad line grows upon photolysis. An elementary calculation based upon a Born-Mayer potential for short-range interactions, adjusted from the interionic distances, leads to force constants of the same magnitude as those adjusted from the experimental frequencies.

Binding Energies, Effective Masses and Screenings Effects of Fröhlich Bipolarons

Abstract: The bipolaron ground state binding energy and the effective masses are calculated self-consistently in a scheme where the electron-phonon interaction is described by the Fr?hlich interaction. We explicitly use the total linear momentum conservation and both two-and three-dimensional systems are considered. We review results for binding energies and show that the bipolaron effective mass increases with the electron-phonon coupling constant ? more rapidly than two free polaron masses. As expected, the increase is greater in two than in three dimensions. We estimate the screening effects due to an electronic or hole density n in a range of values such that nR2b 1 (here Rb is the bipolaron radius). We find that the bipolaron binding energy decreases with n and eventually becomes positive indicating the existence of a metastable bipolaron state. Finally we discuss the possible connections between our results and high Tc superconductivity.

X-Ray photoelectron spectroscopy of new soluble polyaniline perchlorates: evidence for the coexistence of polarons and bipolarons

Abstract: An X-ray photoelectron spectroscopic (XPS) study was carried out on the following soluble polyaniline perchlorates and polyaniline base: (A) a perchlorate prepared in a single step by the use of copper(II) perchlorate as an oxidative coupling agent; (B) a polyemeraldine base obtained by treating perchlorate A; and (C) a perchlorate obtained by the protonation of the polyemeraldine base. The N 1s XPS peak of perchlorate C exhibited a more intense envelope on the higher binding energy side than the N 1s peak of perchlorate A. The difference spectrum, with a peak maximum at 402.0 eV, can be assigned to the bipolaron nitrogen in perchlorate C, when referred to the previous spectroscopic study of the two perchlorates. The area of the bipolaron peak was 5% of the whole peak of perchlorate C, although the charged nitrogen was 30% of the total nitrogen. Polarons are the major charge carriers in the two conducting polyaniline perchlorates, and perchlorate C contains bipolarons (in 5% of the total nitrogen) that coexist with polarons.

Magnetic properties of phases possessing block type structures in the Nb2O5-2x system, with O<or=x<or=0.083

Abstract: The magnetic susceptibility of phases with block type structures in the Nb2O5-2x system, with 0 or=0.05 reveals localized electrons, as suggested earlier.

Direct measurements of bipolaron-band development in doped polypyrrole with inverse photoemission

Abstract: Inverse-photoemission spectroscopy has been used to investigate dopant-dependent electronic properties of polypyrrole (PPy) films doped with ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$ (tetrafluoroborate) or (PSS${)}^{\mathrm{\ensuremath{-}}}$ [poly(4-styrenesulfonate)]. The results show that the bonding and antibonding bipolaron bands appear at \ensuremath{\sim}1.0 and \ensuremath{\sim}2.6 eV above the valence-band maximum for heavily doped PPy films (\ensuremath{\sim}1 dopant per 3 pyrrole units). The bandwidth of each of these bipolaron bands is \ensuremath{\sim}0.4 eV and the band gap for heavily doped films is \ensuremath{\sim}3.6 eV. The bipolaron bands move toward the band edges and the gap shrinks with decreasing dopant concentration. Annealing of heavily doped films at 300 \ifmmode^\circ\else\textdegree\fi{}C leads to dopant loss from the surface. This effect is less pronounced for the larger polymeric anions such as (PSS${)}^{\mathrm{\ensuremath{-}}}$ than for the smaller counterions such as ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$, suggesting that the former is more stable or has a greater thermal activation energy for bipolaron dissociation into single polarons.

Separation of neutral-to-polaron and polaron-to-bipolaron redox events in alkoxy substituted di-2-thienylphenylene polymers

Abstract: In situ electron paramagnetic resonance (EPR)/electrochemistry methods have been used to elucidate the potential dependence of the presence of polaron and bipolaron charge carriers in poly(di-2-thienyl-2,5-diheptoxyphenylene).