Showing papers on "Bipolaron published in 1995"
TL;DR: In this article, the changes that occurred in the IR spectra of polypyrrole doped with dodecylsulfate were evaluated as it was progressively reduced electrochemically at constant current.
102 citations
TL;DR: In this paper, the protonation processes of polyaniline (PANI) were examined by means of both UV-visible and fluorescence spectra and in situ UVvisible and ESR spectra in order to understand he mechanism of proton doping in PANI.
Abstract: The protonation processes of polyaniline (PANI) were examined by means of both UV-visible and fluorescence spectra and in situ UV-visible and in situ electron spin resonance (ESR) spectra in order to understand he mechanism of proton doping in PANI. Results obtained from UV-visible and fluorescence spectra of PANI films indicate that the protonation processes take place on the imine segment of the polyemerldine chain consistent with MacDiarmid's suggestions. It is noted that the protonation processes consist of chemical and diffusion processes. The doping processes at the initial stage are controlled by a chemical reaction, whereas the doping processes at the middle period are dominated by a diffusion process. In situ ESR spectra of PANI films have demonstrated that a polaron is formed after protonation processes, but a splitting process of bipolaron into polaron at the measured time region was not observed. For partial protonated PANI, moreover, the fact that variation of absorption value of the peak at 630 nm with doping time supported suggestions in which a parameter of the protonation state for the molecular structure of doped PANI should be considered. © 1995 John Wiley & Sons, Inc.
83 citations
TL;DR: In this article, the effect of γ and electron beam irradiation on conducting polymers such as polyaniline, polypyrrole, and polythiophene was investigated and it was shown that the charge-transport in low doped conducting polymer can be explained by paramagnetic polaron migration along and between the polymer chains.
75 citations
TL;DR: The first direct measurements of a polaron to bipolaron transition in a conjugated polymer using ultraviolet photoelectron spectroscopy were made in this article, which indicated a transition from polaron-to bipolaron charge storage states in these poly(p-phenylenevinylene), or PPV, was carried out in UHV by physical vapor deposition of rubidium atoms.
70 citations
TL;DR: In this article, the authors reexamined the electronic absorption spectra of non-degenerate polymers and found that two transitions are strong among the three expected for a polaron, whereas one transition is strong for a bipolaron.
55 citations
TL;DR: In this article, the geometric and electronic properties of the polaron lattice of polyaniline have been investigated in the presence of polaron-and bipolaron-related bands.
Abstract: We report quantum-chemical studies on the p- and n-type doping of the leucoemeraldine base form of polyaniline. The attention focuses on the characterization of the geometric and electronic structure related to the charge-storage species which are formed upon doping. When considering p-type doping, which leads to the emeraldine salt form, we find in agreement with previous studies that two polaron- or bipolaron-related bands are induced within the original bandgap. Only the lower of these is deep in the gap and is susceptible to be observed in (photo-)absorption experiments below 3.5 eV. Comparison is made between ring- and nitrogen-centered positive polarons which possess nearly identical stabilities; at low doping level, the two types of polarons induce nearly the same electronic structure; at high doping level, the band structure evolves differently but leads to similar calculated values for the intragap electronic transitions. The geometric and electronic structure obtained for the ring-centered polaron lattice on a single emeraldine salt chain indicates that every other ring adopts a semiquinoid-type geometry and carries most of the unpaired electron density. The characteristics of the positive bipolaron are also investigated; the geometrical results are in close agreement with experiment. In the case of n-doping, the negative defects do not correspond to the formation of localized polarons or bipolarons; the geometry modifications are significantly weaker and more extended than in the p-doping case. The corresponding band structures are calculated to differ very little from that of the neutral case.
54 citations
TL;DR: In this paper, the ionization threshold energies of poly(pyridine)-2,5-diyl (PPy) and poly(2,2′-bipyridine−5,5′−diyl) (PBPy) were measured using synchrotron radiation for two kinds of π-conjugated polymers, which exhibit n-type electrically conducting properties.
Abstract: Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy....
50 citations
TL;DR: In this paper, the electronic structure and geometry of polythiophene, using cyclo-dodecathiophene as a model compound, are calculated and their stability is investigated.
45 citations
TL;DR: In this paper, the authors showed that the spatial extension of polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are explicitly taken into account or not.
Abstract: Ab initio self‐consistent field (SCF), second‐ and third‐order Mo/ller–Plesset calculations on the charge‐transfer complexes of one and two alkali atoms with oligothiophenes and oligophenylenes are reported. Complexes up to quaterthiophene and quaterphenyl with Li, Na, and Cs have been investigated. The mono‐alkali complexes are related to polaron and the di‐alkali complexes to bipolaron defects. Extensive basis set investigations have been performed. The quinoid structures which result from the interaction of the alkali atoms with the oligomers are well reproduced by the SCF method, whereas for accurate interaction energies electron correlation effects are very important. Our calculations show that the spatial extension of the polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are explicitly taken into account or not. From previous quantum chemical calculations one finds that, e.g., in the case of oligothiophene dications the bipolaron defects extend over 9–11 thiophene units whereas our calculations under inclusion of the electron donating alkali atoms show that in this case the defects are much more localized. A very crude estimate of the energy balance between two polaron defects and one bipolaron gives about 15 kcal/mol in favor of the bipolaron structure. We also find a rather similar behavior along the alkali series as concerns interaction energies, defect structures, and charge distribution patterns.
40 citations
TL;DR: In this paper, a slightly doped spin gap state in double-chain system was studied in terms of Abelian bosonization method and it was shown that the spin gap formation in terms with the interchain exchange coupling J ⊥ persists only when the two chains are doped with equal number of carriers.
26 citations
TL;DR: In this paper, the formation of moieties of high and low band gap energy along the backbone of polythiophene backbone has been studied for the generation of paramagnetic and diamagnetic conducting states through the control of the redox state.
Abstract: The attachment to polythiophenes of substituents with different electronic properties produces polymeric materials with unusual electrical and optical properties Controlled generation of polarons, dimerized polarons, and bipolarons was achieved through the formation of moieties of high and low band gap energy along the polythiophene backbone The occurrence of such quantum wells decreased the recombination of the polarons in bipolarons as would be observed in symmetrically substituted polythiophenes One of the most interesting consequences of this band-gap engineering is the ability of turning the transport properties of these polymers These novel characteristics make possible the formation of paramagnetic and diamagnetic conducting states through the control of the redox state They may also enhance the polaron population available for singlet exciton conversion in electroluminescent devices
TL;DR: In this article, an energy band gap of 5.25+or 0.15 eV has been determined for MgNb2O6 single crystals grown by the Czochralski method, showing a deep blue color due to the presence of broad optical absorption in the visible spectral region.
Abstract: As-grown MgNb2O6 single crystals grown by the Czochralski method show a deep-blue colour due to the presence of broad optical absorption in the visible spectral region. The absorption of the samples is modified by thermal treatments in air and in vacuum. By heating in air the samples become transparent and by heating in a 1*10-2 mbar vacuum a complex and broad absorption is induced. An energy band gap of 5.25+or-0.15 eV has been determined. Oxidized samples have an absorption edge at 4.20 eV (at 15 K) but some pre-edge absorption is also present. This latter absorption and the associated photoluminescence emission with a maximum at 2.5 eV are attributed to interstitial Nb ions or Nb ions in Mg lattice sites. The optical absorptions induced by X-ray irradiation and vacuum reduction have been discussed in terms of the defects introduced. An absorption band at 2.30 eV induced by X-ray irradiation is attributed to holes trapped in oxygen (O--bound small polarons). The absorption band at 1.69 eV has been attributed to an electron trapped in Nb5+ ions (Nb4+ small polarons). Another absorption observed at 1.20 eV has been ascribed to a Mg-related electron trap centre. The possible presence of bipolaron centres and other electronic traps is discussed. A comparison with the optical absorption bands observed in LiNbO3 crystals has been established.
TL;DR: In this paper, the authors use the translationally invariant Bogolyubov-Tyablikov method to propose a polaron theory and present calculations of autolocalised electron states for different types of interaction.
Abstract: We use the translationally invariant Bogolyubov–Tyablikov method to propose a polaron theory. We present calculations of autolocalised electron states for different types of interaction. The structure of these states is shown to be strongly related to the structure and details of the local phonon spectrum. We calculate this spectrum in the strong-coupling limit. Applications of the large polaron model and possibilities of experimental tests are considered for the strong coupling. We generalise the Bogolyubov–Tyablikov treatment to the strongly coupled bipolaron and give criteria of the stability and formation of the bipolaron states.
TL;DR: It is demonstrated that if the spatial dispersion is allowed, the carrier spectrum in a medium with many-component polarization proves to have an autolocalization band structure and modification of the Bose-condensation condition for bipolarons causes the existence of maximum polaron concentration.
Abstract: We develop the polaron theory taking into account the spatial dispersion of lattice polarizability caused by several phonon branches This allows us to describe new effects which cannot be obtained in the classical polaron theory because of the limitations of its model medium that has only one dispersionless phonon branch interacting with the carrier It is demonstrated that if the spatial dispersion is allowed, the carrier spectrum in a medium with many-component polarization proves to have an autolocalization band structure It occurs due to limitation on the polaron velocity in accordance with Landau's theory of the quantum liquid This results in several effects One of them is ``Cherenkov'' radiation by a sufficiently rapid polaron of the coherent medium vibrations Owing to this radiation there exists a possibility that two polarons can couple under resonance conditions with the medium vibrations It leads to the formation of a two-center bipolaron coupled due to the exchange of real phonons The influence of the autolocalization band structure on the properties of a system with high carrier concentration is also studied This is of interest as far as the properties of high-temperature superconductors are concerned Like any complex oxides they are characterized by many-component polarization The obtained limitation on the velocity of any autolocalized state causes both the existence of maximum polaron concentration (of the order of ${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and modification of the Bose-condensation condition for bipolarons
TL;DR: In this paper, the temperature dependence of the ac conductivity, σac(ω), has a broad structure at all frequencies in compositions with x = 0.1, 0.4 and 0.5 whose peak position is not thermally activated.
Abstract: AC conductivity and dielectric relaxation measurements of the bulk amorphous compositions in the pseudo-binary system (As2S3)1-x(PbS)x (x = 0, 0.1, 0.4 and 0.5) in the frequency range 500 Hz-10 kHz and in the temperature span 180–450 K are reported. The temperature dependence of the ac conductivity, σac(ω), has a broad structure at all frequencies in compositions with x = 0.1, 0.4 and 0.5 whose peak position is not thermally activated. A similar structure was also observed in the data on the dielectric constant, ϵ1, which peaked at a frequency of 1 kHz in the composition with x = 0.5. Analysis of the results using the correlated barrier hopping model revealed that the electronic conduction takes place by single polaron and bipolaron hopping processes at high and low temperatures, respectively, in compositions containing Pb. The microstructure and phase-separation in the glasses containing Pb influence the electrical transport and dielectric dispersion. This study has revealed the possible presence of a phase transformation at around 300 K at a frequency of 1 kHz in the dielectric dispersion behaviour of composition with x = 0.5.
TL;DR: In this article, a model for high Tc superconductivity in which polarons and bipolarons coexist is studied, where bipolarons are assumed to be in a bound or resonant state depending on the total charge carrier density and are allowed to decay into a pair of independent polarons.
TL;DR: In this paper, the optical absorption of polarons and bipolarons in doped conjugated polymers is investigated theoretically in the Pariser-Pan-Pople model with long-range electron-electron (e-e) interactions, electron-lattice coupling, and modulation in transfer integral lifting the degeneracy of the ground state.
Abstract: The optical absorption of polarons and bipolarons in doped conjugated polymers is investigated theoretically. The Pariser-Pan-Pople model in the present paper includes long-range electron-electron (e-e) interactions, electron-lattice coupling of the Su-Schrieffer-Heeger type, and modulation in transfer integral lifting the degeneracy of the ground state. The calculations by mean of the single configuration-interaction method with unrestricted Hartree-Fock molecular orbitals show that a polaron (bipolaron) gives rise to two (one) absorption peaks below the optical gap even in the presence of the e-e interaction. The peak positions depend on the interaction strength.
TL;DR: In this article, a second electron in the phonon bath is taken into account, its interaction with the phonons might overcome the electron-electron repulsion, and the pair of electrons form a stable bound state, called bipolaron.
Abstract: An electron in a polar crystal lattice interacting with the longitudinal optical phonon modes is called a polaron. Since the effective electron-phonon interaction is attractive, it enhances the electron’s effective mass. If a second electron in the phonon bath is taken into account, its interaction with the phonons might overcome the electron-electron repulsion. If so, the pair of electrons forms a stable bound state, called bipolaron. Due to the competition between the opposite forces the bipolaron exists only in a small region of electron-phonon coupling constant and electron-electron repulsion. It is the aim to contribute to the investigation of this stability region.
TL;DR: In this paper, the Feynman model Hamiltonian for a polaron is generalized to the case of a bipolaron in an external magnetic field and the resulting Hamiltonian is exactly diagonalized and the eigenfrequencies and eigenvectors are found.
Abstract: The Feynman model Hamiltonian for a polaron is generalized to the case of a bipolaron in an external magnetic field. The resulting Hamiltonian is exactly diagonalized and the eigenfrequencies and eigenvectors are found. Numerical results are given as function of the magnetic field and limiting results are obtained in the low- and high-magnetic field limit. The time evolution of the electron position coordinates is derived from which we obtain the optical absorption.
TL;DR: In this paper, it was shown that minor changes in the phonon spectrum of the model induce extra elastic forces between the bipolarons, which could break the bipolaronic 2kF CDW ordering.
Abstract: Standard Peierls theory predicts that the wave-vector of the charge density wave (CDW) induced by the electron-phonon coupling in the ground-state of a one-dimensional conductor is twice the Fermi wave-vector 2kF. At moderately large electron-phonon coupling, beyond the TBA (transition by breaking of analyticity), it is known that this CDW becomes an array of interacting bipolarons. In the simplest models, such as the 1D adiabatic Holstein model, the bipolaron interactions are repulsive and the ground state remains a 2kF CDW of equidistant bipolarons. We show that (apparently) minor changes in the phonon spectrum of the model induce extra elastic forces between the bipolarons, which could break the bipolaronic 2kF CDW ordering. This effect is studied in detail in the modified 1D Holstein model, where the optical phonon has a non-zero dispersion which is chosen positive in order that an extra force that appears between the bipolarons be attractive. The Coulomb forces between the bipolarons are initially neglected. An accurate numerical study modelled on the standard analysis of multiphase points confirms that, in a large part of the phase diagram, the CDW ground state separates in two phases, which correspond to two CDW with different 2kF wave-vectors (which are generally commensurate) and thus different electronic densities. These separations, into phases called either 'parent' or 'non-parent', are studied on the basis of the Farey construction of rational numbers. The long-range Coulomb interaction between the charged bipolarons, forbids any macroscopic phase separation in two phases with different electronic densities. The bipolaron structure then has to be a periodic sequence of alternate domains of the two CDW phases on the microscopic scale. The resulting structure is a new CDW with a modulation wave-vector that is not 2kF. We suggest that the aberrant wave-vector observed in the real CDW system (TaSe4)2I could be interpreted along the ideas developed in this paper. Although this compound shares many of the usual properties of quasi-one-dimensional conductors with CDW, the component of the modulation wave-vector 0.085c* in the chain direction is, unusually, quite different from the value 2kF=c* expected from band filling. The CDW in (TaSe4)2I could correspond to a nonstandard bipolaron ordering due to a strong attractive elastic interaction between the bipolarons along the chain competing with the long-range Coulomb forces.
TL;DR: In this paper, the effects of the polymer properties on the electronic structure were modeled using the self-consistent reaction field theory in the multicavity approximation (MCa SCRF), and the electronic properties of the respective polymers were extrapolated from the oligomer data.
Abstract: Four common five-membered heterocycles-pyrrole, phosphole, thiophene, and furan- and their oligomers with the chain length of 2, 4, 6, and 10 units have been studied quantum chemically using the semiempirical PM3 parameterization. The oligomers of pyrrole and phosphole with the homolytically dissociated N-H bond and P-H bond, respectively, and oligomers of thiophene and furan with one electron removed per monomer unit (4n + 2 {pi}-electron bipolaron systems) have also been studied. The electronic properties of the respective polymers were extrapolated from the oligomer data. Bulk polymer effects on the electronic structure were modeled using the self-consistent reaction field theory in the multicavity approximation (MCa SCRF). 48 refs., 3 figs., 6 tabs.
TL;DR: In this article, the energy spectra of a large bipolaron in two dimensions are investigated by a variational procedure in high magnetic fields in the framework of the adiabatic theory.
TL;DR: There can be at least three types of stable planar bipolarons in an ionic medium: (1) a four-lobed large bipolaron, (2) a circularly symmetric largeipolaron, and (3) a small bipolaron.
Abstract: The shape of a large bipolaron is considered for self-trapped carriers confined within a plane that is immersed within a strongly ionic medium. For a wide enough electronic energy band, correlation effects determine a large bipolaron's shape. Then a stable large bipolaron is found in which the two self-trapped carriers occupy mutually perpendicular dumbbell-shaped orbitals. This arrangement gives the self-trapped carriers a planar four-lobed charge density. With progressively narrower electronic bands, the short-range electron-lattice interactions become increasingly important. Then the stable solution shifts to that of a large bipolaron with a circularly symmetric shape. With a still narrower electronic band, the stable solution collapses to that of a small (single-site) bipolaron. Thus, there can be at least three types of stable planar bipolarons in an ionic medium: (1) a four-lobed large bipolaron, (2) a circularly symmetric large bipolaron, and (3) a small bipolaron. The forces between noncircularly symmetric bipolarons depend on their shapes and relative orientations. As a result, large bipolarons' shapes can affect how and if they order in their ground state.
01 Sep 1995
TL;DR: In this article, the stability of large bipolarons in the presence of a static magnetic field is investigated on the basis of Feynman's path-integral approach, although the region of validity of the Jensen-Feynman inequality remains to be investigated for bipolarons.
01 Jan 1995
TL;DR: In this article, the effect of electron correlation on the lattice relaxation of the lowest optically allowed 1Bu state of finite polyenes is investigated, and the results show that in the intermediate to strong electron-electron interaction limit, the most stable geometry of the low-optically accessible excitation is a self-trapped exciton (or a bipolaron), rather than separated solitons.
Abstract: The effect of electron correlation on the lattice relaxation of the lowest optically allowed 1Bu state of finite polyenes is investigated in this paper. We examine the competition between electron–electron interaction and electron‐phonon coupling on the formation of localized lattice distortion in the 1Bu state for finite polyene with chain length up to 30 double bonds, using a number of theoretical models for the electron–electron interaction: short range Hubbard, extended Hubbard, and long‐range Pariser–Parr–Pople models. The results show that in the intermediate to strong electron–electron interaction limit, the most stable geometry of the lowest optically accessible excitation is a self‐trapped exciton (or a bipolaron), rather than separated solitons.
TL;DR: In this article, the ground-state energy of a bipolaron is calculated by the variational method as a function of the Frohlich coupling constants alpha and eta, the ratio of the optic to the static dielectric constant.
Abstract: The regular perturbation method for the strong-coupling bipolaron is developed systematically. The ground-state energy of a bipolaron is calculated by the variational method as a function of the Frohlich coupling constants alpha and eta , the ratio of the optic to the static dielectric constant. It is found that the bipolaron is stable in a broad region of eta . The estimated upper bound of eta turns out to be 0.165.