Bipolaron Transition Research Articles

Polymer incorporated with graphene oxide composite for high electrochemical performance

In this present work the Polyaniline/Graphene Oxide composite (PANI/GO) synthesized by chemical polymerization. XRD pattern shows crystal plane (200) and (002) reveals the presence of GO in the PANI/GO composite, UV-absorption spectra exhibits three distinguished peaks at 235 nm is corresponding to the π to π* transition of C–C in GO, 354 nm and 642 nm indexed with bi-polaronic transition of PANI in red shift, surface morphology revels the PANI homogenously dispersed on the surface of GO. CV curve indicates the PANI/GO composite electrode current rate and surface area 5 fold higher than the PANI electrode, which is suggested that the PANI/GO composite electrode is highly suitable for energy storage devices. EIS spectra show the semi-circle arc, and Rct value of PANI is found to be 28.61 Ω and PANI/GO composite electrode found to be 16.15 Ω. These results represent the PANI/GO composite electrode having more storage capacitive than the PANI, it shows high electrical conductivity, and also enhances the electrochemical performance.

The incorporation of polypyrrole (PPy) in CS/PVA composite films to enhance the structural, optical, and the electrical conductivity

AbstractUsing the solution casting method, a blend of polyvinyl alcohol/chitosan (PVA/Cs) blend was prepared with different amounts of polypyrrole (PPy) to enhance the structural and electrical conductivity of the films. The miscibility and interaction between PVA/Cs were confirmed by FT-IR measurements by shifting and disappearing some bands suggesting that the PVA functional group interacted with every other functional group on the Cs side chain. For PVA/Cs/PPy, some band intensities varied, indicating the interaction between PPy and PVA/Cs. XRD showed that the addition of PPy to the blend leads to a noticeable decrease in the intensity of the diffraction peak at 2θ = 20.0° confirming the interactions have occurred between the PVA/Cs mixture and PPy. The UV–visible spectra indicate that increasing the amounts of PPy leads to a dramatic decrease in the energy band gap and an increase in the Urbach energy due to the creation of new energy levels that emerged between conduction and valance bands. After PPy was introduced, the polaron and bipolaron transition peaks at 416, 465, and 560 nm become apparent in the PL spectrum. The dielectric and the electrical properties were reported. The values of ε′ and ε′′ were stronger at low frequencies confirming the possibilities of interface polarization processes. The tan δ behavior diagrams have exhibited one peak trend in all samples shift toward higher frequencies as the temperature and PPy rise. The presence of the peaks was explained based on the electrical conductivity mechanism and dielectric behavior. The Cole–Cole plot displays a half one semicircle shape that explains the absence of contact effects. This semicircle expands as both temperature and PPy increase.

Charge-Transfer Magnetoelectrics of Polymeric Multiferroics

The renaissance of multiferroics has yielded a deeper understanding of magneto-electric coupling of inorganic single-phase multiferroics and composites. Here, we report charge-transfer polymeric multiferroics, which exhibit external field-controlled magnetic, ferroelectric, and microwave response, as well as magneto-dielectric coupling. The charge-transfer-controlled ferroic properties result from the magnetic field-tunable triplet exciton which has been validated by the dynamic polaron-bipolaron transition model. In addition, the temperature-dependent dielectric discontinuity and electric-field-dependent polarization confirms room temperature ferroelectricity of crystalline charge-transfer polymeric multiferroics due to the triplet exciton, which allows the tunability of polarization by the photoexcitation.

Kinetic and Mechanistic Aspects of a Poly(o-toluidine)-Modified Gold Electrode. 1. Simultaneous Cyclic Spectroelectrochemistry and Electrogravimetry Studies in H2SO4 Solutions

Electrodeposited poly(o-toluidine) (POT) on gold electrodes was investigated in a 0.5 M H2SO4 aqueous solution using cyclic electrogravimetry with in situ vis–NIR spectroscopy. This coupling of different techniques allows the electrical, color, and mass changes during the electrochemical reactions of these polymers to be correlated. Therefore, this is a powerful tool to obtain valuable information on the physical models of polymer films and their electrochemical properties. The accurate analysis of the results from these techniques showed the contribution of three different redox transitions (leucoemeraldine–polaron transition, polaron–bipolaron transition, and bipolaron–pernigraniline transition) and the participation of cation (hydrated proton), anion (bisulfate ion), and free solvent (water) transfers in these redox transitions. Moreover, the contributions of each redox transition to the current, absorbance derivative, and mass derivative were separated.

Kinetic and Mechanistic Aspects of a Poly(o-Toluidine)-Modified Gold Electrode. 2. Alternating Current Electrogravimetry Study in H2SO4 Solutions

Electrodeposited poly(o-toluidine) (POT) on gold electrodes was investigated in 0.5 M H2SO4 aqueous solutions using alternating current electrogravimetry (simultaneous electrochemical impedance and mass transfer functions). The kinetic aspects of the three different redox transitions proposed for this polymer (leucoemeraldine–polaron transition, polaron–bipolaron transition, and bipolaron–pernigraniline transition) and the species involved, cation (hydrated proton), anion (bisulfate ion), and free solvent (water), are studied by means of the mass impedance technique. An ionic transfer model is proposed with coherent results where anion transfer is the fastest process and hydrated proton transfer is the slowest process (Grotthuss mechanism). The flux and counterflux of water molecules due to the conformational structure changes of the polymer and exclusion effect governed by the anion transfer are also discussed.

Synergistic Effects in the Gas Sensitivity of Polypyrrole/Single Wall Carbon Nanotube Composites

Polypyrrole/single wall carbon nanotube composites were synthesized by in-situ chemical polymerization using pyrrole (PPy) as precursor and single wall carbon nanotubes (SWNTs) as additive component. Electron microscope images reveal that SWNTs component acts as nucleation sites for PPy growth in the form of spherical and cylindrical core-shell structures. The SWNTs/PPy core-shell results in thin n-p junctions which modify the PPy bandgap and reduce the work function of electrons. As a result of the strong coupling, Raman and IR spectra show that the PPy undergoes a transition from polaron to bipolaron state, i.e., indicating an increase in the conductivity. In the UV-Vis spectra, the 340 nm adsorption band (π*-π transition) exhibits a red shift, while the 460 nm adsorption band (bipolaron transition) experiences a blue shift indicating a change in electronic structure and a relocation of polaron levels in the band gap of PPy. The modification in PPy electronic structure brings in a synergistic effect in sensing feature. Upon exposure to oxygen (an oxidizing agent) and NH3 gas (a reducing agent), the PPy/SWNTs nanocomposite shows an enhancement in sensitivity exceeding ten folds in comparison with those of PPy or SWNTs.

Heavy-Electron Formation and Polaron–Bipolaron Transition in the Anharmonic Holstein Model

The emergence of the bipolaronic phase and the formation of the heavy-electron state in the anharmonic Holstein model are investigated using the dynamical mean-field theory in combination with the exact diagonalization method. For a weak anharmonicity, it is confirmed that the first-order polaron–bipolaron transition occurs from the observation of a discontinuity in the behavior of several physical quantities. When the anharmonicity is gradually increased, the polaron–bipolaron transition temperature is reduced as well as the critical values of the electron–phonon coupling constant for polaron–bipolaron transition. For a strong anharmonicity, the polaron–bipolaron transition eventually changes to a crossover behavior. The effect of anharmonicity on the formation of the heavy-electron state near the polaron–bipolaron transition and the crossover region is discussed in detail.

Polaron to Bipolaron Transition in Conjugated Polymers

A possible mechanism of polaron to bipolaron transition is investigated based on the well known SSH model. We have studied the dynamical process of collision between two positively charged polarons with same spin in the presence of external electric fields. It is found that when the relative velocity of collision is not very large, the collision leads to a direct transition of two polarons to a bipolaron.

Rattling-Induced Heavy Fermion State in the Anharmonic Holstein Model

We investigate the heavy fermion states in the half-filled anharmonic Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method at low temperatures down to 1/10 4 of the conduction bandwidth. For the weak anharmonic cases with single-well or shallow double-well potentials, the bipolaronic first order phase transition takes place at a critical value of the electron–phonon coupling below a critical temperature as recently observed in the harmonic Holstein model. On the other hand, for the strong anharmonic case with deep double-well potential, the bipolaronic transition is suppressed and changes to a crossover where the heavy fermion state with a large effective mass is realized due to the effect of largely anharmonic local oscillations called rattling.

First Order Bipolaronic Transition at Finite Temperature in the Holstein Model

We investigate the Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. Below a critical temperature T cr , a coexistence of the polaronic and the bipolaronic solutions is found for the same value of the electron–phonon coupling g in the range g c1 ( T )< g < g c2 ( T ). In the coexistence region, the system shows a first order phase transition from the bipolaronic to the polaronic states as T decreases at T = T p (< T cr ), where the double occupancy and the lattice fluctuation together with the anharmonicity of the effective ion potential change discontinuously without any symmetry breaking. The obtained bipolaronic transition seems to be consistent with the rattling transition in the β-pyrochlore oxide KOs 2 O 6 .

The influence of the PANI structure on the conductive mechanism and on the electrical equivalent circuit analysis

The influence of the PANI structural changes during the oxidation of PANI on conductive mechanism and on the electrochemical impedance analysis was investigated. The results obtained by the electron spin resonance (ESR), Raman spectroscopy, ac electrogravimetry and probe beam deflection were used to explain the influence of the PANI structural changes on conductive mechanism. It was explained that the ingress of water molecules that occurs in the potential region of the current peak A, has an important influence on the PANI structure i.e., the PANI chain stretching process. The water molecules increase the distance between the positive charge (polaron) and the negative charge (counter-ion), which facilitates the potentially dependent PANI chain stretching process. The increase in opposite charge distance was explained to be crucial for the process of PANI chain stretching i.e., the chain is transformed from coil type to expanded coil type chain. The influence of the stretching process to the ESR signal properties was commented. It was explained that a decrease in the interaction between polarons, which was promoted by the stretching process, leads to the narrow ESR signal. The influence of structure and/or PANI chain ends on polaron to bipolaron transition was clarified. It was found that the polaron to bipolaron transition is hindered in the condition when the thicker PANI layer is investigated, which leads to greater structural disorder and/or the higher population of the chain ends. The calculated experimental impedance response of the different thickness PANI layers was successfully used in the EIS analysis. The correlation between the PANI structure and physical meaning of EEC parameters was effectively applied to approve or disprove the application of the proposed electrical equivalent circuit (EEC). The structural PANI changes and the EEC analysis of impedance data were used to test the proposed method that examines the interaction of the χ 2-value and obtained EEC parameters, which are related to the PANI structure properties. The proposed method discarded EEC, which resulted in the lowest χ 2-value, from the further analysis due to different trends of experimental and simulated data presented in the low frequency region of Nyquist spectra. In addition, the proposed method rejected EEC from further analysis if its parameters have not shown the correlation with the changes in the PANI structure. The presentations of data in Bode and admittance spectra were found to be especially useful when investigating systems that have pronounced capacitive behavior in the low frequency region.

Unsaturated Linkages in Dioxythiophene−Benzothiadiazole Donor−Acceptor Electrochromic Polymers: The Key Role of Conformational Freedom

The perspective of generating new colors commonly difficult to attain such as cyan blue and greens of various tones and saturations has motivated the design of conjugated polymer finely tuned in their molecular structure to reassemble the optical features desired in the context of non-emissive electrochromic device (ECD) applications. Herein, we report on a series of soluble donor−acceptor (DA) conjugated polymers involving 3,4-dioxythiophenes (DOTs) and 2,1,3-benzothiadiazole (BTD) constructed in combination with unsaturated linkages, namely ethynylene and trans-ethylene, and compare these to their fully polyheterocyclic DA control analogues with careful emphasis on optical, electrochemical, and electrochromic (EC) properties. As confirmed by spectroelectrochemical analysis, ethynylene linkers hindered the formation of a defined bipolaronic transition in the near-IR and were thus found disruptive with respect to the EC potential of their subsequent alternating copolymers. On the other hand, the presence of trans-ethylene spacers incorporated in the DA polymeric backbones allowed further narrowing of the energy gap so that new colors distinct from that exhibited by the control polymers were accessed including saturated green, a complementary color in the realization of polymeric EC displays. Systematic spectroelectrochemical analysis of each novel DA polymer is provided that offers clear evidence of the requirement for conformational freedom and stable quinoidal geometries upon electrochemical oxidation as cathodically coloring electrochromic polymers (ECPs) are switched reversibly to their transmissive doped state.

Gutzwiller scheme for electrons and phonons: The half-filled Hubbard-Holstein model

We analyze the ground-state properties of strongly correlated electrons coupled with phonons by means of a generalized Gutzwiller wave function, which includes phononic degrees of freedom. We study in detail the paramagnetic half-filled Hubbard-Holstein model, where the electron-electron interaction can lead to a Mott transition and the electron-phonon coupling to a bipolaronic transition. We critically discuss the quality of the proposed wave function in describing the various transitions and crossovers that occur as a function of the parameters. Previous variational attempts are recovered as particular choices of the wave function, while keeping all the variational freedom allows to access regions of the phase diagram, otherwise inaccessible within previous variational approaches.

Transformation of polarons to bipolarons in disordered matter

The polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity versus frequency measurements in disordered media with inhomogeneous disorder induce spatiotemporal fluctuations of the density of polarons yielding polaron to bipolaron transformation. The external field results in the transition, alternatively to an increasing doping level. The assertion is confirmed by performing successive complex impedance measurements in disordered media. A systematic increase in the real part of the ac conductivity in the frequency domain, followed by mutual reduction in the magnetic inductivity of conducting polypyrrole, are explained.

Effects of polarons and bipolarons on spin polarized transport in an organic device

A polaron–polaron interaction model is suggested to study the spin injection and transport in an organic semiconductor (OSC) device. The evolutions of spin polarons and spinless bipolarons are calculated from the drift-diffusion equations, in which both the polaron–bipolaron transition and the spin-flipping of a spin polaron are included. Then the spin polarized current is obtained. It is found that the polarons are responsible for the spin polarized transport in an OSC. Different from the case in a normal inorganic semiconductor, spinless bipolarons will affect the spin polarization of the OSC device. Finally, effects of the spin-flip time and the mobility of the carriers on the spin polarization in an organic device are discussed.

Interchain interaction effects on polaron–bipolaron transition on conducting polymers

We investigate the effects of interchain interaction on the polaron–bipolaron transition on conjugated polymer. We use a Su-Schrieffer-Heeger model modified to include interchain interaction, an external electric field and electron–electron interaction via extended Hubbard terms. We study the dynamics within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in interacting conducting polymer chains bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition which is produced is single-polaron to bipolaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is also observed under special circumstances. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field.

Raman study of order‐disorder transition of bipolarons in Ti4O7

AbstractWe have studied the phase transition of bipolarons in Ti4O7 by means of Raman spectroscopy. It is found that on warming across the phase transition, the Raman spectrum shows distinct changes, i.e., broadening of the linewidths, decrease in the phonon frequencies, and appearance of the fine structure. These features indicate the existence of structural disorder above the transition temperature, and hence are consisitent with a model that the phase transition is an order‐disorder transition of bipolarons. The mechanism of the line broadening is discussed in relation to the bipolaron motion in the disordered phase. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of mixing transition ions in glasses. II. The P 2O 5–Fe 2O 3–MnO system

The first article of this series reported electrical properties of phosphate glasses containing V and Fe ions, both of which contributed to conductivity. A mixed transition-ion effect was detected in such glasses which had several characteristics similar to the mixed alkali effect, observed in glasses containing two alkali ions. This article reports electrical and optical properties of phosphate glasses containing Fe and Mn ions, one of which (Mn) does not contribute directly to conductivity. DC resistivity by the Van der Pauw four-probe method and optical absorption (UV–VIS–IR) measurements have been performed on xP 2O 5 − (100 − x) (Fe 2O 3 + MnO) (PFM) glasses. The transport mechanism has been identified to be hopping of small polarons between Fe 2+ and Fe 3+ sites. For 0.3 < n Fe < 0.5, where, n Fe, the atomic fraction of Fe ions is given by (Fe/(P + Fe + Mn)), a marginal linear increase in resistivity has been observed with decreasing n Fe. However, when n Fe is reduced below ∼0.3, the resistivity increases exponentially with decreasing n Fe, which is similar to ‘metal–insulator transition’ (MIT), observed in many crystalline systems. The resistivity transition in these glasses has been explained by a (small) polaron to (small) bipolaron transition (PBT) in the composition range 0 ⩽ n Fe ⩽ 0.3. The occurrence of PBT is confirmed by the shifting of the polaronic optical absorption band to higher energies with increasing small bipolaron concentration.

Dynamic response functions for the Holstein-Hubbard model

We present results on the dynamical correlation functions of the particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated using the dynamical mean-field theory which is solved by the numerical renormalization group method. We clarify the competing influences of the electron-electron and electron-phonon interactions particularity at the different metal to insulator transitions. The Coulomb repulsion is found to dominate the behavior in large parts of the metallic regime. By suppressing charge fluctuations, it effectively decouples electrons from phonons. The phonon propagator shows a characteristic softening near the metal to bipolaronic transition but there is very little softening on the approach to the Mott transition.

Variation of the metal-insulator transition and formation of bipolarons by Cd doping in the thiospinel systemCu1−xCdxIr2S4

The pseudobinary thiospinel system ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{Ir}}_{2}{\mathrm{S}}_{4}$ was investigated by the x-ray-diffraction, electrical resistivity, magnetic-susceptibility, and specific-heat measurements. It was shown that the system exhibits a miscibility-gap behavior for the Cd substitution, however, nearly monophasic samples were obtained by quenching at 1373 K, except for $0.4&lt;x&lt;~0.6.$ With increasing the Cd concentration, the room-temperature electrical conductivity and Pauli susceptibility decrease monotonically, consistent with the hole-filling picture. The first-order metal-insulator transition at about 230 K in the parent compound ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is changed into a second-order transition around 185 K when $x\ensuremath{\sim}0.25,$ whereafter the second-order transition disappears at $x\ensuremath{\sim}0.8.$ No superconductivity was observed down to 1.8 K. The end-member compound ${\mathrm{CdIr}}_{2}{\mathrm{S}}_{4}$ is shown as an insulator with a band gap of 0.3 eV. Analysis for the data of magnetic susceptibility and electrical resistivity suggests the formation of bipolarons below 185 K for $0.25&lt;x&lt;0.8,$ which accounts for the absence of superconductivity in terms of the transition from the BCS Cooper pairs to small bipolarons.