Bipolaron States Research Articles

Contact kinetics in conducting polymers

We consider electronic properties of interfaces with doped conducting polymers. Two peculiar features are emphasized: the possibility of polaronic and bipolaronic states of electrons (holes) and the mobility of ions within the depletion layer. The mutual diffusion of various types of charges and the transformation of electronic states are considered. On this basis we describe the stationary characteristics and the hysteresis at the Schottky contacts and the effect of photoluminescence quenching and enhancement in a light-emitting device (LED).

Magnetoabsorption of large bipolarons

The magnetoabsorption of large bipolarons is investigated by the path-integral method. The response of a bipolaron in a magnetic field to an external electromagnetic field is derived using a generalized anisotropic modelling influence phase in the framework of the memory-function approach. The resulting magnetoabsorption spectrum of large bipolarons is presented for special values of the electron-phonon coupling constant and the magnetic field taken as example and analysed. Its general feature is that this spectrum consists of a series of relatively narrow peaks. The peculiarities of the magnetoabsorption spectrum of the bipolaron as a function of the frequency of the electromagnetic field are attributed to the transitions involving scattering states or internal excited bipolaron states.

Stability of reduced polypyrrole

Electrochemically polymerized polypyrrole(PPy) shows changes in visible light spectrum during electrochemical reduction revealing changes in electron states of the conjugated double bond. The concentration of polarons increases fast in the early stage during reduction on expense of bipolarons but drops off momentarily to show an increase of bipolaron concentration which again decreases with further reduction. The reduced PPy is unstable to show changes in the visible light spectrum in less than one minute after cutting off applied potential. The absorption peak at 420nm assigned as π-π ∗ transition in the conjugated double bond shrinks fast to show a strong absorption around 800nm assigned as bipolaron. It is observed that the spectral changes slow down when hydroquinone is added to the system. The addition of hydroquinone in the system stabilized polaron not to couple with each other during natural oxidation of PPy and makes the polymer segments not to turn easily into a bipolaron state from a polaron state. Hydroquinone is oxidized in an aqueous system to produce protons which seem to stabilize free radicals in the polaron state. It is suggested that deprotonation of −N +H in the pyrrole ring develops into productions of an imine structure to loose the electrochemical reactivity.

Electron energy-loss investigation of the electronic structure of a PPP-related ladder polymer

The electronic structure of a ladder polymer of PPP has been investigated by high-energy electron energy-loss spectroscopy (EELS) in transmission. The band gap of the undoped compound is about 0.4 eV smaller compared to PPP. Information on the character of the undoped LPPP as well as on bipolaron states formed upon K doping in the former band gap are obtained from momentum dependent EELS investigations. The extension of the bipolaron is estimated to 20 ±4 Å.

Some aspects of the radiation processing of conducting polymers

Recently, in our laboratory (IARC), complex studies have been made on conducting polymers such as polyaniline, polypyrrole and polythiophene. Our research programme has been directed toward a better molecular understanding of the structural properties controlling electronic conductivity of polymers. The most important issue of our studies within conducting polymer family includes effect of γ and electron beam irradiation on their electrical conductivity. The irradiation was performed on the samples in the form of pellets onto which aluminium electrodes had been vacuum-deposited. The relative conductivities of irradiated pellets prepared from highly conducting polyaniline, polypyrrole and polythiophene powders decreased with increase of irradiation dose under vacuum. On the other hand in the case of irradiation under dopant atmosphere conductivity of pellets was increased. The dramatic increase in conductivity was observed after irradiation of insulating polyaniline form (even under air atmosphere). The ESR studies suggest that the charge-transport in low doped conducting polymer can be explained by paramagnetic polaron migration along and between the polymer chains. At higher doping levels, in the case of polythiophene and polypyrrole the energetically favourable is a bipolaron state as charge carrier.

The polaron scenario for high T c superconductivity

In high T c materials electrons on the whole are only weakly coupled to the phonons except for a few specific local modes. The latter gives rise to the formation of polaronic charge carriers on specific molecular centers in those materials. There is considerable experimental evidence for their existence in form of localized states and we conjecture that localized bi-polaronic states lie inside the valence band of itinerant electrons. With increased doping this valence band is gradually being emptied out, eventually resulting in a situation where the chemical potential coincides with the level of the bi-polaronic state. At this point an insulator→metal transition is triggered off and due to hybridization between the localized bi-polarons and the itinerant electrons the initially localized bi-polaronic states acquire itinerant upon lowering the temperature; eventually undergoing a transition to a superfluid state at a critical temperature T c. A characteristic feature of this scenario of an hybridized Boson-Fermion mixture is the pinning of the chemical potential at the Bosonic (bi-polaronic) level which only weakly depends on doping in the fully developed metallic regime. T c is then essentially controlled by the number of Bosons which does depend on doping and leads to the known universal behaviour of T c as a function of Boson concentration and mass.

Transition between bipolaron and polaron states in doped heterocycle polymers

On the basis of a model similar to that of Su, Schrieffer, and Heeger, we investigate transitions between bipolaron and polaron states in doped heterocycle polymers such as polypyrrole, polythiophene, and polyselenophene. The dopant impurity is assumed to be located between chains, and the interchain coupling is due to the electron hopping which is assumed to be confined to nearest-neighboring carbon groups around the counterion. The electron-electron interaction described by a Hubbard-type Hamiltonian is also included in our calculation. It is shown that the system may be in a bipolaron state or a state of a polaron pair of spin triplet (singlet) nature. We find that the interchain coupling has little influence on the stability of a bipolaron. Transitions from a bipolaron to a pair of polarons can be induced by a strong electron-electron interaction, which is also responsible for the energy difference between the singlet and triplet polarons states.

The Electronic Structure of Alkali Doped Ultra Thin Sexiphenyl Films

Abstract The formation and evolution of the gap states formed with increasing Cs concentration in thin 6P films are followed with ultraviolet photoemission spectroscopy (UPS) and indirectly with energy loss spectroscopy involving electronic transitions (EELS). Two distinct doping regimes are apparent, in the lower doping regime bipolaron-like (dianionic) states are formed 0.8 and 2.9 eV above the valence band maxima. In the higher doping regime, approaching Cs saturation, a 0.5 eV splitting in the gap states is observed which is attributed to interacting bipolaron states, and as such is a precursor to bipolaron band formation.

FeCl3-doped polyvinylidene fluoride

Infrared (350-4000 cm -1) and optical (1.1 5 x 1 04 - 2.95 x 1 04 cm -1) spectra, differential thermal analysis (DTA) and d.c. electrical resistivity of FeCI3- doped polyvinylidene fluoride (PVDF) films, over the doping mass fraction range 0 ~< w ~< 0.40, have been measured. The i.r. spectra provided evidence of: (a) the presence of both ~ and y phases in the undoped, and a y phase in the doped PVDF films; (b) a head-to-head content of 20%; and (c) a different doping mode beyond a 0.25 doping level. The optical spectra resulted in two induced energy bands, and a probable interband electronic transition, due to doping. Dipole relaxation and premelting endothermic peaks were identified by DTA. Electrical conduction is thought to proceed by interpolaron hopping among the polaron and bipolaron states induced by doping. The hopping distance, R o, is calculated according to the Kuivalainen model. A numerical equation is adopted to formulate the dependence of R o on doping level and temperature. It is found that Ro < CC separation length. This implies that, in doped PVDF, charge carrier hopping is not an intrachain process.

Stability and nature of polarons in a two-site two-electron model.

A two-site two-electron system is studied in the presence of an electron-phonon (e-ph) interaction and intersite hopping of the electrons to determine the stability of different polaronic states. Exact energies and eigenfunctions for the triplet and Anderson bipolaronic states are found. For the singlet bipolaronic state, the role of different phonon states in lowering the energy is discussed. The nature of polarons with the variation in e-ph coupling strength is studied.

Surface polarons and bipolarons at GaAs(110) with adsorbed alkali metals

We performed density-functional theory calculations for Na or K adsorbates on GaAs(110). The alkali-metal adatoms are found to donate their valence electrons into Ga dangling bonds. This results in a lattice relaxation around the occupied Ga orbital with a very strong polaron shift of about 1 eV. Moreover, it is predicted that two polarons can merge, forming a surface bipolaron. The polaron as well as the bipolaron states are localized, which explains the observed non-metallic character of the surface. The formation of the localized bipolaron can be also described in terms of a “negative- U” behavior of the Ga dangling orbital. We discuss how the formation of bipolarons may affect STM images and surface core-level photoemission spectra.

Improved proof of existence of chaotic polaronic and bipolaronic states for the adiabatic Holstein model and generalizations

We simplify the proofs and extend the scope of the main theorems of Aubry, Abramovici and Raimbault on existence of polaronic and bipolaronic states near the anti-integrable limit for the adiabatic Holstein model of electron-phonon systems and generalizations. In particular, we obtain sharper estimates and allow exponential lattices, arbitrary square-integrable electron hopping, and some amounts of inhomogeneity and dispersion.

Degradation of copolymers of aniline and metanilic acid

The degradation of aniline-metanilic acid copolymer (PMA) by potentiostatic anodic polarisation was compared with the degradation of polyaniline (PAn) in order to understand the function of the pendant groups in polymer degradation. The degradation of PMA was believed to follow the same reaction pathway as the unmodified PAn, namely by attack of water molecules on oxidised polymer in the bipolaronic state. The presence of sulphonate curtailed the bipolaron production and increased the steric hindrance in degradative hydrolysis, so that the degradation of polymer was inhibited as a result. It is concluded that the pendant groups on the PAn backbone only moderated the electroactivity of the linear PAn chains in degree but not in kind.

Electrochemistry of end-capped oligothienyls. New insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene

The kinetics of anodic coupling to dimers of thiophene oligomers ( n = 3–5), methyl protected at one α-terminal position, is second order in oligomer concentration and evidences high activation enthalpies and negative activation entropies. Activation free energies are linearly related to the inverse of the oligomer length n (the dimerization rate decreases as n is increased). Thin films of methyl end-capped thiophene oligomers ( n = 4, 6, 8 and 10) display reversible oxidations from a single one-electron step (tetramer) to a single two-electron step (octamer and decamer) through two separate one-electron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidized hexamer. The close resemblance of the electrochemical and ESR behaviour of the hexamer with that of polythiophene suggests that oxidation of the latter occurs via hexameric spin-dimerized polarons. The conductive and capacitive properties of the end-capped oligomers ( n = 6, 8 and 10) were investigated by in situ conductivity and chronopotentiometry. While conductivity of octamer and decamer is displayed at the two-electron (bipolaron) state, the hexamer, insulating at this state, is conducting at the mixed-valence polaron-bipolaron state; capacitive responses are evidenced at the bipolaron state for the octamer and decamer only. The difference of conductive and capacitive behaviour between the hexamer and the higher oligomers is explained by charge localization in hexameric segments.

Extent of incorporation of hydrolysis products in polyaniline films deposited by cyclic potential sweep

The ratio of the second anodic current peak at 0.52 V to the first anodic current peak at 0.17 V, I p,B/ I p,A, in the voltammogram of cyclic potential sweep (CPS) deposition of polyaniline (PANi) is used to measure the extent of incorporation of products of polymer degradation by hydrolysis during electro-polymerization. In CPS deposition from HClO 4 solutions, the ratio depends on such depositon conditions as the scan rate, the rotation speed of the electrode, the number of deposition cycles and the aniline concentration. The competitive reaction of aniline and water with an intermediate generated by PANi oxidation to the bipolaronic state, and the solubility of the resulting products, determine the amount of hydrolysis products to be incorporated in the polymer deposit. The presence of hydrolysis products in PANi inhibits fibrous growth and gives rise to a powdery film morphology. The ratio I p.B/ I p,A is therefore useful as an index of film quality in practical applications.

Bipolaronic charge density waves and polaronic spin density waves in the Holstein-Hubbard model

At large electron-phonon coupling, the phonon of the Holstein-Hubbard model can be (generally) treated classically. Exact results can be obtained for this model at any band filling and any finite dimension where the Hubbard term with coefficient U is treated rigorously. The existence of chaotic bipolaronic states is proven when U is smaller than a certain value U c and for an electron-phonon coupling k sufficiently large. The existence of magnetic polaronic states is also proven for positive U and k sufficiently large. For 0<U<U c , there exists an overlap region with chaotic mixed polaronic-bipolaronic states. There is a region for U≃U c /2 and large k of special interest, where the bipolarons in their ground-state become singlet pairs of polarons bounded by a magnetic resonance. Then, the Peierls-Nabarro barrier which usually pins polarons and bipolarons to the lattice depresses to almost zero and the validity of the classical lattice approximation breaks down so that the initial exact results do not apply in that specific region. The formation of a superconducting state with possibly a high critical temperature, is then conjectured at a large coupling k in a narrow domain in-between the two insulating domains corresponding to Bipolaronic Charge Density Waves and to magnetic Polaronic Spin Density Waves respectively

The phase diagram of the 2D Holstein-t-J model near half filling

The authors have used an exact diagonalization technique to explore the ground-state phase diagram of the two-dimensional Holstein-t-J model near half filling. The nature of the ground state is analysed, calculating self consistently the local lattice distortions, on-site spin and charge densities, as well as the magnetic structure form factor. They demonstrate that doped holes tend to form hole-polaron or bipolaron states. Compared to that in the uncorrelated model (spinless fermions), the critical electron-phonon coupling strength for the self-trapping transition of the doped charge carriers is considerably reduced by the antiferromagnetic exchange interaction. The relevance to the cuprate superconductors is discussed.

Polaron and bipolaronlike states in n-doped bithiophene.

Bithiophene is studied with varying Cs dosing as a model for the n-doping of thiophene-based conducting polymers. The evolution of the electronic structure with increasing Cs concentration is followed both indirectly, by electron-energy-loss spectroscopy (EELS), and directly by ultraviolet-photoemission spectroscopy (UPS). In both spectroscopies the behavior of the states formed in the gap indicates two doping regimes. The EELS results parallel the optical-absorption results for the p-doping of longer thiophene oligomers and polythiophene, and suggest a polaron (anion) to bipolaronlike (dianion) transformation. The UPS results are consistent with this interpretation, showing first in the low-doping regime the appearance of two states in the band gap which move closer together in the higher-doping regime. This constitutes direct observation of a transition from polaron to bipolaron states, albeit in a model system. Consideration of the photoionization cross sections indicates that the deeper (bi)polaron level has C 2p character while the shallow level, filled by donation from the Cs, has significant S 3p character.

Self-trapped state and phonon localization in TiO2 quantum dot with a dipole layer

The discrete phonon spectra of TiO2 quantum dot with a dipole layer is reported. The absorption intensities of different LO phonons are related to the phonon-electron coupling in accord with the prediction of the continuum polarization model. The two-phonon bound state appeared due to the localization effect. The optical absorption of the self-trapped state [reported in Appl. Phys. Lett. 59, 1826 (1991)] may be related to the strong electron-phonon interaction in this system, i.e., a bipolaronic state might form with the presence of surface donor.

Fabrication of a novel electro-optical intensity modulator from the conjugated polymer, poly(2,5-dimethoxy-p-phenylene vinylene)

We report an electro-optic absorption modulator based around a metal-insulator-semiconductor field-effect transistor structure in which the active semiconductor is the polymer poly(2,5-dimethoxy-p-phenylene vinylene). It incorporates a single optical waveguide formed by the polymer, insulator, and a polyimide top layer which is ribbed to define the guide laterally. Charge injection creates polaron and bipolaron states which change the subgap absorption between 2 and 1.4 μm and between 800 nm and the band edge at 600 nm.