Theoretical Studies of Charged Defect States in Doped Polyacetylene and Polyparaphenylene

Defect statecalculations have been per formed for polyacetylene and polybparaphenylene in the fraamework of the Su, schriever, and Heeger Hamiltonian. In polyacetylene, the study of the energetics of the separation of the radical(natural defect)-ion(charged defect) pair induced upon doping indicates that the two defects tend to remain close to each other. This results in the formatn of polarons whose binding energy is estimated to be of the order of 0.05 eV Absorption spectra at low doping levels are consistent with polaron formation. Interaction between polarons leads to the formation of charged solitions. In poly(p-phenylene), defects are always correlated in pairs. Upn doping, polarons are formed (binding energy ～ 0.03 eV), wit the relaxation of the lattice extending over about four rings. Calculations suggest the possibility of bipolarons (doubly charged defects) that yield conductivity without Pauli susceptibility.     

Electronic Excitations in Polyacetylene

Recent experimental results on light scattering and absorption in polyacetylene are reviewed, and interpreted in terms of the soliton model. The data indicate that injection of an electron-electron (hole-hole) pair by chemical doping or photoinjection of an electron-hole pair invariably distort the lattice leading to formation of a soliton-antisoliton pair. It is concluded that unlike traditional semiconductors, the polyacetylene lattice is inherently unstable in the presence of electron and/or hole excitations.     

Isomerization and Spin Generation in Polyacetylene

The isomerization of undoped (CH)_x was investigated in a combined Raman and ESR experiment. We find that the ESR linewidth, but not the spin concentration, is correlated with the degree of isomerization as determined from the Raman intensities. The spin concentratin does not change until the isomerization is well change untill the isomerization is well advanced. Hence the decrease in linewidth must be due to the increased mobility of the spins (solitons) rather than exchange interaction. The Spin qeneration is suggested to be due to Diels-Alder crosslinking. The Raman intensities indicate that there are two isomerization processes, with activation energies of 16 and 28 kcal/mole. Anomalous Raman scattering by cis (CH)_x excited with de ep red laser light provides additional evidence for defect structures.     

Morphology and Dopant Distribution in Polyacetylene

We report the results of scanning electron microscopic stadies of the morpology and dopant distribution in polyacetylene, (CH)_x. A variety of fibrillar and rod-like morphologies were observed for trans-(CH)_x and (CH)_x doped with iodine and AsF₅. Results show that iodine and atrsenic distributions are uniform across the surface of the films to a resolution of 5000 A, with no significatn difference in the concentration observed on the dull and shiny sides of the film. Hence any inhomogenieties in the dopant distribution must be on a finer scale. The fiberillar morphology was observed through the bulk of a film even after doping with iodine. Swelling of the fibrils is seen upon doping, with larger effects observed in the AsF₅ doped samples.     

Frequency Dependent Conductivity of Polyacetylene

We report on an extensive study of the frequency and temperature dependent conductivity of largely cis- (CH)_x trans- (CH)_x, and NH₃ compensated and iodine doped materials. The results reveal a strongly temperature (T) dependent dc conductivity and weakly T-dependent ac conductivity for largely cis- (CH)_x and for NH₃ compensated samples, similar to the behavior that is observed in many crystalline and amorphous semiconductors. The trans- (CH)_x and lightly iodine doped trans- (CH)_x have larger dc and ac conductivity and a strongly T-dependent ac conductivity. These latter results contrast with the usual expectations of variable range hopping and polaronic hopping, but are in good agreement with the predictions of Kivelson's Theory of charge transport via intersoliton electron hopping.     

Thermal Isomerization of Polyacetylene Films

Isomerization of the as-formed (-78°C) cis-polyacetylene to the trans isomer is relevant to the electronic structure, the crystal structure and the electrical properties of this prototypical conductive polymer. We have studied the process using Fourier transform infrared (FTIR) and 13_C magic angle nuclear magnetic resonance (NMR) spectroscopies at temperatures between 80 and 110°C. The observed kinetics will be discussed in terms of three basic processes: a second order process which randomly converts cis-cis-cis (CCC) sequences to trans-cis-trans (TCT) sequences via a pair of biradicals, a fourth order process that converts CTCC sequences to TCTT sequences via four biradicals, and two fourth order processes which allow sequential migration along the chain. The correlation of observed electron spin resonance and x-ray results, among others, with these mechanisms will be shown.     

Real Time Dynamics in Polyacetylene

Dynamical solutios to a one-dimensional coupled electron-phonon model of polyacetylene are explored numerically. The results indicate that interesting objects in polyacetylene such as neutral spincarrying solitons, charged spinles solitons, polarons and breathers can be dynamically generated by single electron and electron pair tunneling, pgotoabsorption, and chain breaking     

Infrared Absorption of Doped Polyacetylene

The frequency dependent conductivity of added charge (e.g. solitons) to a dimerized Peieres condensate with N coupled phonons is studied. The electron-Phonon system, with electron -electron interactions and pinning effects, yields in the adiabatic and continuum limits an infrared absprption whose structure is independent of the charge configuration. The kinetic mass of the charge affects only the pinning parameter and the over-all magnitude of the absorption. This mass is es timated from experimental data and compared with theoretical soliton and polaron masses.     

Lattice Dynamical Calculations of Trans and Cis Polyacetylene

Lattice dynamical calculations of the lattice modes for both trans and cis polyacetylene were performed. Using rigid body approximation for the chain we obtained all 8 lattice frequencies and modes of motion for both trans and cis polyacetylene. The frequencies concerned with Raman active lattice modes are presented for the first time. We also performed far infrared transmission measurements of undoped trans and cis polyacetylene and confirmed the results obtained by Bechtold et al. Comparison of the lattice frequencies obtained by lattice dynamical calculations with the results obtained by far infrared measurement and inelastic neutron scattering is discussed.     

Polyacetylene and Relativistic Field Theory Models

Connections between continuum, mean-field, adiabatic Peierls-fröhlich theory in the half-filled band limit and known field theory results are discussed. Particular attention is gi en to the Φ⁴ model and to the solvable N +2 Gross-Neveu model. The later is equivalent to the Peierls system at a static, semi- classical level. Based on this equivalence we note the prediction of both kink and polaron solitions in models of trans-(CH)_x. Polarons in cis-(CH) are compared with those in the trans isomer. Optical absorption from polarons is described, and general experimental consequences of polarons in (CH)_x and other conjugated polymers is discussed.     

On Trapping of Phase Kinks in Polyacetylene

A microscopic mechanism for trapping of solitons in polyacetylene is given. Molecular orbital calculations with full geom ety optimizaiton using semi-empirical tight.binding Hamiltonian for finite chain indicate that the neutral spin-carying phase kink is attracted by an energy on the meV scale to carbon atoms having a non-hydrogen side- group in accordance with recent magne tic resonance experiments. The oxygen induced trapping of the spins is also discussed.     

Distribution of Iodine in Doped Polyacetylene Filme

The distribution of iodine in polyacetylene doped under conditons typical for most published work is investigated by energy dispersive X-ray analysis under an electron microscope. The 9.1 GHz dielectric constant of a series of iodine doped (CH)_x is measured as a function of temperature. From the X-ray analysis we conclude that the reaction takes place in two steps: a fast reaction at the fiber surfaces and a slow reaction with the interior of (CH)_x fibers. The dielectric dconstant of lightly doped (CH)_x in the interior of the fibers. The temperature dependence of ε is believed to be related to the intrinsic temperatue dependence of the conductivity of the highly doped fiber surface layers.     

Resonant Raman Scattering in the Polyacetylene System

The amplitude mode approach for describing the resonant raman scattering in dimerized chains is reviewed and applied to the polyacetylene system. trans-(CH)_x is disordered with a variable electron-phonon coupling constant. cis-(CH) _x is ordered with an extrinsic gap of ～ 5% of the full gap. A distribution of extrinsic gaps is shown to exist In trans-(CH) _x chains in partially isomerized polyacetylene.     

Transport,Magnetic and Structural Studies of Polyacetylene

Experimental progress is reviewed with emphasis on magnetic and transport studies. The magnetic results are fully consistent with the soliton doping mechanism; the decrease in Curie-law contribution has been demonstrated; the Pauli term remains small (Y < 0.07) and is apparently zero in the limit of completely uniform doping. The electrical conductivity (P and T dependence) and thermopower (n-type and p-type) are in excellent agreement with Kivelson's theory of intersoliton electron hopping.     

Structural Investigations of Polyacetylene and Polyphenylene by Neutron Scattering

Wide angle neutron scattring patterns of deuterated cis- and trans-polyacetylene (PA) and deuterated polyphenylene (PPh) doped with AsF₅ have been measured. For undoped trans-PA a refined crystal structure is proposed to reproduce the neurtron scattering data. Doping of PA with AsF₅ is explained on the basis of an intercalation model. Deuterated PPh has been prepared by different polyerisation procedures, variing the crystallinity of the undoped material in a wide range. Strong extra peaks are observed during doping with AsF₅, wheres the crystallinity of the samples remains essentially unchanged. An intercalation model similar to AsF₅.-doped graphite is proposed.     

Solitons in Polyacetylene Induced and Probed by Positive Mupons

Abstract

Positive muons were injected into both trans- and cis- (CH) as well as (CD)_x. In cis-polyacetylene, the μ⁺ was found to form a radical state with an unpaired electron localized near the μ⁺ In trans-polyacetylene, the longitudinal μ⁺ spin relaxation rate at 293 K showed H^?1/2 dependence on the applied field H. After considering possible mechanisms, we concluded that it is due to the soliton-like one-dimensional motion of induced unpaired electron.     

Absence of Knight-Sheft in the Metallic State of Polyacetylene

NMR spectra on ¹⁸C of cis-(CH)_x have been recorded using the magic angle spinning technique. Inspite of doping with AsF₅ up to a level of ～ 7at%, (CH)_x does not convert to the trans modification. No Knight shift or other line shift is observed up to this dopant concentrtion. Above 7at% a line shift of about 30 ppm is found. To extend the soliton model to cis-(CH)_x bi-solitons or bi-polarons are proposed.     

Electron Hopping Conduction in the Soliton Model of Polyacetylene

It is shown that at low temperatures a novel conduction mechanism, phonon assisted hopping between soliton bound states (intersoliton hopping), may be the dominant conduction process in lightly doped polyacetylene. The theory is developed from a more general phenomenological viewpoint than previously, and the model independent aspects of the transport are stressed. It is found that the theory successfully accounts for experiments in polyacetylene which simultaneously show the hallmarks of solitons (e.g. no increase in the number of spins upon doping, a highly one-dimensional spin mobility, a qualitative dependence on cis to trans isomerization) and the characteristics of hopping transport (e.g. a strongly frequency and electric field dependent conductivity). It is further suggested that, since the theoretical considerations that lead to the conclusion that intersoliton hopping is important in polyacetylene are quite general, the same basic transport process may be important in many quasi-one dimensional semiconductors.     

Proton Magnetic Resonance in Partially Deuterated Trans - Polyacetylene

Proton NMR measurements as a function of ^l H concentration in partially deuterated trans-(CH_zD_1-z)_x show that at high ¹H content the proton relaxation is ^{z 1-z x}homogeneous and reflects the dynamics of electronic spins. At low concentration, protons of residual catalyst (about 6% per CH unit) dominate the NMR results, misleading earlier analysis.