Showing papers by "Alan J. Heeger published in 1980"

Soliton excitations in polyacetylene

Abstract: A theoretical analysis of the excitation spectrum of long-chain polyenes is presented. Because of the twofold degeneracy of the ground state of the dimerized chain, elementary excitations corresponding to topological solitons are obtained. The solitons can have three charge states $Q=0$. $\ifmmode\pm\else\textpm\fi{}e$. The neutral soliton has spin one-half while the charged solitons have spin zero. One electronic state is localized at the gap center for each soliton or antisoliton present. The soliton's energy of formation, length, mass, activation energy for motion, and electronic properties are calculated. These results are compared with experiment.

Nature of the open state in long polynucleotide double helices: possibility of soliton excitations

Abstract: The existence of transiently open states in DNA and synthetic polynucleotide double helices has been demonstrated by hydrogen exchange measurements; base pairs reversibly separate and reclose, exposing nucleotide protons to exchange with solvent protons. Recently it has been possible to define the equilibrium, kinetic, and activation parameters of the major open state that determines base pair hydrogen exchange. However, there is no direct information at the moment about the conformation of the open form. Here we consider the possibility that the low energy and slow opening and closing rates observed reflect a deformation involving several adjacent base pairs. Assuming a mobile open unit capable of diffusing along the double helix, we find that available data are consistent with structures of 10 or so adjacent open pairs. It is further suggested that these structures correspond to thermally induced soliton excitations of the double helix, which retain coherence by sharing the energy of a twist deformation among several base pairs. Solitons are nonlinear excitations that can travel as coherent solitary waves, and have been recognized as an important mechanism for mediating conformational changes in polymers and condensed systems generally. Comparison of the double helix with simple mechanical analogs suggests that soliton excitations may well exist within DNA chains, and the present analysis shows that the hydrogen exchange open state is consistent with these.

Electrical transport in doped polyacetylene

Abstract: The results of an experimental study of electrical conductivity and thermopower in doped polyacetylene are reported. Included are measurements on both as‐grown and partially oriented films doped with iodine and AsF5; [CH(AsF5)y]xand [CH(I3)y]x, where y covers the full doping range. The data indicate three important concentration regimes; the dilute limit (y<0.001), the transitional region (0.001

Solitons in Polyacetylene: Magnetic Susceptibility

Abstract: The absoulte spin susceptibility of As${\mathrm{F}}_{5}$-doped trans-polyacetylene, ${[\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{y}]}_{x}$, was determined by spin resonance techniques for $0.0004l~yl~0.138$. The Curie-law contribution decreases above $y=0.001$ to a level less than 1 ppm in the highly conducting regime. The temperature-independent Pauli term is small (\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ emu/mole) for $yl0.01$, remains small up to $y=0.05$, and shows an abrupt increase near $y=0.07$ to a value of \ensuremath{\sim} 3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ emu/mole at higher concentrations. The results are consistent with the soliton doping mechanism.

Electron spin resonance studies of magnetic soliton defects in polyacetylene

Abstract: We present electron spin resonance studies of the magnetic defect in undoped polyacetylene. Temperature dependences of the absorption spectra of (CD)x and (CH)x over the range 2–295 K have been obtained. The properties of the magnetic defect in fully isomerized trans samples and in cis‐rich samples are compared. The data are consistent with a description in terms of a mobile neutral magnetic defect in the trans polymer and a stationary one in the cis‐rich polymer. The spin resonance linewidths are determined by unresolved hyperfine splittings resulting from interaction of the electron spin with many nuclear spins within the envelope of the spatially extended magnetic defect. The results of analyses of the detailed line shapes and linewidths are discussed in terms of the bond alternation domain wall (soliton) model of the magnetic defect.

Photoelectron spectroscopy of iodine‐doped polyacetylene

Abstract: X‐ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used in a study of the electronic structure of iodine‐doped polyacetylene. Core level and valence band spectra are used to assess the nature and the consequences fo the iodine doping. The spectra are compared with our previous results on pure and AsF5‐doped polyacetylene, and other published work on iodine‐doped polyacetylene.

Electrochemical doping of conjugated polymers

Abstract: Conjugated polymers are doped with dopant ions to a preselected room temperature electrical conductivity ranging from that characteristic of semiconductor behavior to that characteristic of metallic behavior, by means of an electrochemical procedure wherein the polymer is employed as one or both of the electrodes of an electrolytic cell, including as the electrolyte a compound which is ionizable into the dopant ions. Upon electrolysis of the electrolyte, the polymer, if used as the anode, becomes doped with anionic dopant ions to a p-type material; or if used as the cathode, becomes doped with cationic dopant ions to an n-type material. The above-described electrochemical doping procedure finds particularly useful application in the charging of novel secondary batteries in which a doped conjugated polymer is employed as one or both of the electrodes. Such secondary batteries, in their charged state, comprise a metal whose Pauling electronegativity value is no greater than 1.6, or a conjugated polymer doped with dopant cations of said metal, as the anode-active material; a conjugated polymer doped with dopant anions as the cathode-active material; and a compound which is ionizable into the dopant ions as the electrolyte. In the initial discharged state of such secondary batteries, the polymer is in undoped form, and charging of the battery is effected by electrochemical doping of the polymer with the dopant ions of the electrolyte.

Batteries having conjugated polymer electrodes

Abstract: Conjugated polymers are doped with ionic dopant species to a preselected room temperature electrical conductivity ranging from that characteristic of semiconductor behavior to that characteristic of metallic behavior, by means of reversible electrochemical doping procedures. The doping procedures are carried out in an electrochemical cell wherein the polymer to be doped is employed as one or both of the electrodes, and the electrolyte is a compound which is ionizable into the ionic dopant species. Upon operation of the cell, the polymer, if used as the anode, becomes doped with an anionic dopant species to a p-type material; or if used as the cathode, becomes doped with a cationic dopant species to an n-type material. The electrochemical doping reactions and their reverse electrochemical undoping reactions are utilized as the charging and discharging mechanisms of novel lightweight secondary batteries which employ doped or dopable conjugated polymers as one or both of their electrodes.

Tensile properties and partial alignment of polyacetylene, (CH)x, films

Abstract: The tensile properties of both cis- and trans-polyacetylene, (CH)x, were investigated in order to determine the variables that influence that ultimate extension ratio (l/l0) of polyacetylene films. It was found that when cis-films of polyacetylene were extended in an inert (argon) atmosphere, ultimate extension ratios as high as 3.3 were obtained and with subsequent isomerization at 160–200°C under stress, a final l/l0, of about 4 was obtained. The extension ratio is extremely sensitive to exposure to oxygen and decreases markedly after periods as short as 1 min. It appears as though crosslinking may occur in the polymer when it is exposed to air and this produces brittleness in the specimen. When (CH)x was isomerized from cis to trans, it became brittle, even in the absence of air.

X-ray scattering study of one-dimensional lattice dynamics in Hg 3 − δ As F 6

Abstract: The diffuse scattering of x rays by the linear chains of Hg atoms in the quasi-one-dimensional compound ${\mathrm{Hg}}_{3\ensuremath{-}\ensuremath{\delta}}\mathrm{As}{\mathrm{F}}_{6}$ was studied at temperatures from 100 to 350 K, and wave vectors from 7 to 20 ${\mathrm{\AA{}}}^{\ensuremath{-}1}$. The results are consistent with thermal diffuse scattering from a disordered one-dimensional "liquid" and, at high temperatures, agree quantitatively with the model of classical, independent, harmonic chains proposed by Emery and Axe. Deviations are observed in the temperature region from 130 to 200 K where the correlation length is about 25% longer than predicted.

Conducting Organic Polymers: Doped Polyacetylene

Abstract: The electronic structure of polyenes has been a subject of interest for many years. The unsaturated bonds which characterize the polyenes have an important effect on their electronic structure and the corresponding electronic properties. In a polyene (see Fig, 1) three of the four carbon valence electrons are in sp2 hybridized orbitals; two of the σ-type bonds are links in the backbone chain while the third forms a bond with some side group (e.g., H in Fig. 1). The remaining valence electron has the symmetry of the 2pZ orbital and forms a π bond in which the charge density is perpendicular to the plane of the molecule. In terms of an energy-band description, the σ bonds form low-lying completely filled bands, while the π bond would correspond to a half-filled band. The π bond could be metallic provided there is negligible distortion of the chain, and an independent-particle model proved to be satisfactory.

‘Organic metals.’ Synthesis of a new type of organic metal derived from polyacetylene, (CH)x

Abstract: When ca. 85%cis films of the semiconducting polymer, polyacetylene, (CH)x, are doped with bromine to compositions such as [CHBry]x(y=ca. 0·15–0·35) and then partly dehydrobrominated at 150°C, they yield new semi-conducting polymers such as [CH0·85Br0·11]x which can then be doped with I2 or AsF5 to yield new highly conducting organic metals such as [CH0·89Br0·09I0·12]x and [CH0·82Br0·13(AsF5)0·12]x, respectively.