Effects of Various Dopants on Solitons in Polyacetylene

The effects of various dopants on solitons in polyacetylene were studied by using CNDO/2 level semiempirical quantum chemical method. The width of solitons is reduced when dopant is present, and the charge density wave(CDW) is further gathered on the carbon atom in soliton center. The effects of p-type of dopants are greater than those of n-type of ones. The charge transfer in doped polyacetylene can be achieved by the propagation of CDW along the chain. The conductivity of doped polyacetylene is proportional to the quantity of charge transfer between dopant and polyacetylene chain.     

聚乙炔掺杂导电的双向机制

提出了一种掺杂聚乙炔的双向导电机理。垂直聚乙炔分子链方向的电荷输运是通过掺杂原子(或分子)在链间振动实现的;平行分子链方向的电荷输运是通过电荷密度波的传播实现的。掺杂聚乙炔的电导率随掺杂剂与聚乙炔链之间电荷转移量的增大而增大。     

Conductivity study of stretch-oriented new polyacetylene

Oriented polyacetylene samples were synthetized and doped with iodine and FeCl₃. Their electric conductivity and anisotropy were measured within the temperature range 4-300 K. The results were described using the Ping Sheng model. It is concluded that the factors limiting the conductivity parallel and perpendicular to the stretching direction are the same and thus no interfibrillar conduction occurs. Some processes related to the mechanical stretching are discussed. © 1994 John Wiley & Sons, Inc.     

Novel concepts in electronic polymers: Polyaniline and its derivatives

Polyanilines have been known for over one hundred years. Recent studies of this chemically flexible polymer have demonstrated unusual chemical, electrical, and optical phenomena, both in insulating forms and conducting forms. The polyanilines differ substantially from earlier studied polyacetylene, polythiophene, polypyrrole, polydiacetylene, and other polymers in that their electronic structure is based on the overlap of alternating nitrogen atoms and C₆ rings. Two classes of the emeraldine form of polyaniline are distinguished by preparation route and crystal structure. For both classes, the Pauli susceptibility indicative of metallic state is correlated with the formation of a three-dimensionally crystalline region. Charge conduction studies of oriented films and fibers demonstrate that three-dimensional order between chains is critical for high conductivities. Within the crystalline regions the conduction electrons are substantially delocalized in three-dimensions while the dc transport is dominated by quasi-one-dimensional variable range hopping in the disordered regions. This delocalized charge leads to high absorbance (loss) at microwave frequencies, important for a broad range of applications. The ability to derivatize polyaniline at ring and nitrogen positions allows one to test concepts for the control of conductivity as well as improved processing. The monomethyl derivative, poly(orthotoluidine), has nearly identical electronic structure and crystal structure, yet its conductivity is three orders of magnitude lower than that of emeraldine hydrochloride. This is attributed to increased interchain separation and decreased interchain correlation causing 1-D localization. Similar 1-D localization is observed in the self-doped sulfonated polyaniline system. Extensive photoinduced optical studies of the insulating base forms of polyaniline show the importance of ring rotation in stabilizing photoexcited positive polarons. Long-lived photoinduced absorptions are observed which are suggested to be a possible basis for erasable optical information storage technology.     

Nickel-based layered superconductor, LaNiOAs

Rietveld analysis of the powder X-ray diffraction of a new layered oxyarsenide, LaNiOAs, which was synthesized by solid-state reactions, revealed that LaNiOAs belongs to the tetragonal ZrCuSiAs-type structure (P4/nmm) and is composed of alternating stacks of La-O and Ni-As layers. The electrical and magnetic measurements demonstrated that LaNiOAs exhibits a superconducting transition at 2.4 K, and above this, LaNiOAs shows metallic conduction and Pauli paramagnetism. The diamagnetic susceptibility measured at 1.8 K corresponded to ∼20% of perfect diamagnetic susceptibility, substantiating that LaNiOAs is a bulk superconductor.     

In-situ Apparent Mobility of Charge Carriers in Polyaniline Films Measured with a New Four-band Electrode

The in‐situ apparent mobility in polyaniline films was accurately measured in a wide doping region using a new four‐band electrode. It was found the apparent mobility in polyaniline films rises with increasing the doping level or carrier density. The influence of film thickness on the conductivity and apparent mobility of charge carriers was also investigated. The relative higher conductivity observed in a thinner film under low and intermediate doping potentials is assigned to the higher inter‐chain mobility related to the more ordered structure of the film. The mobility variations provide experimental evidence to confirm the inter‐chain path for hopping transport of polarons and the intra‐chain path for evolution of metallic conduction.     

Coupled cluster treatments of periodic systems from strongly localized reference functions: 1-D and 2-D spin and electron lattices

A modified coupled cluster method is applied to the research of the ground-state energy of periodic systems described by model Hamiltonians. The reference function is always a strongly localized function. The method is applied first to Heisenberg Hamiltonians and spin-frustrated one-dimensional (1-D) chain and square lattices, starting from Neel functions or from products of bond singlets. The same method is then applied to Hubbard Hamiltonian for 1-D chain and two-dimensional (2-D) square lattices starting from Neel function or products of bond molecular orbitals (MOs). In both cases the wave operators involve a very limited number of local operators. Despite its simplicity, the method is able to treat quite satisfactorily highly degenerate situations, approaching correctly the highly delocalized regime from the Neel function or the highly correlated regime from a product of bond MOs. However, the method is not precise enough to treat the subtle phenomenon of bond alternation of polyacetylene. The coupled cluster method from strongly localized reference functions represents an elegant and quite efficient exploratory tool, but its accuracy is limited by the poor treatment of collective effects. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 115–132, 1998     

天然橡胶掺杂导电机制的理论研究

绝大多数导电聚合物均具有共轭骨架,因而化学家们认为共轭作用是聚合物导电的先决条件,然而Thakur用I_2处理具有非共轭骨架的聚合物——聚异戊二烯(PI),其电导率提高十几个数量级,达到了10~(-2)～10~(-1)S·cm~(-1),引起了人们的极大关注。相关实验亦报道了基本类似的结果,但对PI的导电机制尚无统一的认识。本文利用EHMO/CO法,通过计算系列模型的二维能带结构,提出了掺杂模型以探讨天然橡胶的导电机制。     

SEMICONDUCTING POLYMER DEVICES:NEW DEVELOPMENTS

Since the discovery of metallic conductivity in doped conjugated polymers (such as in doped polyacetylene),semiconducting properties of undoped conugated polymers have been drawn equal attentions. In additonal to the interests in the nonlinear properites of funda-mental excitations in these materials,possibilities of device applocations (such as solar cells and field -effect transistors) have been investigated in the past decade. However,the perfor-mance of polymer devices developed in early times was far from that of commercial devices made with inorganic materials (which was commonly attributed to the low carrier mobility of these materials),that managers and engineers in device industry dropped their initial in-terests gradually on this type of materials in later 1980' s.     

Molecular transport junctions: asymmetry in inelastic tunneling processes

Inelastic electron tunneling spectroscopy (IETS) measurements are usually carried out in the low-voltage ("Ohmic", i.e., linear) regime where the elastic conduction/voltage characteristic is symmetric to voltage inversion. Inelastic features, normally observed in the second derivative d(2)I/dV(2) are also symmetric (in fact antisymmetric) in many cases, but asymmetry is sometimes observed. We show that such asymmetry can occur because of different energy dependences of the two contact self-energies. This may be attributed to differences in contact density of states (different contact material) or different energy dependence of the coupling (STM-like geometry or asymmetric positioning of molecular vibrational modes in the junction). The asymmetry scales with the difference between the energy dependence of these self-energies and disappears when this dependence is the same for the two contacts. Our nonequilibrium Green function approach goes beyond proposed WKB scattering theory in properly accounting for Pauli exclusion, as well as providing a path to generalizations, including consideration of phonon dynamics and higher-order perturbation theory.     

Doping of (CH)x films to the metallic state with metal hexafluorides

When polyacetylene films, (CH)_x, are exposed to the vapours of hexafluorides, the resistances of the films drop rapidly. The following hexafluorides were shown to dope (CM)_x to the metallic state: SeF₆, TeF₆, WF₆, ReF₆, OsF₆, IrF₆, MoF₆, UF₆ and XeF₆. Conductivity $\text{vs}$ degree of doping curves obtained for WF₆, MoF₆ and UF₆ exhibit a shape similar to that observed for AsF₅; namely, an increase in electrical conductivity of several orders of magnitude at low concentrations until a point when additional doping has little further effect. Parallel e.s.r. line-shape measurements confirm metallic behaviour above a critical transition. The highest conductivity observed in the series is 350 Ω^?1 cm^?1 for [CH(WF₆)_0.087]_x. The maximum observed for the XeF₆ doped polyacetylene was about 0.1 Ω^?1 cm^?1. The other hexafluorides gave materials which show intermediate conductivities. The XeF₆ doped polyacetylene is not stable, presumably because of internal fluorination of the (CH)_x by the dopant.     

Polymerization of acetylenes with rare earth coordination catalysts

The polymerization of acetylene and its derivatives by rare earth coordination catalysts and the characterization of the polymers so obtained in our laboratory are reviewed. Because of the metallic conductivity possessed by doped polyacetylene and the unique properties such as conductivity (semiconductivity), paramagnetism, migration and transfer of energy and chemical reactivity and complex formation ability often shown by acetylenic polymers, which seem promising as specialty polymers, there has been considerable interest in the polymers of acetylene and its derivatives. A wide variety of catalyst systems have been developed for the polymerization of acetylenes. But there has been no information concerning the use of rare earth compounds as catalysts in the polymerization of acetylene and its derivatives. We for the first time in 1981 have succeeded in the polymerization of acetylene with rare earth coordination catalysts, which in turn is a development based upon earlier work on the diene polymerization using rare earth coordination catalysts(Ref. 1). Using rare earth catalysts, acetylene can be polymerized conveniently into high cis polyacetylene films with metallic sheen at room temperature and phenylacetylene can also be polymerized into high molecular weight, high cis polyphenylacetylene films at ambient temperature. Thus new varieties of polyacetylenes have been developed and a novel family of coordination catalysts consisting of a rare earth compound plus trialkyl aluminum for the polymerization of acetylenes has been exploited. This article reviews our studies on the polymerization of acetylene and its derivatives with rare earth coordination catalysts and on the characterization of the polyacetylenes prepared.     

Coupled protonic and electronic conduction in the molecular conductor [2-(2-1H-Benzimidazolyl)-1H-benzimidazolium]-TCNQ

A novel molecular based proton-electron mixed conductor, (H3BBIM(+))(TCNQ)(Cl(-))(0.5)(H(2)O) (1), where H3BBIM(+) is 2-(2-1H-benzimidazolyl)-1H-benzimidazolium and TCNQ is 7,7,8,8-tetracyano-p-quinodimethane, was synthesized. The salt exhibited peculiar phase transitions as a result of proton-electron coupling phenomena within the crystal. Salt 1 is composed of a closed-shell H3BBIM(+) cation and an open-shell TCNQ anion radical, and was obtained by electrocrystallization in a buffered CH(3)CN solution. Crystal 1 was constructed from the segregated uniform stacks of H3BBIM(+) and TCNQ. The regular stack of partially electron-transferred TCNQ(-0.5) provided a one-dimensional electron-conducting column. Between the regular H3BBIM(+) columns, a channel-like sequence of holes was formed at the side-by-side space that is filled with disordered Cl(-) ions and H(2)O molecules, and which offer a proton-conducting path. The electrical conductivity at room temperature (10 S cm(-1)) was greater by a magnitude of four than the protonic conductivity (1x10(-3) S cm(-1)). Electronic conduction changed from metallic (T>250 K) to semiconducting (250>T>100 K), then insulating (T<100 K). Protonic conductivity was observed above 200 K. The continuous metal-semiconductor transition at 250 K is caused by the formation of the Cl(-) superstructure, whereas the disappearance of protonic conductivity at 200 K is related to the rearrangement of the [Cl(-)-(H(2)O)(2)] sublattice within the channel. The magnetic susceptibility continuously shifted from Pauli paramagnetism (T>250 K) to the one-dimensional linear Heisenberg antiferromagnetic chain (T<250 K). Lattice dimerization in regular TCNQ columns was confirmed by the appearance of vibrational a(g) mode at low temperatures. The strong localization of conduction electrons on each TCNQ dimer caused a Mott transition at 100 K. The melting and freezing of the [Cl(-)-(H(2)O)(2)] sublattice within the channel was correlated to the conduction electrons on the TCNQ stack and the protonic conductivity.     

Conductivity in polyacetylene. VI. Semiconductor—Metal transition of alkali-doped polymer

A local model is set up for the conductivity in alkali-doped polyacetylene (PA) based on results of ab initio and semiempirical calculations. At low doping levels, solitons and polarons appear naturally in the (nondegenerate) ground state. Alkali atoms donate their valence electrons to neutral solitons, which have the highest electron affinity in the PA structure. Due to the high polarizability of the PA chains, there is a charge buildup on a few carbon atoms close to the alkali ion. At the same time, a new soliton, screened from the alkali ion, is formed some distance away from the latter. This solition may migrate through the PA polymer partly by hopping for one chain segment to another (E_a ≥ 0.15 eV) and partly by soliton motion. In the calculation of the spectra, we used geometry-optimized structures and configuration interaction (i.e., taking into account electron-lattice interaction and explicit Coulomb correlation) and obtained good agreement with experimental spectra. As the concentration of alkali is increased, absorption occurs at energies below 1 eV. At higher doping levels, corresponding to a few mol%, the electrons delocalize over many alkali spacings and the trapping capability of the polymer decreases the conductivity becomes bandlike. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 655–665, 1997     

ESR and magnetic properties of electrically conducting metallophthalocyanine polymers

Polymeric metallophthalocyanines of Cu, Ni, Co in which benzene rings are shared in common with the macrocyclic phthalocyanine rings containing peripheral carboxylic groups have been synthesized and their electrical conductivities are shown to increase from 5 to 8 orders of magnitude by thermal treatment. The cobalt polymer exhibits greater conductivity than its nickel or copper analogs. The heated polymers show very broad electron spin resonance (ESR) signals and large paramagnetic susceptibilities. The bulk magnetic susceptibility of these polymers shows Curie-like behavior when the samples are heated from room temperature to 473 K. This is explained on the basis of coexistence of fixed mobile and conduction electron spins in the system and the interaction of the soliton type defects present in these polymers with the polymeric network to give dipolar charge carriers. This is also supported by the decrease in ESR intensity with increasing temperature when the samples are heated in the ESR cavity. The effect of the presence of unpaired electrons on the d shell of the central metal atoms on the line width and the line shape of the ESR spectra is also explained. © 1994 John Wiley & Sons, Inc.     

Modifying thermal transport in electrically conducting polymers: effects of stretching and combining polymer chains

If their thermal conductivity can be lowered, polyacetylene (PA) and polyaniline (PANI) offer examples of electrically conducting polymers that can have potential use as thermoelectrics. Thermal transport in such polymers is primarily influenced by bonded interactions and chain orientations relative to the direction of heat transfer. We employ molecular dynamics simulations to investigate two mechanisms to control the phonon thermal transport in PANI and PA, namely, (1) mechanical strain and (2) polymer combinations. The molecular configurations of PA and PANI have a significant influence on their thermal transport characteristics. The axial thermal conductivity increases when a polymer is axially stretched but decreases under transverse tension. Since the strain dependence of the thermal conductivity is related to the phonon scattering among neighboring polymer chains, this behavior is examined through Herman's orientation factor that quantifies the degree of chain alignment in a given direction. The conductivity is enhanced as adjacent chains become more aligned along the direction of heat conduction but diminishes when they are orthogonally oriented to it. Physically combining these polymers reduces the thermal conductivity, which reaches a minimum value for a 2:3 PANI/PA chain ratio.     

聚N-甲基苯胺的质子酸掺杂

聚N-甲基苯胺(PMAn)可用酸碱进行可逆的掺杂及反掺杂。掺杂使电导增大。掺杂过程在本质上是链的质子化过程,与阴离子无关。用FTIR、UV-VIS、ESR表征了掺杂前后结构的变化。结果表明,掺杂后链上来偶电子增加,电子及电荷更加离域化,并与电导率增大相吻合。PMAn的导电载流子可能是离域化的阳离子自由基。     

Nickel-based oxyphosphide superconductor with a layered crystal structure, LaNiOP

A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La(3+)O(2-))(+) and (Ni(2+)P(3-))(-). We found that the resulting LaNiOP shows a superconducting transition at approximately 3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4-300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached approximately 40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.     

1064-nanometer-excited Fourier transform Raman spectroscopy of conducting polymers

Application of 1064-nm-excited Fourier transform (FT)-Raman spectroscopy to the characterization of conducting polymers is described. 1064-nm-excited FT-Raman spectra with high signal-to-noise ratios are obtained from polyacetylene (PA), poly(1,4-phenylene) (PPP), poly(1,4-phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) in their neutral (insulating) state. The resonant Raman spectra of acceptor- or donor-doped (conducting) PA and PPV are also obtained wih 1064-nm excitation. The resonant Raman spectra of Na-doped PA change in two stages with increasing dopant concentration, the first change corresponding to the increase in electrical conductivity and the second to the appearance of a Pauli susceptibility. The 1064-nm-excited FT-Raman spectrum of Na-doped PPV indicates existence of negative bipolarons which are equivalent to divalent anions extending over a few repeating units in the polymer chains.     

Lattice dynamics of polyacetylene as a one-dimensional system: Part II. Doping-induced disorder

The quadratic valence force field derived in Part I of this study for perfect trans-polyacetylene is used and adapted for the study of the doping-induced IR absorption of polyacetylene. The lattice dynamics of a one-dimensional disordered system is treated with the introduction of structural defects to represent the doping sites. Concentration, distribution, size of the defects and inhomogeneity of the doping are all considered. Two models for the defects are analyzed; one representing the “soliton” and the other, equally acceptable on chemical grounds, representing centro-symmetric defects. It is concluded that the IR spectrum observed for doped polyacetylene does not provide unequivocal evidence for the existence of solitons. Alternative explanations are equally acceptable from the spectroscopic viewpoint. The structural information derived from lattice dynamics and spectra is discussed in detail.