Experimental studies of n-doped (CH)x: D.c. electrical conductivity

The d.c. electrical conductivity of n-doped (CH)_x was studied as a function of the doping level at 25°C. The dopants were sodium-naphthalene, lithium and sodium benzophenone dianionic salts in tetrahydrofuran (THF). Details of the experimental procedure are described.It was discovered that the THF vapour pressure present in the atmosphere in contact with the doped film drastically increases the film conductivity, at low doping levels. This may be due to an effect of the solvation state of cations.Conductivity measurements were also performed at very low doping levels (evaluated from metallic infrared bands) on (CHNa_y)_s, (CDNa_y)_x and (CDK_y)_x. A mobility value of 4 × 10^?6 cm^2/rmVs was deduced for 57% trans-(CHNiny)_x.     

In situ electron spin resonance experiments on polyacetylene during electrochemical doping

We present results obtained from in situ e.s.r. measurements on both n-type [Na_y⁺(CH)^−y]_x and p-type [(CH)^+y(ClO₄⁻)_y]_x during electrochemical doping. For [Na_y⁺(CH)^−y]_x, data were obtained over an extended concentration range out to ∼15 mol% (or well into the metallic regime); for p-type [(CH)^+y(ClO₄⁻)_y]_x we have succeeded in achieving doping sufficiently high in situ to observe the abrupt first-order onset of the Pauli spin susceptibility. Comparison of the results obtained from p-type and n-type electrochemical doping shows remarkable similarities between the two. For both p-type and n-type doping, at dilute concentrations charge is stored in nonmagnetic spinless charged solitons. For both p-type and n-type doping, a first-order phase transition is observed at a dopant concentration of y ∼ 0.06. For both p-type and n-type doping, there is an abrupt increase in the Pauli spin susceptibility at the transition with a corresponding increase in N(0) by a factor of more than 200 to a metallic value of N(0) ≈ 0.08 states/eV C (both signs of spin). For both p-type and n-type doping, there is a qualitative change in spin dynamics at the transition. The precise symmetry of the results obtained from n-type and p-type doping demonstrates without ambiguity the charge conjugation symmetry of trans-(CH)_x. The mechanism for the first-order electronic phase transition is discussed.     

Resonant Raman spectroscopy of conducting organic polymers. (CH)x,and an oriented analog

The results of low-temperature Raman scattering experiments on cis and trans (CH)_x are reviewed, and the Raman bands corresponding to carbon -carbon stretches of the polymer backbone discussed. The Raman spectra of a variety of commercial polarizing sheet, Polaroid K, are presented. The spectra indicate that the dehydrated polyvinyl alcohol chain in Polaroid K is an oriented analog of trans (CH)_x for purposes of Raman spectroscopy.For films of trans (CH)_x, changes in the Raman profile with excitation wavelength reveal the presence of a distribution of effective chain lengths. The shorter the length of conjugated chain, the higher the carbon-carbon stretch frequencies, and the further to the blue the optical absorption and wavelength necessary for maximum resonant enhancement of the Raman scattering. The shape and positions of the carbon-carbon band corresponding to double bond stretching suggest the presence of lengths of chain (terminated by conjugation or bonding defects) having x (number of monomer units) from x ? 10 to x ? 100. This is based on a limiting C=C frequency in an infinite chain of 1450 cm^?1, determined from the wavelength-independent, low-frequency origin of the band. A similar band-shifting phenomenon in Polaroid K is displayed.Spectra of iodinated (CH)_x and iodinated Polaroid K are presented which suggest a similarity between the dopant species present in the two different materials.     

Photoconductivity and function properties of polyacetylene films

Photoconductivity and photovoltaic effects of AsF₅-doped and undoped trans-(CH)_x films have been measured at room temperature in the wavelength region from 0.3 to 3.5 μm. The photovoltaic response threshold at 1.48 eV, measured on Schottky barrier junctions with a low work function metal, is interpreted as the single particle band-gap of trans-(CH)_x. I-V and C-V characteristics of the junctions indicate that good Schottky barriers are formed between lightly doped p-type (CH)_x and low work function metals. Evidence for ～ 2 × 10¹⁸ cm^?3 deep traps in both doped and undoped trans-(CH)_x is obtained from analysis of these characteristics.     

Infrared photoluminescence and the 1Ag state in polyacetylene

Experimental evidence concerning photoluminescence at 1.95 and 1.5 eV in cis-(CH)_x and at 1.2 eV in trans-(CH)_x is reviewed. It is suggested that the infrared luminescence is associated with the 2¹A_g excited state and perhaps with the photogenerated band-edge absorption in both cis and trans polyacetylene.     

Structure and conductivity of poly(acetylene)

Poly(acetylene), (CH)_x, synthesized under different polymerization conditions, was investigated by means of IR and ¹³C NMR spectroscopy, X-ray diffraction, and conductivity measurements. The molecular structure is strongly influenced by the polymerization temperature and the catalyst system. The polymerization temperature mainly governs the cis to trans ratio. In low temperature Shirakawa (CH)_x the cis crystals consist cis-transoid chains, whereas in Luttinger (CH)_x the cis crystals contain cis-transoid as well as trans-cisoid chain configurations. The catalyst systems affect the degree of distortion of the conjugated (CH)_x chains by sp³ hybridized carbon atoms. In Shirakawa (CH)_x the sp³ content is very low. (CH)_x prepared after the procedures of Luttinger and Reppe is characterized by an increasing content of sp³ hybridized carbon atoms. Hatano (CH)_x exhibits the highest sp³ content of all samples under investigation. A linear relationship exists between the relative sp³ content and the X-ray crystallinity. The higher the sp³ content, the lower is the crystallinity and the less perfect are the crystals. The sp³ content and the crystallinity determine the electrical conductivity of trans (CH)_x and iodine complexed (CH)_x. Low sp³ content and high crystallinity of (CH)_x are the preconditions for high conductivity of the corresponding iodine complexes.     

Spectroscopic properties of polyacetylenes synthesized via three modifications of Ziegler-Natta catalytic system

Results of spectroscopic measurements of three types of highly conducting polyacetylene are presented. A comparison is made of the traditional Shirakawa (or S-(CH)_x), Naarmann (or N-(CH)_x) and Tsukamoto (or v-(CH)_x) polyacetylenes. The non-conducting undoped materials have been characterized by ¹³C CPMAS NMR. FT-IR and resonance Raman scattering. A higher conductivity for iodine-doped N- (CH)_x and v-(CH)_x) in comparison with S-(CH)_x is observed even though a greater sp³ content is found in N-(CH)_x and v-(CH)_x films. Since the sp³ hybridized carbons may belong to catalyst residues, the sp³ content does not correlate with conductivity. On the other hand, it is shown from the resonance Raman scattering data that a longer average conjugation length in N-(CH)_x and v-(CH)_x may correlate with the higher conductivities of the iodine-doped films.     

Raman Scattering of segmented trans(CH)x and trans(CD)x

We present a complete Raman investigation of segmented trans(CH)_x and trans(CD)_x obtained by introduction of additional atoms (D atoms in (CH)_x and H atoms in (CD)_x) which form sp³ type defects. Raman spectra are recorded at different excitation wavelengths from the near infrared (λ_L = 676.4 nm) to the near u.v. (λ_L = 351.1 nm) and analyzed in the framework of the bimodal distribution model. It is shown in particular that upon introduction of additional atoms, the relative concentration of long conjugated segments is substantially lowered with respect to that of short ones. On the other hand, it is observed that even for a high concentration of sp³ defects (y = 0.27), long conjugated segments are still present in the polymeric chains. This result suggests that a clustering of defects occurs in agreement with the optical band gap values measured in these materials. This hypothesis is corroborated by results obtained from energy calculations performed on similar compounds.     

Organic metals and semiconductors: The chemistry of polyacetylene, (CH)x,and its derivatives

Polyacetylene, (CH)_x, is the simplest organic polymer. Both the cis- and trans-forms may be prepared as silvery, flexible, polycrystalline, semiconducting films. The cis-films can be stretched to over three times their original length with partial alignment of the (CH)_x fibrils. Through chemical or electrochemical doping, the electrical conductivity of the films can be increased over twelve orders of magnitude with properties ranging from insulator (σ < 10^?10 ohm^?1 cm^?1) to semiconductor, to metal (σ > 10³ ohm^?1 cm^?1). By the use of donors or acceptors, n-type or p-type polymer, respectively, is produced. When doped, the partly aligned (CH)_x film shows marked anisotropic electrical and optical properties. The more chemically oriented aspects of (CH)_x and its derivatives and those physical studies which are most illustrative of the effects of doping will be discussed.     

Theory of 3-D structure and intrinsic defects of transpolyacetylene

First-principles local-density functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3-D trans-(CH)_χ are reported. The P2₁/a structure is predicted to have a lower energy than the P2₁/n structure. A Green function calculation reveals that the 3-D character of the electronic band edge states destabilizes polarons and bipolarons and prevents self-trapping in perfectly ordered 3-D trans-(CH)_χ.     

Excited state dynamics in conjugated molecules

Using an equation of motion theory, optical absorption lineshapes are calculated for cis and trans-(CH)_x and are used to probe intramolecular and vibrational relaxation in these systems. We conclude that the broad edge in trans is due to a large-amplitude multiphonon relaxation consistent with the notion of soliton photogeneration, while the cis edge describes a weaker relaxation attributed to kink confinement in the excited state.     

Non-linear dynamics,breathers and photoinduced absorption in polyacetylene

We study the dynamics of trans-(CH)_x within the adiabatic approximation to the model of Su, Schrieffer and Heeger (SSH). We show, both by a qualitative analytic estimate and by quantitative numerical results, that a single soliton has a maximum velocity, v_m, which can be higher than the velocity of sound. Further, using a long-time numerical simulation, we demonstrate that, from an initial-hole pair, such as would be generated in photoinduced experiments, a soliton and antisoliton emerge with velocities ±v_m and that a persistent, spatially localized, time-periodic non-linear oscillation — a ‘breather’ — is left behind. We argue that this breather may account for the high energy, intra-gap structure observed in photoinduced absorption in both trans- and cis-(CH)_x, including isotope dependence recently observed in experiments on trans-(CH)_x and trans-(CD)_x.     

One-step synthesis and properties of monolithic photoluminescent ruby colored cuprous oxide antimony oxide glass nanocomposites

Cuprous oxide (Cu₂O) antimony glass (K₂O-B₂O₃-Sb₂O₃) monolithic nanocomposites having brilliant yellow to ruby red color have been synthesized by a single-step melt-quench technique involving in situ thermochemical reduction of Cu²⁺ (CuO) by the reducing glass matrix without using any external reducing agent. The X-ray diffraction (XRD), infrared transmission and reflection spectra, and selected area electron diffraction analysis support the reduction of Cu²⁺ to Cu⁺ with the formation of Cu₂O nanoclusters along with Cu_ySb_2−x(O,OH)_6-7 (y ≤ 2, x ≤ 1) nanocrystalline phases while Cu⁰ nanoclusters are formed at very high Cu concentration. The UV-vis spectra of the yellow and orange colored nanocomposites show size-controlled band gap shift of the semiconductor (Cu₂O) nanocrystallites embedded in the glasses while the red nanocomposite exhibits surface plasmon resonance band at 529 nm due to metallic Cu. Transmission electron microscopic image advocates the formation of nanocystallites (5-42 nm). Photoluminescence emission studies show broad red emission band around 626 nm under various excitation wavelengths from 210 to 270 nm.     

Preparation and electrochemical properties of some (Sc1−xTix)Ni alloys

A series of (Sc_1−xTi_x)Ni alloys and their hydrides (Sc_1−xTi_x)NiH_y, where x=0, 0.1, 0.3, 0.5, and y=2.9, 2.58, 2.23 and 1.55, have been prepared. The ability to store hydrogen gradually decreases with increasing Ti content in line with the reduction of the unit cell volume from 32.16 ų for ScNi to 29.68 ų for (Sc_0.5Ti_0.5)Ni. The electrochemical discharge capacity, however, varies in a more complex manner with (Sc_0.7Ti_0.3)Ni composition displaying the highest value of 231 mAh/g. The discharge capacity losses after 10 electrochemical charge–discharge cycles also vary non-linearly between 46% for (Sc_0.9Ti_0.1)Ni and 5% for (Sc_0.5Ti_0.5)Ni.     

Solitons or not in polyacetylene (and does it matter?)

The results of a series of experiments are described which demonstrate that solitons are the important excitations in trans-(CH)_x and that the properties of these non-linear excitations can be directly studied through measurements on trans-(CH)_x samples, either during photoexcitation of after doping. The importance of these concepts in the more general context of conducting polymers is addressed. Although the two-fold degenerate ground state of trans-(CH)_x is quite special, the relevant concepts can be generalized to confined soliton pairs (bipolarons) and applied to a wide variety of conjugated polymers in which the ground state degeneracy is not present.     

Resonant Raman scattering of Er3+ doped and partially isomerized polyacetylene

The results of experimental studies of resonant Raman scattering (RRS) spectra from Er³⁺ implantation doped and partially isomerized polyacetylene are presented. Thin cis-(CH)_x films were synthesized by the method of rare earth complex catalysis polymerization. Doping technique is ion implantation. The surface density of implanted Erbium ion is about 5×10¹³/cm². In the case of cis-(CH)_x, RRS bands at 1160, 1270 and 1540 cm⁻¹ have been observed λ_L = 488 nm (10K) and it was shown clearly that a shift in frequency of the peaks assigned to the remaining trans segments occurs during the isomerization process. In the case of Er³⁺ doped cis-(CH)_x, Er³⁺ induced Raman bands at 1145, 1320 and 1510 cm⁻¹ have been measured (λ_L = 488 nm, RT) for the first time and it is observed that the Raman bands from Er³⁺ doped trans/cis-(CH)_x are shifted to lower frequencies. We point out that the effect of Er³⁺ dopant is associated with the defects.The experimental results are discussed by using the model based on the hypothesis that the sample properties can be interpreted in terms of a bimodal distribution of long and short conjugation length segments (for trans-(CH)_x) and the bipolaron microscopic model on the basis of multi-phonon lattice relaxation theory (for cis-(CH)_x). Our studied results support further the bimodal distribution model and the bipolaron microscopic model.     

Microstructure, ferroelectric, and piezoelectric properties of (1 − x − y)Bi0.5Na0.5TiO3-xBaTiO3-yBi0.5Ag0.5TiO3 lead-free ceramics

Lead-free (1 − x − y)Bi_0.5Na_0.5TiO₃-xBaTiO₃-yBi_0.5Ag_0.5TiO₃ (BNT-BT-BAT-x/y, x = 0-0.10, y = 0-0.075) piezoelectric ceramics were synthesized by conventional oxide-mixed method. The microstructure, ferroelectric, and piezoelectric properties of the ceramics were investigated. Results show that a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases of BNT-BT-BAT-x/0.04 ceramics is formed at x = 0.06-0.08. The addition of BAT has no obvious change on the crystal structure of BNT-BT ceramics while it causes the grain size of the ceramics to become more homogenous. Near the MPB, the ceramics with x = 0.06 and y = 0.05-0.06 possess optimum electrical properties: P_r ∼ 42.5 μC/cm², E_c ∼ 32.0 kV/cm, d₃₃ ∼ 172 pC/N, k_p ∼ 32.6%, and k_t ∼ 52.6%. The temperature dependences of k_p and polarization versus electric hysteresis loops reveal that the depolarization temperature (T_d) of BNT-BT-BAT-0.06/y ceramics decreases with increasing y. In addition, the polar and non-polar phases may coexist in the BNT-BT-BAT-x/y ceramics above T_d.     

The hydriding behaviour of U(Fe1−xNix)Al system (0≤x≤0.75) and magnetic studies on U(Fe1−xNix)AlH0.8

The hydriding behaviour of the solid-solution series of compounds, U(Fe_1−xNi_x)Al, has been investigated. In the parent pseudoternary compounds, as Ni is gradually substituted for Fe, the magnetic correlations grow. As a result, the magnetic properties change from exchange enhanced Pauli-paramagnetic and spin fluctuating (x=0), via weakly magnetic with no long range magnetic ordering down to 4 K (0<x≤0.3) to ferromagnetic (0.35≤x≤0.75) and eventually to antiferromagnetic for x≥0.9. The present hydriding report covers compositions up to the ferromagnetic regime. It is found that hydrogen absorption does not take place for x below ≅0.7. The lowest nickel containing composition, which forms a well defined hydride phase, viz., U(Fe_0.3Ni_0.7)Al, absorbs y=0.8 H atoms per formula unit. This matches exactly with the lower hydride phase of pure UNiAl. No other hydride phase is formed either above or below y=0.8. The magnetization studies show that U(Fe_0.3Ni_0.7)AlH_0.8 has much higher values of the magnetic ordering temperature (T_C), paramagnetic Curie temperature (θ_P) and the uranium magnetic moment, relative to the unhydrided composition. These findings are in consonance with the large increase in the ⁵⁷Fe Mössbauer isomer-shift value, on hydriding. It is suggested that the electron charge transfer from H to the Fe 3d band (inferred from the Mössbauer studies) weakens the 5f–3d hybridization, thus enhancing the ferromagnetic correlations. The observed large increase in the a-axis cell parameter also implies a reduction in U–U hybridization, further justifying the observed increase in T_C, θ_P and U moment values.     

Vibrational spectra of polyaniline and its 15N- and 2H-substituted derivatives in as-polymerized,alkali-treated and reduced states

Raman spectra of polyaniline and its ¹⁵N- and ²H-substituted derivatives were studied in as-polymerized (conductive), alkali-treated (insulating) and electrochemically reduced (insulating) states. Emeraldine and N,N′-diphenyl-p-benzoquinone diimine were also studied. The CN stretching and some other vibrations were assigned on the basis of the isotopic shifts of the Raman wavenumbers. The presence of quinone diimine (Q) and p-disubstituted benzene (φ) structures was identified in alkali-treated samples (2A) by the 632.8 and 457.9 nm excited spectra in resonance and preresonance with the 630 and 330 nm absorptions, respectively. Together with the infrared NH stretching bands, these observations give experimental evidence of: (1) the head-to-tail polymerization and (2) the partial structure of 2A, -Q-φ-(NH-φ)_x-Q-φ-(NH-φ)_y-. The partial structure is the same for samples prepared electrochemically and chemically except that the composition (x, y, etc.) depends on the synthetic conditions in both cases. In the quinone diimine part (Q′) of as-polymerized samples (2S), electrons are more delocalized than in the corresponding part of 2A, since the CN stretching wavenumber is about 137 cm⁻¹ lower than that of 2A. In the course of reduction of 2S, such modified quinone diimine parts are changed into p-disubstituted benzene structure and the fully reduced samples (1S) are mostly composed of p-disubstituted benzene. The presence of the electron-delocalized quinone diimine parts in 2S is closely related to its high electrical conductivity.     

Frequency dependent conductivity of lightly doped cis-polyacetylene: Role of multiply connected doped trans-polyacetylene regions

The evolution of the frequency and temperature dependence of the conductivity of cis-polyacetylene [cis-(CH)_x] lightly doped with iodine is reported. As the dopant concentration is increased, the frequency-dependent contribution to the conductivity remains unchanged while the frequency-independent contribution increases dramatically. These results are inconsistent with the formation of small isolated metallic islands embedded in undoped cis-(CH)_x. The data support the idea that even at the lightest dopant levels studied (d.c. conductivity increased by only one order of magnitude), the doped regions are multiply connected, not isolated. Comparison with the temperature- and frequency-dependent conductivities of cis and trans-(CH)_x implies that the doped regions are isomerized to the trans configuration and that there may be some structural evolution in the undoped fibril cores, in agreement with earlier spectroscopic and structural studies.