Optical investigation of intra and interchain charge dynamics in conducting polymers

Reflectance measurements on tensile-drawn films of polypyrrole (PPy) doped with PF₆ are analyzed in terms of intra and interchain contributions to the optical conductivity σ(ω). The intrachain σ(ω) is typical of a disordered Drude metal with σ(ω→0)≈2×10³ S/cm, while the interchain σ(ω) is dominated by spectral features of strongly localized conductors, implying the interchain charge dynamics is incoherent.     

A.c. conductivity of poly(N-methylpyrrole)

The dielectric constant ɛ′(ω) and measured a.c. conductivity σ_m(ω) of lightly doped poly (N-methylpyrrole) films have been investigated in the temperature range 77–350 K and in the frequency range 100 Hz–1 MHz. In the low-temperature region the measured a.c. conductivity can be described by the relation σ(ω) = Aω_s, where the exponent s is found to be less than unity. At high temperatures the conductivity remains strongly frequency dependent but the dependence becomes weak at low frequencies. The temperature and frequency dependences of a.c. conductivity σ(ω) can be qualitatively explained by the contributions from two mechanisms, one giving rise to a broad dielectric loss peak having a distribution of relaxation times, and the other giving a linear dependence of conductivity on frequency.     

Impedance spectroscopy properties of polypyrrole doped with boric acid

The electrical conductivity and dielectric properties of polypyrrole doped with boric acid have been investigated. The direct current conductivity (dc) of the polymer increases with increasing temperature. The alternating current (ac) conductivity of the polymer obeys the power law, i.e., σ_ac(ω) = Aω^s. The alternating conductivity of polypyrrole doped with boric acid is controlled by the correlated barrier hopping model. The activation energy for alternating current mechanism decreases with increasing frequency which confirms the hopping conduction to the dominant mechanism as compared with the dc activation energy. The density of localized states N(E_F) for polypyrrole doped with boric acid was in the range of 2.5–9.2 × 10²² cm^?3 for various temperatures. The dielectric relaxation mechanism was explained on the basis of complex dielectric modulus. The imaginary modulus plot at different temperatures shows a dielectric mechanism with non-Debye relaxation. Boric acid can be a good candidate for controlling the electrical conductivity of the conducting polymer.     

Optical properties and the electronic structure of Co<Subscript>2</Subscript>TiGe and Co<Subscript>2</Subscript>TiSn Heusler alloys

The frequency dependences of the real part ε₁(ω) and imaginary part ε₂(ω) of the complex dielectric constant of the Heusler alloys Co₂TiGe and Co₂TiSn have been studied in the spectral range of 0.1–5 eV. It has been established that the interband electron transitions play a dominant role in the formation of the optical properties of these alloys. An anomalous behavior of the optical properties in the IR range of spectrum, namely, the absence of the Drude component of the optical conductivity σ(ω), has been revealed. The results of the studies have been discussed based on the calculations of the electronic structure of the alloys.     

Conductivity and magnetoconductivity below 1 K in films of poly(3,4-ethylenedioxythiophene) doped with CF3SO3

The temperature dependence of the electrical conductivity σ(T) and the magnetoconductivity σ(B,T) of poly(3,4-ethylenedioxythiophene) (PEDOT) films heavily doped with CF₃SO₃ has been measured down to T = 150 mK in magnetic fields up to B = 9 T. It is shown that, below 1 K, σ(T) α ln T and σ(B,T) depends significantly on the orientation of B parallel or perpendicular to the film surface. These facts are interpreted as a manifestation of two-dimensional character of electron transport in highly doped PEDOT films because of the possible formation of strongly graphitized layers with plane structure of polymer chains or fibrils.     

PEDOT/PAMPS: An electrically conductive polymer composite with electrochromic and cation exchange properties

Poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propane sulfonate) (PAMPS) composite films were electrochemically prepared from a mixture of water and N,N-dimethylformamide (DMF) containing 3,4-ethylenedioxythiophene (EDOT) and the polyelectrolyte, PAMPS. The presence of PAMPS in the PEDOT matrix was confirmed by spectroscopic and electrochemical methods. Depending on the current density, the conductivity of PEDOT/PAMPS free standing composite films reached values of 80 S/cm. Spectroelectrochemistry of neutralized PEDOT/PAMPS composite films showed a maximum absorption at 2.0 eV (615 nm) and a band gap of 1.65 eV, as calculated from the onset of the π–π* transition. Thin PEDOT/PAMPS composite films were found to switch rapidly between oxidized and reduced states in less than 0.4 s with an initial optical contrast of 76% at λ_max: 615 nm. The morphology of the polymer composites demonstrates a cauliflower structure. Despite the high molecular weight of the polyelectrolyte, the film density was found to be similar to classical PEDOT (i.e., ca. 1.4 g/cm³), while the surface roughness averaged below 5%. As expected with the use of a sulfonated polyelectrolyte as dopant, cation exchange properties were observed with hexaammineruthenium(III) chloride as an active electrolyte.     

The dc and ac conductivity of polyaniline–polyvinyl alcohol blends

The dc and ac conductivity of polyaniline (PANI)–polyvinyl alcohol (PVA) blends have been reported from room temperature to 80 K in the frequency range of 1 kHz–5 MHz. The dc conductivity has been described by the variable range hopping (VRH) model. The large value of VRH exponent (>0.5) indicates that hopping transport occurs between the superlocalised states of polymer. The frequency (ω) dependence of conductivity satisfies the ω^s power law. The variation of s with temperature suggests that the ac conduction is due to the correlated barrier hopping (CBH). The onset frequency ω(0) is proportional to σ(0). The permittivity increases sharply at low frequency and high temperature.     

Analysis of impedance spectra on corroding metals

The dispersion occurring during impedance measurements on corroding metal surfaces can be mathematically represented by the introduction of a frequency-dependent impedance Z₂(ω). In order to determine this impedance physically, an analysis of the term Z₂(ω) = K₂⁻¹(jω)^−α2 is carried out. Supplementary to the conventional transmission line model for solid electrodes, a new model is presented. The occurrence of frequency-dependent elements is explained here as a superposition of many partial reactions in the form of R_p — C_p elements in series circuit. The frequency of the individual relaxation times is determined by means of the empirical distribution expression by Cole and Cole. It is shown that the results of impedance measurements on zinc-plated electrodes with a defective polymer layer after an intensive chemical loading can be represented by this model. The causes of dispersion are discussed in a general way.     

Extraction of electronic parameters of organic diode fabricated with NIR absorbing functional manganase phthalocyanine organic semiconductor

The semiconducting and metal/organic semiconductor properties of the newly synthesized NIR absorbing α-substituted manganase phthalocyanine bearing functional 2,3-dihydroxypropylthio moieties {M[Pc(S–CH₃CH₂(OH)CH₂(OH))]₄X}(M = Mn^III) have been investigated by electrical conductivity–temperature, optical absorption and current–voltage characteristics methods. The electrical conductivity increases with the temperature, suggesting that the peripheral α-substituted-functional manganase phthalocyanine is an organic semiconductor. The optical band gap and trap energy values were determined and were found to be 2.98 eV and 1.95 eV, respectively. The ITO/MnPc/Al diode shows a rectifying behavior due to the formation of MnPc/Al interface with a rectification ratio of 29.4 at ±2 V. The series resistance R_s and ideality factor n values were found to be 102.6 kΩ and 8.89, respectively. The interface state density for the diode was of order of 2.73 × 10¹¹ eV^?1 cm^?2 with the interface time constant of 1.93 × 10^?5.It is evaluated that newly synthesized α-substituted manganase phthalocyanine bearing functional 2,3-dihydroxypropylthio moieties is an organic semiconductor and can be used in electronic device applications as an organic diode.     

Anisotropy of optical properties of hexagonal RMnO3 Manganites (R = Ho, Er, Tm, Yb)

The evolution of real and imaginary parts of the complex permittivity of hexagonal single crystals of RMnO₃ manganites (R = Ho, Er, Tm, Yb) differing in the radius of rare-earth ion r _R has been studied using spectroscopic ellipsometry in the range of 0.5–5.0 eV at room temperature. Spectra of dielectric functions show a strong polarization dependence. The optical-absorption edge for polarization E ⊥ c is determined by the intense narrow peak at 1.6 eV, whereas for polarization E ‖ c, the peak is shifted toward high energies by 0.15–0.20 eV and its intensity is suppressed greatly. It has been shown that, when r _R decreases, the energy position of the intense peak at 1.6 eV in the spectrum of the imaginary part of the dielectric function for polarization E ⊥ c shifts toward low energies by no more than 0.1 eV, which reflects changes in the local surroundings of the Mn³⁺ ion. For the both polarizations, a broad absorption band with the center at 2.4 eV has been revealed; the band was detected earlier in the antiferromagnetic phase in nonlinear spectra upon the optical generation of the second harmonic. Spectra of permittivity have been analyzed within available concepts on the electronic structure of hexagonal RMnO₃ compounds and have been compared with corresponding spectra of previously studied orthorhombic RMnO₃ compounds.     

Metal ion sensing functional mono and double-decker lanthanide phthalocyanines: Synthesis,characterization and electrical properties

We report, in this study, the preparation, physical characterization metal ion sensing properties of peripherally functionalized ionophore ligand, 4,5-bis(6-hydroxyhexylthio)-1,2-dicyanobenzene (1) and its mono 2,3,7,8,12,13,17,18-octakis(6-hydroxyhexylthio)phthalocyaninatometal (II) {M = Zn^II (2), Cu^II (3)} and double-decker lanthanide bis-phthalocyanines, {([4,5,4′,5′ 4″,5″,4?,5?]-tetrakis-(6-hydroxyhexylthio)phthalocyaninatolanthanium(III)}){M[Pc(S–C₆H₁₃OH)₄]₂} {M = Eu^III (4), Yb^III (5), and Lu^III (6)}. All benzenes on phthalocyanines are functionalized with hydroxyhexylsulfanyl moieties for potential use as soft metal ion binding, such as Ag⁺ and Pd²⁺. The temperature dependence of the dc and ac conduction properties of 4, 5 and 6 thin films have been investigated in the frequency range of 40–10⁵ Hz and temperature range 290–436 K. The dc results showed an activated conductivity dependence on temperature for all films. Obtained data reveal that ac conductivity obeys the relation σ_ac(ω) = Aω^s and exponent is found to decrease by increasing temperature. The data obtained results were compared with the prediction of the Quantum Mechanical Tunelling (QMT) and Correlated Barrier Hopping (CBH) models. The analysis showed that the CBH model is the dominant conduction mechanism for the electron transport in the films. The new synthesized compounds have been characterized by elemental analysis, FTIR, ¹H and ¹³C NMR, MS, UV–vis and EPR spectral data.     

Spectroscopic investigations of the BEDT-TTF charge transfer salts with NO3− anions (β″- and δ-phase)

We report the polarized reflectance spectra (650–6500 cm⁻¹) as well FT-NIR Raman spectra of β″-(BEDT-TTF)₃(NO₃)₂ and δ-(BEDT-TTF)₂(NO₃)_0.9(NO₂)_0.1 salts as a function of temperature. Additionally, the crystal structure of δ-phase salt was determined at room temperature and the temperature dependence of electrical conductivity was measured. The reflectance spectra of metallic β″-phase salt were fitted with the Drude model; the metal–insulator transition at T = 20–27 K has practically no influence on IR spectra. The δ-phase salt shows semiconducting behavior; inside conducting organic layers the BEDT-TTF molecules are arranged in dimers. The specific vibrational features of the δ-salt are discussed in terms of hydrogen bonding between BEDT-TTF and anions.     

The application of the transform of Kramers- Kronig for computing the polarization resistance

Some general considerations on the use of the Kramers-Kronig transform for computing the real and imaginary parts of the electrode impedance Z(ω) are given and the relationship between R_p and Zr(0) is established by considering a simple electric network. Experimental applications concern the behaviour of iron in 5 wt% HCl solutions, containing a specific inhibitor for this environment with concentration values ranging from 0.1 to 3.0 g/kg, at temperatures of 65, 75, 80 and 90°C. Electrode impedance determinations were performed over the [0.08, 2 × 10⁴] Hz frequency interval under current control. The values of R_s were usually determined by processing experimental data belonging to the [0.08, 2 × 10⁴] Hz frequency interval. The values of R_p, based on the use of the first Kramers-Kronig relation between Z_r(ω) and Z_i(ω), were computed using the D01GAF subroutine of the NAG library. The accuracy of this procedure was verified by comparing the experimental and computed values of Z_r(ω). Comparison of the two sets of values of Z_r(ω) confirmed that the use of the Kramers-Kronig transform was pertinent to analyse experimental data because the reproduction of all examined experimental data was rather faithful over the [0.08, 2 × 10⁴] Hz frequency interval. At last, the first Kramers-Kronig relation provide a valid mathematical tool to compute the correct magnitude order of R_p as a function of inhibitor concentration.     

One-pot synthesis of highly stable silver nanoparticles-conducting polymer nanocomposite and its catalytic application

Herein, we report the preparation of highly stable Ag_nano–PEDOT nanocomposite by one-pot fashion in acidic condition using 3,4-ethylenedioxythiophene (EDOT) as a reductant and polystyrene sulfonate (PSS^?) as a dopant for PEDOT as well as particle stabilizer for silver nanoparticles (AgNPs). The above nanocomposite denoted as Ag_nano–PEDOT/PSS^? nanocomposite. The formation of AgNPs with concomitant EDOT oxidation was followed by UV–visible (UV–vis) spectroscopy at different time intervals. Ag_nano–PEDOT/PSS^? nanocomposite shows absorption bands at 380 and above 700 nm, which correspond to surface plasmon resonance (SPR) peak of AgNPs and oxidized PEDOT, respectively. Ag_nano–PEDOT/PSS^? nanocomposite was characterized by infrared (IR) spectroscopy, transmission electron microscopy (TEM), and XRD. TEM study reveals that AgNPs are distributed uniformly around PEDOT polymer with an average particle size diameter of 10–15 nm. In addition, Ag_nano–PEDOT/PSS^? nanocomposite was tested for the catalytic reduction of 4-nitrophenol. For comparing stability, we were also synthesized AgNPs in the absence of PSS^? (denoted as Ag_nano–PEDOT) using EDOT as reductant. UV–vis spectrum of Ag_nano–PEDOT nanocomposite revealed that AgNPs prepared in the absence of PSS^? was not stable.     

Linear and nonlinear optical spectra of polyazomethines fabricated by chemical vapor deposition

We report on the results of third-harmonic generation (THG) measurements on vapor-deposited thin films of two polyazomethines, poly(nitrilo-1,4-phenylenenitrilomethylidyne-1,4-phenylenemethylidyne) (PNPP) and poly(nitrilo-2,5-azinylnitrilomethylidyne-1,4-phenylenemethylidyne) (PNAP) covering pump energies from 0.6 to 1.4 eV. The THG data show the presence of a strong three-photon resonance centered near 0.9 eV for both samples; additional weaker structure is observed for PNPP near 1.2 eV. The peak values of the third-order susceptibility are 2.8 × 10⁻¹¹ e.s.u. for PNPP and 1.8 × 10⁻¹¹ e.s.u. for PNAP; off-resonance values are nearly an order of magnitude smaller. We have also obtained the χ⁽³⁾(3ω) spectrum of an oriented PNPP film grown on an obliquely evaporated SiO₂ pattern. An enhancement by nearly a factor of three in the peak nonlinear optical susceptibility is observed relative to that of the unoriented PNPP film, demonstrating that a moderate degree of molecular orientation was achieved.     

Study of IF5 chemical doping of aluminium polyfluorophthalocyanine: (PcAlF)n

Chemical IF₅ doping of aluminium polyfluorophthalocyanine, (PcAlF)_n, has been studied by thermogravimetric analysis, infrared spectroscopy, elemental analysis and electrical conductivity measurements. An optimum electrical conductivity (σ = 6 × 10⁻² ω⁻¹ cm⁻¹) is achieved for a doping rate lying in the range 0.5 - 0.8 mole of IF₅ per (PcAlF) ring. The decrease of electrical conductivity of the doped material measured for higher doping rates is shown to be related to the disappearance of the eclipsed configuration in the stacking of the rings, induced by excess inserted dopant species. These results confirm those previously published concerning the AsF₅ doping of the polymer and can be explained with the help of the same structural model.     

Correlation between dielectric relaxation and d.c. electrical conduction in polypyrrole family of polymers

The dielectric constant ɛ′ (ω) of polypyrrole, poly(N-methyl pyrrole) and their copolymer, poly(N-methyl pyrrole-pyrrole), has been measured in the frequency range 100 Hz–100 kHz and in the temperature range 77–350 K. The d.c. conductivity has also been measured in the temperature range 77–350 K. The static dielectric constant (ɛ₀) and relaxation frequency (_R have been estimated from the dielectric constant data. The estimation of site spacing (d) suggests that the charge carrier localization is about per 4–8 pyrrole rings. An attempt has been made to establish a correlation between dielectric relaxation and d.c. electrical conduction in these systems and it has been found that both originate from the same hopping process.     

Chemical polymerization of 3,4-ethylenedioxythiophene in an aqueous medium containing a phenol derivative as an additive

In this paper, we describe poly(3,4-ethylenedioxythiophene) (PEDOT) prepared in an aqueous medium containing Fe₂(SO₄)₃ and alkylnaphthalenesulfonate as an oxidant and a dopant, respectively. It is found that the addition of p-nitrophenol leads to an improvement of the moisture stability of PEDOT as well as an increase in yield. On the other hand, the initial conductivity of PEDOT decreased slightly. An increase in yield and a decrease in conductivity were also observed when other phenol derivatives with a strong electron-withdrawing substitute, such as m-nitrophenol and p-cyanophenol, were added. The effects of phenol additives on PEDOT were different from those on polypyrrole, in which an increase in conductivity and a slight decrease in yield were observed.     

AC conductivity and dielectric properties of GeSexTl0.3 amorphous thin films

AC conductivity and dielectric properties have been studied for amorphous thin films with different thicknesses of glassy system GeSe_xTl_0.3 with X = 3, 4 that prepared with thermal evaporation technique. The measurements are taken at temperature range (303-403 K) and frequency range (10²-10⁵ Hz).AC conductivity σ_ac(ω) is found to be proportional to ω^s where s < 1. The temperature dependence of the ac conductivity and the parameter s can be discussed with the aim of the correlated barrier-hopping (CBH) model.The dielectric constant ?′and the dielectric loss ?″ showed frequency and temperature dependence. The maximum barrier height W_M calculated from the dielectric measurements according to Giuntini equation are in good agreement with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott in the case of chalcogenide glasses.