C.O. Yoon

Hopping transport in doped conducting polymers in the insulating regime near the metal-insulator boundary: polypyrrole, polyaniline and polyalkylthiophenes

Abstract Transport data for polypyrrole-hexafluorophosphate (PPy-PF6), protonated polyaniline and iodine-doped regio-regular polyalkylthiophenes (PATs), all in the insulating regime near the disorder-induced metal-insulator (M-I) boundary, are presented and analyzed. These doped conducting polymers have the transport properties of a Fermi glass insulator; the disorder being characterized by the resistivity ratio, ρr ue5fc ρ(1.4 K)/ρ(300 K). In the regime close to the M-I transition (ρr 103), ρ(T) shows two distinctly different behaviors. In homogeneous material, Motts VRH conduction is recovered. In inhomogeneous samples, where ‘metallic island’ are formed after partial dedoping or by strong morphological disorder, ln ρ ∝ (T 0 ′/T) 1 2 , characteristic of a granular system.

Electrical transport in conductive blends of polyaniline in poly(methyl methacrylate)

The electrical transport properties of solution-processed conducting polyblends of polyaniline-camphor sulfonic acid (PANI-CSA) in poly(methyl methacrylate) (PMMA) are investigated for PANI-CSA over the full range of volume fractions (f) above the percolation threshold. The positive temperature coefficient of resistivity at high temperature and the linear temperature dependence of thermoelectric power indicate that, because of the phase-separated network morphology, the microscopic transport properties of PANI-CSA in the blends remain nearly unchanged by dilution. The low temperature resistivity indicates transport by generalized variable range hopping, ϱ(T) = ϱ0exp[(T0T)γ]. A systematic change in γ is observed as a function of dilution; γ increases from approximately 13 at high concentrations to approximately 23 near the percolation threshold (f ≈ 0.01). Measurements of the magnetoresistance show that the localization length decreases with dilution.

Pressure and magnetic field dependence of the low temperature resistivity of PF6-doped polypyrrole

Abstract The temperature coefficient of the resistivity (TCR) of doped polypyrrole changes sign below 25 K, from negative to positive, in samples with relatively weak disorder (as inferred from the resistivity ratio, ϱ r = ϱ (1.4 K)/ ϱ (300 K)). We have studied the crossover from negative to positive TCR as a function of the disorder, and as a function of pressure and magnetic field. At ambient pressure, the sign change is systematically observed only for samples having ϱ r ⩽2. For example, when ϱ r decreases from 1.97 to 1.33, the temperature ( T 1 ) defining the transition from negative to positive TCR increases from 7.5 to 24 K. Pressure enhances interchain transport and thereby decreases ϱ r . For samples having ϱ r ≈ 2–10, the TCR transition can be induced by applying high pressure. A magnetic field of 8 T suppresses the TCR transition. This anomalous temperature dependence of the low temperature electrical transport is discussed in terms of correlation corrections to the conductivity in the disordered metallic regime near the metal-insulator transition.

Resistivity and magnetoresistance of metallic polyaniline and polypyrrole at millikelvin temperatures

Abstract The resistivity and magnetoresistance of polyaniline protonated by camphor sulfonic acid (PANI-CSA) and polypyrrole doped with PF 6 (PPy-PF 6 ) have been studied down to 25 mK in magnetic fields up to 16 T.

Thermoelectric power of doped polyaniline near the metal-insulator transition

Abstract The thermoelectric power (S) of various doped polyaniline (PANI) near the metal-insulator ( M-I ) transition was investigated as a function of temperature (T), and compared with the characteristic temperature dependence of conductivity (σ). For the PANI protonated by camphor sulfonic acid (CSA), S(300K) = +8 ∼ +12 μ V/K and linearly dependent upon temperature, indicating that microscopic transport is via the diffusion of charge carriers in the extended electronic states with relatively narrow bandwidth. In the homogeneous limit, the thermoelectric power is insensitive to the presence of microscopic disorder associated with the critical behavior of σ(T) near the M-I transition. For the samples doped by conventional protonic acids (HCl, H 2 SO 4 ), the large scale inhomogeneity induces the strong temperature dependence of resistivity, ϱ( T ) ∞ exp [( T 0 T ) 1 2 ] , and additional negative contribution to the temperature dependence of thermoelectric power.

Effect of anisotropy on conductivity and magnetoconductance in heavily doped polyacetylene

Abstract The conductivity of iodine doped Shirakawa polyacetylene films with various stretching ratio ( l l 0 = 1∼6 ) was measured as a function of temperature, pressure and magnetic field. Unoriented samples show a power-law temperature dependence of the conductivity, with the characteristic resistivity ratio ρ I ≡ ρ(1.4K)/ρ(300K) ∼ 80, and a positive magnetoreisitance at low temperature. Although the conductivity anisotropy ( σ ∥ σ ⊥ ) increases with stretching up to σ ∥ σ ⊥ ∼ 110 for l l 0 = 6 , the temperature dependence of the conductivity ( ρ r = 3.0 ± 0.3) for stretched sample is nearly independent on the direction and the ratio of stretching. The negative magnetoresistance for the stretched sample indicates that weak localization contribution to the conductivity is dominant at low temperature. The anisotropy of magnetoconductance is close to the conductivity anisotropy implying that the bulk anisotropy originates from partially aligned fibrillar morphology. Application of high pressure decreases ρ r and changes the dominant dephasing mechanism of weak localization from the inelastic electron-electron scattering in the dirty limit to that in clean limit.

Magnetoresistance in polyaniline networks near the percolation threshold

Abstract The self-assembled network of conducting polyaniline (PANI), protonated by camphor sulfonic acid (CSA), in a matrix of insulating polymethylmethacrylate (PMMA) has a remarkably low percolation threshold. The critical volume fraction (fc) of PANI-CSA phase segregated in PMMA is fc ∼ 0.003 (0.3%). The conductivity at room temperature near the percolation threshold is quite high, 3 × 10−3 S/cm. The systematic increase in the exponent of the temperature dependence of conductivity (γ) results from transport on the fractal structure and to the related superlocalization of the electronic wavefunctions with systematic variations in the fractal dimensionality upon dilution of the PANI-CSA. Below the percolation threshold, the temperature dependence of the resistivity is like that of granular metals with γ = 0.5. The positive magnetoresistance (MR) shows a maximum upon decreasing the volume fraction of PANI-CSA in agreement with effective medium theory. Analysis of the MR indicates that the localization length near the percolation threshold is approximately 25 A at 4.2 K.

Metallic transport, magnetic susceptibility and reflectance spectra in heavily doped PPY(PF/sub 6/)

Summary form only given. We summarize some metallic features observed in heavily doped polypyrrole-hexafluorophosphate, PPy(PF/sub 6/). Measurements of the conductivity, thermoelectric power, electron spin resonance (ESR) and optical reflectance are carried out for the electroclunically prepared sample at low temperature (-40/spl deg/C), with the characteristic resistivity ratio P/sub r/=P(1.4K)/P(300K)=l.8. The sample shows a positive temperature coefficient of the resistivity (TCR) at temperatures below T/spl ap/15K. The thermoelectric power (S) increases linearly in proportion to the temperature, with S /spl ap/ 7 /spl mu/V/K at room temperature. High reflectance in far IR as well as a plasma resonance around 1.7 eV were observed in reflectance measurement. The temperature independent paramagnetic susceptibility down to 50K also confirms the existence of finite density of states at Fermi level. The results suggest that the electronic structure of PPy(PF/sub 6/) is characteristic of a disordered metal.

Effect of anisotropy on the temperature dependence of conductivity and magnetoconductance in heavily doped polyacetylene

Summary form only given. The temperature dependence of the conductivity and the magnetoresistance (MR) of heavily doped Shirakawa polyacetylene are measured for samples with different stretching ratio (1/1/sub 0/= 1/spl sim 6/). Unoriented samples show a strong temperature dependence of conductivity with the characteristic resistivity ratio Pr=P(I.4K)/p(300K)/spl sim/10/sup 2/. The conductivity anisotropy(/spl sigma////spl sigma//spl perp/ increases with stretching ratio upto /spl sigma/////spl sigma//spl perp//spl ap/110 for 1/1/sub 0/=6. For the samples of 1/1/sub 0//spl ges/3 P/sub r//spl sim/3 and nearly independent of 1/1/sub/0 and chain direction. Application of high pressure decreases P/sub r/, for both parallel and perpendicular direction without any variation in its anisotropy of conductivity. The magnetoconductance measured at low temperature (T<4.2K) is negative for unoriented samples and positive for stretched samples. The magnitude of magnetoconductance depends on the direction of magnetic field with respect to the chain direction.

Temperature dependence of resistivity and magnetoresistance in doped regioregular polyalkylthiophenes

Summary form only given, as follows. The temperature dependence of resistivity and magnetoresistance of iodine doped regioregular polybutylthiophene (PBT), polyhexylthiophene (PHT), polyoctylthiophene (POT) and polydecy1thiophene (PDT) has been studied in the present work. In general, the room temperature conductivity of regioregular polyalkylthiophene (PAT) is considerably higher than that of regiorandom PAT due to the enhanced structural order and conjugation length in the former. The temperature dependence of conductivity of regioregular PAT is nearly two orders of magnitJde weaker than the regiorandom PAT. This indicates that regioregular PATs are near the critical regime of disorder-induced metal-insulator transition (MIT), while regiorandom PATs are in the insulating regime. At high pressures the conductivity increases and the temperature dependence of conductivity is weaker due to enhanced interchain transport, especially for regioregular PATs. The role played by the chemical structure, morphology and disorder in the temperature dependence of conductivity and magnetoresistance for regioregular PATs has been studied in detail.