F. Zuo

Insulator-to-metal transition in polyaniline

Abstract The emeraldine base (EB) form of polyaniline can be varied from insulating ( σ −10 ohm −1 cm −1 ) to conducting ( σ ≈ 10 +1 ohm − cm − ) through protonation. That is, the number of electrons on the polymer backbone is constant while the number of protons is increased. We present here extensive magnetic, optical and transport data that demonstrate that the resulting emeraldine salt (ES) is metallic with a finite density of states at the Fermi energy. The results are consistent with segregation into fully protonated emeraldine salt and unprotonated emeraldine base polymer regions. it is proposed that the observed transition is an isolated bipolaron-to-polaron lattice transition. The correspondence of this concept to the disproportionation between protonated imine plus amine to form two semiquinones is shown.

Insulator-to-metal transition in polyaniline: Effect of protonation in emeraldine

Abstract The emeraldine base form of the polymer can be varied from insulating ( σ ∼ 10 −10 ohm −1 cm −1 ) to conducting ( σ ∼ 10 0 ohm −1 cm −1 ) states through protonation. Based upon extensive magnetic, optical and transport data, we demonstrate that the resulting emeraldine salt is metallic with a finite density of states at the Fermi energy. The roles of a novel bipolaron-to-polaron lattice transition and phase segregation into conducting and non-conducting regions are discussed.

Charge transport in the “emeraldine” form of polyaniline

Abstract The conductivity and the dielectric constant of the “emeraldine” form of polyaniline is studied in a wide range of frequency (dc, 10 1 –10 5 Hz, 6.5×10 9 Hz), temperature (20K–340K) and protonation level (x ≡[C1]/[N] =0.0–0.5). Two distinct behaviors are observed: Firstly, for low protonation (x ≤ 0.13), hopping of charges between polarons and bipolarons leads to dipolar relaxation ( ϵ RF (T)). Oscillation of polarons around pinning centers is suggested to be responsible for the high frequency dielectric constant of low protonated samples. Secondly, for intermediate to highly protonated samples (x > 0.22), the presence of “metallic islands” is apparent. Oscillation of charge carriers within the coherence length contributes to ϵ μω (T=0). The intrinsic conductivity within the islands is estimated to be of the order of 250 S/cm. The T-dependence of the dielectric constant indicates an increase of the coherence length with increasing temperature. The role of localization and the nature of “textured metallic islands” are discussed.

Solution studies of the emeraldine oxidation state of polyaniline

Abstract Optical studies of emeraldine base in solutions show a red shift in both 2eV and 3.8eV absorption peaks compared with the optical absorption for emeraldine films and powders. As for solids, the 2.0eV peak is absent for the salt form. Unlike the salt film, the strong absorption peak is narrow without the free electron absorption tail, suggestive of strong localization of polarons in the solution form. Dilution studies show there is no shift in the absorption peaks with concentration in both emeraldine base and salt, indicating that optical absorption is an intrachain interaction. Temperature dependent UV-VIS spectra show the first thermochromatic effect in a system without side groups that suggests possible ring flipping. Photoinduced optical studies of emeraldine base polymer in solution shows photoinduced absorption peaks at 1.4eV and 3.0eV and photoinduced bleaching at 1.8eV. The photoinduced absorption peak observed at 0.9eV in film is no longer present. Its disappearance implies the importance of interchain interactions for the formation of the electronic state. In contrast, the 1.4eV absorption is from the interchain excitation of polarons.

Decamethylchromocenium tetracyanoethenide, (Cr(C5Me5)2]:.+[TCNE]˙+: a molecular ferromagnet with Tc= 3.65 K

The electron-transfer salt 4[Cr(C5Me5)2]+[TCNE]˙–(TCNE = tetracyanoethylene) has been prepared and structurally characterized by single-crystal X-ray diffraction. It possesses a solid-state motif of parallel 1D chains of alternating radical rations and anions similar to, but not isostructural to, the ferromagnet [Fe(C5Me5)2]˙+[TCNE]˙–·4[Cr(C5Me5)2]+[TCNE]˙– is a soft ferromagnet with no hysteretic effects observed and Tc= 3.65 K.

Paramagnetic limiting of the upper critical field of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2)

We report detailed measurements of the interlayer magnetoresistance of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) for temperatures down to 0.5 K and fields up to 30 T. The upper critical field is determined from the resistive transition for a wide range of temperatures and field directions. For magnetic fields parallel to the layers, the upper critical field increases approximately linearly with decreasing temperature. The upper critical field at low temperatures is compared to the Pauli paramagnetic limit, at which singlet superconductivity should be destroyed by the Zeeman splitting of the electron spins. The measured value is comparable to a value for the paramagnetic limit calculated from thermodynamic quantities but exceeds the limit calculated from BCS theory. The angular dependence of the upper critical field shows a cusplike feature for fields close to the layers, consistent with decoupled layers.

Magnetic and transport properties of the alloys

Magnetization and transport measurements have been performed to study the martensitic and pre-martensitic transitions for a series of ferromagnetic Heusler alloys. Both magnetization and resistivity measurements show a clear jump at the martensitic transition and a discontinuous slope change at the pre-martensitic transition. The characteristic temperatures correspond well with those derived from previous direct structural results from neutron scattering, electron microscopy and ultrasonic studies. The martensitic transition temperature decreases with increasing Mn concentration.

Highly conducting polyacetylene

Abstract “New”-(CH) x and “ARA” (i.e. A dditional R educing A gent)-(CH)x have been synthesized at room temperature. “New”-(CH)x is formed by polymerizing acetylene on flat polymeric or glass substrates using an AlEt3/Ti (n-BuO)4 catalyst in silicone oil which has previously been heat-treated. Additional reducing agents may be added to the catalyst after its thermal treatment to produce “ARA”-(CH)x. The stretch-oriented “New”-(CH)x polymerized on glass and “ARA”-(CH)x polymerized on polyethylene can be p-doped with I2/CCl4 to give conductivities of ∼50,000 and ∼75,000 S/cm respectively. Optical absorption, photoinduced infrared absorption, magnetic susceptibility and conductivity studies on selected forms of the above doped and undoped types of (CH)x are discussed. The electronic structure of highly conducting “New”-and “ARA”-(CH)x is observed to be essentially identical to that of conventional Shirakawa-(CH)x.

Frequency Dependent Conductivity of Emeraldine: Absence of Protonic Conductivity

Abstract An experimental study of the frequency dependence of the conductivity of emeraldine polymer as a function of moisture content and protonation level is presented. A decrease in the conductivity and dielectric constant are observed to occur with pumping. The detailed frequency and composition behavior of these data are inconsistent with the proposed proton exchange assisted conduction of electrons (PEACE). The experimental results are shown to be consistent with the role of charging energy limited tunneling among small granular polymeric metal particles formed upon protonation of emeraldine.

Transport and magnetic studies of segmented polyacetylene

Abstract Earlier studies of conducting polymers focused on polyacetylene, (CH) x , where extended conjugation lengths and crystallinity are present. We present here the results of a detailed study of temperature dependent magnetic susceptibility and ac and dc transport of [CHD y ] and [CHD y +z A z − ] x where A is oxygen and iodine and 0.02≤y≤0.17. The CHD y introduce sp 3 defects along the otherwise sp 2 bonded polyacetylene chains and result in “segmentation” of the chain. Our results show that the presence of the sp 3 bonded carbons has only a moderate effect on the conjugation of the pi band of polyacetylene.