S. Masubuchi

Magnetic properties in polypyrrole doped by series of dopants

Abstract Magnetic susceptibility and ESR linewidth are measured as a function of temperature in polypyrrole (PPy) doped with PF 6 −, AsF 6 −, ClO 4 −, BF 4 −, ToS − . Spin susceptibility by Schumacher-Slichter (ESR-NMR) technique shows both the temperature independent Pauli term ranging from 0.7 to 1.4×10 −5 emu/mol-ring and the Curie term of 1 spin per 600~1500 rings, which is consistent with that obtained by SQUID measurement in the same sample. The temperature dependence of ESR linewidth is ascribed to the Elliott mechanism typical for the metallic electron spins, increasing monotonically with the temperature. This interpretation is also consistent with the ESR linewidth in polythiophene doped with ClO 4 − and AsF 6 −. Therefore, both the susceptibility and the ESR linewidth demonstrate the PPys doped with several dopants to have a metallic electronic state.

Pressure-induced metallic resistivity of PF6− doped poly(3-methylthiophene)

Abstract Electrical resistivity of poly(3-methylthiophene) doped with PF6− has been studied under pressure up to 13 kbar using a self-clamped beryllium-copper pressure cell. Application of the pressure induces a metallic temperature dependence in resistivity at low temperatures. Below about 4 K, T 1 2 dependence in electrical conductivity was observed, which can be explained taking the electron-electron interaction in a disordered system into account. We show that the extended heterogeneous model by Kaiser and Graham gives a good fit to the temperature dependence of the conductivity at low temperatures in the whole range of the pressure applied.

Mossbauer study of polythiophene and its derivatives

Summary form only given. Polythiophene and its derivatives are highly conducting polymers when doped with appropriate chemical species such as iodine or FeCl/sub 3/. We have investigated the chemical structure of the dopants in these polymers using / sup 129/I and /sup 57/Fe, which are Mossbauer nuclei of good resolution. It has been found that the iodine in these polymers exists as linear forms of triiodide and pentaiodide anions and that the abundance ratio of triiodide decreases with increasing of the doping level. Since the polymers were doped with the iodine of neutral molecules, these anions are created by electron transfer between the polymers and the dopants. Our experimental results are consistent with the proposed mechanism of the electric conductivity which is due to migration of polarons on the polymer chains.

Intrinsic transport properties in electrochemically prepared polythiophene doped with PF6

Abstract We have studied the transport properties in polythiophene doped with PF 6 − . The intrinsic transport properties inherent in the crystallized regions were abstracted from the precise experiments of the d.c. resistivity ρ dc ( T ), voltage-shorted-compaction (VSC) resistance ratio r VSC ( T ) and absolute thermoelectric power S ( T ) as a function of temperature at 1.6–300 K. At high temperatures between 35 and 80 K, r VSC ( T ) clearly exhibits a metallic behavior following the temperature dependence approximately expressed by T 2.7 , characteristic of the low- dimensional electronic structure. Unlike the behavior above 35 K, r VSC ( T ) increases steeply with decreasing temperature below 35 K. Corresponding to r VSC ( T ), S ( T ) also shows a metallic behavior at high temperatures, while at low temperatures an anomalous behavior is observed as reported in tetracyanoquinodimethane (TCNQ) salt. The results are interpreted in terms of a metal-insulator transition within the crystallized region that is believed to originate the fundamental transport character of conducting polymer as a novel low-dimensional metal.

Metallic transport properties in electrochemically as-grown and heavily doped polythiophene and poly(3-methylthiophene)

Abstract We have studied the temperature dependence of the ordinary 4-terminal conductivity, VSC conductivity, thermoelectric power (TEP) and low-freqeuency ESR for ClO4-doped (as grown) polythiopene (PT) and poly(3-methylthiophene) (PMeT) as well as for those doped further with HClO4. Al high temperatures the VSC conducttivity changes with T−2 in the as-grown PT, whereas it changes with T−1-in the as-grown PMeT. Such a metallic behaviour with temperature was also obtained in the low-frequency ESR linewidth. Results of the TEP experiments were suggestive as well of the metallic characteristics. The temperatures results of both the VSC and the TEP measurements indicate that a metal-insulator (or semiconductor) transition takes place at 30K for PT and 10K for PMet.

Resonance Raman Scattering of Br2 Doped Single-Walled Carbon Nanotube Bundles

Abstract We have measured Raman spectra of bromine doped single-walled carbon nanotubes (SWNTs) using various laser lines to clarify the electronic states of the doped SWNT. In the case of evacuated sample after full doping, two breathing mode peaks were observed simultaneously by visible laser excitations. We assigned the higher frequency peak to the doped SWNT bundles, and the other peak to the undoped portions in the sample. Intensity ratio between them decreased with decreasing excitation energy, and in the infrared region, the breathing mode band of the doped bundle was not observed. These results can be explained by a simple rigid band model.

ESR linewidth in conducting polymers with five-membered ring

Abstract The ESR linewidths in polythiophene, poly(3-methylthiophene) and polypyrrole doped with ClO4− and AsF6− were measured as a function of temperature to study the dynamics of charged carriers with spin. The temperature dependence of the ESR linewidth can be understood in terms of the Elliott mechanism, characteristic for metallic electrons. Such an interpretation leads us to conclude that the electron scattering is not governed by potential of dopants, but by that on polymer backbone. In addition, the temperature dependence of the resistance ratio rVSC obtained by voltage-shorted-compaction method was found to coincide with that of the linewidth qualitatively. We show that a simple one-dimensional model for the resistivity that takes the electron scattering only by 2kF-phonons into consideration can qualitatively well explain both the data.

A possible phase transition in metallic polythiophene, and poly(3-methylthiophene) at low temperature

Summary form only given. Polythiophene (PT) and poly(3-methylthiophene) (PMeT) films prepared by electrochemical synthesis are flexible and highly conducting (/spl sigma/>100S/cm). We have studied the transport properties in as-grown PT and PMeT doped with PF/sub 6//sup -/. Precise measurements were made of the Voltage-Shorted-Compaction (VSC) resistivity (P/sub vsc/) and the absolute thermoelectric power S(T) between 1.6-300K, both of which are unique in abstracting the transport properties inherent to the crystallized regions of polymeric materials. In PT, (P/sub vsc/) exhibited a metallic behaviour at high temperatures between 30-100K with the temperature dependence approximately expressed by T/sup 2.5/, characteristic of the low dimensional electronic structure. Unlike the behaviour above 30K, (P/sub vsc/) increased steeply with decreasing temperature below 30K. Corresponding to (P/sub vsc/), S(T) also showed metallic behaviour at high temperatures, while at low temperatures an anomalous behaviour was observed as in TTF-TCNQ. The results were strongly indicative that a metal-insulator transition would occur in the crystallized regions at 30K. In PMeT, (P/sub vsc/) was metallic between 5-300K, with a T/sup 2/-dependence especially between 5-40K. Below 5K it became almost independent of temperature. The difference of the transport property of the bulk material of PT and PMeT is to be understood by their different transition temperatures.

Metallic temperature dependence of resistivity in heavily doped polyacetylene by NMR

Summary form only given. H-NMR spin-lattice relaxation rate T/sub 1//sup -1/ of heavily doped polyacetylene was measured in an attempt to study the spin dynamics of its conduction electrons. Films synthesized by standard Shirakawa method and non-solvent method were doped with FSO/sub 3/H, HClO/sub 4/, iodine, bromine and potassium in vapor phase. Against our intention, in all of the samples a metallic behavior in the T/sub 1//sup -1/ arising from quasi-one-dimensional motion of conduction electrons was not found for some time after doping, and it was revealed that the T/sub 1//sup -1/ was governed by a relaxation mechanism due to residual Curie spins. The results of T/sub 1//sup -1/ for the samples other than the bromine-doped sample did not show the metallic behavior after all, whereas for the bromine-doped one a change in the temperature dependence of T/sub 1//sup -1/ from the non-metallic behavior to the metallic one was found as time passed. It was confirmed that this change was caused by the partial substitution of hydrogen atoms in the polyacetylene chain by bromine atoms. We first succeeded in deducing the temperature dependence of intrinsic resistivity for metallic polyacetylene from the data of T/sub 1/sup -1/.

Bundle Effects of Single-Wall Carbon Nanotubes

To see the bundle effects on the electronic and the vibrational properties of single-wall carbon nanotubes (SWNTs), we have measured the resonance Raman scattering of isolated SWNTs and thick bundles. For the measurements of the isolated SWNTs, we used an evacuated sample after bromine doping. A broad and asymmetric tangential mode band, which is a sign of the resonance of the metallic SWNTs and can be fitted by a Fano line shape, is not observed in the isolated SWNTs. This suggests the inter-tube interactions play an important role to the Fano interference. On the other hand, the purified sample shows very thick acquired bundles. We observed 4% higher breathing mode frequencies than in the pristine sample. Further, in the case of multi-wall nanotubes, we observed 5% higher breathing mode frequencies than the SWNTs. These results can be explained by the interlayer interactions.