Hisashi Araki

Electrical and Optical Properties of Conducting Polymer - Fullerene and Conducting Polymer - Carbon Nanotube Composites

Abstract Various novel photo-physical properties such as a drastic photoluminescence quenching and photoconductivity enhancement and a photo-induced enhancement of low-field microwave absorption due to a highly effective photo-induced electron transfer have been observed in various conducting polymers doped with fullerenes and also acceptor-type molecules and conducting polymers. New types of junction devices utilizing effective charge separation at the interface of conducting polymer/C60, C60-doped conducting polymer/C60 and acceptor type conducting polymer / donor type conducting polymer have been proposed and their novel characteristics have been demonstrated. Novel concepts such as an interpenetrating network, a condensed interface and the effect of introduction of photo-harvesting antenna molecule at the interface have been discussed. Nano-composite films of carbon nanotube (NT) and conducting polymer were prepared. With increasing the volume fraction of NT in conducting polymer the conductivity incr...

Electrical Conductivity and ESR Spectrum of Buckminsterfullerene-Doped Poly(3-alkylthiophene)

Buckminsterfullerene (C60) is effectively doped in poly(3-alkylthiophene). However, the characteristics of C60-doped poly(3-alkylthiophene) are different from the typical behavior observed in the case of conventional dopants. Electrical conductivity of poly(3-alkylthiophene) increases upon C60 doping and decreases again at higher concentration of C60. Its temperature dependence is also anomalous. The absorption spectrum changes markedly upon C60 doping. ESR linewidth decreases from 7.1 G to 1.9 G upon C60 doping. Spin density, evaluated by ESR, is enhanced upon C60 doping. These doping characteristics are tentatively explained by taking electronic energy diagrams of both poly(3-alkylthiophene) and C60 into consideration.

Multiwalled carbon nanotubes with bamboo-like structure and effects of heat treatment

Microstructures of bamboo-like multiwalled carbon nanotubes prepared by pyrolysis of Ni-phthalocyanine are studied by high-resolution transmission electron microscopy. The intershell spacings of graphitic shells in these nanotubes are estimated. From pristine nanotubes with a tube diameter of 40–260 nm, the spacing values of 0.339–0.342 nm are obtained almost independent of their tube diameters. However, after heat treatment at 2800 °C they are reduced to a value equivalent to the interlayer spacing of graphite, and tube walls with a clear polygonal cross section come to be observed. The heat treatment of samples is also a very useful method to prepare metal-free nanotubes. The bamboo-like graphitic shells are interpreted to be determined by a cone-shaped Ni nanoparticle included at the base of the multiwalled carbon nanotube.

Field emission from aligned carbon nanotubes prepared by thermal chemical vapor deposition of Fe-phthalocyanine

The field emission from aligned multiwalled carbon nanotubes prepared by thermal chemical vapor deposition of Fe-phthalocyanine is reported, with emphasis on the current–voltage characteristics and current fluctuation. By running of a pristine emitter at high current and/or voltage for a long time, the onset voltage is lowered from 400 to 125 V and the field-enhancement factor increases from 360 to 1110, demonstrating geometrical reconstruction of the nanotube apex. Deviation from the Fowler–Nordheim relation is caused at a constant local electric field of ∼4×103 V/μm and is related to high-field conditioning. Electron emission current under a high current density condition (>10 mA/cm2) is stable and noiseless. The reason for current noise in the lower current density is discussed.

Electrical and optical properties of conducting polymer-C60-carbon nanotube system

Nanocomposite of multi-wall carbon nanotubes (MWNT) and C 60 -doped conducting polymer have been studied. MWNTs were prepared by the method of organic chemical vapor deposition (CVD) using metal phthalocyanines as starting material. We have examined the electrical and optical properties of the nanocomposite film of MWNT - C 60 -poly(3-hexylthiophene) (P3HT) with different volume fractions of MWNTs, The enhancement of photoconductivity of composite films has been found at near percolation threshold. The enhanced photoresponse was observed due to the efficient charge separation and reduced recombination of photogenerated charge carriers in MWNT-C 60 -P3HT composite film.

Characteristics of buckminsterfullerene doped conducting polymer

Abstract Conducting polymers such as poly(3-alkylthiophene) are effectively doped with buckminsterfullerene (C 60 ). Change of absorption spectrum and drastic quenching of photoluminescence have been observed upon C 60 doping. On the other hand, the slight enhancement of electrical conductivity and the change of ESR by C 60 doping is not so remarkable compared with conventional strong dopants. Photoconductivity of poly(3-alkylthiophene) is enhanced and the response time becomes shorter by C 60 doping, which suggests that C 60 is a weak dopant, providing mainly photoinduced charge transfers between C 60 and the polymer. These results are explained by taking electronic energy states of poly(3-alkylthiophene) and C 60 into consideration at account of polaronic effects in both C 60 and polymer. Small enhancement of electrical conductivity and significant quenching of photoluminescence have been also observed upon C 70 doping, but change of optical absorption is less remarkable than the case of C 60 doping.

Electrical and optical properties of molecularly doped conducting polymers

Abstract Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C60, C70 etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C60-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.

Granular superconductivity in a conducting polymer-fullerene-alkali metal composite

Abstract A superconducting (SC) phase is found in poly(3-alkylthiophene)-(C 60 ) y -K x composites by both low field microwave absorption (LFMA) and SQUID magnetometry. T C varies from 12 to 17 K depending on y and x : T C ( y = 0.05) = 17 K, at optimal x . LFMA shows phase reversal just below T C in composites with y > 0.025 indicating the granular SC phase of disordered K 3 C 60 clusters. The possibility of a two-component SC phase in which the electrons of conducting polymer chains are actively involved in SC pairing induced via hybridization with C 60 molecules is discussed for the case y

The temperature dependence of electron emission from a discontinuous carbon film device between silver film electrodes

Abstract Electron emission characteristics are studied by applying an a.c. (1 kHz) bias voltage of 11–16 V to the film device at temperatures of 77–423 K. The emission currents obey the Fowler-Nordheim equation in the lower voltage range but saturate in the higher voltage range. This saturation effect is dominant at high temperatures. It is shown that this can be caused by positive space charges distributed in excess in the cathode metal-insulator interface region of the discontinuous film and an increased space charge density with rising temperature. When the a.c. characteristics are plotted on a semilogarithmic graph, they shift toward higher voltage with increasing amplitude. From this result it is suggested that excess negative space charges are localized in the discontinuous films and increase in quantity as the amplitude increases.

Novel properties of new type conducting and insulating polymers and their composites

Novel properties of recently developed conducting and insulating polymers and their composites are discussed. Properties of conducting polymer whose main chains are composed of unsaturated /spl pi/-bonds depend strongly on the main chain structure, substituent and also molecular dopants. Various applications of conducting polymers such as electroluminescence (EL) elements, electrolyte capacitors, photoconductors, photovoltaic cells, superconductors and insulators at cryogenic temperature, are discussed by taking effects of molecular dopants such as C/sub 60/ into consideration. A new type of insulating polymer, syndiotactic polypropylene prepared by newly developed metallocene catalysts has been studied and found to exhibit much superior electrical, thermal and mechanical characteristics compared with those of conventional isotactic polypropylene, atactic polypropylene and polyethylene. These excellent characteristics originate from lower crystallinity, smaller spherulites and different crystal lattice than in isotactic polypropylene. Negligible degradation of syndiotactic polypropylene by contact with copper is interpreted in terms of difference of catalysts and suppression of diffusion of copper cation. New types of conducting polymer, insulating polymer composites were prepared. Their conductivity was controlled over more than 10 orders of magnitude by small amounts of a conducting polymer, polypyrrole, which can be interpreted in terms of the percolation model depending on the shape and density of polypyrrole coated insulating polymer particles. Nonlinear current-voltage characteristics were also studied.