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Intrinsic transport properties in polypyrrole
ELSEVIER
SyntheticMetals101 (1999) 509-510
Intrinsic transport properties in polypyrrole K. Yoshiokar,*, S. Masubuchi’, T. Fukuhara”, S. Kazama’ ‘Department of Physics, Chuo University, Kasuga, Brrrzkyo-ku, Tokyo, 112-8551 Japan *Aerospace Division, Japan Aviation Electronics hdustry, Ltd., Musashino, Akishima, Tokyo, 196-888.5 Japan 3Department of Liberal Arts and Sci., Toyarna Pret University, Kosugi, Toyarna, 939-0398 Japan
Abstract We have studied transport properties in polypyrrole (PPy) doped with CF,SO; and ClO,-. We measuredthe dc resistivity r&T), the absolute thermoelectric power S(?J,and the dc resistivity with the VSC (voltage-shorted-compaction)technique pvsc(7’)as a function of temperature. We aim to abstract the intrinsic transport properties inherent in the crystallized regions in PPy. At high temperatures between 10 and 200K, ~~~(7’) shows a metallic behavior following a temperaturedependenceapproximately expressedby T”*, which is weaker than in the ordinary metals. Below 4K, p&T) in Clod- dopedPPy shows to be semiconducting. The result is interpreted in terms of a metal-insulator transition within the crystallized regions. Keywords: Electrochemical polymerization, Transport measurements, Thermopower, Voltage-Shorted-Compaction, phase transitions, Polypyrole and derivatives.
Metal-imulator
Conductive polypyrrole (PPy) has been extensively studied for its interesting transport properties [ 13. In view of the sub-macroscopic morphology and the electrical transport, conducting polymer is composed of two different regions. The one is non-crystallized regions and the other is the crystallized ones. Although the bulk transport property is semiconducting, the transport in the crystallized regions is believed to be metallic, which originates the metallic nature of the conducting polymers as a whole. However, it is difficult to show the intrinsically metallic nature of the crystallized regions themselvesthrough experimental manifestations. We have conducted a series of transport experiments aiming to clarify the metallic transport intrinsic to the crystallized regions in conductive PPy. In this work we report on PPy doped with CFSO?- and Clod-. To abstract the intrinsic properties, we have measuredthe temperaturedependenceof(i) dc resistivity, (ii) the absolute thermoelectric power and (iii) the dc resistivity with the VSC (voltage-shorted-compaction) technique. The temperature dependence of dc resistivity is semiconducting, but on the other hand the temperature dependence of both the absolute thermoelectric power and the VSC resistanceis metallic.
smaller than 100 PA to ensure the ohmic relationship. The absolute thermoelectric power, S(7J, was measured with an appropriate temperature difference across the sample between 5 and 300K. On the VSC measurements,the surface of the sample film was first filed to remove a damaged layer, and then it was painted very thinly with a silver paste. Temperaturedependence of the VSC resistivity (VSC resistivity ratio : pvsc(T) = Rvsc(7J/ Rvsc(280K)) was measured for the painted film with the same method as dc resistivity. The VSC measurementrelies entirely on a successful production of short circuit(s) with the conducting pastebridging the islands of crystallized regions that exist in the sea of non-crystallized area. The resistance of the successful VSC sample consists of series resistances from two areas:The one is from the crystallized regions while another is from the silver paste. We have no exact way to measurethe resistanceof the respective area. Therefore, the absolute value of the resistanceitself has no significant meaning. Only the temperature dependence of the resistance is meaningful. For a successful VSC measurement, pvsc(T) shows a larger temperature dependence than for a silver paste (Fig.1). In such a case, the silver paste produces a conducting path including at least one crystallized region. As the successful production of bridging is stochastic, we made a number of measurementsfor ensuring the reliability and the reproducibility.
2. Experimental
3. Results and Discussion
PPy films were synthesized by electrochemical polymerization with a constant current density. The reaction was carried out under an argon atmosphereat -20 “C. The temperature dependence of dc resistivity, pd,(7’), was measuredwith the ordinary four-probe method between 1.4 and 300K. The dc current applied to the PPy film was set to be
The pd,(T) in PPy decreasesmonotonically with temperature regardlessof dopant species,a semiconductor-typebehavior. The resistance ratio, R(n/R(280K), changes by less than a decade between 1.4 and 300K. Such a temperaturedependenceis among the smallest for the ordinary conducting polymers. S(7) at 280K ranges between t3 and +7pV/K. The value is as
1. Introduction
0379-6779/99/$ - seefront matter0 1999ElsevierScienceS.A. All rightsreserved. PII: SO379-6779(98)01341-l
510
K. Yoshioka et al. I Synthetic Metals 101 (1999) 509-510
small as an ordinary metal. Decreasing temperatureis followed by a monotonic decrease in S(T) in proportion to T. A linear relationship to T with an almost zero intercept is indicative of a typical metallic behavior. The dc resistivity, however, shows a semiconducting temperaturedependence,due to the existence of non-crystallized regions between crystallized regions. In the resistivity measurement with an applied current flow from outside, the former plays a role of potential barriers. The transport property within the non-crystallized regions is semiconducting. It is difficult to unveil entirely the intrinsic transport property by meansof the macroscopicmeasurementsof pdc(T)becauseof the overwhelming effect from the non-crystallized regions. The absolute thermoelectric power can reflect the transport properties in crystallized regions, because there exists no current flow applied from outside [2]. Semiconducting transport properties in the non-crystallized regions do not play a major role in S(n, thus allowing to abstract the transport properties inherent in the crystallized regions. For further studying the intrinsic transport properties in the crystallized regions, we applied the VSC resistivity measurement in a wide temperaturerange between 0.5 and 300K. In Fig. 1, the temperature dependence of pvsc(T) for PPy(C104-) and PPy(CF,S03-)is depicted. Also included is the resistanceratio of a silver pastethat is painted thickly upon a PPy(cl0~‘) film. More than 30 samples were investigated for the respective sample, which showed similar tendencies. Since the reproducibility is reasonablefor all the samples,the results presentedhere can be considered to successfully abstract the temperature dependence of the resistivity in the crystallized regions. pvso(T) shows a metallic temperature dependence in a wide temperature range above 4K regardless of dopant species. This metallic p&T) correspondsto the metallic S(T). This result demonstratesthat the intrinsic transport properties in crystallized regions in PPy is metallic regardless of dopants. p~sc(T) follows a power law expressed by p&T)=T”’ between 10 and TOOK. This temperaturedependenceis weaker than that in elemental metals. in our previous work, we showed that pv~c(T)in other conducting polymers shows a steepertemperaturedependencethan ordinary metals, i.e., pvsc(q 0~7” (n 2 2) for polyacetylene (PA) [3] and polythiophene (PT) [4]. Such a steeptemperaturedependenceof pvsc(lT)originates probably from a low dimensional electronic system [4], which is not observedin PPy. pvsc(7’)in PPy shows a weak temperature dependence above 4K in all the samples regardlessof dopant species. This observation is peculiar to PPy. The intrinsic transport in PPy might be different from other conducting polymers like PA and PT. Below 4K, we observed two types of temperature dependence in pvsc(7J. Typel: it is metallic down to 0.5K for PPy(CF$03-). Type2: the temperature dependenceof p&T) changesfrom metallic to semiconducting for PPy(ClOd-), i.e., an increase in p&T) on lowering temperature below 4K. A similar change in the temperature dependence in pvsc(T) has been observed in other conducting polymers such as PT [4]. Our preliminary result of specific heat, C(Q, in PPy(C104-), shows an anomalous behavior at low temperatures[5]. Combining the results of p&7+) and C(r) for PPy(C104-)at low temperatures,a metal-insulator (semiconductor) phasetransition takes place at low temperatures in crystallized regions in PPy(C104-).
0.70 0.50 0.40 0.30 0.20
0.10 0.09
T WI Fig. 1 Temperaturedependenceof the VSC resistanceratio pvsr(n. 4. Conclusions
As in the ordinary conducting polymers, PPy showed a semiconducting temperature dependencein the bulk resistivity and a metallic temperature dependence in the absolute thermoelectric power. The former reflects the transport in noncrystallized regions, while the latter that in crystallized regions. Our findings based on the VSC measurement is that the resistivity intrinsic to the crystallized regions in PPy is metalhc over a wide temperature range between 0.5 and 200K for PPy(CF3S03‘), and between 4 and 200K for PPy(CIOJ-j. In PPy(C104-),we found a tu&ov@ in the temperature dependence at 4K. Referring to the anomalousC(T), we consider the changein the temperature dependenceas a phasetransition in crystallized regions. The other point in the intrinsic transport properties in PPy is that pv~(T) has a weaker temperaturedependence regardlessof dopant speciesthan in otherconducting polymers like PA and PT. This is probably due to a low degreeof crystallinity and different characteristic of the electronic structure in PPy. Acknowledgment
Our thanks are due to Dr. Y. Aoki, Prof. H. Sato and Prof. K. Mizoguchi of Tokyo Metropolitan University, and Dr. J. Takada and Prof. T. Matsuyamaof the ResearchReactor institute of Kyoto University. References
[l] R. S. Kohlman and A. J. Epstein, Handbobk of Conducting Polymers, edited by T. Skotheim (Dckker, New York, 1988) [2] A.B. Kaiser, Phys. Rev. B40 (1989) 2806. [3] S. Masubuchi, K. Mizoguchi, K. Mizuno, K. Kime and H. Shirakawa, Synth. Met., 22 (1987) 41. [4] S. Masubuchi, S. Kazama,Synth. Met., 74 (1995) 151. [5] Y. Aoki et. al., unpublished data.
SyntheticMetals101 (1999) 509-510
Intrinsic transport properties in polypyrrole K. Yoshiokar,*, S. Masubuchi’, T. Fukuhara”, S. Kazama’ ‘Department of Physics, Chuo University, Kasuga, Brrrzkyo-ku, Tokyo, 112-8551 Japan *Aerospace Division, Japan Aviation Electronics hdustry, Ltd., Musashino, Akishima, Tokyo, 196-888.5 Japan 3Department of Liberal Arts and Sci., Toyarna Pret University, Kosugi, Toyarna, 939-0398 Japan
Abstract We have studied transport properties in polypyrrole (PPy) doped with CF,SO; and ClO,-. We measuredthe dc resistivity r&T), the absolute thermoelectric power S(?J,and the dc resistivity with the VSC (voltage-shorted-compaction)technique pvsc(7’)as a function of temperature. We aim to abstract the intrinsic transport properties inherent in the crystallized regions in PPy. At high temperatures between 10 and 200K, ~~~(7’) shows a metallic behavior following a temperaturedependenceapproximately expressedby T”*, which is weaker than in the ordinary metals. Below 4K, p&T) in Clod- dopedPPy shows to be semiconducting. The result is interpreted in terms of a metal-insulator transition within the crystallized regions. Keywords: Electrochemical polymerization, Transport measurements, Thermopower, Voltage-Shorted-Compaction, phase transitions, Polypyrole and derivatives.
Metal-imulator
Conductive polypyrrole (PPy) has been extensively studied for its interesting transport properties [ 13. In view of the sub-macroscopic morphology and the electrical transport, conducting polymer is composed of two different regions. The one is non-crystallized regions and the other is the crystallized ones. Although the bulk transport property is semiconducting, the transport in the crystallized regions is believed to be metallic, which originates the metallic nature of the conducting polymers as a whole. However, it is difficult to show the intrinsically metallic nature of the crystallized regions themselvesthrough experimental manifestations. We have conducted a series of transport experiments aiming to clarify the metallic transport intrinsic to the crystallized regions in conductive PPy. In this work we report on PPy doped with CFSO?- and Clod-. To abstract the intrinsic properties, we have measuredthe temperaturedependenceof(i) dc resistivity, (ii) the absolute thermoelectric power and (iii) the dc resistivity with the VSC (voltage-shorted-compaction) technique. The temperature dependence of dc resistivity is semiconducting, but on the other hand the temperature dependence of both the absolute thermoelectric power and the VSC resistanceis metallic.
smaller than 100 PA to ensure the ohmic relationship. The absolute thermoelectric power, S(7J, was measured with an appropriate temperature difference across the sample between 5 and 300K. On the VSC measurements,the surface of the sample film was first filed to remove a damaged layer, and then it was painted very thinly with a silver paste. Temperaturedependence of the VSC resistivity (VSC resistivity ratio : pvsc(T) = Rvsc(7J/ Rvsc(280K)) was measured for the painted film with the same method as dc resistivity. The VSC measurementrelies entirely on a successful production of short circuit(s) with the conducting pastebridging the islands of crystallized regions that exist in the sea of non-crystallized area. The resistance of the successful VSC sample consists of series resistances from two areas:The one is from the crystallized regions while another is from the silver paste. We have no exact way to measurethe resistanceof the respective area. Therefore, the absolute value of the resistanceitself has no significant meaning. Only the temperature dependence of the resistance is meaningful. For a successful VSC measurement, pvsc(T) shows a larger temperature dependence than for a silver paste (Fig.1). In such a case, the silver paste produces a conducting path including at least one crystallized region. As the successful production of bridging is stochastic, we made a number of measurementsfor ensuring the reliability and the reproducibility.
2. Experimental
3. Results and Discussion
PPy films were synthesized by electrochemical polymerization with a constant current density. The reaction was carried out under an argon atmosphereat -20 “C. The temperature dependence of dc resistivity, pd,(7’), was measuredwith the ordinary four-probe method between 1.4 and 300K. The dc current applied to the PPy film was set to be
The pd,(T) in PPy decreasesmonotonically with temperature regardlessof dopant species,a semiconductor-typebehavior. The resistance ratio, R(n/R(280K), changes by less than a decade between 1.4 and 300K. Such a temperaturedependenceis among the smallest for the ordinary conducting polymers. S(7) at 280K ranges between t3 and +7pV/K. The value is as
1. Introduction
0379-6779/99/$ - seefront matter0 1999ElsevierScienceS.A. All rightsreserved. PII: SO379-6779(98)01341-l
510
K. Yoshioka et al. I Synthetic Metals 101 (1999) 509-510
small as an ordinary metal. Decreasing temperatureis followed by a monotonic decrease in S(T) in proportion to T. A linear relationship to T with an almost zero intercept is indicative of a typical metallic behavior. The dc resistivity, however, shows a semiconducting temperaturedependence,due to the existence of non-crystallized regions between crystallized regions. In the resistivity measurement with an applied current flow from outside, the former plays a role of potential barriers. The transport property within the non-crystallized regions is semiconducting. It is difficult to unveil entirely the intrinsic transport property by meansof the macroscopicmeasurementsof pdc(T)becauseof the overwhelming effect from the non-crystallized regions. The absolute thermoelectric power can reflect the transport properties in crystallized regions, because there exists no current flow applied from outside [2]. Semiconducting transport properties in the non-crystallized regions do not play a major role in S(n, thus allowing to abstract the transport properties inherent in the crystallized regions. For further studying the intrinsic transport properties in the crystallized regions, we applied the VSC resistivity measurement in a wide temperaturerange between 0.5 and 300K. In Fig. 1, the temperature dependence of pvsc(T) for PPy(C104-) and PPy(CF,S03-)is depicted. Also included is the resistanceratio of a silver pastethat is painted thickly upon a PPy(cl0~‘) film. More than 30 samples were investigated for the respective sample, which showed similar tendencies. Since the reproducibility is reasonablefor all the samples,the results presentedhere can be considered to successfully abstract the temperature dependence of the resistivity in the crystallized regions. pvso(T) shows a metallic temperature dependence in a wide temperature range above 4K regardless of dopant species. This metallic p&T) correspondsto the metallic S(T). This result demonstratesthat the intrinsic transport properties in crystallized regions in PPy is metallic regardless of dopants. p~sc(T) follows a power law expressed by p&T)=T”’ between 10 and TOOK. This temperaturedependenceis weaker than that in elemental metals. in our previous work, we showed that pv~c(T)in other conducting polymers shows a steepertemperaturedependencethan ordinary metals, i.e., pvsc(q 0~7” (n 2 2) for polyacetylene (PA) [3] and polythiophene (PT) [4]. Such a steeptemperaturedependenceof pvsc(lT)originates probably from a low dimensional electronic system [4], which is not observedin PPy. pvsc(7’)in PPy shows a weak temperature dependence above 4K in all the samples regardlessof dopant species. This observation is peculiar to PPy. The intrinsic transport in PPy might be different from other conducting polymers like PA and PT. Below 4K, we observed two types of temperature dependence in pvsc(7J. Typel: it is metallic down to 0.5K for PPy(CF$03-). Type2: the temperature dependenceof p&T) changesfrom metallic to semiconducting for PPy(ClOd-), i.e., an increase in p&T) on lowering temperature below 4K. A similar change in the temperature dependence in pvsc(T) has been observed in other conducting polymers such as PT [4]. Our preliminary result of specific heat, C(Q, in PPy(C104-), shows an anomalous behavior at low temperatures[5]. Combining the results of p&7+) and C(r) for PPy(C104-)at low temperatures,a metal-insulator (semiconductor) phasetransition takes place at low temperatures in crystallized regions in PPy(C104-).
0.70 0.50 0.40 0.30 0.20
0.10 0.09
T WI Fig. 1 Temperaturedependenceof the VSC resistanceratio pvsr(n. 4. Conclusions
As in the ordinary conducting polymers, PPy showed a semiconducting temperature dependencein the bulk resistivity and a metallic temperature dependence in the absolute thermoelectric power. The former reflects the transport in noncrystallized regions, while the latter that in crystallized regions. Our findings based on the VSC measurement is that the resistivity intrinsic to the crystallized regions in PPy is metalhc over a wide temperature range between 0.5 and 200K for PPy(CF3S03‘), and between 4 and 200K for PPy(CIOJ-j. In PPy(C104-),we found a tu&ov@ in the temperature dependence at 4K. Referring to the anomalousC(T), we consider the changein the temperature dependenceas a phasetransition in crystallized regions. The other point in the intrinsic transport properties in PPy is that pv~(T) has a weaker temperaturedependence regardlessof dopant speciesthan in otherconducting polymers like PA and PT. This is probably due to a low degreeof crystallinity and different characteristic of the electronic structure in PPy. Acknowledgment
Our thanks are due to Dr. Y. Aoki, Prof. H. Sato and Prof. K. Mizoguchi of Tokyo Metropolitan University, and Dr. J. Takada and Prof. T. Matsuyamaof the ResearchReactor institute of Kyoto University. References
[l] R. S. Kohlman and A. J. Epstein, Handbobk of Conducting Polymers, edited by T. Skotheim (Dckker, New York, 1988) [2] A.B. Kaiser, Phys. Rev. B40 (1989) 2806. [3] S. Masubuchi, K. Mizoguchi, K. Mizuno, K. Kime and H. Shirakawa, Synth. Met., 22 (1987) 41. [4] S. Masubuchi, S. Kazama,Synth. Met., 74 (1995) 151. [5] Y. Aoki et. al., unpublished data.