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Low temperature thermoelectric power of the metal-halide doped polyacetylene
ELSEVIER
Synthetic
Metals
84 (1997)
747-748
Low temperature thermoelectric power of the metal-halide doped polyacetylene E. S. Choi”, Y. H. Seola’, Y. S. Song!, Y. W. Park” ‘Department ofPhysics, Seoul National Universig, Seoul 151-742, South Korea bResearch Institute for Basic Sciences, Seoul National Givers@, Seoul 1.51-742, South Korea
Abstract We have measured thermoelectric power (TEP) of the doped polyacetylene (PA) from room temperature down to liquid helium temperature. The dopants are Au&, NbC15, Fe& and 12. The overall TEP data show the diffisive metallic property, i.e. the TEP is positive and it decreases quasi-linearly upon cooling. For the metal-halide doped PA, the TEP is temperature independent with small magnitude about 0.2-0.6pV/K below 1% while the quasi-linear temperature dependent TEP is observed for the iodine doped PA in the same low temperature regime. The concentration dependence of TEP for Au& doped PA shows that the TEP becomes negative at TaOK-30K depending on the doping concentration with a broad minimum peak at low temperature. The result indicates that the effect of dopant to the PA chain is important to understand the metallic nature of heavily doped PA. Kqwords
: thermopower,
polyacetylene and derivatives
1. Introduction The unusal metallic properties of heavily doped polyacetylene (PA) has not been well understood because of the randomly entangled fibriallr morphology of the polymer. Recently, however, there has been a considerable progress in understanding the unusual metallic properties of PA by considering the heavily doped PA as a highly disordered metal[lJ]. But most of the arguments have ignored the effect of dopants to the polymer chains. This is mainly because the measured conductivity for various doped polymers turned out to be rather insensitive to the dopant species. On the other hand, TEP is a very good probe to study the effect of dopants to the polymer chain since TEP is extremely sensitive to the scattering of charge carriers. The temperature dependent TEP of transition metal halide doped polyacetylene is known to be different from that of iodine or AsFs doped polyacetylene. The difference has been analyzed based on the possible different scattering mechanism between the conduction electrons in the polymer chain and the localized spins in the dopants[3]. Alternatively, the knee shape enhancement on top of the linear diffusive TEP for the FeCls doped PA has been interpreted as due to the electron-phonon interaction enhancement effect[4]. Since the heavily doped PA has a lot of disorders originated mainly from the morphological complexity of the polymer, it can be viewed as an amorphous metal. On the other hand, the significant knee shape enhancement effect is seen only in magnetic impurity doped PA such as [CH(F~C~&C.& There is no clear knee shape nonlinearity observed in nonmagnetic impurity doped PA such as [CH(I&& which suggests that the scattering by magnetic impurities is more important than the electron-phonon enhancement effect.
* Present address : Hyundai Electronics Co. Ltd., Icheon, Kyungi-Do, 0379-6779/97/$17.00
PII s0379-6779(96)04127-6
0 1997
Elsevier
Science
S.A
All
rights
reserved
In this paper, we have measured the TEP for FeCIa, NbCls, AuCla and 12 doped PA samples from room temperature down to T=4.2K. The results show clear difference below 20K between the metal-halide (FeCls, NbCls and AuCls) doped PA and the iodine doped PA. 2. Experiment The high density polyacetylene (HDPA) film was synthesized using the modified Shirakawa method[5]. Approximately 10 pm thickness PA film is obtained. Subsequent stretching of the film of 4-5 times elongation (L& = 4-5) and the gas phase doping with iodine gives maximum conductivity of OCR= 40000 S/cm. The FeCls and NbCl5 doping were done by dissolving them in nitromethane solvent. In the case of the AuCls doping, a&o&rile is used as a solvent. Both solvents are purified and used within 24 hrs after purification. The dopant concentration was determined by weight uptake upon doping. We obtained PA samples of various dopant concentration by controlling the doping time and the concentration of dopant solution. Techniques for 4-probe dc conductivity and TEP measurements are the same as the ones published earlier[6]. 3. Results and discussions Fig. 1. shows the temperature dependent TEP for iodine, AuCls, FeCls and NbCls doped PA films t?om room temperature down to liquid helium temperature. The inset shows the low temperature TEP data in enlarged scale. As one can see clearly the overall T-dependence of TEP are different between iodine doped PA and the other AuCls, FeCls and NbCls doped PA films. This difference has been analyzed in detail in our earlier
Korea
748
E.S. Choi et al./SyntheticMetals
paper[3]. Interestingly, the low temperature TEP data show very clear difference between iodine and metal-halide doped PA. The quasi-linear T-dependence of TEP for iodine doped PA is observed down to 4.2 K. On the other hand, the metal-halide doped PA show temperature independent TEP below 20K. In the case of NbCls doped PA, there exists even a broad minimum peaked at around 1OK. To measure the TEP below 2OK, we have calibrated our Cu holder very carefully. From the careful measurements of TEP for the above samples shown in Fig. 1, one can conclude that the low temperature TEP are deftitely different between iodine-doped PA and metal-halide.doped PA. Incidentally, the metal-halide doped PA films have partially filled d-electrons in the dopants while iodine doped PA does not have d-electrons in the outer shell of the iodine. Therefore the scattering mechanism between conduction electrons in the PA chain and the localized dopant ions may be spin-spin interaction.
84 (1997)
747-748
.**
. .* x . . . n*“e
0
20
40
60
80
100
Fig. 2. Concentrationdependenceof TEP of [CH(AuC14h],; A(y=O.O16, oR?c114oS/cm, SRr=22.1 pv/K), l (y=0.061, 1210S/cm,19,1pViK), O(y=O.O93,4054S/cm, 168pVLK), v (y=O.l28,5366S/cm,lO.SpV/K ) In summary, we observed the quasi&rear temperature dependence of TEP for iodinedopedPA from roomtemperature down to 4.2K. For metal-halidedoped PA, the temperature dependence of TEP is quasilinear at high temperatureregion becomingtemperatureindependentbelow 20K, which is clearly different from that of iodine dopedPA. The similarity of TEP betweenthe concentrationdependence and the magneticfield dependenceindicates the existence of spin-spin interaction betweenthe conductionelectronspinin metallic PA chain and the localizedspinin the dopantlocatednearbythe chain,
sb
160
160
260
250
360
T(K) Fig. 1. TEP as a function of temperaturefor 12(~,o~&605 S/cm,) AuCls (0,1140S/cm), FeCls (*,4210S/cm) and NbCl5(0,1979S/cm) dopedPA Fig.2 showsthe concentrationdependence of AuCls dopedPA from T=lOOK to 4.2K. The dopantconcentrationwasestimated by assumingthe dopantis in the form of AuCL-. The obtained highestroomtemperatureconductivity is 5300S/cm.It increases uponcoolingwith maximumconductivity of 5500S/cmat 272K. And then it decreases as the temperatureis loweredbecoming temperatureindependentbelow30K. The or+.sk/0~~0.67. The TEP data showthat as the dopingconcentrationincreases,the magnitudeof TEP at low temperaturedecreasesbecoming negativeat T-20K for ~0.061 and 0.093 and at T-30K for y=O.128sample,respectively.The peaktemperaturealsoshifts to higher values.Thesetendenciesare similarwith thoseunder high magnetictield[7], which indicate that the dopantswith localized spinsbehave like external fields to the conduction electronspinsin the PA chain.Thusasthe dopantconcentration increases, the strengthof the effective external field increasesin a meanfield approximationsense.
Acknowledgment
This work is supportedby the Ministry of Education(MOE) and the Korea Scienceand EngineeringFoundation(KOSEF), Korea. Partial support for Mr. E. S. Choi was done by the ReseachCenter for Physico-ChemicalProperties at Korea University. References
[I] Y. Nogami, H. Kaneko, H. Ito, T. Ishiguro,T. Sasaki,N. Toyota,A. Takahashi,J. Tsukamoto,Whys.Rev. B 43 (1991) 11829 [2] ReghuM., K. Vakiparta, Y. Cao,D. Moses,Whys.Rev. B 49 (1994) 16162 [3] Y. W. Park,W. K. Han, C. H. Choi,H. Shirakawa,Phys.Rev. B 41 (1990)5067 [4] A. B. Kaiser,Phys.Rev. B 40 (1989)2806 [5] K. Akagi, M. Suez&i, H. Shirakawa, H. Kyotani, M. Shimomura,T. Yamabe,5”th. Met. 28 (1989) 1 [6] Y. W. Park, Sy&. Met. 45 (I 991) 173 [7] E. S. Choi, Y. S. Song, Y. W. Park, S. T. Ham&s, Proceedings of the ICSM’96 (to be publishedin Synthetic Metals)
Synthetic
Metals
84 (1997)
747-748
Low temperature thermoelectric power of the metal-halide doped polyacetylene E. S. Choi”, Y. H. Seola’, Y. S. Song!, Y. W. Park” ‘Department ofPhysics, Seoul National Universig, Seoul 151-742, South Korea bResearch Institute for Basic Sciences, Seoul National Givers@, Seoul 1.51-742, South Korea
Abstract We have measured thermoelectric power (TEP) of the doped polyacetylene (PA) from room temperature down to liquid helium temperature. The dopants are Au&, NbC15, Fe& and 12. The overall TEP data show the diffisive metallic property, i.e. the TEP is positive and it decreases quasi-linearly upon cooling. For the metal-halide doped PA, the TEP is temperature independent with small magnitude about 0.2-0.6pV/K below 1% while the quasi-linear temperature dependent TEP is observed for the iodine doped PA in the same low temperature regime. The concentration dependence of TEP for Au& doped PA shows that the TEP becomes negative at TaOK-30K depending on the doping concentration with a broad minimum peak at low temperature. The result indicates that the effect of dopant to the PA chain is important to understand the metallic nature of heavily doped PA. Kqwords
: thermopower,
polyacetylene and derivatives
1. Introduction The unusal metallic properties of heavily doped polyacetylene (PA) has not been well understood because of the randomly entangled fibriallr morphology of the polymer. Recently, however, there has been a considerable progress in understanding the unusual metallic properties of PA by considering the heavily doped PA as a highly disordered metal[lJ]. But most of the arguments have ignored the effect of dopants to the polymer chains. This is mainly because the measured conductivity for various doped polymers turned out to be rather insensitive to the dopant species. On the other hand, TEP is a very good probe to study the effect of dopants to the polymer chain since TEP is extremely sensitive to the scattering of charge carriers. The temperature dependent TEP of transition metal halide doped polyacetylene is known to be different from that of iodine or AsFs doped polyacetylene. The difference has been analyzed based on the possible different scattering mechanism between the conduction electrons in the polymer chain and the localized spins in the dopants[3]. Alternatively, the knee shape enhancement on top of the linear diffusive TEP for the FeCls doped PA has been interpreted as due to the electron-phonon interaction enhancement effect[4]. Since the heavily doped PA has a lot of disorders originated mainly from the morphological complexity of the polymer, it can be viewed as an amorphous metal. On the other hand, the significant knee shape enhancement effect is seen only in magnetic impurity doped PA such as [CH(F~C~&C.& There is no clear knee shape nonlinearity observed in nonmagnetic impurity doped PA such as [CH(I&& which suggests that the scattering by magnetic impurities is more important than the electron-phonon enhancement effect.
* Present address : Hyundai Electronics Co. Ltd., Icheon, Kyungi-Do, 0379-6779/97/$17.00
PII s0379-6779(96)04127-6
0 1997
Elsevier
Science
S.A
All
rights
reserved
In this paper, we have measured the TEP for FeCIa, NbCls, AuCla and 12 doped PA samples from room temperature down to T=4.2K. The results show clear difference below 20K between the metal-halide (FeCls, NbCls and AuCls) doped PA and the iodine doped PA. 2. Experiment The high density polyacetylene (HDPA) film was synthesized using the modified Shirakawa method[5]. Approximately 10 pm thickness PA film is obtained. Subsequent stretching of the film of 4-5 times elongation (L& = 4-5) and the gas phase doping with iodine gives maximum conductivity of OCR= 40000 S/cm. The FeCls and NbCl5 doping were done by dissolving them in nitromethane solvent. In the case of the AuCls doping, a&o&rile is used as a solvent. Both solvents are purified and used within 24 hrs after purification. The dopant concentration was determined by weight uptake upon doping. We obtained PA samples of various dopant concentration by controlling the doping time and the concentration of dopant solution. Techniques for 4-probe dc conductivity and TEP measurements are the same as the ones published earlier[6]. 3. Results and discussions Fig. 1. shows the temperature dependent TEP for iodine, AuCls, FeCls and NbCls doped PA films t?om room temperature down to liquid helium temperature. The inset shows the low temperature TEP data in enlarged scale. As one can see clearly the overall T-dependence of TEP are different between iodine doped PA and the other AuCls, FeCls and NbCls doped PA films. This difference has been analyzed in detail in our earlier
Korea
748
E.S. Choi et al./SyntheticMetals
paper[3]. Interestingly, the low temperature TEP data show very clear difference between iodine and metal-halide doped PA. The quasi-linear T-dependence of TEP for iodine doped PA is observed down to 4.2 K. On the other hand, the metal-halide doped PA show temperature independent TEP below 20K. In the case of NbCls doped PA, there exists even a broad minimum peaked at around 1OK. To measure the TEP below 2OK, we have calibrated our Cu holder very carefully. From the careful measurements of TEP for the above samples shown in Fig. 1, one can conclude that the low temperature TEP are deftitely different between iodine-doped PA and metal-halide.doped PA. Incidentally, the metal-halide doped PA films have partially filled d-electrons in the dopants while iodine doped PA does not have d-electrons in the outer shell of the iodine. Therefore the scattering mechanism between conduction electrons in the PA chain and the localized dopant ions may be spin-spin interaction.
84 (1997)
747-748
.**
. .* x . . . n*“e
0
20
40
60
80
100
Fig. 2. Concentrationdependenceof TEP of [CH(AuC14h],; A(y=O.O16, oR?c114oS/cm, SRr=22.1 pv/K), l (y=0.061, 1210S/cm,19,1pViK), O(y=O.O93,4054S/cm, 168pVLK), v (y=O.l28,5366S/cm,lO.SpV/K ) In summary, we observed the quasi&rear temperature dependence of TEP for iodinedopedPA from roomtemperature down to 4.2K. For metal-halidedoped PA, the temperature dependence of TEP is quasilinear at high temperatureregion becomingtemperatureindependentbelow 20K, which is clearly different from that of iodine dopedPA. The similarity of TEP betweenthe concentrationdependence and the magneticfield dependenceindicates the existence of spin-spin interaction betweenthe conductionelectronspinin metallic PA chain and the localizedspinin the dopantlocatednearbythe chain,
sb
160
160
260
250
360
T(K) Fig. 1. TEP as a function of temperaturefor 12(~,o~&605 S/cm,) AuCls (0,1140S/cm), FeCls (*,4210S/cm) and NbCl5(0,1979S/cm) dopedPA Fig.2 showsthe concentrationdependence of AuCls dopedPA from T=lOOK to 4.2K. The dopantconcentrationwasestimated by assumingthe dopantis in the form of AuCL-. The obtained highestroomtemperatureconductivity is 5300S/cm.It increases uponcoolingwith maximumconductivity of 5500S/cmat 272K. And then it decreases as the temperatureis loweredbecoming temperatureindependentbelow30K. The or+.sk/0~~0.67. The TEP data showthat as the dopingconcentrationincreases,the magnitudeof TEP at low temperaturedecreasesbecoming negativeat T-20K for ~0.061 and 0.093 and at T-30K for y=O.128sample,respectively.The peaktemperaturealsoshifts to higher values.Thesetendenciesare similarwith thoseunder high magnetictield[7], which indicate that the dopantswith localized spinsbehave like external fields to the conduction electronspinsin the PA chain.Thusasthe dopantconcentration increases, the strengthof the effective external field increasesin a meanfield approximationsense.
Acknowledgment
This work is supportedby the Ministry of Education(MOE) and the Korea Scienceand EngineeringFoundation(KOSEF), Korea. Partial support for Mr. E. S. Choi was done by the ReseachCenter for Physico-ChemicalProperties at Korea University. References
[I] Y. Nogami, H. Kaneko, H. Ito, T. Ishiguro,T. Sasaki,N. Toyota,A. Takahashi,J. Tsukamoto,Whys.Rev. B 43 (1991) 11829 [2] ReghuM., K. Vakiparta, Y. Cao,D. Moses,Whys.Rev. B 49 (1994) 16162 [3] Y. W. Park,W. K. Han, C. H. Choi,H. Shirakawa,Phys.Rev. B 41 (1990)5067 [4] A. B. Kaiser,Phys.Rev. B 40 (1989)2806 [5] K. Akagi, M. Suez&i, H. Shirakawa, H. Kyotani, M. Shimomura,T. Yamabe,5”th. Met. 28 (1989) 1 [6] Y. W. Park, Sy&. Met. 45 (I 991) 173 [7] E. S. Choi, Y. S. Song, Y. W. Park, S. T. Ham&s, Proceedings of the ICSM’96 (to be publishedin Synthetic Metals)