- Email: [email protected]
Quantum transport in AuCl3 doped polyacetylene studied by ESR
Synthetic Metals 117 (2001) 21±25
Quantum transport in AuCl3 doped polyacetylene studied by ESR$ A. Bartla,*, L. Dunscha, Y.W. Parkb, E.S. Choib, D.S. Suhb a
Institut fuÈr FestkoÈrper- und Werkstofforschung, Helmoltzstr. 20, D-01069 Dresden, Germany b Department of Physics, Seoul National University, Seoul 151-742, South Korea
Abstract Starting with a review of general principles of transport processes in polyacetylene (PA) which can be studied by electron spin resonance (ESR) new results on highly doped PA are presented. The AuCl3 doped PA samples stored under vacuum at room temperature in ESR sample tubes reach a steady state in the temperature dependent intensity and linewidth behavior after three to four month storage. The ESR spectra show two lines with an equal g-factor. In particular, the linewidth of the small line demonstrates a remarkable property. The temperature dependence of the linewidth shows a maximum of DB 0:30 mT at 30 K, and it is constant at DB 0:17 mT above this temperature in the whole temperature region between 70 and 300 K. The ESR data are discussed in a two spin model of delocalized states at the PA chain and ®xed spins near the dopants. Broadening and motional narrowing in¯uence the ESR linewidth. The interpretation points to the metallic behavior of the highly doped PA at low temperatures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyacetylene; Electron paramagnetic resonance spectroscopy; Doping; Metallic behavior
1. Introduction Heavily doped polyacetylene (PA) is known to have unique metallic properties, but the conduction mechanism is different from ordinary metals. Dynamical change of electronic states occurs upon doping, which is fundamentally due to the existence of dopants interacting with conjugated polymer chains [1]. From the ®rst discovery of the insulator-metal transition by doping metallic PA has been widely investigated experimentally and theoretically [2]. However, it is dif®cult to understand the intrinsic properties of the metallic PA due to the complex morphology of the doped material. Its metallic properties, such as the temperature independent magnetic susceptibility, the high electrical conductivity and the linear temperature dependent thermoelectric power, were widely studied [3±5]. One of the remarkable features in polyacetylene is the spin-charge inversion of conjugation defects: neutral defects carry a spin and charged defects are spinless [6]. The magnetic moment of the neutral defect (neutral soliton) can be seen in the ESR signal and in the static susceptibility as a paramagnetic contribution originated in unpaired electrons. In the metallic state when there is a ®nite
$
Dedicated to the Nobel Laureates Alan Heeger, Alan Mac Diarmid and Hideki Shirakawa. * Corresponding author.
density of states at the Fermi level and the Peierls transition is suppressed, the delocalized electrons will lead to a Pauli paramagnetism which contributes to the static susceptibility too. In the last years there has been a considerable progress in understanding the unusual properties of this material. Evidence for the motion of the solitons can be obtained from the ESR spectra. The linewidth of the ESR spectrum is generally de®ned as the distance DHpp between both peaks of the derivative ESR spectrum. Many different effects can contribute to the ®nite linewidth: unresolved hyperfine splitting combined with motional narrowing; interactions between unpaired electrons of different nature: conduction, fixed and mobile electrons; exchange narrowing. In this way, the ESR is able to study energy and mass transport processes in PA by using neutral solitons or other paramagnetic species as probes. During the thermal isomerization while cis-PA is transformed to trans-PA, these paramagnetic defects are created. The properties of neutral solitons were studied by ESR of undoped PA focused on the temperature dependence of the ESR linewidth. By these studies two narrowing mechanisms were found: a temperature dependent and a temperature independent mechanism. In the last years a consistent picture of the neutral soliton dynamics was installed and by using different magnetic
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 3 3 - 6
22
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
methods the analyses with objective conclusions were proposed [7]. From the ESR investigations a two spin model could be created. In the PA matrix there exist mobile spins and ®xed spins. By using this model the effect of oxidation can be explained. Mobile spins are localized near an oxygen atom and will be transformed to ®xed spins and therefore the ESR linewidth will be broader upon oxidation. In this connection it was found that PA is very sensitive against reactions with oxygen which are mainly responsible for the ``aging'' processes. The ESR shows different concentrations and linewidths for the paramagnetic states during the aging process [8]. Moreover iodine doped samples are very much sensitive to the aging than PA doped with other compounds. The magnetoresistance (MR) changes its sign and the magnitude is really different upon aging [1] because extrinsic barriers could be introduced by aging and temperature treatment. The metal halide doped samples show a very broad peak originated from Fe4 or Au4 so that the intrinsic p-electron spin signal is hidden by the metal ions signal. Measurements of the magnetic susceptibility of AuCl3 doped PA using dc SQUID magnetometer show big diamagnetic background with a Curie-like term on the top of it. About the Pauli component of heavily doped PA there are many references [1,2,6,9]. The doping concentration dependent dc-conductivity of AuCl3 seems to be similar to polyacetylenes doped with other doping compounds. Moreover, the MR changes its sign and the magnitude is really different upon aging [1,10,11] because extrinsic barriers could be introduced by aging and temperature treatment.
Table 1 Selected properties of AuCl3 doped polyacetylene samples Concentration (wt.%) 2.7 3.0 3.8 5.4 6.8
Total mass (mg) 0.7 0.8
Conductivity (S/cm)
Thermopower (mV/K)
1155 1816 4311 6975 8780
18.5 15.2 15.8 15.1 13.5
3. Results and discussion For characterizing the quality of the AuCl3 doped PA samples the electrical conductivity dependence from the doping concentration was studied at room temperature (Fig. 1). The electrical conductivity increases upon doping which is quite normal and understandable. The AuCl3 doped PA samples show different ESR spectra (Fig. 2). Two superimposed ESR lines with the same gfactor, g 2:0021 but with different linewidths can be clearly seen. The reasons are two kinds of spins in the PA matrix: ®xed spins with a linewidth DBbroad 1:6 mT and mobile spins with a linewidth DBsmall 0:2 mT under vacuum and at room temperature. With increasing doping concentration the concentration of the ®xed spins increases. The ESR parameters of the small line can be measured only at low doping concentrations. Furthermore, it was found that the samples are not stable with time. In the high AuCl3 doped PA samples sealed in a high purity quartz tube under vacuum ``aging'' processes could be observed. Included in these ``aging'' processes are
2. Experimental High density polyacetylene ®lms were synthesized using a modi®ed Shirakawa method with heat treated Ziegler± Natta catalysts and than stretched by four to ®ve times. The AuCl3 doping was done in puri®ed acetonitrile as a solvent [1]. Each sample was sealed in a high purity quartz tube under vacuum. The electrical resistivity measurements were done by the conventional four-probe technique. The doped ®lm was cut into the shape of a strip along the stretched direction. The typical size of the samples was 8 0:5 0:01 mm3. The current was applied in parallel with the stretched direction. Electron spin resonance spectra were recorded by an ESR-300 X-band spectrometer (ZWG Berlin) with 100 kHz modulation and a microwave power less than 1 mW at room temperature and in the temperature region of 4±300 K. For ESR measurements the PA samples were handled in sealed quartz sample tubes with a diameter of 4 mm to avoid contamination with oxygen. The investigated PA samples are listed in Table 1.
Fig. 1. Electrical conductivity at 290 K as a function of doping concentration at AuCl3 doped PA.
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
Fig. 2. ESR spectra of AuCl3 doped PA with different doping concentrations in (wt.%).
four to six cooling down and warming up procedures because temperature dependent ESR measurements. These processes reach a steady state in temperature dependent ESR intensity and linewidth behavior after some months. These ``aging'' processes are much smaller in low AuCl3 doped PA than in highly doped samples. The ®xed spins seem to be the reason of this behavior. Since the intensity of the small ESR line does not change remarkable the broad line diminishes during the ``aging'' processes (Fig. 3). It is proposed that the broad ESR line proceeds from unpaired electrons localized near structure defects, near adsorbed or bonded oxygen atoms or molecules and other irregularities in the PA matrix. It should be possible, that these ®xed spins during the cooling down and warming up
Fig. 3. ESR spectra of 3.0% AuCl3 doped PA after different storage times and temperature treatments (a) after preparation, (b) after 105 days and two temperature treatments between 300 and 4 K (c) after 290 days.
23
Fig. 4. ESR linewidth and intensity as a function of temperature of a PA sample doped with 3.0% AuCl3.
procedures can be transformed in mobile spins. In the case of temperature dependent measurements on highly doped PA samples (5.4% AuCl3) below 100 K only the small line can be seen [12]. Because all these facts the further investigations were done only at the small ESR line which represents the mobile spins. Low doped PA samples in which no remarkable ``aging'' processes could be found show the small ESR line only and in some cases an unimportant part of ®xed spins. As an example Fig. 4 shows the linewidth and intensity as a function of temperature for a PA sample doped with 3.0% AuCl3. This behavior is well known and reported in [13] for cis-PA. The linewidth increases continual with increasing temperature and the ESR intensity shows Curie-like behavior In highly doped PA with 5.4% AuCl3 the situation is quite different. After reaching a steady state during four months the temperature dependence of the linewidth of the small line shows a complete other behavior. The linewidth of this line increases above 4 K and has a maximum of DB 0:35 mT at about 30 K. Above this temperature the linewidth decreases again to DB 0:17 mT. From 70 K to room temperature the linewidth is nearly constant. The measurements after different storage times show a reversible behavior of the samples (Fig. 5). This is the ®rst time that an ESR linewidth maximum at T 30 K was observed and then DBsmall is constant at temperatures T > 70 K. For explanation of this effect further investigations especially on freshly prepared and doped samples are necessary. A further remarkable result measured at highly doped samples is the expected behavior of the temperature dependent ESR intensity.
24
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
Fig. 5. ESR linewidth vs. temperature of PA doped with 5.4% AuCl3.
It is to be pointed out that if the measured values are plotted as ``Intensity Temperature'' versus ``Temperature'' because w wcurie wpauli wT C wpauli T
C wpauli T
(1) (2)
in each case a maximum is found between 100 and 150 K (Fig. 6). Storing the PA sample for longer times because the
Fig. 6. Paramagnetic behavior measured after different storage times after preparation.
``aging'' processes causes this maximum to be shifted to higher temperatures. Important is that the double integrals of the ESR spectra which are expressions of the spin concentrations show the same number of spins (within an error of 20%) in the whole investigated temperature range. This temperature behavior is not Curie-like in the whole temperature region. It shows a Pauli-like component at 50 K< T < 150 K and then it becomes Curie-like paramagnetism at higher temperature. At T < 50 K the intensity grows more rapidly than the Curie component upon cooling, which could be originated from the spin density wave (SDW) formation at around 100 K because the effective mass of SDW would be heavier than that of the single particles, so it shows more localized behavior (lower mobility). The Pauli-like paramagnetism is indicating metallic behavior. The ESR data are discussed in a two spin model of delocalized states at the PA chain and fixed spins in the dopants. The interpretation underlines the metallic behavior of the doped PA at low temperatures. The increase of linewidth at low temperature (below 30 K) is also consistent with the metallic characteristics. The thermo electric power (TEP) is metallic-like (i.e. linear in temperature) from room temperature down to about 30 K and then it becomes flattened (or it becomes negative (sign crossing) and then flattened at around this temperature) depending on the doping concentration. For T < 10 K, the TEP begins to increase [1]. Thus, the metallic behavior below 30 K is not simply metallic but it must be related to the interaction between the delocalized charge carrier in the polymer chain and the dopants. These facts are supported by the results of the PA sample doped with 6.8% AuCl3. In Fig. 7, the integrated ESR signal intensity multiplied by the measuring temperature against the measuring temperature is plotted. The gradient of the curve shows Pauli paramagnetism in the whole
Fig. 7. Metallic behavior of 6.8% AuCl3 doped PA.
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
investigated temperature range from 4 to 300 K. The broad line of the ESR spectrum of 6.8% doped PA in Fig. 2 shows for the great number of doping molecules a great number of localized unpaired electrons. These results suggest that in this highly doped PA matrix quite different quantum transport processes take place compared to low doped PA. The time and temperature dependence of the ESR results are consistent with MR data. The MR is initially negative for fresh samples and then as the sample becomes aged, the sign becomes positive and the magnitude gets bigger. On the other hand the thermoelectric power (TEP) is less sensitive to aging in time and temperature. This is due to the extrinsic barriers introduced by aging and temperature treatment. The MR and the electrical resistivity are very sensitive to such effect but the TEP is less sensitive to it. The sensitive change of the ESR signal is also related to this effect since the ESR and MR as well as resistance are all measuring the charge transport. EXAFS measurement results can be understood by assuming the ionic state of AuCl3 to be AuCl2 ÿ [9]. However, further studies including elemental analysis should be done to verify this hypothesis. The excess of Cl could also form a covalent bond to the polymer backbone which can act as a localization center. The negative MR and the metallic TEP with some anomalies at low temperatures as well as the metallic ESR seem to indicate that the disordered metal theory of heavily doped PA does not work. Neither the critical regime senario near the metal-insulator transition boundary can interpret the data consistently. The broad band metallic picture with the impurity scattering between the conduction electrons in the chain and the dopant can be a source of these localized effect. In an additional picture the temperature dependence of the ESR linewidth of highly doped PA with a maximum at 30 K shows a Kondo-effect like behavior. That means at low temperatures an exchange interaction between the magnetic moments of the dopant ions and the Pauli or conduction electrons is added to the scattering mechanism based on potential perturbation. As a result of this effect the exchange interaction changes the mobility of the conduction electrons and by this way the linewidth of their ESR signal. In this temperature region the ESR linewidth increases. Below a determined characteristic limit temperature this effect can be revoked. It is shown that the described ESR experiments give additional hints for the explanation of the metallic state in highly doped PA. 4. Conclusions Temperature dependence of the ESR linewidth and the relative concentrations of paramagnetic species were measured at polyacetylene doped with AuCl3. The linewidth increases above 4 K and has a maximum of DB 0:35 mT
25
at about 30 K. Above this temperature the linewidth decreases again to DB 0:17 mT. From 70 K to room temperature the linewidth is nearly constant. The measurements after different storage times show a reversible behavior. This is the ®rst time to be observed an ESR linewidth maximum at T 30 K. The temperature behavior of the spin concentration is not Curie-like in the whole temperature region. It shows a Pauli-like component at 50 K < T < 150 K and then becomes Curie-like paramagnetism at higher temperatures. The ionic state of the dopant molecules in the PA samples is determined by EXAFS as AuClÿ 2 with some Au cluster probably on the surface of the sample. The impurity scattering between the delocalized electrons in the polymer chain and the localized electrons in the dopants close to this PA chain could be observed. The ESR data measured at samples reaching a steady state with time support the picture of localized states in the broad band metal. In an additional picture the temperature dependence of the ESR linewidth of highly doped PA with a maximum at 30 K shows a Kondoeffect like behavior. For more details of this effect further investigations especially on freshly prepared and doped samples are necessary. Acknowledgements Partial support of this work was done by KISTEP under the contract No. 98-1-01-04-A-026, Ministry of Science and Technology (MOST), Korea. The experimental help of Brunhild Schandert and Frank Ziegs is grateful acknowledged.
References [1] E.S. Choi, Y.H. Seol, Y.S. Song, Y.W. Park, S.T. Hannahs, Synth. Met. 84 (1997) 685. [2] C.K. Chiang, C.R. Fincher Jr., Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. MacDiarmid, Phys. Rev. Lett. 39 (1977) 1098. [3] A.J. Epstein, H. Rommelmann, M.A. Druy, A.J. Heeger, A.G. MacDiarmid, Solid State Commun. 38 (1981) 683. [4] Th. Schimmel, J. Gmeiner, W. Rieb, G. Denninger, M. Schwoerer, H. Naarmann, N. Theophilou, Solid State Commun. 65 (1988) 1311. [5] Y.W. Park, Synth. Met. 45 (1991) 173. [6] S. Roth, H. Bleier, Adv. Phys. 36 (1987) 385. [7] K. Mizoguchi, S. Masubuchi, K. Kume, K. Akagi, H. Shirakawa, Synth. Met. 84 (1997) 865. [8] A. Bartl, Mater. Sci. Forum 122 (1993) 1. [9] D.S. Suh, E.S. Choi, J.G. Park, S.B. Lee, Y.W. Park, J.M. Lee, Synth. Met. 101 (1999) 498. [10] Y.W. Park, E.S. Choi, D.S. Suh, Synth. Met. 96 (1998) 81. [11] E.S. Choi, G.T. Kim, D.S. Suh, D.C. Kim, J.G. Park, Y.W. Park, Synth. Met. 100 (1999) 2. [12] A. Bartl, L. Dunsch, E.S. Choi, D.S. Suh, Y.W. Park, Synth. Met. 101 (1999) 465. [13] D. Davidov, S. Roth, W. Neumann, H. Sixl, Solid State Commun. 52 (1984) 375.
Quantum transport in AuCl3 doped polyacetylene studied by ESR$ A. Bartla,*, L. Dunscha, Y.W. Parkb, E.S. Choib, D.S. Suhb a
Institut fuÈr FestkoÈrper- und Werkstofforschung, Helmoltzstr. 20, D-01069 Dresden, Germany b Department of Physics, Seoul National University, Seoul 151-742, South Korea
Abstract Starting with a review of general principles of transport processes in polyacetylene (PA) which can be studied by electron spin resonance (ESR) new results on highly doped PA are presented. The AuCl3 doped PA samples stored under vacuum at room temperature in ESR sample tubes reach a steady state in the temperature dependent intensity and linewidth behavior after three to four month storage. The ESR spectra show two lines with an equal g-factor. In particular, the linewidth of the small line demonstrates a remarkable property. The temperature dependence of the linewidth shows a maximum of DB 0:30 mT at 30 K, and it is constant at DB 0:17 mT above this temperature in the whole temperature region between 70 and 300 K. The ESR data are discussed in a two spin model of delocalized states at the PA chain and ®xed spins near the dopants. Broadening and motional narrowing in¯uence the ESR linewidth. The interpretation points to the metallic behavior of the highly doped PA at low temperatures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyacetylene; Electron paramagnetic resonance spectroscopy; Doping; Metallic behavior
1. Introduction Heavily doped polyacetylene (PA) is known to have unique metallic properties, but the conduction mechanism is different from ordinary metals. Dynamical change of electronic states occurs upon doping, which is fundamentally due to the existence of dopants interacting with conjugated polymer chains [1]. From the ®rst discovery of the insulator-metal transition by doping metallic PA has been widely investigated experimentally and theoretically [2]. However, it is dif®cult to understand the intrinsic properties of the metallic PA due to the complex morphology of the doped material. Its metallic properties, such as the temperature independent magnetic susceptibility, the high electrical conductivity and the linear temperature dependent thermoelectric power, were widely studied [3±5]. One of the remarkable features in polyacetylene is the spin-charge inversion of conjugation defects: neutral defects carry a spin and charged defects are spinless [6]. The magnetic moment of the neutral defect (neutral soliton) can be seen in the ESR signal and in the static susceptibility as a paramagnetic contribution originated in unpaired electrons. In the metallic state when there is a ®nite
$
Dedicated to the Nobel Laureates Alan Heeger, Alan Mac Diarmid and Hideki Shirakawa. * Corresponding author.
density of states at the Fermi level and the Peierls transition is suppressed, the delocalized electrons will lead to a Pauli paramagnetism which contributes to the static susceptibility too. In the last years there has been a considerable progress in understanding the unusual properties of this material. Evidence for the motion of the solitons can be obtained from the ESR spectra. The linewidth of the ESR spectrum is generally de®ned as the distance DHpp between both peaks of the derivative ESR spectrum. Many different effects can contribute to the ®nite linewidth: unresolved hyperfine splitting combined with motional narrowing; interactions between unpaired electrons of different nature: conduction, fixed and mobile electrons; exchange narrowing. In this way, the ESR is able to study energy and mass transport processes in PA by using neutral solitons or other paramagnetic species as probes. During the thermal isomerization while cis-PA is transformed to trans-PA, these paramagnetic defects are created. The properties of neutral solitons were studied by ESR of undoped PA focused on the temperature dependence of the ESR linewidth. By these studies two narrowing mechanisms were found: a temperature dependent and a temperature independent mechanism. In the last years a consistent picture of the neutral soliton dynamics was installed and by using different magnetic
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 3 3 - 6
22
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
methods the analyses with objective conclusions were proposed [7]. From the ESR investigations a two spin model could be created. In the PA matrix there exist mobile spins and ®xed spins. By using this model the effect of oxidation can be explained. Mobile spins are localized near an oxygen atom and will be transformed to ®xed spins and therefore the ESR linewidth will be broader upon oxidation. In this connection it was found that PA is very sensitive against reactions with oxygen which are mainly responsible for the ``aging'' processes. The ESR shows different concentrations and linewidths for the paramagnetic states during the aging process [8]. Moreover iodine doped samples are very much sensitive to the aging than PA doped with other compounds. The magnetoresistance (MR) changes its sign and the magnitude is really different upon aging [1] because extrinsic barriers could be introduced by aging and temperature treatment. The metal halide doped samples show a very broad peak originated from Fe4 or Au4 so that the intrinsic p-electron spin signal is hidden by the metal ions signal. Measurements of the magnetic susceptibility of AuCl3 doped PA using dc SQUID magnetometer show big diamagnetic background with a Curie-like term on the top of it. About the Pauli component of heavily doped PA there are many references [1,2,6,9]. The doping concentration dependent dc-conductivity of AuCl3 seems to be similar to polyacetylenes doped with other doping compounds. Moreover, the MR changes its sign and the magnitude is really different upon aging [1,10,11] because extrinsic barriers could be introduced by aging and temperature treatment.
Table 1 Selected properties of AuCl3 doped polyacetylene samples Concentration (wt.%) 2.7 3.0 3.8 5.4 6.8
Total mass (mg) 0.7 0.8
Conductivity (S/cm)
Thermopower (mV/K)
1155 1816 4311 6975 8780
18.5 15.2 15.8 15.1 13.5
3. Results and discussion For characterizing the quality of the AuCl3 doped PA samples the electrical conductivity dependence from the doping concentration was studied at room temperature (Fig. 1). The electrical conductivity increases upon doping which is quite normal and understandable. The AuCl3 doped PA samples show different ESR spectra (Fig. 2). Two superimposed ESR lines with the same gfactor, g 2:0021 but with different linewidths can be clearly seen. The reasons are two kinds of spins in the PA matrix: ®xed spins with a linewidth DBbroad 1:6 mT and mobile spins with a linewidth DBsmall 0:2 mT under vacuum and at room temperature. With increasing doping concentration the concentration of the ®xed spins increases. The ESR parameters of the small line can be measured only at low doping concentrations. Furthermore, it was found that the samples are not stable with time. In the high AuCl3 doped PA samples sealed in a high purity quartz tube under vacuum ``aging'' processes could be observed. Included in these ``aging'' processes are
2. Experimental High density polyacetylene ®lms were synthesized using a modi®ed Shirakawa method with heat treated Ziegler± Natta catalysts and than stretched by four to ®ve times. The AuCl3 doping was done in puri®ed acetonitrile as a solvent [1]. Each sample was sealed in a high purity quartz tube under vacuum. The electrical resistivity measurements were done by the conventional four-probe technique. The doped ®lm was cut into the shape of a strip along the stretched direction. The typical size of the samples was 8 0:5 0:01 mm3. The current was applied in parallel with the stretched direction. Electron spin resonance spectra were recorded by an ESR-300 X-band spectrometer (ZWG Berlin) with 100 kHz modulation and a microwave power less than 1 mW at room temperature and in the temperature region of 4±300 K. For ESR measurements the PA samples were handled in sealed quartz sample tubes with a diameter of 4 mm to avoid contamination with oxygen. The investigated PA samples are listed in Table 1.
Fig. 1. Electrical conductivity at 290 K as a function of doping concentration at AuCl3 doped PA.
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
Fig. 2. ESR spectra of AuCl3 doped PA with different doping concentrations in (wt.%).
four to six cooling down and warming up procedures because temperature dependent ESR measurements. These processes reach a steady state in temperature dependent ESR intensity and linewidth behavior after some months. These ``aging'' processes are much smaller in low AuCl3 doped PA than in highly doped samples. The ®xed spins seem to be the reason of this behavior. Since the intensity of the small ESR line does not change remarkable the broad line diminishes during the ``aging'' processes (Fig. 3). It is proposed that the broad ESR line proceeds from unpaired electrons localized near structure defects, near adsorbed or bonded oxygen atoms or molecules and other irregularities in the PA matrix. It should be possible, that these ®xed spins during the cooling down and warming up
Fig. 3. ESR spectra of 3.0% AuCl3 doped PA after different storage times and temperature treatments (a) after preparation, (b) after 105 days and two temperature treatments between 300 and 4 K (c) after 290 days.
23
Fig. 4. ESR linewidth and intensity as a function of temperature of a PA sample doped with 3.0% AuCl3.
procedures can be transformed in mobile spins. In the case of temperature dependent measurements on highly doped PA samples (5.4% AuCl3) below 100 K only the small line can be seen [12]. Because all these facts the further investigations were done only at the small ESR line which represents the mobile spins. Low doped PA samples in which no remarkable ``aging'' processes could be found show the small ESR line only and in some cases an unimportant part of ®xed spins. As an example Fig. 4 shows the linewidth and intensity as a function of temperature for a PA sample doped with 3.0% AuCl3. This behavior is well known and reported in [13] for cis-PA. The linewidth increases continual with increasing temperature and the ESR intensity shows Curie-like behavior In highly doped PA with 5.4% AuCl3 the situation is quite different. After reaching a steady state during four months the temperature dependence of the linewidth of the small line shows a complete other behavior. The linewidth of this line increases above 4 K and has a maximum of DB 0:35 mT at about 30 K. Above this temperature the linewidth decreases again to DB 0:17 mT. From 70 K to room temperature the linewidth is nearly constant. The measurements after different storage times show a reversible behavior of the samples (Fig. 5). This is the ®rst time that an ESR linewidth maximum at T 30 K was observed and then DBsmall is constant at temperatures T > 70 K. For explanation of this effect further investigations especially on freshly prepared and doped samples are necessary. A further remarkable result measured at highly doped samples is the expected behavior of the temperature dependent ESR intensity.
24
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
Fig. 5. ESR linewidth vs. temperature of PA doped with 5.4% AuCl3.
It is to be pointed out that if the measured values are plotted as ``Intensity Temperature'' versus ``Temperature'' because w wcurie wpauli wT C wpauli T
C wpauli T
(1) (2)
in each case a maximum is found between 100 and 150 K (Fig. 6). Storing the PA sample for longer times because the
Fig. 6. Paramagnetic behavior measured after different storage times after preparation.
``aging'' processes causes this maximum to be shifted to higher temperatures. Important is that the double integrals of the ESR spectra which are expressions of the spin concentrations show the same number of spins (within an error of 20%) in the whole investigated temperature range. This temperature behavior is not Curie-like in the whole temperature region. It shows a Pauli-like component at 50 K< T < 150 K and then it becomes Curie-like paramagnetism at higher temperature. At T < 50 K the intensity grows more rapidly than the Curie component upon cooling, which could be originated from the spin density wave (SDW) formation at around 100 K because the effective mass of SDW would be heavier than that of the single particles, so it shows more localized behavior (lower mobility). The Pauli-like paramagnetism is indicating metallic behavior. The ESR data are discussed in a two spin model of delocalized states at the PA chain and fixed spins in the dopants. The interpretation underlines the metallic behavior of the doped PA at low temperatures. The increase of linewidth at low temperature (below 30 K) is also consistent with the metallic characteristics. The thermo electric power (TEP) is metallic-like (i.e. linear in temperature) from room temperature down to about 30 K and then it becomes flattened (or it becomes negative (sign crossing) and then flattened at around this temperature) depending on the doping concentration. For T < 10 K, the TEP begins to increase [1]. Thus, the metallic behavior below 30 K is not simply metallic but it must be related to the interaction between the delocalized charge carrier in the polymer chain and the dopants. These facts are supported by the results of the PA sample doped with 6.8% AuCl3. In Fig. 7, the integrated ESR signal intensity multiplied by the measuring temperature against the measuring temperature is plotted. The gradient of the curve shows Pauli paramagnetism in the whole
Fig. 7. Metallic behavior of 6.8% AuCl3 doped PA.
A. Bartl et al. / Synthetic Metals 117 (2001) 21±25
investigated temperature range from 4 to 300 K. The broad line of the ESR spectrum of 6.8% doped PA in Fig. 2 shows for the great number of doping molecules a great number of localized unpaired electrons. These results suggest that in this highly doped PA matrix quite different quantum transport processes take place compared to low doped PA. The time and temperature dependence of the ESR results are consistent with MR data. The MR is initially negative for fresh samples and then as the sample becomes aged, the sign becomes positive and the magnitude gets bigger. On the other hand the thermoelectric power (TEP) is less sensitive to aging in time and temperature. This is due to the extrinsic barriers introduced by aging and temperature treatment. The MR and the electrical resistivity are very sensitive to such effect but the TEP is less sensitive to it. The sensitive change of the ESR signal is also related to this effect since the ESR and MR as well as resistance are all measuring the charge transport. EXAFS measurement results can be understood by assuming the ionic state of AuCl3 to be AuCl2 ÿ [9]. However, further studies including elemental analysis should be done to verify this hypothesis. The excess of Cl could also form a covalent bond to the polymer backbone which can act as a localization center. The negative MR and the metallic TEP with some anomalies at low temperatures as well as the metallic ESR seem to indicate that the disordered metal theory of heavily doped PA does not work. Neither the critical regime senario near the metal-insulator transition boundary can interpret the data consistently. The broad band metallic picture with the impurity scattering between the conduction electrons in the chain and the dopant can be a source of these localized effect. In an additional picture the temperature dependence of the ESR linewidth of highly doped PA with a maximum at 30 K shows a Kondo-effect like behavior. That means at low temperatures an exchange interaction between the magnetic moments of the dopant ions and the Pauli or conduction electrons is added to the scattering mechanism based on potential perturbation. As a result of this effect the exchange interaction changes the mobility of the conduction electrons and by this way the linewidth of their ESR signal. In this temperature region the ESR linewidth increases. Below a determined characteristic limit temperature this effect can be revoked. It is shown that the described ESR experiments give additional hints for the explanation of the metallic state in highly doped PA. 4. Conclusions Temperature dependence of the ESR linewidth and the relative concentrations of paramagnetic species were measured at polyacetylene doped with AuCl3. The linewidth increases above 4 K and has a maximum of DB 0:35 mT
25
at about 30 K. Above this temperature the linewidth decreases again to DB 0:17 mT. From 70 K to room temperature the linewidth is nearly constant. The measurements after different storage times show a reversible behavior. This is the ®rst time to be observed an ESR linewidth maximum at T 30 K. The temperature behavior of the spin concentration is not Curie-like in the whole temperature region. It shows a Pauli-like component at 50 K < T < 150 K and then becomes Curie-like paramagnetism at higher temperatures. The ionic state of the dopant molecules in the PA samples is determined by EXAFS as AuClÿ 2 with some Au cluster probably on the surface of the sample. The impurity scattering between the delocalized electrons in the polymer chain and the localized electrons in the dopants close to this PA chain could be observed. The ESR data measured at samples reaching a steady state with time support the picture of localized states in the broad band metal. In an additional picture the temperature dependence of the ESR linewidth of highly doped PA with a maximum at 30 K shows a Kondoeffect like behavior. For more details of this effect further investigations especially on freshly prepared and doped samples are necessary. Acknowledgements Partial support of this work was done by KISTEP under the contract No. 98-1-01-04-A-026, Ministry of Science and Technology (MOST), Korea. The experimental help of Brunhild Schandert and Frank Ziegs is grateful acknowledged.
References [1] E.S. Choi, Y.H. Seol, Y.S. Song, Y.W. Park, S.T. Hannahs, Synth. Met. 84 (1997) 685. [2] C.K. Chiang, C.R. Fincher Jr., Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. MacDiarmid, Phys. Rev. Lett. 39 (1977) 1098. [3] A.J. Epstein, H. Rommelmann, M.A. Druy, A.J. Heeger, A.G. MacDiarmid, Solid State Commun. 38 (1981) 683. [4] Th. Schimmel, J. Gmeiner, W. Rieb, G. Denninger, M. Schwoerer, H. Naarmann, N. Theophilou, Solid State Commun. 65 (1988) 1311. [5] Y.W. Park, Synth. Met. 45 (1991) 173. [6] S. Roth, H. Bleier, Adv. Phys. 36 (1987) 385. [7] K. Mizoguchi, S. Masubuchi, K. Kume, K. Akagi, H. Shirakawa, Synth. Met. 84 (1997) 865. [8] A. Bartl, Mater. Sci. Forum 122 (1993) 1. [9] D.S. Suh, E.S. Choi, J.G. Park, S.B. Lee, Y.W. Park, J.M. Lee, Synth. Met. 101 (1999) 498. [10] Y.W. Park, E.S. Choi, D.S. Suh, Synth. Met. 96 (1998) 81. [11] E.S. Choi, G.T. Kim, D.S. Suh, D.C. Kim, J.G. Park, Y.W. Park, Synth. Met. 100 (1999) 2. [12] A. Bartl, L. Dunsch, E.S. Choi, D.S. Suh, Y.W. Park, Synth. Met. 101 (1999) 465. [13] D. Davidov, S. Roth, W. Neumann, H. Sixl, Solid State Commun. 52 (1984) 375.