Magnetic Properties of Conducting Polymer Nanostructures - The

Current-voltage characteristics of individual conducting polymer nanotubes and nanowires. Long Yun-Ze , Yin Zhi-Hua , Li Meng-Meng , Gu Chang-Zhi , Du...
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J. Phys. Chem. B 2006, 110, 23228-23233

Magnetic Properties of Conducting Polymer Nanostructures Yunze Long,*,†,‡ Zhaojia Chen,† Jiaoyan Shen,† Zhiming Zhang,§ Lijuan Zhang,§ Hongmei Xiao,§ Meixiang Wan,§ and Jean Luc Duvail| Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China, College of Materials Science and Engineering, Shandong UniVersity, Jinan 250061, China, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Institut des Mate´ riaux Jean Rouxel, 2 rue de la Houssinie` re, 44322 Nantes, France ReceiVed: April 12, 2006; In Final Form: August 31, 2006

Magnetic susceptibility measurements on conducting polyaniline and polypyrrole nanostructures with different dopant type and doping level as functions of temperature and magnetic field are reported. The susceptibility data cannot be simply described as Curie-like susceptibility at lower temperatures and temperature-independent Pauli-like susceptibility at higher temperatures; some unusual transitions are observed in the temperature dependence of susceptibility, for example, paramagnetic susceptibility decreases gradually with lowering temperature, which suggests the coexistence of polarons and spinless bipolarons and possible formation of bipolarons with changing temperature or doping level. In particular, it is found that the direct current magnetic susceptibilities are strongly dependent on applied magnetic field, dopant type, and doping level.

1. Introduction Magnetic properties of conducting polymers such as polyacetylene,1-3 polyaniline (PANI),4-15 polypyrrole (PPy),16-21 and polythiophene17,22 have been extensively studied since the discovery of highly conductive doped polyacetylene, because the magnetic properties can provide important details of chargecarrying species and unpaired spins. For example, Weinberger et al.1 reported that the total static magnetic susceptibilities of doped polyacetylene could be separated into three components: atomic core diamagnetism, local-moment Curie-law paramagnetism, and temperature-independent Pauli paramagnetism. The magnetic susceptibility clearly changes from a Curielike to a Pauli-like behavior as the temperature arises. The density of states at the Fermi level N(EF) can be obtained from the Pauli susceptibility according to the Pauli formula: χP ) µB2N(EF), where µB is the Bohr magneton. Similar results were widely observed in doped polyaniline and polypyrrole. For instance, Jinder et al.4 found that HCl-doped polyaniline shows a Pauli susceptibility which is approximately linearly proportional to the degree of protonation. Through susceptibility studies, the coexistence of polarons and bipolarons in doped polyaniline7-9 and polypyrrole17,18 has been suggested. The magnetic properties of polyaniline systems have been extensively studied by Kahol et al.10-13 Nearly all polyaniline derivatives such as polyalkylanilines and sulfonated polyanilines were found to have a nearly linear χ(T) dependence on T, attributed to disorder-induced localized polaron pairs. Although a lot of efforts have been done on the magnetic properties of polyaniline and polypyrrole through different methods such as static magnetic measurements,4,5,10-14,17 elec* To whom correspondence should be addressed. Present address: College of Materials Science and Engineering, Shandong University, Jinan 250061, China. E-mail: [email protected]. † Institute of Physics, Chinese Academy of Sciences. ‡ College of Materials Science and Engineering, Shandong University. § Institute of Chemistry, Chinese Academy of Sciences. | Institut des Mate ´ riaux Jean Rouxel.

tron paramagnetic resonance (EPR),7-9,17-19 and NMR spectroscopy,15 some of the results are still contradictory possibly due to the complexity of polymeric materials such as material preparation and doping. For example, Raghunathan et al.7 reported that the temperature-independent susceptibility of PTSA-doped PANI decreases by 1 order of magnitude with increasing dopant concentration, which is quite different from the results of HCl-doped PANI reported by Jinder et al.4 In particular, paramagnetic spins are found even in pristine emeraldine base polyaniline (PANI-EB).10 The arisen question is focused on the nature of the temperature-independent susceptibility. Is it a true N(EF)-related Pauli susceptibility? In previous reports, the magnetic susceptibility is usually studied as a function of temperature. In this paper, the magnetic properties of polyaniline and polypyrrole nanotubes have been explored as functions of both temperature (3-300 K) and magnetic field (-90-90 kOe). Some interesting results have been observed. It is found that the magnetic susceptibilities are strongly influenced by material preparation (e.g., dopant type and doping level) and measuring condition (e.g., applied direct current (dc) magnetic field). In particular, some unusual transitions in the χdc vs T plots are reported and discussed. 2. Experimental Section The polyaniline and polypyrrole nanostructures were prepared by a template-free method,23,24 and their electrical properties have been reported in earlier papers.25,26 The detailed procedures of preparation have been reported in previous papers.23-29 Aniline monomer was distilled under reduced pressure. Ammonium persulfate as an oxidant and R-naphthalene sulfonic acid (NSA) as the dopant were used without any further treatment. In a typical synthesis procedure, aniline monomer and NSA (the molar ratios of [aniline]/[NSA] are 1:0.015, 1:0.25, 1:0.5, 1:1, and 1:2, respectively) were mixed in deionized water with stirring, and then the oxidant was also dissolved in deionized water and was added slowly into a previously cooled mixture. After all of the oxidant was added, the reaction mixture

10.1021/jp062262e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/17/2006

Magnetic Properties of Polymer Nanostructures

J. Phys. Chem. B, Vol. 110, No. 46, 2006 23229 the susceptibility increases sharply with decreasing temperature. Above 40 K, the susceptibility becomes temperature independent. The total dc magnetic susceptibility (χdc) comprises three contributions: intrinsic atomic core diamagnetic χdia, localmoment Curie-law paramagnetic χC, and Pauli-like paramagnetic χP:

χdc ) Mdc/B ) χdia + χP + χC ) χ0 + C/T

Figure 1. Temperature dependence of dc magnetic susceptibility of naphthalene sulfonic acid doped polyaniline (PANI-NSA) with different molar ratios of [aniline]/[NSA]: (a) plotted as χdc vs T and (b) plotted as χdcT vs T.

was stirred for 24 h. The precipitate was then washed with deionized water, methanol, and ethyl ether several times and finally dried at room temperature in a dynamic vacuum for 24 h. The morphology, structure, and conductivity of the resulting PANI-NSA nanotubes were characterized in ref 27. It is found that the room-temperature conductivity increases from 10-2 to 10-1 S/cm with increasing dopant concentration. Salicylic acid (SA) doped polyaniline (PANI-SA) 28 and para-toluenesulfonic acid (PTSA) doped polypyrrole (PPy-PTSA)29 nanostructures could be carried out along similar lines. Here we note that the diameter, thickness of the wall, and morphology are related to the molar ratio of dopant. However, the following context will indicate that the magnetic susceptibilities of doped PANI and PPy are less dependent on the morphology but strongly dependent on the dopant type and doping level. The magnetic susceptibility measurements were conducted by using a Physical Property Measurement System (PPMS from Quantum Design) from 300 K down to 3 K at a dc magnetic field of 10 or 0.5 kOe. The magnetic field dependences of magnetization were measured by the same system from -90 to 90 kOe at different temperatures. The background signal coming from the sample holder was systematically removed. 3. Results and Discussion 3.1. Naphthalene Sulfonic Acid Doped Polyaniline (PANINSA). Figure 1a shows the dc magnetic susceptibility χ(T) of PANI nanotubes as a function of temperature for an applied magnetic field of B ) 10 kOe. Obviously, at lower temperatures,

where Mdc is the magnetization intensity measured at a dc magnetic field B, χ0 ) χdia + χP corresponds to the temperatureindependent susceptibility in Figure1a, and C the Curie constant. Figure 1b shows the χdcT vs T plot; the curves are nearly linear. In this paper, we study the total magnetic susceptibility; χdc has not been corrected for the diamagnetic contribution χdia by using Pascal constants,30 because in some cases χdc is dependent on the applied magnetic field. The context of this will be discussed in the following. From Figure1a, we can see that all the PANI samples show Curie-like susceptibility at low temperatures and temperatureindependent susceptibility at high temperatures. Such a behavior has been widely reported for conducting polymers. By now, there are several interpretations of the susceptibility. A popular one is the existence of two spin species.4,6,15 According to the heterogeneous disorder model,15 it is general accepted that the polymer is composed of crystalline and amorphous regions. Since the charge carriers in the amorphous regions are expected to be strongly localized to single polymer chain or defects, the amorphous regions likely give rise to the Curie spins (trapped spins) and Curie-type susceptibility. The Pauli-like susceptibility is usually attributed to the delocalized or mobile Pauli spins in metallic crystalline regions. This model is supported by NMR studies by Beau et al.15 However, recent studies on emeraldine base PANI (PANI-EB) have demonstrated that paramagnetic spins were found even in carefully synthesized PANI-EB samples.10 This seems to be contradictory to the above analysis because the undoped PANI-EB is an insulator and there should be no delocalized or mobile carriers. Is it reasonable to consider that the observed temperature-independent susceptibility is just a density-of-states-related Pauli susceptibility? Through several magnetic susceptibility studies on polyaniline and its derivatives, Kahol et al.10,12 have pointed out that the above simple interpretation of the “linear” part of the χT vs T plot in terms of Pauli susceptibility may not be correct especially for PANI and other less conducting polymeric materials, because most of the spins are localized and there are only a low or zero density of states at the Fermi level. For such a disordered system, a pair of polarons (called a bipolaron) can be formed by being held together by a strong exchange interaction characterized by the coupling constant J; the magnetic behavior should therefore be determined by a distribution of J couplings within the pairs. Thus, a simple exchange coupled pairs model was proposed by Kahol et al.:10

χ ) fP

∫ χPAIRP(J) dJ + (1 - fP)N2µB2/kBT

where fP is the fraction of total spins involved in pairs, P(J) is the distribution function for intrapair couplings, χPAIR ) (N2µB2/ kBT)[3 + exp(-2J/kBT)]-1 is the magnetic susceptibility of a pair of spins, J varies from Jmin to a maximum value J0, and N is the number of spins in the disordered system. Assuming P(J) ) constant and J0 . T, the above equation can be simplified as

χT ) fPµB2[(2/3)ln(3/4)(N/J0)]T + N2µB2(1 + fp)/3kB

23230 J. Phys. Chem. B, Vol. 110, No. 46, 2006 which exhibits a linear dependence of χT vs T. This model has successfully explained the aging or moisture effect on magnetic properties of PANI, PANI blends, and PANI derivatives.10-13 Here, it should be noted that a similar model was also suggested and applied to doped polyaniline and polypyrrole.31 For the present case, we propose that the susceptibility data can be interpreted by the exchange coupled pairs model, as shown in Figure1b, by the linear variation of χdcT vs T in a wide temperature range. Figure 1 also indicates that at room temperature the undoped PANI (PANI-EB) shows diamagnetism; with increasing dopant level, the lightly doped PANI-NSA shows paramagnetism and the heavily doped samples show diamagnetism. Kahol et al.11 also reported that the total dc magnetic susceptibility of overdoped PANI-PSSA measured by SQUID shows diamagnetism. Here, it is interesting to note the temperature-independent susceptibility χ0 decreases from positive to negative when the doping level increases. It could be argued that the possible reason is the core diamagnetism has not been subtracted from the total magnetic susceptibility. In fact, the authors have done the diamagnetic corrections by using Pascal constants; however, the lightly doped sample still shows a larger temperatureindependent susceptibility than the heavily doped samples (data not shown). It should be noted that, in the studies of Raghunathan et al.,7 even though the total magnetic susceptibility data have been corrected for the diamagnetic contribution using Pascal constants, they still found that the temperatureindependent Pauli-like contribution to the dc magnetic susceptibility does not, unlike in the case of HCl-doped PANI,4 increase quasi-linearly with the doping level but decreases in both cases. Through ESR measurements, Raghunathan et al.7 suggested that initially polaron formation may be occurring, and at higher protonation levels, the formation of bipolaron may be taking place. Shimoi and Abe32 have theoretically pointed out the possibility of a doping-induced phase transition from a polaron lattice to a bipolaron lattice since polarons are destabilized at high concentration of carriers. That is to say, unpaired spins (paramagnetic species) and paired spins can possibly coexist in the conducting state and unpaired spins (polarons) may be coupled into spinless spins (bipolarons) as concentration increases,7,9,17 thus the temperature-independent susceptibility is expected to decrease with an increasing doping level. Here, we note the conductivity increases with increasing doping concentration due to the increase in the total number of charge carriers (polarons and bipolarons). The paramagnetism and diamagnetism of the doped PANINSA were confirmed by the magnetic field dependence of magnetization at different temperatures. Figure 2 shows the magnetization M(B) as a function of magnetic field at high temperature (300 K) and low temperature (4 or 5 K) for lightly doped and heavily doped PANI-NSA samples, respectively. For the lightly doped sample, paramagnetic behavior is evident at both 300 and 4 K, whereas the heavily doped sample shows diamagnetism at 300 K and paramagnetism at 5 K. These results are consistent with the temperature-dependent susceptibility in Figure 1a. 3.2. Salicylic Acid Doped Polyaniline (PANI-SA). To further explore the magnetic properties of doped PANI, salicylic acid (SA) doped PANI samples were also studied. It is found that the magnetic behaviors of PANI-SA are quite different than those of PANI-NSA. Parts a and b of Figure 3 show the magnetic field dependence of magnetization for lightly doped PANI-SA ([An]/[SA] ) 1:0.1). Unlike the lightly doped PANI-NSA, this sample mainly shows diamagnetism at 300

Long et al.

Figure 2. Typical magnetic field dependence of magnetization for the following: (a) lightly doped and (b) heavily doped PANI-NSA samples at room temperature (300 K) and low temperature (4 or 5 K).

Figure 3. Magnetic field dependence of magnetization for salicylic acid doped polyaniline (PANI-SA, [aniline]/[SA] ) 1:0.1) at 300 and 3 K, and temperature dependence of magnetic susceptibility measured at dc magnetic field B ) 0.5 kOe.

K and a very weak paramagnetic contribution only at a weak magnetic field B < 1 kOe. At T ) 3 K, a strong transition is observed in the Mdc vs B plot. The magnetization intensity Mdc increases with increasing strength of magnetic field when B is less than 33 kOe. However, Mdc decreases when B continues to increase due to the atomic core diamagnetic contribution and then changes the sign from positive to negative at about B ) 76 kOe. Figure 3c shows the temperature dependence of susceptibility measured at B ) 0.5 kOe. When the temperature is lowered from 300 to 50 K, the paramagnetic χdc decreases gradually; Below 50 K, the Curie-type susceptibility becomes the main contribution. However, when the concentration was increased to [An]/[SA] ) 1:1, the magnetic properties of PNAI-SA become quite unusual. As shown in parts a and b of Figure 4, at T ) 300 K the sample shows a transition from weak paramagnetism to

Magnetic Properties of Polymer Nanostructures

J. Phys. Chem. B, Vol. 110, No. 46, 2006 23231

Figure 5. Temperature dependence of magnetic susceptibility for molybdenic acid doped polyaniline (PANI-MA, [aniline]/[MA] ) 1:1) measured at dc magnetic field B ) 10 kOe.

Figure 4. Magnetic field dependence of magnetization for PANISA ([aniline]/ [SA] ) 1:1) at 300 and 3 K, and temperature dependence of magnetic susceptibility measured at dc magnetic field B ) 0.5 kOe.

diamagnetism at B ) 3.6 kOe. In particular, the sample only shows diamagnetism at T ) 3 K and no Curie-type paramagnetism is observed. The temperature dependence of magnetic susceptibility measured at magnetic field B ) 0.5 kOe is displayed in Figure 4c. It confirms the paramagnetism at higher temperature and diamagnetism at lower temperature. Here, it should be noted that, if the temperature-dependent susceptibility was measured at stronger dc magnetic field (e.g., B ) 10 kOe), only diamagnetism will be observed at room temperature, because the magnetic field dependence of magnetization in Figures 3a and 4a, that is, the Mdc vs B plot is not linear. Since χdc is defined as χdc ) Mdc/B, in this case, the dc magnetic susceptibility χdc is dependent on the strength of the applied dc magnetic field. This may be one of the reasons that the reported dc susceptibility and density of state at the Fermi level are quite different as the applied magnetic field is 7.5 T in ref 4, 5 kG in refs 10-13, and 50 kOe in ref 14. Comparing Figure 4 with Figure 3, it seems that the increase of concentration will increase the paramagnetic spins at higher temperature and decrease the Curie-type species at lower temperature. However, a question arises: why do the paramagnetic spins disappear gradually with lowering temperature, as demonstrated in Figures 3c and 4c? In fact, a similar result was also observed in molybdenic acid doped polyaniline nanotubes (PANI-MA, [aniline]/[MA] ) 1:1). As shown in Figure 5, the magnetic susceptibility above 70 K is not a constant but increases gradually from 70 to 200 K. A possible interpretation is the formation of spinless (diamagnetic) bipolarons. Oliveira Jr. and Santos33 theoretically supported the coexistence of polarons and bipolarons and discussed their relative stability. Yang and Li8 found that there is an equilibrium between polaron and bipolaron through EPR studies of polyaniline films. Nalwa17 has observed the gradual decrease of paramagnetic susceptibility from 300 K down to 75 K in PPy-HBF4 and pointed out that the unpaired spins (polarons) show a strong coupling tendency

and the pairing of spins takes place with a change in temperature. Here, we note the above results can be repeated in PANI-SA samples prepared by other methods (e.g., doping the PANI-EB sample by using SA solution). The results are only dependent on the dopant and doping level and nearly independent of the morphology. 3.3. Magnetic Properties of Doped or Oxidized Polypyrrole. The magnetic properties of doped PPy have been reported by several groups. For example, Devreux et al.16 found that the magnetic susceptibility of PPy-HClO4 shows a sharp transition around 30 K and obeys the Curie-Weiss law (χ ) C/(T - TC), where C is the Curie constant and TC is the Curie temperature) from 30 to 300 K. A strong transition at about 75 K in the χdc vs T plot of PPy-HBF4 was reported by Nalwa.17 However, magnetic field dependence of magnetization has not been explored. Figure 6 shows the magnetic properties of paratoluenesulfonic acid (PTSA) doped PPy. The χdc vs T plot is similar to earlier reports, as a sharp transition around 25 K is observed. However, the magnetic field dependence of magnetization (Mdc vs B) is not linear at either 300 or 10 K, as shown in parts a and b of Figure 6, a transition from paramagnetic behavior to diamagnetic behavior with increasing magnetic field is evident. This means that the dc magnetic susceptibility χdc of PPy is also dependent on the strength of the applied dc magnetic field. If a small magnetic field (e.g., B ) 1 kOe, not 10 kOe in Figure 6c) is applied, then the susceptibility χdc should be positive (paramagnetism) from 300 to 4 K even without diamagnetic corrections using Pascal constants. So we suggest that it may be not correct to evaluate the density of states at the Fermi level through static magnetic susceptibility measurements. Figure 7a shows the temperature dependence of magnetic susceptibility of PPy (synthesized by an in situ chemical oxidative polymerization method34) measured at dc magnetic field B ) 10 KOe. It does not vary like the χdc vs T curves in Figures 1 and 6. Above 73 K, the susceptibility is temperature independent. However, a kink is observed between 73 and 17 K. Raghunathan et al.7 have observed some peaks in the χdc vs T plot, which were found to occur in the case of PANI-(PTSA)x for x ) 0.18 and 0.38 and in the case of PANI-(SSA)x for x ) 0.2 and 0.25. In addition, Nalwa17 also observed similar peaks in PPy and polythiophene. Raghunathan et al.7 suggested that the peak observed in the magnetic susceptibility data may be due to some spin-ordering transition which may or may not be associated with the formation of bipolarons. To understand the origin of the transition, magnetization as a function of magnetic field was measured at 300, 50, and 10 K, as shown in parts b, c, and d of Figure 7. It is obvious that the Mdc vs B plots are symmetrical at 300 and 5 K. However, the Mdc vs B

23232 J. Phys. Chem. B, Vol. 110, No. 46, 2006

Figure 6. Magnetic field dependence of magnetization for paratoluenesulfonic acid doped polypyrrole (PPy-PTSA, [pyrrole]/[PTSA] ) 1:0.3) at 300 and 10 K, and temperature dependence of magnetic susceptibility measured at dc magnetic field B ) 10 kOe.

curve is strongly not symmetrical at 50 K. These results indicate that some kind of spin-ordering transition does take place between 73 and 17 K, as pointed out by Nalwa,17 the coupling of unpaired spins takes place at some stage, particular in oxidized PPy. Further studies are needed to clarify the nature of this transition.7 3.4. Further Discussion. As we know, polaron and spinless bipolaron (a pair of coupled polarons) are two possible configurations of charge carriers in doped conjugated polymers

Long et al. with nondegenerate ground state, such as polyaniline and polypyrrole. In some earlier theoretical models,35 a bipolaron was always energetically more stable than two separate polarons. However, in the Oliveira-Santos model,33 a polaron is energetically more stable than a bipolaron, polarons and bipolarons coexist in short oligomers, and polarons are predominant in long chains. According to the Shimoi-Abe model,32 taking into account Coulomb effects on bipolarons and polarons, it is pointed out that a polaron is the most stable configuration at low doping levels and a doping-induced phase transition from a polaron lattice to a bipolaron lattice is possible because polarons is destabilized at high concentration of carriers. Namely, besides polymer physical structures, length of chains, e-e interactions, electron-lattice couplings, and many other factors will influence the relative stability of polarons and bipolarons. Our results in Figure 1a support the Shimoi-Abe model, which is also supported by the earlier magnetic results of PTSA- and SSA-doped polyaniline.7 It should be mentioned that, in the case of HCl- and H2SO4-doped polyaniline,4,36 the results do not support the Shimoi-Abe model; the temperatureindependent Pauli-like susceptibility increases monotonically with the increase of protonation level, that is, polarons are still stable and predominant at high concentration. These results indicate that the doping level and dopant type may strongly influence the relative stability of polarons and bipolarons by changing the microcrystalline structure during sample preparation (e.g., degree of conjugation, degree of cross linking, interactions between charge carriers, counteranions, and other particles). For example, it seems that inorganic acid dopants such as HCl and H2SO4 can increase the stability of polarons, whereas organic acid dopants such as NSA, SA, PTSA, and SSA can increase the relative stability of bipolarons especially at high doping levels. However, dealing with how the dopant type and dopant size influence the relative stability of polarons and bipolarons needs more and further theoretical studies to clarify. Our results in Figures 3-5 demonstrate that temperature or thermal energy will also influence the relative stability of polarons and bipolarons since there is an equilibrium between them.8 A pair of coupled polarons or bipolarons may be formed with the change in temperature. In this case, fP, the fraction of total spins involved in pairs in the exchange coupled pairs model, is not a constant. That means the “Pauli-like” magnetic

Figure 7. (a) Temperature dependence of magnetic susceptibility for in situ chemically oxidized PPy measured at dc magnetic field B ) 10 kOe and magnetic field dependence of magnetization at (b) 300, (c) 150, and (d) 10 K.

Magnetic Properties of Polymer Nanostructures susceptibility is temperature dependent, as shown in Figures 3-5. The dc magnetic susceptibility data χ(T) cannot be fitted by the exchange coupled pairs model. The results of magnetic field dependence of magnetization also indicate that the magnetic susceptibility is dependent on the applied magnetic field in some cases. In this article, we cannot conclude whether the magnetic field will influence the relative stability of polarons and bipolarons or not; this needs further studies to clarify. However, to obtain the “Pauli-like” susceptibility and the density of states at the Fermi level through static magnetic susceptibility measurements, it is necessary and useful to confirm whether the susceptibility is or is not dependent on the magnetic field. 4. Conclusions The magnetic properties of polyaniline and polypyrrole nanostructures have been studied as functions of temperature and magnetic field. Some interesting results were observed: (1) The dc susceptibility χdc ) Mdc/B is dependent on the applied magnetic field for some samples, which has been evidenced by the magnetic field dependence of magnetization (Mdc vs B plot). (2) Some unusual transitions were observed in the temperature dependence of susceptibility. These transitions suggest the coexistence of paramagnetic polarons and spinless bipolarons and possible formation of bipolarons (or polarons) with changes in doping level and temperature. These results indicate that the magnetic susceptibilities of doped PANI and PPy are strongly dependent on the doping level, dopant type, and temperature, which result in a change of the relative ratio of localized spins, mobile spins, and spinless bipolarons. The results also indicate that it may be not correct to estimate the density of states at the Fermi level through static magnetic susceptibility measurements, because the temperatureindependent susceptibility is dependent on the applied magnetic field and cannot include the contribution of spinless bipolarons (which have a contribution to electrical conductivity). Acknowledgment. This project was financially supported by the National Natural Science Foundation of China (Grant No. 10374107 and 10604038). The authors are grateful to Dr. Xuetong Zhang (College of Chemistry and Molecular Engineering, Center for Nanoscale Science and Technology, Peking University) for providing the polypyrrole sample synthesized by an in situ chemical oxidative polymerization method. References and Notes (1) Weinberger, B. R.; Kaufer, J.; Heeger, A. J.; Pron, A.; MacDiarmid, A. G. Phys. ReV. B 1979, 20, 223. (2) Tomkiewicz, Y.; Schultz, T. D.; Broom, H. B.; Clarke, T. C.; Street, G. B. Phys. ReV. Lett. 1979, 43, 1532. (3) Ikehata, S.; Kaufer, J.; Woerner, T.; Pron, A.; Druy, M. A.; Sivak, A.; Heeger, A. J.; MacDiarmid, A. G. Phys. ReV. Lett. 1980, 45, 1123. (4) Ginder, J. M.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Solid State Commun. 1987, 63, 97.

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