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Conductivity and magnetoresistance of polyacetylene fiber network
Synthetic Metals 105 Ž1999. 207–210 www.elsevier.comrlocatersynmet
Letter
Conductivity and magnetoresistance of polyacetylene fiber network G.T. Kim a , M. Burghard b, D.S. Suh a , K. Liu b, J.G. Park a , S. Roth b, Y.W. Park a
a,),1
Department of Physics and Condensed Matter Research Institute, Seoul National UniÕersity, Seoul 151-742, South Korea b Max-Planck-Institut fur Heisenbergstraße 1, D-70569, Stuttgart, Germany ¨ Festkorperforschung, ¨ Received 27 March 1999; accepted 3 May 1999
Abstract The four-probe conductivity and magnetoresistance ŽMR. in micron-scale for the iodine-doped polyacetylene ŽPA. fiber network were measured. A remarkably weaker temperature dependence of resistivity compared to that of the bulk PA film was observed. The transverse MR at T s 1.5 K was negative and its relative magnitude was approximately 0.1% at H s 10 T. The slow oscillatory tendency with small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Conductivity; Magnetoresistance; Polyacetylene; Fiber network
1. Introduction Although the typical room temperature ŽRT. resistivity of heavily doped polyacetylene ŽPA. was similar to those of the normal metals, nonmetallic temperature dependence of resistivity was usually observed w1–3x. The nonmetallic temperature dependence of resistivity could be due to either the disordered metal characteristics of activated mobility w4x, localization w5x or the fluctuation-induced tunneling effect across the interfibrillar contact barriers, assuming the individual fiber to be broadband metallic in the heavily doped regime w6,7x. Recent reports on the resistance measurement of the single carbon nanotube w8–10x opened a new possibility of measuring the resistivity in micron-scale. In this paper, we presented the fourprobe resistance in micron-scale for the iodine-doped PA fiber network. The temperature dependence of resistivity of PA fiber network was remarkably weaker Ž r ŽT s 1.5 K.rr RT s 1.1. than that of the bulk film Ž r ŽT s 1.5 K.rr RT s 2.4.. The transverse magnetoresistance ŽMR. at T s 1.5 K was negative and its relative magnitude was approximately 0.1% at H s 10 T. It is about 1r40 of the bulk PA film signal. The slow oscillatory tendency with
)
Corresponding author. Tel.: q82-28806607; fax: q82-28737037; E-mail: [email protected] 1 Present address: National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306-4005, USA.
small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network.
2. Experiment The PA fiber network sample was prepared from the low-density foam like PA synthesized by the method of Wnek et al. w11x. Fig. 1Ža. shows the SEM image of the PA sample on the SiO 2 substrate. It shows fiber network structure. Typical diameter of individual fiber is approximately 60–80 nm. The dark red gel of the low-density foam like PA was cut into a small piece and dropped into toluene to sonicate for a few seconds for dispersion. After the sonication, the toluene solution was centrifuged for 10 min at 3000 rpm and then the toluene solvent was substituted with propanol using m-pipet for better dispersion. The sonication and the substitution procedures were repeated about five times. After the repeated processing, the solution was diluted with propanol in different concentrations to make the optimal density of the sample for the transport measurement. The optimal density of a sample corresponded to the sample just large enough to make contacts to the four consecutive electrodes. If the density of the sample in the propanol solution was too large, the four electrodes to measure the resistivity were not defined because the whole gold electrodes area was covered with samples. On the other hand, if the density of the sample
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 9 1 - 0
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G.T. Kim et al.r Synthetic Metals 105 (1999) 207–210
was too small, there were not enough samples which made contacts to four gold electrodes. A SiO 2 substrate patterned with gold leads separated by 3 mm with each other was processed in 6 ml of Ž3-Aminopropyl.-triethoxysilane in 10 ml of distilled water for 1 h for better adhesion of the sample to the substrate ŽSimilar surface treatment with self-assembled monolayers of octadecylsiloxane was reported for the area-selective deposition of conducting polymers w12x.. And then, one droplet of the optimal density solution was dropped on the SiO 2 substrate where the substrate was under argon flowing atmosphere. After drying for 10 min in argon atmosphere, the sample chamber was evacuated to pump out the solvent. The leads for the four-probe conductivity measurement were selected through the optical microscope, and bonded to the 20-pin chip carrier Ž4 mm = 4 mm. with gold wires. During the whole preparation process, the sample was kept in 99.999% argon flowing atmosphere. The PA fiber network seems to be more sensitive to air than the PA bulk film, since upon air exposure, the resistance of the iodine-doped PA fiber network increases more rapidly than that of the bulk PA film. Fig. 1Žb. shows two PA fiber networks mounted on top of the SiO 2 substrate. Two sets of four electrodes between electrodes 1 and 6 were connected to each of the two samples. The numbering of the electrodes was from the top of Fig. 1Žb.. The thickness of the sample measured by the Atomic Force Microscope ŽAFM. was 100 nm in average. Since the diameter of individual fiber was 60–80 nm ŽFig. 1Ža.., the PA fiber network samples were formed by less than two PA fiber layers in average. Since the separation between the two voltage contacts is 3 mm, one could estimate from Fig. 1Ža. that there are approximately 25 interfibrillar junctions in each layer. Thus, the total number of interfibrillar junctions between the voltage probes are approximately 50 in average. The sample was doped by exposing the iodine vapor to the sample in a sealed tube. Since the iodine was reactive with metal electrodes such as tungsten, titanium or aluminum, we had to choose gold as electrodes to pattern on the SiO 2 substrate. Gold electrode is inert enough not to be affected by iodine vapor during the doping period. There was no observable color change in gold electrode during the iodine doping. A piece of bulk PA film was located right next to the sample for monitoring the doping concentration by measuring the weight uptake every 2 h. After 24 h doping, the doping concentration of Iy 3 for the bulk PA film was 2%. Since the rate of iodine uptake in a bulk PA film should be much slower than for the fibers, the value for doping concentration of PA fiber network must be much greater than 2%. The temperature dependence of resistivity at zero magnetic field was measured by the d.c. four-probe method using Keithley 220 Current Source. The low current of nanoampere level was measured with Keithley 617 electrometer. For the voltage drop measurement of the low Žhigh. resistance sample, Keithley 182 sensitive nanovoltmeter ŽKeith-
ley 617 electrometer. was used. The current was applied between electrodes 1 and 6, and the voltage was measured between the electrodes 3 and 4. The MR was measured at T s 1.5 K by the a.c. technique. The EG & G5210 Lockin Amplifier with the reference frequency of 13.7 Hz was used. The magnetic field was swept in both increasing and decreasing directions. Temperature and magnetic field were controlled by the Oxford 12-T superconducting magnet system.
3. Results and discussion Fig. 2 shows the normalized resistivity Ž r ŽT .rr RT . as a function of temperature for the Iy 3 -doped PA fiber network. For comparison, the resistivity of Iy 3 -doped bulk film in Ref. w1x Ž r RT , 3.7 = 10y5 V cm. was plotted in broken lines. The temperature dependence of resistivity of PA fiber network was remarkably weaker than that of the bulk film: r ŽT s 1.5 K.rr RT s 1.1 for the PA fiber network and 2.4 for the bulk PA film. The four-probe resistance at RT was 120 V. If we assume the width of the sample as 6 mm, and the thickness as 100 nm, the resistivity is estimated to be 2.4 = 10y3 V cm. Since the density of PA fiber network is 0.02–0.04 grcm3 ŽRef. w11x., it is approximately 30 times smaller than ; 1.0 grcm3 of the bulk PA film used for monitoring the doping concentration. The high density bulk PA film was synthesized by the modified Shirakawa method w13x. With the density correction factor of 1r30, the measured resistivity, 2.4 = 10y3 V cm, could be as low as r RT , 8 = 10y5 V cm Ži.e., the conductivity could be as high as s RT , 1.3 = 10 4 Srcm.. It is the same magnitude as the one normally measured in the stretch-oriented high density bulk PA film upon heavy doping Ž y ) 7%. w1–3x. It indicates that the actual doping concentration of the PA fiber network was much greater than 2%. The low RT resistivity and its weak temperature dependence was reproduced in another iodine-doped PA fiber network. Notably, the current-carrying capacity of PA fiber network seemed to be quite high, because we found that some damage occurred to the gold electrodes rather than PA fiber network when electric shock was unexpectedly applied to the system. It suggested the high possibility of using the PA fiber network as electrical connection wires in nanoscale molecular devices. The heterogeneous model of charge transport in conducting polymers w6x assumed that the resistance was composed of metallic fibers and barriers due to interfibrilla contacts. The metallic fibers were in series with the junctions, so that R dc s R i ŽT . q R j ŽT . where R i ŽT . resulted from the doped metallic fibers with an intrinsic metallic resistivity, and R j ŽT . was the junction resistance. Although the extended state character of single fiber could as well be due to three-dimensional delocalization w5x, we
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Fig. 1. Ža. Scanning electron microscope ŽSEM. image of PA fiber network on SiO 2 substrate: the diameter of PA fiber is 60–80 nm. Žb. Optical microscope image of the dispersed PA fiber network deposited on the SiO 2 substrate. The SiO 2 substrate was patterned with gold electrodes separated by 3 mm. Fig. 2. The normalized resistivity Ž r ŽT .rr RT . vs. temperature, R RT s 120 V Ž r RT s 2.4 = 10y3 V cm.. v, The iodine-doped PA fiber network; Broken lines: data from Ref. w1x Ž r RT , 3.7 = 10y5 V cm.. The inset figure: the I–V curve at T s 1.5 K: Current was changed slowly in sequence of 1, 2, 3, and 4. Fig. 3. Transverse MR of Iy 3 -doped PA fiber network between H s 0 and H s 10 T at T s 1.5 K. ^: Sweeping up the magnetic fields; \: Sweeping down; Solid line: average of ^ and \; Broken lines: transverse MR of iodine-doped bulk PA film at T s 1.7 K Ž r RT s 9 = 10y5 V cm. from Ref. w16x.
assumed R i ŽT . s A expŽy" v 0rk B T . as for the quasi one-dimensional metallic conduction where v 0 is the 2 k F phonon frequency w14x, and R j ŽT . s B expŽT1rŽT q T0 .. for the junction resistance w15x. Within this model, T1 is dominated by the contact barrier height and T0 is related to the barrier width w15x. Fitting our experimental data for the PA fiber network to the above heterogeneous model gave the parameters T1 s 48.2 K and T0 s 288 K. From the same analysis of the data for bulk PA film of Basescu et al. w1x Žbroken line in Fig. 2., one obtains T1 s 123 K and T0 s 103 K. The model results indicated that the effective barrier height and width became much smaller in PA fiber network compared to those of bulk PA film. The almost temperature-independent resistivity data of PA fiber network corresponds within this model to the predominance of the tunneling conduction ŽT0 s 288 K. from RT down
to T s 1.5 K. A tunneling conduction at high temperature could lead to a possibility of making quantum tunneling devices at RT with PA fiber network. The inset of Fig. 2 shows the I–V characteristics of the PA fiber network measured at T s 1.5 K. The current was changed slowly in sequence of 1, 2, 3, and 4. There appeared a deviation from the linear Ohmic I–V characteristics, which could be understood as due to the heating effect, i.e., the applied current could induce heating in the sample which led to smaller voltage drop than that of the Ohmic value at T s 1.5 K ŽNotice that the resistance decreased upon heating at low temperature as shown in Fig. 2.. If the current was reduced, the effect was reversed. Considering the nanosize of the individual fiber Ž60–80 nm diameter., the applied 10 nA current could be already high enough to induce such heating effect.
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Fig. 3 shows the MR of the iodine-doped PA fiber network at T s 1.5 K. The magnetic field was applied in the perpendicular direction to the sample plane. Thus, transverse MR was measured. A negative MR with small magnitude of D r Ž H s 10 T.rr Ž H s 0. ; 0.001 was observed at T s 1.5 K. The broken line shows the transverse MR data at T s 1.7 K of bulk PA film w16x, which rapidly becomes more negative, reaching y0.04 at H s 10 T. Such large transverse MR were also observed by others w4,5x. The remarkably small magnitude of MR for PA fiber network compared to that of the bulk PA film indicated that the magnitude of MR depended strongly on the interfibrillar contacts resistance and geometry. According to the analysis of the data in Fig. 2, the effective contact barrier height and width were reduced significantly in PA fiber network. It could be due to the smaller number of contact barriers in the PA fiber network than in the bulk PA film. Another important feature was the possible presence of a slow oscillatory behavior of MR although the amplitude was quite small. There was also an asymmetry in MR depending on the sweeping direction of the magnetic field. Although the oscillatory feature of the data was not so clear, we speculate that it might be real. To check the electrical noise level of the signal, the time variation of resistance at zero magnetic field was measured for approximately equal time interval to the duration of the MR measurement. The measured time variation of resistance at zero magnetic field normalized to RŽ H s 0, T s 1.5 K. was less than 1.6 = 10y4 . Since, the observed oscillatory feature in MR has amplitude ; 1 = 10y3 as shown in Fig. 3, the signal to noise of the oscillatory features is approximately six. The sweeping up and down cycles of magnetic field showed some asymmetry with slight shifts of the peak positions in each field sweeping. But the general tendency of the oscillatory nature was reproducible and the estimated period of oscillation is approximately 6 T. We also speculated that the closed loop structures of the PA fiber network could be sensitive to the sweeping direction and the magnitude of magnetic field causing the asymmetric oscillatory nature between the sweeping up and the sweeping down cycles. In summary, there are approximately 50 interfibrillar junctions in seriesrparallel between the two voltage contacts of the device. The results of PA fiber network open a new possibility of investigating quantum transport properties such as the tunneling conduction across the interfibrillar junctions, the Aharonov–Bohm effect-like interference under magnetic field, etc. One might speculate even the ballistic transport between the two contact barriers as a possible origin of the almost temperature-independent re-
sistivity data shown in Fig. 2. Furthermore, the interfibrillar contact barrier could be regarded as a naturally grown nanojunction between the two metallic fibers of nanosize. The metallic PA fiber could be used as an electrical connector of the two different molecular devices. Acknowledgements We would specially give our gratitudes to M. Riek, M. Schmidt, J.H. Smet, A. Christian, F. Zha, J. Muster, B. Steffi, K. Vojislav at MPI, Stuttgart, for valuable discussions and furnishing their facilities. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida. One of the authors ŽYWP. appreciates Prof. J.R. Schrieffer for his kind arrangement to work at the NHMFL. This work was supported by STEPI under the contract no. 98-I-01-04-A026, Ministry of Science and Technology ŽMOST., Korea. References w1x N. Basescu, Z.-X. Liu, D. Moses, A.J. Heeger, H. Naarmann, N. Theophilou, Nature 4 Ž1987. 403. w2x Y.W. Park, C. Park, Y.S. Lee, C.O. Yoon, H. Shirakawa, Y. Suezaki, K. Akagi, Solid State Commun. 65 Ž1988. 147. w3x Th. Schimmel, W. Riess, J. Gmeiner, G. Denninger, M. Schwoerer, Solid State Commun. 65 Ž1988. 1311. w4x Y. Nogami, H. Kaneko, H. Ito, T. Ishiguro, T. Sasaki, N. Toyota, A. Takahashi, J. Tsukamoto, Phys. Rev. B 43 Ž1991. 11829. w5x H.H.S. Javadi, A. Chakraborty, C. Li, N. Theophilou, D.B. Swanson, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 43 Ž1991. 2183. w6x A.B. Kaiser, Synth. Met. 45 Ž1991. 183. w7x N. Theophilou, D.B. Swanson, A.G. MacDiarmid, A. Chakraborty, H.H.S. Javadi, R.P. Mccale, S.P. Treat, F. Zuo, A.J. Epstein, Synth. Met. 28 Ž1989. D35. w8x L. Langer, L. Stockman, J.P. Heremans, V. Bayot, C.H. Olk, C. van Haesendonck, Y. Bruynseraede, J.-P. Issi, Synth. Met. 70 Ž1995. 1393. w9x T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Nature 382 Ž1996. 54. w10x S.J. Tans, M.H. Davoret, H. Dai, A. Thess, R.E. Smalley, L.J. Geerligs, C. Dekker, Nature 386 Ž1997. 474. w11x G.E. Wnek, J.C.W. Chien, F.E. Karasz, M.A. Druy, Y.W. Park, A.G. MacDiarmid, A.J. Heeger, J. Polym. Sci., Polym. Lett. Ed. 17 Ž1979. 779. w12x Z. Huang, P.-C. Wang, A.G. MacDiarmid, Y. Xia, G. Whitesides, Langmuir 13 Ž1997. 6480. w13x K. Akagi, M. Suezaki, H. Shirakawa, H. Kyotani, M. Shimomura, Y. Tanabe, Synth. Met. 28 Ž1989. D1. w14x S. Kivelson, A.J. Heeger, Synth. Met. 22 Ž1988. 371. w15x P. Sheng, Phys. Rev. B 21 Ž1980. 2180. w16x E.S. Choi, G.T. Kim, D.S. Suh, D.C. Kim, J.G. Park, Y.W. Park, Synth. Met. 100 Ž1999. 3.
Letter
Conductivity and magnetoresistance of polyacetylene fiber network G.T. Kim a , M. Burghard b, D.S. Suh a , K. Liu b, J.G. Park a , S. Roth b, Y.W. Park a
a,),1
Department of Physics and Condensed Matter Research Institute, Seoul National UniÕersity, Seoul 151-742, South Korea b Max-Planck-Institut fur Heisenbergstraße 1, D-70569, Stuttgart, Germany ¨ Festkorperforschung, ¨ Received 27 March 1999; accepted 3 May 1999
Abstract The four-probe conductivity and magnetoresistance ŽMR. in micron-scale for the iodine-doped polyacetylene ŽPA. fiber network were measured. A remarkably weaker temperature dependence of resistivity compared to that of the bulk PA film was observed. The transverse MR at T s 1.5 K was negative and its relative magnitude was approximately 0.1% at H s 10 T. The slow oscillatory tendency with small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Conductivity; Magnetoresistance; Polyacetylene; Fiber network
1. Introduction Although the typical room temperature ŽRT. resistivity of heavily doped polyacetylene ŽPA. was similar to those of the normal metals, nonmetallic temperature dependence of resistivity was usually observed w1–3x. The nonmetallic temperature dependence of resistivity could be due to either the disordered metal characteristics of activated mobility w4x, localization w5x or the fluctuation-induced tunneling effect across the interfibrillar contact barriers, assuming the individual fiber to be broadband metallic in the heavily doped regime w6,7x. Recent reports on the resistance measurement of the single carbon nanotube w8–10x opened a new possibility of measuring the resistivity in micron-scale. In this paper, we presented the fourprobe resistance in micron-scale for the iodine-doped PA fiber network. The temperature dependence of resistivity of PA fiber network was remarkably weaker Ž r ŽT s 1.5 K.rr RT s 1.1. than that of the bulk film Ž r ŽT s 1.5 K.rr RT s 2.4.. The transverse magnetoresistance ŽMR. at T s 1.5 K was negative and its relative magnitude was approximately 0.1% at H s 10 T. It is about 1r40 of the bulk PA film signal. The slow oscillatory tendency with
)
Corresponding author. Tel.: q82-28806607; fax: q82-28737037; E-mail: [email protected] 1 Present address: National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306-4005, USA.
small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network.
2. Experiment The PA fiber network sample was prepared from the low-density foam like PA synthesized by the method of Wnek et al. w11x. Fig. 1Ža. shows the SEM image of the PA sample on the SiO 2 substrate. It shows fiber network structure. Typical diameter of individual fiber is approximately 60–80 nm. The dark red gel of the low-density foam like PA was cut into a small piece and dropped into toluene to sonicate for a few seconds for dispersion. After the sonication, the toluene solution was centrifuged for 10 min at 3000 rpm and then the toluene solvent was substituted with propanol using m-pipet for better dispersion. The sonication and the substitution procedures were repeated about five times. After the repeated processing, the solution was diluted with propanol in different concentrations to make the optimal density of the sample for the transport measurement. The optimal density of a sample corresponded to the sample just large enough to make contacts to the four consecutive electrodes. If the density of the sample in the propanol solution was too large, the four electrodes to measure the resistivity were not defined because the whole gold electrodes area was covered with samples. On the other hand, if the density of the sample
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 9 1 - 0
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G.T. Kim et al.r Synthetic Metals 105 (1999) 207–210
was too small, there were not enough samples which made contacts to four gold electrodes. A SiO 2 substrate patterned with gold leads separated by 3 mm with each other was processed in 6 ml of Ž3-Aminopropyl.-triethoxysilane in 10 ml of distilled water for 1 h for better adhesion of the sample to the substrate ŽSimilar surface treatment with self-assembled monolayers of octadecylsiloxane was reported for the area-selective deposition of conducting polymers w12x.. And then, one droplet of the optimal density solution was dropped on the SiO 2 substrate where the substrate was under argon flowing atmosphere. After drying for 10 min in argon atmosphere, the sample chamber was evacuated to pump out the solvent. The leads for the four-probe conductivity measurement were selected through the optical microscope, and bonded to the 20-pin chip carrier Ž4 mm = 4 mm. with gold wires. During the whole preparation process, the sample was kept in 99.999% argon flowing atmosphere. The PA fiber network seems to be more sensitive to air than the PA bulk film, since upon air exposure, the resistance of the iodine-doped PA fiber network increases more rapidly than that of the bulk PA film. Fig. 1Žb. shows two PA fiber networks mounted on top of the SiO 2 substrate. Two sets of four electrodes between electrodes 1 and 6 were connected to each of the two samples. The numbering of the electrodes was from the top of Fig. 1Žb.. The thickness of the sample measured by the Atomic Force Microscope ŽAFM. was 100 nm in average. Since the diameter of individual fiber was 60–80 nm ŽFig. 1Ža.., the PA fiber network samples were formed by less than two PA fiber layers in average. Since the separation between the two voltage contacts is 3 mm, one could estimate from Fig. 1Ža. that there are approximately 25 interfibrillar junctions in each layer. Thus, the total number of interfibrillar junctions between the voltage probes are approximately 50 in average. The sample was doped by exposing the iodine vapor to the sample in a sealed tube. Since the iodine was reactive with metal electrodes such as tungsten, titanium or aluminum, we had to choose gold as electrodes to pattern on the SiO 2 substrate. Gold electrode is inert enough not to be affected by iodine vapor during the doping period. There was no observable color change in gold electrode during the iodine doping. A piece of bulk PA film was located right next to the sample for monitoring the doping concentration by measuring the weight uptake every 2 h. After 24 h doping, the doping concentration of Iy 3 for the bulk PA film was 2%. Since the rate of iodine uptake in a bulk PA film should be much slower than for the fibers, the value for doping concentration of PA fiber network must be much greater than 2%. The temperature dependence of resistivity at zero magnetic field was measured by the d.c. four-probe method using Keithley 220 Current Source. The low current of nanoampere level was measured with Keithley 617 electrometer. For the voltage drop measurement of the low Žhigh. resistance sample, Keithley 182 sensitive nanovoltmeter ŽKeith-
ley 617 electrometer. was used. The current was applied between electrodes 1 and 6, and the voltage was measured between the electrodes 3 and 4. The MR was measured at T s 1.5 K by the a.c. technique. The EG & G5210 Lockin Amplifier with the reference frequency of 13.7 Hz was used. The magnetic field was swept in both increasing and decreasing directions. Temperature and magnetic field were controlled by the Oxford 12-T superconducting magnet system.
3. Results and discussion Fig. 2 shows the normalized resistivity Ž r ŽT .rr RT . as a function of temperature for the Iy 3 -doped PA fiber network. For comparison, the resistivity of Iy 3 -doped bulk film in Ref. w1x Ž r RT , 3.7 = 10y5 V cm. was plotted in broken lines. The temperature dependence of resistivity of PA fiber network was remarkably weaker than that of the bulk film: r ŽT s 1.5 K.rr RT s 1.1 for the PA fiber network and 2.4 for the bulk PA film. The four-probe resistance at RT was 120 V. If we assume the width of the sample as 6 mm, and the thickness as 100 nm, the resistivity is estimated to be 2.4 = 10y3 V cm. Since the density of PA fiber network is 0.02–0.04 grcm3 ŽRef. w11x., it is approximately 30 times smaller than ; 1.0 grcm3 of the bulk PA film used for monitoring the doping concentration. The high density bulk PA film was synthesized by the modified Shirakawa method w13x. With the density correction factor of 1r30, the measured resistivity, 2.4 = 10y3 V cm, could be as low as r RT , 8 = 10y5 V cm Ži.e., the conductivity could be as high as s RT , 1.3 = 10 4 Srcm.. It is the same magnitude as the one normally measured in the stretch-oriented high density bulk PA film upon heavy doping Ž y ) 7%. w1–3x. It indicates that the actual doping concentration of the PA fiber network was much greater than 2%. The low RT resistivity and its weak temperature dependence was reproduced in another iodine-doped PA fiber network. Notably, the current-carrying capacity of PA fiber network seemed to be quite high, because we found that some damage occurred to the gold electrodes rather than PA fiber network when electric shock was unexpectedly applied to the system. It suggested the high possibility of using the PA fiber network as electrical connection wires in nanoscale molecular devices. The heterogeneous model of charge transport in conducting polymers w6x assumed that the resistance was composed of metallic fibers and barriers due to interfibrilla contacts. The metallic fibers were in series with the junctions, so that R dc s R i ŽT . q R j ŽT . where R i ŽT . resulted from the doped metallic fibers with an intrinsic metallic resistivity, and R j ŽT . was the junction resistance. Although the extended state character of single fiber could as well be due to three-dimensional delocalization w5x, we
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Fig. 1. Ža. Scanning electron microscope ŽSEM. image of PA fiber network on SiO 2 substrate: the diameter of PA fiber is 60–80 nm. Žb. Optical microscope image of the dispersed PA fiber network deposited on the SiO 2 substrate. The SiO 2 substrate was patterned with gold electrodes separated by 3 mm. Fig. 2. The normalized resistivity Ž r ŽT .rr RT . vs. temperature, R RT s 120 V Ž r RT s 2.4 = 10y3 V cm.. v, The iodine-doped PA fiber network; Broken lines: data from Ref. w1x Ž r RT , 3.7 = 10y5 V cm.. The inset figure: the I–V curve at T s 1.5 K: Current was changed slowly in sequence of 1, 2, 3, and 4. Fig. 3. Transverse MR of Iy 3 -doped PA fiber network between H s 0 and H s 10 T at T s 1.5 K. ^: Sweeping up the magnetic fields; \: Sweeping down; Solid line: average of ^ and \; Broken lines: transverse MR of iodine-doped bulk PA film at T s 1.7 K Ž r RT s 9 = 10y5 V cm. from Ref. w16x.
assumed R i ŽT . s A expŽy" v 0rk B T . as for the quasi one-dimensional metallic conduction where v 0 is the 2 k F phonon frequency w14x, and R j ŽT . s B expŽT1rŽT q T0 .. for the junction resistance w15x. Within this model, T1 is dominated by the contact barrier height and T0 is related to the barrier width w15x. Fitting our experimental data for the PA fiber network to the above heterogeneous model gave the parameters T1 s 48.2 K and T0 s 288 K. From the same analysis of the data for bulk PA film of Basescu et al. w1x Žbroken line in Fig. 2., one obtains T1 s 123 K and T0 s 103 K. The model results indicated that the effective barrier height and width became much smaller in PA fiber network compared to those of bulk PA film. The almost temperature-independent resistivity data of PA fiber network corresponds within this model to the predominance of the tunneling conduction ŽT0 s 288 K. from RT down
to T s 1.5 K. A tunneling conduction at high temperature could lead to a possibility of making quantum tunneling devices at RT with PA fiber network. The inset of Fig. 2 shows the I–V characteristics of the PA fiber network measured at T s 1.5 K. The current was changed slowly in sequence of 1, 2, 3, and 4. There appeared a deviation from the linear Ohmic I–V characteristics, which could be understood as due to the heating effect, i.e., the applied current could induce heating in the sample which led to smaller voltage drop than that of the Ohmic value at T s 1.5 K ŽNotice that the resistance decreased upon heating at low temperature as shown in Fig. 2.. If the current was reduced, the effect was reversed. Considering the nanosize of the individual fiber Ž60–80 nm diameter., the applied 10 nA current could be already high enough to induce such heating effect.
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Fig. 3 shows the MR of the iodine-doped PA fiber network at T s 1.5 K. The magnetic field was applied in the perpendicular direction to the sample plane. Thus, transverse MR was measured. A negative MR with small magnitude of D r Ž H s 10 T.rr Ž H s 0. ; 0.001 was observed at T s 1.5 K. The broken line shows the transverse MR data at T s 1.7 K of bulk PA film w16x, which rapidly becomes more negative, reaching y0.04 at H s 10 T. Such large transverse MR were also observed by others w4,5x. The remarkably small magnitude of MR for PA fiber network compared to that of the bulk PA film indicated that the magnitude of MR depended strongly on the interfibrillar contacts resistance and geometry. According to the analysis of the data in Fig. 2, the effective contact barrier height and width were reduced significantly in PA fiber network. It could be due to the smaller number of contact barriers in the PA fiber network than in the bulk PA film. Another important feature was the possible presence of a slow oscillatory behavior of MR although the amplitude was quite small. There was also an asymmetry in MR depending on the sweeping direction of the magnetic field. Although the oscillatory feature of the data was not so clear, we speculate that it might be real. To check the electrical noise level of the signal, the time variation of resistance at zero magnetic field was measured for approximately equal time interval to the duration of the MR measurement. The measured time variation of resistance at zero magnetic field normalized to RŽ H s 0, T s 1.5 K. was less than 1.6 = 10y4 . Since, the observed oscillatory feature in MR has amplitude ; 1 = 10y3 as shown in Fig. 3, the signal to noise of the oscillatory features is approximately six. The sweeping up and down cycles of magnetic field showed some asymmetry with slight shifts of the peak positions in each field sweeping. But the general tendency of the oscillatory nature was reproducible and the estimated period of oscillation is approximately 6 T. We also speculated that the closed loop structures of the PA fiber network could be sensitive to the sweeping direction and the magnitude of magnetic field causing the asymmetric oscillatory nature between the sweeping up and the sweeping down cycles. In summary, there are approximately 50 interfibrillar junctions in seriesrparallel between the two voltage contacts of the device. The results of PA fiber network open a new possibility of investigating quantum transport properties such as the tunneling conduction across the interfibrillar junctions, the Aharonov–Bohm effect-like interference under magnetic field, etc. One might speculate even the ballistic transport between the two contact barriers as a possible origin of the almost temperature-independent re-
sistivity data shown in Fig. 2. Furthermore, the interfibrillar contact barrier could be regarded as a naturally grown nanojunction between the two metallic fibers of nanosize. The metallic PA fiber could be used as an electrical connector of the two different molecular devices. Acknowledgements We would specially give our gratitudes to M. Riek, M. Schmidt, J.H. Smet, A. Christian, F. Zha, J. Muster, B. Steffi, K. Vojislav at MPI, Stuttgart, for valuable discussions and furnishing their facilities. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida. One of the authors ŽYWP. appreciates Prof. J.R. Schrieffer for his kind arrangement to work at the NHMFL. This work was supported by STEPI under the contract no. 98-I-01-04-A026, Ministry of Science and Technology ŽMOST., Korea. References w1x N. Basescu, Z.-X. Liu, D. Moses, A.J. Heeger, H. Naarmann, N. Theophilou, Nature 4 Ž1987. 403. w2x Y.W. Park, C. Park, Y.S. Lee, C.O. Yoon, H. Shirakawa, Y. Suezaki, K. Akagi, Solid State Commun. 65 Ž1988. 147. w3x Th. Schimmel, W. Riess, J. Gmeiner, G. Denninger, M. Schwoerer, Solid State Commun. 65 Ž1988. 1311. w4x Y. Nogami, H. Kaneko, H. Ito, T. Ishiguro, T. Sasaki, N. Toyota, A. Takahashi, J. Tsukamoto, Phys. Rev. B 43 Ž1991. 11829. w5x H.H.S. Javadi, A. Chakraborty, C. Li, N. Theophilou, D.B. Swanson, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 43 Ž1991. 2183. w6x A.B. Kaiser, Synth. Met. 45 Ž1991. 183. w7x N. Theophilou, D.B. Swanson, A.G. MacDiarmid, A. Chakraborty, H.H.S. Javadi, R.P. Mccale, S.P. Treat, F. Zuo, A.J. Epstein, Synth. Met. 28 Ž1989. D35. w8x L. Langer, L. Stockman, J.P. Heremans, V. Bayot, C.H. Olk, C. van Haesendonck, Y. Bruynseraede, J.-P. Issi, Synth. Met. 70 Ž1995. 1393. w9x T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Nature 382 Ž1996. 54. w10x S.J. Tans, M.H. Davoret, H. Dai, A. Thess, R.E. Smalley, L.J. Geerligs, C. Dekker, Nature 386 Ž1997. 474. w11x G.E. Wnek, J.C.W. Chien, F.E. Karasz, M.A. Druy, Y.W. Park, A.G. MacDiarmid, A.J. Heeger, J. Polym. Sci., Polym. Lett. Ed. 17 Ž1979. 779. w12x Z. Huang, P.-C. Wang, A.G. MacDiarmid, Y. Xia, G. Whitesides, Langmuir 13 Ž1997. 6480. w13x K. Akagi, M. Suezaki, H. Shirakawa, H. Kyotani, M. Shimomura, Y. Tanabe, Synth. Met. 28 Ž1989. D1. w14x S. Kivelson, A.J. Heeger, Synth. Met. 22 Ž1988. 371. w15x P. Sheng, Phys. Rev. B 21 Ž1980. 2180. w16x E.S. Choi, G.T. Kim, D.S. Suh, D.C. Kim, J.G. Park, Y.W. Park, Synth. Met. 100 Ž1999. 3.