Are Bulk Defective Carbon Nanotubes Less Electrically Conducting

Mar 8, 2003 - Current−voltage measurements of various carbon nanotube−polymer composite films at room temperature as well as at liquid nitrogen te...
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NANO LETTERS

Are Bulk Defective Carbon Nanotubes Less Electrically Conducting?

2003 Vol. 3, No. 4 549-553

Paul C. P. Watts, Wen-Kuang Hsu,* Harold W. Kroto, and David R. M. Walton School of Chemistry, Physics and EnVironmental Science, UniVersity of Sussex, Brighton BN1 9QJ, U.K. Received January 15, 2003; Revised Manuscript Received February 18, 2003

ABSTRACT Current−voltage measurements of various carbon nanotube−polymer composite films at room temperature as well as at liquid nitrogen temperature reveal that the composite films consisting of defective carbon nanotubes exhibit higher conductivity than do graphite carbon nanotubes. This novel finding contradicts previous results.

Lattice defects weaken carbon nanotube (CNT) structures; therefore, most defective CNTs adopt a woven or tangled morphology.1 Previous electrical conductivity measurements of individual CNTs showed that defective CNTs indeed possess higher resistivities than do graphite CNTs,2,3 which is due to an increase in electron scattering by the defects. Theoretical studies by Rochefort et al. proposed that the introduction of defects (e.g., C-O) into CNT lattices would reduce CNT conduction by 30-50%,4 a result that is consistent with experiment.2 Recently, Grimes et al. reported that the graphitization of CNTs by high-temperature annealing reduces the magnitude of CNT permittivity. They also found that the defective CNTs made by the CVD process exhibit a higher conductivity than do graphitic CNTs.5 For example, the composite film made by mixing defective CNTs with polymer has a conductivity of 7.78 × 104 S/m, and the graphitic CNT-polymer composite has a conductivity of 2.22 × 104 S/m. This finding remains to be explained. In this letter, four-terminal current-voltage (I-V) measurements were carried out on polystyrene (PS) films containing various CNTs, and the results indicate that the defective CNT-polystyrene (PS) films do exhibit higher conductivity than graphitic CNTs made by arc discharge. This outcome is consistent with Grimes et al.’s report, but it contradicts previous reports.2,3 An explanation is proposed in this paper to account for the above finding. Various multiwalled CNTs including arc-made CNTs,6 arcmade boron-doped CNTs (BCNTs),7 Fe-filled CNTs made by the pyrolysis of ferrocene,8 and conventional CVD-CNTs (purchased from Guangzhou Yorkpoint New Energy Company, China) were individually incorporated into PS via a solvent-vacuum-casting technique9 followed by the hotpressing process. Here, we briefly describe the technique. The raw CNTs were ground and dispersed in toluene (20 * Corresponding author. E-mail: [email protected]. 10.1021/nl034028v CCC: $25.00 Published on Web 03/08/2003

© 2003 American Chemical Society

mL) by an ultrasonicate probe (20-kW power) for 30 min. The CNT-containing toluene was subsequently added to the toluene-polystyrene solution, followed by another 30 min of ultrasonication. The mixture was then transferred to a Petri dish and placed under vacuum for a week to remove solvent and to form a solid plastic film. The CNT-to-polymer ratio (wt %) varies from 1:2 to 1:8, and the composite film is 2 mm thick. Each film (1.5 × 3 cm) has been marginally painted with silver paste to support four-terminal I-V measurements. All CNT samples were analyzed by highresolution transmission electron microscopy (HRTEM) to provide information regarding the extent of CNT lattice imperfections prior to composite film formation. However, HRTEM is unable to detect covalent-type defects (e.g., C-O); therefore, thermogravimetric analysis (TGA) was employed to determine the onset of CNT oxidation because defects are very sensitive to oxidation. Composite films have been examined by the SEM technique without undergoing the gold-sputtering process. Arc-made CNTs, BCNTs, and Fe-filled CNTs have been previously studied by HRTEM,6-8 and the degree of their graphitization is similar. Nevertheless, TGA indicates that the Fe-filled CNTs begin to oxidize at ca. 430 °C,10 which is significantly lower than the values obtained from arc-made CNTs and BCNTs.10 This outcome implies that the Fe-filled CNTs contain covalent-bonding-like defects, which trigger and accelerate the CNT oxidation.10 According to TEM and HRTEM images, it is clear that the arc-CNTs (Figure 1a and b) and Fe-filled CNTs are graphitic (Figure 1c and d) and that the CVD-CNTs are defective (Figure 1e and f). Meanwhile, Figure 1e and f also supports the above description that the CVD-CNTs are in weaving morphologies. Figure 2 shows TGA profiles of CVD-CNTs (curve a), Fe/CNTs (curve b), arc-CNTs (curve c), and arc BCNTs (curve d). First, we focus on CVD-CNTs (curve a), and the

Figure 1. TEM and HRTEM images of arc-CNTs (a, b), Fe/CNTs (c, d), and CVD-CNTs (e, f). Two CNTs overlaps shown by arrows 1 and 2 (b).

Figure 2. TGA profiles of CVD-CNTs (curve a), Fe/CNTs (curve b), arc-CNTs (curve c), and arc-BCNTs (curve d).

following features are distinguished. First, the slope of curve a is larger than that of curves b-d. Second, the onset of CVD-CNT oxidation is ca. 400 °C, which is significantly lower than that of curves c and d (ca. 750-850 °C) and is only slightly lower (ca. 30 °C) than that of the Fe-filled CNTs (curve b). Third, the remaining weight at 700 °C is ca. 5% (grey region). By comparison with arc- and ferrocene-made CNTs, the larger slope in curve a indicates that the CVDCNTs contain a larger defect density and that their oxidation is faster. Meanwhile, the CVD-CNTs contain small amounts of catalytic metal particles (Fe) at the tube tips, which become oxide (Fe2O3) when the tube walls are attacked and damaged by O2. Oxide formation accounts for the 5% remaining mass at 700 °C (grey region, Figure 2). The oxidation of Fe/CNTs begins with a weight increase of ca. 5% at 430 °C, followed by a common weight-loss profile between 560 and 790 °C. The remaining weight at 790 °C is ca. 50% for curve b. The 5% weight increase and 50% 550

remaining weight at the beginning and at the end of the oxidation are also due to Fe2O3 formation. The 5% weight increase at the beginning of oxidation is not seen in curve a, which is due to the fact that the ferrocene/CNTs contain a significant quantity of Fe,8 so CO (or CO2) and Fe2O3 simultaneously form during the O2 attack; apparently the latter is slightly faster than the former, which leads to a small weight increase at the beginning of oxidation. The presence of the 35% remaining weight and higher oxidation temperature in arc-BCNTs (curve d) than in the arc-CNTs (curve c) is due to the fact that the B2O3 forms during oxidation and B has an antioxidation effect.11 According to our previous X-ray diffraction analyses, arcmade CNTs and BCNTs displayed well-defined 00l and inplane reflections including 002, 004, 100, and 110.7 The trace of 3D reflection (e.g., 101) was also present. For Fe/CNTs, only 00l reflections (002, 004) were distinguished, and the in-plane reflections were small and broad.12 The CVD-CNTs exhibited only a very broad 002 peak, and the 004 peak was absent. The X-ray diffraction clearly indicates that arc-made CNTs as well as BCNTs are graphitic structures; however, the Fe/CNTs and CVD-CNTs lack of in-plane and 3D ordering. The morphology of arc-made CNT/PS films differs from that of defective CNT/PS composites. According to our SEM studies, the fractured surfaces of arc-made CNT/PS films show straight protruding CNTs (Figure 3a), which is reminiscent of bare arc-made CNTs. Similar morphology is also found in arc-made BCNT/PS films. For defective CNT/ PS composites (either CVD-CNTs or Fe/CNTs) the rigidlike protruding CNTs are absent, and the embedded tubelike features are frequently present in fractured film surfaces Nano Lett., Vol. 3, No. 4, 2003

Figure 3. SEM images taken from the fractured surfaces of arcmade CNT/PS (a) and CVD-CNT/PS films (b).

(arrows, Figure 3b). The diameter of embedded tubelike structures is slightly thicker than that of the bare defective CNTs. The embedded tubes are actually CNTs coated with polymer, as revealed previously by HRTEM (400 keV) at Cambridge.13 It is known that the electrical conduction pathway within a CNT/polymer film is essentially established via a CNT network.9 Accordingly, one should be concerned as to whether the embedded CNTs found in defective CNT/ PS films produce CNT networks that are superior to those produced by arc-made CNTs in the films, which results in a higher conductivity in CVD-CNT/PS films (or Fe/CNT/PS films) than in arc-made CNT/PS films. In fact, as described in a previous paper, the defective CNTs show a better dispersion in polymer than do graphitic CNTs.14 Nevertheless, if all CNTs are fully coated with polymer, then the conducting CNT-CNT contact will become capacitor-like (similar to that that of the CNT/PS-CNT structure15), which leads to an increase in film resistance.15 The CNT-polymer composite films exhibit a CNT percolation threshold value (ca. 12.5 wt %, corresponding to a 1:8 CNT-to-polymer ratio). Note: The percolation threshold value essentially relates to CNT networking structures within the polymer, and it depends on CNT concentration and is independent of the CNT type. Accordingly, all types of CNTs would possess a similar threshold value.9 When the CNT-to-polymer ratio exceeds this value, the film resistance drops exponentially because of CNT network formation. A previous study showed that when the CNT content is below and above the percolation threshold the composite film resistance can Nano Lett., Vol. 3, No. 4, 2003

Figure 4. I-V spectra of CVD-CNT/PS and Fe-filled CNT/PS films at 1:4 (a, c) and 1:6 (b, d) ratios. Arc-made CNT/PS (1:2 ratio) and BCNT/PS (1:4 ratio) films are also shown (e-f).

be expressed as CNT resistors connected in series (R ) R1 + R2 + ... Rn) and in parallel (1/R ) 1/R1 + 1/R2 + ... 0.1/Rn).9,15 In other words, below the CNT percolation threshold value, the CNT dispersion is mostly localized. Above this value, CNTs are well dispersed in the polymer matrix, so the CNT percolation threshold can be regarded as a parameter for determining the degree of CNT dispersion. This description is supported by the fact that when the CNT content exceeds the threshold value the conduction across the film is uniform. (See Figure 3a and b in ref 9.) Previous studies indicated that individual arc-made BCNTs are metallic and exhibit higher conductivity than arc-made CNTs because of an increase in the number of hole-charge carriers via the incorporation of BC3 units into the CNT lattice.16 The conductivity of bulk BCNTs is also higher than values obtained from the bulk arc-made CNTs.7 Nevertheless, our I-V studies clearly indicate that the defective CNT/PS composite films (i.e., CVD- and Fe-filled CNTs) exhibit lower film resistance as compared with that of arc-made CNT and BCNT/PS films. This outcome is not likely to be due to the presence of an uneven CNT distribution in the PS matrix because composite films studied here contain a greater CNTto-polymer ratio than the CNT percolation threshold value. Figure 4a and b shows I-V characteristics of CVD-CNT/ PS composite films at 1:4 and 1:6 CNT-to-polymer ratios as compared with those of Fe-filled CNT/PS films (Figure 4c and d). Meanwhile, the I-V characteristics of arc-made CNT/PS films (1:2) and BCNT/PS films (1:4), extracted from 551

a previous paper,9 are also displayed (Figure 4e and f). Figure 4 shows the following features. First, all of the I-V spectra are linear (i.e., ohmic conduction). Second, the BCNT/PS film has a positive temperature coefficient of resistivity and metallic character (Figure 4f) whereas others exhibit a negative temperature coefficient of resistivity (i.e., semiconducting, the film resistance increases as the temperature decreases). Third, the encapsulated Fe does not increase the CNT conduction because electron hopping across 3.4-Å spaces from the inner layers to the outer layers is difficult. Moreover, if the encapsulated Fe does contribute to CNT conduction (i.e., the charge transfer between the C walls and Fe), then one would expect the presence of metallic I-V characteristics in Fe/CNT/PS films, not in semiconducting films. The CVD-CNT/PS(1:4) film (Figure 4a) exhibits a very large current-carrying capacity of ca. 420 mA (1 V) at room temperature (RT), which decreases to 380 mA at liquid nitrogen (LN) temperature. The current value obtained from the CVD-CNT/PS(1:4) composite film corresponds to 1 × 102 A cm-2. This value is promoted to 1 × 105 A cm-2 if the PS matrix volume is excluded. The value of 1 × 105 A cm-2 is comparable to that of metals. However, the current is reduced to 50 and 35 mA (1 V) at RT and LN, respectively, for the CVD-CNT/PS(1:6) film (Figure 4b). The current reduction is due to less CNT loading in the PS matrix. By comparison with CVD-CNT/PS films, the current capacity for the Fe/CNT/PS composites is lowersca. 100-120 mA for film(1:4) and 4-5 mA for film(1:6) at 5 V (Figure 4c and d, respectively). The current capacities for arc-CNT/PS and arc-BCNT/PS films are much lower by a factor of 3-6 at 10 V (Figure 4e and f). It is noteworthy that the applied voltage lies in the 0-1 V range for CVD-CNT/PS films, 1-5 V range for Fe/CNT/PS films, and 0-10 V range for arc-made CNT/PS films (Figure 4). This phenomenon indicates that the CVD-CNT/PS films actually possess lower resistance than the Fe/CNT/PS and arc-made CNT/PS composites. The I-V measurements have been carried out repeatedly on composite films with different CNT-to-polymer ratios (1:2-8) at RT and LN, and the results are similar (i.e., defective CNT/PS composite films always exhibit a lower resistance than the arc-made CNT/PS films). The film resistance (Rfilm) can be expressed as follows: 1

() 1

∑R

+ Rc

(1)

1 1 1 1 ) + + ... Rn R1 R2 Rn

(2)

Rfilm

)

n

Rc is the CNT networking resistance (i.e., CNT-CNT contact resistance), and Rn are the individual CNT intrinsic resistances. Equation 2 represents the individual CNT resistors connected in parallel in composite films (i.e., CNT content exceeds the CNT percolation threshold value). Equation 1 means that when n f ∞, Σ(1/Rn) f 0, and 1/Rfilm is mainly determined by Rc. n f ∞ and Σ(1/Rn) f 0 also implies that 552

when the CNT content exceeds the percolation threshold value the CNT intrinsic resistance becomes insignificant. For example, assume that we have two CNT composite films; film1 contains mostly higher intrinsic resistances of CNTs (say Rhigh ) 100 Ω), and lower intrinsic resistances of CNTs (say Rlow ) 10 Ω) constitute film2. It is clear when n f ∞, Σ(1/Rn(high)) ≈ Σ(1/Rn(low)) ≈ 0, so the resistance of film1 approximates the R value of film2. This phenomenon is present only in CNT networks and is not found in carbon black-polymer systems. Accordingly, the lower resistance in CVD-CNT/PS composite films is mainly due to Rc (i.e., the CNT-CNT contact resistance). Additional supporting evidence for the presence of lower Rc values in CVD-CNT/ PS films is that the film resistance is higher in BCNT/PS composites than in CVD-CNT/PS. As mentioned above, the BCNTs are metallic, which means that the Rn of BCNTs must be lower than that of the semiconducting CVD-CNTs, so the only possible contribution to BCNT film resistance must arise from Rc. Electron transport through a CNT is of the diffusion type (occasionally the ballistic type, depending on the extent of the CNT mean free path); the electron tunnels between CNTs.14,16 When n approaches ∞, the tunneling mechanism (which is temperature-independent) dominates the CNT transport property. Previous studies showed that when the CNT-to-polymer ratio is high (1:2-6) the I-V temperaturedependent character weakens.9 This means that the Rc term gradually dominates the film resistance as the CNT concentration increases (i.e., n f ∞). However, a question remains as to why the Rc value is lower in CVD-CNT/PS films (or Fe-filled CNT/PS films) than in arc-made CNT/PS films (or in BCNT/PS films). When the electron tunnels through the interfaces between CNTs, scattering occurs, which leads to an increase in CNT resistance.16 An interesting experiment carried out previously by Rinlzer et al. showed that the oxidized CNTs exhibit a lower turn-on potential for electron emission than intact CNTs.17 Namely, the emission arises not only from open-ended tubes but also from damaged CNT lattices. This finding may explain the presence of lower Rc values in defective CNT/PS films; in other words, the CNT defects can act as low-potential sites for electron tunneling between CNTs, which gives rise to a lower Rc in the defective CNT/PS composite films. In conclusion, the current response of a nanodevice consisting of a single CNT bridging two electrical leads is mainly determined by two factors: (a) the intrinsic resistance of the CNT and (b) the contact resistance between the CNT and the leads; (b) surpasses (a) in most cases studied, considering only two conduction channels (corresponding to 12.9 kΩ) for a metallic CNT. However, the situation is now different for the CNTs embedded in polymer. First, the contact resistance between the CNTs and leads is minimized via highly conducting paste electrodes. Second, the intrinsic resistance of CNTs (Rn) and the CNT-CNT contact resistance (Rc) within the composite film therefore become crucial factors. Rn becomes insignificant in determining the composite film resistance when the CNT content exceeds the CNT percolation threshold value. The presence of lower Nano Lett., Vol. 3, No. 4, 2003

resistance in CVD-CNT/PS films is due to the CNT defects, which act as low-potential sites for the electron-tunneling process. Acknowledgment. We thank the EPSRC and the Wolfson foundation of the U.K. for financial support. References (1) Amelinckx, S.; Zhang, X. B.; Bernaerts, D.; Zhang, X. F.; Ivanov, V.; Nagy, J. B. Science (Washington, D.C.) 1994, 265, 635. (2) Dai, H. G.; Wong, E. W.; Lieber, C. M. Science (Washington, D.C.) 1996, 272, 523. (3) Ebbesen, T. W.; Lezec, H. J.; Hiura, H.; Bennett, J. W.; Ghaemi, H. F.; Thio, T. Nature (London) 1996, 382, 54. (4) Rochefort, A.; Avouris, P. J. Phys. Chem. A 2000, 104, 9807. (5) Grimes, C. A.; Dickey, E. C.; Mungle, C.; Ong, K. G.; Qian, D. J. Appl. Phys. 2001, 90, 4134. (6) Iijima, S. Nature (London) 1991, 354, 56. (7) Hsu, W. K.; Firth, S.; Redlich, Ph.; Terrones, M.; Terrones, H.; Zhu, Y. Q.; Clark, R. J. H.; Kroto, H. W.; Walton, D. R. M. J. Mater. Chem. 2000, 10, 1425.

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(8) Rao, C. N. R.; Sen, R.; Satishkumar, B. C.; Govindaraj, A. Chem. Commun. 1998, 15, 1525. (9) Watts, P. C. P.; Hsu, W. K.; Randall, D.; Kroto, H. W.; Walton, D. R. M. Phys. Chem. Chem. Phys. 2002, 4, 5655. (10) Watts, P. C. P.; Hsu, W. K.; Barnes, A.; Chambers, B. AdV. Mater., in press, 2003. (11) Terrones, M.; Hsu, W. K.; Ramos, S.; Castillo, R.; Terrones, H. Fullerene Sci. Technol. 1998, 6, 787. (12) Grobert, N.; Mayne, M.; Terrones, M.; Sloan, J.; Dunin-Borkowski, R. E.; Kamalakaran, R.; Seeger, T.; Terrones, H.; Ru¨hle, M.; Walton, D. R. M.; Kroto, H. W.; Hutchison, J. L. Chem. Commun. 2001, 471. (13) Chen, G. Z.; Shaffer, M. S. P.; Coleby, D.; Dixon, G.; Zhou, W.; Fray, D. J. AdV. Mater. 2000, 12, 522. (14) Watts, P. C. P.; Hsu, W. K.; Kotzeva, V.; Chen, G. Z. Chem. Phys. Lett. 2002, 366, 42. (15) Watts, P. C. P.; Hsu, W. K.; Chen, G. Z.; Fray, D. J.; Kroto, H. W.; Walton, D. R. M. J. Mater. Chem 2001, 11, 2482. (16) Hsu, W. K.; Nakalima, T. Carbon 2002, 40, 462. (17) Rinlzer, A. G.; Hafner, J. H.; Nikolaev, P.; Lou, L.; Kim, S. G.; Toma´nek, D.; Nordlander, P.; Colbert, D. T.; Smalley, R. E. Science (Washington, D.C.) 1995, 269, 1550.

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