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Characterization of transport properties of multiwalled carbon nanotube networks by microwave plasma chemical vapor deposition
Diamond & Related Materials 15 (2006) 1138 – 1142 www.elsevier.com/locate/diamond
Characterization of transport properties of multiwalled carbon nanotube networks by microwave plasma chemical vapor deposition Yasuhiko Hayashi a,⁎, T. Tokunaga b , K. Kaneko b , Z. Horita b a
Department of Environmental Technology & Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Material Science and Engineering, University of Kyushu, 6-10-1 Hakozaki, Higashi, Fukuoka 812-8581, Japan Available online 23 February 2006
Abstract We report the synthesis of multiwalled carbon nanotubes (MWCNTs) and the characterization of temperature-dependent electrical transport properties of MWCNT networks by using a two-point configuration without the lithographical technique. MWCNTs were grown by microwave plasma chemical vapor deposition with the bias enhanced growth technique. The Raman intensity ratio between the D- (∼1360 cm− 1) and G(∼1590 cm− 1) peaks (ID / IG) as well as the full width at half maximum of the G-peak decreased from 1.03 to 0.03 and 18 to 13 cm− 1, respectively, with the increase in the oxidative purification time. This indicates that the crystallinity of graphite sheets is improved by the oxidative purification process and burn-off of the defects in MWCNT networks. The metal electrodes were attached on both the top and the bottom of the insulating thin films, and the as-grown and oxidative-purified MWCNT networks were connected between the electrodes for I–V measurements at various temperatures. At room temperature, the conductance for the MWNT networks at around zero bias was 0.65 G0 (G0: fundamental conductance unit), which was less than the value of 1 G0 for metallic MWCNTs. Further, the conductance increased linearly with the bias voltage until it attained its peak. In the 190–390 K range, the temperature characteristic of the I–V shows that the electron transport of the as-grown MWCNT networks was activated by a lower activation energy than that in oxidative-purified MWCNT networks. © 2006 Elsevier B.V. All rights reserved. Keywords: Multiwalled carbon nanotubes (MWCNTs); MWCNT networks; Temperature-dependent transport properties; Conductance
1. Introduction Carbon nanotubes (CNTs) with either single- or multilayered graphene cylinders have been studied extensively with regard to their electrical transport properties since they are the most promising materials for nanoelectronics. It is expected that they would behave as ideal quantum wires with electrical conduction occurring through one-dimensional (1D) modes. In each mode, the conductance is quantized in units of 0.5 G0 (G0 = 2e2 / h = 1 / 12.9 kΩ− 1, where e is the electron charge and h is Planck's constant) [1]. Brown et al. proposed that CNTs must be considered to be ideal test beds for studying ballistic phonon transport and thermal conductance quantization [2]. Therefore, in recent years, many scientists have investigated electrical transport in CNTs by using two- or four-terminal methods [3]. ⁎ Corresponding author. Tel./fax: +81 52 735 5104. E-mail address: [email protected] (Y. Hayashi). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.01.006
Multiple steps in dc current have previously been observed in single-walled CNTs (SWCNTs) and were reported in devices with highly resistive metal contacts [4,5]; these were attributed to the populating of higher 1D subbands. Frank et al. discovered that in multiwalled CNTs (MWCNTs), quantization principally occurs in units of 1 G0, with additional plateaus appearing near 0.5 G0 under certain conditions [6]. Recently, Brown et al. reported the size of the conductance steps observed at room temperature and the correlation between the electrical and thermal conductance steps by using individual MWCNTs [2]. The conductance of a single MWCNT around zero bias is 0.4 G0, and it increases almost linearly with the applied voltage until it peaks [3]. Although the electrical properties of isolated MWCNTs have been investigated in detail, very few electrical and thermal measurements on MWCNT networks, which are referred to as bundle-like MWCNTs, have been reported to date. Many technological issues with regard to controlling and characterizing “individual” CNTs are yet to be resolved because most studies on CNT
Y. Hayashi et al. / Diamond & Related Materials 15 (2006) 1138–1142
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growth have been and are still using randomly dispersed catalysts over the growth area [7]. It is difficult to position single MWCNTs between metal electrodes with a sufficient quality so as to allow ballistic transport measurements. In this study, instead of an isolated MWCNT, we report the temperature dependence of the electrical transport properties of bundle-like MWCNT networks. These properties are measured by using a two-point configuration without the lithographical technique. The influence of the purification of MWCNT networks on their electrical transport properties has been evaluated by varying the oxidative purification time of the MWCNTs. Based on the temperature dependence of the electrical characterization, we propose a general description of the electric conduction mechanism of MWCNT networks. 2. Experimental The MWCNT networks were grown by bias-enhanced microwave PECVD (BE-MPECVD) using a 2.45 GHz, 1.5 kW microwave power supply, as described elsewhere [8]. The substrate temperature was controlled by a radio-frequency graphite heater. The silicon substrate was cleaned for 1 min using hydrofluoric acid, and the substrate surface was oxidized by an acid solution (H2SO4 : H2O2 = 4 : 1). A 1-nm-thick Fe layer was deposited on the thin barrier layer of SiO2 formed on Si surface in order to prevent the formation of silicide. The substrate was then transferred into the growth chamber and gradually heated to 973 K, following which it was placed in vacuum for 10 min. Hydrogen (H2) gas was then fed into the chamber to maintain a pressure of 20 Torr at the preset temperature and a microwave plasma was turned on to 600 kW. The feed gas methane (CH4) was introduced, and the H2 gas flow rate was adjusted to achieve a CH4 : H2 ratio of 1 : 1 at a total pressure of 20 Torr. Then, the MWCNT networks were grown for 10 min under a negative bias of 400 V. In order to remove the amorphous carbon or defective graphene cylinders from as-grown MWCNT networks, the samples were oxidized at 973 K by varying the oxidation time under the flow of 10 sccm O2 gas at a pressure of 20 Torr. Asgrown or purified MWCNT networks were wired between Au electrodes, which were formed on both the top and the bottom of the insulating thin film. The MWCNTs were observed by scanning electron microscopy (SEM) and conventional transmission electron microscopy (TEM). The current–voltage (I–V) characteristics of the MWCNT networks were measured at various temperatures by using a two-terminal configuration. The Raman spectra were measured in the back-scattering geometry by using the 514.5 nm line of an Ar+ ion laser at room temperature in the spectral range of 900 to 1800 cm− 1 with a resolution of 1.0 cm− 1; the signals were separated by a monochromator. 3. Results and discussion Fig. 1 shows the SEM and TEM images of MWCNTs grown by BE-MPECVD; these images confirmed the presence of dense
Fig. 1. TEM image of as-grown MWCNTs and SEM images of as-grown and oxidative-purified MWCNTs.
graphite sheets. The as-grown MWCNTs were densely packed in bundles and the MWCNT was typically 20 nm in diameter and up to several μm long. In the TEM observation, the fringes of the wall surface of the MWCNTs indicated several defects in the graphite sheet. The SEM observation indicates that a large number of MWCNTs were burnt out with the increase in the oxidative purification time, thereby suggesting that the elimination of the majority of the defective MWCNTs and amorphous carbon covering the surfaces of MWCNT bundles. The first-order Raman spectra of the MWCNTs comprise two broad peaks located at 1580 cm− 1 (G-peak) and 1350 cm− 1
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(D-peak), as shown in Fig. 2. The G- and D-peaks are attributed to the inplane symmetric C–C stretching (E2g) and the graphite materials with small crystallite sizes (disordered graphite), respectively [9,10]. For estimating the defect concentration, the D-peak intensity is usually normalized with respect to the Gpeak intensity (ID / IG) [11,12]. The Raman ID / IG ratio varies significantly depending on the different absorption cross sections in the disordered sample; thus, the Raman ratio is sensitive to the existence of defects and amorphous carbon on the MWCNTs. With the increase in the oxidative purification time from 0 to 360 min, the ID / IG ratio decreased from 1.03 to 0.03, as shown in Fig. 3(a); this was because the defects and amorphous carbon on the surface of the MWCNTs were eliminated by the oxidative purification process. Consequently, the Raman spectra were dominated by the sharp G-peak at 1580 cm− 1. The full width at half maximum (FWHM) of the Gpeak decreased from 18 to 13 cm− 1 with the increase in the oxidative purification time, as shown in Fig. 3(b), indicating that the crystallinity of the graphite sheets was improved by the oxidation and burn-off of the defects. A schematic illustration of the device is shown in Fig. 4. The thickness of the insulator was 50 μm and an Au electrode was used. Fig. 5 shows the conductance curve of the as-grown MWCNT networks obtained by a scanning bias from − 4.0 to 4.0 V and measured at temperatures of 200, 300, and 380 K. The resistance of this network at 300 K was estimated to be about 133 kΩ. The conductance curve calculated by the two-terminal differential conductance G = dI / dV exhibited two symmetric peaks at around ± 3.0 V or higher. At room temperature, the conductance for the MWCNT networks at around zero bias voltage was 0.65 G0 of the fundamental conductance unit. Although contributions of 1 G0 and 0.5 G0 have been reported [6], it is expected that CNTs contribute 2 G0 per conducting carbon shell [6,13]. The conductance increased linearly with the applied bias until it peaked at a bias of ± 3.0 V; it then decreased with a further increase in the applied bias over ±3.0 V. The conductance peaks observed in this experiment were lower than
Fig. 2. Raman spectra of MWCNTs with different oxidative purification times (0, 135, and 360 min). The intensities have been normalized with respect to the highest peak.
Fig. 3. (a) A plot of the Raman ID / IG ratio of the MWCNTs versus the oxidative purification time. (b) A plot of the FWHM of the G-peak versus the oxidative purification time.
that for the expected voltage of ± 5.8 V based on the ballistic transport theory [13]. Therefore, our experiment indicates that the transport through the MWCNT networks between the Au electrodes was affected by a large backscattering. The contact scattering or impurities and surfactants on the MWCNTs can
Fig. 4. Schematic illustration of the device used for conductance measurements.
Y. Hayashi et al. / Diamond & Related Materials 15 (2006) 1138–1142
Fig. 5. Conductance of the MWCNT networks as a function of the applied voltage at various temperatures.
reduce the measured conductances to less than the value of 1 G0 for metallic MWCNTs [6,3]. In particular, the conductance of 0.65 G0 obtained in our experiment is in good agreement with 0.4 G 0 obtained for an individual MWCNT [3]. The conductance of semiconducting MWCNTs at around zero bias is lower than that of metallic MWCNTs due to the presence of an energy gap. The difference between the metallic and semiconducting MWCNTs with regard to their electrical transport at around zero bias should not be considerable due to a very small energy gap (0.05 eV) of the semiconducting MWCNTs [14]. Liang et al. discussed in their report that the conductance of semiconducting MWCNTs at around zero bias can be extremely high; however, it would be less than the value of 1 G0 for metallic MWCNTs [3]. Therefore, the conductance of 0.65 G0 obtained in our experiment indicates that the MWCNT networks are semiconducting. The conductance at around zero bias voltage increased from 0.42 to 1.25 G0 with the increase in the temperature from 200 K to 380 K. No significant difference was observed in the slopes of the dI / dV curves; however, the conductance peaks shifted toward a higher absolute voltage with the increase in the temperature. Although the origin of the nonlinear I–V characteristic, which was significantly different from that of single-walled CNTs [15], has not been clarified thus far, it can be interpreted to originate due to a change in the current distribution along the cross section of the MWCNT networks; the outermost shell mainly contributes to the conduction at lower currents, while the inner shells gradually begin to contribute with the increase in current such that the total conductance increases with the current [16]. Fig. 6(a) and (b) show plots of the normalized current of the MWCNT networks versus 1000 / T and the activation energy Ea of the MWCNT networks versus the Raman ID / IG ratio, respectively. The normalized current decreased monotonically with the temperature (200 to 390 K); it then remained almost constant in the temperature range below 200 K. We fitted these data to the Arrhenius-type equation f(T) = f0exp(− Ea / kBT) to calculate Ea, where kB is Boltzmann's constant. In the temperature range of 200 to 390 K, the temperature character-
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istic of I–V shows that the oxidative-purified MWCNT networks transported electrons with a low activation energy of 21 meV. The Ea of 21 meV was found to be smaller than the bandgap (∼80 meV) of the pure undoped MWNT, reported by Carroll et al [17]. Further, the electron transport of as-grown MWCNT networks was activated with a lower activation energy than that of the oxidative-purified MWCNTs in the temperature range of 390 to 200 K. The Ea of the MWCNT networks initially decreased with the Raman ID / IG ratio and it then remained almost constant at around 5 meV with a further increase in the Raman ID / IG ratio (N 0.7), as shown in Fig. 6(b). This result indicates that the oxidative purification process (obtained Raman ID / IG ratio of 0.7) requires more than 50 min to remove most defects or amorphous carbon from the surface of MWCNTs. In the case of the oxidative-purified MWCNT networks, the carrier transport could be interpreted as the thermally activated interlayer hopping transport of electrons from the inner to the outer nanotube walls with an activation energy of 21 meV [18]. However, in the case of the MWCNT networks composed of defective MWCNTs, the carrier transport can be interpreted as the thermally activated hopping transport of electrons through defects, possibly on the outermost shell of MWCNTs, with a very low activation energy of 5 meV. Our result indicates that the defect states for carrier transport by the hopping mechanism decrease with the increase in the oxidative purification time. Therefore, it is difficult to achieve the conduction of carriers in oxidative-purified MWCNT networks; an additional bias is required to initiate conduction.
Fig. 6. (a) A plot of the normalized current of the MWCNT networks versus 1000 / T. (b) A plot of the normalized current versus the Raman ID / IG ratio.
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4. Summary and conclusion
References
In conclusion, the transport properties of MWCNT networks (not isolated MWCNTs) have been measured using the twopoint configuration in the temperature range of 200 to 390 K. The Raman ID / IG and FWHM of MWCNTs decreased with the increase in the oxidative purification time. The conductance increased linearly with the bias until it peaked, following which it decreased with a further increase in the bias. The electric conductance at around zero bias voltage was 0.65 G0 and it increased with the temperature. The conductance of 0.65 G0 at room temperature, less than the value of 1 G0 for metallic MWCNTs, indicates that the MWCNT networks are semiconducting. The electric conductance measurements with respect to the temperature for as-grown MWCNTs reveal that electron transport was activated with a lower activation energy than that for oxidative-purified MWCNTs in the temperature range of 390 to 200 K. The activation energy increased with the oxidative purification time due to the improved crystallinity of the graphite sheets. The elimination of the defect states on the MWCNTs by oxidative purification may become an important process for the production of nanotube-embedded devices.
[1] M.J. Biercuk, N. Mason, J. Martin, A. Yacoby, C.M. Marcus, Phys. Rev. Lett. 94 (2005) 26801. [2] E. Brown, L. Hao, J.C. Gallop, J.C. Macfarlane, Appl. Phys. Lett. 87 (2005) 023107. [3] Y.X. Liang, Q.H. Li, T.H. Wang, Appl. Phys. Lett. 84 (2004) 3379 (and references therein). [4] R.D. Antonov, A.T. Johnson, Phys. Rev. Lett. 83 (1999) 3274. [5] J. Appenzeller, J. Knoch, M. Radosavljevic, et al., Phys. Rev. Lett. 92 (2004) 226802. [6] S. Frank, P. Poncharal, Z.L. Wang, W.A. Heer, Science 280 (1998) 1744. [7] M. Ishida, H. Hongo, F. Nihei, Y. Ochiai, Jpn. J. Appl. Phys. 43 (2004) L1356 (and references therein). [8] Y. Hayashi, T. Tokunaga, T. Jimbo, Y. Yogata, S. Toh, K. Kaneko, Appl. Phys. Lett. 84 (2004) 2886. [9] J. Maultzsch, S. Reich, C. Thomsen, Phys. Rev., B 64 (2001) 121407. [10] C. Thomsen, S. Reich, Phys. Rev. Lett. 85 (2000) 5214. [11] M. Hirscher, M. Becher, M. Haluska, U. Dettlaff-Weglikowska, A. Quintel, G. Duesberg, Y.-M. Choi, P. Downes, M. Hulman, S. Roth, I. Stepanek, P. Bernier, Appl. Phys., A Mater. Sci. Process. 72 (2001) 129; L. Zhang, H. Li, K.-T. Yue, S.-L. Zhang, X. Wu, J. Zi, Z. Shi, Z. Gu, Phys. Rev., B 65 (2002) 073401. [12] C. Zhi, X.D. Bai, E.G. Wang, Appl. Phys. Lett. 80 (2002) 3590. [13] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [14] C. Schonenberger, A. Bachtold, C. Strunk, J.-P. Salvetat, L. Forro, Appl. Phys., A Mater. Sci. Process. 69 (1999) 283. [15] A. Javey, J. Guo, M. Paulsson, Q. Wang, D. Mann, M. Lundstrom, H. Dai, Phys. Rev. Lett. 92 (2004) 106804. [16] R. Hobara, S. Yoshimoto, T. Ikuno, M. Katayama, N. Yamauchi, W. Wongwiriyapan, S. Honda, I. Matsuda, S. Hasegawa, K. Oura, Jpn. J. Appl. Phys. 43 (2004) L1081. [17] D.L. Carroll, Ph. Redlich, X. Blase, J.-C. Charlier, S. Curran, P.M. Ajayan, S. Roth, M. Ruhle, Phys. Rev. Lett. 81 (1998) 2332. [18] B. Wei, R. Spolenak, P. Kohler-Redlich, M. Ruhle, E. Arzt, Appl. Phys. Lett. 74 (1999) 3149.
Acknowledgments This work was partly supported by the “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, carried out at the Research Laboratory of High Voltage Electron Microscopy at Kyushu University. This work was also partly supported by a Grant-in Aid for Scientific Research (Houga-16651065) from MEXT.
Characterization of transport properties of multiwalled carbon nanotube networks by microwave plasma chemical vapor deposition Yasuhiko Hayashi a,⁎, T. Tokunaga b , K. Kaneko b , Z. Horita b a
Department of Environmental Technology & Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Material Science and Engineering, University of Kyushu, 6-10-1 Hakozaki, Higashi, Fukuoka 812-8581, Japan Available online 23 February 2006
Abstract We report the synthesis of multiwalled carbon nanotubes (MWCNTs) and the characterization of temperature-dependent electrical transport properties of MWCNT networks by using a two-point configuration without the lithographical technique. MWCNTs were grown by microwave plasma chemical vapor deposition with the bias enhanced growth technique. The Raman intensity ratio between the D- (∼1360 cm− 1) and G(∼1590 cm− 1) peaks (ID / IG) as well as the full width at half maximum of the G-peak decreased from 1.03 to 0.03 and 18 to 13 cm− 1, respectively, with the increase in the oxidative purification time. This indicates that the crystallinity of graphite sheets is improved by the oxidative purification process and burn-off of the defects in MWCNT networks. The metal electrodes were attached on both the top and the bottom of the insulating thin films, and the as-grown and oxidative-purified MWCNT networks were connected between the electrodes for I–V measurements at various temperatures. At room temperature, the conductance for the MWNT networks at around zero bias was 0.65 G0 (G0: fundamental conductance unit), which was less than the value of 1 G0 for metallic MWCNTs. Further, the conductance increased linearly with the bias voltage until it attained its peak. In the 190–390 K range, the temperature characteristic of the I–V shows that the electron transport of the as-grown MWCNT networks was activated by a lower activation energy than that in oxidative-purified MWCNT networks. © 2006 Elsevier B.V. All rights reserved. Keywords: Multiwalled carbon nanotubes (MWCNTs); MWCNT networks; Temperature-dependent transport properties; Conductance
1. Introduction Carbon nanotubes (CNTs) with either single- or multilayered graphene cylinders have been studied extensively with regard to their electrical transport properties since they are the most promising materials for nanoelectronics. It is expected that they would behave as ideal quantum wires with electrical conduction occurring through one-dimensional (1D) modes. In each mode, the conductance is quantized in units of 0.5 G0 (G0 = 2e2 / h = 1 / 12.9 kΩ− 1, where e is the electron charge and h is Planck's constant) [1]. Brown et al. proposed that CNTs must be considered to be ideal test beds for studying ballistic phonon transport and thermal conductance quantization [2]. Therefore, in recent years, many scientists have investigated electrical transport in CNTs by using two- or four-terminal methods [3]. ⁎ Corresponding author. Tel./fax: +81 52 735 5104. E-mail address: [email protected] (Y. Hayashi). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.01.006
Multiple steps in dc current have previously been observed in single-walled CNTs (SWCNTs) and were reported in devices with highly resistive metal contacts [4,5]; these were attributed to the populating of higher 1D subbands. Frank et al. discovered that in multiwalled CNTs (MWCNTs), quantization principally occurs in units of 1 G0, with additional plateaus appearing near 0.5 G0 under certain conditions [6]. Recently, Brown et al. reported the size of the conductance steps observed at room temperature and the correlation between the electrical and thermal conductance steps by using individual MWCNTs [2]. The conductance of a single MWCNT around zero bias is 0.4 G0, and it increases almost linearly with the applied voltage until it peaks [3]. Although the electrical properties of isolated MWCNTs have been investigated in detail, very few electrical and thermal measurements on MWCNT networks, which are referred to as bundle-like MWCNTs, have been reported to date. Many technological issues with regard to controlling and characterizing “individual” CNTs are yet to be resolved because most studies on CNT
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growth have been and are still using randomly dispersed catalysts over the growth area [7]. It is difficult to position single MWCNTs between metal electrodes with a sufficient quality so as to allow ballistic transport measurements. In this study, instead of an isolated MWCNT, we report the temperature dependence of the electrical transport properties of bundle-like MWCNT networks. These properties are measured by using a two-point configuration without the lithographical technique. The influence of the purification of MWCNT networks on their electrical transport properties has been evaluated by varying the oxidative purification time of the MWCNTs. Based on the temperature dependence of the electrical characterization, we propose a general description of the electric conduction mechanism of MWCNT networks. 2. Experimental The MWCNT networks were grown by bias-enhanced microwave PECVD (BE-MPECVD) using a 2.45 GHz, 1.5 kW microwave power supply, as described elsewhere [8]. The substrate temperature was controlled by a radio-frequency graphite heater. The silicon substrate was cleaned for 1 min using hydrofluoric acid, and the substrate surface was oxidized by an acid solution (H2SO4 : H2O2 = 4 : 1). A 1-nm-thick Fe layer was deposited on the thin barrier layer of SiO2 formed on Si surface in order to prevent the formation of silicide. The substrate was then transferred into the growth chamber and gradually heated to 973 K, following which it was placed in vacuum for 10 min. Hydrogen (H2) gas was then fed into the chamber to maintain a pressure of 20 Torr at the preset temperature and a microwave plasma was turned on to 600 kW. The feed gas methane (CH4) was introduced, and the H2 gas flow rate was adjusted to achieve a CH4 : H2 ratio of 1 : 1 at a total pressure of 20 Torr. Then, the MWCNT networks were grown for 10 min under a negative bias of 400 V. In order to remove the amorphous carbon or defective graphene cylinders from as-grown MWCNT networks, the samples were oxidized at 973 K by varying the oxidation time under the flow of 10 sccm O2 gas at a pressure of 20 Torr. Asgrown or purified MWCNT networks were wired between Au electrodes, which were formed on both the top and the bottom of the insulating thin film. The MWCNTs were observed by scanning electron microscopy (SEM) and conventional transmission electron microscopy (TEM). The current–voltage (I–V) characteristics of the MWCNT networks were measured at various temperatures by using a two-terminal configuration. The Raman spectra were measured in the back-scattering geometry by using the 514.5 nm line of an Ar+ ion laser at room temperature in the spectral range of 900 to 1800 cm− 1 with a resolution of 1.0 cm− 1; the signals were separated by a monochromator. 3. Results and discussion Fig. 1 shows the SEM and TEM images of MWCNTs grown by BE-MPECVD; these images confirmed the presence of dense
Fig. 1. TEM image of as-grown MWCNTs and SEM images of as-grown and oxidative-purified MWCNTs.
graphite sheets. The as-grown MWCNTs were densely packed in bundles and the MWCNT was typically 20 nm in diameter and up to several μm long. In the TEM observation, the fringes of the wall surface of the MWCNTs indicated several defects in the graphite sheet. The SEM observation indicates that a large number of MWCNTs were burnt out with the increase in the oxidative purification time, thereby suggesting that the elimination of the majority of the defective MWCNTs and amorphous carbon covering the surfaces of MWCNT bundles. The first-order Raman spectra of the MWCNTs comprise two broad peaks located at 1580 cm− 1 (G-peak) and 1350 cm− 1
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(D-peak), as shown in Fig. 2. The G- and D-peaks are attributed to the inplane symmetric C–C stretching (E2g) and the graphite materials with small crystallite sizes (disordered graphite), respectively [9,10]. For estimating the defect concentration, the D-peak intensity is usually normalized with respect to the Gpeak intensity (ID / IG) [11,12]. The Raman ID / IG ratio varies significantly depending on the different absorption cross sections in the disordered sample; thus, the Raman ratio is sensitive to the existence of defects and amorphous carbon on the MWCNTs. With the increase in the oxidative purification time from 0 to 360 min, the ID / IG ratio decreased from 1.03 to 0.03, as shown in Fig. 3(a); this was because the defects and amorphous carbon on the surface of the MWCNTs were eliminated by the oxidative purification process. Consequently, the Raman spectra were dominated by the sharp G-peak at 1580 cm− 1. The full width at half maximum (FWHM) of the Gpeak decreased from 18 to 13 cm− 1 with the increase in the oxidative purification time, as shown in Fig. 3(b), indicating that the crystallinity of the graphite sheets was improved by the oxidation and burn-off of the defects. A schematic illustration of the device is shown in Fig. 4. The thickness of the insulator was 50 μm and an Au electrode was used. Fig. 5 shows the conductance curve of the as-grown MWCNT networks obtained by a scanning bias from − 4.0 to 4.0 V and measured at temperatures of 200, 300, and 380 K. The resistance of this network at 300 K was estimated to be about 133 kΩ. The conductance curve calculated by the two-terminal differential conductance G = dI / dV exhibited two symmetric peaks at around ± 3.0 V or higher. At room temperature, the conductance for the MWCNT networks at around zero bias voltage was 0.65 G0 of the fundamental conductance unit. Although contributions of 1 G0 and 0.5 G0 have been reported [6], it is expected that CNTs contribute 2 G0 per conducting carbon shell [6,13]. The conductance increased linearly with the applied bias until it peaked at a bias of ± 3.0 V; it then decreased with a further increase in the applied bias over ±3.0 V. The conductance peaks observed in this experiment were lower than
Fig. 2. Raman spectra of MWCNTs with different oxidative purification times (0, 135, and 360 min). The intensities have been normalized with respect to the highest peak.
Fig. 3. (a) A plot of the Raman ID / IG ratio of the MWCNTs versus the oxidative purification time. (b) A plot of the FWHM of the G-peak versus the oxidative purification time.
that for the expected voltage of ± 5.8 V based on the ballistic transport theory [13]. Therefore, our experiment indicates that the transport through the MWCNT networks between the Au electrodes was affected by a large backscattering. The contact scattering or impurities and surfactants on the MWCNTs can
Fig. 4. Schematic illustration of the device used for conductance measurements.
Y. Hayashi et al. / Diamond & Related Materials 15 (2006) 1138–1142
Fig. 5. Conductance of the MWCNT networks as a function of the applied voltage at various temperatures.
reduce the measured conductances to less than the value of 1 G0 for metallic MWCNTs [6,3]. In particular, the conductance of 0.65 G0 obtained in our experiment is in good agreement with 0.4 G 0 obtained for an individual MWCNT [3]. The conductance of semiconducting MWCNTs at around zero bias is lower than that of metallic MWCNTs due to the presence of an energy gap. The difference between the metallic and semiconducting MWCNTs with regard to their electrical transport at around zero bias should not be considerable due to a very small energy gap (0.05 eV) of the semiconducting MWCNTs [14]. Liang et al. discussed in their report that the conductance of semiconducting MWCNTs at around zero bias can be extremely high; however, it would be less than the value of 1 G0 for metallic MWCNTs [3]. Therefore, the conductance of 0.65 G0 obtained in our experiment indicates that the MWCNT networks are semiconducting. The conductance at around zero bias voltage increased from 0.42 to 1.25 G0 with the increase in the temperature from 200 K to 380 K. No significant difference was observed in the slopes of the dI / dV curves; however, the conductance peaks shifted toward a higher absolute voltage with the increase in the temperature. Although the origin of the nonlinear I–V characteristic, which was significantly different from that of single-walled CNTs [15], has not been clarified thus far, it can be interpreted to originate due to a change in the current distribution along the cross section of the MWCNT networks; the outermost shell mainly contributes to the conduction at lower currents, while the inner shells gradually begin to contribute with the increase in current such that the total conductance increases with the current [16]. Fig. 6(a) and (b) show plots of the normalized current of the MWCNT networks versus 1000 / T and the activation energy Ea of the MWCNT networks versus the Raman ID / IG ratio, respectively. The normalized current decreased monotonically with the temperature (200 to 390 K); it then remained almost constant in the temperature range below 200 K. We fitted these data to the Arrhenius-type equation f(T) = f0exp(− Ea / kBT) to calculate Ea, where kB is Boltzmann's constant. In the temperature range of 200 to 390 K, the temperature character-
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istic of I–V shows that the oxidative-purified MWCNT networks transported electrons with a low activation energy of 21 meV. The Ea of 21 meV was found to be smaller than the bandgap (∼80 meV) of the pure undoped MWNT, reported by Carroll et al [17]. Further, the electron transport of as-grown MWCNT networks was activated with a lower activation energy than that of the oxidative-purified MWCNTs in the temperature range of 390 to 200 K. The Ea of the MWCNT networks initially decreased with the Raman ID / IG ratio and it then remained almost constant at around 5 meV with a further increase in the Raman ID / IG ratio (N 0.7), as shown in Fig. 6(b). This result indicates that the oxidative purification process (obtained Raman ID / IG ratio of 0.7) requires more than 50 min to remove most defects or amorphous carbon from the surface of MWCNTs. In the case of the oxidative-purified MWCNT networks, the carrier transport could be interpreted as the thermally activated interlayer hopping transport of electrons from the inner to the outer nanotube walls with an activation energy of 21 meV [18]. However, in the case of the MWCNT networks composed of defective MWCNTs, the carrier transport can be interpreted as the thermally activated hopping transport of electrons through defects, possibly on the outermost shell of MWCNTs, with a very low activation energy of 5 meV. Our result indicates that the defect states for carrier transport by the hopping mechanism decrease with the increase in the oxidative purification time. Therefore, it is difficult to achieve the conduction of carriers in oxidative-purified MWCNT networks; an additional bias is required to initiate conduction.
Fig. 6. (a) A plot of the normalized current of the MWCNT networks versus 1000 / T. (b) A plot of the normalized current versus the Raman ID / IG ratio.
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Y. Hayashi et al. / Diamond & Related Materials 15 (2006) 1138–1142
4. Summary and conclusion
References
In conclusion, the transport properties of MWCNT networks (not isolated MWCNTs) have been measured using the twopoint configuration in the temperature range of 200 to 390 K. The Raman ID / IG and FWHM of MWCNTs decreased with the increase in the oxidative purification time. The conductance increased linearly with the bias until it peaked, following which it decreased with a further increase in the bias. The electric conductance at around zero bias voltage was 0.65 G0 and it increased with the temperature. The conductance of 0.65 G0 at room temperature, less than the value of 1 G0 for metallic MWCNTs, indicates that the MWCNT networks are semiconducting. The electric conductance measurements with respect to the temperature for as-grown MWCNTs reveal that electron transport was activated with a lower activation energy than that for oxidative-purified MWCNTs in the temperature range of 390 to 200 K. The activation energy increased with the oxidative purification time due to the improved crystallinity of the graphite sheets. The elimination of the defect states on the MWCNTs by oxidative purification may become an important process for the production of nanotube-embedded devices.
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Acknowledgments This work was partly supported by the “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, carried out at the Research Laboratory of High Voltage Electron Microscopy at Kyushu University. This work was also partly supported by a Grant-in Aid for Scientific Research (Houga-16651065) from MEXT.