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Effect of nanowire catalyst for carbon nanotubes growth by ICP-CVD
Diamond & Related Materials 14 (2005) 841 – 845 www.elsevier.com/locate/diamond
Effect of nanowire catalyst for carbon nanotubes growth by ICP-CVD J.H. Yena, I.C. Leub,*, M.T. Wua, C.C. Lina, M.H. Hona a
Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan 70101, Taiwan, Republic of China b Department of Electronic Engineering, Kun Shan University of Technology, Tainan Hsien 710, Taiwan, Republic of China Available online 8 December 2004
Abstract Aligned multi-walled carbon nanotubes (CNTs) fully filled with Fe, Co, and Ni were synthesized by inductively coupled plasma chemical vapor deposition (ICP-CVD) using nanowires as catalysts. The nanowires were prepared by electroplating using anodized alumina oxide (AAO) as a template. By properly selecting the length of nanowire (420F20 nm), CNTs fully filled with metal catalyst can be directly obtained. The effect of nanowires (Fe, Co and Ni) on growth of aligned CNTs is also systematically studied. It is found that the catalyst has a strong effect on the nanotube alignment and crystallinity. The CNTs catalyzed by Ni and Fe have better alignment and higher degree of graphitization, whereas those from Co show a little curled character in the tips and somewhat tangle between CNTs. In the field emission test, the CNTs filled with Ni, Fe and Co show low turn-on field about 1, 1.1 and 1.5 V/Am, respectively. Besides low turn-on field, the CNTs synthesized by this method also have a high emission current density (larger than 10 mA/cm2). D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes (CNTs); Catalyst; Nanowires; Field emission
1. Introduction Since the first discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been investigated by many researchers due to its high potential for the use in cold cathode flat panel displays, electron gun, and nanodevices [2,3]. There are many methods used to synthesize CNTs such as arc discharge [4], laser ablation [5], chemical vapor deposition (CVD) [6] and plasma enhanced chemical vapor deposition (PECVD) [7]. Among the methods mentioned above, PECVD is an effective way to synthesize well-aligned CNTs at a relatively low temperature with high purity and high yield. The possibility of producing metal- or semiconductorfilled CNTs is not of less technological interest than the tubes themselves. In the past years the electroless plating method has been mainly used to acquire metal–carbon nanotube composite [8]. But this method often contains some impurities such as Ni–P or Co–P. Instead of using electroless plating, some researchers used pyrolysis [9] or electrochemical deposition method to fill metals into the tube [10]. However, * Corresponding author. Tel./fax: +886 6 2380208. E-mail address: [email protected] (I.C. Leu). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.10.012
the effective way to directly obtain metal-filled CNTs during the CNTs growth has not been extensively explored. In this study, we present an alternative method to fabricate the metal-filled CNTs by inductively coupled plasma chemical vapor deposition (ICP-CVD). The Fe, Co, and Ni nanowires used as catalysts were prepared by electroplating using anodized alumina oxide (AAO) as template. The morphology, internal structure and crystallinity of both CNTs and metal nanowires were investigated using scanning electron microscopy and transmission electron microscopy. Raman spectroscopy was also employed to study the crystallinity of the CNTs. Furthermore, the field emission properties of metal-filled CNTs were also investigated using a diode configuration.
2. Experimental The CNTs were synthesized by using ICP-CVD. The anodic alumina oxide nanotemplate used for nanowire growth was fabricated by anodizing the Al film deposited on Si wafer through a multi-layer scheme (Al/Au/Ti/Si), where the Al top layer, Au conducting layer and Ti adhesive
842
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
Table 1 Bath compositions for the deposition of metal nanowires Metal
Electrolyte
Fe
FeSO4!7H2O 30 g/l H3BO3 6 g/l CoSO4!7H2O 100 g/l H3BO3 20 g/l NiSO4!6H2O 330 g/l NiCl2!6H2O 45 g/l H3BO3 35 g/l
Co Ni
Electroplating voltage (V)
Temp. (8C)
pH
1.2
25
3.5
1.0
25
4
1.0
25
3.8
layer were successively deposited on the silicon wafer by ebeam evaporation. The layer thicknesses of Al, Au and Ti were 1200, 10 and, 5 nm, respectively. The anodization was first conducted in a bath of 0.3 M oxalic acid solution with cooling circulation at a constant voltage of 40 V and at 13 8C. After anodization through the entire Al film thickness, the specimen was then immersed into a 5 wt.% H3PO4 solution to remove the barrier layer at the oxide/Au interface. The Fe, Co and Ni nanowires were electrochemically deposited from the pore bottom of the nanochannels of AAO template by applying a constant DC voltage using H3BO3-stabilized FeSO4d 6H2O, CoSO4d 6H2O, NiSO4d 6H2O and NiCl2d 6H2O solutions. The detailed electroplating condition was shown in Table 1. After obtaining metal nanowries, the AAO template was removed using 5 wt.% NaOH solution. After obtaining the nanowires on the Si substrate, the sample was transferred into the ICP-CVD growth chamber. The carrier and reaction gas were H2 and CH4, respectively. The growth parameters in this study were as follows: growth temperature 660 8C, time 40 min, rf power 250 W, DC bias 400 V and pressure 3 Torr [11]. The morphology and structure of CNTs were observed by using FE-SEM (HITACHI S4200) and high-resolution transmission electron microscope (HR-TEM, Hitachi HF2000), respectively. A Raman spectrometer (Jobin Yvon LabRam HR) was also used to identify the crystallinity of CNTs. The field emission (FE) test was performed in a high vacuum chamber with a base pressure 510 6 Torr. The FE test was conducted using a diode configuration with CNTs as cathode and Cu disk as anode, in which the gap between cathode and anode was 60 Am. The emission current was measured with a Keithley 237 electrometer capable of sourcing up to 1100 V and 10 mA.
difficult to obtain CNTs with metal fully filled into their hollow interiors. Considering the continuous growth of CNTs, the amount of metals may play an important role in determining the geometrical distribution of metals within the channel of CNTs. We therefore used metal nanowires as catalyst for CNTs growth instead of using metal nanoparticles. The motivation of using nanowiress as catalyst is to provide enough amount of metal during CNTs growth. The preparation of free-standing metal nanowires on the substrate was conducted by electroplating using AAO as a template. The advantages of using electroplating with AAO to obtain metal nanowires are the capability to control the diameter and the length of nanowires precisely. Fig. 1 shows the SEM images of Fe, Co and Ni nanowire arrays on a Si wafer after removing the AAO nanotemplate by using 5 wt.% NaOH solution. The SEM shows that the Fe, Co and Ni nanowires stand perpendicularly to the substrate with high uniformity. The density of the metal nanowires is about 1.21010/cm2. The diameter and the length of Fe, Co, and Ni nanowires are about 60 and 420F20 nm, respectively. The length of metal
3. Results and discussion The growth of CNTs by CVD or PECVD often involves three main steps [12]: (i) decomposition of hydrocarbon gas at the surface of catalyst nanoparticles (ii) diffusion of the resultant carbon atom in the nanoparticles to form the nucleation seed (iii) precipitation of carbon atoms at the nanoparticles interface to form nanotubes. The growth by using nanoparticles as catalyst often associated with the catalyst particles at the top or bottom of CNTs, but it is
Fig. 1. SEM images of (a) Fe, (b) Co, and (c) Ni nanowires on Si substrate after removing the AAO template.
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
843
of nanowires was controlled by deposition time. The deposition rates of Fe, Co, and Ni are 6.7, 10.3 and 8.4 nm/s, respectively, for the conditions mentioned previously. According to the results from preliminary studies, the range for the length of the nanowires chosen in this study has two boundaries. First, the length should be short enough for the nanowires to stand on the substrate without tangle. Second, it must long enough to provide enough metal to fill into the CNTs fully. The growth results are shown in Figs. 2 and 3. Fig. 2 shows the CNTs grown by ICP-CVD using different nanowires as catalyst. The straight and aligned CNTs arrays were grown using Fe, Ni and Co nanowires as catalyst, but those from Co nanowires showed a little curled character in the tips and somewhat tangle between CNTs. According to the catalytic growth mechanism [13], the deposition of curled, twist or helical CNTs may be the result of a variation of catalytic activity or carbon segregation on the active sites at the catalyst periphery during nanotube growth. Therefore, it could be deduced from the result that the Co exhibits non-uniform catalytic activity and carbon
Fig. 3. TEM and HRTEM images of (a) Fe, (b) Co, and (c) Ni nanowirefilled CNTs.
Fig. 2. SEM images of CNTs grown at 660 8C for 40 min by ICP-CVD using (a) Fe, (b) Co, and (c) Ni nanowires as catalysts.
segregation behavior. A similar study was reported by Huang et al. [14] using different particulate catalysts for CNTs growth by plasma-enhanced hot-filament chemical vapor deposition. Their results showed that the aligned CNTs were only obtained by using Ni nanoparticles as catalyst. The factors affecting the aligned CNTs growth were suggested to be due to varied catalyst activity and nanoparticles diameter. The worse case of alignment of CNTs in their study was that grown on Co catalyst, which has the smallest diameter. However, the diameter of nanowire catalyst in this study is about the same, which means that the difference in catalyst
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J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
diameter is not necessarily the underlying factor in determining the alignment behavior of CNTs. In spite of the similar morphology of CNTs formed from Fe, Co, and Ni nanowires, there are different consequences of the catalysts on the internal structures of the CNTs, such as the degree of graphitization and the level of defects. Fig. 3 shows the interior structures of CNTs synthesized by using different nanowires as catalyst. From the images, all the CNTs have clean surfaces without amorphous carbon. The catalysts (Fe, Co and Ni) are filled into CNTs fully by this method and the lengths of nanowires are almost the same as that of CNTs. The inserted HRTEM images elucidate the effects of catalyst on the crystallinity of CNTs. The CNTs exhibit a multiwalled structure for all tubes grown form the three different catalysts. The graphitic layers of the CNTs grown using the Fe nanowires reveal a highly ordered crystalline structure, with the graphitic sheets separated by ~0.33 nm. For CNTs grown using Co or Ni nanowires, nanotubes showing less crystallinity than that grown using Fe nanowire are obtained. The CNTs grown on Co nanowires also shows the presence of wavy graphitic sheets over a short distance, and thus are of more defective structure compared with those from Fe or Ni nanowires. To obtain the information about the crystallinity of the entire CNTs, Raman spectroscopy was used. Fig. 4 shows Raman spectra for the CNTs growth using Fe, Ni, and Co nanowires as catalysts. All the spectra show the major two Raman bands at ~1341 cm 1 (D band) and ~1571 cm 1 (G band). The G band indicates original graphite features but the D band has been explained as the contribution from disordered features of graphitic sheets [15,16]. However, for the CNTs grown on Co nanowire, the D band becomes stronger and broader. It was noted that the intensity ratio of the D band to the G band (I D/I G) has a linear relation with the inverse of the of the in-plane crystallite dimension [15]. The values of (I D/I G) for the CNTs grown using Fe, Ni, Co
Fig. 5. I–V characteristics of CNTs filled with different catalysts.
nanowires as catalyst are 0.87, 0.91 and 1.01, respectively. It reveals that the degree of long-range ordered crystalline perfection of the CNTs grown using Fe or Ni nanowires is higher than that of CNTs grown using Co nanowires, which is consistent with the HRTEM images. Recently, Lee et al. [17] also reported that the CNTs grown on Fe and Ni catalyst particles using CVD exhibited a higher degree of crystalline perfection than that of CNTs grown on Co catalyst. From the above discussion, we find that the crystallinity of CNTs is also dominated by the activity of metal nanowires. Fig. 5 illustrates the electron emission current density vs. electric field (I–V) curves of nanowire-filled CNTs array. In general, the turn-on field and threshold field of the emitter are defined as the external field needed to extract a current density of 10 AA/cm2 and 10 mA/cm2, respectively. For Ni-, Fe- and Co-filled CNTs, the turn-fields are approximately 1, 1.1 and 1.5 V/Am, respectively. The result shows that CNTs synthesized by this method have a low turn-on field. The threshold fields of CNTs filled with Ni, Fe, and Co nanowires are about 7.5, 9.16 and 10.83 V/Am, respectively. This result shows that the CNTs filled with metal also have a high current density. There are many factors reported to affect the field emission properties of CNTs such as emitter size and crystallinity [18]. From the above discussion, the main difference of CNTs synthesized using Fe, Co, Ni nanowires as catalyst is its internal structure. The CNTs grown using Co nanowires have an inferior crystallinity, which results in the highest turn-on field and threshold field. Increasing the crystallinity of CNTs, as those synthesized by Ni and Fe nanowires, the turn-on field and threshold field are reduced significantly.
4. Conclusion
Fig. 4. Raman spectra for the CNTs grown from Fe, Co, and Ni nanowires.
In conclusion, multi-walled carbon nanotubes filled with metal nanowires were successfully fabricated by using ICP-
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
CVD. This study reveals that the nanowire catalyst strongly affects not only the alignment but also the microstructure of CNTs. Nanotubes grown from Fe and Ni nanowires have the best alignment. Their walls also exhibit to be composed of reasonably good graphitized layers. Besides the size effect reported in the literature, the degree of graphitization of nanotube emitters also has an impact on the field emission property. Carbon nanotubes grown from Ni and Fe nanowires demonstrate better field emission properties than those from Co nanowires.
Acknowledgements The authors acknowledge the financial support from the National Science Council of Taiwan, Republic of China under contract No. NSC 92-2120-M-006-003.
References [1] S. Iijima, Nature (London) 345 (1991) 56. [2] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512.
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[3] Y. Chen, D.T. Shaw, L. Guo, Appl. Phys. Lett. 76 (2000) 2469. [4] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M.L. Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fisher, Nature (London) 388 (1997) 756. [5] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49. [6] Z.P. Huang, J.W. Xu, Z.F. Ren, J.H. Wang, M.P. Siegal, P.N. Provencio, Appl. Phys. Lett. 73 (1998) 3845. [7] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [8] J. Li, M. Moskovits, T. Haslett, Chem. Mater. 10 (1998) 1963. [9] P.C.P. Watts, W.K. Hsu, Appl. Phys., A 78 (2004) 79. [10] Q. Xu, L. Zhang, J. Zhu, J. Phys. Chem., B 107 (2003) 8294. [11] J.H. Yen, I.C. Leu, C.C. Lin, M.H. Hon, Appl. Phys., A 80 (2005) 415. [12] R.T. Baker, Carbon 27 (1989) 315. [13] S. Amelinckx, X.B. Zhang, D. Bernaerts, X.F. Zhang, V. Ivanov, J.B. Nagy, Science 265 (1994) 635. [14] Z.P. Huang, D.Z. Wang, J.G. Wen, M. Sennett, H. Gibson, Z.F. Ren, Appl. Phys., A 74 (2002) 387. [15] F. Tuinstra, J.L. Koening, J. Chem. Phys. 53 (1970) 1126. [16] D.S. McCulloch, S. Prawer, A. Hoffman, Phys. Rev., B 50 (1994) 5905. [17] C.J. Lee, J. Park, J.A. Yu, Chem. Phys. Lett. 360 (2002) 250. [18] L. Dong, J. Jiao, C. Pan, D.W. Tuggle, Appl. Phys., A 78 (2004) 9.
Effect of nanowire catalyst for carbon nanotubes growth by ICP-CVD J.H. Yena, I.C. Leub,*, M.T. Wua, C.C. Lina, M.H. Hona a
Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan 70101, Taiwan, Republic of China b Department of Electronic Engineering, Kun Shan University of Technology, Tainan Hsien 710, Taiwan, Republic of China Available online 8 December 2004
Abstract Aligned multi-walled carbon nanotubes (CNTs) fully filled with Fe, Co, and Ni were synthesized by inductively coupled plasma chemical vapor deposition (ICP-CVD) using nanowires as catalysts. The nanowires were prepared by electroplating using anodized alumina oxide (AAO) as a template. By properly selecting the length of nanowire (420F20 nm), CNTs fully filled with metal catalyst can be directly obtained. The effect of nanowires (Fe, Co and Ni) on growth of aligned CNTs is also systematically studied. It is found that the catalyst has a strong effect on the nanotube alignment and crystallinity. The CNTs catalyzed by Ni and Fe have better alignment and higher degree of graphitization, whereas those from Co show a little curled character in the tips and somewhat tangle between CNTs. In the field emission test, the CNTs filled with Ni, Fe and Co show low turn-on field about 1, 1.1 and 1.5 V/Am, respectively. Besides low turn-on field, the CNTs synthesized by this method also have a high emission current density (larger than 10 mA/cm2). D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes (CNTs); Catalyst; Nanowires; Field emission
1. Introduction Since the first discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been investigated by many researchers due to its high potential for the use in cold cathode flat panel displays, electron gun, and nanodevices [2,3]. There are many methods used to synthesize CNTs such as arc discharge [4], laser ablation [5], chemical vapor deposition (CVD) [6] and plasma enhanced chemical vapor deposition (PECVD) [7]. Among the methods mentioned above, PECVD is an effective way to synthesize well-aligned CNTs at a relatively low temperature with high purity and high yield. The possibility of producing metal- or semiconductorfilled CNTs is not of less technological interest than the tubes themselves. In the past years the electroless plating method has been mainly used to acquire metal–carbon nanotube composite [8]. But this method often contains some impurities such as Ni–P or Co–P. Instead of using electroless plating, some researchers used pyrolysis [9] or electrochemical deposition method to fill metals into the tube [10]. However, * Corresponding author. Tel./fax: +886 6 2380208. E-mail address: [email protected] (I.C. Leu). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.10.012
the effective way to directly obtain metal-filled CNTs during the CNTs growth has not been extensively explored. In this study, we present an alternative method to fabricate the metal-filled CNTs by inductively coupled plasma chemical vapor deposition (ICP-CVD). The Fe, Co, and Ni nanowires used as catalysts were prepared by electroplating using anodized alumina oxide (AAO) as template. The morphology, internal structure and crystallinity of both CNTs and metal nanowires were investigated using scanning electron microscopy and transmission electron microscopy. Raman spectroscopy was also employed to study the crystallinity of the CNTs. Furthermore, the field emission properties of metal-filled CNTs were also investigated using a diode configuration.
2. Experimental The CNTs were synthesized by using ICP-CVD. The anodic alumina oxide nanotemplate used for nanowire growth was fabricated by anodizing the Al film deposited on Si wafer through a multi-layer scheme (Al/Au/Ti/Si), where the Al top layer, Au conducting layer and Ti adhesive
842
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
Table 1 Bath compositions for the deposition of metal nanowires Metal
Electrolyte
Fe
FeSO4!7H2O 30 g/l H3BO3 6 g/l CoSO4!7H2O 100 g/l H3BO3 20 g/l NiSO4!6H2O 330 g/l NiCl2!6H2O 45 g/l H3BO3 35 g/l
Co Ni
Electroplating voltage (V)
Temp. (8C)
pH
1.2
25
3.5
1.0
25
4
1.0
25
3.8
layer were successively deposited on the silicon wafer by ebeam evaporation. The layer thicknesses of Al, Au and Ti were 1200, 10 and, 5 nm, respectively. The anodization was first conducted in a bath of 0.3 M oxalic acid solution with cooling circulation at a constant voltage of 40 V and at 13 8C. After anodization through the entire Al film thickness, the specimen was then immersed into a 5 wt.% H3PO4 solution to remove the barrier layer at the oxide/Au interface. The Fe, Co and Ni nanowires were electrochemically deposited from the pore bottom of the nanochannels of AAO template by applying a constant DC voltage using H3BO3-stabilized FeSO4d 6H2O, CoSO4d 6H2O, NiSO4d 6H2O and NiCl2d 6H2O solutions. The detailed electroplating condition was shown in Table 1. After obtaining metal nanowries, the AAO template was removed using 5 wt.% NaOH solution. After obtaining the nanowires on the Si substrate, the sample was transferred into the ICP-CVD growth chamber. The carrier and reaction gas were H2 and CH4, respectively. The growth parameters in this study were as follows: growth temperature 660 8C, time 40 min, rf power 250 W, DC bias 400 V and pressure 3 Torr [11]. The morphology and structure of CNTs were observed by using FE-SEM (HITACHI S4200) and high-resolution transmission electron microscope (HR-TEM, Hitachi HF2000), respectively. A Raman spectrometer (Jobin Yvon LabRam HR) was also used to identify the crystallinity of CNTs. The field emission (FE) test was performed in a high vacuum chamber with a base pressure 510 6 Torr. The FE test was conducted using a diode configuration with CNTs as cathode and Cu disk as anode, in which the gap between cathode and anode was 60 Am. The emission current was measured with a Keithley 237 electrometer capable of sourcing up to 1100 V and 10 mA.
difficult to obtain CNTs with metal fully filled into their hollow interiors. Considering the continuous growth of CNTs, the amount of metals may play an important role in determining the geometrical distribution of metals within the channel of CNTs. We therefore used metal nanowires as catalyst for CNTs growth instead of using metal nanoparticles. The motivation of using nanowiress as catalyst is to provide enough amount of metal during CNTs growth. The preparation of free-standing metal nanowires on the substrate was conducted by electroplating using AAO as a template. The advantages of using electroplating with AAO to obtain metal nanowires are the capability to control the diameter and the length of nanowires precisely. Fig. 1 shows the SEM images of Fe, Co and Ni nanowire arrays on a Si wafer after removing the AAO nanotemplate by using 5 wt.% NaOH solution. The SEM shows that the Fe, Co and Ni nanowires stand perpendicularly to the substrate with high uniformity. The density of the metal nanowires is about 1.21010/cm2. The diameter and the length of Fe, Co, and Ni nanowires are about 60 and 420F20 nm, respectively. The length of metal
3. Results and discussion The growth of CNTs by CVD or PECVD often involves three main steps [12]: (i) decomposition of hydrocarbon gas at the surface of catalyst nanoparticles (ii) diffusion of the resultant carbon atom in the nanoparticles to form the nucleation seed (iii) precipitation of carbon atoms at the nanoparticles interface to form nanotubes. The growth by using nanoparticles as catalyst often associated with the catalyst particles at the top or bottom of CNTs, but it is
Fig. 1. SEM images of (a) Fe, (b) Co, and (c) Ni nanowires on Si substrate after removing the AAO template.
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
843
of nanowires was controlled by deposition time. The deposition rates of Fe, Co, and Ni are 6.7, 10.3 and 8.4 nm/s, respectively, for the conditions mentioned previously. According to the results from preliminary studies, the range for the length of the nanowires chosen in this study has two boundaries. First, the length should be short enough for the nanowires to stand on the substrate without tangle. Second, it must long enough to provide enough metal to fill into the CNTs fully. The growth results are shown in Figs. 2 and 3. Fig. 2 shows the CNTs grown by ICP-CVD using different nanowires as catalyst. The straight and aligned CNTs arrays were grown using Fe, Ni and Co nanowires as catalyst, but those from Co nanowires showed a little curled character in the tips and somewhat tangle between CNTs. According to the catalytic growth mechanism [13], the deposition of curled, twist or helical CNTs may be the result of a variation of catalytic activity or carbon segregation on the active sites at the catalyst periphery during nanotube growth. Therefore, it could be deduced from the result that the Co exhibits non-uniform catalytic activity and carbon
Fig. 3. TEM and HRTEM images of (a) Fe, (b) Co, and (c) Ni nanowirefilled CNTs.
Fig. 2. SEM images of CNTs grown at 660 8C for 40 min by ICP-CVD using (a) Fe, (b) Co, and (c) Ni nanowires as catalysts.
segregation behavior. A similar study was reported by Huang et al. [14] using different particulate catalysts for CNTs growth by plasma-enhanced hot-filament chemical vapor deposition. Their results showed that the aligned CNTs were only obtained by using Ni nanoparticles as catalyst. The factors affecting the aligned CNTs growth were suggested to be due to varied catalyst activity and nanoparticles diameter. The worse case of alignment of CNTs in their study was that grown on Co catalyst, which has the smallest diameter. However, the diameter of nanowire catalyst in this study is about the same, which means that the difference in catalyst
844
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diameter is not necessarily the underlying factor in determining the alignment behavior of CNTs. In spite of the similar morphology of CNTs formed from Fe, Co, and Ni nanowires, there are different consequences of the catalysts on the internal structures of the CNTs, such as the degree of graphitization and the level of defects. Fig. 3 shows the interior structures of CNTs synthesized by using different nanowires as catalyst. From the images, all the CNTs have clean surfaces without amorphous carbon. The catalysts (Fe, Co and Ni) are filled into CNTs fully by this method and the lengths of nanowires are almost the same as that of CNTs. The inserted HRTEM images elucidate the effects of catalyst on the crystallinity of CNTs. The CNTs exhibit a multiwalled structure for all tubes grown form the three different catalysts. The graphitic layers of the CNTs grown using the Fe nanowires reveal a highly ordered crystalline structure, with the graphitic sheets separated by ~0.33 nm. For CNTs grown using Co or Ni nanowires, nanotubes showing less crystallinity than that grown using Fe nanowire are obtained. The CNTs grown on Co nanowires also shows the presence of wavy graphitic sheets over a short distance, and thus are of more defective structure compared with those from Fe or Ni nanowires. To obtain the information about the crystallinity of the entire CNTs, Raman spectroscopy was used. Fig. 4 shows Raman spectra for the CNTs growth using Fe, Ni, and Co nanowires as catalysts. All the spectra show the major two Raman bands at ~1341 cm 1 (D band) and ~1571 cm 1 (G band). The G band indicates original graphite features but the D band has been explained as the contribution from disordered features of graphitic sheets [15,16]. However, for the CNTs grown on Co nanowire, the D band becomes stronger and broader. It was noted that the intensity ratio of the D band to the G band (I D/I G) has a linear relation with the inverse of the of the in-plane crystallite dimension [15]. The values of (I D/I G) for the CNTs grown using Fe, Ni, Co
Fig. 5. I–V characteristics of CNTs filled with different catalysts.
nanowires as catalyst are 0.87, 0.91 and 1.01, respectively. It reveals that the degree of long-range ordered crystalline perfection of the CNTs grown using Fe or Ni nanowires is higher than that of CNTs grown using Co nanowires, which is consistent with the HRTEM images. Recently, Lee et al. [17] also reported that the CNTs grown on Fe and Ni catalyst particles using CVD exhibited a higher degree of crystalline perfection than that of CNTs grown on Co catalyst. From the above discussion, we find that the crystallinity of CNTs is also dominated by the activity of metal nanowires. Fig. 5 illustrates the electron emission current density vs. electric field (I–V) curves of nanowire-filled CNTs array. In general, the turn-on field and threshold field of the emitter are defined as the external field needed to extract a current density of 10 AA/cm2 and 10 mA/cm2, respectively. For Ni-, Fe- and Co-filled CNTs, the turn-fields are approximately 1, 1.1 and 1.5 V/Am, respectively. The result shows that CNTs synthesized by this method have a low turn-on field. The threshold fields of CNTs filled with Ni, Fe, and Co nanowires are about 7.5, 9.16 and 10.83 V/Am, respectively. This result shows that the CNTs filled with metal also have a high current density. There are many factors reported to affect the field emission properties of CNTs such as emitter size and crystallinity [18]. From the above discussion, the main difference of CNTs synthesized using Fe, Co, Ni nanowires as catalyst is its internal structure. The CNTs grown using Co nanowires have an inferior crystallinity, which results in the highest turn-on field and threshold field. Increasing the crystallinity of CNTs, as those synthesized by Ni and Fe nanowires, the turn-on field and threshold field are reduced significantly.
4. Conclusion
Fig. 4. Raman spectra for the CNTs grown from Fe, Co, and Ni nanowires.
In conclusion, multi-walled carbon nanotubes filled with metal nanowires were successfully fabricated by using ICP-
J.H. Yen et al. / Diamond & Related Materials 14 (2005) 841–845
CVD. This study reveals that the nanowire catalyst strongly affects not only the alignment but also the microstructure of CNTs. Nanotubes grown from Fe and Ni nanowires have the best alignment. Their walls also exhibit to be composed of reasonably good graphitized layers. Besides the size effect reported in the literature, the degree of graphitization of nanotube emitters also has an impact on the field emission property. Carbon nanotubes grown from Ni and Fe nanowires demonstrate better field emission properties than those from Co nanowires.
Acknowledgements The authors acknowledge the financial support from the National Science Council of Taiwan, Republic of China under contract No. NSC 92-2120-M-006-003.
References [1] S. Iijima, Nature (London) 345 (1991) 56. [2] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512.
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