- Email: [email protected]
Shaping carbon nanotube bundles during growth using a magnetic field
CARBON
4 6 ( 2 0 0 8 ) 5 4 4 –5 6 1
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Letters to the Editor
Shaping carbon nanotube bundles during growth using a magnetic field Nobuo Ohmae* Department of Mechanical Engineering, Graduate School of Engineering, Kobe University, Rokkodai 1-1, Nada, Kobe 657-8501, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2007
2007 Elsevier Ltd. All rights reserved.
Accepted 6 December 2007 Available online 25 January 2008
Carbon nanotubes (CNT) usually grow straight in the vertical direction [1–3], except carbon nanocoil [4–6] and spaghettilike CNT [7]. But, if a curved structure were to be obtained by applying a suitable stimulus, the application of CNT to MEMS/NEMS or to bio-nano can greatly be widened. The original ideas of synthesizing a complex structure of CNT arose firstly from our experimental results by high resolution transmission electron microscopy which showed that the CNT grown by the plasma-enhanced chemical vapor deposition (PE-CVD) very frequently possesses the catalyst at its top [8]. The second favored condition is that the catalyst is iron with our experimental system. Therefore, magnetic field appears effective to control the growth direction to create a complex structure of CNT. Attempts have been made to synthesize an intricately curved CNT structure using a magnetic field. The external magnetic field was applied to the substrate outside the PE-CVD apparatus [8]. Fig. 1 shows the designed yoke-shaped permanent magnet made of Nd–Fe–B, whose magnetic flux density is 500 mT. Because the external magnetic field is applied from the outside of furnace of PE-CVD, the demagnetization due to high temperature (usually around 700 C) does not become a problem. The magnetic field at the center of the quartz tube is calculated at 10 mT.1 The substrate was held in the vertical direction, i.e., perpendicular to the axis of quartz tube, while the direction of magnetic
field was opposite to that of gravity. The deposition of CNT was carried out using H2 and CH4 as the reaction gases with the flow rates of 100 sccm and 10 sccm, respectively. The temperature of the quartz tube was maintained at 700 C with the gas pressure of 2.7 · 103 Pa. The growth rate of CNT was about 50 nm/s. Before depositing CNT, Fe thin film typically with the thickness of 5 nm was vacuum-deposited on the Si(0 0 1) substrate, the surface of which had been oxidized at 800 C in O2 atmosphere. Further, with using H2 at a pressure of 1.0 · 102 Pa, Fe films were transformed into nanoparticles by treating in vacuum at 700 C for 30 min. Simply because the morphological structure is easy to recognize with a patterned CNT bundle, the Si mask which has the 5 lm · 5 lm squares at each neighboring distance of 2 lm was used. Hook-shaped CNT is shown in Fig. 2. In this case, the magnetic field was introduced after 3 min from the onset of deposition. The direction of the line of magnetic force was opposite to the initial growth direction of CNT. Obviously, CNT re-grows in the direction of the line of magnetic force. By changing the direction of the line of magnetic force during PE-CVD processes, the arch-shaped structure of CNT resulted. Typical SEM photograph is shown in Fig. 3. The magnetic field was first applied after 1 min from the start of
* Fax: +81 78 803 6111. E-mail address: [email protected]. 1 Neomax Co., Ltd. product specification. 0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.12.005
CARBON
4 6 (2 0 0 8) 5 4 4–56 1
545
Fig. 1 – Yoke-shaped Nd–Fe–B magnet designed for applying magnetic field outside the furnace. The direction of magnetic field is illustrated.
Fig. 4 – Ladder-shaped CNT bundles.
Fig. 2 – Hook-shaped CNT bundles.
Fig. 3 – Arch-shaped CNT bundles.
deposition, and the second after 3 min. The direction of the line of magnetic force of the latter differs from the former
at 90. This dramatic picture supports the aim of this study that the shape of CNT is controllable by the external magnetic field. The ladder-shaped structure is indicated in Fig. 4, by repeatedly changing the direction of the line of magnetic force. Due to the strong effect of self-bias, in this case, the ladder elongates to the vertical direction. The most probable reason why the complex configurations of CNT presented in Figs. 2–4 can be attributed to the force acting on Fe nanoparticles. In our experimental conditions, Fe nanoparticles exist in the shape of ellipse (or sometimes will-0-wisp) whose long axis is along the growth direction of CNT. The magnetic force pulls the Fe particles in one direction or the other, and this force changes the growth direction of the CNTs. This is why the growth direction changes when a magnetic field is applied. The TEM photograph in Fig. 5 shows the evidence of curved CNT, where the bulge of CNT wall (indicated by a circle) is visible. It also was possible to construct the curved CNT structure by simply placing a Sm–Co magnet inside the quartz tube. However, one of the weak points of this method is that the magnet is not movable inside the quartz tube, so that CNT with complex structure was difficult to synthesize. Bower et al. explained the reason for the vertical growth of CNTs in PE-CVD is due to the self-bias, the electric potential
546
CARBON
4 6 ( 2 0 0 8 ) 5 4 4 –5 6 1
ticles experience a force in the field, and tend to move in the direction, where the magnetic field lines get stronger.
Acknowledgements This work was supported in part by the Grant-in-Aid for Exploratory Research from the Ministry of Education, Culture, Sports, Science and Technology under 16656057. I would like to thank Neomax Co. Ltd. and Renesas Technology Corp. for providing us with the permanent magnets and the mask for CNT deposition. Thanks are due to Taketoshi Akimoto, Keiichi Takigawa, Takenori Wada and Naohiro Matsumoto for their experiments on the manufacturing of CNT.
R E F E R E N C E S
Fig. 5 – TEM photograph showing the change in growth direction of CNT when the direction of magnetic force was changed.
between plasma and substrate [9]. They estimated in their work that the self-bias was 10 V. In the present study, the plasma potential was measured by a Langmuir probe during PE-CVD. The Langmuir probe revealed that the floating potential was 14 eV and that the plasma potential was 24 eV. Therefore, the self-bias is 10 eV, and is consistent with the results by Bower et al. Because of the high electric field created by this self-bias (104–105 V/m) and the lack in providing much higher electric fields between two parallel plates, external electric field was found to be less effective for synthesizing the curved structure of CNTs in the present experimental conditions. In summary, the CNT structures with complex shape were successfully obtained by the external magnetic field. The primary reason for this was that the ferromagnetic Fe nanopar-
[1] Ren ZF, Huang ZP, Wang DZ, Wen JG. Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot. Appl Phys Lett 1999;75:1086–8. [2] Chena M, Chenb CM, Kooa HS, Chen CF. Catalyzed growth model of carbon nanotubes by microwave plasma chemical vapor deposition using CH4 and CO2 gas mixtures. Diam Relat Mater 2003;12:1829–35. [3] Zhu L, Xu J, Xiu Y, Sun Y, Hess DW, Wong CP. Growth and electrical characterization of high-aspect-ratio carbon nanotube arrays. Carbon 2006;44:253–8. [4] Bai JB. Growth of nanotube/nanofibre coils by CVD on an alumina substrate. Mater Lett 2003;57:2629–33. [5] Zhong DY, Liu S, Wang EG. Patterned growth of coiled carbon nanotubes by a template-assisted technique. Appl Phys Lett 2003;83:4423–5. [6] Ihara S, Itoh S, Kitakami J. Helically coiled cage forms of graphitic carbon. Phys Rev B 1993;48:5643–7. [7] Juang ZY, Lai JF, Weng CH, Lee JH, Lai HJ, Lai TS, et al. On the kinetics of carbon nanotube growth by thermal CVD method. Diam Relat Mater 2004;13:2140–6. [8] Kinoshita H, Kume I, Sakai H, Tagawa M, Ohmae N. High growth rate of vertically aligned carbon nanotubes using a plasma shield in microwave plasma-enhanced chemical vapor deposition. Carbon 2004;42:2753–6. [9] Bower C, Zhu W, Jin S, Zhou O. Plasma-induced alignment of carbon nanotubes. Appl Phys Lett 2000;77:830–2.
4 6 ( 2 0 0 8 ) 5 4 4 –5 6 1
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Letters to the Editor
Shaping carbon nanotube bundles during growth using a magnetic field Nobuo Ohmae* Department of Mechanical Engineering, Graduate School of Engineering, Kobe University, Rokkodai 1-1, Nada, Kobe 657-8501, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2007
2007 Elsevier Ltd. All rights reserved.
Accepted 6 December 2007 Available online 25 January 2008
Carbon nanotubes (CNT) usually grow straight in the vertical direction [1–3], except carbon nanocoil [4–6] and spaghettilike CNT [7]. But, if a curved structure were to be obtained by applying a suitable stimulus, the application of CNT to MEMS/NEMS or to bio-nano can greatly be widened. The original ideas of synthesizing a complex structure of CNT arose firstly from our experimental results by high resolution transmission electron microscopy which showed that the CNT grown by the plasma-enhanced chemical vapor deposition (PE-CVD) very frequently possesses the catalyst at its top [8]. The second favored condition is that the catalyst is iron with our experimental system. Therefore, magnetic field appears effective to control the growth direction to create a complex structure of CNT. Attempts have been made to synthesize an intricately curved CNT structure using a magnetic field. The external magnetic field was applied to the substrate outside the PE-CVD apparatus [8]. Fig. 1 shows the designed yoke-shaped permanent magnet made of Nd–Fe–B, whose magnetic flux density is 500 mT. Because the external magnetic field is applied from the outside of furnace of PE-CVD, the demagnetization due to high temperature (usually around 700 C) does not become a problem. The magnetic field at the center of the quartz tube is calculated at 10 mT.1 The substrate was held in the vertical direction, i.e., perpendicular to the axis of quartz tube, while the direction of magnetic
field was opposite to that of gravity. The deposition of CNT was carried out using H2 and CH4 as the reaction gases with the flow rates of 100 sccm and 10 sccm, respectively. The temperature of the quartz tube was maintained at 700 C with the gas pressure of 2.7 · 103 Pa. The growth rate of CNT was about 50 nm/s. Before depositing CNT, Fe thin film typically with the thickness of 5 nm was vacuum-deposited on the Si(0 0 1) substrate, the surface of which had been oxidized at 800 C in O2 atmosphere. Further, with using H2 at a pressure of 1.0 · 102 Pa, Fe films were transformed into nanoparticles by treating in vacuum at 700 C for 30 min. Simply because the morphological structure is easy to recognize with a patterned CNT bundle, the Si mask which has the 5 lm · 5 lm squares at each neighboring distance of 2 lm was used. Hook-shaped CNT is shown in Fig. 2. In this case, the magnetic field was introduced after 3 min from the onset of deposition. The direction of the line of magnetic force was opposite to the initial growth direction of CNT. Obviously, CNT re-grows in the direction of the line of magnetic force. By changing the direction of the line of magnetic force during PE-CVD processes, the arch-shaped structure of CNT resulted. Typical SEM photograph is shown in Fig. 3. The magnetic field was first applied after 1 min from the start of
* Fax: +81 78 803 6111. E-mail address: [email protected]. 1 Neomax Co., Ltd. product specification. 0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.12.005
CARBON
4 6 (2 0 0 8) 5 4 4–56 1
545
Fig. 1 – Yoke-shaped Nd–Fe–B magnet designed for applying magnetic field outside the furnace. The direction of magnetic field is illustrated.
Fig. 4 – Ladder-shaped CNT bundles.
Fig. 2 – Hook-shaped CNT bundles.
Fig. 3 – Arch-shaped CNT bundles.
deposition, and the second after 3 min. The direction of the line of magnetic force of the latter differs from the former
at 90. This dramatic picture supports the aim of this study that the shape of CNT is controllable by the external magnetic field. The ladder-shaped structure is indicated in Fig. 4, by repeatedly changing the direction of the line of magnetic force. Due to the strong effect of self-bias, in this case, the ladder elongates to the vertical direction. The most probable reason why the complex configurations of CNT presented in Figs. 2–4 can be attributed to the force acting on Fe nanoparticles. In our experimental conditions, Fe nanoparticles exist in the shape of ellipse (or sometimes will-0-wisp) whose long axis is along the growth direction of CNT. The magnetic force pulls the Fe particles in one direction or the other, and this force changes the growth direction of the CNTs. This is why the growth direction changes when a magnetic field is applied. The TEM photograph in Fig. 5 shows the evidence of curved CNT, where the bulge of CNT wall (indicated by a circle) is visible. It also was possible to construct the curved CNT structure by simply placing a Sm–Co magnet inside the quartz tube. However, one of the weak points of this method is that the magnet is not movable inside the quartz tube, so that CNT with complex structure was difficult to synthesize. Bower et al. explained the reason for the vertical growth of CNTs in PE-CVD is due to the self-bias, the electric potential
546
CARBON
4 6 ( 2 0 0 8 ) 5 4 4 –5 6 1
ticles experience a force in the field, and tend to move in the direction, where the magnetic field lines get stronger.
Acknowledgements This work was supported in part by the Grant-in-Aid for Exploratory Research from the Ministry of Education, Culture, Sports, Science and Technology under 16656057. I would like to thank Neomax Co. Ltd. and Renesas Technology Corp. for providing us with the permanent magnets and the mask for CNT deposition. Thanks are due to Taketoshi Akimoto, Keiichi Takigawa, Takenori Wada and Naohiro Matsumoto for their experiments on the manufacturing of CNT.
R E F E R E N C E S
Fig. 5 – TEM photograph showing the change in growth direction of CNT when the direction of magnetic force was changed.
between plasma and substrate [9]. They estimated in their work that the self-bias was 10 V. In the present study, the plasma potential was measured by a Langmuir probe during PE-CVD. The Langmuir probe revealed that the floating potential was 14 eV and that the plasma potential was 24 eV. Therefore, the self-bias is 10 eV, and is consistent with the results by Bower et al. Because of the high electric field created by this self-bias (104–105 V/m) and the lack in providing much higher electric fields between two parallel plates, external electric field was found to be less effective for synthesizing the curved structure of CNTs in the present experimental conditions. In summary, the CNT structures with complex shape were successfully obtained by the external magnetic field. The primary reason for this was that the ferromagnetic Fe nanopar-
[1] Ren ZF, Huang ZP, Wang DZ, Wen JG. Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot. Appl Phys Lett 1999;75:1086–8. [2] Chena M, Chenb CM, Kooa HS, Chen CF. Catalyzed growth model of carbon nanotubes by microwave plasma chemical vapor deposition using CH4 and CO2 gas mixtures. Diam Relat Mater 2003;12:1829–35. [3] Zhu L, Xu J, Xiu Y, Sun Y, Hess DW, Wong CP. Growth and electrical characterization of high-aspect-ratio carbon nanotube arrays. Carbon 2006;44:253–8. [4] Bai JB. Growth of nanotube/nanofibre coils by CVD on an alumina substrate. Mater Lett 2003;57:2629–33. [5] Zhong DY, Liu S, Wang EG. Patterned growth of coiled carbon nanotubes by a template-assisted technique. Appl Phys Lett 2003;83:4423–5. [6] Ihara S, Itoh S, Kitakami J. Helically coiled cage forms of graphitic carbon. Phys Rev B 1993;48:5643–7. [7] Juang ZY, Lai JF, Weng CH, Lee JH, Lai HJ, Lai TS, et al. On the kinetics of carbon nanotube growth by thermal CVD method. Diam Relat Mater 2004;13:2140–6. [8] Kinoshita H, Kume I, Sakai H, Tagawa M, Ohmae N. High growth rate of vertically aligned carbon nanotubes using a plasma shield in microwave plasma-enhanced chemical vapor deposition. Carbon 2004;42:2753–6. [9] Bower C, Zhu W, Jin S, Zhou O. Plasma-induced alignment of carbon nanotubes. Appl Phys Lett 2000;77:830–2.