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Study on the electron cyclotron resonance plasma chemical vapor deposition of carbon nanotubes
Materials Science and Engineering B 140 (2007) 44–47
Study on the electron cyclotron resonance plasma chemical vapor deposition of carbon nanotubes Zhi Wang a,∗ , Dechun Ba b , Peijiang Cao c , ChunhongYu a , Ji Liang d a
b
ShenYang Institute of Aeronautical Engineering, Shenyang 110034, PR China School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110004, PR China c Department of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China d Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 25 October 2005; received in revised form 5 March 2007; accepted 7 March 2007
Abstract In this paper, the carbon nanotubes growth on porous silicon substrates by electron cyclotron resonance plasma chemical vapor deposition (ECR-CVD) method are studied as the function of the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber and the substrate temperature. The results showed that the flow ratio of CH4 /(H2 + CH4 ) and the total pressure are the key factors on the concentration of carbon radical in the chamber, which will influence the growth rate, the density and the orientation of carbon nanotubes. The outer diameters of carbon nanotubes could be controlled by changing the substrate temperatures, it was shown that the aligned carbon nanotubes cannot be formed at lower temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Growth characteristics
1. Introduction
2. Experimental
Recently carbon nanotubes have received great interests for their superior mechanical strength and versatile electronic properties. For most of the applications, particularly in the field of emission display and nanoscale electronic devices, it is necessary to have a method which can directly synthesize large area aligned carbon nanotubes [1–8]. Since ECR-CVD system has the advantages of high dissociation percentage of the precursor gas and great uniformity on plasma energy distribution, it becomes a potential candidate [9]. Kuo and co-workers [10] and Shih and co-workers [11–13] grew aligned CNTs on Si and porous anodic alumina by ECR-CVD, respectively. Unfortunately, there are few papers that provide systematic studies about the conditions of obtaining aligned CNTs. In this paper, without the substrate bias, we applied ECRCVD method to synthesize large area aligned CNTs. The effects on the carbon nanotubes growing from the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber and the substrate temperature were investigated systematically.
2.1. Fabrication of catalyst-coated substrate
∗
Corresponding author. Tel.: +86 24 86141599; fax: +86 24 86141517. E-mail address: [email protected] (Z. Wang).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.03.008
Porous silicon substrates with a thin nanoporous layer were obtained by electrochemical etching of Si (1 1 1) wafers. Fe3 O4 nanoparticles with diameter less than 10 nanometer were synthesized by thermal hydrolyzation. Then, these nanoparticles were spread on a porous silicon substrate and were left to dry in atmosphere [14]. 2.2. Growth of carbon nanotubes In our ECR-CVD system, the microwave power (2.45 GHz) is set as 700 W and the magnetic field is set as 875 G. By using an additional electrical heater inside the sample holder the substrate can reach 600 ◦ C. Details of the ECR-CVD system can be found in our recent work [15]. The catalyst-coated porous silicon wafer was placed in the vacuum chamber with base pressure 10−3 Pa. At first, the temperature of substrate to 550 ◦ C and introduce H2 at a flow rate of 50 sccm. Pretreat the substrate by H2 plasma for 10 min with the total pressure kept as 30 Pa. It should be noted that the substrate temperature could increase
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
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Table 1 Growth conditions for every specimen, including the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber (Pa) and the substrate temperature (◦ C) Specimens
The flow ratio of CH4 / (H2 + CH4 ) (sccm/sccm) (%)
The total pressure in vacuum chamber (Pa)
The substrate temperature (◦ C)
Growth rate (nm/min)
Outer diameter (nm)
1 2 3 4 5 6 7 8
25 50 100 50 50 50 50 50
30 30 30 5 15 80 30 30
650 650 650 650 650 650 510 560
100 333 333 10 16 333 67 67
60–90 60–90 60–90 60–90 60–90 60–90 20–30 20–90
to 700 ◦ C due to plasma heating. During this process, reduce the Fe3 O4 nanoparticles into nanosized iron particles. Introduce CH4 into the chamber for the carbon nanotubes growth. Finally, a black film was formed on the porous silicon surface. To investigate the effects from different aspects, in our experiments, carbon nanotubes were grown under different flow ratios of CH4 /(H2 + CH4 ) (from 25 to 100%), different total pressures (from 5 to 80 Pa), and varying substrate temperatures (from 510 to 650 ◦ C). The detailed process parameters of specimens are listed in Table 1.
A field-emission type scanning electron microscope (XLSFEG) with the acceleration voltage set as 20 kV, which is used to study the morphologies of carbon nanotubes. Transmission electron microscopy (JEOL-200CX) with the acceleration voltage as set 200 kV, whose objective is to obtain TEM images of carbon nanotubes. Raman spectrum (Renishaw System 100) that is used to evaluate the structure of carbon nanotubes. For the preparation procedure of the TEM specimen, two steps are involved. First, specimens are scratched from the silicon substrate and cleaned ultrasonically in an ethanol bath for 10 min, then two drops of this liquid are put onto a carbon-coated copper grid.
Since Raman spectra is very sensitive to change in translational symmetry, it is very useful to characterize carbon nanotubes. It is well known that the G peak centered at 1586 cm−1 is the zone center E2g mode of the perfect graphite crystal and the D band at 1354 cm−1 is a zone edge A1g mode activated by disorder in the graphite poly-crystal. So generally the ID /IG ratio is used to evaluate the quality of as-grown carbon nanotube film [16]. Fig. 2 shows Raman spectra of carbon nanotube film. The ID /IG ratios of specimens 2 and 3 is 0.600 and 0.696, respectively, which indicate that under the flow ratio of 50%, we can get the minimum defects, while under flow ratio of 100% we will get the maximum defects. These Raman results agree well with the result of SEM. From above SEM images and Raman spectra, following conclusions can be drawn. The flow ratio of CH4 /(H2 + CH4 ) in the vacuum chamber has great effects on the growth rate of carbon nanotubes. When the ratio is less than 50% the growth rate increases as the CH4 /(H2 + CH4 ) flow ratio increase. With the increasing of the CH4 /(H2 + CH4 ) flow ratio, carbon nanotubes display worse alignment and has more defects on the surfaces of carbon nanotubes. It is believed that during the growth of CNTs, while total pressure is fixed, CH4 fraction controls the concentration of carbon radical in the chamber, which will influences growth rate, area density and aligned growth of carbon nanotubes.
3. Results and discussion
3.2. The effect of the total pressure in vacuum chamber
3.1. The effect of the flow ratio of CH4 /(H2 + CH4 )
To make a comparison based on different total pressure in vacuum chamber, we kept the flow ratio of CH4 /(H2 + CH4 ) and the substrate temperature constant. The specimens labeled as 2, 4, 5 and 6 have been fabricated with the total pressures varying from 5 to 80 Pa. Comparing the SEM image of specimen 4 in Fig. 3(a) with that of specimen 5 in Fig. 3(b), we can see that short curly carbon nanotubes grow on the porous silicon surface and the lengths of carbon nanotubes for specimen 4 are 300 nm shorter than those for specimen 5. According to the SEM image of specimen 6 shown in Fig. 3(c) the aligned carbon nanotubes with the length of about 10 m and a crust composed of Fe and carbon particles are formed on the top of the CNTs film. Raman spectrum of specimen 6 is shown in Fig. 2. We can see the D band and the G band clearly from this figure and its ID /IG ratio is about 0.720. Such high ID /IG ratio indicates that the carbon radical coming from CH4 decomposition exceeds the
2.3. Characterization
Three specimens were obtained by varying the flow ratios of CH4 /(H2 + CH4 ) from 25 to 100%, we labeled them as 1, 2 and 3. It should be noted that the total pressure and the substrate temperature were kept the same for these three specimens. According to the SEM images shown in Fig. 1(a–c), specimen 1 shows the high orientation of CNTs with the length of about 3 m; specimen 2 shows the lodged aligned CNTs with the length of 10 m; for specimen 3 it can be seen that the CNTs have worse alignment and the lengths are about 10 m, also there are many defects formed on the outer surfaces of CNTs. To better understand the structure of carbon nanotubes, the details of the TEM image of specimen 2 is drawn in Fig. 1(d). It can be seen that these as-grown carbon nanotubes with hollow tubular structures are multiwalled tubes having outer diameter of about 80 nm.
46
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
Fig. 1. SEM images of carbon nanotubes grown at the different flow ratios of CH4 /(H2 + CH4 ) of: (a) 25%, (b) 50%, and (c) 100%. (d) TEM image of carbon nanotubes grown at the flow ratio of 50%.
Based on the results of SEM and Raman, here are our findings: with the increase of the total gas pressure in vacuum chamber, the growth rate of carbon nanotubes increases, also more carbon particles defects appear. Carbon nanotubes cannot grow when the total pressure is lesser than 1 Pa. We believe that when the flow ratio of CH4 /(H2 + CH4 ) is kept constant, the total pressure influences the growth of high density and aligned carbon nanotubes by controlling the concentration of carbon radical in the chamber. The effect of the total pressure on the carbon nanotubes growing is very similar to that of the flow ratio. 3.3. The effect of the substrate temperature
Fig. 2. Raman spectra of carbon nanotubes.
demand of carbon nanotube growth, and many carbon particles with disorder structures are formed under this condition. These Raman results agree well with the result of SEM.
In this section, the specimens 2, 7 and 8 are synthesized by choosing the substrate temperatures from 510 to 650 ◦ C, whereas the flow ratio of CH4 /(H2 + CH4 ) and the total pressure were kept constant. The substrate temperatures were varied by controlling additional electrical heater. Fig. 4(a and b) shows the SEM images of specimens 7 and 8, respectively. It can be seen
Fig. 3. SEM images of carbon nanotubes grown at the total pressures in vacuum chamber of: (a) 5 Pa, (b) 15 Pa and (c) 80 Pa.
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
47
Fig. 4. SEM images of carbon nanotubes grown at the substrate temperatures of: (a) 510 ◦ C and (b) 560 ◦ C.
that carbon nanotubes of both specimens grow disorderly and the lengths are about 2 m. The outer diameters of carbon nanotubes of specimen 7 vary from 20 to 30 nm and specimen 8 vary from 20 to 90 nm. As for specimen 2, it can be seen from Fig. 1(b) that high orientation CNTs have the outer diameters varying from 60 to 90 nm and the length is 10 m. It can also be found that CNTs cannot grow on the porous silicon surface when the substrate temperature is below 500 ◦ C. From the above experiments, following conclusions can be drawn. The substrate temperature controls the outer diameters of carbon nanotubes. Carbon nanotubes show less aligned growth and the growth rate decreases at lower temperature. It is well known that the melting point of nano-particle decreases with the decrease of nano-particle size. At this temperature iron particles melt and incorporate into large particles in order to lower the surface free energy [17]. The agglomeration of the nanoparticles aggravate with the increasing of the growth temperature, so it cause the diameters of CNTs extend, the growth rate increases and the CNTs tend to aligned growth. As a summary, we can draw the conclusion that the temperature plays important role in controlling the diameter of CNTs by influencing the size of catalyst and it can make CNTs grow curly because of forming more dislocations and pentagonals at low temperature [18,19]. 4. Conclusions In this paper, we demonstrate how we synthesize the carbon nanotubes on porous silicon by ECR-CVD method with Fe3 O4 nanoparticles as catalyst. The carbon nanotubes growth under different process parameters was investigated. The results showed that the flow ratio of CH4 /(H2 + CH4 ) and the total pressure in vacuum chamber can control the growth rate, the density and the aligned growth of carbon nanotubes by adjusting the carbon concentration in the vacuum chamber; the results also showed that the substrate temperatures can affect the diameters of carbon nanotubes by changing the sizes of catalyst particles. These conclusions are very useful in leading us to the right directions of fabricating carbon nanotubes by ECR-CVD method.
Acknowledgments This project was supported by China Postdoctoral Science Foundation (Grant No. 20060390041), Foundation of Liaoning Provincial Education Committee of China (Grant No. 05 L327) and the Science and Technology Foundation of Shenzhen (Grant No. 200501). References [1] S. Iijima, Nature 354 (1991) 56. [2] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. [3] C. White, T.N. Todorov, Nature 393 (1998) 240. [4] Y.H. Gao, Y. Bando, Nature 415 (2002) 599. [5] B.Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath, P.M. Ajayan, Nature 416 (2002) 495. [6] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512. [7] Z.F. Ren, Z.P. Huang, D.Z. Wang, J.G. Wen, J.W. Xu, J.H. Wang, L.E. Calvet, J. Chen, J.F. Klemic, M.A. Reed, Appl. Phys. Lett. 75 (1999) 1086. [8] Y. Chen, D.T. Shaw, L.P. Guo, Appl. Phys. Lett. 76 (2000) 2469. [9] A. Huczko, Appl. Phys. A 74 (2002) 617. [10] C.M. Hsu, C.H. Lin, H.L. Chang, C.T. Kuo, Thin Solid Films 420 (2002) 225. [11] S.H. Tsai, C.W. Chao, C.L. Lee, H.C. Shih, Appl. Phys. Lett. 74 (1999) 3462. [12] S.H. Tsai, F.K. Chiang, T.G. Tsai, F.S. Shieu, H.C. Shih, Thin Solid Films 366 (2000) 11. [13] X.W. Liu, L.H. Chan, K.H. Hong, H.C. Shih, Thin Solid Films 420–421 (2002) 212. [14] C.M. She, Y.K. Su, H.T. Yang, T.Z. Yang, Acta Phys. Sin. 524 (2003) 83. [15] Z. Wang, D.C. Ba, F. Liu, P.J. Cao, T.Z. Yang, Y.S. Gu, H.J. Gao, Vacuum 77 (2005) 139. [16] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao, G. Wang, Science 274 (1996) 1701. [17] N. Jiang, R. Koie, T. Inaoka, Y. Shintani, K. Nishimura, A. Hiraki, Appl. Phys. Lett. 81 (2002) 526. [18] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [19] U. Kim, R. Pcionek, D.M. Aslam, D. Tomanek, Diamond Relat. Mater. 10 (2001) 1947.
Study on the electron cyclotron resonance plasma chemical vapor deposition of carbon nanotubes Zhi Wang a,∗ , Dechun Ba b , Peijiang Cao c , ChunhongYu a , Ji Liang d a
b
ShenYang Institute of Aeronautical Engineering, Shenyang 110034, PR China School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110004, PR China c Department of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, PR China d Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 25 October 2005; received in revised form 5 March 2007; accepted 7 March 2007
Abstract In this paper, the carbon nanotubes growth on porous silicon substrates by electron cyclotron resonance plasma chemical vapor deposition (ECR-CVD) method are studied as the function of the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber and the substrate temperature. The results showed that the flow ratio of CH4 /(H2 + CH4 ) and the total pressure are the key factors on the concentration of carbon radical in the chamber, which will influence the growth rate, the density and the orientation of carbon nanotubes. The outer diameters of carbon nanotubes could be controlled by changing the substrate temperatures, it was shown that the aligned carbon nanotubes cannot be formed at lower temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Growth characteristics
1. Introduction
2. Experimental
Recently carbon nanotubes have received great interests for their superior mechanical strength and versatile electronic properties. For most of the applications, particularly in the field of emission display and nanoscale electronic devices, it is necessary to have a method which can directly synthesize large area aligned carbon nanotubes [1–8]. Since ECR-CVD system has the advantages of high dissociation percentage of the precursor gas and great uniformity on plasma energy distribution, it becomes a potential candidate [9]. Kuo and co-workers [10] and Shih and co-workers [11–13] grew aligned CNTs on Si and porous anodic alumina by ECR-CVD, respectively. Unfortunately, there are few papers that provide systematic studies about the conditions of obtaining aligned CNTs. In this paper, without the substrate bias, we applied ECRCVD method to synthesize large area aligned CNTs. The effects on the carbon nanotubes growing from the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber and the substrate temperature were investigated systematically.
2.1. Fabrication of catalyst-coated substrate
∗
Corresponding author. Tel.: +86 24 86141599; fax: +86 24 86141517. E-mail address: [email protected] (Z. Wang).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.03.008
Porous silicon substrates with a thin nanoporous layer were obtained by electrochemical etching of Si (1 1 1) wafers. Fe3 O4 nanoparticles with diameter less than 10 nanometer were synthesized by thermal hydrolyzation. Then, these nanoparticles were spread on a porous silicon substrate and were left to dry in atmosphere [14]. 2.2. Growth of carbon nanotubes In our ECR-CVD system, the microwave power (2.45 GHz) is set as 700 W and the magnetic field is set as 875 G. By using an additional electrical heater inside the sample holder the substrate can reach 600 ◦ C. Details of the ECR-CVD system can be found in our recent work [15]. The catalyst-coated porous silicon wafer was placed in the vacuum chamber with base pressure 10−3 Pa. At first, the temperature of substrate to 550 ◦ C and introduce H2 at a flow rate of 50 sccm. Pretreat the substrate by H2 plasma for 10 min with the total pressure kept as 30 Pa. It should be noted that the substrate temperature could increase
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
45
Table 1 Growth conditions for every specimen, including the flow ratio of CH4 /(H2 + CH4 ), the total pressure in vacuum chamber (Pa) and the substrate temperature (◦ C) Specimens
The flow ratio of CH4 / (H2 + CH4 ) (sccm/sccm) (%)
The total pressure in vacuum chamber (Pa)
The substrate temperature (◦ C)
Growth rate (nm/min)
Outer diameter (nm)
1 2 3 4 5 6 7 8
25 50 100 50 50 50 50 50
30 30 30 5 15 80 30 30
650 650 650 650 650 650 510 560
100 333 333 10 16 333 67 67
60–90 60–90 60–90 60–90 60–90 60–90 20–30 20–90
to 700 ◦ C due to plasma heating. During this process, reduce the Fe3 O4 nanoparticles into nanosized iron particles. Introduce CH4 into the chamber for the carbon nanotubes growth. Finally, a black film was formed on the porous silicon surface. To investigate the effects from different aspects, in our experiments, carbon nanotubes were grown under different flow ratios of CH4 /(H2 + CH4 ) (from 25 to 100%), different total pressures (from 5 to 80 Pa), and varying substrate temperatures (from 510 to 650 ◦ C). The detailed process parameters of specimens are listed in Table 1.
A field-emission type scanning electron microscope (XLSFEG) with the acceleration voltage set as 20 kV, which is used to study the morphologies of carbon nanotubes. Transmission electron microscopy (JEOL-200CX) with the acceleration voltage as set 200 kV, whose objective is to obtain TEM images of carbon nanotubes. Raman spectrum (Renishaw System 100) that is used to evaluate the structure of carbon nanotubes. For the preparation procedure of the TEM specimen, two steps are involved. First, specimens are scratched from the silicon substrate and cleaned ultrasonically in an ethanol bath for 10 min, then two drops of this liquid are put onto a carbon-coated copper grid.
Since Raman spectra is very sensitive to change in translational symmetry, it is very useful to characterize carbon nanotubes. It is well known that the G peak centered at 1586 cm−1 is the zone center E2g mode of the perfect graphite crystal and the D band at 1354 cm−1 is a zone edge A1g mode activated by disorder in the graphite poly-crystal. So generally the ID /IG ratio is used to evaluate the quality of as-grown carbon nanotube film [16]. Fig. 2 shows Raman spectra of carbon nanotube film. The ID /IG ratios of specimens 2 and 3 is 0.600 and 0.696, respectively, which indicate that under the flow ratio of 50%, we can get the minimum defects, while under flow ratio of 100% we will get the maximum defects. These Raman results agree well with the result of SEM. From above SEM images and Raman spectra, following conclusions can be drawn. The flow ratio of CH4 /(H2 + CH4 ) in the vacuum chamber has great effects on the growth rate of carbon nanotubes. When the ratio is less than 50% the growth rate increases as the CH4 /(H2 + CH4 ) flow ratio increase. With the increasing of the CH4 /(H2 + CH4 ) flow ratio, carbon nanotubes display worse alignment and has more defects on the surfaces of carbon nanotubes. It is believed that during the growth of CNTs, while total pressure is fixed, CH4 fraction controls the concentration of carbon radical in the chamber, which will influences growth rate, area density and aligned growth of carbon nanotubes.
3. Results and discussion
3.2. The effect of the total pressure in vacuum chamber
3.1. The effect of the flow ratio of CH4 /(H2 + CH4 )
To make a comparison based on different total pressure in vacuum chamber, we kept the flow ratio of CH4 /(H2 + CH4 ) and the substrate temperature constant. The specimens labeled as 2, 4, 5 and 6 have been fabricated with the total pressures varying from 5 to 80 Pa. Comparing the SEM image of specimen 4 in Fig. 3(a) with that of specimen 5 in Fig. 3(b), we can see that short curly carbon nanotubes grow on the porous silicon surface and the lengths of carbon nanotubes for specimen 4 are 300 nm shorter than those for specimen 5. According to the SEM image of specimen 6 shown in Fig. 3(c) the aligned carbon nanotubes with the length of about 10 m and a crust composed of Fe and carbon particles are formed on the top of the CNTs film. Raman spectrum of specimen 6 is shown in Fig. 2. We can see the D band and the G band clearly from this figure and its ID /IG ratio is about 0.720. Such high ID /IG ratio indicates that the carbon radical coming from CH4 decomposition exceeds the
2.3. Characterization
Three specimens were obtained by varying the flow ratios of CH4 /(H2 + CH4 ) from 25 to 100%, we labeled them as 1, 2 and 3. It should be noted that the total pressure and the substrate temperature were kept the same for these three specimens. According to the SEM images shown in Fig. 1(a–c), specimen 1 shows the high orientation of CNTs with the length of about 3 m; specimen 2 shows the lodged aligned CNTs with the length of 10 m; for specimen 3 it can be seen that the CNTs have worse alignment and the lengths are about 10 m, also there are many defects formed on the outer surfaces of CNTs. To better understand the structure of carbon nanotubes, the details of the TEM image of specimen 2 is drawn in Fig. 1(d). It can be seen that these as-grown carbon nanotubes with hollow tubular structures are multiwalled tubes having outer diameter of about 80 nm.
46
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
Fig. 1. SEM images of carbon nanotubes grown at the different flow ratios of CH4 /(H2 + CH4 ) of: (a) 25%, (b) 50%, and (c) 100%. (d) TEM image of carbon nanotubes grown at the flow ratio of 50%.
Based on the results of SEM and Raman, here are our findings: with the increase of the total gas pressure in vacuum chamber, the growth rate of carbon nanotubes increases, also more carbon particles defects appear. Carbon nanotubes cannot grow when the total pressure is lesser than 1 Pa. We believe that when the flow ratio of CH4 /(H2 + CH4 ) is kept constant, the total pressure influences the growth of high density and aligned carbon nanotubes by controlling the concentration of carbon radical in the chamber. The effect of the total pressure on the carbon nanotubes growing is very similar to that of the flow ratio. 3.3. The effect of the substrate temperature
Fig. 2. Raman spectra of carbon nanotubes.
demand of carbon nanotube growth, and many carbon particles with disorder structures are formed under this condition. These Raman results agree well with the result of SEM.
In this section, the specimens 2, 7 and 8 are synthesized by choosing the substrate temperatures from 510 to 650 ◦ C, whereas the flow ratio of CH4 /(H2 + CH4 ) and the total pressure were kept constant. The substrate temperatures were varied by controlling additional electrical heater. Fig. 4(a and b) shows the SEM images of specimens 7 and 8, respectively. It can be seen
Fig. 3. SEM images of carbon nanotubes grown at the total pressures in vacuum chamber of: (a) 5 Pa, (b) 15 Pa and (c) 80 Pa.
Z. Wang et al. / Materials Science and Engineering B 140 (2007) 44–47
47
Fig. 4. SEM images of carbon nanotubes grown at the substrate temperatures of: (a) 510 ◦ C and (b) 560 ◦ C.
that carbon nanotubes of both specimens grow disorderly and the lengths are about 2 m. The outer diameters of carbon nanotubes of specimen 7 vary from 20 to 30 nm and specimen 8 vary from 20 to 90 nm. As for specimen 2, it can be seen from Fig. 1(b) that high orientation CNTs have the outer diameters varying from 60 to 90 nm and the length is 10 m. It can also be found that CNTs cannot grow on the porous silicon surface when the substrate temperature is below 500 ◦ C. From the above experiments, following conclusions can be drawn. The substrate temperature controls the outer diameters of carbon nanotubes. Carbon nanotubes show less aligned growth and the growth rate decreases at lower temperature. It is well known that the melting point of nano-particle decreases with the decrease of nano-particle size. At this temperature iron particles melt and incorporate into large particles in order to lower the surface free energy [17]. The agglomeration of the nanoparticles aggravate with the increasing of the growth temperature, so it cause the diameters of CNTs extend, the growth rate increases and the CNTs tend to aligned growth. As a summary, we can draw the conclusion that the temperature plays important role in controlling the diameter of CNTs by influencing the size of catalyst and it can make CNTs grow curly because of forming more dislocations and pentagonals at low temperature [18,19]. 4. Conclusions In this paper, we demonstrate how we synthesize the carbon nanotubes on porous silicon by ECR-CVD method with Fe3 O4 nanoparticles as catalyst. The carbon nanotubes growth under different process parameters was investigated. The results showed that the flow ratio of CH4 /(H2 + CH4 ) and the total pressure in vacuum chamber can control the growth rate, the density and the aligned growth of carbon nanotubes by adjusting the carbon concentration in the vacuum chamber; the results also showed that the substrate temperatures can affect the diameters of carbon nanotubes by changing the sizes of catalyst particles. These conclusions are very useful in leading us to the right directions of fabricating carbon nanotubes by ECR-CVD method.
Acknowledgments This project was supported by China Postdoctoral Science Foundation (Grant No. 20060390041), Foundation of Liaoning Provincial Education Committee of China (Grant No. 05 L327) and the Science and Technology Foundation of Shenzhen (Grant No. 200501). References [1] S. Iijima, Nature 354 (1991) 56. [2] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. [3] C. White, T.N. Todorov, Nature 393 (1998) 240. [4] Y.H. Gao, Y. Bando, Nature 415 (2002) 599. [5] B.Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath, P.M. Ajayan, Nature 416 (2002) 495. [6] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512. [7] Z.F. Ren, Z.P. Huang, D.Z. Wang, J.G. Wen, J.W. Xu, J.H. Wang, L.E. Calvet, J. Chen, J.F. Klemic, M.A. Reed, Appl. Phys. Lett. 75 (1999) 1086. [8] Y. Chen, D.T. Shaw, L.P. Guo, Appl. Phys. Lett. 76 (2000) 2469. [9] A. Huczko, Appl. Phys. A 74 (2002) 617. [10] C.M. Hsu, C.H. Lin, H.L. Chang, C.T. Kuo, Thin Solid Films 420 (2002) 225. [11] S.H. Tsai, C.W. Chao, C.L. Lee, H.C. Shih, Appl. Phys. Lett. 74 (1999) 3462. [12] S.H. Tsai, F.K. Chiang, T.G. Tsai, F.S. Shieu, H.C. Shih, Thin Solid Films 366 (2000) 11. [13] X.W. Liu, L.H. Chan, K.H. Hong, H.C. Shih, Thin Solid Films 420–421 (2002) 212. [14] C.M. She, Y.K. Su, H.T. Yang, T.Z. Yang, Acta Phys. Sin. 524 (2003) 83. [15] Z. Wang, D.C. Ba, F. Liu, P.J. Cao, T.Z. Yang, Y.S. Gu, H.J. Gao, Vacuum 77 (2005) 139. [16] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao, G. Wang, Science 274 (1996) 1701. [17] N. Jiang, R. Koie, T. Inaoka, Y. Shintani, K. Nishimura, A. Hiraki, Appl. Phys. Lett. 81 (2002) 526. [18] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [19] U. Kim, R. Pcionek, D.M. Aslam, D. Tomanek, Diamond Relat. Mater. 10 (2001) 1947.