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Transformation of carbon nanotubes to diamond in microwave hydrogen plasma
Materials Letters 61 (2007) 2208 – 2211 www.elsevier.com/locate/matlet
Transformation of carbon nanotubes to diamond in microwave hydrogen plasma Qiaoqin Yang a,⁎, Songlan Yang a , Chijin Xiao b , Akira Hirose b a
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 b Plasma Physics laboratory, University of Saskatchewan, 116 Science Place, Saskatoon, SK, Canada S7N 5E2 Received 17 April 2006; accepted 24 August 2006 Available online 15 September 2006
Abstract Multiwall carbon nanotubes (MWCNTs), synthesized by microwave plasma enhanced chemical vapor deposition, were used as the precursor to synthesize diamond in a pure hydrogen microwave discharge. Diamond-scratched silicon wafers, with and without precoated MWCNTs, were placed side by side on a substrate holder. Diamond was formed on both wafers with the consumption of MWCNTs. The present results suggest that the solid–gas–solid transformation mechanism is involved in this process. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond; Carbon nanotubes; Microwave hydrogen plasma
1. Introduction Diamond is by far the hardest known material and has many unique and beneficial properties: it has the lowest coefficient of thermal expansion, is chemically inert, offers the lowest friction coefficient and the best wear resistance, has the highest thermal conductivity, and exhibits excellent biocompatibility. The combination of these extreme properties makes diamond an ideal material for many applications. However, natural diamond is scarce and costly. This has motivated scientists for more than a century to explore the diamond synthesis process using various techniques with different starting carbon sources. Since the first successful synthesis of diamond through direct transformation of graphite at high pressures (N 12.5 GPa) and high temperatures (N3000 °C) [1,2], various carbon allotropes have been used as the precursors for diamond synthesis. It has been synthesized by squeezing fullerene in a diamond-anvil cell at a pressure of approximately 20 GPa at room temperature [3,4], and by generating microwave discharges in the C60-containing argon gas [5,6], by hydrogen treatment of graphite in a hot filament [7–9] or a microwave plasma enhanced chemical vapor deposition ⁎ Corresponding author. Tel.: +1 306 966 5470; fax: +1 306 966 5427. E-mail address: [email protected] (Q. Yang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.060
(MPECVD) reactor [10]. Since the discovery of CNTs in 1991 [11], many techniques have also been tried to synthesize diamond from CNTs. These include laser irradiation [12,13], shock waves [14], spark plasma sintering [15], and radio-frequency hydrogen plasma [16–18]. In-situ defects-controlled solid–solid phase transformation mechanism, in which the defects are the original sites for nucleation and growth of the diamond particles, has been proposed for the transformation of CNTs to diamond in the radiofrequency hydrogen plasma [16–18], whereas the transformation of graphite to diamond by hydrogen treatment of graphite in the hot filament or microwave plasma reactor is believed to be a solid–gas–solid process [8–10]: the graphite (solid) is etched by an atomic hydrogen generated by microwave plasma or hot filament to form hydrocarbon radicals (gas), which act as the precursors for diamond (solid) growth, similar to diamond growth from the gas mixture of methane and hydrogen. There are many experimental similarities between the works in Refs. [8–10] using graphite as the precursor and those in Refs. [16–18] using CNTs as the precursor. Both CNTs and graphite contain sp2-bonded structures and served as carbon sources. In all the processes, only pure hydrogen was admitted into the reactor. Thus it is desirable to investigate whether a solid–gas–solid process is also involved in the formation of
Q. Yang et al. / Materials Letters 61 (2007) 2208–2211
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diamond through the hydrogen plasma treatment of CNTs. For this purpose, the dispersed MWCNTs were precoated on the surface of the silicon substrates, similar to the process described by others [16–18], and treated in a pure hydrogen plasma in a MPECVD reactor together with the silicon wafers without MWCNTs. A Scanning Electron Microscope (SEM) and a Raman Spectroscope were employed to analyze the experimental results. 2. Experimental details The MWCNTs used in these experiments were synthesized on an inconel 600 plate at a temperature of 350 °C (heated by the plasma and measured with a thermocouple mounted right behind the plate) without any additional catalyst using a 2.45 GHz MPECVD reactor manufactured by Plasmionique Inc. in a gas mixture of hydrogen (99.99%) and 1 vol.% methane (99.98%) with a total gas flow rate of 100 sccm. Fig. 1 shows a schematic diagram of the MPCVD system. The working gas pressure and the microwave power were maintained at 4 kPa and 1 kW, respectively. The growth duration was 22 h. A typical Transmission Electron Microscopic (TEM) image of the synthesized MWCNTs is shown in Fig. 2, revealing a long curved structure. The diameters of the MWCNTs range from a few nanometers to dozens of nanometers. All the P-type (100)-oriented mirror polished silicon wafers were ultrasonically scratched in a solution containing diamond powder (an average size of 1 μm) to create a surface for a high diamond nucleation density. Some of the scratched wafers were then coated with the MWCNTs described in Fig. 2. For that,
Fig. 2. Typical TEM image of the synthesized MWCNTs.
MWCNTs were firstly ultrasonically dispersed in methanol, and then the silicon wafers were put into the solution followed by air drying. Diamond nucleation and growth experiments on diamondscratched silicon wafers, with and without MWCNTs coatings, were simultaneously conducted in the MPECVD system. The vacuum chamber was pumped down to a pressure of 6.6 × 10− 4 Pa using a turbo-molecular pump and then filled with 99.99% pure hydrogen (H2) at a flow rate of 20 sccm. The working gas pressure and the microwave power were maintained at 4 kPa and 1 kW, respectively. The samples were only heated by plasma during the process. The substrate temperature during the deposition, typically at 520 °C, was measured with a thermocouple mounted right behind the substrate. The MWCNTs coated on silicon wafers were the sole carbon source in the system. Samples were treated in the MPECVD reactor for durations between 2 and 15 h. The treated samples were characterized by a JEOL 840A SEM and a Micro-Raman spectroscope. The Raman spectra were obtained using a Renishaw micro-Raman system 2000 spectrometers operated at a laser wavelength of 514.5 nm generated by an argon laser. The spot size was approximately 1 μm. 3. Results and discussions
Fig. 1. Schematic diagram of the MPCVD reactor.
Typical SEM micrographs and Raman spectra of the samples precoated with MWCNTs before and after 2 h hydrogen plasma treatment are shown in Figs. 3 and 4, respectively. The images are similar. The MWCNTs do not form continuous films but are locally distributed as fine-structured networks. No clear evidence for formation of the diamond particles can be found from the SEM image of samples after a 2 h treatment (Fig. 3b). Consistently, only two broad peaks centered at around 1350 cm− 1 (D-band) and 1590 cm− 1 (G-band), the signatures of the graphitic structure of carbon, appear in the Raman spectra of the samples after a 2 h treatment (Fig. 4b),
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Fig. 5(a) shows typical SEM micrographs of a MWCNT precoated sample after a 5 h treatment in the hydrogen plasma. In addition to the fine-structured MWCNTs networks, some newly formed nanometer sized particles are sparsely distributed on the whole surface, preferentially close to the carbon networks. Raman spectrum (Fig. 5b) taken from those newly formed particles reveals a clearly defined diamond peak around 1332 cm− 1, confirming that those particles are diamond in nature. Hence, it is reasonable to believe that diamond nucleation occurred on both the MWCNTs and the areas without MWCNTs coating. And the precoated MWCNTs served as the possible carbon source for diamond nucleation and growth on both sites. Furthermore, as seen from Fig. 5(a), the diamond particles nucleate and grow on silicon surface areas close to the MWCNTs, and the closer the area, the higher the particle density. After a 15 h hydrogen plasma treatment, the fine-structured networks disappeared, diamond particles with an average grain size of approximately 300 nm were distributed on the silicon surface with precoated MWCNTs (Fig. 6a). Diamond particles also emerge on silicon wafers without MWCNTs, which were placed beside the precoated MWCNTs, as confirmed by SEM observation (Fig. 6b) and Raman spectra (not shown here). If diamond formation is only controlled by the in-situ solid–solid phase transformation mechanism, it is clear that during the present experimental process, the original sites of MWCNTs should remain unchanged regardless of the length of the treatment time, and the diamond grains should not appear on the silicon surface without
Fig. 3. SEM micrographs of MWCNTs coated samples (a) before and (b) after a 2 h treatment in hydrogen plasma at 1000 W. The inset is the enlarged view of the indicated rectangular area.
indicating that little or no diamond formed after a short period (2 h or less) of hydrogen plasma treatment. Comparing with the Raman spectrum of the MWCNTs before treatment (Fig. 4a), the full width at half maximum (FWHM) of the D- and G-bands of the MWCNTs after the treatment is much bigger, suggesting that the treatment introduces defects into MWCNTs and decreases the graphitization degree of MWCNTs, consistent with previous reports [16–18].
Fig. 4. Typical Raman spectra of the MWCNTs coated samples (a) before and (b) after a 2 h hydrogen plasma treatment at 1000 W.
Fig. 5. (a) SEM micrographs of MWCNTs coated samples treated in the hydrogen plasma at 1000 W for 5 h, and (b) Raman spectra for the newly formed particles.
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reasonable to believe that the solid–gas–solid mechanism is involved in the transformation of CNTs to diamond in the hydrogen plasma.
4. Conclusion Silicon wafers with and without the precoated MWCNTs have been simultaneously treated in a pure hydrogen microwave plasma. Diamond has been formed on both kinds of silicon wafers with the consumption of MWCNTs. The results have demonstrated that the solid–gas–solid mechanism, similar to the diamond synthesis through hydrogen etching of graphite, is active in the transformation of CNTs to diamond in hydrogen plasma. In order to get more insight on the solid–gas–solid transformation mechanism, further examination of the intermediate MWCNTs morphologies using high resolution TEM in conjunction with gas phase chemistry analyses is being carried out and will be presented in a future report. Acknowledgments This work is supported by the Canada Research Chair Program and by the Natural Sciences and Engineering Research Council of Canada. References
Fig. 6. Typical SEM micrographs for silicon coated (a) with and (b) without MWCNTs after a 15 h hydrogen plasma treatment.
[1] [2] [3] [4] [5]
MWCNTs coatings. The fact that diamond has been grown on silicon wafers beside the MWCNTs and the framework of MWCNTs disappeared indicates that the solid–gas–solid mechanism, similar to diamond synthesis through hydrogen etching of graphite [7–10], should be also involved in the transformation in the present study. Furthermore, the solid–solid transformation mechanism is difficult to explain in the formation of diamond nanorods in Refs. [17,18]. Solid– gas–solid transformation process should be also involved in the growth of diamond nanorods through hydrogen plasma treatment of MWCNTs in their processes. Previous studies indicated that the major species contributing to diamond growth through the CVD process are methyl (CH3) radicals [19,20]. In the conventional diamond CVD process, the diluted mixture of methane (CH4) in hydrogen gas flows through the reactor chamber, and decomposes to form CH3 radicals and atomic H by hot filament or microwave plasma. It has long been known that hydrogen reacts with graphite at elevated temperatures to form a variety of hydrocarbon spices and radicals, including CH3 and CH4. Study using the modulated molecular beam-mass spectrometric method has shown that only methane and CH3 radicals are produced when atomic hydrogen reacts with graphite at temperatures below 527 °C [21]. In the present experiments and the experiments in Refs. [17,18], atomic hydrogen generated by microwave or radio-frequency plasma is much more reactive than molecular hydrogen and reacts with MWCNTs under the treatment conditions to form methane and CH3 radicals in the vicinity of the MWCNTs. Those that formed methane and CH3 radicals act as the precursors for diamond nucleation and growth. Hence, it is
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
F.P. Bundy, Science 137 (1962) 1057. F.P. Bundy, J. Chem. Phys. 38 (1963) 631. M.N. Regueiro, P. Monceau, J. Hodeau, Nature 355 (1992) 237. M.N. Regueiro, L. Abello, G. Lucazeau, J.L. Hodeau, Phys. Rev., B 46 (1992) 9903. D.M. Gruen, S.Z. Liu, A.R. Krauss, J.S. Luo, X.Z. Pan, Appl. Phys. Lett. 64 (1994) 1502. D.M. Gruen, S.Z. Liu, A.R. Krauss, X.Z. Pan, J. Appl. Phys. 75 (1994) 1758. S.D. Shin, N.M. Hwang, D.Y. Kim, Diamond Relat. Mater. 11 (2002) 1337. Q. Yang, W. Chen, C. Xiao, R. Sammynaiken, A. Hirose, Carbon 43 (2005) 748. Q. Yang, W. Chen, C. Xiao, A. Hirose, R. Sammynaiken, Diamond Relat. Mater. 14 (2005) 1683. Q. Yang, W. Chen, C. Xiao, A. Hirose, M. Bradley, Carbon 43 (2005) 2635. S. Iijima, Nature 354 (1991) 56. B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mater. Sci. Lett. 16 (1997) 402. B. Wei, J. Zhang, J. Liang, D. Wu, Carbon 36 (1998) 997. Y.Q. Zhu, T. Sekine, T. Kobayashi, E. Takazawa, M. Terrones, H. Terrones, Chem. Phys. Lett. 287 (1998) 689. F.M. Zhang, J. Shen, J.F. Sun, Y.Q. Zhu, G. Wang, G. McCartney, Carbon 43 (2005) 1254. L.T. Sun, J.L. Gong, Z.Y. Zhu, D.Z. Zhu, S.X. He, Z.X. Wang, et al., Appl. Phys. Lett. 84 (2004) 2901. L.T. Sun, J.L. Gong, Z.Y. Zhu, D.Z. Zhu, Z.X. Wang, W. Zhang, et al., Diamond Relat. Mater. 14 (2005) 749. L.T. Sun, J.L. Gong, D.Z. Zhu, Z.Y. Zhu, S.X. He, Adv. Mater. 16 (2004) 1849. W.A. Yarbrough, K. Tankala, T. Debroy, J. Mater. Res. 7 (1992) 379. M. Tsuda, M. Nakajima, S. Oikawa, J. Am. Chem. Soc. 108 (1986) 5780. M. Balooch, D.R. Olander, J. Chem. Phys. 63 (1975) 4772.
Transformation of carbon nanotubes to diamond in microwave hydrogen plasma Qiaoqin Yang a,⁎, Songlan Yang a , Chijin Xiao b , Akira Hirose b a
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9 b Plasma Physics laboratory, University of Saskatchewan, 116 Science Place, Saskatoon, SK, Canada S7N 5E2 Received 17 April 2006; accepted 24 August 2006 Available online 15 September 2006
Abstract Multiwall carbon nanotubes (MWCNTs), synthesized by microwave plasma enhanced chemical vapor deposition, were used as the precursor to synthesize diamond in a pure hydrogen microwave discharge. Diamond-scratched silicon wafers, with and without precoated MWCNTs, were placed side by side on a substrate holder. Diamond was formed on both wafers with the consumption of MWCNTs. The present results suggest that the solid–gas–solid transformation mechanism is involved in this process. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond; Carbon nanotubes; Microwave hydrogen plasma
1. Introduction Diamond is by far the hardest known material and has many unique and beneficial properties: it has the lowest coefficient of thermal expansion, is chemically inert, offers the lowest friction coefficient and the best wear resistance, has the highest thermal conductivity, and exhibits excellent biocompatibility. The combination of these extreme properties makes diamond an ideal material for many applications. However, natural diamond is scarce and costly. This has motivated scientists for more than a century to explore the diamond synthesis process using various techniques with different starting carbon sources. Since the first successful synthesis of diamond through direct transformation of graphite at high pressures (N 12.5 GPa) and high temperatures (N3000 °C) [1,2], various carbon allotropes have been used as the precursors for diamond synthesis. It has been synthesized by squeezing fullerene in a diamond-anvil cell at a pressure of approximately 20 GPa at room temperature [3,4], and by generating microwave discharges in the C60-containing argon gas [5,6], by hydrogen treatment of graphite in a hot filament [7–9] or a microwave plasma enhanced chemical vapor deposition ⁎ Corresponding author. Tel.: +1 306 966 5470; fax: +1 306 966 5427. E-mail address: [email protected] (Q. Yang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.060
(MPECVD) reactor [10]. Since the discovery of CNTs in 1991 [11], many techniques have also been tried to synthesize diamond from CNTs. These include laser irradiation [12,13], shock waves [14], spark plasma sintering [15], and radio-frequency hydrogen plasma [16–18]. In-situ defects-controlled solid–solid phase transformation mechanism, in which the defects are the original sites for nucleation and growth of the diamond particles, has been proposed for the transformation of CNTs to diamond in the radiofrequency hydrogen plasma [16–18], whereas the transformation of graphite to diamond by hydrogen treatment of graphite in the hot filament or microwave plasma reactor is believed to be a solid–gas–solid process [8–10]: the graphite (solid) is etched by an atomic hydrogen generated by microwave plasma or hot filament to form hydrocarbon radicals (gas), which act as the precursors for diamond (solid) growth, similar to diamond growth from the gas mixture of methane and hydrogen. There are many experimental similarities between the works in Refs. [8–10] using graphite as the precursor and those in Refs. [16–18] using CNTs as the precursor. Both CNTs and graphite contain sp2-bonded structures and served as carbon sources. In all the processes, only pure hydrogen was admitted into the reactor. Thus it is desirable to investigate whether a solid–gas–solid process is also involved in the formation of
Q. Yang et al. / Materials Letters 61 (2007) 2208–2211
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diamond through the hydrogen plasma treatment of CNTs. For this purpose, the dispersed MWCNTs were precoated on the surface of the silicon substrates, similar to the process described by others [16–18], and treated in a pure hydrogen plasma in a MPECVD reactor together with the silicon wafers without MWCNTs. A Scanning Electron Microscope (SEM) and a Raman Spectroscope were employed to analyze the experimental results. 2. Experimental details The MWCNTs used in these experiments were synthesized on an inconel 600 plate at a temperature of 350 °C (heated by the plasma and measured with a thermocouple mounted right behind the plate) without any additional catalyst using a 2.45 GHz MPECVD reactor manufactured by Plasmionique Inc. in a gas mixture of hydrogen (99.99%) and 1 vol.% methane (99.98%) with a total gas flow rate of 100 sccm. Fig. 1 shows a schematic diagram of the MPCVD system. The working gas pressure and the microwave power were maintained at 4 kPa and 1 kW, respectively. The growth duration was 22 h. A typical Transmission Electron Microscopic (TEM) image of the synthesized MWCNTs is shown in Fig. 2, revealing a long curved structure. The diameters of the MWCNTs range from a few nanometers to dozens of nanometers. All the P-type (100)-oriented mirror polished silicon wafers were ultrasonically scratched in a solution containing diamond powder (an average size of 1 μm) to create a surface for a high diamond nucleation density. Some of the scratched wafers were then coated with the MWCNTs described in Fig. 2. For that,
Fig. 2. Typical TEM image of the synthesized MWCNTs.
MWCNTs were firstly ultrasonically dispersed in methanol, and then the silicon wafers were put into the solution followed by air drying. Diamond nucleation and growth experiments on diamondscratched silicon wafers, with and without MWCNTs coatings, were simultaneously conducted in the MPECVD system. The vacuum chamber was pumped down to a pressure of 6.6 × 10− 4 Pa using a turbo-molecular pump and then filled with 99.99% pure hydrogen (H2) at a flow rate of 20 sccm. The working gas pressure and the microwave power were maintained at 4 kPa and 1 kW, respectively. The samples were only heated by plasma during the process. The substrate temperature during the deposition, typically at 520 °C, was measured with a thermocouple mounted right behind the substrate. The MWCNTs coated on silicon wafers were the sole carbon source in the system. Samples were treated in the MPECVD reactor for durations between 2 and 15 h. The treated samples were characterized by a JEOL 840A SEM and a Micro-Raman spectroscope. The Raman spectra were obtained using a Renishaw micro-Raman system 2000 spectrometers operated at a laser wavelength of 514.5 nm generated by an argon laser. The spot size was approximately 1 μm. 3. Results and discussions
Fig. 1. Schematic diagram of the MPCVD reactor.
Typical SEM micrographs and Raman spectra of the samples precoated with MWCNTs before and after 2 h hydrogen plasma treatment are shown in Figs. 3 and 4, respectively. The images are similar. The MWCNTs do not form continuous films but are locally distributed as fine-structured networks. No clear evidence for formation of the diamond particles can be found from the SEM image of samples after a 2 h treatment (Fig. 3b). Consistently, only two broad peaks centered at around 1350 cm− 1 (D-band) and 1590 cm− 1 (G-band), the signatures of the graphitic structure of carbon, appear in the Raman spectra of the samples after a 2 h treatment (Fig. 4b),
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Fig. 5(a) shows typical SEM micrographs of a MWCNT precoated sample after a 5 h treatment in the hydrogen plasma. In addition to the fine-structured MWCNTs networks, some newly formed nanometer sized particles are sparsely distributed on the whole surface, preferentially close to the carbon networks. Raman spectrum (Fig. 5b) taken from those newly formed particles reveals a clearly defined diamond peak around 1332 cm− 1, confirming that those particles are diamond in nature. Hence, it is reasonable to believe that diamond nucleation occurred on both the MWCNTs and the areas without MWCNTs coating. And the precoated MWCNTs served as the possible carbon source for diamond nucleation and growth on both sites. Furthermore, as seen from Fig. 5(a), the diamond particles nucleate and grow on silicon surface areas close to the MWCNTs, and the closer the area, the higher the particle density. After a 15 h hydrogen plasma treatment, the fine-structured networks disappeared, diamond particles with an average grain size of approximately 300 nm were distributed on the silicon surface with precoated MWCNTs (Fig. 6a). Diamond particles also emerge on silicon wafers without MWCNTs, which were placed beside the precoated MWCNTs, as confirmed by SEM observation (Fig. 6b) and Raman spectra (not shown here). If diamond formation is only controlled by the in-situ solid–solid phase transformation mechanism, it is clear that during the present experimental process, the original sites of MWCNTs should remain unchanged regardless of the length of the treatment time, and the diamond grains should not appear on the silicon surface without
Fig. 3. SEM micrographs of MWCNTs coated samples (a) before and (b) after a 2 h treatment in hydrogen plasma at 1000 W. The inset is the enlarged view of the indicated rectangular area.
indicating that little or no diamond formed after a short period (2 h or less) of hydrogen plasma treatment. Comparing with the Raman spectrum of the MWCNTs before treatment (Fig. 4a), the full width at half maximum (FWHM) of the D- and G-bands of the MWCNTs after the treatment is much bigger, suggesting that the treatment introduces defects into MWCNTs and decreases the graphitization degree of MWCNTs, consistent with previous reports [16–18].
Fig. 4. Typical Raman spectra of the MWCNTs coated samples (a) before and (b) after a 2 h hydrogen plasma treatment at 1000 W.
Fig. 5. (a) SEM micrographs of MWCNTs coated samples treated in the hydrogen plasma at 1000 W for 5 h, and (b) Raman spectra for the newly formed particles.
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reasonable to believe that the solid–gas–solid mechanism is involved in the transformation of CNTs to diamond in the hydrogen plasma.
4. Conclusion Silicon wafers with and without the precoated MWCNTs have been simultaneously treated in a pure hydrogen microwave plasma. Diamond has been formed on both kinds of silicon wafers with the consumption of MWCNTs. The results have demonstrated that the solid–gas–solid mechanism, similar to the diamond synthesis through hydrogen etching of graphite, is active in the transformation of CNTs to diamond in hydrogen plasma. In order to get more insight on the solid–gas–solid transformation mechanism, further examination of the intermediate MWCNTs morphologies using high resolution TEM in conjunction with gas phase chemistry analyses is being carried out and will be presented in a future report. Acknowledgments This work is supported by the Canada Research Chair Program and by the Natural Sciences and Engineering Research Council of Canada. References
Fig. 6. Typical SEM micrographs for silicon coated (a) with and (b) without MWCNTs after a 15 h hydrogen plasma treatment.
[1] [2] [3] [4] [5]
MWCNTs coatings. The fact that diamond has been grown on silicon wafers beside the MWCNTs and the framework of MWCNTs disappeared indicates that the solid–gas–solid mechanism, similar to diamond synthesis through hydrogen etching of graphite [7–10], should be also involved in the transformation in the present study. Furthermore, the solid–solid transformation mechanism is difficult to explain in the formation of diamond nanorods in Refs. [17,18]. Solid– gas–solid transformation process should be also involved in the growth of diamond nanorods through hydrogen plasma treatment of MWCNTs in their processes. Previous studies indicated that the major species contributing to diamond growth through the CVD process are methyl (CH3) radicals [19,20]. In the conventional diamond CVD process, the diluted mixture of methane (CH4) in hydrogen gas flows through the reactor chamber, and decomposes to form CH3 radicals and atomic H by hot filament or microwave plasma. It has long been known that hydrogen reacts with graphite at elevated temperatures to form a variety of hydrocarbon spices and radicals, including CH3 and CH4. Study using the modulated molecular beam-mass spectrometric method has shown that only methane and CH3 radicals are produced when atomic hydrogen reacts with graphite at temperatures below 527 °C [21]. In the present experiments and the experiments in Refs. [17,18], atomic hydrogen generated by microwave or radio-frequency plasma is much more reactive than molecular hydrogen and reacts with MWCNTs under the treatment conditions to form methane and CH3 radicals in the vicinity of the MWCNTs. Those that formed methane and CH3 radicals act as the precursors for diamond nucleation and growth. Hence, it is
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
F.P. Bundy, Science 137 (1962) 1057. F.P. Bundy, J. Chem. Phys. 38 (1963) 631. M.N. Regueiro, P. Monceau, J. Hodeau, Nature 355 (1992) 237. M.N. Regueiro, L. Abello, G. Lucazeau, J.L. Hodeau, Phys. Rev., B 46 (1992) 9903. D.M. Gruen, S.Z. Liu, A.R. Krauss, J.S. Luo, X.Z. Pan, Appl. Phys. Lett. 64 (1994) 1502. D.M. Gruen, S.Z. Liu, A.R. Krauss, X.Z. Pan, J. Appl. Phys. 75 (1994) 1758. S.D. Shin, N.M. Hwang, D.Y. Kim, Diamond Relat. Mater. 11 (2002) 1337. Q. Yang, W. Chen, C. Xiao, R. Sammynaiken, A. Hirose, Carbon 43 (2005) 748. Q. Yang, W. Chen, C. Xiao, A. Hirose, R. Sammynaiken, Diamond Relat. Mater. 14 (2005) 1683. Q. Yang, W. Chen, C. Xiao, A. Hirose, M. Bradley, Carbon 43 (2005) 2635. S. Iijima, Nature 354 (1991) 56. B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mater. Sci. Lett. 16 (1997) 402. B. Wei, J. Zhang, J. Liang, D. Wu, Carbon 36 (1998) 997. Y.Q. Zhu, T. Sekine, T. Kobayashi, E. Takazawa, M. Terrones, H. Terrones, Chem. Phys. Lett. 287 (1998) 689. F.M. Zhang, J. Shen, J.F. Sun, Y.Q. Zhu, G. Wang, G. McCartney, Carbon 43 (2005) 1254. L.T. Sun, J.L. Gong, Z.Y. Zhu, D.Z. Zhu, S.X. He, Z.X. Wang, et al., Appl. Phys. Lett. 84 (2004) 2901. L.T. Sun, J.L. Gong, Z.Y. Zhu, D.Z. Zhu, Z.X. Wang, W. Zhang, et al., Diamond Relat. Mater. 14 (2005) 749. L.T. Sun, J.L. Gong, D.Z. Zhu, Z.Y. Zhu, S.X. He, Adv. Mater. 16 (2004) 1849. W.A. Yarbrough, K. Tankala, T. Debroy, J. Mater. Res. 7 (1992) 379. M. Tsuda, M. Nakajima, S. Oikawa, J. Am. Chem. Soc. 108 (1986) 5780. M. Balooch, D.R. Olander, J. Chem. Phys. 63 (1975) 4772.