- Email: [email protected]
Laser irradiation of carbon nanotubes
Materials Chemistry and Physics 72 (2001) 218–222
Laser irradiation of carbon nanotubes P.D. Kichambare a , L.C. Chen a,∗ , C.T. Wang b , K.J. Ma b , C.T. Wu c , K.H. Chen c b
a Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan Department of Mechanical Engineering, Chung-Cheng Institute of Technology, Taoyan, Taiwan c Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
Abstract We report on the structural modifications of carbon nanotubes (CNT) by laser irradiation. In particular, we investigated the transformation of CNT by using Nd:YAG laser operating at 266 and 1064 nm with different energy fluences and number of pulses. After 15 pulses of 266 nm laser irradiation with a laser fluence of 80 and 60 mJ cm−2 , transformation of CNT into sub-micron size plates and cauliflower type aggregation of carbon deposits, respectively, were observed. At a fixed laser fluence of 400 mJ cm−2 (1064 nm) transformation of CNT into carbon mass protrusions with progressive sharper aspect ratio was obtained by increasing number of laser pulses. Our investigations demonstrate that a selective area transformation can be controlled by the laser fluence and number of pulses. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Plasma-assisted CVD; Laser irradiation; Auger electron spectroscopy (AES)
1. Introduction The discovery of carbon nanotubes (CNT) [1] has triggered very intense attention because of their remarkable properties and potential applications ranging from electronic devices [2,3] to catalytic supports for reactions in fuel cells [4,5], advanced AFM tips [6], natural gas storage [7], topographical imaging and molecular manipulation [8]. The behavior of CNT at high temperature in air was studied for reinforcing fiber in composites. The opening of the caps, thinning of nanotubes, and pit formation on the surface of nanotubes were observed during the gasification of CNT in air [9,10], in an oxygen steam [11] or under a flow of carbon dioxide [12] over a limited temperature range and time period. Recently transformation of CNT to nanoparticles and diamond by ball milling process [13] and laser irradiation [14], respectively, has been reported. In the latter report, a CO2 continuous laser was employed to irradiate the iron surface coated with nanotubes in Ar atmosphere [14]. After irradiation, these CNT were heated in a muffle furnace and finally quenched in methanol and ice mixture. The diamond appears only after quenching in the laser irradiated sample whereas direct transformation to diamond-like C60 has been reported, suggesting that there is an intermediate phase between CNT and diamond formed by laser irradiation. ∗ Corresponding author. Tel.: +886-2-2366-8228; fax: +886-2-2362-0200. E-mail address: [email protected] (L.C. Chen).
Meanwhile the laser processing is emerging as a powerful tool to study the structural modification of CNT and to alter the size of nanoparticle of metals and semiconductors. We report an approach to transform CNT to sub-micron size plates and cauliflower type aggregation of carbon deposits by laser processing which has an advantage of selective area structure modification without causing substantial contamination. In this work, the CNT before and after laser irradiation were analyzed by high resolution scanning electron microscopy (HRSEM), micro-Raman spectroscopy and Auger electron spectroscopy (AES). We further studied the role of the laser fluence for different wavelength and number of pulses on such structural transformation of CNT.
2. Experimental details In the present study, the CNT films were grown by microwave plasma chemical vapor deposition (MWPCVD) method on iron coated silicon substrates (with Fe film of 300 Å thick). The coating of such an iron film is necessary as Fe helps for both initial nucleation and growth of CNT. These substrates were first treated under hydrogen plasma for 10 min to activate the surface of iron. The growth of nanotubes were performed in an AsTex 5 kW microwave reactor using a mixture of semiconductor grade CH4 , H2 , N2 and SiH4 as source gases with typical flow rates of 20, 80, 80 and 4 sccm, respectively. For all deposition, the chamber pressure was maintained at 25 Torr, while the microwave
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 4 4 0 - 0
P.D. Kichambare et al. / Materials Chemistry and Physics 72 (2001) 218–222
power was kept at 1.0 kW and the substrate temperature was kept at 750◦ C for 120 min during the growth process. The CNT prepared in this way were used for laser irradiation. The detail procedure of preparation of these CNT will be published elsewhere. The experiments for laser processing were performed using excimer laser operating at 193 nm (the laser fluences 30 and 50 mJ cm−2 ), Nd:YAG lasers operating at 266 nm (the laser fluences 30, 60 and 80 mJ cm−2 ) and 1064 nm (the laser fluences 80 and 400 mJ cm−2 ) wavelengths. The samples were exposed to 1,5, 10, and 15 successive laser pulses. The morphology and chemical composition of the films were further analyzed by HRSEM and AES. A Perkin-Elmer scanning Auger nanoprobe system (SAN 760) using Ar ion beam with typical beam energy of 3 and 4 keV has been used for AES investigation. The typical probe size used was of order of few microns, while the system was capable of acquiring depth profile data from an area as small as 500 Å. The Raman spectra were recorded on a Renishaw system 2000 micro-Raman spectrometer with a 25 mW He–Ne laser (632.8 nm wavelength) as excitation source. With a 5 m opening of the exit slit, the Raman spectral resolution was better than ±1 cm−1 .
219
nanotubes are evenly deposited over the total surface of substrate and have quite a uniform diameter of about 85 nm; while the typical length is well beyond 10 m thereby resulting in a high aspect ratio. No structural transformations were observed below the threshold fluence of 30 mJ cm−2 . The HRSEM images of laser processed surfaces of the nanotubes showed that the surface morphologies depended on the laser fluence as well as number of pulses and laser wavelength. The HRSEM image of these CNT films processed by a laser beam (266 nm) with the laser fluence of 30, 60 and 80 mJ cm−2 and for 15 pulses are shown in Fig. 2a–c, respectively. When the laser fluence was 30 mJ cm−2 , the bundles of some tubes became thicker
3. Results and discussion Fig. 1 illustrates surface morphology of the as-grown CNT films. A noteworthy feature of this micrograph is that these
Fig. 1. SEM image of CNT of uniform diameter grown on silicon substrate by microwave plasma enhanced chemical vapor deposition.
Fig. 2. High resolution scanning electron microscope images of CNT surface after laser (266 nm) processing with a laser fluence of (a) 30 mJ cm−2 , (b) 60 mJ cm−2 and (c) 80 mJ cm−2 . All of the samples were irradiated for 15 pulses.
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along with some formation of granular carbonaceous mass (Fig. 2a). Bending of CNT with large curvature was also found, presumably caused by uneven shrinkage of the nanotubes. When the sample was irradiated by laser with an energy fluence of 60 mJ cm−2 , cauliflower type aggregation of carbonaceous deposits has occurred (Fig. 2b). When the laser fluence was 80 mJ cm−2 , complete conversion and formation of well-defined carbonaceous plate of 4 m in width and 7 m in length (Fig. 2c) has been observed. For a fixed laser fluence of 80 mJ cm−2 and with the number of pulses decreased to 10, then only tiny plates of 0.5 m in width and 1 m in length were observed. With further decrease in number of pulses to 5 and 1, an aggregation of granular carbonaceous mass and bending of CNT were observed. When CNT were processed by a laser beam of 1064 nm with the laser fluence of 400 mJ cm−2 , the carbon mass protrusions with progressively higher aspect ratio was obtained. When the sample was irradiated with 15 pulses, the sharp horn shape peaks (11 m in width and 17 m in height) were observed as shown in Fig. 3a. As the number of pulses decreased to 10, the horn shape structures were reduced to a hill shape structures (7 m in width and 4 m in height) as depicted in Fig. 3b. With further decrease in number of pulses to 5 and 1, the cauliflower type aggregation of carbon deposit (Fig. 3c) and bending of CNT with little curvature were observed. While with the laser wavelength of 193 nm with a fluence of 30 and 50 mJ cm−2 and 15 pulses, a tiny and large lump of carbonaceous mass, respectively, was observed. To determine the chemical composition of an individual nanotube or a carbonaceous plate, scanning Auger spectroscopy with depth profiling was used. A typical Auger spectrum (not shown here) taken on the CNT before laser irradiation indicated the presence of C (91.29 at.%), Si (2.42 at.%), N (4.34 at.%) and O (1.95 at.%) . While C (86.07 at.%), Si (1.55 at.%), N (3.24 at.%) and O (9.14 at.%) were present in carbonaceous plate formed after laser irradiation of CNT. The peak position around 265 eV in Auger spectrum indicates that the carbonaceous mass is an amorphous carbon [15]. An increase in at.% of O signal of laser processed carbonaceous mass suggests that oxygen must has been absorbed from atmosphere during laser induced transformation of CNT. Fig. 4a–d depicts the Raman spectra of pure CNT, nanotubes irradiated by laser (266 nm) fluence of 30, 60 and 80 mJ cm−2 , respectively. Despite the differences in the apparent microstructure and morphology, all spectra show the peaks at 1350 and 1570 cm−1 representing the characteristics of graphitic carbon. In addition to these peaks, pure CNT also exhibit a strong and wide peak at 2700 cm−1 , which is attributed to the disorder induced by tube curvature [16]. After laser irradiation of nanotubes with 30, 60 and 80 mJ cm−2 laser fluences, the intensity of this peak has reduced. There is a slight upshift of G line towards higher frequency with increase in the laser fluence indicating thereby the films treated with laser irradiation are dominated by trihedral bonding and formation of nano-size graphitic
Fig. 3. High resolution scanning electron microscope images of CNT surface after laser (1064 nm) processing with a laser fluence of 400 mJ cm−2 , and number of pulses (a) 15, (b) 10 and (c) 5.
crystal [17]. On the other hand, the D line shift towards lower wave number indicating thereby the change in bonding due to these structural transformation. It is therefore clear from these observations that the higher the laser beam intensity, the more the original graphite structure is decomposed, resulting in the appearance of an amorphous carbon. The results described above indicate that the laser fluence controls the structure of CNT, resulting in formation of various carbon materials. This can be explained
P.D. Kichambare et al. / Materials Chemistry and Physics 72 (2001) 218–222
221
Fig. 4. Raman spectra of (a) pure CNT, nanotube ablated by laser beam 266 nm with a laser fluence (b) 30 mJ cm−2 , (c) 60 mJ cm−2 and (d) 80 mJ cm−2 for 15 pulses.
straightforwardly as follows: when CNT were subjected to the laser irradiation (266 nm) of least fluence of 30 mJ cm−2 , carbon atoms absorb energy and initiate the oxidation at defect sites thereby breaking the C–C bonds in various layers of CNT. A gradual increase in the laser fluence (60 mJ cm−2 ) enhances an interlayer and intertube interaction and the structure of CNT collapsed into cauliflower type aggregation of carbonaceous deposits. Further increase in laser fluence (80 mJ cm−2 ) increases the temperature of surface of CNT within the laser irradiated area to very high. Similarly, an increase in the number of pulses also increases an accumulation of energy in CNT irradiated matrix. This creates a high flux of carbon atoms and ions. The subsequent removal of laser beam at this stage initiates the condensation of carbon atoms and ions leading to the formation of well-defined carbonaceous plates. The formation of small carbon clusters that grow with increasing laser fluence suggests a substantial mass transport between gaseous and solid phases. However, in case of higher laser fluence (1064 nm, 400 mJ cm−2 ), the temperature of the surface of CNT within the laser irradiated area reaches a much higher temperature. This sudden elevation of local temperature leads to the formation of larger aggregation of carbon because of its higher fluidity. The detail mechanism for such phase transformation is currently under investigation.
Our studies also point out the role of the laser fluence and number of pulses as the effective parameters for such transformation.
Acknowledgements Financial support for this work from the National Research Council of Taiwan, through grant number NSC 88-2112-M-002-022 and NSC 89-2112-M-002-047 and the Ministry of Education of ROC through grant number 89-N-FA01-2-4-5, is gratefully acknowledged. We would like to thank Prof. Y.T. Chen for providing laser facilities to carry the part of this work and the fruitful discussion. References [1] [2] [3] [4]
[5] [6] [7]
4. Conclusions
[8]
In conclusion, our investigation demonstrates the potential of laser irradiation for transformation of CNT into submicron size plates and an aggregation of carbon deposits that may not be feasible by conventional thermal heating.
[9] [10] [11] [12]
S. Iijima, Nature 354 (1991) 56. M.S. Dresselhaus, Nature 391 (1998) 19. C.T. White, T.N. Todorov, Nature 393 (1998) 240. J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutartre, P. Genestle, P. Bernier, P.M. Ajayan, J. Am. Chem. Soc. 116 (1999) 7935. G. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346. H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. S.S. Wong, E. Joselevich, A.T. Woolley, C.L. Cheung, C.M. Lieber, Nature 394 (1998) 52. S.C. Tsang, Z. Guo, Y.K. Chen, M.L.H. Green, H.A.O. Hill, T.W. Hambley, P. Sadler, J. Angew Chem. Int. Ed. Engl. 36 (1997) 2197. P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Nature 362 (1993) 522. N. Yao, V. Lordi, S.X.C. Ma, J. Mater. Res. 13 (1998) 2432. S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520. D.T. Colbert, Science 266 (1994) 1218.
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[13] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493. [14] B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mater. Sci. Lett. 16 (1997) 402. [15] Y.T. Jan, H.C. Hsieh, C.F. Chen, Diam. Relat. Mater. 8 (1999) 772.
[16] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.W. Williams, M. Menon, K.R. Subbaswamy, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275 (1997) 187. [17] Y.G. Gogotsi, M. Yoshimura, Nature 367 (1994) 628.
Laser irradiation of carbon nanotubes P.D. Kichambare a , L.C. Chen a,∗ , C.T. Wang b , K.J. Ma b , C.T. Wu c , K.H. Chen c b
a Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan Department of Mechanical Engineering, Chung-Cheng Institute of Technology, Taoyan, Taiwan c Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
Abstract We report on the structural modifications of carbon nanotubes (CNT) by laser irradiation. In particular, we investigated the transformation of CNT by using Nd:YAG laser operating at 266 and 1064 nm with different energy fluences and number of pulses. After 15 pulses of 266 nm laser irradiation with a laser fluence of 80 and 60 mJ cm−2 , transformation of CNT into sub-micron size plates and cauliflower type aggregation of carbon deposits, respectively, were observed. At a fixed laser fluence of 400 mJ cm−2 (1064 nm) transformation of CNT into carbon mass protrusions with progressive sharper aspect ratio was obtained by increasing number of laser pulses. Our investigations demonstrate that a selective area transformation can be controlled by the laser fluence and number of pulses. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Plasma-assisted CVD; Laser irradiation; Auger electron spectroscopy (AES)
1. Introduction The discovery of carbon nanotubes (CNT) [1] has triggered very intense attention because of their remarkable properties and potential applications ranging from electronic devices [2,3] to catalytic supports for reactions in fuel cells [4,5], advanced AFM tips [6], natural gas storage [7], topographical imaging and molecular manipulation [8]. The behavior of CNT at high temperature in air was studied for reinforcing fiber in composites. The opening of the caps, thinning of nanotubes, and pit formation on the surface of nanotubes were observed during the gasification of CNT in air [9,10], in an oxygen steam [11] or under a flow of carbon dioxide [12] over a limited temperature range and time period. Recently transformation of CNT to nanoparticles and diamond by ball milling process [13] and laser irradiation [14], respectively, has been reported. In the latter report, a CO2 continuous laser was employed to irradiate the iron surface coated with nanotubes in Ar atmosphere [14]. After irradiation, these CNT were heated in a muffle furnace and finally quenched in methanol and ice mixture. The diamond appears only after quenching in the laser irradiated sample whereas direct transformation to diamond-like C60 has been reported, suggesting that there is an intermediate phase between CNT and diamond formed by laser irradiation. ∗ Corresponding author. Tel.: +886-2-2366-8228; fax: +886-2-2362-0200. E-mail address: [email protected] (L.C. Chen).
Meanwhile the laser processing is emerging as a powerful tool to study the structural modification of CNT and to alter the size of nanoparticle of metals and semiconductors. We report an approach to transform CNT to sub-micron size plates and cauliflower type aggregation of carbon deposits by laser processing which has an advantage of selective area structure modification without causing substantial contamination. In this work, the CNT before and after laser irradiation were analyzed by high resolution scanning electron microscopy (HRSEM), micro-Raman spectroscopy and Auger electron spectroscopy (AES). We further studied the role of the laser fluence for different wavelength and number of pulses on such structural transformation of CNT.
2. Experimental details In the present study, the CNT films were grown by microwave plasma chemical vapor deposition (MWPCVD) method on iron coated silicon substrates (with Fe film of 300 Å thick). The coating of such an iron film is necessary as Fe helps for both initial nucleation and growth of CNT. These substrates were first treated under hydrogen plasma for 10 min to activate the surface of iron. The growth of nanotubes were performed in an AsTex 5 kW microwave reactor using a mixture of semiconductor grade CH4 , H2 , N2 and SiH4 as source gases with typical flow rates of 20, 80, 80 and 4 sccm, respectively. For all deposition, the chamber pressure was maintained at 25 Torr, while the microwave
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 4 4 0 - 0
P.D. Kichambare et al. / Materials Chemistry and Physics 72 (2001) 218–222
power was kept at 1.0 kW and the substrate temperature was kept at 750◦ C for 120 min during the growth process. The CNT prepared in this way were used for laser irradiation. The detail procedure of preparation of these CNT will be published elsewhere. The experiments for laser processing were performed using excimer laser operating at 193 nm (the laser fluences 30 and 50 mJ cm−2 ), Nd:YAG lasers operating at 266 nm (the laser fluences 30, 60 and 80 mJ cm−2 ) and 1064 nm (the laser fluences 80 and 400 mJ cm−2 ) wavelengths. The samples were exposed to 1,5, 10, and 15 successive laser pulses. The morphology and chemical composition of the films were further analyzed by HRSEM and AES. A Perkin-Elmer scanning Auger nanoprobe system (SAN 760) using Ar ion beam with typical beam energy of 3 and 4 keV has been used for AES investigation. The typical probe size used was of order of few microns, while the system was capable of acquiring depth profile data from an area as small as 500 Å. The Raman spectra were recorded on a Renishaw system 2000 micro-Raman spectrometer with a 25 mW He–Ne laser (632.8 nm wavelength) as excitation source. With a 5 m opening of the exit slit, the Raman spectral resolution was better than ±1 cm−1 .
219
nanotubes are evenly deposited over the total surface of substrate and have quite a uniform diameter of about 85 nm; while the typical length is well beyond 10 m thereby resulting in a high aspect ratio. No structural transformations were observed below the threshold fluence of 30 mJ cm−2 . The HRSEM images of laser processed surfaces of the nanotubes showed that the surface morphologies depended on the laser fluence as well as number of pulses and laser wavelength. The HRSEM image of these CNT films processed by a laser beam (266 nm) with the laser fluence of 30, 60 and 80 mJ cm−2 and for 15 pulses are shown in Fig. 2a–c, respectively. When the laser fluence was 30 mJ cm−2 , the bundles of some tubes became thicker
3. Results and discussion Fig. 1 illustrates surface morphology of the as-grown CNT films. A noteworthy feature of this micrograph is that these
Fig. 1. SEM image of CNT of uniform diameter grown on silicon substrate by microwave plasma enhanced chemical vapor deposition.
Fig. 2. High resolution scanning electron microscope images of CNT surface after laser (266 nm) processing with a laser fluence of (a) 30 mJ cm−2 , (b) 60 mJ cm−2 and (c) 80 mJ cm−2 . All of the samples were irradiated for 15 pulses.
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along with some formation of granular carbonaceous mass (Fig. 2a). Bending of CNT with large curvature was also found, presumably caused by uneven shrinkage of the nanotubes. When the sample was irradiated by laser with an energy fluence of 60 mJ cm−2 , cauliflower type aggregation of carbonaceous deposits has occurred (Fig. 2b). When the laser fluence was 80 mJ cm−2 , complete conversion and formation of well-defined carbonaceous plate of 4 m in width and 7 m in length (Fig. 2c) has been observed. For a fixed laser fluence of 80 mJ cm−2 and with the number of pulses decreased to 10, then only tiny plates of 0.5 m in width and 1 m in length were observed. With further decrease in number of pulses to 5 and 1, an aggregation of granular carbonaceous mass and bending of CNT were observed. When CNT were processed by a laser beam of 1064 nm with the laser fluence of 400 mJ cm−2 , the carbon mass protrusions with progressively higher aspect ratio was obtained. When the sample was irradiated with 15 pulses, the sharp horn shape peaks (11 m in width and 17 m in height) were observed as shown in Fig. 3a. As the number of pulses decreased to 10, the horn shape structures were reduced to a hill shape structures (7 m in width and 4 m in height) as depicted in Fig. 3b. With further decrease in number of pulses to 5 and 1, the cauliflower type aggregation of carbon deposit (Fig. 3c) and bending of CNT with little curvature were observed. While with the laser wavelength of 193 nm with a fluence of 30 and 50 mJ cm−2 and 15 pulses, a tiny and large lump of carbonaceous mass, respectively, was observed. To determine the chemical composition of an individual nanotube or a carbonaceous plate, scanning Auger spectroscopy with depth profiling was used. A typical Auger spectrum (not shown here) taken on the CNT before laser irradiation indicated the presence of C (91.29 at.%), Si (2.42 at.%), N (4.34 at.%) and O (1.95 at.%) . While C (86.07 at.%), Si (1.55 at.%), N (3.24 at.%) and O (9.14 at.%) were present in carbonaceous plate formed after laser irradiation of CNT. The peak position around 265 eV in Auger spectrum indicates that the carbonaceous mass is an amorphous carbon [15]. An increase in at.% of O signal of laser processed carbonaceous mass suggests that oxygen must has been absorbed from atmosphere during laser induced transformation of CNT. Fig. 4a–d depicts the Raman spectra of pure CNT, nanotubes irradiated by laser (266 nm) fluence of 30, 60 and 80 mJ cm−2 , respectively. Despite the differences in the apparent microstructure and morphology, all spectra show the peaks at 1350 and 1570 cm−1 representing the characteristics of graphitic carbon. In addition to these peaks, pure CNT also exhibit a strong and wide peak at 2700 cm−1 , which is attributed to the disorder induced by tube curvature [16]. After laser irradiation of nanotubes with 30, 60 and 80 mJ cm−2 laser fluences, the intensity of this peak has reduced. There is a slight upshift of G line towards higher frequency with increase in the laser fluence indicating thereby the films treated with laser irradiation are dominated by trihedral bonding and formation of nano-size graphitic
Fig. 3. High resolution scanning electron microscope images of CNT surface after laser (1064 nm) processing with a laser fluence of 400 mJ cm−2 , and number of pulses (a) 15, (b) 10 and (c) 5.
crystal [17]. On the other hand, the D line shift towards lower wave number indicating thereby the change in bonding due to these structural transformation. It is therefore clear from these observations that the higher the laser beam intensity, the more the original graphite structure is decomposed, resulting in the appearance of an amorphous carbon. The results described above indicate that the laser fluence controls the structure of CNT, resulting in formation of various carbon materials. This can be explained
P.D. Kichambare et al. / Materials Chemistry and Physics 72 (2001) 218–222
221
Fig. 4. Raman spectra of (a) pure CNT, nanotube ablated by laser beam 266 nm with a laser fluence (b) 30 mJ cm−2 , (c) 60 mJ cm−2 and (d) 80 mJ cm−2 for 15 pulses.
straightforwardly as follows: when CNT were subjected to the laser irradiation (266 nm) of least fluence of 30 mJ cm−2 , carbon atoms absorb energy and initiate the oxidation at defect sites thereby breaking the C–C bonds in various layers of CNT. A gradual increase in the laser fluence (60 mJ cm−2 ) enhances an interlayer and intertube interaction and the structure of CNT collapsed into cauliflower type aggregation of carbonaceous deposits. Further increase in laser fluence (80 mJ cm−2 ) increases the temperature of surface of CNT within the laser irradiated area to very high. Similarly, an increase in the number of pulses also increases an accumulation of energy in CNT irradiated matrix. This creates a high flux of carbon atoms and ions. The subsequent removal of laser beam at this stage initiates the condensation of carbon atoms and ions leading to the formation of well-defined carbonaceous plates. The formation of small carbon clusters that grow with increasing laser fluence suggests a substantial mass transport between gaseous and solid phases. However, in case of higher laser fluence (1064 nm, 400 mJ cm−2 ), the temperature of the surface of CNT within the laser irradiated area reaches a much higher temperature. This sudden elevation of local temperature leads to the formation of larger aggregation of carbon because of its higher fluidity. The detail mechanism for such phase transformation is currently under investigation.
Our studies also point out the role of the laser fluence and number of pulses as the effective parameters for such transformation.
Acknowledgements Financial support for this work from the National Research Council of Taiwan, through grant number NSC 88-2112-M-002-022 and NSC 89-2112-M-002-047 and the Ministry of Education of ROC through grant number 89-N-FA01-2-4-5, is gratefully acknowledged. We would like to thank Prof. Y.T. Chen for providing laser facilities to carry the part of this work and the fruitful discussion. References [1] [2] [3] [4]
[5] [6] [7]
4. Conclusions
[8]
In conclusion, our investigation demonstrates the potential of laser irradiation for transformation of CNT into submicron size plates and an aggregation of carbon deposits that may not be feasible by conventional thermal heating.
[9] [10] [11] [12]
S. Iijima, Nature 354 (1991) 56. M.S. Dresselhaus, Nature 391 (1998) 19. C.T. White, T.N. Todorov, Nature 393 (1998) 240. J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutartre, P. Genestle, P. Bernier, P.M. Ajayan, J. Am. Chem. Soc. 116 (1999) 7935. G. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346. H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. S.S. Wong, E. Joselevich, A.T. Woolley, C.L. Cheung, C.M. Lieber, Nature 394 (1998) 52. S.C. Tsang, Z. Guo, Y.K. Chen, M.L.H. Green, H.A.O. Hill, T.W. Hambley, P. Sadler, J. Angew Chem. Int. Ed. Engl. 36 (1997) 2197. P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Nature 362 (1993) 522. N. Yao, V. Lordi, S.X.C. Ma, J. Mater. Res. 13 (1998) 2432. S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520. D.T. Colbert, Science 266 (1994) 1218.
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[13] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493. [14] B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mater. Sci. Lett. 16 (1997) 402. [15] Y.T. Jan, H.C. Hsieh, C.F. Chen, Diam. Relat. Mater. 8 (1999) 772.
[16] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.W. Williams, M. Menon, K.R. Subbaswamy, A. Thess, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Science 275 (1997) 187. [17] Y.G. Gogotsi, M. Yoshimura, Nature 367 (1994) 628.