Growth of carbon nanotubes by chemical vapor deposition

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1 Ž. Diamond and Related Materials Growth of carbon nanotubes by chemical vapor deposition Minjae Jung a, Kwang Yong Eun b, Jae-Kap Lee b, Young-Joon Baik b, Kwang-Ryeol Lee b,, Jong Wan Park a a Department of Metallurgy, Hanyang Uni ersity, 17, Haengdang-dong, Seoul, 1-791, South Korea b Thin Film Technology Research Center, Korea Institute of Science and Technology, P.O. Box 11, Cheongryang, Seoul, , South Korea Abstract The growth behavior of carbon nanotubes Ž CNT. deposited from CH by thermal CVD method was investigated. Nickel particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N, H, Ar, NH and their mixtures to investigate the effect of the environment on the CNT growth behavior. The deposition was performed at 850 C in atmospheric pressure. In pure N environment, thick carbon layer deposition occurred on the substrate without CNT growth. The Ni particles encapsulated by the carbon deposition could not work as the catalyst in this condition. However, the growth of CNT was enhanced as the H concentration increased in the mixture of N and H environment. In pure H environment, randomly tangled CNTs could be obtained. The growth of CNT was much enhanced when using NH as the environment gas. Vertically aligned CNTs could be deposited in NH environment, whereas the CNT growth could not be obtained in the mixture of N and H environment of the same ratio of N H. These results were discussed in terms of the passivation of the catalyst caused by the excessive deposition of carbon on the catalyst surface. For the deposition of the CNT, the decomposition rate of CH should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface by excessive carbon deposition. The present work showed that the composition of environment gas significantly affects the reaction kinetics in the CNT growth. It is also noted that nitride surface layer formation on Ni catalyst in NH environment can affect the CNT growth behavior. 001 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotube; CVD; Aligned MWNT; Deposition behavior 1. Introduction Since Iijima reported the synthesis of carbon nanotube Ž CNT. by arc discharge process 1, many reports on the CNT synthesis were published in various deposition methods such as arc discharge, laser ablation, pyrolysis of hydrocarbon 4 and chemical vapor deposition Ž CVD. method 5 7. The physical and Corresponding author. Tel.: ; fax: address: krlee@kist.re.kr Ž K. Lee.. chemical properties of CNT were also reported by many researchers The most attractive property of CNT would be the excellent electron field emission behavior 10,11. Flat panel display or vacuum microelectronic devices would be the most promising application of CNT 1. CVD process by which CNTs were grown using metal particles as the reaction catalyst has several advantages over other synthesis methods. The CNTs can grow at relatively low temperature, and the size of CNT can be controlled by varying the size of the metal catalyst. Furthermore, it is relatively easy to obtain vertically aligned CNTs which is important for the application of $ - see front matter 001 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 16 ( ) M. Jung et al. Diamond and Related Materials field emission display. However, the process parameter control for the growth of CNT still remains in an empirical manner, because the reaction kinetics and the growth mechanism are not fully understood. Deposition behavior of CNT in CVD process can be compared with that of the carbon filaments 1. Many investigations were reported on growth mechanism and kinetics of the carbon filament, the effect of metal catalysts and the mass production methods It is well known that the synthesis of carbon filament was suppressed by passivation of the metal catalyst. Both the polymeric encapsulation at low temperature and the excess decomposition of hydrocarbon at high temperature passivated the metal catalyst 0. On the other hand, excess hydrogen prevents the passivation resulting in the filament growth 1. In the present work, CNTs were deposited from C H by thermal CVD process using Ni as the catalyst. We investigated the growth behaviors in various gas environments of N, H, Ar, NH and their mixtures. We focused on the passivation behavior of the Ni catalyst and the reaction between the catalyst and the environment gas. We show that the passivation can be effectively controlled by changing the ratio of C H to H in the environment. The CNT growth was much enhanced in NH environment, where the ease of thermal decomposition of NH resulted in the formation of nitride surface layer on the catalyst and reduced the C H decomposition kinetics.. Experimental CNT was grown by using thermal decomposition of C H at atmospheric pressure. SiliconŽ 100. wafer with surface oxide layer of thickness 1 m was used as the substrate. Nickel was deposited on the substrate by DC magnetron sputtering method. The thickness of the Ni film ranged from to 7 nm. In order to change the deposited Ni film to small particles, the specimen was annealed at 800 C for 15 min in hydrogen environment. The hydrogen pressure during the annealing was kept at 1 torr by adjusting the flow rate of hydrogen. Fig. 1 is the typical SEM microstructure of the specimen, which shows that the Ni particles of diameter ranging from 15 to 90 nm were uniformly distributed on the substrate. The size and the number density of the particles were strongly dependent on the thickness of the Ni film. As the thickness increased, the size of the particles increased with lower number density. The number density of the Ni particles was m for.4-nmthick film and m for 6.8-nm-thick film. In the present work, we used the Ni particles obtained by annealing the 6.8-nm-thick film as the catalyst. Fig. shows the schematic of the horizontal flow Fig. 1. SEM microstructure of Ni particles on SiO Si substrate formed by annealing of sputter deposited Ni film of Ž. a.4 nm and Ž b. 6.8 nm in thickness. reactor used for CNT growth. The quartz tube of diameter 5 mm and length 800 mm was used for the reactor. The reaction zone of high temperature was obtained by an electric furnace which generated a uniform temperature reaction zone of length 50 mm in the middle of the furnace. The reaction gas was introduced to the gas inlet at a fixed flow rate using a mass flow controller. The reaction gas was decomposed at the reaction zone and the residual gas was removed to the hood via a water bath. The water bath at the gas outlet was used to prevent the reverse flow of air into the reactor. The substrate was placed in the substrate holder which is connected to a loading system. Before loading the substrate into the reaction zone, the temperature was increased to the reaction temperature ranging from 700 to 1000 C in Ar environment. The substrate was then pushed to the reaction zone by the loading system. Before the deposition, the substrate was pretreated for 1 h in various gas environments such as H, N, H N,H Ar or NH. The flow rate of the environment gas was 00 sccm except for NH Ž 100 sccm.. The deposition of the CNT was performed for 0 min by adding C H to the environment gas. The concentration of C H was varied from.4 to 0 vol.%. We observed that the initial transient deposition occurred for the first min due to the diluting effect of the existing environment gas. However, steady state of the growth was obtained after 5 min. In this paper, we reported the growth behaviors in the steady state. After deposition, the reactor was cooled down in Ar environment. Fig.. Schematic of horizontal flow reactor for carbon nanotube growth.

3 ( ) M. Jung et al. Diamond and Related Materials Results and discussion Fig. shows the growth morphologies when the mixture of N and H was used as the environment gas. The concentration of C H was.4 vol.% and the deposition temperature was 850 C. Without hydrogen in the environment Ž Fig. a., we could not obtain CNT but thick carbon layer deposited on the substrate. In this experimental condition, the similar carbon layer was deposited without Ni particles, which shows that the Ni particles did not act as the catalyst for CNT growth. Observation of earlier stage of growth showed that all the Ni particles were passivated by the large granular carbon deposition. However, when the H fraction in the environment was larger than 0.6, the CNT growth was observed as shown in Fig. b. The microstructure of Fig. b shows that the CNTs Žindi- cated by arrow 1. were grown while some Ni particles were still passivated by the granular carbon deposition Ž arrow.. The number of passivated Ni particles decreased with increasing H fraction in the environment. As can be seen in Fig. c,d, most of deposits were CNTs when the H fraction is higher than In pure H environment, the growth rate of CNT further increased by factor of as can be judged from the thickness of the CNT layers, although the CNTs were not vertically aligned but randomly tangled. We did the same experiment in the mixture of Ar and H environment, where we observed the same dependence of the growth behavior on the H fraction Fig.. SEM microstructures of the carbon deposits for 0 min with.4 vol.% CH at 850 C after pretreatment for 1 h in the environments of various H Ž N H. fractions: Ž a. H Ž N H. 0; Ž b. H Ž N H. 0.6; Ž c. H Ž N H. 0.85; and Ž d. H Ž N H Fig. 4. Ž. a Typical SEM microstructure of Ni particles after pretreatment in the mixture of N and H environment. Ž b. Auger spectroscopies of the samples after the pretreatment in N or H environment. in the environment. This result shows that N was not decomposed in the present experimental condition but acted like an inert gas. Fig. 4a shows the typical SEM microstructure of Ni particles after pretreatment in the mixture of N and H environment. The number density and morphology of the particles were not changed by the pretreatment. The Auger spectra of the pretreated substrate surface were summarized in Fig. 4b. The spectra were obtained after sputtering for 0 s in the analysis chamber and shifted upward for ease of comparison. Due to the limited spatial resolution of the Auger spectroscopy, the spectrum included the compositional information of SiO in addition to the surface of Ni particles. However, the spectrum of the specimen pretreated in N environment was identical to that in H environment. This result confirmed that the nitrogen did not react with Ni catalyst in this experimental condition. One of the roles of hydrogen in the CNT growth would be to change the decomposition kinetics of C H. When the hydrogen fraction is high, the rate of decomposition of C H, C H C H, is suppressed. Higher growth rate of CNT observed in pure H environment Ž Fig. c,d. shows that the reduced decomposition rate is not the rate limiting factor of the CNT growth. However, lower decomposition rate of C H seems to prevent the passivation of the catalyst surface caused by excessive carbon deposition, which in turn enhances the growth of CNT. The importance of the decomposition kinetics was also confirmed by changing the deposition temperature. Fig. 5a shows the CNT growth at 750 C when the hydrogen concentration in the environment is only 0.4. In this hydrogen fraction, we could not obtain CNT growth at 850 C. On the other hand, CNT growth was significantly sup- pressed even in pure H environment at 950 C Ž see Fig. 5b.. It should be noted that well-developed CNTs were grown at 850 C in pure H environment, as can be seen in Fig. d. Fig. 6 showed the TEM microstructure of CNT obtained in pure H environment. Similar TEM mi-

4 18 ( ) M. Jung et al. Diamond and Related Materials Fig. 5. SEM microstructures of the deposits for 0 min with.4 vol.% C H after pretreatment for 1 h Ž. a at 750 C when H Ž N H. 0.5 and Ž b. at 950 C when H Ž N H crostructures were observed in all the environmental conditions where CNT could be obtained. The microstructure shows that the CNT grows in bamboo structure : three or five layers of graphite deposited on the surface of Ni catalyst were separated from the catalyst surface resulting in the growth of CNT with closed end. For the continuous CNT growth, this process should be repeated during the whole growth period. However, if the deposition on Ni catalyst is too fast due to higher decomposition rate of C H, the catalyst surface would be encapsulated by the deposit, which prevent the evolution of CNT in bamboo structure. In this condition, the passivation of the catalyst can occur in a catastrophic manner. Therefore, one of the requirements for the CNT growth is that the decomposition kinetics of C H should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface. The present work shows that the concentration ratio of hydrogen to hydrocarbon in the reaction gas and the reaction temperature are the effective process parameters to control the decomposition kinetics. NH environment significantly improved the growth behavior of CNTs. Fig. 7a,b shows the well-aligned CNTs deposited in NH environment at 950 C for 0 min. In both C H concentrations of 9.1 and 1 vol.%, vertically aligned CNT growth was observed. The growth rate of the CNT increased from 0.5 m min to 0.87 m min as the C H concentration increased from 9.1 to 1 vol.%. This growth behavior can be compared with that in the mixture of N and H environment of the same ratio of N H as in NH. Fig. 7c shows that thick carbon layer was observed without CNT growth in the mixture of N and H environment, even if the C H concentration was lower than that in NH envi- ronment. Fig. 7d is the TEM microstructure of the well-aligned CNT deposited in NH environment, which shows that the growth also occurred in bamboo structure. The growth mechanism in NH environment can be said to be identical to that in the mixture of N and H environment, although the well-aligned CNTs were grown in NH environment in much wider range of Fig. 6. TEM microstructure of CNT synthesized with.4 vol.% C H after pretreatment for 1 h at 850 C when H Ž N H temperature and C H concentration for the CNT growth. The SEM microstructure of Ni particles after the pretreatment in NH environment showed that the size and the distribution of Ni particles were not changed by the pretreatment. However, we observed that the pretreatment in NH environment changed composi- Fig. 7. Ž. a SEM microstructures of the CNTs synthesized for 0 min with 9.1 vol.% CH after pretreatment for 1 h in NH environment. Ž b. SEM microstructures of the CNTs synthesized for 0 min with 1.0 vol.% C H after pretreatment for 1 h in NH environment. Ž. c SEM microstructures of the deposits synthesized for 0 min with 7.0 vol.% CH after pretreatment for 1 h in the environment of mixture of N and H environment H Ž N H Ž d. TEM microstructure of the CNT of Fig. 7b.

5 ( ) M. Jung et al. Diamond and Related Materials Fig. 8. Ž. a Auger spectroscopies of the samples after pretreatment in NH environment and in the mixture of N and H environment H Ž N H Ž b. XRD spectrum of the sample after the pretreatment for 1 h at 950 C in NH environment. tion and structure of the catalyst. The Auger and XRD spectra of the pretreated substrate surface were shown in Fig. 8. The Auger spectrum in Fig. 8a shows that nitrogen was significantly incorporated in the surface of Ni particles in NH environment. Ž The Auger spectrum in Fig. 8a includes the nitrogen incorporated in the SiO substrate. However, the Auger spectrum analysis of the substrate without Ni particles shows that the amount of the nitrogen incorporated in the SiO was much smaller than that in Fig. 8a. It can thus be said that the nitrogen was significantly incorporated in the surface of the Ni particles by the pretreatment in NH environment.. On the other hand, the nitrogen incorporation was not observed in the mixture of N and H environment. XRD spectrum of Fig. 8b shows that the nitrogen incorporation resulted in the formation of Ni N phase. It is well known that NH is much easier to be decomposed than N, which would increase the concentration of atomic N and H in the environment. The ease of decomposition of NH changed the composition and structure of the Ni catalyst surface. The role of NH environment in the CNT growth is yet to be resolved. However, a number of the possibilities can be addressed in the viewpoint of the passivation of catalyst surface. First of all, if the nitride surface enhances the separation of deposited carbon from the catalyst surface during the CNT growth in bamboo structure, the passivation of the catalyst would be much suppressed. Therefore, even in high concentration of C H where CNT growth could not be obtained in the mixture of N and H environment, well-aligned CNT deposition was obtained in NH environment. The higher growth rate in NH environment agrees with this possibility. A second possibility is to suppress the surface or bulk diffusion flux of carbon from the edge of the Ni particles. Finally, higher concentration of the atomic hydrogen in NH environment would also suppress the decomposition of C H, which prevents the passivation of Ni particles. Although the second and third possibility would prevent the passivation of the catalyst, the growth rate of CNT would be reduced. However, as previously discussed, the kinetic rate of carbon deposition or hydrocarbon decomposition are not the rate-limiting factors of the CNT growth in the present experimental condition. On the other hand, preventing the passivation of the catalyst can be said to be the most important factor for the CNT growth in thermal CVD process. 4. Conclusions The most significant result of the present work is that the growth behavior of CNT is strongly dependent on the gas composition of the environment. Chemical reaction in the environment significantly affects the catalyst surface reaction which is crucial to the CNT growth. The present work shows that one of the requirements for the CNT growth is that the reaction kinetics including both diffusion in catalyst and decomposition of hydrocarbon should be controlled to supply carbon for CNT growth without passivation of the catalyst surface. The concentration ratio of hydrogen to hydrocarbon in the reaction gas and the reaction temperature are the key process parameters which should be optimized in this view point. It was also observed that NH environment much enhanced the CNT growth than the mixture of N and H environment. We observed the formation of nitride surface layer on Ni catalyst after pretreatment in NH environment. This reaction is due to the ease of thermal decomposition of NH. Although the mechanism of the enhanced CNT growth in NH environment is not yet clear, the nitride surface seems to play an important role in the CNT

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