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Pretreatment of Ni-carboxylates metal-organics for growing carbon nanotubes on silicon substrates
Diamond and Related Materials 13 (2004) 1242–1248
Pretreatment of Ni-carboxylates metal-organics for growing carbon nanotubes on silicon substrates Tsung-Han Chena, Shih-Chin Changa, I.-Nan Linb,c,* a
Department of Materials Science and Engineering, National Tsing-Hua University, 101 Kuang-Fu Road Section 2, Hsin-Chu 300, Taiwan, ROC b Materials Science Center, National Tsing-Hua University, 101 Kuang-Fu Road Section 2, Hsin-Chu 300, Taiwan, ROC c Department of Physics, TamKang University, Tamsui 251, Taiwan, ROC
Abstract A special pretreatment process was adopted for converting the Ni-carboxylates, Ni(C7 H15 COO)2 , into Ni-clusters, such that they were agglomerated but were not coalesced even after experiencing high temperature process. The growth of carbon nanotubes (CNTs) was thus facilitated. The CNTs obtained are very small in diameter (approx. 30 nm) and are straight, indicating that the CNTs contain very few defects. They exhibit large and stable electron field emission properties, that is, the field emission can be turned on at Eos3.3–4.2 Vymm, with emission current capacity of Je s2.2 mAycm2 , at 7.0 Vymm applied field. These results imply that spin coating of Ni(C7 H15 COO)2 is a simple and inexpensive method to form catalysts for the growth of CNTs and has the potential for scaling up. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Electron field emission properties; Chemical vapor deposition; Metal-organic precursors
1. Introduction Since the first observation of carbon nanotubes (CNTs), w1x extensive investigations have been pursued due to their unique physical properties and potential technological applications. CNTs can be synthesized by various methods such as arc discharge, w2x laser vaporization, w3x and chemical vapor deposition (CVD) w4,5x. In CVD method, metal nanoparticles play an important catalytic role for the growth of CNTs. Small size for the metal nanoparticles is desired for growing high quality CNTs w6x. Two approaches were frequently used for preparing the metal nanoparticles employed in the CVD synthesis of CNTs, viz. utilization of a mesoporous substrate, such as anodized Al2O3 w7x and porous Si w8x, to confine the metal nanoparticles by the small pores or heat treatment of a thin metal layer in hydrogen w9x (or ammonia w10x) gas at high temperature to form metal nanoparticles. However, the former process usually resulted in high CNT-to-substrate electrical contact resis*Corresponding author. Tel.: q886-35742574; fax: q88635717783. E-mail address: [email protected] (I.-Nan Lin).
tance, which degraded the field emission properties, whereas the latter process usually induced pronounced metal-to-substrate interaction, which hinders the catalytic efficiency of the metal nanoparticles. In this paper, a simple and inexpensive method was developed to prepare Ni-catalysts on substrates. The Niclusters were converted from Ni-carboxylates in CH4 y N2 atmosphere such that they can be maintained at very small size (approx. 30 nm) even when they were densely populated. The effect of processing parameters on the characteristics of thus obtained Ni-clusters and the related catalytic behavior for growing CNTs were investigated. 2. Experimental procedures Ni(C7H15COO)2, Ni(OR)2 , carboxylate was prepared by reacting Ni(NO3)2 with 2-ethyltexanoic acid and ammonia water. Precursors containing 0.1–0.4 M Ni(OR)2, in isopropyl alcohol, were spin-coated onto Ptype (100) silicon substrates and then pre-backed at 80 8C for 20 s, followed by post-treatment in N2 (5% H2) or NH3 gases at approximately 700 8C to reduce Ni(OR)2 to Ni-clusters. The carboxylates-coated sub-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.021
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Fig. 1. SEM micrographs of Ni-clusters converted from 0.2 M Nicarboxylates, which were heat-treated in (a) N2 (5% H2) and (b) NH3, with the insets showing the formation of nickel silicides or budlike carbon soots after grown in CH4yN2 (150y150 sccm) for 20 min.
strates were heated up very fast by using a susceptor to absorb the microwave power. The substrate temperature, which is very crucial for growing the CNTs, was monitored by an optical pyrometer. Methane (CH4) and nitrogen mixing gas, approximately 550 Torr and 300 sccm flow rate, was then introduced into CVD chamber for 1–20 min to grow CNTs, while the catalyst-coated substrates were heated up to approximately 900 8C in a very fast rate, again, by using microwave heating technique. A scanning electron microscopy (JEOL JSM-6500F) was used for examining the morphology of post-treated Ni-catalyst and the carbon nanotubes grown on Sisubstrates. A transmission electron microscope (JEOL200X) was used to examine the structure of carbon nanotubes. A Raman spectrometer (Renishaw microRaman 2000) with an excitation wavelength of 632.8 nm of He–Ne laser was used to evaluate the structure of the carbon nanotubes. The electron field emission properties of CNTs coated on Si substrate was measured using a diode setup. An ITO-coated glass, which served as anode, was separated from the cathode, the CNTscoated silicon, by a 180 mm spacer. The current–voltage
Fig. 2. SEM micrograph (a) Ni-clusters converted from 0.2 M Nicarboxylates, which were heat-treated in CH4yN2s150y150 sccm environment at 700 8C prior to the growth of CNTs; the SEM micrograph of carbon nanotubes or carbonaceous materials grown on insitu converted Ni-clusters in (b) CH4yN2 s150y150 sccm and (c) 100% CH4 atmosphere.
(I–V) characteristics of the carbon nanotubes was measured using Keithley 237, under 10y6 mbar and was analyzed using Fowler–Nordheim model to estimate the turn-on field for the CNTs, which was designated as the voltage, where the (ln IyV 2)y(1yV) plot (F–N plot) deviates from straight line.
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Fig. 3. Planar view of the carbon nanotubes grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration. The precursors were directly heated in CH4 yN2 s150y150 sccm atmosphere (550 Torr) at approximately 700 8C. The insets show the corresponding Raman spectra.
3. Results and discussion Previous studies w11x indicated clearly that the morphology of the Fe-clusters alters the growth behavior of carbon nanotubes (CNTs) markedly. Only small and well-dispersed Fe-clusters can grow CNTs with good quality, that is, with uniformly small diameter and low concentration of carbon soots. In this research, we observed that, for Ni-catalyst, the thermal history of the catalyst also affects pronouncedly the growth behavior of CNTs even for the small sized Ni-clusters. Fig. 1a,b, show that the morphologies of the Ni-clusters prepared from 0.2 M Ni-carboxylates are alike, no matter whether they were pre-treated in N2 (5% H2) or NH3 atmosphere. They are small (40;70 nm) and well dispersed. However, these Ni-clusters do not grow CNT. The inset in Fig. 1a indicates that the N2 (5% H2) pre-treated Niclusters form nickel silicide particles, whereas inset in
Fig. 1b reveals that, the NH3 pre-treated ones grow only bud-like carbon soots. The main problems of these pretreatment are, presumably, the occurrence of pronounced Ni-to-Si interaction and Ni-to-Ni coalescence, which is implied by the presence of larger Ni-particulates among the small ones. To improve the catalytic effect of Ni-clusters, the heat treatment process for producing the catalysts was modified, viz. the Ni-carboxylates films were directly heated in CH4 yN2s150y150 sccm atmosphere (approx. 700 8C) in a fast rate. Fig. 2a illustrates that such a pretreatment process markedly improved the characteristics of the Ni-clusters, which are approximately 30 nm and are densely populated. The size of the Ni-clusters is much more uniform as compared with those shown in Fig. 1, indicating that there is no coalescence occurred. Fig. 2b shows that such a Ni-cluster markedly enhances the formation kinetics for the CNT, which were in-situ
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Fig. 4. The electron field emission properties of the CNTs grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration.
grown in the same environment by increasing the substrate temperature to approximately 900 8C. CNTs are straight and are very small in diameter (approx. 30 nm). The prime factor modifying the catalytic effect of Niclusters due to the pretreatment in CH4 yN2 atmosphere is presumed to be the fact that the carbon species dissociated form CH4 can react with the fresh formed Ni-clusters, preventing the coalescence of the Ni-clusters. The Ni-clusters can thus be maintained at very small size (approx. 30 nm). The constituent reaction species in the chamber is as important as the surface chemistry of Ni-clusters in influencing the catalytic effect of Ni-clusters, which, in turn, alters the growth behavior of CNTs. Fig. 2c indicated that, for the same Ni-catalysts as those in Fig. 2a, only carbon soot are resulted when CH4-content is increased to 100%. No carbonaceous materials are formed when the CH4-content is reduced to CH4 yN2s100y200 sccm (not shown). The physical characteristics of the Ni-clusters also
impose some influence on the formation kinetics of the CNTs, but to a less extent. Fig. 3a–c demonstrate that CNTs were readily formed on Ni-clusters prepared from 0.1–0.4 M Ni-carboxylates, provided that they were insitu pretreated in CH4 yN2s150y150 sccm atmosphere and the growth conditions were properly controlled. The CNTs grown on (0.1 M) carboxylate-derived Ni-clusters are of high number density and are uniformly small (approx. 20 nm in diameter), with 300-nm long (Fig. 3a), whereas those grown on (0.2 M) carboxylatederived Ni-clusters are of smaller number density and are slightly longer in diameter (approx. 30 nm), with about the same length (Fig. 3b). Raman spectra shown as insets in Fig. 3a,b imply that they are basically of the same characteristics. Smaller diameter for the (0.1 M) carboxylate-derived CNTs is apparently owing to the smaller size of the corresponding Ni-clusters. Further increase in the concentration of Ni-carboxylate to 0.4 M results in pronounced change on the
Table 1 The correlation between CNTs characteristics and their electron field emission properties Precursor
Diameter (nm)
Number density (1ycm2)
Turn-on field (Vymm)
Emission current density (mAycm2)
0.1 M 0.2 M 0.4 M
20 30 50
9=109 5=109 1=109
3.3–4.0 2.0–3.2 3.3–3.6
2.2 5–0.6 0.035
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Fig. 5. Cross-sectional view of the carbon nanotubes grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration. The precursors were directly heated in CH4 yN2 s150y150 sccm atmosphere (550 Torr) at approximately 700 8C.
morphology of CNTs. Fig. 3c indicates that the number density of the CNTs is markedly reduced and the diameter of CNTs is pronouncedly increased (to approx. 50 nm). Carbon soots surrounding large Ni-clusters are observed occasionally. Raman spectrum shown in the corresponding inset does not reveal any difference in CNT characteristics. Such a phenomenon indicates that the Ni-clusters thus formed, approximately 80 nm in diameter, are too large to catalyze the formation of CNTs. That the catalyzing efficiency of Ni-clusters for forming the CNTs is intimately correlated with their size, is again demonstrated. The electron field emission density varies with the morphology of CNTs profoundly, as illustrated in Fig. 4a–c. For the ultra-small CNTs shown in Fig. 3a, the electron field emission can be turned on at around (Eo)Is3.3–4.0 Vymm and emission current density achieves (Je)Is2.2 mAycm2 at 7.0 Vymm applied field (Fig. 4a). The field emission characteristics are quite stable with respect to the voltage cycling. In contrast, the medium-sized CNTs (Fig. 3b) show better electron field emission properties, i.e. the turn-on field is slightly smaller w(Eo)IIs2.0–3.2 Vymmx and the emission cur-
rent density is larger w(Je)IIs5 mAycm2 x, as indicated in Fig. 4b. However, the field emission characteristics of these CNTs degrade appreciably with respect to voltage cycling, that is, the Jo-value decreases markedly to (Je)II9s0.6 mAycm2 after several voltage cycles. For the CNTs containing carbon soots (Fig. 3c), the electron field emission current density of CNT is significantly smaller, i.e. around (Je)IIIs0.035 mAycm2 (Fig. 4c). The characteristics of CNTs and the associated electron field emission parameters, turn-on field (Eo) and field emission current density (Je), are summarized in Table 1, which indicates clearly that higher number density of CNTs is desirable for increasing their Jevalue. While the increases in emission capacity with the number density of CNTs is understandable, that the turnon field for these CNTs does not change with the diameter of CNTs is quite unexpected. Moreover, the turn-on field for these CNTs is large comparing to the conventional CNTs. Detailed examination on the crosssectional micrographs of these CNTs (Fig. 5a–c) reveals an interesting phenomenon, that is, a thin continuous layer was formed underneath the CNTs. In some cases, the underlying layer is so thick that the CNTs layer was
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Fig. 6. SEM micrographs showing the evolution of carbon nanotubes grown on in-situ converted Ni-clusters, where the growth periods are (a) 10 min and (b) 20 min; (c) TEM micrograph showing the Ni-clusters aggregate in the underlying layer.
lifted-up, separating from the substrate. Apparently, the presence of such an underlying layer hinders the transport of electrons from substrates to CNTs, resulting in large turn-on field. To understand the formation process for the underlying layer, the evolution of CNTs from Ni-clusters was systemically examined. For the Ni-catalysts shown as those in Fig. 2a, no carbonaceous materials were observable when the Ni-clusters were exposed to growth environment only for 1 min (not shown). Fig. 6a and inset shows that CNTs about ;30 nm in diameter and ;200 nm in length were resulted, when films were grown for 10 min. The length of CNTs increases slightly, with the diameter remained as the same, for those grown for longer period (20 min, Fig. 6b). TEM micrograph shown in Fig. 6c reveals that thick layer underneath the CNTs is the aggregate of Niclusters. Individual Ni-particulates (approx. 30 nm in size) are still clearly observable, indicating that there is no interdiffusion occurred between the Ni-clusters, even though they are very small, viz. Ni-clusters form agglomerates without coalescing. CNTs stem from some of the individual Ni-particles. The Ni-catalysts are located at the tip of CNTs, which indicates that the CNTs grew via a tip-growth process. The Ni-particulates are
about the same size as CNTs (approx. 30 nm). Niparticulates in the aggregates are spherical, whereas those inside the CNTs are rod-shaped. The pronounced change in the geometry of Ni-particulates infers that the dissolution and re-precipitation process have occurred. Moreover, straight CNT implies that most of the carbons are hexagons, and very few defects, such as pentagons or heptagons, are formed during the growth of CNTs. Apparently, it is the smallness of the Ni-particulates, which facilitates the formation of hexagons. The importance of small size of Ni-particulates for catalyzing the formation of CNTs is again demonstrated. Usually, the metallic clusters can be maintained at very small diameter only when they are well separated from each other. Coalescence is easily induced via the interdiffusion between small metallic clusters whenever the metallic clusters are densely populated, as they are extremely reactive. However, Fig. 6c indicates that the Ni-clusters converted from Ni-carboxylates in CH4 yN2 atmosphere density populated and are very small (approx. 30 nm), even after experiencing the high temperature process. These results indicate that these nano-sized Ni-clusters are somehow passivated. Probably, it is the catalytic effect of Ni-clusters on the dissociation of CH4, which forms a thin layer of carbo-
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naceous species surrounding the Ni-clusters and prevents the coalescence of Ni-clusters in the latter stage. Such an argument, however, need further detailed studies. 4. Conclusions Carbon nanotubes (CNTs) with good electron field emission properties were successfully grown on Niclusters converted form Ni-carboxylates. It is observed that the conversion process is important in growing CNTs. Only the Ni-clusters formed in CH4 yN2 atmosphere can grow CNTs, whereas those formed in N2 (5% H2) or NH3 atmosphere can only results in carbon soot. Acknowledgments The authors gratefully acknowledge financial support from the National Science Council, ROC, through the project No. NSC 91-2622-E-007-027 and NSC 91-2218E-003-001.
References w1x S. Iijima, Nature 354 (1991) 56. w2x T.W. Ebbesen, P.M. Ajayan, Nature 358 (1992) 220. w3x A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, et al., Science 273 (1996) 483. w4x W.Z. Li, S.S. Xie, L.X. Qain, B.H. Chang, B.S. Zou, W.Y. Zhou, et al., Science 274 (1996) 1701. w5x Z.W. Pan, S.S. Xie, B.H. Chang, L.F. Sun, W.Y. Zhou, G. Wang, Chem. Phys. Lett. 299 (1999) 97. w6x C. Bower, O. Zhou, Appl. Phys. Lett. 77 (2000) 2767. w7x J. Li, C. Papadopoulos, J.M. Xu, M. Moskovits, Appl. Phys. Lett. 75 (1999) 367. w8x S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. w9x X.H. Chen, S.Q. Feng, Y. Ding, J.C. Peng, Z.Z. Chen, Thin Solid Films 339 (1999) 6. w10x M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, et al., Appl. Phys. Lett. 90 (2001) 5308. w11x T.H. Chen, S.C. Chang, I.N. Lin, Diamond Relat. Mater. 12 (2003) 283.
Pretreatment of Ni-carboxylates metal-organics for growing carbon nanotubes on silicon substrates Tsung-Han Chena, Shih-Chin Changa, I.-Nan Linb,c,* a
Department of Materials Science and Engineering, National Tsing-Hua University, 101 Kuang-Fu Road Section 2, Hsin-Chu 300, Taiwan, ROC b Materials Science Center, National Tsing-Hua University, 101 Kuang-Fu Road Section 2, Hsin-Chu 300, Taiwan, ROC c Department of Physics, TamKang University, Tamsui 251, Taiwan, ROC
Abstract A special pretreatment process was adopted for converting the Ni-carboxylates, Ni(C7 H15 COO)2 , into Ni-clusters, such that they were agglomerated but were not coalesced even after experiencing high temperature process. The growth of carbon nanotubes (CNTs) was thus facilitated. The CNTs obtained are very small in diameter (approx. 30 nm) and are straight, indicating that the CNTs contain very few defects. They exhibit large and stable electron field emission properties, that is, the field emission can be turned on at Eos3.3–4.2 Vymm, with emission current capacity of Je s2.2 mAycm2 , at 7.0 Vymm applied field. These results imply that spin coating of Ni(C7 H15 COO)2 is a simple and inexpensive method to form catalysts for the growth of CNTs and has the potential for scaling up. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Electron field emission properties; Chemical vapor deposition; Metal-organic precursors
1. Introduction Since the first observation of carbon nanotubes (CNTs), w1x extensive investigations have been pursued due to their unique physical properties and potential technological applications. CNTs can be synthesized by various methods such as arc discharge, w2x laser vaporization, w3x and chemical vapor deposition (CVD) w4,5x. In CVD method, metal nanoparticles play an important catalytic role for the growth of CNTs. Small size for the metal nanoparticles is desired for growing high quality CNTs w6x. Two approaches were frequently used for preparing the metal nanoparticles employed in the CVD synthesis of CNTs, viz. utilization of a mesoporous substrate, such as anodized Al2O3 w7x and porous Si w8x, to confine the metal nanoparticles by the small pores or heat treatment of a thin metal layer in hydrogen w9x (or ammonia w10x) gas at high temperature to form metal nanoparticles. However, the former process usually resulted in high CNT-to-substrate electrical contact resis*Corresponding author. Tel.: q886-35742574; fax: q88635717783. E-mail address: [email protected] (I.-Nan Lin).
tance, which degraded the field emission properties, whereas the latter process usually induced pronounced metal-to-substrate interaction, which hinders the catalytic efficiency of the metal nanoparticles. In this paper, a simple and inexpensive method was developed to prepare Ni-catalysts on substrates. The Niclusters were converted from Ni-carboxylates in CH4 y N2 atmosphere such that they can be maintained at very small size (approx. 30 nm) even when they were densely populated. The effect of processing parameters on the characteristics of thus obtained Ni-clusters and the related catalytic behavior for growing CNTs were investigated. 2. Experimental procedures Ni(C7H15COO)2, Ni(OR)2 , carboxylate was prepared by reacting Ni(NO3)2 with 2-ethyltexanoic acid and ammonia water. Precursors containing 0.1–0.4 M Ni(OR)2, in isopropyl alcohol, were spin-coated onto Ptype (100) silicon substrates and then pre-backed at 80 8C for 20 s, followed by post-treatment in N2 (5% H2) or NH3 gases at approximately 700 8C to reduce Ni(OR)2 to Ni-clusters. The carboxylates-coated sub-
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.021
T.-H. Chen et al. / Diamond and Related Materials 13 (2004) 1242–1248
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Fig. 1. SEM micrographs of Ni-clusters converted from 0.2 M Nicarboxylates, which were heat-treated in (a) N2 (5% H2) and (b) NH3, with the insets showing the formation of nickel silicides or budlike carbon soots after grown in CH4yN2 (150y150 sccm) for 20 min.
strates were heated up very fast by using a susceptor to absorb the microwave power. The substrate temperature, which is very crucial for growing the CNTs, was monitored by an optical pyrometer. Methane (CH4) and nitrogen mixing gas, approximately 550 Torr and 300 sccm flow rate, was then introduced into CVD chamber for 1–20 min to grow CNTs, while the catalyst-coated substrates were heated up to approximately 900 8C in a very fast rate, again, by using microwave heating technique. A scanning electron microscopy (JEOL JSM-6500F) was used for examining the morphology of post-treated Ni-catalyst and the carbon nanotubes grown on Sisubstrates. A transmission electron microscope (JEOL200X) was used to examine the structure of carbon nanotubes. A Raman spectrometer (Renishaw microRaman 2000) with an excitation wavelength of 632.8 nm of He–Ne laser was used to evaluate the structure of the carbon nanotubes. The electron field emission properties of CNTs coated on Si substrate was measured using a diode setup. An ITO-coated glass, which served as anode, was separated from the cathode, the CNTscoated silicon, by a 180 mm spacer. The current–voltage
Fig. 2. SEM micrograph (a) Ni-clusters converted from 0.2 M Nicarboxylates, which were heat-treated in CH4yN2s150y150 sccm environment at 700 8C prior to the growth of CNTs; the SEM micrograph of carbon nanotubes or carbonaceous materials grown on insitu converted Ni-clusters in (b) CH4yN2 s150y150 sccm and (c) 100% CH4 atmosphere.
(I–V) characteristics of the carbon nanotubes was measured using Keithley 237, under 10y6 mbar and was analyzed using Fowler–Nordheim model to estimate the turn-on field for the CNTs, which was designated as the voltage, where the (ln IyV 2)y(1yV) plot (F–N plot) deviates from straight line.
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Fig. 3. Planar view of the carbon nanotubes grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration. The precursors were directly heated in CH4 yN2 s150y150 sccm atmosphere (550 Torr) at approximately 700 8C. The insets show the corresponding Raman spectra.
3. Results and discussion Previous studies w11x indicated clearly that the morphology of the Fe-clusters alters the growth behavior of carbon nanotubes (CNTs) markedly. Only small and well-dispersed Fe-clusters can grow CNTs with good quality, that is, with uniformly small diameter and low concentration of carbon soots. In this research, we observed that, for Ni-catalyst, the thermal history of the catalyst also affects pronouncedly the growth behavior of CNTs even for the small sized Ni-clusters. Fig. 1a,b, show that the morphologies of the Ni-clusters prepared from 0.2 M Ni-carboxylates are alike, no matter whether they were pre-treated in N2 (5% H2) or NH3 atmosphere. They are small (40;70 nm) and well dispersed. However, these Ni-clusters do not grow CNT. The inset in Fig. 1a indicates that the N2 (5% H2) pre-treated Niclusters form nickel silicide particles, whereas inset in
Fig. 1b reveals that, the NH3 pre-treated ones grow only bud-like carbon soots. The main problems of these pretreatment are, presumably, the occurrence of pronounced Ni-to-Si interaction and Ni-to-Ni coalescence, which is implied by the presence of larger Ni-particulates among the small ones. To improve the catalytic effect of Ni-clusters, the heat treatment process for producing the catalysts was modified, viz. the Ni-carboxylates films were directly heated in CH4 yN2s150y150 sccm atmosphere (approx. 700 8C) in a fast rate. Fig. 2a illustrates that such a pretreatment process markedly improved the characteristics of the Ni-clusters, which are approximately 30 nm and are densely populated. The size of the Ni-clusters is much more uniform as compared with those shown in Fig. 1, indicating that there is no coalescence occurred. Fig. 2b shows that such a Ni-cluster markedly enhances the formation kinetics for the CNT, which were in-situ
T.-H. Chen et al. / Diamond and Related Materials 13 (2004) 1242–1248
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Fig. 4. The electron field emission properties of the CNTs grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration.
grown in the same environment by increasing the substrate temperature to approximately 900 8C. CNTs are straight and are very small in diameter (approx. 30 nm). The prime factor modifying the catalytic effect of Niclusters due to the pretreatment in CH4 yN2 atmosphere is presumed to be the fact that the carbon species dissociated form CH4 can react with the fresh formed Ni-clusters, preventing the coalescence of the Ni-clusters. The Ni-clusters can thus be maintained at very small size (approx. 30 nm). The constituent reaction species in the chamber is as important as the surface chemistry of Ni-clusters in influencing the catalytic effect of Ni-clusters, which, in turn, alters the growth behavior of CNTs. Fig. 2c indicated that, for the same Ni-catalysts as those in Fig. 2a, only carbon soot are resulted when CH4-content is increased to 100%. No carbonaceous materials are formed when the CH4-content is reduced to CH4 yN2s100y200 sccm (not shown). The physical characteristics of the Ni-clusters also
impose some influence on the formation kinetics of the CNTs, but to a less extent. Fig. 3a–c demonstrate that CNTs were readily formed on Ni-clusters prepared from 0.1–0.4 M Ni-carboxylates, provided that they were insitu pretreated in CH4 yN2s150y150 sccm atmosphere and the growth conditions were properly controlled. The CNTs grown on (0.1 M) carboxylate-derived Ni-clusters are of high number density and are uniformly small (approx. 20 nm in diameter), with 300-nm long (Fig. 3a), whereas those grown on (0.2 M) carboxylatederived Ni-clusters are of smaller number density and are slightly longer in diameter (approx. 30 nm), with about the same length (Fig. 3b). Raman spectra shown as insets in Fig. 3a,b imply that they are basically of the same characteristics. Smaller diameter for the (0.1 M) carboxylate-derived CNTs is apparently owing to the smaller size of the corresponding Ni-clusters. Further increase in the concentration of Ni-carboxylate to 0.4 M results in pronounced change on the
Table 1 The correlation between CNTs characteristics and their electron field emission properties Precursor
Diameter (nm)
Number density (1ycm2)
Turn-on field (Vymm)
Emission current density (mAycm2)
0.1 M 0.2 M 0.4 M
20 30 50
9=109 5=109 1=109
3.3–4.0 2.0–3.2 3.3–3.6
2.2 5–0.6 0.035
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Fig. 5. Cross-sectional view of the carbon nanotubes grown on in-situ converted Ni-clusters, which were prepared from Ni-carboxylates with (a) 0.1 M, (b) 0.2 M and (c) 0.4 M concentration. The precursors were directly heated in CH4 yN2 s150y150 sccm atmosphere (550 Torr) at approximately 700 8C.
morphology of CNTs. Fig. 3c indicates that the number density of the CNTs is markedly reduced and the diameter of CNTs is pronouncedly increased (to approx. 50 nm). Carbon soots surrounding large Ni-clusters are observed occasionally. Raman spectrum shown in the corresponding inset does not reveal any difference in CNT characteristics. Such a phenomenon indicates that the Ni-clusters thus formed, approximately 80 nm in diameter, are too large to catalyze the formation of CNTs. That the catalyzing efficiency of Ni-clusters for forming the CNTs is intimately correlated with their size, is again demonstrated. The electron field emission density varies with the morphology of CNTs profoundly, as illustrated in Fig. 4a–c. For the ultra-small CNTs shown in Fig. 3a, the electron field emission can be turned on at around (Eo)Is3.3–4.0 Vymm and emission current density achieves (Je)Is2.2 mAycm2 at 7.0 Vymm applied field (Fig. 4a). The field emission characteristics are quite stable with respect to the voltage cycling. In contrast, the medium-sized CNTs (Fig. 3b) show better electron field emission properties, i.e. the turn-on field is slightly smaller w(Eo)IIs2.0–3.2 Vymmx and the emission cur-
rent density is larger w(Je)IIs5 mAycm2 x, as indicated in Fig. 4b. However, the field emission characteristics of these CNTs degrade appreciably with respect to voltage cycling, that is, the Jo-value decreases markedly to (Je)II9s0.6 mAycm2 after several voltage cycles. For the CNTs containing carbon soots (Fig. 3c), the electron field emission current density of CNT is significantly smaller, i.e. around (Je)IIIs0.035 mAycm2 (Fig. 4c). The characteristics of CNTs and the associated electron field emission parameters, turn-on field (Eo) and field emission current density (Je), are summarized in Table 1, which indicates clearly that higher number density of CNTs is desirable for increasing their Jevalue. While the increases in emission capacity with the number density of CNTs is understandable, that the turnon field for these CNTs does not change with the diameter of CNTs is quite unexpected. Moreover, the turn-on field for these CNTs is large comparing to the conventional CNTs. Detailed examination on the crosssectional micrographs of these CNTs (Fig. 5a–c) reveals an interesting phenomenon, that is, a thin continuous layer was formed underneath the CNTs. In some cases, the underlying layer is so thick that the CNTs layer was
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Fig. 6. SEM micrographs showing the evolution of carbon nanotubes grown on in-situ converted Ni-clusters, where the growth periods are (a) 10 min and (b) 20 min; (c) TEM micrograph showing the Ni-clusters aggregate in the underlying layer.
lifted-up, separating from the substrate. Apparently, the presence of such an underlying layer hinders the transport of electrons from substrates to CNTs, resulting in large turn-on field. To understand the formation process for the underlying layer, the evolution of CNTs from Ni-clusters was systemically examined. For the Ni-catalysts shown as those in Fig. 2a, no carbonaceous materials were observable when the Ni-clusters were exposed to growth environment only for 1 min (not shown). Fig. 6a and inset shows that CNTs about ;30 nm in diameter and ;200 nm in length were resulted, when films were grown for 10 min. The length of CNTs increases slightly, with the diameter remained as the same, for those grown for longer period (20 min, Fig. 6b). TEM micrograph shown in Fig. 6c reveals that thick layer underneath the CNTs is the aggregate of Niclusters. Individual Ni-particulates (approx. 30 nm in size) are still clearly observable, indicating that there is no interdiffusion occurred between the Ni-clusters, even though they are very small, viz. Ni-clusters form agglomerates without coalescing. CNTs stem from some of the individual Ni-particles. The Ni-catalysts are located at the tip of CNTs, which indicates that the CNTs grew via a tip-growth process. The Ni-particulates are
about the same size as CNTs (approx. 30 nm). Niparticulates in the aggregates are spherical, whereas those inside the CNTs are rod-shaped. The pronounced change in the geometry of Ni-particulates infers that the dissolution and re-precipitation process have occurred. Moreover, straight CNT implies that most of the carbons are hexagons, and very few defects, such as pentagons or heptagons, are formed during the growth of CNTs. Apparently, it is the smallness of the Ni-particulates, which facilitates the formation of hexagons. The importance of small size of Ni-particulates for catalyzing the formation of CNTs is again demonstrated. Usually, the metallic clusters can be maintained at very small diameter only when they are well separated from each other. Coalescence is easily induced via the interdiffusion between small metallic clusters whenever the metallic clusters are densely populated, as they are extremely reactive. However, Fig. 6c indicates that the Ni-clusters converted from Ni-carboxylates in CH4 yN2 atmosphere density populated and are very small (approx. 30 nm), even after experiencing the high temperature process. These results indicate that these nano-sized Ni-clusters are somehow passivated. Probably, it is the catalytic effect of Ni-clusters on the dissociation of CH4, which forms a thin layer of carbo-
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naceous species surrounding the Ni-clusters and prevents the coalescence of Ni-clusters in the latter stage. Such an argument, however, need further detailed studies. 4. Conclusions Carbon nanotubes (CNTs) with good electron field emission properties were successfully grown on Niclusters converted form Ni-carboxylates. It is observed that the conversion process is important in growing CNTs. Only the Ni-clusters formed in CH4 yN2 atmosphere can grow CNTs, whereas those formed in N2 (5% H2) or NH3 atmosphere can only results in carbon soot. Acknowledgments The authors gratefully acknowledge financial support from the National Science Council, ROC, through the project No. NSC 91-2622-E-007-027 and NSC 91-2218E-003-001.
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