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High-aligned carbon nanotube film with netlike bulges
ARTICLE IN PRESS
Vacuum 81 (2007) 953–957 www.elsevier.com/locate/vacuum
High-aligned carbon nanotube film with netlike bulges Peijiang Cao, Deliang Zhu, Wenjun Liu, Xiaocui Ma Department of Materials Science and Engineering, Shenzhen University; Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China Received 20 July 2006; accepted 28 July 2006
Abstract High-aligned carbon nanotubes film with netlike bulges made of catalyst particles has been synthesized on a silica wafer by pyrolyzing ferrocene/melamine mixtures. The structure and composition of carbon nanotubes are investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS). It is found that these nanotubes have uniform outer diameters of about 25 nm and lengths of about 40 mm. High-resolution TEM images show that each carbon nanotube is composed of graphite-like layers arranged in a stacked-cup-like structure. XPS spectrum shows that the crust covering the tops of the aligned carbon nanotube film consists of carbon, iron and ferric oxide. The EELS spectrum shows that these nanotubes are pure-carbon tubes. The formation mechanism of the netlike bulges has been provided. r 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotube; Pyrolysis; TEM
1. Introduction Since their discovery in 1991 [1], carbon nanotubes have attracted increasing attention because of their unique structural, mechanical, and electronic properties. Numerous novel applications of carbon nanotubes have been investigated, such as field emitters [2,3], nanoelectronic devices [4–8], nanotube actuators [9], batteries [10], probe tips for scanning probe microscopy [11,12], nanotubereinforced materials [13], etc. For these applications, largescale and highly aligned arrays of nanotubes are desirable. Aligned nanotubes were obtained previously by using chemical vapor deposition (CVD) over catalysts embedded in mesoporous silica [14,15], and over laser-patterned catalysts [16,17]. Ren et al. [18] used plasma-enhanced CVD and synthesized self-aligned nanotubes on glass substrates. Although many research groups have fabricated high-aligned carbon nanotubes, the formation of highaligned carbon nanotube film with fantastic surface morphology has not been reported. Corresponding author. Tel.: +86 755 26538537; fax: +86 755 26536239. E-mail address: [email protected] (P. Cao).
0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.09.014
In the present study, a unique three-step procedure is employed to synthesize high-aligned carbon nanotube film with netlike bulges in a single stage furnace and the formation mechanism of the netlike bulges has been discussed. 2. Experimental procedure 2.1. Formation of silica layer A single-stage furnace was used to synthesize silica layer [19]. A single-crystal silicon (1 1 1) wafer was cleaned ultrasonically in acetone and ethanol baths consecutively for 10 min each. It was then dipped into 20% hydrofluoric acid solution for 5 min to expose a clean silicon surface. The wafer was washed by distilled water and put in quartz boat A, which was placed at the center of the furnace. First O2 was introduced into the quartz tube at a flow rate of 100 sccm for 20 min to ensure the purity of O2 gas in the quartz tube. The temperature inside furnace was raised to 700 1C at a rate of 12 1C/min and maintained for 2 h. Then it was allowed to return to room temperature naturally. A silica layer formed on the silicon surface.
ARTICLE IN PRESS 954
P. Cao et al. / Vacuum 81 (2007) 953–957
2.2. Fabrication of aligned carbon nanotube film A new procedure was used to grow aligned carbon nanotube film. First, a silica wafer was placed in quartz boat B, which was moved into the furnace, 8.5 cm from boat A, and a 1:5 mixture (by weight) of powdered ferrocene (dicyclopentadienyliron, Aldrich 98%, 300 mg) and melamine (s-triaminotriazine; Avocado, 99%; 1500 mg) was introduced into boat A. High-purity Ar gas was pumped into the tube at a flow rate of 100 sccm for 20 min to ensure a stable atmosphere. Second, a three-step procedure was employed to control the furnace temperature. The temperature at the center of furnace was increased to 180 1C and maintained for 30 min so as to evaporate ferrocene fully. It was then raised to 380 1C and kept constant for 30 min to evaporate melamine completely. After that, it was increased to 900 1C and held steady for 60 min to form carbon nanotubes. During all three steps the rate of raising the temperature was kept at 12 1C/min and no more Ar gas was introduced. Finally, the quartz tube was cooled to room temperature with Ar, flowing at a rate of 10 sccm. During the above procedure, the pressure of Ar in quartz tube was kept at atmosphere pressure. Upon removing the sample from the furnace, a brown layer was observed to have grown on the whole surface of the silica wafer. 2.3. Characterization A field emission type SEM (Hitachi S-4200) was used to observe the morphology of aligned carbon nanotube film. The acceleration voltage was 20 kV during measurements. TEM images were obtained from Tecnai-20 (PHILIPS, Holland) and Tecnai F20 (FEI Corp., America). During
the TEM observation, the acceleration voltage was 200 kV. A Gatan parallel electron-energy-loss spectrometer (EELS) installed on the Tecnai F20 TEM is employed to determine the composition of an individual carbon nanotube. XPS measurements were performed using an ESCA LAB 5 X-ray photoelectron spectrometer with a monochromatized Mg Ka (1253.6 eV) X-ray source. The preparation procedure for the TEM specimen was as follows. Aligned carbon nanotube film grown on silica wafer was scratched from silica substrate and cleaned ultrasonically in an ethanol bath for 10 min. Finally two drops of this liquid were dropped onto a carbon coated copper grid. 3. Results and discussion 3.1. SEM morphologies Typical SEM images of high-aligned carbon nanotube film are shown in Figs. 1(a)–(d). Fig. 1(a) shows an overall image of the sample. It can be seen that large-area carbon nanotube film forms and that many netlike bulges appear on the surface of carbon nanotube film. Fig. 1(b) shows a magnified image of two bulges. It can be seen faintly that these bulges are made of particles and that there are carbon nanotubes protruding from the bulge (marked by white arrow). Carbon nanotube film has been scratched from the silica surface and their SEM image is given in Fig. 1(c). It can be seen that carbon nanotube film is composed of many high-aligned carbon nanotube arrays. High-magnified image of one carbon nanotube array is shown in Fig. 1(d). It can be seen that a crust presents on the top end of carbon nanotube array (marked by black arrow) and that many crusts have been peeled off from carbon
Fig. 1. SEM images of carbon nanotube film. (a) a general view, (b) high magnification SEM micrographs of two bulges, (c) high-aligned carbon nanotubes arrays scratched from silica surface and (d) a cross-sectional view of aligned carbon nanotubes.
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957
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nanotube array (marked by white arrow). As-grown high density, high-aligned carbon nanotubes have the length of about 40 mm. No crust presents at the bottom end of carbon nanotube array, which indicates that the growth of a tube is catalyzed by a particle that ends up in the crust. 3.2. XPS analysis Fig. 2 shows an overall XPS scan of the crust on top of carbon nanotube film. Indicated in the spectrum are C1 s (285.0 eV), O1 s (532.7 eV) and O (KVV) (746.7 eV) along with iron (peaks identified by ‘‘’’ sign). The binding energy of carbon is 285.0 eV, while those of iron are 53.7 (Fe3p), 90.7 (Fe3s), 552.7 (Fe (L3VV)), 606.7 (Fe (L3M23V)) and 707.7 eV (Fe2p), respectively. The inset in Fig. 2 shows the profile of Fe2p XPS spectrum. The Fe2p3/2 (707.5 eV) and Fe 2p1/2 (720.7 eV) peaks are double peaks (besides the main peak shoulder peaks lie at the high binding energy sides with binding energy of 710.3 and 724.2 eV for 2p3/2 and 2p1/2, respectively), indicating that Fe has two chemical states, one is the pure Fe and the other is ferric oxide. So it can be concluded that the crust covering on the top end of carbon nantoube film is composed of carbon, iron and ferric oxide. The intensive O peak in the overall XPS spectrum comes from ferric oxide in surface crust and surface absorbed oxygen. 3.3. TEM structures TEM was employed to study the structures of carbon nanotubes in greater detail. Typical morphologies of carbon nanotubes are displayed in Figs. 3(a)–(b). Lowresolution TEM image (Fig. 3(a)) shows that carbon nanotubes are compartmentalized by a lateral segmentation; i.e., divided into many irregular cup-like structures oriented in the same direction—stacked, in a sense. They have a highly uniform outer diameter—i.e., the tube’s outer
O (KVV)
4000
710
720
2000
730 O 1s
700
C 1s
Intensity (a.u.)
3000
1000
Fig. 3. (a) Low-resolution TEM micrograph of a typical region with aligned nanotubes and (b) high-resolution TEM micrograph of top end of an individual carbon nanotube.
diameter, estimated to be about 25 nm. High-resolution TEM image of top end of individual carbon nanotube is shown in Fig. 3(b). It can be seen clearly that the catalyst particle (the black region) presents on the tip of nanotube and that every cup-like structure replicates the shape of the catalyst particle. 3.4. EELS analysis EELS was used to examine the spectrum of an individual carbon nanotube. Electron beams of 100 keV in energy can be focused on an area about several tens of nanometers in diameter, so we are able to obtain information on each individual tube. The p* and s* peaks in the region of the C K-shell ionization edge are displayed clearly (Fig. 4). On the other hand, no N K-shell ionization edge, lying at 401 eV, can be seen. The absence of an N K-shell ionization edge proves that no nitrogen element from melamine has been doped into the carbon nanotubes in this work and that as-grown nanotubes are pure carbon tubes. For the C K-shell ionization edge, the sharp p* peak (285.6 eV) indicates that C is in a sp2 hybridization state and that carbon nanotube is mainly composed of hexagonal-graphene layers, while the existence of the s* peak (295.7 eV) indicates the formation of pentagonal defects in graphene layers [20]. 3.5. Formation mechanism of the netlike bulges
0 0
200
400 600 Binding Energy (eV)
800
1000
Fig. 2. Wide survey XPS spectrum of aligned CNTs surface (—binding energy position of iron). Inset, a Gaussian fit of the Fe2p spectrum reveals the presence of four peaks at 707.5, 710.3, 720.7 and 724.2 eV.
Now that we have considered the physical structure and chemical makeup of carbon nanotubes, we develop a simple scenario of their growth process as shown in Fig. 5. First the ferrocene powders sublime from boat A at the
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957
956
σ* C-K edge
π*
300
400
500 Energy Loss (eV)
600
700
Fig. 4. EELS spectrum of an individual carbon nanotube, showing the presence of the ionization edge at 285.6 eV corresponding to the C K-shell.
dissolve into the top of the iron particles and diffuse to the other side, forming a shell consisting of graphene layers on the bottom surface of the iron particle. Each shell assumes the exact shape of the bottom of the iron particle that condenses it [21]. As the shell grows thicker, stress builds. When the stress reaches a critical value, it is relaxed by the iron particle pushing away from the bottom of the shell, thus defining a space that becomes a compartment of carbon nanotube as the space is enclosed by the next carbon shell formed under the iron particle at its new position. As the above process continues, long, stackedcup-like nanotubes form with catalytic iron particles on their tops. On the action of van der Waals forces, these carbon nantoubes grow with the same direction and finally lead to the formation of high-aligned carbon nantoube film (Fig. 5(g). The formation mechanism of the netlike bulges made of catalyst particles is illustrated as follows. During the first stage from Figs. 5(b)–(c), the ferrocene layer has been formed on silica surface, while the melamine layer has not been deposited. The ferrocene particles expand with the increasing of the substrate temperature from 180 to 380 1C, which finally leads to the appearance of the bulge as shown in Fig. 5(c). During the second stage from Figs. 5(d) to (e), after the melamine layer has been deposited on the ferrocene layer, with the rising of the substrate temperature from 380 to 900 1C, the expansions of the ferrocene layer along with the melamine layer may lead to the formation of the bulge (Fig. 5(e)). Further work is needed to ensure clearly during which stage the netlike bulges made of catalyst particles from. 4. Conclusions
Fig. 5. Schematic illustration of the formation process for the netlike bulges made of catalyst crust.
High-aligned carbon nanotube film has been fabricated successfully on a silica wafer using a high-temperature pyrolysis method in a single-stage furnace. The surface of the carbon nanotube film was covered by the netlike bulges made of catalyst particles, which are composed of iron, ferric oxide and carbon particles. As-grown nanotubes are pure-carbon tubes and exhibit stacked-cup-like structures. They have an uniform diameter of 25 nm and length of about 40 mm. Based on the above account, our three-step procedure has yielded interesting results and the formation mechanism of the netlike bulges has been provided. Acknowledgments
center of the furnace and condense on the silica substrate in boat B, which leads to the formation of a thin layer of ferrocene powder on the silica surface (Fig. 5(b)). Then melamine is deposited on top of the ferrocene layer (Fig. 5(d)). When the temperature is raised from 380 to 900 1C and maintained for 60 min, iron particles form from the decomposition of ferrocene at a proper temperature (Fig. 5(f)). Each iron particle catalyzes the condensation of carbon atoms coming from ferrocene and melamine, which
This work was supported by the Science and Technology Foundation of Shenzhen (Grant nos. 200448, 200501 and 200503). References [1] Iijima S. Nature 1991;354:56. [2] De Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179.
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957 [3] Fan SS, Chapline MG, Franklin NR, Tombler TW, Cassell AM, Dai HJ. Science 1999;283:512. [4] Collins PG, Zettl A, Bando H, Thess A, Smalley RE. Science 1997;278:100. [5] Frank S, Poncharal P, Wang ZL, De Heer WA. Science 1998;280:1744. [6] Tans SJ, Verschueren ARM, Dekker C. Nature 1998;393:49. [7] White C, Todorov TN. Nature 1998;393:240. [8] Menon M, Srivastava D. Phys Rev Lett 1997;79:443. [9] Baughman RH, Cui CX, Zakhidov AA, Lqval Z, Barisci JN, Spinks BM, et al. Science 1999;284:1340. [10] Che GL, Lakschmi BB, Fisher ER, Martin CR. Nature 1998;393:346. [11] Wong SS, Joselevich E, Woolley AT, Cheung CI, Lieber CM. Nature 1998;394:52. [12] Wong SS, Woolley AT, Joselevich E, Cheung C, Lieber CM. J Am Chem Soc 1998;120:8557.
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[13] Dresselhaus MS. Nature 1992;358:195. [14] Li WZ, Xie SS, Qian LX, Chang BH, Zou BS, Zhou WY, et al. Science 1996;274:1701. [15] Pan ZW, Xie SS, Chang BH, Wang CY, Lu L, Liu W, et al. Nature 1998;394:631. [16] Terrones M, Grobert N, Zhang JP, Terrones H, Olivares J, Hsu WK, et al. Chem Phys Lett 1998;285:299. [17] Terrones M, Redlich P, Grobert N, Trasovares S, Hsu WK, Terrones H, et al. Adv Mater 1999;11:655. [18] Ren ZF, Huang ZP, Wu JW, Wang JH, Bush P, Siegal MP, et al. Science 1995;282:1105. [19] Cao PJ, Gu YS, Liu F, Liu HW, Zhang HR, Shen F, et al. Appl Phys A 2005;80:195. [20] Iijima S, Ichihashi T, Ando Y. Nature 1992;356:776. [21] Terrones M, Terrones H, Grobert N, Hsu WK, Walton DRM, Ru¨hle M, et al. Appl Phys Lett 1999;75:3932.
Vacuum 81 (2007) 953–957 www.elsevier.com/locate/vacuum
High-aligned carbon nanotube film with netlike bulges Peijiang Cao, Deliang Zhu, Wenjun Liu, Xiaocui Ma Department of Materials Science and Engineering, Shenzhen University; Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China Received 20 July 2006; accepted 28 July 2006
Abstract High-aligned carbon nanotubes film with netlike bulges made of catalyst particles has been synthesized on a silica wafer by pyrolyzing ferrocene/melamine mixtures. The structure and composition of carbon nanotubes are investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS). It is found that these nanotubes have uniform outer diameters of about 25 nm and lengths of about 40 mm. High-resolution TEM images show that each carbon nanotube is composed of graphite-like layers arranged in a stacked-cup-like structure. XPS spectrum shows that the crust covering the tops of the aligned carbon nanotube film consists of carbon, iron and ferric oxide. The EELS spectrum shows that these nanotubes are pure-carbon tubes. The formation mechanism of the netlike bulges has been provided. r 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotube; Pyrolysis; TEM
1. Introduction Since their discovery in 1991 [1], carbon nanotubes have attracted increasing attention because of their unique structural, mechanical, and electronic properties. Numerous novel applications of carbon nanotubes have been investigated, such as field emitters [2,3], nanoelectronic devices [4–8], nanotube actuators [9], batteries [10], probe tips for scanning probe microscopy [11,12], nanotubereinforced materials [13], etc. For these applications, largescale and highly aligned arrays of nanotubes are desirable. Aligned nanotubes were obtained previously by using chemical vapor deposition (CVD) over catalysts embedded in mesoporous silica [14,15], and over laser-patterned catalysts [16,17]. Ren et al. [18] used plasma-enhanced CVD and synthesized self-aligned nanotubes on glass substrates. Although many research groups have fabricated high-aligned carbon nanotubes, the formation of highaligned carbon nanotube film with fantastic surface morphology has not been reported. Corresponding author. Tel.: +86 755 26538537; fax: +86 755 26536239. E-mail address: [email protected] (P. Cao).
0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.09.014
In the present study, a unique three-step procedure is employed to synthesize high-aligned carbon nanotube film with netlike bulges in a single stage furnace and the formation mechanism of the netlike bulges has been discussed. 2. Experimental procedure 2.1. Formation of silica layer A single-stage furnace was used to synthesize silica layer [19]. A single-crystal silicon (1 1 1) wafer was cleaned ultrasonically in acetone and ethanol baths consecutively for 10 min each. It was then dipped into 20% hydrofluoric acid solution for 5 min to expose a clean silicon surface. The wafer was washed by distilled water and put in quartz boat A, which was placed at the center of the furnace. First O2 was introduced into the quartz tube at a flow rate of 100 sccm for 20 min to ensure the purity of O2 gas in the quartz tube. The temperature inside furnace was raised to 700 1C at a rate of 12 1C/min and maintained for 2 h. Then it was allowed to return to room temperature naturally. A silica layer formed on the silicon surface.
ARTICLE IN PRESS 954
P. Cao et al. / Vacuum 81 (2007) 953–957
2.2. Fabrication of aligned carbon nanotube film A new procedure was used to grow aligned carbon nanotube film. First, a silica wafer was placed in quartz boat B, which was moved into the furnace, 8.5 cm from boat A, and a 1:5 mixture (by weight) of powdered ferrocene (dicyclopentadienyliron, Aldrich 98%, 300 mg) and melamine (s-triaminotriazine; Avocado, 99%; 1500 mg) was introduced into boat A. High-purity Ar gas was pumped into the tube at a flow rate of 100 sccm for 20 min to ensure a stable atmosphere. Second, a three-step procedure was employed to control the furnace temperature. The temperature at the center of furnace was increased to 180 1C and maintained for 30 min so as to evaporate ferrocene fully. It was then raised to 380 1C and kept constant for 30 min to evaporate melamine completely. After that, it was increased to 900 1C and held steady for 60 min to form carbon nanotubes. During all three steps the rate of raising the temperature was kept at 12 1C/min and no more Ar gas was introduced. Finally, the quartz tube was cooled to room temperature with Ar, flowing at a rate of 10 sccm. During the above procedure, the pressure of Ar in quartz tube was kept at atmosphere pressure. Upon removing the sample from the furnace, a brown layer was observed to have grown on the whole surface of the silica wafer. 2.3. Characterization A field emission type SEM (Hitachi S-4200) was used to observe the morphology of aligned carbon nanotube film. The acceleration voltage was 20 kV during measurements. TEM images were obtained from Tecnai-20 (PHILIPS, Holland) and Tecnai F20 (FEI Corp., America). During
the TEM observation, the acceleration voltage was 200 kV. A Gatan parallel electron-energy-loss spectrometer (EELS) installed on the Tecnai F20 TEM is employed to determine the composition of an individual carbon nanotube. XPS measurements were performed using an ESCA LAB 5 X-ray photoelectron spectrometer with a monochromatized Mg Ka (1253.6 eV) X-ray source. The preparation procedure for the TEM specimen was as follows. Aligned carbon nanotube film grown on silica wafer was scratched from silica substrate and cleaned ultrasonically in an ethanol bath for 10 min. Finally two drops of this liquid were dropped onto a carbon coated copper grid. 3. Results and discussion 3.1. SEM morphologies Typical SEM images of high-aligned carbon nanotube film are shown in Figs. 1(a)–(d). Fig. 1(a) shows an overall image of the sample. It can be seen that large-area carbon nanotube film forms and that many netlike bulges appear on the surface of carbon nanotube film. Fig. 1(b) shows a magnified image of two bulges. It can be seen faintly that these bulges are made of particles and that there are carbon nanotubes protruding from the bulge (marked by white arrow). Carbon nanotube film has been scratched from the silica surface and their SEM image is given in Fig. 1(c). It can be seen that carbon nanotube film is composed of many high-aligned carbon nanotube arrays. High-magnified image of one carbon nanotube array is shown in Fig. 1(d). It can be seen that a crust presents on the top end of carbon nanotube array (marked by black arrow) and that many crusts have been peeled off from carbon
Fig. 1. SEM images of carbon nanotube film. (a) a general view, (b) high magnification SEM micrographs of two bulges, (c) high-aligned carbon nanotubes arrays scratched from silica surface and (d) a cross-sectional view of aligned carbon nanotubes.
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957
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nanotube array (marked by white arrow). As-grown high density, high-aligned carbon nanotubes have the length of about 40 mm. No crust presents at the bottom end of carbon nanotube array, which indicates that the growth of a tube is catalyzed by a particle that ends up in the crust. 3.2. XPS analysis Fig. 2 shows an overall XPS scan of the crust on top of carbon nanotube film. Indicated in the spectrum are C1 s (285.0 eV), O1 s (532.7 eV) and O (KVV) (746.7 eV) along with iron (peaks identified by ‘‘’’ sign). The binding energy of carbon is 285.0 eV, while those of iron are 53.7 (Fe3p), 90.7 (Fe3s), 552.7 (Fe (L3VV)), 606.7 (Fe (L3M23V)) and 707.7 eV (Fe2p), respectively. The inset in Fig. 2 shows the profile of Fe2p XPS spectrum. The Fe2p3/2 (707.5 eV) and Fe 2p1/2 (720.7 eV) peaks are double peaks (besides the main peak shoulder peaks lie at the high binding energy sides with binding energy of 710.3 and 724.2 eV for 2p3/2 and 2p1/2, respectively), indicating that Fe has two chemical states, one is the pure Fe and the other is ferric oxide. So it can be concluded that the crust covering on the top end of carbon nantoube film is composed of carbon, iron and ferric oxide. The intensive O peak in the overall XPS spectrum comes from ferric oxide in surface crust and surface absorbed oxygen. 3.3. TEM structures TEM was employed to study the structures of carbon nanotubes in greater detail. Typical morphologies of carbon nanotubes are displayed in Figs. 3(a)–(b). Lowresolution TEM image (Fig. 3(a)) shows that carbon nanotubes are compartmentalized by a lateral segmentation; i.e., divided into many irregular cup-like structures oriented in the same direction—stacked, in a sense. They have a highly uniform outer diameter—i.e., the tube’s outer
O (KVV)
4000
710
720
2000
730 O 1s
700
C 1s
Intensity (a.u.)
3000
1000
Fig. 3. (a) Low-resolution TEM micrograph of a typical region with aligned nanotubes and (b) high-resolution TEM micrograph of top end of an individual carbon nanotube.
diameter, estimated to be about 25 nm. High-resolution TEM image of top end of individual carbon nanotube is shown in Fig. 3(b). It can be seen clearly that the catalyst particle (the black region) presents on the tip of nanotube and that every cup-like structure replicates the shape of the catalyst particle. 3.4. EELS analysis EELS was used to examine the spectrum of an individual carbon nanotube. Electron beams of 100 keV in energy can be focused on an area about several tens of nanometers in diameter, so we are able to obtain information on each individual tube. The p* and s* peaks in the region of the C K-shell ionization edge are displayed clearly (Fig. 4). On the other hand, no N K-shell ionization edge, lying at 401 eV, can be seen. The absence of an N K-shell ionization edge proves that no nitrogen element from melamine has been doped into the carbon nanotubes in this work and that as-grown nanotubes are pure carbon tubes. For the C K-shell ionization edge, the sharp p* peak (285.6 eV) indicates that C is in a sp2 hybridization state and that carbon nanotube is mainly composed of hexagonal-graphene layers, while the existence of the s* peak (295.7 eV) indicates the formation of pentagonal defects in graphene layers [20]. 3.5. Formation mechanism of the netlike bulges
0 0
200
400 600 Binding Energy (eV)
800
1000
Fig. 2. Wide survey XPS spectrum of aligned CNTs surface (—binding energy position of iron). Inset, a Gaussian fit of the Fe2p spectrum reveals the presence of four peaks at 707.5, 710.3, 720.7 and 724.2 eV.
Now that we have considered the physical structure and chemical makeup of carbon nanotubes, we develop a simple scenario of their growth process as shown in Fig. 5. First the ferrocene powders sublime from boat A at the
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957
956
σ* C-K edge
π*
300
400
500 Energy Loss (eV)
600
700
Fig. 4. EELS spectrum of an individual carbon nanotube, showing the presence of the ionization edge at 285.6 eV corresponding to the C K-shell.
dissolve into the top of the iron particles and diffuse to the other side, forming a shell consisting of graphene layers on the bottom surface of the iron particle. Each shell assumes the exact shape of the bottom of the iron particle that condenses it [21]. As the shell grows thicker, stress builds. When the stress reaches a critical value, it is relaxed by the iron particle pushing away from the bottom of the shell, thus defining a space that becomes a compartment of carbon nanotube as the space is enclosed by the next carbon shell formed under the iron particle at its new position. As the above process continues, long, stackedcup-like nanotubes form with catalytic iron particles on their tops. On the action of van der Waals forces, these carbon nantoubes grow with the same direction and finally lead to the formation of high-aligned carbon nantoube film (Fig. 5(g). The formation mechanism of the netlike bulges made of catalyst particles is illustrated as follows. During the first stage from Figs. 5(b)–(c), the ferrocene layer has been formed on silica surface, while the melamine layer has not been deposited. The ferrocene particles expand with the increasing of the substrate temperature from 180 to 380 1C, which finally leads to the appearance of the bulge as shown in Fig. 5(c). During the second stage from Figs. 5(d) to (e), after the melamine layer has been deposited on the ferrocene layer, with the rising of the substrate temperature from 380 to 900 1C, the expansions of the ferrocene layer along with the melamine layer may lead to the formation of the bulge (Fig. 5(e)). Further work is needed to ensure clearly during which stage the netlike bulges made of catalyst particles from. 4. Conclusions
Fig. 5. Schematic illustration of the formation process for the netlike bulges made of catalyst crust.
High-aligned carbon nanotube film has been fabricated successfully on a silica wafer using a high-temperature pyrolysis method in a single-stage furnace. The surface of the carbon nanotube film was covered by the netlike bulges made of catalyst particles, which are composed of iron, ferric oxide and carbon particles. As-grown nanotubes are pure-carbon tubes and exhibit stacked-cup-like structures. They have an uniform diameter of 25 nm and length of about 40 mm. Based on the above account, our three-step procedure has yielded interesting results and the formation mechanism of the netlike bulges has been provided. Acknowledgments
center of the furnace and condense on the silica substrate in boat B, which leads to the formation of a thin layer of ferrocene powder on the silica surface (Fig. 5(b)). Then melamine is deposited on top of the ferrocene layer (Fig. 5(d)). When the temperature is raised from 380 to 900 1C and maintained for 60 min, iron particles form from the decomposition of ferrocene at a proper temperature (Fig. 5(f)). Each iron particle catalyzes the condensation of carbon atoms coming from ferrocene and melamine, which
This work was supported by the Science and Technology Foundation of Shenzhen (Grant nos. 200448, 200501 and 200503). References [1] Iijima S. Nature 1991;354:56. [2] De Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179.
ARTICLE IN PRESS P. Cao et al. / Vacuum 81 (2007) 953–957 [3] Fan SS, Chapline MG, Franklin NR, Tombler TW, Cassell AM, Dai HJ. Science 1999;283:512. [4] Collins PG, Zettl A, Bando H, Thess A, Smalley RE. Science 1997;278:100. [5] Frank S, Poncharal P, Wang ZL, De Heer WA. Science 1998;280:1744. [6] Tans SJ, Verschueren ARM, Dekker C. Nature 1998;393:49. [7] White C, Todorov TN. Nature 1998;393:240. [8] Menon M, Srivastava D. Phys Rev Lett 1997;79:443. [9] Baughman RH, Cui CX, Zakhidov AA, Lqval Z, Barisci JN, Spinks BM, et al. Science 1999;284:1340. [10] Che GL, Lakschmi BB, Fisher ER, Martin CR. Nature 1998;393:346. [11] Wong SS, Joselevich E, Woolley AT, Cheung CI, Lieber CM. Nature 1998;394:52. [12] Wong SS, Woolley AT, Joselevich E, Cheung C, Lieber CM. J Am Chem Soc 1998;120:8557.
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