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Combined growth of carbon nanotubes and carbon nanowalls by plasma-enhanced chemical vapor deposition
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Carbon 45 (2007) 2932–2937 www.elsevier.com/locate/carbon
Combined growth of carbon nanotubes and carbon nanowalls by plasma-enhanced chemical vapor deposition Alexander Malesevic a,b,*, Sorin Vizireanu c, Raymond Kemps a, Annick Vanhulsel a, Chris Van Haesendonck b, Gheorghe Dinescu c b
a VITO Materials, Flemish Institute for Technological Research, Boeretang 200, BE-2400 Mol, Belgium Laboratory of Solid-State Physics and Magnetism, Katholieke Universiteit Leuven, Celestijnenlaan 200D, BE-3001 Leuven, Belgium c National Institute for Laser, Plasma and Radiation Physics, Magurele MG-36, RO-077125 Bucharest, Romania
Received 11 April 2007; accepted 1 October 2007 Available online 12 October 2007
Abstract A technique is reported for the combined growth of carbon nanotubes (CNTs) and carbon nanowalls (CNWs) by plasma-enhanced chemical vapor deposition. The observation serves as a direct proof of a close correlation between the growth of both materials because both are obtained in a single experiment without making any changes to the growth parameters. The growth of freestanding CNTs is driven by a nickel catalyst deposited on an oxidized silicon wafer. It is assumed that the remaining carbon radicals are inserted in the sidewalls and tips of the tubes after the saturation of the catalyst by abundant carbon, thereby forming a CNW layer on top of the CNTs. A possible growth scheme, based on qualitative analysis by electron microscopy, Raman spectroscopy and X-ray diffraction, is presented. It is further shown that the CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the sample. 2007 Elsevier Ltd. All rights reserved.
1. Introduction The carbon nanotube (CNT) synthesis research has strongly evolved over the last years with major breakthroughs in the development of water based chemical vapor deposition for the production of high quality millimeter long single-walled carbon nanotubes [1,2] and the first reports on the successful control of the tube chirality [3–5]. Parallel to these developments, an at first sight new type of carbon material was reported by Wu et al. [6], who observed wall-like carbon structures, perpendicular to the substrate surface. These structures were called carbon nanowalls (CNWs) and transmission electron microscopy studies revealed that CNWs consist of few graphene sheets stacked together, like thin graphite flakes [7–9]. * Corresponding author. Address: VITO Materials, Flemish Institute for Technological Research, Boeretang 200, BE-2400 Mol, Belgium. Tel.: +32 14 33 56 87; fax: +32 14 32 11 86. E-mail address: [email protected] (A. Malesevic).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.004
Since the discovery of CNWs, various groups reported the synthesis of these remarkable structures and performed characterization mainly by electron microscopy and Raman spectroscopy [10–19]. There is an obvious link between CNTs and CNWs, as they both consist solely of carbon atoms arranged in graphene sheets with or without curvature. Up till now the similarities between CNTs and CNWs could only be pointed out indirectly, by comparing e.g. the structural properties and field emission characteristics. By producing both CNTs and CNWs in the same experiment without any change in the growth parameters, we here give direct proof of a high degree of correlation between the growth mechanism of both materials. Our results suggest that the parameter window for the growth of CNTs and CNWs can be identical. Electron microscopy observations reveal how metal particles catalyze the growth of CNTs which in turn we believe to catalyze the growth of CNWs. We assume that carbon radicals are not only dissolved and precipitated by the metal catalyst in order to
A. Malesevic et al. / Carbon 45 (2007) 2932–2937
form tubular CNTs but are also inserted in the sidewalls and tips of the CNTs, forming graphite branches that become CNWs. A similar formation of graphite branches on the sidewalls of CNTs was already reported by Morjan et al. [20], who related this phenomenon to an increased carbon radical concentration in the plasma. There need to be more carbon radicals available than necessary for the growth of CNTs and less than would result in an immediate catalyst poisoning and CNT growth inhibition. Just like single-walled and multi-walled CNTs can be synthesized together depending among other things on the catalyst grain size, we show for the first time that CNWs can be grown together with CNTs by properly fixing the process parameters. Contrary to the separation of single-walled and multi-walled CNTs, CNWs easily detach from the sample by dipping in water, leaving the CNTs behind. This way, both materials can be separately studied and further processed [21,22]. 2. Experimental In order to combine the catalyst deposition and CNT growth in one single process, DC magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD) techniques were implemented in a single reactor. Nickel (Ni) was chosen as the catalyst for the growth of CNTs. A scheme of the experimental setup is presented in Fig. 1. A radiofrequency (RF) plasma source (13.56 MHz, RF power 50–500 W) is mounted on top of a cylindrical reaction chamber and generates a plasma beam oriented along the vertical axis. A home made DC magnetron is mounted laterally
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with its axis oriented horizontally. A substrate holder is placed at the crossing of the two axes and comprises a substrate heater. The substrate holder can be rotated to expose the substrate either to the magnetron plasma or to the radiofrequency plasma beam. Details on the plasma beam source were given elsewhere [23,24]. The RF discharge is generated by introducing argon (Ar) in a small discharge chamber containing two electrodes, one of them acting as a nozzle. The discharge expands through the nozzle from the generation zone into the deposition chamber due to a pressure gradient as a bright, long plasma beam. Acetylene (C2H2) diluted with hydrogen (H2) is introduced in the expanding Ar plasma beam via an injection ring. The injection ring is positioned 6 cm above the substrate holder. The peculiarity of this deposition system is that the discharge and the deposition regions are spatially separated. The carbon containing radicals are formed in the injection zone and are deposited onto the heated Ni catalyst. The samples discussed in this paper were prepared as follows. A 5 nm thin catalyst film was deposited onto an oxidized silicon wafer by DC sputtering a Ni target in Ar at room temperature and at a pressure of 9 · 102 mbar. After heating up the substrate to 700 C, the plasma beam was switched on in Ar at 250 W RF power and a treatment during 5 min in this plasma injected with H2 was performed. The temperature control is performed via a thermocouple inserted in the substrate heater. In a final step C2H2 was added in the injection line and the deposition was performed for 1 h with gas ratios Ar/H2/C2H2 fixed to 500/8/1. The plasmas used in the experiments are cold plasmas that do not significantly alter the substrate temperature sustained by the heater and measured by the thermocouple. The carbon coating obtained in this way is analyzed with field emission scanning electron microscopy (FESEM, JSM-6340F), Raman spectroscopy (DILOR XY 800) and X-ray diffraction (XRD, PHILIPS X’PERT). For the Raman measurements, a microscopic magnification of 100· and an Ar ion laser (SPECTRA PHYSICS BEAMLOCK) with a fixed wavelength of 514 nm were used together with a liquid nitrogen cooled CCD camera detector (EG&G 1530-C/CUV 10245). X-rays were produced with ˚ ) and after passing a monochomator, the a copper Ka target (k = 1.5406 A XRD patterns were recorded with an accuracy of 0.01 under appropriate Bragg reflection conditions. The presented XRD patterns were treated with a baseline subtraction and smoothing (averaging over 10 points).
3. Results and discussion
Fig. 1. Scheme of the experimental setup.
The as prepared carbon coating consists of two distinct layers as evidenced by cross-section SEM analysis, presented in Fig. 2. CNTs grow from the Ni catalyst islands perpendicular to the sample surface with an average diameter of 80 nm and a height of 1 lm (Fig. 2E). The tubes are most probably of the carbon nanofibre type, a typical result in PECVD experiments, without a hollow center but with a bamboo or stacked cup inner structure [25]. It is noticed in Fig. 2D that the sidewalls have a clear branched structure. Because there is no sign of Ni particles neither at the tip nor at the base of the tubes, it is hard to distinguish between a tip or base growth mechanism [25]. Residing on the tips of the CNT forest is a high density, 4 lm thick CNW foam-like structure (Fig. 2B). The most remarkable features of this structure are its clearly defined boundaries and relative constant thickness. The CNWs are heavily interconnected with each other but do not mix with the CNTs, they merely touch the tips of the CNT forest. That this is a peculiar situation is illustrated in Fig. 2A where it can be seen that the CNT and CNW layers break at different positions when cleaving the sample for crosssection SEM analysis. This way, our results may provide
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a nice opportunity for studying friction at the nanoscale by sliding the CNW foam over the CNTs. We believe and this is discussed below in greater detail that the CNWs nucleate from the sidewalls and tips of the CNTs. The limits of the process window for obtaining combined growth of CNTs and CNWs on Ni coated substrates was investigated by varying the process parameters: temperature, plasma power and gas flow. The results were found highly reproducible for temperatures starting from a lower limit of 600 C, plasma power between 250 and 400 W and a H2:C2H2 precursor gas mixture of 200– 1000% in flowing Ar. High Ar flow rates are necessary to minimize the radical time of transport from the injection ring to the substrate and prevent decay of excited species before arriving at the substrate. Sufficient dilution of C2H2 in H2 was shown necessary for successful CNT growth in previous studies by other authors, e.g. for etching unwanted amorphous carbon [26] and reducing C2H2 reactivity. We have found that an optimum H2:C2H2 precursor gas ratio is 10:1. Our experiments indicate that the presence of a catalyst is a necessary condition for the combined growth since no CNTs nor CNWs could be synthesized in the absence of a Ni catalyst. Only cauliflower-like amorphous carbon struc-
tures are deposited on the oxidized silicon substrates in the absence of Ni as illustrated in Fig. 3. The separation of the CNTs and CNWs is done by dipping the sample in pure water at 65 C during 1 min. This way the CNWs detach while the CNTs remain attached to the substrate. This technique was originally developed by Murakami et al. [27] in order to detach CNTs from quartz substrates. Separating the CNWs from the CNTs makes it possible to study electrical, magnetic and thermal properties of CNWs without the influence of catalyst and substrate in the future. A qualitative analysis of the carbon coating is performed with Raman spectroscopy and XRD. Each coating was analyzed twice: after synthesis with both CNTs and CNWs attached on the sample surface and after detachment of the CNWs. This way, it is possible to distinguish between the respective contribution of the CNTs and CNWs to the measuring signal. A comparison between the Raman spectrum of the combined CNT/CNW layer and the CNT layer after detachment of the CNWs is presented in Fig. 4. The two spectra were normalized for a better comparison. Each spectrum shows three characteristic carbon bands, i.e., the disorder induced D-band around 1350 cm1, the G-
Fig. 2. Cross-section SEM images at different magnification of the combined system of CNTs and CNWs. The CNTs grow vertically on the substrate surface and are covered by a 4 lm thick CNW layer.
A. Malesevic et al. / Carbon 45 (2007) 2932–2937
Fig. 3. In the absence of a catalyst, only cauliflower-like amorphous carbon structures were deposited. From a discontinuity in the amorphous carbon film it is clear that no CNTs nor CNWs grow underneath.
band around 1580 cm1 that is related to in-plane sp2 vibrations, and the disorder induced D 0 -band around 1610 cm1 [15,16,28–30]. For a qualitative analysis of graphite-like structures, the intensity ratio of the D-band to the G-band is measured and denoted as the R-value: R = ID/IG [31]. CNWs usually suffer from defects and are known for their relatively large R-value, exceeding unity [15,16], while pure CNTs and higly ordered graphite can have R-values close to zero [28–30]. The Raman spectra in Fig. 4 display a relatively large R-value of 0.7 for the CNTs in contrast to a value of 0.4 for the combined CNT/CNW system. These results indicate that the CNTs are very defective and add to the assumption that the tubes are likely of the carbon nanofibre type, while the CNWs are somewhat more ordered with respect to their sp2 structure. To further investigate the crystalline nature of the CNTs and CNWs, XRD was performed on the samples after synthesis as well as after detachment of the CNWs. Because
Fig. 4. Comparison of the Raman spectrum of the combined CNT and CNW system after synthesis and the Raman spectrum of the CNT layer after detachment of the CNWs. The two spectra were normalized for a better comparison. Three Raman bands can be identified, i.e., the D-band around 1350 cm1, the G-band around 1580 cm1and the D 0 -band around 1610 cm1.
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both XRD patterns are almost identical, slightly differing in intensity, only the most intense pattern, belonging to the combined CNT/CNW system, is discussed here and presented in Fig. 5. Three sets of peaks representing hexagonal carbon (C), pure Ni and Si, respectively, are identified. The C (0 0 2) peak can be used for calculating the interplane spacing between parallel graphite planes [32]. A slightly increased interlayer distance of 0.340 ± 0.001 nm is found for the combined CNT/CNW system, compared to the value of 0.336 nm for single-crystalline graphite. Since the main contribution to the XRD spectrum comes from the CNW layer, the higher interlayer distance is attributed to a combination of curvature, rotation and/or translation of the graphene sheets in the CNWs [33]. In order to develop a growth scheme, it must be known whether the CNT or CNW layer grows first and how the first layer influences the growth of the second layer. It is known that catalyst saturation or encapsulation by carbon must be prevented for the successful growth of CNTs [34]. It is hard to argue that this condition will still be valid if a 4 lm thick CNW layer is synthesized from the Ni catalyst preceding the growth of the CNTs. We therefore assume that the CNTs grow first and that abundant carbon radicals are not only dissolved and precipitated by the catalyst particles but are also inserted in the sidewalls of the tubes during the CNT growth, forming branches like in Fig. 2D. We believe that abundant carbon radicals rapidly terminate the CNT growth due to catalyst saturation and that the remaining carbon species are mainly inserted in the graphite branches originating in the vicinity of the CNT tips. The branches at the CNT tips grow fast and hinder a diffusion of carbon radicals along the nanotube axis to the lower lying branches. The branches grow longer and become a CNW layer on top of the CNTs. The process, driving the combined growth of CNTs and CNWs, is the subject of a follow-up study by means of microwave PECVD. This technique allows a more detailed investiga-
Fig. 5. XRD spectrum of the combined CNT and CNW system. Three sets of peaks can be identified belonging to hexagonal oriented carbon, cubic face-centered nickel and silicon, respectively. All peaks are marked by the Miller indices of the corresponding lattice planes.
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tion of the effect of the carbon radical density and confirms the proposed growth scheme. The formation of graphite branches on the sidewalls of CNTs has been observed previously in DC PECVD experiments and was linked to a high carbon radical concentration in the plasma sheath [20]. In the absence of a plasma sheath in our specific setup, the carbon radical concentration depends only on the injected H2:C2H2 precursor gas mix, but we believe that it plays a similar role as in the DC PECVD experiments [20]. The carbon radical concentration must be higher than needed for the growth of CNTs and lower than the catalyst saturation level beyond which no CNT nucleation is possible. The critical restrictions on the carbon radical concentration may explain why no previous studies reported on the combined growth of CNTs and CNWs. The necessity of a catalyst for the growth of CNTs in PECVD has been long identified and this rule can be expanded to the growth of CNWs on CNTs since no CNTs nor CNWs were observed in the absence of a catalyst on the substrates (see Fig. 3). This result is contradictory to several observations of CNW growth on substrates without the need for a catalyst [7,10,11], but these reports are based on experiments with a completely different process window on which no observations of the growth of CNTs in the presence of a catalyst are reported. We therefore conclude that the Ni particles catalyze the growth of CNTs which in turn catalyze the growth of CNWs. The presence of CNTs is found to be a necessary condition for the nucleation of CNWs in this case. 4. Conclusions and outlook In conclusion, a process window is identified for the combined growth of CNTs and CNWs by PECVD. The CNWs grow as a high density coating on top of the CNTs. We believe that the CNWs grow from branches on the tips and sidewalls of the CNTs, as suggested by cross-section SEM analysis. By synthesizing CNTs together with CNWs in one single experiment, without any change of the growth parameters, it was shown for the first time that the process window for the growth of CNTs and CNWs can be identical. Raman spectroscopy and XRD reveal that both CNTs and CNWs are highly crystalline and that the CNWs show less deviation from the ideal sp2 bonding than the CNTs. The CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the substrate, which facilitates further separate processing of the CNWs and of the CNTs. Acknowledgements The research in Belgium has been funded by the Flemish Institute for Technological Research (VITO), by the Belgian Interuniversity Attraction Poles (IAP) research program and by the Fund for Scientific Research – Flanders (FWO). The research in Magurele has been funded by the Romanian R&D Program CEEX.
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[28] Kastner J, Pichler T, Kuzmany H, Curran S, Blau W, Weldon DN, et al. Resonance Raman and infrared spectroscopy of carbon nanotubes. Chem Phys Lett 1994;221(1–2):53–8. [29] Dresselhaus MS, Dresselhaus G, Jorio A, Souza Filho AG, Saito R. Raman spectroscopy on isolated single wall carbon nanotubes. Carbon 2002;40(12):2043–61. [30] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Raman spectroscopy of carbon nanotubes. Phys Reports 2005;409(2):47–99. [31] Kastner J, Pichler T, Kuzmany H, Curran S, Blau W, Weldon DN, et al. Resonance Raman and infrared spectroscopy of carbon nanotubes. Chem Phys Lett 1994;221(1–2):53–8. [32] Cullity BD. Elements of X-ray diffraction. Reading, Massachusetts: Addison-Wesley Publishing Company, Inc.; 1978. [33] Li ZQ, Lu CJ, Xia ZP, Zhou Y, Luo Z. X-ray diffraction patterns of graphite and turbostratic carbon. Carbon 2007;45:1686–95. [34] Harris PJF. Solid state growth mechanisms for carbon nanotubes. Carbon 2007;45(2):229.
Carbon 45 (2007) 2932–2937 www.elsevier.com/locate/carbon
Combined growth of carbon nanotubes and carbon nanowalls by plasma-enhanced chemical vapor deposition Alexander Malesevic a,b,*, Sorin Vizireanu c, Raymond Kemps a, Annick Vanhulsel a, Chris Van Haesendonck b, Gheorghe Dinescu c b
a VITO Materials, Flemish Institute for Technological Research, Boeretang 200, BE-2400 Mol, Belgium Laboratory of Solid-State Physics and Magnetism, Katholieke Universiteit Leuven, Celestijnenlaan 200D, BE-3001 Leuven, Belgium c National Institute for Laser, Plasma and Radiation Physics, Magurele MG-36, RO-077125 Bucharest, Romania
Received 11 April 2007; accepted 1 October 2007 Available online 12 October 2007
Abstract A technique is reported for the combined growth of carbon nanotubes (CNTs) and carbon nanowalls (CNWs) by plasma-enhanced chemical vapor deposition. The observation serves as a direct proof of a close correlation between the growth of both materials because both are obtained in a single experiment without making any changes to the growth parameters. The growth of freestanding CNTs is driven by a nickel catalyst deposited on an oxidized silicon wafer. It is assumed that the remaining carbon radicals are inserted in the sidewalls and tips of the tubes after the saturation of the catalyst by abundant carbon, thereby forming a CNW layer on top of the CNTs. A possible growth scheme, based on qualitative analysis by electron microscopy, Raman spectroscopy and X-ray diffraction, is presented. It is further shown that the CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the sample. 2007 Elsevier Ltd. All rights reserved.
1. Introduction The carbon nanotube (CNT) synthesis research has strongly evolved over the last years with major breakthroughs in the development of water based chemical vapor deposition for the production of high quality millimeter long single-walled carbon nanotubes [1,2] and the first reports on the successful control of the tube chirality [3–5]. Parallel to these developments, an at first sight new type of carbon material was reported by Wu et al. [6], who observed wall-like carbon structures, perpendicular to the substrate surface. These structures were called carbon nanowalls (CNWs) and transmission electron microscopy studies revealed that CNWs consist of few graphene sheets stacked together, like thin graphite flakes [7–9]. * Corresponding author. Address: VITO Materials, Flemish Institute for Technological Research, Boeretang 200, BE-2400 Mol, Belgium. Tel.: +32 14 33 56 87; fax: +32 14 32 11 86. E-mail address: [email protected] (A. Malesevic).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.004
Since the discovery of CNWs, various groups reported the synthesis of these remarkable structures and performed characterization mainly by electron microscopy and Raman spectroscopy [10–19]. There is an obvious link between CNTs and CNWs, as they both consist solely of carbon atoms arranged in graphene sheets with or without curvature. Up till now the similarities between CNTs and CNWs could only be pointed out indirectly, by comparing e.g. the structural properties and field emission characteristics. By producing both CNTs and CNWs in the same experiment without any change in the growth parameters, we here give direct proof of a high degree of correlation between the growth mechanism of both materials. Our results suggest that the parameter window for the growth of CNTs and CNWs can be identical. Electron microscopy observations reveal how metal particles catalyze the growth of CNTs which in turn we believe to catalyze the growth of CNWs. We assume that carbon radicals are not only dissolved and precipitated by the metal catalyst in order to
A. Malesevic et al. / Carbon 45 (2007) 2932–2937
form tubular CNTs but are also inserted in the sidewalls and tips of the CNTs, forming graphite branches that become CNWs. A similar formation of graphite branches on the sidewalls of CNTs was already reported by Morjan et al. [20], who related this phenomenon to an increased carbon radical concentration in the plasma. There need to be more carbon radicals available than necessary for the growth of CNTs and less than would result in an immediate catalyst poisoning and CNT growth inhibition. Just like single-walled and multi-walled CNTs can be synthesized together depending among other things on the catalyst grain size, we show for the first time that CNWs can be grown together with CNTs by properly fixing the process parameters. Contrary to the separation of single-walled and multi-walled CNTs, CNWs easily detach from the sample by dipping in water, leaving the CNTs behind. This way, both materials can be separately studied and further processed [21,22]. 2. Experimental In order to combine the catalyst deposition and CNT growth in one single process, DC magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD) techniques were implemented in a single reactor. Nickel (Ni) was chosen as the catalyst for the growth of CNTs. A scheme of the experimental setup is presented in Fig. 1. A radiofrequency (RF) plasma source (13.56 MHz, RF power 50–500 W) is mounted on top of a cylindrical reaction chamber and generates a plasma beam oriented along the vertical axis. A home made DC magnetron is mounted laterally
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with its axis oriented horizontally. A substrate holder is placed at the crossing of the two axes and comprises a substrate heater. The substrate holder can be rotated to expose the substrate either to the magnetron plasma or to the radiofrequency plasma beam. Details on the plasma beam source were given elsewhere [23,24]. The RF discharge is generated by introducing argon (Ar) in a small discharge chamber containing two electrodes, one of them acting as a nozzle. The discharge expands through the nozzle from the generation zone into the deposition chamber due to a pressure gradient as a bright, long plasma beam. Acetylene (C2H2) diluted with hydrogen (H2) is introduced in the expanding Ar plasma beam via an injection ring. The injection ring is positioned 6 cm above the substrate holder. The peculiarity of this deposition system is that the discharge and the deposition regions are spatially separated. The carbon containing radicals are formed in the injection zone and are deposited onto the heated Ni catalyst. The samples discussed in this paper were prepared as follows. A 5 nm thin catalyst film was deposited onto an oxidized silicon wafer by DC sputtering a Ni target in Ar at room temperature and at a pressure of 9 · 102 mbar. After heating up the substrate to 700 C, the plasma beam was switched on in Ar at 250 W RF power and a treatment during 5 min in this plasma injected with H2 was performed. The temperature control is performed via a thermocouple inserted in the substrate heater. In a final step C2H2 was added in the injection line and the deposition was performed for 1 h with gas ratios Ar/H2/C2H2 fixed to 500/8/1. The plasmas used in the experiments are cold plasmas that do not significantly alter the substrate temperature sustained by the heater and measured by the thermocouple. The carbon coating obtained in this way is analyzed with field emission scanning electron microscopy (FESEM, JSM-6340F), Raman spectroscopy (DILOR XY 800) and X-ray diffraction (XRD, PHILIPS X’PERT). For the Raman measurements, a microscopic magnification of 100· and an Ar ion laser (SPECTRA PHYSICS BEAMLOCK) with a fixed wavelength of 514 nm were used together with a liquid nitrogen cooled CCD camera detector (EG&G 1530-C/CUV 10245). X-rays were produced with ˚ ) and after passing a monochomator, the a copper Ka target (k = 1.5406 A XRD patterns were recorded with an accuracy of 0.01 under appropriate Bragg reflection conditions. The presented XRD patterns were treated with a baseline subtraction and smoothing (averaging over 10 points).
3. Results and discussion
Fig. 1. Scheme of the experimental setup.
The as prepared carbon coating consists of two distinct layers as evidenced by cross-section SEM analysis, presented in Fig. 2. CNTs grow from the Ni catalyst islands perpendicular to the sample surface with an average diameter of 80 nm and a height of 1 lm (Fig. 2E). The tubes are most probably of the carbon nanofibre type, a typical result in PECVD experiments, without a hollow center but with a bamboo or stacked cup inner structure [25]. It is noticed in Fig. 2D that the sidewalls have a clear branched structure. Because there is no sign of Ni particles neither at the tip nor at the base of the tubes, it is hard to distinguish between a tip or base growth mechanism [25]. Residing on the tips of the CNT forest is a high density, 4 lm thick CNW foam-like structure (Fig. 2B). The most remarkable features of this structure are its clearly defined boundaries and relative constant thickness. The CNWs are heavily interconnected with each other but do not mix with the CNTs, they merely touch the tips of the CNT forest. That this is a peculiar situation is illustrated in Fig. 2A where it can be seen that the CNT and CNW layers break at different positions when cleaving the sample for crosssection SEM analysis. This way, our results may provide
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a nice opportunity for studying friction at the nanoscale by sliding the CNW foam over the CNTs. We believe and this is discussed below in greater detail that the CNWs nucleate from the sidewalls and tips of the CNTs. The limits of the process window for obtaining combined growth of CNTs and CNWs on Ni coated substrates was investigated by varying the process parameters: temperature, plasma power and gas flow. The results were found highly reproducible for temperatures starting from a lower limit of 600 C, plasma power between 250 and 400 W and a H2:C2H2 precursor gas mixture of 200– 1000% in flowing Ar. High Ar flow rates are necessary to minimize the radical time of transport from the injection ring to the substrate and prevent decay of excited species before arriving at the substrate. Sufficient dilution of C2H2 in H2 was shown necessary for successful CNT growth in previous studies by other authors, e.g. for etching unwanted amorphous carbon [26] and reducing C2H2 reactivity. We have found that an optimum H2:C2H2 precursor gas ratio is 10:1. Our experiments indicate that the presence of a catalyst is a necessary condition for the combined growth since no CNTs nor CNWs could be synthesized in the absence of a Ni catalyst. Only cauliflower-like amorphous carbon struc-
tures are deposited on the oxidized silicon substrates in the absence of Ni as illustrated in Fig. 3. The separation of the CNTs and CNWs is done by dipping the sample in pure water at 65 C during 1 min. This way the CNWs detach while the CNTs remain attached to the substrate. This technique was originally developed by Murakami et al. [27] in order to detach CNTs from quartz substrates. Separating the CNWs from the CNTs makes it possible to study electrical, magnetic and thermal properties of CNWs without the influence of catalyst and substrate in the future. A qualitative analysis of the carbon coating is performed with Raman spectroscopy and XRD. Each coating was analyzed twice: after synthesis with both CNTs and CNWs attached on the sample surface and after detachment of the CNWs. This way, it is possible to distinguish between the respective contribution of the CNTs and CNWs to the measuring signal. A comparison between the Raman spectrum of the combined CNT/CNW layer and the CNT layer after detachment of the CNWs is presented in Fig. 4. The two spectra were normalized for a better comparison. Each spectrum shows three characteristic carbon bands, i.e., the disorder induced D-band around 1350 cm1, the G-
Fig. 2. Cross-section SEM images at different magnification of the combined system of CNTs and CNWs. The CNTs grow vertically on the substrate surface and are covered by a 4 lm thick CNW layer.
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Fig. 3. In the absence of a catalyst, only cauliflower-like amorphous carbon structures were deposited. From a discontinuity in the amorphous carbon film it is clear that no CNTs nor CNWs grow underneath.
band around 1580 cm1 that is related to in-plane sp2 vibrations, and the disorder induced D 0 -band around 1610 cm1 [15,16,28–30]. For a qualitative analysis of graphite-like structures, the intensity ratio of the D-band to the G-band is measured and denoted as the R-value: R = ID/IG [31]. CNWs usually suffer from defects and are known for their relatively large R-value, exceeding unity [15,16], while pure CNTs and higly ordered graphite can have R-values close to zero [28–30]. The Raman spectra in Fig. 4 display a relatively large R-value of 0.7 for the CNTs in contrast to a value of 0.4 for the combined CNT/CNW system. These results indicate that the CNTs are very defective and add to the assumption that the tubes are likely of the carbon nanofibre type, while the CNWs are somewhat more ordered with respect to their sp2 structure. To further investigate the crystalline nature of the CNTs and CNWs, XRD was performed on the samples after synthesis as well as after detachment of the CNWs. Because
Fig. 4. Comparison of the Raman spectrum of the combined CNT and CNW system after synthesis and the Raman spectrum of the CNT layer after detachment of the CNWs. The two spectra were normalized for a better comparison. Three Raman bands can be identified, i.e., the D-band around 1350 cm1, the G-band around 1580 cm1and the D 0 -band around 1610 cm1.
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both XRD patterns are almost identical, slightly differing in intensity, only the most intense pattern, belonging to the combined CNT/CNW system, is discussed here and presented in Fig. 5. Three sets of peaks representing hexagonal carbon (C), pure Ni and Si, respectively, are identified. The C (0 0 2) peak can be used for calculating the interplane spacing between parallel graphite planes [32]. A slightly increased interlayer distance of 0.340 ± 0.001 nm is found for the combined CNT/CNW system, compared to the value of 0.336 nm for single-crystalline graphite. Since the main contribution to the XRD spectrum comes from the CNW layer, the higher interlayer distance is attributed to a combination of curvature, rotation and/or translation of the graphene sheets in the CNWs [33]. In order to develop a growth scheme, it must be known whether the CNT or CNW layer grows first and how the first layer influences the growth of the second layer. It is known that catalyst saturation or encapsulation by carbon must be prevented for the successful growth of CNTs [34]. It is hard to argue that this condition will still be valid if a 4 lm thick CNW layer is synthesized from the Ni catalyst preceding the growth of the CNTs. We therefore assume that the CNTs grow first and that abundant carbon radicals are not only dissolved and precipitated by the catalyst particles but are also inserted in the sidewalls of the tubes during the CNT growth, forming branches like in Fig. 2D. We believe that abundant carbon radicals rapidly terminate the CNT growth due to catalyst saturation and that the remaining carbon species are mainly inserted in the graphite branches originating in the vicinity of the CNT tips. The branches at the CNT tips grow fast and hinder a diffusion of carbon radicals along the nanotube axis to the lower lying branches. The branches grow longer and become a CNW layer on top of the CNTs. The process, driving the combined growth of CNTs and CNWs, is the subject of a follow-up study by means of microwave PECVD. This technique allows a more detailed investiga-
Fig. 5. XRD spectrum of the combined CNT and CNW system. Three sets of peaks can be identified belonging to hexagonal oriented carbon, cubic face-centered nickel and silicon, respectively. All peaks are marked by the Miller indices of the corresponding lattice planes.
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tion of the effect of the carbon radical density and confirms the proposed growth scheme. The formation of graphite branches on the sidewalls of CNTs has been observed previously in DC PECVD experiments and was linked to a high carbon radical concentration in the plasma sheath [20]. In the absence of a plasma sheath in our specific setup, the carbon radical concentration depends only on the injected H2:C2H2 precursor gas mix, but we believe that it plays a similar role as in the DC PECVD experiments [20]. The carbon radical concentration must be higher than needed for the growth of CNTs and lower than the catalyst saturation level beyond which no CNT nucleation is possible. The critical restrictions on the carbon radical concentration may explain why no previous studies reported on the combined growth of CNTs and CNWs. The necessity of a catalyst for the growth of CNTs in PECVD has been long identified and this rule can be expanded to the growth of CNWs on CNTs since no CNTs nor CNWs were observed in the absence of a catalyst on the substrates (see Fig. 3). This result is contradictory to several observations of CNW growth on substrates without the need for a catalyst [7,10,11], but these reports are based on experiments with a completely different process window on which no observations of the growth of CNTs in the presence of a catalyst are reported. We therefore conclude that the Ni particles catalyze the growth of CNTs which in turn catalyze the growth of CNWs. The presence of CNTs is found to be a necessary condition for the nucleation of CNWs in this case. 4. Conclusions and outlook In conclusion, a process window is identified for the combined growth of CNTs and CNWs by PECVD. The CNWs grow as a high density coating on top of the CNTs. We believe that the CNWs grow from branches on the tips and sidewalls of the CNTs, as suggested by cross-section SEM analysis. By synthesizing CNTs together with CNWs in one single experiment, without any change of the growth parameters, it was shown for the first time that the process window for the growth of CNTs and CNWs can be identical. Raman spectroscopy and XRD reveal that both CNTs and CNWs are highly crystalline and that the CNWs show less deviation from the ideal sp2 bonding than the CNTs. The CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the substrate, which facilitates further separate processing of the CNWs and of the CNTs. Acknowledgements The research in Belgium has been funded by the Flemish Institute for Technological Research (VITO), by the Belgian Interuniversity Attraction Poles (IAP) research program and by the Fund for Scientific Research – Flanders (FWO). The research in Magurele has been funded by the Romanian R&D Program CEEX.
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