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Fabrication of carbon nanotubes by electrical breakdown of carbon-coated Au nanowires
Materials Letters 64 (2010) 1583–1586
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of carbon nanotubes by electrical breakdown of carbon-coated Au nanowires K.J. Briston a, Y. Peng a, A.G. Cullis b, B.J. Inkson a,⁎ a b
NanoLAB Centre, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Article history: Received 26 January 2010 Accepted 20 April 2010 Available online 26 April 2010 Keywords: Gold nanowire Carbon nanotube Electrically-induced graphitization In-situ TEM Electrical measurements
a b s t r a c t A carbon nanotube has been generated by the electrically-induced breakdown of a carbon-coated Au nanowire. Under high current density the Au in the nanowire migrates towards both the anode and cathode resulting in a free-standing carbon nanotube and a 73% reduction in resistance. The resistivity of the carbon nanotube was b 8 × 10− 5 Ω m and it could cope with a current density N1.8 × 1011 A/m2, indicating a structural change from amorphous to graphitic carbon. The dimensions of carbon nanotubes produced in this way have an internal diameter controlled by the parent metal nanowire template. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Carbon nanotubes (CNTs) are among the most promising materials for applications in nanoscale science and electronics including as interconnects for electronic devices and as carriers of nanovolumes of material within their cores, for example for drug delivery [1]. A wide range of CNT fabrication methods have been developed [2], however they typically do not allow the diameters of the produced tubes to be well-controlled. Furthermore, multiwall (MW) CNTs with large outer diameters generally still have small inner diameters, limiting the size of objects that can be contained within the core. Recently, it has been found that amorphous carbon nanowires can be graphitized to form tubular graphitic structures very similar to CNTs (though not as perfect) [2–6]. This has generally been achieved by the graphitization of amorphous carbon-rich nanowires deposited by electron beam induced deposition (EBID) (e.g. [2–4]) or ion beam induced deposition (IBID) (e.g. [5]). Nogami et al. [6], however, have very recently managed to produce heavily distorted graphitic CNTs from the surface carbon contamination on free-standing Si/SiO2 nanochains. In this work we show that CNTs of more controlled diameter can be fabricated by the graphitization of carbon contamination on metal nanowires during their electrical breakdown.
70 nm diameter Au nanowires were fabricated by electrodeposition into an anodic aluminum oxide (AAO) template [7]. The AAO template was dissolved in 0.1 M NaOH freeing the nanowires into solution. The nanowires were rinsed in distilled water several times to clean off the residual NaOH and then deposited on a Si substrate. An individual Au nanowire was picked up off the substrate using a scanning electron microscope (SEM) nanomanipulator (Kleindiek MM3A) fitted with an electrochemically etched stiff and conductive NiCr tip [8]. After pickup, the protruding length of the nanowire was approximately 600 nm. The tip holding the nanowire was transferred to a NanoLAB transmission electron microscope (TEM) holder designed for making in-situ electrical measurements. A Au probe, tip diameter 190 nm, was used to contact the free-standing end of the Au nanowire inside a JEOL 2010F TEM. I–V sweeps from a positive voltage (at the probe tip) to a negative voltage were then performed to test the nanowire. The NiCr tip holding the nanowire was kept at 0 V throughout. The measured line resistance, with the Au and NiCr tips pressed firmly into direct contact, was found to be ∼77 Ω. 3. Results
⁎ Corresponding author. Tel.: + 44 114 222 5925; fax: + 44 114 222 5943. E-mail address: Beverley.Inkson@sheffield.ac.uk (B.J. Inkson). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.035
The Au nanowire and Au probe were initially covered in a layer of amorphous carbon b10 nm thick (Fig. 1(a), (b)) from fabrication and pickup in the SEM. This carbon coating appears to have been electrically insulating since initial I–V curves for the carbon-coated Au nanowire, from +0.50 V to − 0.50 V and +0.80 V to − 0.80 V, were very non-linear. For 10 different Au probe/nanowire contacts,
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Fig. 1. (a, b) TEM images of a Au nanowire and carbon coating before formation of a nanotube. (c) Example I–V curve from the Au nanowire/carbon in (a). (d, e) TEM images of the Au nanowire and CNT after electrically-induced breakdown. (f) I–V curve for the + 1.5 V to − 1.5 V voltage sweep in which the CNT was formed.
the I–V curves gave resistances of between ∼ 300 kΩ and ∼1.2 MΩ at 0.16 V (Fig. 1(c)), far above the low, linear resistance expected for Au nanowires of similar dimensions [8]. The lowest resistivity calculated for the carbon-coated Au nanowire was 3.8 × 10− 3 Ω m while, in previous work, resistivities as low as 2.26 × 10− 8Ω m have been measured for similar Au nanowires lying on substrates [8]. The high resistivities measured here are attributed to the difficulty in forming a direct contact between the tips and freely suspended Au nanowire, due to the amorphous carbon overcoating. This results in high contact resistances between the nanowire and tips. When a larger voltage sweep (V N 1 V) was applied, the Au nanowire was instantly observed to break down (Fig. 1(d)–(f)). Joule heating caused the Au nanowire to melt when the voltage was between +1.50 V and + 1.00 V, and again when between −1.00 V and −1.50 V. The Au in the middle of the nanowire flowed towards both probe contacts leaving an empty free-standing CNT behind (Fig. 1(d), (e)). No movement of Au occurred between +1.00 V and −1.00 V. During the modification of the Au nanowire with the +1.50 V to − 1.50 V sweep, and in subsequent + 1.00 V → −1.00 V voltage sweeps, the I–V curves were far more linear than before and with a drastically reduced resistance (Fig. 1(f)). Once the CNT had formed (Fig. 1(d)), resistances of between ∼8.0 kΩ and ∼76.0 kΩ at 0.20 V were measured for 3 different Au probe/nanowire contacts. This represents a reduction of at least 73% between the lowest measured resistances before and after the modification, and occurs despite the
fact that the breakdown of the Au nanowire forces the current to flow through the CNT. The length of the CNT formed was ∼ 240 nm, with an average cross-sectional area of 2400 nm2 evaluated from in-situ diameter measurements (Fig. 1(d)). The maximum resistivity of the CNT, calculated using the lowest measured resistance of 8.0 kΩ and assuming this resistance is entirely due to the CNT, is calculated to be ∼ 8 × 10− 5 Ω m. This is an upper limit for the resistivity of the nanotube as, in reality, the contacts and rest of the nanowire also contribute to the measured 8.0 kΩ resistance. The initial microstructure of the carbon coating on the Au nanowire investigated in this work, prior to electrical testing, was amorphous. Although the resistivities reported for ‘amorphous’ carbon vary depending on the degree of graphitization, the calculated maximum resistivity of the CNT here is five orders of magnitude lower than 0.689 Ω m, which was the measured resistivity for an amorphous carbon nanowire deposited by EBID using the contamination gases present in a TEM [2]. This clearly indicates the CNT generated here does not have amorphous-like resistivity. Jin et al. measured the resistivity of a graphitized carbon nanowire as 5.2 × 10− 6Ω m, which is comparable to the resistivity of highly crystalline CNTs [2]. While the resistivity measurement for the CNT in this work is not as low as that for graphitized solid carbon nanowires, this could be due to the added resistance contributions of the contacts and Au in our measurement. Further evidence for the graphitization of the carbon coating as it becomes a nanotube under current flow, comes from calculations of the current densities the nanotube can cope with. Using the smallest measured cross-section area for the CNT (∼ 1700 nm3) and the maximum negative current at the end of the voltage sweep in which it was formed (302 μA), the maximum current density through the carbon shell was ∼ 1.8 × 1011 A/m2. This is higher than the measured current density at electrical breakdown for amorphous carbon nanowires (∼2.2 × 109 A/m2) [2] by two orders of magnitude. After formation of the CNT, a higher voltage was applied to generate significantly more Joule heating. The application of a +2.00 V → −2.00 V voltage sweep led to breakdown of the CNT (Fig. 2). Substantial changes were observed as soon as the +2.00 V were applied, with most of the Au remaining near the probe tip instantly diffusing out of the CNT (Fig. 2(a)–(c)), accompanied by a reduction in contrast of the nanotube due to thinning of the carbon, which ultimately led to the nanotube breaking (Fig. 2(d)). The two probe tips also moved towards oneanother, probably due to the high current density causing Joule heating and thermal expansion of the tips. Fig. 2(e) shows the I–V curve for the nanotube failure event. The current drops very quickly to 0 μA as a result of the carbon shell thinning and eventually breaking. Although the initial current was measured at 309 μA, it is likely to have been a lot higher than this due to the rapid rate of damage during the approximate 1 s time-lag between the application of the voltage and the first measurement of the current. Following breakdown, high resolution TEM images of the ultrathin leftover fragments of the CNT revealed some limited graphitic structure where the graphite sheets were parallel to the imaging beam (see Fig. 2(f)). 4. Discussion The formation of CNTs is usually achieved by direct growth using metal catalysts, typically Fe [2,3] and Pt [9]. Recent studies have shown that amorphous carbon nanowires can be converted into tubular graphitic structures using the movement of Fe catalyst nanoparticles through the amorphous carbon [2–4]. Non-tubular graphitic ribbons have also been created by Pt nanoparticles passing through amorphous carbon nanowires [10]. The catalyst movement, which causes local graphitization, can be driven by heating [3,4] or current flow [2,10].
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melted first and migrated towards both the probe and support tips. Au migration linked to the direction of electron flow, which is characteristic of electromigration, was not observed. The method demonstrated here, using Au nanowires as a template to fabricate CNTs, could prove very useful in the future as it will allow the nanotubes produced to have well-defined dimensions, and does not involve catalyst doping. The dimensions of CNTs produced have an internal diameter controlled by the parent metal nanowire, and an external diameter controlled by the thickness of the initial carbon coating. The internal diameter can be altered by changing the diameter of the template Au nanowire, which can be easily achieved by growing Au nanowires in AAO templates with different diameter pores. It should also be possible to produce nanotubes on other nanowire templates which can be removed by Joule heating, although these should be of high uniformity or heavily distorted structures will result [6]. 5. Conclusion This work has demonstrated the production of a graphitic CNT from a carbon-coated Au nanowire. This was achieved by directly applying a +1.5 V → − 1.5 V voltage sweep to the nanowire using a NanoLAB electrical probe in TEM. The Au nanowire melted in the centre and diffused towards the electrodes, leaving a free-standing nanotube. It was concluded that the carbon left forming the nanotube had been partially graphitized due to the 73% drop in resistance, a low resistivity for the tube of b8 × 10− 5Ω m, the ability of the tube to handle high current densities N1.8 × 1011 A/m2 and evidence of graphitic structure in HRTEM images of the carbon. This method of fabrication of CNTs from metal templates could be used in the future to create CNTs with well-defined dimensions which could be useful, for example, in electronics and for drug delivery. Fig. 2. Real-time TEM observation of the electrical breakdown of a CNT. (a) t = 0 s, before the start of the voltage sweep, (b) t = 0.08 s, Au diffusing out of the nanotube at 2.0 V, (c) t = 2.32 s (1.8 V) and (d) t = 3.44 s (1.5 V). The CNT broke at t = 3.40 s. (e) I–V curve, with A and D corresponding to (a) and (d). (f) High resolution image of part of the remnant carbon circled in (d).
Here the graphitization of an amorphous carbon coating on Au has been observed. Au is not recognized as a catalyst for graphitization, however graphitic structures and carbon onions have been generated in the presence of Au by mechanical deformation of an amorphous carbon film on Au [11], arc discharge of graphite electrodes drilled out and filled with Au powder [12] and electron beam decomposition of amorphous carbon thin films with Au nanoparticles deposited on top [13]. All of these processes involve significant amounts of thermal energy. The formation of graphitic CNTs has been observed in the presence of other materials also not known for their catalytic effect, namely Ga [5] and Si/SiO2 [6]. Fujita et al. [5] observed partial graphitization of amorphous carbon nanowires with embedded Ga nanoparticles during electrical breakdown at ∼ 30 V with high current flow. Nogami et al. [6] observed the formation of graphitic CNTs from carbon-coated Si/SiO2 nanochains also as a result of electrical breakdown at ∼20 V. For these cases where a recognized catalyst is not involved (including the experiment presented here), it is probable that graphitization of the amorphous carbon is achieved due to the high local temperature produced by Joule heating when a high current density flows through a size-limited nanowire or nanochain. The local temperature produced by Joule heating is especially high for experiments where a nanowire is free-standing as there is no substrate to conduct away heat. For the case here, with a nanowire suspended between two probes (Fig. 1), the temperature generated by Joule heating was probably a maximum near the middle of the nanowire since the tips would act as heat-sinks. This is consistent with our observation that the Au near the middle of the nanowire
Acknowledgement The authors thank the EPSRC for a PhD studentship and funding GR/S85689/01 and EP/G036748/1. References [1] Endo M, Strano MS, Ajayan PM. Potential applications of carbon nanotubes. Top Appl Phys 2008;111:13–62. [2] Jin CH, Wang JY, Chen Q, Peng L-M. In situ fabrication and graphitization of amorphous carbon nanowires and their electrical properties. J Phys Chem B 2006;110:5423–8. [3] Fujita J, Ishida M, Ichihashi T, Ochiai Y, Kaito T, Matsui S. Carbon nanopillar laterally grown with electron beam-induced chemical vapor deposition. J Vac Sci Technol B 2003;21:2990–3. [4] Ichihashi T, Fujita J, Ishida M, Ochiai Y. In situ observation of carbon-nanopillar tubulization caused by liquidlike iron particles. Phys Rev Lett 2004;92:215702; Ichihashi T, Ishida M, Ochiai Y, Fujita J. In situ observation of carbon-nanopillar tubulization process. J Vac Sci Technol B 2004;22:3221–3; Ichihashi T, Ishida M, Ochiai Y, Fujita J. Carbon-nanopillar tubulization caused by liquidlike iron catalyst nanoparticles. J Surf Sci Nanotechnol 2004;4:401–5. [5] Fujita J, Ichihashi T, Nakazawa S, Okada S, Ishida M, Ochiai Y, et al. Inducing graphite tube transformation with liquid gallium and flash discharge. Appl Phys Lett 2006;88:083109; Fujita J, Nakazawa S, Ichihashi T, Ishida M, Kaito T, Matsui S. Graphitic tube transformation of FIB–CVD pillar by Joule heating with flash discharge. Microelectron Eng 2006;84:1507–10. [6] Nogami T, Ohno Y, Ichikawa S, Kohno H. Converting an insulating silicon nanochains to a conducting carbon nanotube by electrical breakdown. Nanotechnology 2009;20:335602. [7] Peng Y, Luxmoore I, Forster MD, Cullis AG, Inkson BJ. Nanomanipulation and electrical behaviour of a single gold nanowire using in-situ SEM–FIB–nanomanipulators. J Phys Conf Ser 2007;126:012031. [8] Peng Y, Cullis T, Inkson B. Accurate electrical testing of individual gold nanowires by in situ scanning electron microscope nanomanipulators. Appl Phys Lett 2008;93:183112; Peng Y, Cullis T, Möbus G, Xu X, Inkson B. Conductive nichrome probe tips: fabrication, characterization and application as nanotools. Nanotechnology 2008;20:395708. [9] Saito Y, Tani Y, Miyagawa N, Mitsushima K, Kasuya A, Nishina Y. High yield of single-wall carbon nanotubes by arc discharge using Rh–Pt mixed catalysts. Chem Phys Lett 1998;294:593–8.
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[10] Gazzadi GC, Frabboni S. Structural evolution and graphitization of metallorganic– Pt suspended nanowires under high-current-density electrical test. Appl Phys Lett 2009;94:173112. [11] Wang JJ, Lockwood AJ, Peng Y, Xu X, Bobji S, Inkson BJ. The formation of carbon nanostructures by in situ TEM mechanical nanoscale fatigue and fracture of carbon thin films. Nanotechnology 2009;20:305703.
[12] Ugarte D. How to fill or empty a graphitic onion. Chem Phys Lett 1993;209:99–103. [13] Troiani HE, Miki-Yoshida M, Camacho-Bragado GA, Marques MAL, Rubio A, Ascencio JA, et al. Direct observation of the mechanical properties of single-walled carbon nanotubes and their junctions at the atomic level. Nano Lett 2003;3:751–5.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of carbon nanotubes by electrical breakdown of carbon-coated Au nanowires K.J. Briston a, Y. Peng a, A.G. Cullis b, B.J. Inkson a,⁎ a b
NanoLAB Centre, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Article history: Received 26 January 2010 Accepted 20 April 2010 Available online 26 April 2010 Keywords: Gold nanowire Carbon nanotube Electrically-induced graphitization In-situ TEM Electrical measurements
a b s t r a c t A carbon nanotube has been generated by the electrically-induced breakdown of a carbon-coated Au nanowire. Under high current density the Au in the nanowire migrates towards both the anode and cathode resulting in a free-standing carbon nanotube and a 73% reduction in resistance. The resistivity of the carbon nanotube was b 8 × 10− 5 Ω m and it could cope with a current density N1.8 × 1011 A/m2, indicating a structural change from amorphous to graphitic carbon. The dimensions of carbon nanotubes produced in this way have an internal diameter controlled by the parent metal nanowire template. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Carbon nanotubes (CNTs) are among the most promising materials for applications in nanoscale science and electronics including as interconnects for electronic devices and as carriers of nanovolumes of material within their cores, for example for drug delivery [1]. A wide range of CNT fabrication methods have been developed [2], however they typically do not allow the diameters of the produced tubes to be well-controlled. Furthermore, multiwall (MW) CNTs with large outer diameters generally still have small inner diameters, limiting the size of objects that can be contained within the core. Recently, it has been found that amorphous carbon nanowires can be graphitized to form tubular graphitic structures very similar to CNTs (though not as perfect) [2–6]. This has generally been achieved by the graphitization of amorphous carbon-rich nanowires deposited by electron beam induced deposition (EBID) (e.g. [2–4]) or ion beam induced deposition (IBID) (e.g. [5]). Nogami et al. [6], however, have very recently managed to produce heavily distorted graphitic CNTs from the surface carbon contamination on free-standing Si/SiO2 nanochains. In this work we show that CNTs of more controlled diameter can be fabricated by the graphitization of carbon contamination on metal nanowires during their electrical breakdown.
70 nm diameter Au nanowires were fabricated by electrodeposition into an anodic aluminum oxide (AAO) template [7]. The AAO template was dissolved in 0.1 M NaOH freeing the nanowires into solution. The nanowires were rinsed in distilled water several times to clean off the residual NaOH and then deposited on a Si substrate. An individual Au nanowire was picked up off the substrate using a scanning electron microscope (SEM) nanomanipulator (Kleindiek MM3A) fitted with an electrochemically etched stiff and conductive NiCr tip [8]. After pickup, the protruding length of the nanowire was approximately 600 nm. The tip holding the nanowire was transferred to a NanoLAB transmission electron microscope (TEM) holder designed for making in-situ electrical measurements. A Au probe, tip diameter 190 nm, was used to contact the free-standing end of the Au nanowire inside a JEOL 2010F TEM. I–V sweeps from a positive voltage (at the probe tip) to a negative voltage were then performed to test the nanowire. The NiCr tip holding the nanowire was kept at 0 V throughout. The measured line resistance, with the Au and NiCr tips pressed firmly into direct contact, was found to be ∼77 Ω. 3. Results
⁎ Corresponding author. Tel.: + 44 114 222 5925; fax: + 44 114 222 5943. E-mail address: Beverley.Inkson@sheffield.ac.uk (B.J. Inkson). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.035
The Au nanowire and Au probe were initially covered in a layer of amorphous carbon b10 nm thick (Fig. 1(a), (b)) from fabrication and pickup in the SEM. This carbon coating appears to have been electrically insulating since initial I–V curves for the carbon-coated Au nanowire, from +0.50 V to − 0.50 V and +0.80 V to − 0.80 V, were very non-linear. For 10 different Au probe/nanowire contacts,
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Fig. 1. (a, b) TEM images of a Au nanowire and carbon coating before formation of a nanotube. (c) Example I–V curve from the Au nanowire/carbon in (a). (d, e) TEM images of the Au nanowire and CNT after electrically-induced breakdown. (f) I–V curve for the + 1.5 V to − 1.5 V voltage sweep in which the CNT was formed.
the I–V curves gave resistances of between ∼ 300 kΩ and ∼1.2 MΩ at 0.16 V (Fig. 1(c)), far above the low, linear resistance expected for Au nanowires of similar dimensions [8]. The lowest resistivity calculated for the carbon-coated Au nanowire was 3.8 × 10− 3 Ω m while, in previous work, resistivities as low as 2.26 × 10− 8Ω m have been measured for similar Au nanowires lying on substrates [8]. The high resistivities measured here are attributed to the difficulty in forming a direct contact between the tips and freely suspended Au nanowire, due to the amorphous carbon overcoating. This results in high contact resistances between the nanowire and tips. When a larger voltage sweep (V N 1 V) was applied, the Au nanowire was instantly observed to break down (Fig. 1(d)–(f)). Joule heating caused the Au nanowire to melt when the voltage was between +1.50 V and + 1.00 V, and again when between −1.00 V and −1.50 V. The Au in the middle of the nanowire flowed towards both probe contacts leaving an empty free-standing CNT behind (Fig. 1(d), (e)). No movement of Au occurred between +1.00 V and −1.00 V. During the modification of the Au nanowire with the +1.50 V to − 1.50 V sweep, and in subsequent + 1.00 V → −1.00 V voltage sweeps, the I–V curves were far more linear than before and with a drastically reduced resistance (Fig. 1(f)). Once the CNT had formed (Fig. 1(d)), resistances of between ∼8.0 kΩ and ∼76.0 kΩ at 0.20 V were measured for 3 different Au probe/nanowire contacts. This represents a reduction of at least 73% between the lowest measured resistances before and after the modification, and occurs despite the
fact that the breakdown of the Au nanowire forces the current to flow through the CNT. The length of the CNT formed was ∼ 240 nm, with an average cross-sectional area of 2400 nm2 evaluated from in-situ diameter measurements (Fig. 1(d)). The maximum resistivity of the CNT, calculated using the lowest measured resistance of 8.0 kΩ and assuming this resistance is entirely due to the CNT, is calculated to be ∼ 8 × 10− 5 Ω m. This is an upper limit for the resistivity of the nanotube as, in reality, the contacts and rest of the nanowire also contribute to the measured 8.0 kΩ resistance. The initial microstructure of the carbon coating on the Au nanowire investigated in this work, prior to electrical testing, was amorphous. Although the resistivities reported for ‘amorphous’ carbon vary depending on the degree of graphitization, the calculated maximum resistivity of the CNT here is five orders of magnitude lower than 0.689 Ω m, which was the measured resistivity for an amorphous carbon nanowire deposited by EBID using the contamination gases present in a TEM [2]. This clearly indicates the CNT generated here does not have amorphous-like resistivity. Jin et al. measured the resistivity of a graphitized carbon nanowire as 5.2 × 10− 6Ω m, which is comparable to the resistivity of highly crystalline CNTs [2]. While the resistivity measurement for the CNT in this work is not as low as that for graphitized solid carbon nanowires, this could be due to the added resistance contributions of the contacts and Au in our measurement. Further evidence for the graphitization of the carbon coating as it becomes a nanotube under current flow, comes from calculations of the current densities the nanotube can cope with. Using the smallest measured cross-section area for the CNT (∼ 1700 nm3) and the maximum negative current at the end of the voltage sweep in which it was formed (302 μA), the maximum current density through the carbon shell was ∼ 1.8 × 1011 A/m2. This is higher than the measured current density at electrical breakdown for amorphous carbon nanowires (∼2.2 × 109 A/m2) [2] by two orders of magnitude. After formation of the CNT, a higher voltage was applied to generate significantly more Joule heating. The application of a +2.00 V → −2.00 V voltage sweep led to breakdown of the CNT (Fig. 2). Substantial changes were observed as soon as the +2.00 V were applied, with most of the Au remaining near the probe tip instantly diffusing out of the CNT (Fig. 2(a)–(c)), accompanied by a reduction in contrast of the nanotube due to thinning of the carbon, which ultimately led to the nanotube breaking (Fig. 2(d)). The two probe tips also moved towards oneanother, probably due to the high current density causing Joule heating and thermal expansion of the tips. Fig. 2(e) shows the I–V curve for the nanotube failure event. The current drops very quickly to 0 μA as a result of the carbon shell thinning and eventually breaking. Although the initial current was measured at 309 μA, it is likely to have been a lot higher than this due to the rapid rate of damage during the approximate 1 s time-lag between the application of the voltage and the first measurement of the current. Following breakdown, high resolution TEM images of the ultrathin leftover fragments of the CNT revealed some limited graphitic structure where the graphite sheets were parallel to the imaging beam (see Fig. 2(f)). 4. Discussion The formation of CNTs is usually achieved by direct growth using metal catalysts, typically Fe [2,3] and Pt [9]. Recent studies have shown that amorphous carbon nanowires can be converted into tubular graphitic structures using the movement of Fe catalyst nanoparticles through the amorphous carbon [2–4]. Non-tubular graphitic ribbons have also been created by Pt nanoparticles passing through amorphous carbon nanowires [10]. The catalyst movement, which causes local graphitization, can be driven by heating [3,4] or current flow [2,10].
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melted first and migrated towards both the probe and support tips. Au migration linked to the direction of electron flow, which is characteristic of electromigration, was not observed. The method demonstrated here, using Au nanowires as a template to fabricate CNTs, could prove very useful in the future as it will allow the nanotubes produced to have well-defined dimensions, and does not involve catalyst doping. The dimensions of CNTs produced have an internal diameter controlled by the parent metal nanowire, and an external diameter controlled by the thickness of the initial carbon coating. The internal diameter can be altered by changing the diameter of the template Au nanowire, which can be easily achieved by growing Au nanowires in AAO templates with different diameter pores. It should also be possible to produce nanotubes on other nanowire templates which can be removed by Joule heating, although these should be of high uniformity or heavily distorted structures will result [6]. 5. Conclusion This work has demonstrated the production of a graphitic CNT from a carbon-coated Au nanowire. This was achieved by directly applying a +1.5 V → − 1.5 V voltage sweep to the nanowire using a NanoLAB electrical probe in TEM. The Au nanowire melted in the centre and diffused towards the electrodes, leaving a free-standing nanotube. It was concluded that the carbon left forming the nanotube had been partially graphitized due to the 73% drop in resistance, a low resistivity for the tube of b8 × 10− 5Ω m, the ability of the tube to handle high current densities N1.8 × 1011 A/m2 and evidence of graphitic structure in HRTEM images of the carbon. This method of fabrication of CNTs from metal templates could be used in the future to create CNTs with well-defined dimensions which could be useful, for example, in electronics and for drug delivery. Fig. 2. Real-time TEM observation of the electrical breakdown of a CNT. (a) t = 0 s, before the start of the voltage sweep, (b) t = 0.08 s, Au diffusing out of the nanotube at 2.0 V, (c) t = 2.32 s (1.8 V) and (d) t = 3.44 s (1.5 V). The CNT broke at t = 3.40 s. (e) I–V curve, with A and D corresponding to (a) and (d). (f) High resolution image of part of the remnant carbon circled in (d).
Here the graphitization of an amorphous carbon coating on Au has been observed. Au is not recognized as a catalyst for graphitization, however graphitic structures and carbon onions have been generated in the presence of Au by mechanical deformation of an amorphous carbon film on Au [11], arc discharge of graphite electrodes drilled out and filled with Au powder [12] and electron beam decomposition of amorphous carbon thin films with Au nanoparticles deposited on top [13]. All of these processes involve significant amounts of thermal energy. The formation of graphitic CNTs has been observed in the presence of other materials also not known for their catalytic effect, namely Ga [5] and Si/SiO2 [6]. Fujita et al. [5] observed partial graphitization of amorphous carbon nanowires with embedded Ga nanoparticles during electrical breakdown at ∼ 30 V with high current flow. Nogami et al. [6] observed the formation of graphitic CNTs from carbon-coated Si/SiO2 nanochains also as a result of electrical breakdown at ∼20 V. For these cases where a recognized catalyst is not involved (including the experiment presented here), it is probable that graphitization of the amorphous carbon is achieved due to the high local temperature produced by Joule heating when a high current density flows through a size-limited nanowire or nanochain. The local temperature produced by Joule heating is especially high for experiments where a nanowire is free-standing as there is no substrate to conduct away heat. For the case here, with a nanowire suspended between two probes (Fig. 1), the temperature generated by Joule heating was probably a maximum near the middle of the nanowire since the tips would act as heat-sinks. This is consistent with our observation that the Au near the middle of the nanowire
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