Single-Walled Carbon Nanotubes as Templates for Nanowire

David P. Burt, Neil R. Wilson, John M. R. Weaver, Phillip S. Dobson, and Julie V. ... Peter D. Tovee , Manuel E. Pumarol , Mark C. Rosamond , Robert J...
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NANO LETTERS

Single-Walled Carbon Nanotubes as Templates for Nanowire Conducting Probes

2003 Vol. 3, No. 10 1365-1369

Neil R. Wilson*,† and Julie V. Macpherson‡,§ Physics and Chemistry Departments, UniVersity of Warwick, CoVentry CV4 7AL, U.K. Received July 11, 2003; Revised Manuscript Received August 15, 2003

ABSTRACT We describe a method for the formation of metal nanowires at the apex of scanning probe microscopy (SPM) tips using single-walled carbon nanotubes (SWNT) as templates. The SWNT-SPM tips are sputter coated with metal, here AuPd or Au, resulting in the formation of continuous nanowires as small as 30 nm in diameter and up to a few micrometers in length. In principle, nanowires of any metal, or indeed any material, compatible with the sputter-coating process could be formed. AuPd nanowire tips are shown to be robust enough to allow conducting-mode imaging, over prolonged periods of time, with measured electrical resistance through the AuPd nanowires of less than 10 kΩ and currents of up to 200 µA. The simplicity of this method and its general applicability to the formation of nanowires of a variety of materials make it a promising technique for the formation of a range of nanoelectrodes as well as scanning probes.

Single-walled carbon nanotubes (SWNTs) have remarkable physical properties. Mechanically, they have a high Young’s modulus1 and buckle rather than break when compressed. Electronically, one in three is metallic, and the remainder are direct band gap semiconductors.2 They have been demonstrated to be ballistic conductors at room temperature,3 and a single metallic SWNT can pass tens of microamps of current.4 As a result, there has been interest in their use in applications as diverse as single-molecule biosensors,5 supertough fibers,6 and nanoelectromechanical devices.7,8 SWNTs have also demonstrated considerable promise as scanning probe microscopy (SPM) tips.9-13 SWNT-SPM tips offer important advantages over conventional Si microfabricated probes: their small diameter, ca. 1 nm, gives increased lateral resolution; their mechanical robustness gives increased longevity; and their high aspect ratio makes them ideal for probing recesses and sharp features. Recent work highlighted the potential of SWNT-SPM tips for highresolution electrical and electrochemical imaging by demonstrating that stable, low-resistance electrical contacts could be formed between a SWNT and a metal-coated SPM tip.13 However, it was not possible to attach metallic over semiconducting SWNTs selectively. Previous work has shown that suspended SWNTs can act as templates for the formation of continuous nanowires of metals such as Ti, Au, and Fe with widths e10 nm14 as well as a superconducting Mo-Ge alloy.15 In this letter, we * Corresponding author. E-mail: [email protected]. † Physics Department. ‡ Chemistry Department. § E-mail: [email protected]. 10.1021/nl034505+ CCC: $25.00 Published on Web 09/16/2003

© 2003 American Chemical Society

demonstrate the use of SWNT-SPM probes as templates for the formation of high aspect ratio metallic nanowire tips and show that they not only have excellent conducting properties but also are robust and durable for SPM imaging applications. The nanowires are formed by sputter coating a thin film of metal onto the SWNT-SPM tip. Thus, in principle, any material compatible with the sputter-coating process could be used to form the nanowire. In this way, nanowire probes are complementary to SWNT-SPM probes just as conventional metal-coated SPM probes are complementary to conventional probes. The resulting nanowire has a high aspect ratio that is not only the optimal geometry for both electric and magnetic force microscopy but also allows the tip to probe small, deep pore structures. Additionally, there is the potential to form nanowires with a smaller tip apex than that of conventional metal-coated probes (i.e., capable of higher-resolution imaging). The core of SWNTs inside the nanowire anchors it firmly to the tip and confers increased mechanical and electrical stability. Moreover, in theory, the ability to produce a variety of different metallic nanowires extends the capabilities of the probe to conductivity (AuPd, PtIr), magnetic (Fe), and electrochemical imaging (Pt, Au, Ir for pH sensing, and Ag for Cl- potentiometric detection). Coatings on carbon nanotube SPM probes have previously been utilized either for adhesive purposes to secure the nanotube for imaging under solution10-12 or for electrical insulation of the carbon nanotube.16 To the best of our knowledge, we believe this is the first reported use of SWNTSPM tips as templates for the formation of conducting metallic nanowires.

SWNTs were attached to the apex of commercial SPM tips using the “pick-up” method as described previously.12,13 In general, the length, l, of the SWNT protruding from the end of the tip was found to vary from a few nanometers to a few micrometers. If required, the SWNTs were shortened (i.e., l was reduced) by electrical etching.12 For the work described here, many SWNTs were intentionally picked up onto the same tip. The strong van der Waals attraction between SWNTs results in them “sticking together” to form a bundle. Using the pick-up technique, the SWNT is attached to the tip by van der Waals attraction. Although this can be strong enough for stable imaging in air, the stability of the SWNT on the tip is a well-known problem, even more so for imaging under solution,10-12 and makes the formation of usable highresolution SWNT-SPM tips a low-yield process. However, for bundle SWNT-SPM tips, each SWNT in the bundle is attached to both the bundle and the tip so that the bundle itself is securely attached to the tip. This creates a stable probe; indeed, we found the formation of usable bundle SWNT-SPM probes by this technique to be almost 100% efficient. This efficiency, combined with the simplicity and speed of the pick-up technique (typically half an hour to make a bundle SWNT-SPM probe), makes this a viable technique for the fabrication of probes to be used on a daily basis. Bundle SWNT-SPM tips were deliberately used as templates for metallic nanowire formation because of their increased rigidity compared to that of an individual SWNT. To quantify this increase, we can estimate the resistance of the end of the SWNT to forces perpendicular to the SWNT axis (i.e., the lateral spring constant kl) and the axial force required to buckle the SWNT (i.e., the Euler buckling load Fb). Modeling the SWNT-SPM tip as a simple cantilever fixed at one end and free at the other, for a single nanotube of radius r and Young’s modulus Ynt (1.25 TPa1), we find that kl ) 3R/l3 and Fb ) π2R/l2 where R ) Yntπr4/4 is the flexural rigidity of the SWNT. Thus, for a 2-nm-diameter SWNT that is 1 µm long, kl ≈ 3 × 10-6 N m-1 and Fb ≈ 0.01 nN (i.e. too flexible to image with in contact or tapping mode). By forming bundles of nanotubes, we increase the radius of the probe, increasing its rigidity at the cost of decreased lateral resolution,17 although often an individual SWNT will protrude a small distance from the end of the bundle. For comparison, a 10-nm-diameter bundle SWNT-SPM probe that is 1 µm long has Fb ≈ 6 nN and kl ≈ 2 × 10-3 N m-1, which are adequate for imaging in tapping mode. It is therefore not surprising that we have been able to produce bundle SWNT-SPM tips that are robust enough for contactmode imaging by sufficiently shortening thick, ca. 10-nmdiameter bundles.18 However, the conductivity of such tips is usually governed by the individual SWNT that contacts the substrate first and as such can still be metallic or semiconducting. In addition, we have found that “end contacting”19 a SWNT against a metallic surface results in a high resistance and highly force-dependent contact. Thus, for applications where robustness, reproducible low-resis1366

tance electrical properties, or specific electrode materials are important, we have developed metallic nanowire probes. The nanowires are formed by sputter coating the bundle SWNT-SPM probes. The sputtering process involves an isotropic deposition of material onto the sample due to the diffusive path of the material from source to sample. Despite the high aspect ratio of the probe, this results in a uniform coating on both the tip and the SWNT, which is not possible when depositing the material using an evaporator. Importantly, the greater rigidity of the SWNT bundle makes the tubes more resistant to the mechanical and thermal stresses induced by sputtering, enabling the formation of longer and straighter nanowires. Once deposited, the nanowire is significantly stiffer and more robust than the original bundle. Not only is each SWNT in the bundle securely attached to the tip by the coating of metal on top of it, but the metal also adds to the rigidity of the bundle despite its comparatively low Young’s modulus. For a nanowire formed from a metal of Young’s modulus Ymet and outer radius R and with inner SWNT bundle radius r, the resulting flexural rigidity is

R)

π(Ynt r4 + Ymet (R4 - r4)) 4

Thus, for a 1-µm-long bundle that is 10 nm in diameter, a 25-nm-thick coating of AuPd (taking YAuPd ≈ 100 Gpa) results in an increase of kl from 2 × 10-3 to 0.2 N m-1 and, although it will not buckle reversibly, an increase of Fb from 6 to 600 nN. For a typical adhesion force of a few nanoNewtons, far less than Fb, this corresponds to a lateral deflection of the nanowire of 10-20 nm, comparable to the tip radius. By halving the length of the nanowire, this is reduced to a few nanometers, sufficient for the nanowire to be used to image in contact mode. A Digital Instruments multimode atomic force microscope with a Nanoscope IIIA controller and a Picoforce module was employed for these studies. Transmission electron microscopy (TEM) images were taken using a JEOL2000FX at 200 kV and a tilt angle of 60°. Sputter coating was performed with a standard Emscope SC500 sputter coater. Typically, metal films of thickness in the range of 20-30 nm were deposited on the SWNT bundles. This initial study focused on AuPd because of its well-known small grain size and hence its ability to form continuous films at low film thicknesses. Single-beam Si microfabricated contact tips (Nanosensors “contact” probes with a nominal spring constant of 0.07-0.4 N m-1) and tapping tips (Nanosensors “force modulation” probes with nominal spring constant 1.25.5 N m-1) were used. By using shorter nanowires on contact tips, nanowire probes capable of imaging in contact mode were fabricated. For tapping-mode applications, nanowire probes with lengths up to a few micrometers could be employed. Electrical transport through the nanowires was investigated by imaging an Au substrate (evaporated Au on a Si substrate with a Ti adhesion layer) in conducting mode where a bias Nano Lett., Vol. 3, No. 10, 2003

Figure 1. TEM images of (a) a bundle of SWNTs on a AuPdcoated tip, (b) the same tip after sputter coating with AuPd and annealing, and (c) an enlarged view of the nanowire. The image widths are (a) 350 nm, (b) 500 nm, and (c) 100 nm. Figure 3. Current through the nanowire and deflection of the tip as the probe is brought down toward, extend (black line), and away from, retract (red line), a Au surface. “A” marks where the probe touches the surface on extend, and “B”, where it leaves the surface on retract. The applied bias was 3 V through 1.1-MΩ resistance in series. The spring constant of the tip was measured to be 0.2 N m-1.

Figure 2. TEM images of the same part of the same AuPd nanowire (a) before and (b) after annealing. The image widths are 150 nm.

is applied between the tip and substrate while the tip is scanned in contact with the surface. The current was measured with a current preamplifier and input into the Nanoscope IIIa controller to allow the simultaneous acquisition of topography and current images. Simultaneous forcedistance, f-d, and current-distance, i-d, measurements were performed using the Nanoscope IIIa controller and Picoforce module to control the distance between the Au surface and the probe while externally recording the current, height of the probe above the surface (z position),20 and deflection of the cantilever (linearly related to the force). The current as a function of applied bias voltage, i-V response, was also measured while the nanowire was held stationary in contact with the Au surface. Figure 1a shows a TEM image of a small bundle of SWNTs attached to an AuPd-coated contact tip.21 The bundle, ca. 10 nm in diameter, consists of many SWNTs and protudes ca. 600 nm from the end of the tip (taking account of the 60° tilt angle of the TEM). Figure 1b shows the apex of the same tip after sputter coating with 20-25 nm AuPd and subsequent annealing; see below. The diameter of the nanowire is measured to be 50 nm, and the length is 600 nm, corresponding to the length of the original tube tip. An enlarged view of the nanowire is shown in Figure 1c; its polycrystalline nature is evident, as is the uniformity in diameter. By sputter coating with less AuPd, we have formed continuous nanowires with diameters as small as 30 nm and are pursuing ways of reducing this further. Figure 2a shows a TEM image of part of a SWNT bundle after sputter coating with AuPd. The nanowire that was formed has an obvious grain structure, though the grain size is small, as expected for AuPd, and the coating is uniform and continuous. Interestingly, we found that for AuPd a Nano Lett., Vol. 3, No. 10, 2003

structural change was induced by passing a large current through the AuPd nanowire. Figure 2b shows the same area on the same nanowire after applying a 3-V bias (with a 100kΩ resistor in series) with the nanowire in contact with a Au surface. The structure of the AuPd alloy has clearly changed from granular to polycrystalline. We attribute this change to an annealing process due to the energy dissipated in the nanowire at the grain boundaries. This effect was consistently observed in all AuPd nanowire tips that were produced and could be induced by either applying the bias during f-d measurements or while scanning in contact or tapping mode. The resulting nanowires have low resistance and high current carrying capacity. Figure 3 shows simultaneous f-d and i-d measurements for an annealed AuPd nanowire, roughly 50 nm in diameter and 250 nm in length, on a contact tip. A 3-V bias was applied through a 1.1-MΩ resistor in series. The cantilever spring constant here was 0.2 N m-1, as measured by the thermal noise method. The black line shows the response as the probe is lowered toward the surface from right to left (“extend”), and the red line, as it is brought away from left to right (“retract”). “A” marks the point on the extend curve at which the nanowire first touches the surface and the current immediately jumps to its maximum value and remains there. The observed maximum current of 2.714 µA when in contact with the surface implies a resistance through the nanowire, cantilever, and probe of 5 kΩ. “B” marks the point on the retract curve at which the nanowire loses contact with the surface. The hysteresis between the point at which the nanowire touches the surface on the extend and leaves the surface on the retract is indicative of the adhesive forces between the nanowire and Au surface. Note that the current remains constant at its maximum value up until the moment at which the tip leaves the surface (i.e., the current response is independent of the applied force). Although we could not measure a buckling force for the nanowire tips, we did find that they were robust and capable of contact-mode imaging as expected. 1367

Figure 4. Current-voltage response of a AuPd nanowire probe.

Figure 4 shows the i-V response of the nanowire tip, shown in Figure 1b and c; the resistance in series was removed for these measurements. The response is ohmic, with a resistance of only 2.5 kΩ even though the wire is 600 nm long. This puts an upper bound on the resistivity of the nanowire of 30 µΩ cm, although it is likely that some of the 2.5-kΩ resistance is not due to the nanowire itself but is due to the conducting path to the nanowire through the thin metal film on the tip. Also of interest is the current density through the nanowire. At 0.5-V bias, the current through the nanowire is 200 µA, corresponding to a current density of 25 × 105 A cm-2. It is probable that much of the current will be flowing through the bundle of SWNTs at the core of the nanowire. SWNTs are known to be able to sustain current densities in excess of 109 A cm-2.4 The SWNTs may also provide a thermal sink for heat dissipated in the nanowire because of their high thermal conductivity,22 greater than 200 W mK-1. These results demonstrate that the nanowire tips are near ideal conducting probes23 with low, ohmic, load-independent resistance up to the point of contact. To test the robustness and longevity of the nanowire tips, a Au surface was imaged continuously in conducting mode, with a 2-V bias applied across a 1.1-MΩ resistance in series. The same annealed AuPd nanowire tip was used as for the i-d response shown in Figure 3. Figure 5 shows topography (a and c) and current (b and d) images taken after 4 h (a and b) and 5 h (c and d) of continuous scanning. The robustness of the nanowire tip is shown by both the clarity of the topography image after 4 h of continuous imaging (a) and the current image (b), the latter demonstrating that there is still good conducting contact between nanowire and surface. The small variations in the current signal reflect differences in the contact interaction between the apex of the nanowire and the substrate. Both the topography and current images recorded after 4 h are similar to those when imaging commenced. After 5 h of imaging, it is clear from the artifacts in the topography that the nanowire has been damaged in some way, although interestingly, the current image still demonstrates good electrical contact between the nanowire and surface. Subsequent TEM imaging showed that the nanowire had been reduced in length, as shown schematically by the insets in Figure 5b and d. However, given that the nanowire is conducting along its entire length despite being worn down, it remains a good conducting tip. This is in marked 1368

Figure 5. (a and c) Topography and (b and d) conductivity images of a Au surface taken with a AuPd nanowire tip after scanning continuously for (a and b) 4 h and (c and d) 5 h. The applied bias was 2 V through 1.1-MΩ series resistance. The images are 3 µm2. Insets in b and d schematics of the nanowire are before and after damage, respectively.

Figure 6. TEM images of (a) a Au nanowire probe and (b and c) an enlarged view of the nanowire. The image heights are (a) 1.5 µm, (b) 8 µm, and (c) 100 nm.

contrast to a metal-coated tip where when the thin metal coating wears away at the tip the insulating Si (or Si3N4) underneath becomes the point of contact. Clearly, to improve the longevity for conductivity imaging, tougher metals such as PtIr could be used to form the nanowire. To demonstrate the generality of this nanowire tip fabrication method, we have also formed Au nanowires. Au is particularly useful as an electrode material,24 and as such, a well-defined Au nanowire would be an invaluable tool to use in studying electrochemistry at the nanometer level. Figure 6 shows TEM images of a Au nanowire on a tappingmode tip. The nanowire is 1 µm in length, 80 nm wide at its base, and 50 nm at its end, the difference corresponding to the change in diameter of the SWNT bundle at its core. Nano Lett., Vol. 3, No. 10, 2003

Although Au has a larger grain size than AuPd and although there appears to be a weak interaction between Au and SWNTs,14 the nanowire that is formed is still continuous although not as uniform as the AuPd nanowires. To improve the uniformity of coating, a multiple-layer process could be employed; previous work14 proved that Ti coats SWNT uniformly at low film thickness, as low as 2 nm, and can act as an adhesive layer for the formation of uniform metal nanowires of diameter e 10 nm. The use of a Ti underlayer allowed the formation of thin uniform nanowires from a wide range of materials that do not “wet” SWNTs. Future work will focus on a variety of multilayer nanowires, in particular, coating the tip and metallic nanowire in insulator, selectively removed at the end of the nanowire, with the aim of forming reproducible and geometrically well defined “nanoelectrode” probes. In conclusion, we have demonstrated the use of SWNTs as templates for the formation of metal nanowire SPM tips. The high aspect ratio probes that are formed are robust, conducting, and of a well-defined geometry and can be used for imaging in either contact or tapping mode. The generality and simplicity of the described method presents an easy route to the formation of nanowires. The core of SWNTs inside the nanowire gives it mechanical and electrical stability, and the variety of different metallic nanowires that could be formed extends the capabilities of the probe to conductivity, magnetic, and electrochemical imaging. Acknowledgment. We thank Steve York for help with the electron microscopy, Boris Mouzykanstkii and David Cobden for helpful discussions, and David Burt for experimental help. J.V.M. thanks the Royal Society for the award of a University Research Fellowship. We also thank the EPSRC for support (studentship for N.R.W., the purchase of an AFM under the Strategic Equipment Initiative, and Picoforce module GR/S24138/01). References (1) Treacy, M. M. J.; Ebbeson, T. W.; Gibson, J. M. Nature 1996, 381, 678. (2) Kane, C. L.; Mele, E. J. Phys. ReV. Lett. 1997, 78, 1932.

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(3) De Pablo, P. J.; Gomez-Navarro, C.; Martinez, M. T.; Benito, A. M.; Maser, W. K.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Appl. Phys. Lett. 2002, 80, 1462. (4) Yao, Z.; Kane, C. L.; Dekker, C. Phys. ReV. Lett. 2000, 84, 2941. (5) Besteman, K.; Lee, J.-O.; Wiertz, F. G. M.; Heering, H. A.; Dekker, C. Nano Lett. 2003, 3, 727. (6) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, J. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (7) Minot, E. D.; Yaish, Y.; Sazonova, V.; Park, J.-Y.; Brink, M.; McEuen, P. L. Phys. ReV. Lett. 2003, 90, 156401. (8) Ciao, J.; Wang, Q.; Dai, H. Phys. ReV. Lett. 2003, 90, 157601. (9) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147. (10) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (11) Li, J.; Cassell, A. M.; Dai, H. Surf. Interface Anal. 1999, 28, 8. (12) Hafner, J. H.; Cheung, C. L.; Oosterkamp, T. H.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 743. (13) Wilson, N. R.; Cobden, D. H.; Macpherson, J. V. J. Phys. Chem. B 2002, 106, 13102. (14) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (15) Bezryadin, A.; Lau, C. N.; Tinkham, M. Science 2000, 404, 971. (16) Rinzler, A. G. Private communication. (17) In some ways, bundle SWNT-SPM tips are similar to multiwalled nanotube SPM tips. However, the advantages of bundle SWNT-SPM tips include the ease of fabrication using the pick-up technique, the increased stability due to the large number of SWNTs in contact with the tip, and the chance of having increased topographical resolution due to an individual SWNT protruding from the end of the bundle. (18) Wilson, N. R.; Macpherson, J. V. Unpublished results. (19) In other words, electrical contact is made with the end of the SWNT rather than the side. The SWNT probe end-contacts a metallic surface when it is lowered into contact with it. (20) The Picoforce module includes a sensor that measures the height of the probe body, allowing accurate control and measurement of the z position. (21) Coating the tips with metal prior to pick-up was found to make the electrical shortening procedure more controllable and, in general, occur at lower voltages. However, picking up on uncoated tips gives a greater likelihood of the tubes passing directly through the apex of the tip (Figure 6) rather than to one side, as demonstrated by the nanowire tip in Figure 1b. (22) Hone, J.; Llaguno, M. C.; Nemes, N. M.; Johnson, A. T.; Fischer, J. E.; Walters, D. A.; Casavant, M. J.; Schmidt, J.; Smalley, R. E. Appl. Phys. Lett. 2000, 77, 666. (23) O’Shea, S. J.; Atta, R. M.; Welland, M. E. ReV. Sci. Instrum. 1995, 66, 2508. (24) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969.

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