VOLUME 3, NUMBER 12, DECEMBER 2003 © Copyright 2003 by the American Chemical Society
Controlled Mounting of Individual Multiwalled Carbon Nanotubes on Support Tips Niels de Jonge,* Yann Lamy,† and Monja Kaiser Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA EindhoVen, The Netherlands Received September 18, 2003; Revised Manuscript Received October 16, 2003
ABSTRACT Individual multiwalled carbon nanotubes were mounted on tungsten support tips in a scanning electron microscope equipped with a nanomanipulator. It was possible to select the diameter of the nanotube to align the nanotube with respect to the tip axis and to tune the contact length of the nanotube and the support tip. We have also developed a way to control the length of the nanotube protruding from the support tip. Control over the nature of the nanotube cap was not obtained.
A carbon nanotube mounted on a support tip can be applied as electron source in high-resolution electron beam instruments,1 such as electron microscopes and electron beam lithography machines. Indeed, it was demonstrated that the carbon nanotube electron source has a stable emitted current,2,3 a long lifetime,4,5 a low energy spread,3,6,7 and a high brightness.1 A mounted nanotube can also serve as probe tip in scanning probe microscopes, such as an atomic force microscope and a scanning tunneling microscope, to enhance the spatial resolution of these instruments.8 The high aspect ratio of the nanotube is an advantage for the imaging of rough surfaces or surfaces with deep pits. Chemically sensitive probes and magnetized probes were constructed from carbon nanotubes.9-11 Of critical importance for these applications is that the carbon nanotube probe is not surrounded closely by other * Corresponding author. E-mail:
[email protected]. Fax: +3140-2742293. † Present address: Ecole Supe ´ rieure de Physique et de Chimie Industrielles de la ville de Paris, 10 Rue Vauquelin, 75005 Paris, France. 10.1021/nl034792h CCC: $25.00 Published on Web 10/28/2003
© 2003 American Chemical Society
nanotubes, but that only one tube protrudes effectively from the support structure. First results have been obtained by mounting carbon nanotubes on a metal support tip with micromanipulators and an optical microscope.8 This mounting technique was improved using nanomanipulators in a scanning electron microscope.3,12,13 The main problem encountered with these mounting techniques (and also with other techniques14) is the difficulty to control the length of the nanotube protruding from the support tip. For application as electron source in an electron microscope, the tube length should measure 200-500 nm for a multiwalled nanotube with a typical diameter of 10 nm. This length is long enough to provide sufficient field enhancement and short enough to reduce vibrations of the tube.15 Vibrations of the tube broaden the virtual source size with a radius of typically 2 nm and, as a consequence, reduce the brightness of the source.1 It was found that the vibration amplitude of a 170 nm long nanotube was smaller than 0.2 nm, e.g., a high-resolution transmission electron microscope (TEM) image of its cap displayed details of 0.2 nm. For application in scanning probe
instruments, the desired length depends on the specific measurement. For enhancement of the spatial resolution,8 or for chemically modified tips,9 the nanotube should be as short and thin as possible (typically 30 nm for a single-walled nanotube16). But, if the nanotube is used to probe in a deep hole with a sharp edge, the length of the nanotube defines the steepness of the surface that can be resolved, and a balance has to be found between this height resolution and the vibration and bending effects. We have refined the mounting procedure and developed a reliable method to control the length of the nanotube. In this letter we give a detailed description of the mounting technique and evaluate the advantages and disadvantages of this method. The mounting procedure was performed in a scanning electroscope (SEM, Philips), equipped with a piezo-driven nanomanipulator (Omicron). This equipment allows manipulation of two objects with respect to each other with steps of 50 nm, with simultaneous observation at a magnification of 2 × 104. Obviously, a higher precision is possible when the system is built into a TEM.17 The support tip was made from a tungsten wire (0.3 mm) that was laser-welded on a titanium filament. The tip was obtained by electrochemical etching in 5 mol/L NaOH with a tip etching device (Omicron). The wire was put 1-2 mm vertical in the NaOH solution for etching, which mainly took place at the meniscus of the solution, such that the wire was etched-through at the meniscus and the lower part dropped off. The radius of curvature of the tip was usually set to 50 nm. The tip was transferred into the SEM and carefully pierced into carbon tape (STR tape from Shinto Paint Co.). The glue applied in this manner on the tip is necessary for a firm attachment of the nanotube. The attachment of the carbon nanotube to the tungsten tip is already provided by van der Waals force, but it does not provide sufficient binding to withstand the extremely large electric field as needed for its operation as electron source. Alternatively, one may use electron beam radiation for fixation.12,18 Next, a sample with carbon nanotubes already in the SEM was approached by the tip and searched for a suitable nanotube, i.e., a long, straight and thin tube that was uniform in its diameter, pointed in the direction of the tip, and stood sufficiently free from other tubes to allow approaching (Figure 1a). The carbon nanotube sample contained multiwalled carbon nanotubes grown with the arc discharge method in the group of Smalley as described elsewhere.19 When the tip had approached the carbon nanotube within a few hundreds of nanometers, the nanotube usually bent and stuck on the tip. This bending could possibly indicate a slight charging of the nanotube due to the electron beam. Applying a voltage difference between the tip and the nanotube sample increased the amount of bending. To provide sufficient contact surface between the tip and the nanotube, the tungsten tip was first positioned about a micrometer beside the nanotube and then the nanotube was carefully approached from the side. As the nanotube bent, its end stuck to the tip a few micrometers from the tip apex, see Figure 1b. By pulling, the nanotube was aligned with the tip and a contact length of about 1 µm was obtained 1622
Figure 1. Mounting of an individual carbon nanotube on a tungsten support tip. (a) Image showing the sample with nanotubes and one free-standing nanotube approached by the tungsten support tip. (b) The nanotube sticks to the tip. (c) The nanotube is aligned with respect to the tip-axis by pulling. (d) Manipulation resulted in a loop in the nanotube. (e) Further pushing and pulling led to a weak spot in the nanotube. (f) The nanotube finally broke at this spot by applying a small current.
(Figure 1c). Breaking off the nanotube from the main sample occurred by Joule heating with a current of more than 20 µA (a current density larger than 6 × 1010 A/m2 for a tube with a radius of 10 nm!), or by applying a mechanical force. Each time a nanotube emitter was made, its was tested for emission already in the SEM using a metal foil placed 5 µm in front of the tube and a voltage difference between the tube and the foil of about 60-80 V. The mounting procedure using the nanomanipulator in the SEM turned out to be very precise. First, it almost never occurred that other nanotubes were accidentally attached as well, which is a problem when using an optical microscope and micromanipulators.8 The problem of multiple nanotubes is often severe when growing tubes directly on a support tip.14,20 A second advantage of this method is the possibility to select a nanotube with the desired diameter to a precision equal to the resolution of the SEM (10 nm in our case). It is not possible to differentiate between a thin nanotube and a thin bundle of nanotubes. TEM images indicated that we had mounted a single nanotube in most cases, and sometimes a bundle of a few nanotubes. Larger bundles and ropes did not occur. Furthermore, the described method allows the control of the contact length of the nanotube and the tip and it is possible to align the nanotube with respect to the axis of the tip. The breaking-off of the carbon nanotube from the main sample is the most difficult part of the procedure. The carbon material is extremely strong and it can be broken Nano Lett., Vol. 3, No. 12, 2003
Figure 2. A multiwalled carbon nanotube mounted to a tungsten support tip. (a) The nanotube contained a dark spot where it was very thin, as indicated by the arrow. The distance between the arrow and the support tip is 1.8 µm. (b) After applying a current through the nanotube it broke close to the position of this weak spot. The length of the nanotube was 2.1 µm.
only by brute force. As shown in Figure 1, the tube can even form a complete loop by bending. We have found a way to the control of the length of the fraction of the carbon nanotube protruding from the support tip. Many nanotubes show a small region where the tube is somewhat thinner. This region is visible in the SEM as dark spot in a nanotube. When a support tip is attached close to this region, breaking by current or by mechanical force occurs most likely at this position, see Figure 2. We think that such dark spot indicates a defect in the nanotube for a single nanotube. Nanotubes are often found in bundles containing a small number of nanotubes. The spot could then indicate a position where some nanotubes in the bundle end, and at the position of the spot the number of nanotubes is smaller than in the remaining bundle. We have also observed a slightly different situation in which a thin nanotube protruded from a (thicker) bundle of nanotubes. It turned out that such a nanotube often breaks at the end of the bundle. The method to control the length was tested with a number of carbon nanotubes. In Figure 3 the results are shown of the mounting of 19 carbon nanotubes with lengths between 0.27 and 9.5 µm. For 12 nanotubes the difference between the desired length and the obtained length was smaller than 50%. A precision of 50% in the length control is considered fully sufficient for the construction of electron sources and scanning probe tips. Thus, the success rate of the procedure is about 2/3. The time to mount a nanotube is less than 30 min. Considering that we have mounted nanometer-sized objects, we think that the obtained precision, the success rate, and the mounting time are highly satisfying. Alternatively, one may shorten a nanotube after it has been mounted,21 but this is a tedious procedure. For the use as electron source it is desired to have a carbon nanotube with a closed capping, see Figure 4a. This type of capping does not have dangling bonds, which is expected to result in a better stability and a more uniform emitted electron Nano Lett., Vol. 3, No. 12, 2003
Figure 3. Experiments on the control of the length of nanotubes while mounting multiwalled carbon nanotubes on tungsten support tips. The measured length corresponded for most cases with the desired length within 50%. The dashed line indicates 100% match, the dotted lines indicate a correspondence of ( 50%.
Figure 4. High-resolution TEM images of multiwalled carbon nanotubes mounted on tungsten carrier tips. (a) The end of a nanotube with a closed cap. (b) Nanotube with a cap that seems to be open. (c) Nanotube with a cap that does not show regular carbon layers, but an amorphous structure. (d) High-resolution TEM image of the middle section of the nanotube shown in (b).
beam. The method of breaking-off the nanotube does not provide a good control over this property. Sometimes a nanotube was found to be open, see Figure 4b. The nanotube structure of another nanotube was damaged at the cap, as can be seen in Figure 4c. To obtain better control over the cap structure several ideas could be investigated. (1) We and others found indications for a self-closing mechanism,22 which could be exploited to control the capping. (2) The nanotube can be selected from a sample containing short and closed nanotubes, in which case the nanotube does not have to be broken off. (3) Nanotubes with specified lengths and diameters can be grown on a support structure,23 for example a tungsten tip. Not of the least importance is that this mounting method enables the use of high quality arc-dischargegrown nanotubes. That it is indeed possible to mount a high quality carbon nanotube is shown in Figure 4d. 1623
We have demonstrated a controlled mounting procedure of individual carbon nanotubes on tungsten support tips. The technique described here can easily be applied to scanning probe tips and even to other nanomaterials. It may find a broad application in the field of nanotechnology. Acknowledgment. This work was supported by FEI company. Discussions are greatly acknowledged with T. H. Oosterkamp, K. Schoots, and T. van Rooij. F. Holthuysen is thanked for help with the SEM. The CNTs were provided by R. E. Smalley. References (1) de Jonge, N.; Lamy, Y.; Schoots, K.; Oosterkamp, T. H. Nature 2002, 420, 393-395. (2) Dean, K. A.; von Allmen, P.; Chalamala, B. R. J. Vac. Sci. Technol. B 1999, 17, 1959-1969. (3) de Jonge, N.; van Druten, N. J. Ultramicroscopy 2003, 95, 85-91. (4) Dean, K. A.; Chalamala, B. R. Appl. Phys. Lett. 1999, 75, 30173019. (5) Fransen, M. J.; van Rooy, T. L.; Kruit, P. Appl. Surf. Sci. 1999, 146, 312-327. (6) Bonard, J. M.; Salvetat, J. P.; Stockli, T.; Forro, L.; Chatelain, A. Appl. Phys. A 1999, A69, 245-254. (7) Groening, O.; Kuettel, O. M.; Emmenegger, C.; Groening, P.; Schlapbach, L. J. Vac. Sci. Technol. B 2000, 18, 665-678. (8) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. Nature 1996, 384, 147-150. (9) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52-55.
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(10) Klein, D. C. G.; Jensenius, H.; Kijne, J.; Sliedregt Bol, K.; van der Marel, G.; Frenken, J. W. M.; Oosterkamp, T. H. Single Mol. 2002, 3, 160. (11) Arie, T.; Nishijima, H.; Ajayan, P. M.; Nakayama, F. J. Vac. Sci. Technol. B 2000, 18, 104-106. (12) Nishijima, H.; Akita, S.; Nakayama, Y. Jpn. J. Appl. Phys. 1 1999, 38, 7247-7252. (13) Yu, M. F.; Dyer, M. J.; Skidmore, G. D.; Rohrs, H. W.; Lu, X. K.; Ausman, K. D.; von Ehr, J. R.; Ruoff, R. S. Nanotechnology 1999, 10, 244-252. (14) Yenilmez, E.; Wang, Q.; Chen, R. J.; Wang, D.; Dai, H. Appl. Phys. Lett. 2002, 80, 2225-2227. (15) Hafner, J. H.; Cheung, C. L.; Oosterkamp, T. H.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 743-746. (16) Snow, E. S.; Campbell, P. M.; Novak, J. P. Appl. Phys. Lett. 2002, 80, 2002-2004. (17) Kuzumaki, T.; Sawada, H.; Ichinose, H.; Horiike, Y.; Kizuka, T. Appl. Phys. Lett. 2001, 79, 4580-4582. (18) Hirayama, H.; Kawamoto, Y.; Ohshima, Y.; Takayanagi, K. Appl. Phys. Lett. 2001, 79, 1169-1171. (19) Colbert, D. T.; Zhang, J.; McClure, S. M.; Nikolaev, P.; Chen, Z.; Hafner, J. H.; Owens, D. W.; Kotula, P. G.; Carter, C. B.; Weaver, J. H.; Rinzler, A. G.; Smalley, R. Science 1994, 266, 1218-1222. (20) Hafner, J. H.; Cheung, C. L.; Lieber, C. M. Nature 1999, 398, 761762. (21) Akita, S.; Nakayama, Y. Jpn. J. Appl. Phys. 1 2002, 41, 4887-4889. (22) Dean, K. A.; Chalamala, B. R. J. Vac. Sci. Technol. B 2003, 21, 868-871. (23) Teo, K. B.; Lee, S. B.; Chhowalla, M.; Semet, V.; Binh, V. T.; Groening, O.; Castignolles, M.; Loiseau, A.; Pirio, G.; Legagneux, P.; Pribat, D.; Hasko, D. G.; Ahmed, H.; Amaratunga, G. A. J.; Milne, W. I. Nanotechnology 2003, 14, 204-211.
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