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2008, 112, 14714–14717 Published on Web 08/27/2008
Multiple Telescoping Extension of Multiwalled Carbon Nanotubes and Its Application in Atomic Force Microscopy Wei Zhang, Zhonghe Xi, Gengmin Zhang,* Chengyao Li, and Dengzhu Guo Key Laboratory for the Physics and Chemistry of NanodeVices, and Department of Electronics, Peking UniVersity, Beijing 100871, China ReceiVed: July 22, 2008; ReVised Manuscript ReceiVed: August 15, 2008
The multiwalled carbon nanotubes (MWCNTs) fabricated in a fast-heating chemical vapor deposition process were stretched using a microprobe system in a scanning electron microscope. The clean and straight MWCNTs survived multiple telescoping extension and eventually evolved into a gradually sharpened configuration. Some of the so-made telescopic MWCNTs were fabricated into atomic force microscopy probes. Their slender shape enabled them to image the deep holes on an anodic aluminum oxide template surface well. Also, their telescopic structure helped avoid mechanical instability and yield a good amplitude response curve. The interaction between the shells in a multiwalled carbon nanotube (MWCNT) is predominantly a van der Waals force. Due to the weakness of this interaction, the individual cylinders of a MWCNT can slide with respect to each other easily. Sometimes, sword-in-sheath failure occurs under tensile load along the axial direction of a MWCNT. That is, the outermost shells of the MWCNT can break while the inner ones do not. In this case, the MWCNT will experience an extension that resembles the “telescoping” of a mariner’s traditional spyglass.1-3 The potential applications of this telescoping extension include nanometer-scale bearings, springs, and oscillators.1,4,5 Nonetheless, so far, the telescoping extension of a MWCNT is usually laborious. Most of the MWCNTs synthesized by the conventional chemical vapor deposition (CVD) method inevitably contain many defects. Some of these MWCNTs are not straight enough for a telescoping extension, and others tend to fracture after only one sword-in-sheath behavior. Recently, we developed a fast-heating CVD method to fabricate very clean and straight MWCNTs as the cores of conical carbon fibers (CCFs).6 As described in this letter, we performed multiple telescoping extension to the so-made MWCNTs and found that they had excellent extensibility. A CNT is considered as a promising candidate for the imaging probe of an atomic force microscope (AFM).7-11 Its nanometer-scale tip and large aspect ratio can result in very high resolution.12 Also, the sample damage caused by a CNT is generally less serious than that caused by a conventional Si tip.13 However, the application of a CNT as an AFM probe also faces some challenges. The major concern is the mechanical instability.14 A researcher is often in a dilemma when trying to find a suitable length of a CNT used as an AFM probe. On the one hand, exploring deep positions demands a slender CNT probe;15 on the other hand, a long CNT is often subject to such mechanical instabilities as buckling and abrupt adhesion to the sample surface.16-21 This letter reports our attempt to tackle this problem by using telescopic MWCNT probes. Short MWCNTs were stretched into long telescopic * To whom correspondence should be addressed. E-mail: zgmin@ pku.edu.cn. Tel.: 86-10-62751773. Fax: 86-10-62762999.
10.1021/jp806470e CCC: $40.75
Figure 1. Interaction with the microprobe system and exposure of the MWCNT core. (a) A CCF (left) and a W microprobe. (b) Exposure of the MWCNT core after the interaction.
structures. With these telescopic MWCNTs used as the AFM probes, both satisfactory images of deep-holed samples and stable amplitude response curves were attained. CCFs with MWCNTs as their cores have been grown on graphon substrates with a CVD method. The details of their synthesis and characterization have been given elsewhere.6 The high-resolution transmission electron microscopy (HRTEM) observation of the sample, whose results can be found in Figure 3 of ref 6, has unambiguously shown that the core of the CCFs were MWCNTs instead of any other carbonaceous structure. Compared with an ordinary tube-in-furnace CVD method, the CVD method used here featured both a high reaction temperature and a fast heating process.22 As shown in Figure 1, the internal structure of the products was investigated using a microprobe system installed in a scanning electron microscope (SEM, Tecnai XL30).23 Part of the outer layers of the CCF was peeled off under the scratching by the microprobe, and the CNT core was exposed. During the peeling, a certain portion of the CCF fell off as a whole instead of layer by layer. Unlike the often tangled CNTs fabricated with an ordinary CVD method, the CNT cores of the CCFs appeared to be clean and straight. More than 30 samples were submitted to the interaction with the microprobe system, and the MWCNT cores in almost all of them were exposed by just one scratch. The size of 10 exposed MWCNT cores was measured. Their average diameter and length were 33.3 nm and 2.69 µm, respectively. The fact 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 38, 2008 14715
Figure 2. Stretching of a MWCNT using the microprobe system. (a) Before the elongation. (Part of the MWCNT was soldered on the W microprobe.) (b) After the elongation. (Note that some part of the extended MWCNT was very thin.)
TABLE 1: Length Changes of MWCNTs Due to Stretching (ratio ) length after stretching/original length) events
original length (µm)
length after stretching (µm)
ratio
1 2 3 4 5 6 7 8 9 10
1.84 2.67 1.59 0.43 6.12 3.22 2.05 1.21 2.31 1.09
4.46 6.41 5.21 2.64 10.87 8.65 7.37 3.85 5.24 6.73
2.42 2.40 3.28 6.14 1.78 2.69 3.60 3.18 2.27 6.17
that the CNT cores survived the interaction with the microprobe showed that they contained few defects and had relatively perfect crystallinity. Generally, the sword-in-sheath experiment of a MWCNT is quite challenging in that an individual MWCNT is usually hard to pick and manipulate, especially when its two ends need to be firmly fixed and further exerted on by a small pulling force.1,2 The approach introduced in this letter can partly help tackle this difficulty. As the MWCNTs directly grew from the graphon in a nearly vertical orientation, one of its terminals was already naturally fixed to the substrate. After the exposure of the MWCNT core, a W microprobe was brought toward its free end until some part of the MWCNT was in contact with the microprobe, as shown in Figure 2a. Then, the part of the MWCNT that was in contact with the microprobe was illuminated by the electron beam for several minutes, so that the CNT was soldered onto the microprobe with the deposited amorphous carbon.24,25 The original length of the exposed CNT, shown in Figure 2a, was 1.09 µm. After the stretching, the exposed part, shown in Figure 2b, was prolonged to 6.73 µm in a sword-in-sheath process. That is, the CNT fabricated with our fast-heating method exhibited a remarkable extensibility. More than 30 such CNTs were stretched in this manner, and some of the results are summarized in Table 1, which shows that the good extensibility was quite reproducible. After testing the extensibility of the so-made MWCNTs, some of them were assembled onto commercial AFM tips and then submitted to multiple telescoping extension until breaking. First, more details of the extension process were disclosed; second, and more importantly, the broken MWCNTs that were retained on the AFM tips were used as AFM probes. One example of the fabrication of such a MWCNT-based AFM probe is shown in Figure 3. At first, a commercial Si AFM tip was glued onto a microprobe in the SEM beforehand. Subsequently, in a similar process as that shown in Figure 2, the free end of a 38 nm diametered MWCNT that protruded from the surrounding CCF was soldered onto the Si tip using amorphous carbon under electron beam illumination. Consequently, as shown in Figure
Figure 3. Fabrication of a telescopic MWCNT into an AFM probe. (a) A MWCNT that protruded from a CCF was mounted on a commercial AFM Si tip; (b) the first inner tube was exposed; (c) the second inner tube was exposed (points “a” and “b” are the positions where the outermost tube broke when the first inner tube was exposed, and points “c” and “d” are the positions where the first inner tube broke so that the second inner tube was further exposed) (d) the third and fourth inner tubes were exposed; (e) the Si probe with the remainder of the broken MWCNT on its end (the diameter of the telescopic MWCNT tip is 7nm); and (f) a schematic illustration of the telescopic stretching of a MWCNT.
3a, the two ends of the MWCNT were fixed on the substrate and the Si tip. Then, the microprobe was slowly moved away, and a stretching process started. As shown in Figure 3b, a section of the inner tube, 27 nm in diameter and 321 nm in length, was exposed as the result of the stretching. Hereafter, the inner tubes are numbered according to the sequence in which they were exposed. The exposed section in Figure 3b is thus referred to as the first inner tube. With the stretching continued, new inner tubes inside of the first one manifested themselves. The second inner tube can be found in Figure 3c, and the third and fourth ones are observable in Figure 3d. More thinner inside tubes, whose diameters were too small to be precisely determined, continued to be exposed until the whole MWCNT eventually fractured. The remainder of the MWCNT on the Si tip, shown in Figure 3e, was later used as an AFM probe. A model of the above process is given in Figure 3f, which shows that the telescoping of the MWCNT arose from the breaking of outer tubes and the gliding between the outer and the inner ones. A similar result used to be obtained previously in an electrical break-down process.26 As a contrast, no electrical current was involved in the fabrication of the AFM probes here. The often-used samples for testing the performance of an AFM probe include quantum dots, metallic films, a Si grid, a micron-sized groove, and an anodic aluminum oxide (AAO) template.11,17,27-29 For testing our telescopic MWCNT as an AFM probe, the AAO templates, whose holes were approximately 50 nm in diameter, were imaged in tapping mode, and the results were compared with those obtained with a commercial Si tip. An example is given in Figure 4. Since the telescoping behavior of the MWCNT was irreversible, no artifact would be produced by the spontaneous retraction of the inner tubes. In Figure 4a, the pores appeared clearly as hexagons, while the shape of those in Figure 4b was rather irregular. The
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Figure 5. The amplitude response curves of MWCNT probes for AFM; (a) a conventional MWCNT; (b) a telescopic MWCNT.
Figure 4. AFM images of an AAO template (a) obtained with a telescopic MWCNT probe and (b) obtained with a commercial Si tip. (The insets give the two-dimensional views of the two images.)
typical size of the vertices of the hexagons was 49 nm in the inset of Figure 4a and 63 nm in the inset of Figure 4b. Therefore, the pores in the inset of Figure 4a appeared obviously larger than those in the inset of Figure 4b. That is, the broadening effect of the Si tip, which was caused by the pyramid shape of the tip, was effectively avoided by using the CNT probe. Furthermore, the largest depth obtained with the telescopic MWCNT probe was nearly 55.37 nm in Figure 4a and only about 20.63 nm in Figure 4b. The comparative results given in Figure 4 show that the MWCNT probe imaged a clearer multipored array and explored deeper positions than did the commercial Si tip. The amplitude response curves in tapping mode were acquired on a clean Si surface using a conventional MWCNT and a telescopic MWCNT as the AFM probe. The conventional MWCNT, which was used here for a comparison, was 1.6 µm in length and 45 nm in diameter. Imaging fine-textured surfaces and reaching deep valleys demand the AFM probe to be both sufficiently long and sharp. A conventional CNT, with the shape of a slim and long cylinder, can satisfy the two requirements. However, snapping or buckling behaviors often occur when the length of the CNT exceeds 1 µm.30 An amplitude response curve can reflect these unstable behaviors with the variation of the tip-sample distance. In the tapping mode, the amplitude of the tip vibration decreases continuously when the probe approaches the sample surface; thus, the amplitude of the tip vibration can be used to indicate the probe-surface distance. Nonetheless, this damping region is usually only part of the whole response curve. When the probe-sample distance is too large, the amplitudeisunaffectedbytheapproaching;whentheprobe-sample distance is lowered to a critical value, the van der Waals attractive force overwhelms the restoring force of the probe, and the probe can abruptly jump into contact with the surface.17 A relatively large damping region is necessary for a good probe. As shown in Figure 5a, the conventional MWCNT probe had a damping region less than 20 nm when it approached the surface. In the curve of its retraction from the surface, no damping region appeared at all. The loop formed between the approaching and retracting curves reflected the bistable behavior of a resonator in a nonlinear potential.31,32 It is impossible for the probe to be in a strict vertical alignment with respect to the sample surface during the imaging, and the component of the surface interaction force perpendicular to the MWCNT will dominate its elastic response. A long MWCNT is susceptible to this lateral force component and is prone to jump into contact with the sample surface.17 Therefore, as illustrated in Figure 5a, an ordinary MWCNT might not be a very suitable candidate for an AFM
probe due to its unstable behavior. An approach to tackling this problem is the employment of shorter MWCNTs.17 Nevertheless, a short MWCNT is not a good probe for imaging a textured surface with deep holes. As shown in Figure 5b, the telescopic MWCNT has provided another alternative to overcoming this difficulty. The curve was acquired from the telescopic MWCNT probe used for obtaining the image of the AAO surface shown in Figure 4a. The damping region in it is as long as over 40 nm, and the loop is also small between the approaching and the retracting curves. Therefore, a stable performance can be expected in imaging. On the one hand, after the stretching, the telescopic MWCNT became sufficiently long for imaging textured surface, for example, the AAO surface. On the other hand, the telescopic MWCNT had a gradually sharpening conical shape, which is believed to be favorable to a more robust elastic response to the perpendicular force component. So far, the precise breaking positions of the outer MWCNT shells still cannot be well predicted and controlled during the telescoping extension. Now, efforts are being devoted to improving the fabrication of our MWCNT probe in this regard. In conclusion, MWCNTs were grown from graphon substrates as the cores of CCFs with a fast-heating CVD method. The robustness of the MWCNT cores shown in the interaction with a microprobe system indicated relatively perfect crystallinity of the MWCNTs. Multiple telescoping extraction of the inner shells of these MWCNTs was achieved, suggesting their remarkable extensibility. The postextraction telescopic MWCNTs were proved very suitable to be used as the AFM probes. First, they were sufficiently long and thin for obtaining good images of deep holes on sample surface. Second, they were mechanically stable and had a satisfactory amplitude response in the tapping mode. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 60771004) and the MOST of China (No. 2006CB932402). The sample characterization was supported by the Instrumental Analysis Fund of Peking University. References and Notes (1) Cumings, J.; Zettl, A. Science 2000, 289, 602. (2) Yu, M. F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Science 2000, 287, 637. (3) Kis, A.; Jensen, K.; Aloni, S.; Mickelson, W.; Zettl, A. Phys. ReV. Lett. 2006, 97, 025501. (4) Williams, P. A.; Papadakis, S. J.; Patel, A. M.; Falvo, M. R.; Washburn, S.; Superfine, R. Appl. Phys. Lett. 2003, 82, 805. (5) Zheng, Q. S.; Jiang, Q. Phys. ReV. Lett. 2002, 88, 045503. (6) Zhang, W.; Xi, Z. H.; Zhang, G. M.; Wang, S.; Wang, M. S.; Wang, J. Y.; Xue, Z. Q. Appl. Phys. A 2007, 86, 171. (7) Tong, J. H.; Sun, Y. IEEE Trans. Nanotechnol. 2007, 6, 519. (8) Kim, J. E.; Park, J. K.; Han, C. S. Nanotechnology 2006, 17, 2937. (9) Hafner, J. H.; Cheung, C. L.; Wooley, A. T.; Lieber, C. M. Prog. Biophys. Mol. Biol. 2001, 77, 73.
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