Amplitude Response of Multiwalled Carbon Nanotube Probe with

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J. Phys. Chem. C 2008, 112, 15631–15636

15631

Amplitude Response of Multiwalled Carbon Nanotube Probe with Controlled Length during Tapping Mode Atomic Force Microscopy A. N. Jiang, S. Gao,* X. L. Wei, X. L. Liang, and Q. Chen* Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, P.R. China ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: July 21, 2008

Multiwalled carbon nanotube (MWCNT) atomic force microscope (AFM) probes were fabricated with controlled length using nanomanipulators inside scanning electron microscope. The amplitude-distance responses of MWCNT AFM probes were systematically studied experimentally. Several special characteristics of CNT AFM probes were observed, such as amplitude jump-into-zero, rebounds after the probe already touched the surface and large hysteresis during retraction. Transition from attractive to repulsive regions was also observed when the CNT is long and the amplitude is large. Tapping mode amplitude-distance curves were found to change regularly with the length of the carbon nanotubes and their tilting angle relative to the substrate surface normal. The results were comparable with previous theoretical predictions. Through direct observations by electron microscopes, MWCNT AFM probes were found to bend homogenously even when they were pushed toward the SiO2 wafer for several hundred nanometers after they had touched the surface of the substrate. By analyzing the results obtained from several probes it was found that the MWCNT AFM probes in tapping mode should be suitable for stable operation with proper length and working condition. 1. Introduction Because of their remarkable properties, carbon nanotubes (CNTs) have great potential in many applications. CNTs atomic force microscope (AFM) probes have high strength, elasticity, aspect ratio, and smaller dimensions1 and provide high endurance and better resolution than conventional AFM probes.2-4 Additionally, CNT AFM probes also have special applications, for example, in deep trench imaging and nondestructive imaging.5 However, imaging using a CNT probe is different from that using a conventional probe because the relatively high aspect ratio can bring great deformation. Tapping-mode AFM is a high-amplitude dynamic mode where amplitude modulation feedback is used to image the sample topography. It has many advantages with respect to contact AFM and is widely used. However, because high nonlinear interaction forces are involved when the oscillating tip is very close to the sample surface, it can be quite complex to analyze the tapping mode AFM image. The amplitude-distance response curves provide valuable information about the interaction between the tip and the surface and are very useful for choosing suitable working conditions and for obtaining a high-resolution interpretable image. A firm understanding of amplitude-distance curves of CNT AFM probes in tapping mode is essential for understanding the interaction between the CNT probes and the substrate, operating CNT AFM probes in suitable conditions, and explaining the obtained image correctly. Experimental and numerical modeling results have been found to be consistent with each other for conventional AFM probes in noncontact and tapping mode imagings.6,7 But simple theoretical simulations have revealed that the amplitude-distance response in noncontact mode AFM of single-walled carbon * Corresponding authors. E-mail: [email protected]; qingchen@ pku.edu.cn.

nanotube (SWCNT) probes is obviously different from that of conventional AFM probes.8 The SWCNT probe was predicted to jump into contact with the surface of the substrate due to bending in response to the surface-nanotube interaction forces during imaging.8 More than two discrete solutions of the oscillation amplitude were found possible for SWCNT probes when snapping occurs.9 Simulations have also shown that the probe-sample forces during imaging can be significantly lowered when CNT probes were used and they can be further reduced by increasing the tilt angle of the probe relative to the vertical.10 On the other hand, experimental results on the amplitude-distance response of CNT AFM probes are rare. A fluctuation in the amplitude-distance curve has been observed and explained as buckling signature of a SWCNT probe, resulting from the AFM cantilever pushing into the surface of the substrate.11 Repulsive and attractive regions have been observed from the amplitude-distance curves of multiwalled carbon nanotubes (MWCNTs) AFM probes.12,13 Complex behavior of SWCNT probes in amplitude-distance curves after the probes have touched the substrate was observed experimentally and was explained by the nonlinearity in the force exerted on the nanotube through molecular mechanics simulations.14 However, so far, all the existing experimental observations of amplitude-distance response measured only one or two CNT probes. There is not a systematic experimental result on the amplitude-distance response of CNT probes that can verify theoretical predictions. In the present work, for the first time, we performed a systematic experimental study on the amplitude-distance responses in tapping mode AFM using MWCNTs with controlled and changed length. Several special characteristics of CNT AFM probes were observed and the factors affecting these characteristics were studied. Our results help to better assess the capabilities and the limitations of CNT AFM probes in imaging.

10.1021/jp804481g CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008

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Figure 1. (a) TEM image showing the CNT has good structure. (b) SEM image showing a CNT was fixed to an AFM tip. The arrows point to the positions where amorphous carbon has been deposited. (c) and (d) SEM images of a CNT AFM probe viewed from two directions (with a 90° angle to each other and both are parallel to the AFM tip base). (e) SEM image showing a CNT AFM probe is cut by a “nanoknife”.

2. Experimental Section The CNTs used in the present work were MWCNTs synthesized by chemical vapor deposition method using ferrocene as catalyst and cyclohexane as carbon source. The diameter of the MWCNTs is about 60 nm on average. Figure 1a is a typical transmission electron microscopy (TEM) image showing that the nanotube has good structure. Two kinds of AFM cantilevers were used to support the CNTs in our experiments: one is RTESP type from Veeco (with frequency of 230-302 kHz, spring constant k ) 20-80 N/m, and cantilever length of 115-135 µm) and the other is NSG 01 type from NT-MDT (with frequency of 120-190 kHz, k ) 2.5-10 N/m, and typical cantilever length of 130 µm). For the amplitude response experiments, the MWCNT should attach to the silicon AFM tip firmly, so we used the following method to attach the MWCNT. First, MWCNTs and silicon AFM tips were put into a scanning electron microscope (SEM, FEI XL 30F) installed with a nanomanipulation system (Kleindiek MM3A).15 Then the MWCNT suitable for making AFM probes was selected and attached by van der Waals force onto the silicon AFM tips in a controlled direction and angle through nanomanipulations.16,17 Electron beam-induced amorphous carbon was deposited at the contact position to fix the CNT firmly onto the silicon tips. Figure 1b shows a CNT attached firmly to an AFM tip. For each of the constructed CNT AFM probes, we took photos from at least two directions to measure the angle and the length of the protruding nanotube probe (as shown in Figures 1c and 1d). Ten MWCNT probes were made using the above method and almost all of them have excellent endurance in imaging and amplitude-response experiments. The length of the MWCNT probes was adjusted using the “nanoknife” technique.17,18 A nanoknife is a short CNT connected to a metal. A voltage of about 5 V was applied between

the nanoknife and CNT to be cut. The nanoknife was moved to touch the CNT through nanomanipulation. The CNT was cut off at the contact point when it contacted the nanoknife. The working mechanism of the nanoknife has been described in detail previously.18 To study the performance of the same CNT probe at different lengths, after AFM study, the same MWCNT probe was moved back into the SEM again and cut to the desired length using the nanoknife technique.17,18 Figure 1e shows a CNT probe has just been cut by a nanoknife. The amplitude-distance curves in tapping mode were recorded with a Veeco Nanoman II AFM (Dimension 3100, with close-looped hybrid XYZ scanning probe microscope head) in the air using clean SiO2 on a Si wafer as the imaging surface. The structure of the CNTs was examined by TEM (FEI G20). The bending experiment inside the TEM was performed using a manipulation stage (Nanofactory). 3. Results and Discussion The amplitude-distance curve of a conventional silicon AFM tip was recorded for comparison. Figure 2a shows a typical amplitude-distance curve obtained from a silicon AFM tip when it is approaching a flat SiO2 surface. The arrows pointing to the left indicate the approaching movement to the surface and the arrows pointing to the right indicate the retraction from the surface. The cantilever is in free oscillation at stages 1 and 6. The interaction between the AFM tip and the substrate surface modifies the vibrating amplitude at stages 2 and 5. At stages 3 and 4, the cantilever is too close to the surface so that the tip can no longer vibrate. It can be seen that the vibrating amplitude decreases or increases roughly linearly all the way at stages 2 and 5. A set of amplitude-distance curves obtained at different drive voltages from the same MWCNT probe approaching the same

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Figure 2. (a) Amplitude-distance curve of a conventional silicon AFM tip. (b) Amplitude-distance curves obtained at various drive voltages from the same MWCNT probe approaching the same flat SiO2 surface. The CNT, which is 3.6 µm long and 60 nm in diameter, has a 16.5° tilt angle to the normal of the SiO2 surface.

Figure 3. (a)-(d) SEM images showing the same CNT AFM probe was cut short several times. (e) The amplitude-distance curves obtained from one CNT probe at different lengths. The CNT is 45 nm in diameter and has a 33° tilt angle.

flat SiO2 surface was shown in Figure 2b. The results are reproducible for the MWCNT probes used in the present work. The drive voltage that applied to the piezoelectric crystal vibrates the cantilever and thus determines the amplitude of the cantilever. Large drive voltage corresponds to large amplitude response. To separate the curves to show them clearly, the curves shown in Figure 2b have been shifted to the right for different amounts. The amplitude response of the MWCNT AFM probe is obviously different from that of a conventional AFM tip (as shown in Figure 2a). In the right flat part of the curve, the CNT probe does not feel forces from the surface of the substrate and vibrates freely with the vibration amplitude being the set amplitude. When the CNT approaches the SiO2 surface, the amplitude of the MWCNT AFM probe decreases roughly linearly (except some small jumps as pointed to by the short arrow) as the free oscillation is dampened by the interaction between the probe and the surface. When the distance between the CNT probe and the substrate surface is reduced further to some point, the amplitude suddenly jumps into zero, which is different from the case of conventional AFM tips. During retraction, the amplitude remains zero for a long distance, then jumps back to the linear change region, and sometimes even jumps back to the initial constant amplitude region. The noise shown in Figure 2b is mainly the background instrument noise. The noise is higher on the curves obtained at lower vibration amplitude because the low vibration amplitude condition deviated from the standard working conditions of the instrument. The observed amplitude jump-into-zero is consistent with previous theoretical predictions on SWCNT probes.8,19 This behavior has also been observed experimentally from short MWCNT (shorter than 1.2 µm)12 and very long (7.5 µm) but thin (with diameter about 10 nm) MWCNT probes.13 A metallic CNT probe in contact with a mercury droplet20 was also found to have the jump. Here, we observed the jump from long thick MWCNTs (several micrometers long and with diameter from 45 to 65 nm). Different kinds of CNTs (SWCNT or MWCNT) with different dimensions (length and diameter) all have the amplitude jump-into-zero in their amplitude-distance response, indicating the amplitude jump is a general characteristic of CNT

AFM probes. Such nonlinear response of the amplitude limits the working region of the CNT AFM probes for tapping mode AFM. To see the effect in real applications, images were taken using the present CNT AFM probes. When the working condition is set to the linear region, a good image can be obtained. When the CNT AFM probes are used in the region where amplitude jumps, no stable image can be obtained. It is important to know how the nonlinear response of the amplitude changes with various factors. However, all the previously reported experimental observations in amplitude-distance response measured only one or two CNT probes and cannot be compared with theoretical predictions systematically. On the other hand, the theoretical works used a simplified model and were supposed to work in noncontact mode to reduce the difficulty for calculation.8,19 Here, the amplitude-distance curves of several CNT AFM probes were measured at different drive voltages (an example is shown in Figure 2b) in tapping mode AFM. The results showed that the linear region of the amplitude-distance response for the same CNT AFM probe increases and the sudden amplitude jump does not change much when the drive voltage increases. To study the lengths effect, a CNT probe was cut short for several times (as shown in Figure 3a-d) and the amplitude-distance responses were recorded at each time. One typical result is shown in Figure 3e.21 It shows that the amplitude jump-into-zero region is larger for longer CNT. The sudden amplitude jump decreased when the CNT was cut short. The amplitude-distance curves of very short MWCNT probes (at least shorter than 1 µm) do not have the sudden amplitude jump and are similar to that of conventional silicon AFM tips. The lengths effects were measured on five MWCNT AFM probes and similar results were obtained. To better investigate the amplitude-distance response, here a factor of R ) 1 - (the amplitude jump amount)/(maximal resonant amplitude) is defined, as shown in the inset of Figure 4. Larger R implies larger allowed working region of the MWCNT AFM probes. The results of five probes were shown in Figure 4, where the data obtained from the same CNT are connected by lines and the length of the CNT probes and their

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Figure 6. Amplitude-distance curve of a long CNT AFM probe showing rebounds. The CNT, which is 3.1 µm long and 45 nm in diameter, has a 33° tilt angle.

Figure 4. Measured R (defined in the inset) of the CNT AFM probes at different lengths. The data obtained from the same CNT are connected by lines for clarity. The diameter and tilt angle of the CNTs are labeled on the right side of each data set. The data were obtained in the standard working conditions of the AFM instrument.

Figure 5. Amplitude-distance curve and phase-distance curve of a CNT AFM probe. The CNT, which is 4 µm long and 65 nm in diameter, has an 18° tilt angle.

tilting angle relative to the surface normal of the substrate are labeled. Since the value of R also depends on the free vibration amplitude (see Figure 2b), the data shown in Figure 4 were all obtained at the standard conditions of the instrument with the same free vibration amplitude. It can be seen that R increases when the MWCNTs are cut short, which means shorter MWCNT probes have a larger allowed working region. Concerning the tilting angle, it also shows that R increases when the tilting angle decreases (which means the CNT is nearer to the perpendicular position). These results are consistent with the theoretical predictions.8,19 The above results indicate shorter MWCNT with a smaller tilting angle is more suitable for imaging. The small jump observed sometimes in the linear region of the amplitude-distance response (as shown in Figure 2b) is due to the transition from attractive to repulsive regions. Figure 5 shows an amplitude-distance curve and the corresponding phase-distance curve of a CNT AFM probe approaching a flat SiO2 surface. A sudden phase change can be observed (pointed to by the thick short arrow) when there is a small jump in the amplitude curve, confirming that the small jump is due to the attractive to repulsive transition. However, this transition is amplitude-dependent. When the amplitude is small (such as

when it is smaller than 25 nm in the case shown in Figure 2b), there is no transition being observed, and the phase keeps going in one direction until the amplitude jump-into-zero occurs. (More details are included in the Supporting Information). This phenomenon indicates the linear region in such case is a noncontact region. The CNTs used here are long (several micrometers) and thick (with diameter of about 60 nm). The above results imply that using small amplitude can avoid the attractive to repulsive transition for long MWCNT probes. Attractive to repulsive transition has also been reported from conventional AFM tips6,7 as well as from MWCNT probes.12,13 Further work is needed to clarify when this transition occurs. For the same CNT probe, the hysteresis distance does not change much for different drive voltages (as shown in Figure 2b) but decreases with its length, which is similar to the change of the amplitude jump (as shown in Figure 3e). In a comparison of the results obtained from different CNTs with roughly the same length, the hysteresis distance is larger when the tilting angle is larger. The retraction behavior of the CNT AFM probes is different from that of conventional AFM tips. For long CNT with large tilting angle, the hysteresis can be tens of nanometers and the probe may jump back to the set amplitude directly without a linear increase in the retraction curves. On the other hand, if the probe approaching is stopped before the amplitude jumps, the hysteresis in the retraction would not occur. Imaging using the present CNT AFM probes shows that the CNT length does not influence the image quality as long as the probes are working in the linear region on the amplitude-distance response curve. In some cases (such as when a long CNT probe is driven by a large drive voltage), some MWCNT probes were also observed to rebound in the amplitude-distance response (as shown in Figure 6 and the first left curve in Figure 3e) after the probes have already contacted the substrate, which is similar to previous reports on SWCNT probe.14 However, the details of the rebound is not exactly the same as in the previous reports. In the present case, the amplitude increases when the probe-to-surface distance decreases during rebounding and the rebounded amplitude decreased to zero suddenly at the highest points. In the previous reports, although not symmetrically, the amplitude increases and decreases relatively slowly.14 The difference may be due to the different structure of the present MWCNT compared with that of the SWCNT in the previous work. All the amplitude-distance response curves obtained here are repeatable and the properties of the present CNT probes do not change through the experiment. This indicates the MWCNT AFM probes could be durable when imaging and we ascribe it to the high strength of the MWCNTs and their firm combination with the AFM tip support. To better understand the process of CNT AFM probe approaching a substrate surface, studies inside SEM and TEM were also performed. As shown in Figure 7a, a piece of Si

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Figure 8. TEM image showing a CNT while bending. The inset is an enlarged TEM image showing the detailed structure of the CNT. Figure 7. SEM images showing the bending process of a CNT AFM probe when it touched the SiO2 surface. The SiO2 covered Si wafer is nearly parallel to the electron beam. The dashed line outlines the edge of the surface. Above the dashed line is the surface of SiO2; below the dashed line is the side of the substrate. The arrow points to the contact point.

covered by a thin layer of SiO2 was placed inside a SEM with its surface roughly parallel to the electron beam, which means the surface normal of the substrate is roughly perpendicular to the electron beam direction. A MWCNT AFM probe (with the protruding CNT being 1.8 µm long and about 50 nm in diameter) was placed with the axis of the CNT roughly parallel to the surface normal of the substrate and was driven to approach the surface of SiO2 inside the SEM by nanomanipulators. The electron beam does not affect the bending measurements because the beam current used here was very low and the CNT being measured is about 50 nm in diameter and is not very sensitive to the electron beam. As shown in Figure 7, the CNT bent when it was pushed further after it had touched the SiO2 surface and the CNT-substrate contact point pinned. In some other cases, the contact point was observed to slide when the CNT was largely tilted away from the substrate normal. As shown in Figure 7b-d, continuous pushing caused the MWCNT probe to bend more severely. However, the bending is roughly homogeneous along the CNT. Locally concentrated severe buckling of the CNT was not observed even when the MWCNT probe was pushed toward the SiO2 wafer for several hundred nanometers after it had touched the surface. The present observation by SEM directly shows how a MWCNT contacts the substrate surface and bends. Although many events have been proposed for the approaching process of the CNT AFM probe in ref 22, the CNT probe was suggested to bend roughly homogenously in the first several hundred nanometers compression region after the CNT touched the substrate surface,22 which is consistent with the present experimental observation. To see the detailed structure change during bending, an experiment was also performed inside TEM using a manipulation stage (Nanofactory). The MWCNT was found to deform roughly homogenously during bending (as shown in Figure 8), probably due to their thick wall. (Note: Roughly the same amount of deformations was observed on both sides of the CNT before it was bent. After bending, the deformations on the compressed side are more visible due to diffraction contrast effect.) This is different from the case when BN nanotubes were compressed, where kinks were observed rather than a uniform curl of the tube.23 The above processes are static bending processes, which

are not exactly the same as the dynamic working process of a CNT AFM probe in tapping mode. It is well-known that the dynamic process is very difficult to be studied directly, but can be divided into several static segments and can be calculated by a mathematical method. The present studies inside electron microscopes on the static bending process directly show how the CNTs bend after they touch the substrate surface. The information is very useful for understanding the working process of a CNT AFM probe in tapping mode. 4. Conclusions MWCNT AFM probes with different lengths were fabricated using nanomanipulation technique. Amplitude-distance curves of the MWCNT AFM probes in tapping mode AFM were studied systematically through experiments. The amplitudedistance curves of the CNT AFM probes show different characteristics from that of conventional AFM tips, such as amplitude jump-into-zero, rebounds after the probe already touched the surface, and large hysteresis during retraction. Transition from attractive to repulsive regions appeared only when the free vibration amplitude is large. The above characteristics were found to change regularly with the length of the CNT probes and their tilting angle relative to the substrate surface normal and with the free vibration amplitude. SEM and TEM observations show that the MWCNT deform roughly homogenously during bending. Our results show that shorter CNTs with smaller tilting angle and being set at smaller free vibration amplitude are more suitable for tapping mode AFM. Acknowledgment. We thank Professor J Zhang for the MWCNT samples. This work was supported by NSF of China (60371005, 60771005, 60728102). Supporting Information Available: Amplitude- and phase-distance curves of a CNT AFM tip obtained at different free vibration amplitudes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hafner, J. H.; Cheung, C.-L.; Woolley, A. T.; Lieber, C. M. Prog. Biophys. Mol. Biol. 2001, 77, 73. (2) Wade, L. A.; Shapiro, I. R.; Ma, Z. Y.; Quake, S. R.; Collier, C. P. Nano Lett. 2004, 4, 725. (3) Tang, J.; Yang, G.; Zhang, Q.; Parhat, A.; Maynor, B.; Liu, J.; Qin, L.-C.; Zhou, O. Nano Lett. 2005, 5, 11.

15636 J. Phys. Chem. C, Vol. 112, No. 40, 2008 (4) Nguyen, C. V.; Stevens, R. M. D.; Barbe, J.; Han, J.; Meyyappan, M.; Sanchez, M. I.; Larson, C.; Hinsberg, W. D. Appl. Phys. Lett. 2002, 81, 901. (5) Bunch, J. S.; Rhodin, T. N.; McEuen, P. L. Nanotechnology 2004, 15, S76. (6) Garcıa´, R.; Paulo, A. S. Phys. ReV. B 1999, 60, 4961. (7) Anczykowski, B.; Kru¨ger, D.; Fuchs, H. Phys. ReV. B 1996, 53, 15485. (8) Snow, E. S.; Campbell, P. M.; Novak, J. P. Appl. Phys. Lett. 2002, 80, 2002. (9) Solares, S. D.; Esplandiu, M. J.; Goddard, W. A.; Collier, C. P. J. Phys. Chem. B 2005, 109, 11493. (10) Solares, S. D.; Matsuda, Y.; Goddard, W. A., III. J. Phys. Chem. B 2005, 109, 16658. (11) Nguyen, C. V.; Chao, K. J.; Stevens, R. M. D.; Delzeit, L.; Cassell, A.; Han, J.; Meyyapan, M. Nanotechnology 2001, 12, 363. (12) Strus, M. C.; Raman, A.; Han, C. S.; Nguyen, C. V. Nanotechnology 2005, 16, 2482. (13) Lee, S I.; Howell, S. W.; Raman, A.; Reifenberger, R.; Nguyen, C. V.; Meyyapan, M. Ultramicroscopy 2005, 103, 95. (14) Kutana, A.; Giapis, K. P.; Chen, J. Y.; Collier, C. P. Nano Lett. 2006, 6, 1669. (15) Peng, L.-M.; Chen, Q.; Liang, X. L.; Gao, S.; Wang, J. Y.; Kleindiek, S.; Tai, S. W. Micron 2004, 35, 495. (16) de Jonge, N.; Lamy, Y.; Kaiser, M. Nano Lett. 2003, 3, 1621. (17) Wei,X. L.; Jiang, A. N.; Gao, S.; Chen, Q. J. Nanosci. Nanotechnol. 2008, in press.

Jiang et al. (18) Wei, X. L.; Chen, Q.; Liu, Y.; Peng, L. M. Nanotechnology 2007, 18, 185503. (19) Snow, E. S.; Campbell, P. M.; Novak, J. P. J. Vac. Sci. Technol. B 2002, 20, 822. (20) Esplandiu, M. J.; Bittner, V. G.; Giapis, K. P.; Collier, C. P. Nano Lett. 2004, 4, 1873. (21) Because the coupling of the AFM cantilever with the cantilever holder is not the same each time the AFM cantilever is loaded, the drive voltage to make the free vibration of the AFM cantilever identical could be different. In the present experiments, the AFM cantilever needs to be transferred between SEM and AFM several times to study the lengths effect of CNT AFM probes so that the AFM cantilever needs to be loaded many times. To reduce the measurement error and to make our experimental conditions more comparable, we did our best to make the drive voltage nearly the same while keeping the free vibrating amplitude identical. This was done by re-adjusting and re-loading the cantilever and monitoring the drive voltage to achieve certain oscillating amplitude. However, as the automatic adjustment of the instrument is not perfect, the free vibrating amplitude is still not exactly the same at the same working conditions. (22) Yap, H. W.; Lakes, R. S.; Carpicks, R. W. Nano Lett. 2007, 7, 1149. (23) Golberg, D.; Costa, P. M. F. J.; Lourie, O.; Mitome, M.; Bai, X. D.; Kurashima, K.; Zhi, C. Y.; Tang, C. C.; Bando, Y. Nano Lett. 2007, 7, 2164.

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