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A study of friction by carbon nanotube tip
Applied Surface Science 188 (2002) 456–459
A study of friction by carbon nanotube tip Makoto Ishikawa*, Masamichi Yoshimura, Kazuyuki Ueda Nano High-Tech Research Center, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan Received 1 September 2001; accepted 25 September 2001
Abstract The carbon nanotube (CNT) probe, where a multi-walled (MW) CNT is attached onto the tip apex of a commercial silicon nitride cantilever, is applied to study friction on the nanometer scale using contact atomic force microscope (AFM). The friction versus load curve using CNT tip shows a completely different behavior from that of conventional tip, which is ascribed to the unique shape of CNT. In addition, strong scanning length dependency of the friction force is found due to the deformation of CNT. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Friction; Nanotube; Atomic force microscopy; Tip; Friction force microscopy; Buckling force; Capillary force
1. Introduction Because of their unique physical properties carbon nanotubes (CNT) have been expected to be promising probes for various types of scanning probe microscopes (SPM). High aspect ratio of the nanotubes enable us to observe deep holes which cannot be examined by conventional tips, as demonstrated by Dai et al. [1]. Uchihashi et al. applied CNT tips to noncontact atomic force microscopy and obtained clear images of deoxyribonucleic acid (DNA) helical turns by making the best of the sharpness of the nanotube tip apex [2]. Jarvis et al. applied CNT probes to measure solvation forces in water by atomic force microscope (AFM) [3]. Ishikawa et al. utilized CNT-attached tungsten tips for scanning tunneling microscopy (STM) and dimer rows of silicon (100) were clearly resolved [4]. Although, a great deal of applications to SPM are reported using CNT probes, there are few studies where the CNT is *
Corresponding author. Tel.: þ81-52-809-1852; fax: þ81-52-809-1853. E-mail address: [email protected] (M. Ishikawa).
used in contact mode of SPM, to the best of our knowledge. Since the nanotube is robust along the axis, great advantages should be extracted if using in contact mode. Here, we demonstrate the application of CNT probes to the friction study where the probes work in contact with sample surfaces. Advantages as well as problems when applying CNT probes to friction study are described in details.
2. Experimental Commercial CNTs (BU-201, Bucky USA) were fixed to the edge of a razor blade using a carbon adhesive tape. The platinum–palladium of 6 nm thickness was coated on a commercial cantilever made of silicon nitride (OMCL-RC800PSA, Olympus Co., Ltd.) using a magnetro-sputtering technique. The razor blade and the coated cantilever were oppositely mounted on two precision three-dimensional stages in field emission type scanning electron microscope (SEM) chamber (S-4500, Hitachi). A CNT protruding from the edge of the razor blade was attached to the tip
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 3 2 - 1
M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
457
Fig. 1. SEM image of CNT probe used in the present study. The length and diameter is 175 and 20 nm, respectively.
apex under SEM observation using the same method demonstrated by Akita et al. [5]. The length of the nanotube is 175 nm and the diameter is 20 nm, as confirmed by SEM (Fig. 1). Friction measurements were performed in ambient condition using a commercial AFM (NanoScope IIIa, Digital Instruments). The friction force is estimated using the frictional loop, namely, bidirectional friction force for a single line, as was reported by Carpick et al. [6]. The displacement of the base of the AFM cantilever, which is abscissa axis of the frictional loop, is referred to as ‘‘scanning length’’ hereafter. Because the absolute value of the friction force was difficult to estimate, we plotted and analyzed the output voltage of photo diode which is proportional to the friction force. The specimen used was mica that was cleaved just before measurement.
3. Results and discussion Fig. 2 shows the comparison of friction-load curves between conventional (OMCL-800PSA, without
Fig. 2. Friction vs. load curves of conventional and CNT probes, obtained by the analysis of scanning length of 20 nm (a) and 500 nm (b). Dashed lines represent calculation curves based on JKR theory. The arrow shows an obvious jump-off point of conventional tip.
coating) and two different CNT probes. The scanning length used to estimate friction value is 20 nm (a) and 500 nm (b). The curves acquired by conventional tip show a similar behavior independent of scanning length and obey well calculated curves (dashed line) based on Johnson–Kendall–Roberts (JKR) theory where load and frictional force can be normalized by each critical value at the point that the tip jump
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M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
off from the surface [7,8]. In contrast, different characteristics are obtained using CNT probes. For short scanning length as in Fig. 2(a), friction value remains constant independent of applied loads below 200 nN. While for the larger length of 500 nm, friction increases with load almost linearly as shown in Fig. 2(b). The measurement of the friction force over 200 nN of normal force was impossible due to the instability of the frictional loop. We note no appreciable jump-off phenomenon in friction-load curves in the case of CNT probes, while clear appreciable jumpoff phenomena are observed in the case of conventional probes (indicated by the arrows in Fig. 2(a) and (b)). The adhesion force of CNT probe in normal force versus distance curves shows almost a half value of that of conventional tips (not shown here). Since the CNT consists of graphite sheet, small capillary force is expected, which could reduce the adhesion force. Similar adhesion behavior was reported by Eastman et al. for paraffin-coated AFM tips [9]. In order to clarify the reason why the scanning length affects friction versus load behavior in the case of CNT probes, friction versus scanning length curves are plotted for several values of applied loads between
Fig. 3. (a) Friction vs. scanning length at several applied loads. (b) Enlarged plots at the short lengths of (a).
47 and 218 nN (Fig. 3(a)). Fig. 3(b) shows the enlarged plots at short scanning lengths. It is clear that in the range of scanning length below 20 nm, the measured friction force is independent of the applied loads except for the largest load of 218 nN (see Fig. 3(b)). In the case with scanning length larger than 30 nm, friction increases with applied load. These are consistent with the results mentioned before. At certain applied loads between 47 and 156 nN, friction has nothing to do with scanning length larger than 30 nm. In this region, normal friction loops are reproducibly observed. By contrast, the case is completely different for the applied load of 218 nN. The friction measured at 218 nN increases steeply with scanning length, and the reproducible friction loop cannot be obtained. The estimated buckling force of the nanotube used in the present study is about 190 nN (Young’s modulus 0.3 TPa is used). Since the value of 218 nN is larger than this bucking force, the CNT could be bent and deformed at this situation, resulting in unstable friction loops in the experiment. After the measurement, the probe apex was again observed by the SEM and the straight nanotube was kept on it without changing the original length, indicating of elastic buckling. On the other hand, for the applied loads below 190 nN, the CNT is expected to behave as rigid body. The next question is why the friction increases with applied load above scanning length of 30 nm. If the axis of the nanotube is positioned normal to the sample surface during scanning, the contact area would not be changed, resulting in constant friction independent of loads. However it is not the case. We consider that the axis is not normal but tilted to some extent. In this case, with increasing the loads the side of the nanotube is close to the sample surface. If the distance between the side and the sample surface is within the capillary bridge, meniscus interaction contributes to the friction force. Finally we just comment on why the friction is constant in the small scanning length independent of the loads. We do not understand straightforwardly but there is a possibility that the contact point does not moved and only tilt angle of the nanotube is switched back and forth. In fact, the atomic image of the mica surface could not be obtained with the present CNT probes of 200 nm length. We got atomic images when using the nanotube less than 60 nm (not shown here).
M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
4. Conclusion In conclusion, we demonstrate the application of CNT probe for the measurement of friction using the AFM. No appreciable jump-off is observed in frictionload curves, presumably due to small capillary force of the nanotube material. The friction increases with applied loads, but the behavior is different from the case of conventional tips. This characteristic is not explainable by JKR theory but the simple tilting effect of nanotube is proposed. In a short scanning length, the possibility of the nanotube oscillation without changing the contact point is indicated. It is suggested from the present study that the contact angle is crucial to the use of nanotube tip in contact atomic force microscopy. The fabrication of nanotube with shorter pendent length on the tip apex is necessary for the experiments using higher applied loads. Acknowledgements This work is partially supported by a Grant-inAid for Scientific Research on Priority Area (B)
459
(Manipulation of Atom and Molecules by Electronic Excitation) from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [2] T. Uchihashi, N. Choi, M. Tanigawa, M. Ashino, Y. Sugawara, H. Nishijima, S. Akita, Y. Nakayama, H. Tokumoto, K. Yokoyama, S. Morita, M. Ishikawa, Jpn. J. Appl. Phys. 39 (2000) L887. [3] S. Jarvis, T. Uchihashi, T. Ishida, H. Tokumoto, Y. Nakayama, J. Phys. Chem. B 104 (2000) 6091. [4] M. Ishikawa, K. Ojima, M. Yoshimura, K. Ueda, in preparation. [5] S. Akita, H. Nishijima, Y. Nakayama, F. Tokumasu, K. Takeyasu, J. Phys. D: Appl. Phys. 32 (1999) 1044. [6] R.W. Carpick, N. Agraı¨t, D.F. Ogletree, M. Salmeron, Langmuir 12 (1996) 3334. [7] R.W. Carpick, N. Agraı¨t, D.F. Ogletree, M. Salmeron, J. Vac. Sci. Technol. B 14 (1996) 1289. [8] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, CA, 1992. [9] T. Eastman, D. Zhu, Langmuir 12 (1996) 2859.
A study of friction by carbon nanotube tip Makoto Ishikawa*, Masamichi Yoshimura, Kazuyuki Ueda Nano High-Tech Research Center, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan Received 1 September 2001; accepted 25 September 2001
Abstract The carbon nanotube (CNT) probe, where a multi-walled (MW) CNT is attached onto the tip apex of a commercial silicon nitride cantilever, is applied to study friction on the nanometer scale using contact atomic force microscope (AFM). The friction versus load curve using CNT tip shows a completely different behavior from that of conventional tip, which is ascribed to the unique shape of CNT. In addition, strong scanning length dependency of the friction force is found due to the deformation of CNT. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Friction; Nanotube; Atomic force microscopy; Tip; Friction force microscopy; Buckling force; Capillary force
1. Introduction Because of their unique physical properties carbon nanotubes (CNT) have been expected to be promising probes for various types of scanning probe microscopes (SPM). High aspect ratio of the nanotubes enable us to observe deep holes which cannot be examined by conventional tips, as demonstrated by Dai et al. [1]. Uchihashi et al. applied CNT tips to noncontact atomic force microscopy and obtained clear images of deoxyribonucleic acid (DNA) helical turns by making the best of the sharpness of the nanotube tip apex [2]. Jarvis et al. applied CNT probes to measure solvation forces in water by atomic force microscope (AFM) [3]. Ishikawa et al. utilized CNT-attached tungsten tips for scanning tunneling microscopy (STM) and dimer rows of silicon (100) were clearly resolved [4]. Although, a great deal of applications to SPM are reported using CNT probes, there are few studies where the CNT is *
Corresponding author. Tel.: þ81-52-809-1852; fax: þ81-52-809-1853. E-mail address: [email protected] (M. Ishikawa).
used in contact mode of SPM, to the best of our knowledge. Since the nanotube is robust along the axis, great advantages should be extracted if using in contact mode. Here, we demonstrate the application of CNT probes to the friction study where the probes work in contact with sample surfaces. Advantages as well as problems when applying CNT probes to friction study are described in details.
2. Experimental Commercial CNTs (BU-201, Bucky USA) were fixed to the edge of a razor blade using a carbon adhesive tape. The platinum–palladium of 6 nm thickness was coated on a commercial cantilever made of silicon nitride (OMCL-RC800PSA, Olympus Co., Ltd.) using a magnetro-sputtering technique. The razor blade and the coated cantilever were oppositely mounted on two precision three-dimensional stages in field emission type scanning electron microscope (SEM) chamber (S-4500, Hitachi). A CNT protruding from the edge of the razor blade was attached to the tip
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 3 2 - 1
M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
457
Fig. 1. SEM image of CNT probe used in the present study. The length and diameter is 175 and 20 nm, respectively.
apex under SEM observation using the same method demonstrated by Akita et al. [5]. The length of the nanotube is 175 nm and the diameter is 20 nm, as confirmed by SEM (Fig. 1). Friction measurements were performed in ambient condition using a commercial AFM (NanoScope IIIa, Digital Instruments). The friction force is estimated using the frictional loop, namely, bidirectional friction force for a single line, as was reported by Carpick et al. [6]. The displacement of the base of the AFM cantilever, which is abscissa axis of the frictional loop, is referred to as ‘‘scanning length’’ hereafter. Because the absolute value of the friction force was difficult to estimate, we plotted and analyzed the output voltage of photo diode which is proportional to the friction force. The specimen used was mica that was cleaved just before measurement.
3. Results and discussion Fig. 2 shows the comparison of friction-load curves between conventional (OMCL-800PSA, without
Fig. 2. Friction vs. load curves of conventional and CNT probes, obtained by the analysis of scanning length of 20 nm (a) and 500 nm (b). Dashed lines represent calculation curves based on JKR theory. The arrow shows an obvious jump-off point of conventional tip.
coating) and two different CNT probes. The scanning length used to estimate friction value is 20 nm (a) and 500 nm (b). The curves acquired by conventional tip show a similar behavior independent of scanning length and obey well calculated curves (dashed line) based on Johnson–Kendall–Roberts (JKR) theory where load and frictional force can be normalized by each critical value at the point that the tip jump
458
M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
off from the surface [7,8]. In contrast, different characteristics are obtained using CNT probes. For short scanning length as in Fig. 2(a), friction value remains constant independent of applied loads below 200 nN. While for the larger length of 500 nm, friction increases with load almost linearly as shown in Fig. 2(b). The measurement of the friction force over 200 nN of normal force was impossible due to the instability of the frictional loop. We note no appreciable jump-off phenomenon in friction-load curves in the case of CNT probes, while clear appreciable jumpoff phenomena are observed in the case of conventional probes (indicated by the arrows in Fig. 2(a) and (b)). The adhesion force of CNT probe in normal force versus distance curves shows almost a half value of that of conventional tips (not shown here). Since the CNT consists of graphite sheet, small capillary force is expected, which could reduce the adhesion force. Similar adhesion behavior was reported by Eastman et al. for paraffin-coated AFM tips [9]. In order to clarify the reason why the scanning length affects friction versus load behavior in the case of CNT probes, friction versus scanning length curves are plotted for several values of applied loads between
Fig. 3. (a) Friction vs. scanning length at several applied loads. (b) Enlarged plots at the short lengths of (a).
47 and 218 nN (Fig. 3(a)). Fig. 3(b) shows the enlarged plots at short scanning lengths. It is clear that in the range of scanning length below 20 nm, the measured friction force is independent of the applied loads except for the largest load of 218 nN (see Fig. 3(b)). In the case with scanning length larger than 30 nm, friction increases with applied load. These are consistent with the results mentioned before. At certain applied loads between 47 and 156 nN, friction has nothing to do with scanning length larger than 30 nm. In this region, normal friction loops are reproducibly observed. By contrast, the case is completely different for the applied load of 218 nN. The friction measured at 218 nN increases steeply with scanning length, and the reproducible friction loop cannot be obtained. The estimated buckling force of the nanotube used in the present study is about 190 nN (Young’s modulus 0.3 TPa is used). Since the value of 218 nN is larger than this bucking force, the CNT could be bent and deformed at this situation, resulting in unstable friction loops in the experiment. After the measurement, the probe apex was again observed by the SEM and the straight nanotube was kept on it without changing the original length, indicating of elastic buckling. On the other hand, for the applied loads below 190 nN, the CNT is expected to behave as rigid body. The next question is why the friction increases with applied load above scanning length of 30 nm. If the axis of the nanotube is positioned normal to the sample surface during scanning, the contact area would not be changed, resulting in constant friction independent of loads. However it is not the case. We consider that the axis is not normal but tilted to some extent. In this case, with increasing the loads the side of the nanotube is close to the sample surface. If the distance between the side and the sample surface is within the capillary bridge, meniscus interaction contributes to the friction force. Finally we just comment on why the friction is constant in the small scanning length independent of the loads. We do not understand straightforwardly but there is a possibility that the contact point does not moved and only tilt angle of the nanotube is switched back and forth. In fact, the atomic image of the mica surface could not be obtained with the present CNT probes of 200 nm length. We got atomic images when using the nanotube less than 60 nm (not shown here).
M. Ishikawa et al. / Applied Surface Science 188 (2002) 456–459
4. Conclusion In conclusion, we demonstrate the application of CNT probe for the measurement of friction using the AFM. No appreciable jump-off is observed in frictionload curves, presumably due to small capillary force of the nanotube material. The friction increases with applied loads, but the behavior is different from the case of conventional tips. This characteristic is not explainable by JKR theory but the simple tilting effect of nanotube is proposed. In a short scanning length, the possibility of the nanotube oscillation without changing the contact point is indicated. It is suggested from the present study that the contact angle is crucial to the use of nanotube tip in contact atomic force microscopy. The fabrication of nanotube with shorter pendent length on the tip apex is necessary for the experiments using higher applied loads. Acknowledgements This work is partially supported by a Grant-inAid for Scientific Research on Priority Area (B)
459
(Manipulation of Atom and Molecules by Electronic Excitation) from the Ministry of Education, Science, Sports and Culture of Japan.
References [1] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [2] T. Uchihashi, N. Choi, M. Tanigawa, M. Ashino, Y. Sugawara, H. Nishijima, S. Akita, Y. Nakayama, H. Tokumoto, K. Yokoyama, S. Morita, M. Ishikawa, Jpn. J. Appl. Phys. 39 (2000) L887. [3] S. Jarvis, T. Uchihashi, T. Ishida, H. Tokumoto, Y. Nakayama, J. Phys. Chem. B 104 (2000) 6091. [4] M. Ishikawa, K. Ojima, M. Yoshimura, K. Ueda, in preparation. [5] S. Akita, H. Nishijima, Y. Nakayama, F. Tokumasu, K. Takeyasu, J. Phys. D: Appl. Phys. 32 (1999) 1044. [6] R.W. Carpick, N. Agraı¨t, D.F. Ogletree, M. Salmeron, Langmuir 12 (1996) 3334. [7] R.W. Carpick, N. Agraı¨t, D.F. Ogletree, M. Salmeron, J. Vac. Sci. Technol. B 14 (1996) 1289. [8] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, CA, 1992. [9] T. Eastman, D. Zhu, Langmuir 12 (1996) 2859.