Nanoscale Uniaxial Pressure Effect of a Carbon Nanotube Bundle on

In situ measurement of tip-enhanced near-field Raman spectra of an isolated single-wall carbon nanotube (SWNT) bundle has been demonstrated by applyin...
0 downloads 0 Views 101KB Size
Download PDF
NANO LETTERS

Nanoscale Uniaxial Pressure Effect of a Carbon Nanotube Bundle on Tip-Enhanced Near-Field Raman Spectra

2006 Vol. 6, No. 6 1269-1273

Taka-aki Yano,† Yasushi Inouye,*,‡,§,| and Satoshi Kawata†,§,| Department of Applied Physics and Graduate School of Frontier Biosciences, Osaka UniVersity, Suita, Osaka 565-0871, Japan, RIKEN, Wako, Saitama 351-0198, Japan, and CREST, Japan Corporation of Science and Technology, Japan Received January 18, 2006; Revised Manuscript Received April 3, 2006

ABSTRACT In situ measurement of tip-enhanced near-field Raman spectra of an isolated single-wall carbon nanotube (SWNT) bundle has been demonstrated by applying a uniaxial pressure up to ∼2 GPa to the bundle via a metal-coated atomic force microscope tip. We investigated the pressure dependences of Raman frequencies and the intensity of the radial breathing mode bands, the D-band and the G-band, which were related to deformation of SWNTs caused by the tip pressure.

Introduction. Resonant Raman spectroscopy has been utilized widely to analyze the structural and electronic properties of SWNTs and has even been used to determine the chirality of SWNTs.1-3 Pressure effects on SWNTs have also been studied by Raman spectroscopy in order to investigate their mechanical properties. Venkateswaran et al.4 for the first time reported pressure dependence of the G-band and the radial breathing mode (RBM) bands by applying homogeneous pressure to SWNTs under the hydrostatic condition. They observed Raman intensity and frequency changes with increasing pressure up to 5.2 GPa. Recently, Cronin et al.5 reported uniaxial strain effects on isolated SWNTs. They measured the micro-Raman spectrum of an SWNT, which was laterally scratched and elongated by an atomic force microscope (AFM) tip. They found a frequency downshifting of the G-band with increasing pressure5 while upshifting in hydrostatic experiments by Venkateswaran.4 Both micro-Raman measurements were performed and averaged over a wholly pressurized SWNT/SWNTs lying inside a diffraction-limited illumination focal spot. In this letter, we have investigated nanoscale uniaxial pressure effects of a tip on an isolated SWNT bundle by using tip-enhanced near-field Raman spectroscopy (TERS). TERS provides super-resolving capability for the observation and analysis of SWNTs6-8 and also induces Raman frequency shifting due to molecular deformation by locally * To whom correspondence should be addressed. Tel: +81-6-6879-7847. Fax: +81-6-6879-7330. E-mail: [email protected]. † Department of Applied Physics, Osaka University. ‡ Graduate School of Frontier Biosciences, Osaka University. § RIKEN. | CREST. 10.1021/nl060108y CCC: $33.50 Published on Web 05/05/2006

© 2006 American Chemical Society

applying a perturbative atomic force to molecules with a metal-coated AFM tip.9 In situ measurement of tip-enhanced near-field Raman spectra of the bundle was demonstrated by applying a uniaxial AFM-tip force from 0.0 nN to 2.4 nN to the bundle. We found a pressure dependence of Raman frequencies of the RBM bands, the D-band, and the G-band. The pressure dependence of the Raman intensity was also investigated. Experiment. The SWNTs were produced by Carbon Nanotechnologies Inc. (CNI) using the high-pressure CO (HiPco) technique, with a purity of ∼90%. Individual bundles of SWNTs were exfoliated from the aggregates by ultrasonicating them for 3 h in 65% HNO3, after which, the HNO3 solution was diluted with ethanol. The solution was ultrasonicated again for 3 h, and spin-cast onto the glass coverslip. Our experimental setup, based on an inverted optical microscope combined with a contact-mode AFM, was especially established for nano-Raman spectroscopy.6,9,10 Near-field Raman spectra of SWNTs were measured using a Nanofinder (Tokyo instruments, Inc), which we have optimized for TERS. A Si cantilever tip for contact-mode operation was silver-coated by the vacuum evaporation method, resulting in the end diameter of 35 nm. A frequencydoubled Nd:YVO4 laser (λ: 532 nm) was used for illumination. When the metallic tip approached a SWNT bundle existing inside the illumination focal spot, an enhanced electric field was generated at the tip apex because of localized surface plasmon polariton (LSPP) excitation.11-13 The enhanced field amplified the Raman scattering of the SWNT bundle located in the vicinity of the tip.

Figure 1. (a) AFM image of an isolated SWNT bundle on the glass coverslip. (b) The line profile corresponding to the dashed line in the AFM image. The tip-enhanced near-field Raman spectra were measured with the metallic tip on a cross-marked position.

Figure 2. (a) Far-field Raman spectrum of the bundle without the metallic tip. The tip-enhanced near-field Raman images were obtained with the tip onto the bundle under the AFM-tip force of (b) 0.6 nN, (c) 1.2 nN, (d) 1.8 nN, and (e) 2.4 nN.

Results and Discussion. Figure 1a shows an AFM image of an isolated SWNT bundle obtained with the silver-coated cantilever tip. The topographic image was measured in contact-mode operation under a constant force of 200 pN. A line profile in Figure 1b, corresponding to the dashed line in the AFM image, showed that the height of the bundle was ∼2 nm. Judging from the height, the number of SWNTs was estimated to be 2 or 3 in the bundle. After acquiring the AFM image, a far-field Raman spectrum was measured by moving the metallic tip 1 µm away from the bundle. Figure 2a shows the far-field Raman spectrum in the frequency regions of the RBM bands, the D-band, and the G-band. The two RBM bands are observed at the frequencies of 235 and 278 cm-1. Considering that the diameter (d, nm) of a SWNT is inversely proportional to the vibrational frequency (ωRBM, cm-1) of its RBM (i.e., d ) 232/(ωRBM 1270

6.5)14), the RBM bands at 235 and 278 cm-1 corresponded to the SWNTs having diameters of 1.02 and 0.85 nm, respectively. The G-band was split mainly into the two frequency components at 1556 and 1595 cm-1. The higher frequency component, ωG+, exhibited a Lorentzian line shape, whereas the lower frequency component, ωG-, exhibited an asymmetric broader Breit-Wigner-Fano (BWF) line shape, attributable to metallic SWNTs.14,15 In addition, a Kataura plot16 suggests that the SWNTs provide metallic character judging from the excitation photon energy (2.33 eV) and the diameters estimated from the RBM frequencies. Furthermore, the D-band was observed as a broad peak centered at 1339 cm-1. A tip-enhanced near-field Raman spectrum, presented in Figure 2b, was measured on the bundle indicated by the cross mark in Figure 1a while a constant AFM-tip force of 0.6 nN was applied onto it by the metallic tip. It was confirmed by monitoring the in situ topological signal that the tip was set exactly onto the bundle. However, the force curve was not distinguishable when the tip was set on the bundle or on the substrate beside the bundle. To keep the tip position stable during the near-field Raman measurements, a closed-loop operation system was utilized in our system. Assuming that the size of the enhanced light field was 35 nm, which is the same as the diameter of the metallic tip apex, and also assuming that the diameter of the illumination focal spot was ∼460 nm, the enhancement factor of the Raman scattering intensity for each Raman band was estimated to be about 30-40. Figure 2b-e shows the near-field Raman spectra obtained by changing the AFM-tip force up to 2.4 nN. The relative Raman intensity ratios of the RBM band at 235 to that at 278 cm-1 were different among the near-field Raman spectra in Figure 2b-e. The Raman intensities of both of the frequency components (ωG+ and ωG-) in the G-band increased gradually as the force increased. When the pressure increased up to an AFM-tip force of 1.8 nN, the spectral width of the lower frequency component, ωG- , became narrower and the spectral shape changed from the broader asymmetric BWF shape to a Lorentzian profile. However, the D-band did not show obvious differences in its spectral shape. For a detailed analysis, Raman frequencies for the G-band (ωG- and ωG+), the RBM bands, and the D-band are plotted as a function of the AFM-tip force up to 2.4 nN as shown in Figure 3a-c, respectively. As can be seen in Figure 3a, ωGdownshifts as much as 18 cm-1 from the initial frequency of 1595 cm-1 shown in far-field Raman spectroscopy measurement, whereas ωG+ does not shift with increasing the AFM-tip force. However, unlike our experiment, both of the frequency components were upshifted for the SWNTs under the hydrostatic pressure4 or downshifted for SWNTs scratched by an AFM tip5 in micro-Raman experiments. These different behaviors are due mainly to the different ways of applying the pressure to the SWNTs. The hydrostatic experiments induced the radial and the axial compressions of SWNTs, and the uniaxial experiment by Cronin5 induced the radial compression and the axial elongation. Considering the fact that the higher frequency component, ωG+, is Nano Lett., Vol. 6, No. 6, 2006

Figure 3. Raman frequencies of (a) the G-band (ωG- and ωG+), (b) the two RBM bands, and (c) the D-band as a function of the applied AFM-tip force.

assigned to the axial vibration, and the lower frequency component, ωG- , to the circumferential vibration for metallic SWNTs,15 both of the G-band frequency components were subject to pressure in those experiments, inducing frequency shifting of both ωG+ and ωG-. By contrast, in our experimental conditions, the pressure was locally applied onto the SWNTs from only the radial direction by the metallic tip. To examine the localized radial pressure effect in our experiment, we calculated the orthogonal shearing stresses along the circumferential and axial directions of SWNTs using Hertz’s elastic contact theory.17 According to Hertz theory, pressure and stress can be calculated from the sizes, Young’s moduli, and Poisson ratios of two objects that are in contact with each other. In our experimental configuration, the metallic tip was considered as a sphere with a diameter corresponding to size of the tip apex, whereas the bundle was considered as a cylinder. In this configuration, an elliptical contact-area was formed between the two objects. Because the metallic tip was prepared by silver covering a silicon cantilever, the Young’s modulus value and the Poisson ratio of the metallic sphere were employed from the values of bulk silver. The values for the radial direction of the bundle treated as the cylinder were employed from the calculation based on a force-constant lattice dynamical model for a bundle having the triangular close-packed lattice of SWNTs.18 According to the calculations based on these parameters, the shearing stress along the axial direction was more than an order of magnitude smaller than that along the circumferential direction. Applying Hertz theory, we also found that the radial displacement of the cylinder was ∼0.3 nm when the force was loaded up to 2.4 nN, which indicates that the SWNTs in the bundle are radially distorted by the metallic tip. Therefore, stronger shearing stress influenced the vibrational structure of the lower frequency component, Nano Lett., Vol. 6, No. 6, 2006

ωG- , than that of the higher frequency component, ωG+, resulting in frequency shifting of ωG-. Furthermore, the negative frequency shifting of ωG- is attributed to the curvature effect on the G-band for metallic SWNTs, which was investigated by Jorio et al.;19 for example, ωG- is lower for the SWNTs having larger curvature (smaller diameter), whereas ωG+ is independent of the curvature. When a SWNT having circular section is radially deformed into ellipse, the ellipse has the smallest curvature at the top of the ellipsoid and the largest curvature at the side. The circumferential vibration of the SWNT is in the direction of the tip axis at the side and is perpendicular to the axis at the top. However, only circumferential vibration at the side can be effectively coupled with the LSPP light field at the tip apex because collective electron oscillation occurs along the tip axis. Accordingly, the Raman signal of ωG- in tip-enhanced near-field Raman spectra was caused by the circumferential Raman vibration at the side. This indicates that as the SWNTs were radially deformed with increasing the pressure, the curvature of the SWNTs at the sides became larger, and then frequency downshifting of ωGwas induced. Figure 3b shows that both the RBM bands at 235 and 278 cm-1 are slightly downshifted as the force increases, which is opposite to the hydrostatic experiments.4,20-23 The maximum frequency deviation from the force-free frequency of 235 cm-1 Raman band is 7 cm-1, which is 2 cm-1 larger than that of the Raman band at 278 cm-1. This feature is in good agreement with the behavior of the hydrostatic experiments in which SWNTs having large diameter were radially deformed more easily under pressure than those with small diameter.23 With the above-mentioned Hertz model, the pressures (PMAX) corresponding to the maximum force of 2.4 nN were estimated to be ∼2.0 GPa. Therefore, the absolute pressure coefficients were evaluated to be ∼ -3 cm-1/GPa for the RBM bands and ∼-9 cm-1/GPa for the higher frequency component ωG+ of the G-band. This result is inconsistent with the hydrostatic experiments of bundles in which the pressure coefficients for the RBM bands were larger than those for the G-band because of the strong van der Waals (vdW) interaction among the bundled SWNTs under the hydrostatic pressure.20 Therefore, the fact that pressure coefficient of the RBM bands in our experiment is smaller than that of hydrostatic condition is attributed to the intramolecular distortion of the individual SWNTs in the bundle rather than the intermolecular vdW interaction. In contrast to the RBM bands, the frequency of the D-band does not change with increasing force. We have also investigated a force dependence of Raman intensity for all Raman bands. Figure 4 shows the Raman intensity enhancement factor normalized by far-field Raman intensity as a function of applied force up to 2.4 nN. Raman intensity of all Raman bands becomes stronger as the AFMtip force increases. We attribute the force-dependent intensity increase to the modification of electronic density of states (DOS). Tight binding calculations of radially deformed SWNTs showed that the van Hove singularity peaks in DOS shifted to a higher or a lower energy depending on their 1271

Figure 4. Raman intensity of the RBM bands, the D-band, and the G-band as a function of applied AFM-tip force.

chirality.24-26 Optical absorption spectra under the hydrostatic pressure27,28 also showed shifting of the optical transition energies under the pressure. Accordingly, in our case, the optical transition energies of the radially deformed SWNTs changed close to the excitation photon energy and the resonant Raman effect was elevated. Because TERS is the same phenomenon as surfaceenhanced Raman scattering (SERS)29 occurring at a metallic tip apex, there are two enhancement mechanisms. One is the above-mentioned electromagnetic or plasmonic enhancement effect; the other is the chemical enhancement effect due to charge transfer between chemical species and metallic surface. In the TERS measurement, the chemical enhancement occurs only when the metallic tip is operated in contact mode because for a steady interaction, the metal molecules should maintain constance distance with the sample molecules. Corio et al.30 reported that the Raman scattering of ωG- mode was enhanced in the surface-enhanced resonant Raman spectra compared with the normal resonant Raman spectra while the Raman intensity of ωG+ mode did not change. They attributed this phenomenon to the selection rules for the charge transfer mechanism; that is, totally symmetric vibrational modes are more likely to be enhanced. Accordingly, the strongly enhanced vibrational component of the ωG- mode was more likely to have a predominantly total symmetry compared with the ωG+ mode. This result is consistent with our result showing that the Raman intensity enhancement factor of ωG- is about twice that of ωG+. Because one of the enhancement mechanism in SERS, that is, electromagnetic mechanism based on LSPP excitation, is sensitive to the shape and size of the metallic tip apex,31,32 a structural change of the apex under force might also affect the intensity of the enhanced Raman scattering. However, the SEM images before and after applying the force up to 2.4 nN show no difference in the shape of the tip, indicating the absence of plastic deformation. Furthermore, although the elastic deformation during the experiment was not measured directly, the hertz contact theory showed that the deformation of the silver tip was estimated to be ∼0.015 nm while applying the maximum force of 2.4 nN to the SWNTs. Because the value of the deformation is smaller than the bond length of silver atom, we believe that there was no significant deformation of the tip during the measure1272

ment. Hence, the field enhancement effect should not change during the measurements. In the hydrostatic experiments on bundles, an abrupt intensity drop of G-band was observed at a pressure of 1.7 GPa because of the structural phase transition.33 Disappearance of RBM intensity beyond 1.5 GPa has also been reported,4 indicating that the bundles were polygonized under pressure.34-35 However, we did not observe the abovementioned phenomena under an AFM-tip force up to 2.4 nN, the value of which corresponds to a pressure of ∼2 GPa. Furthermore, the intensity ratio of the higher frequency component in the G-band to the frequency component in the D-band, which might reflect defect density in the SWNTs, does not show a drastic pressure dependence and keeps almost the same value ranging from 2.8 to 3.5. The shifts in peak position and changes in Raman intensity were observed to be reversible for this and other isolated bundles when the tip was retracted from the bundle. The D-band/G-band ratio in the far-field Raman spectra before and after applying the force was also restored. Therefore, the deformation of SWNTs was elastic under the force up to 2.4 nN. Increasing the force beyond 2.4 nN has the possibility of inducing irreversible phenomenon because of plastic deformation. We are now processing the experiment on isolated individual SWNTs having a variety of chiralities in order to provide further quantitative and qualitative analysis of the nanoscale uniaxial pressure effects. Acknowledgment. Y.I. gratefully acknowledges financial support by a Grant-in-Aid for Scientific Research No. 16360034 and No. 17034034 from the Ministry of Education, Culture, Sports, Science and Technology. References (1) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (2) Jorio, A.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Hunter, M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. Lett. 2001, 86, 1118. (3) Kneipp, K.; Kneipp, H.; Corio, P.; Brown, S. D. M.; Shafer, K.; Motz, J.; Perelman, L. T.; Hanlon, E. B.; Marucci, A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. Lett. 2000, 84, 3470. (4) Venkateswaran, U. D.; Rao, A. M.; Richter, E.; Menon, M.; Rinzler, A.; Smalley, R. E.; Eklund, P. C. Phys. ReV. B 1999, 59, 10928. (5) Cronin, S. B.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B.; Dresselhaus, M. S.; Tinkham, M. Phys. ReV. Lett. 2004, 93, 167401. (6) Hayazawa, N.; Yano, T.; Watanabe, H.; Inouye, Y.; Kawata, S. Chem. Phys. Lett. 2003, 376, 174. (7) Hartschuh, A.; Sanchez, E. J.; Xie, X. S.; Novotny, L. Phys. ReV. Lett. 2003, 90, 95503. (8) Saito, Y.; Hayazawa, N.; Kataura, H.; Murakami, T.; Tsukagoshi, K.; Inouye, Y.; Kawata, S. Chem. Phys. Lett. 2005, 410, 136. (9) Watanabe, H.; Ishida, A.; Hayazawa, N.; Inouye, Y.; Kawata, S. Phys. ReV. B 2004, 69, 155418. (10) Ichimura, T.; Hayazawa, N.; Hashimoto, M.; Inouye, Y.; Kawata, S. Phys. ReV. Lett. 2004, 92, 220801. (11) Inouye, Y.; Kawata, S. Opt. Lett. 1994, 19, 159. (12) Fischer, U. C.; Pohl, D. W. Phys. ReV. Lett. 1989, 62, 458. (13) Bachelot, R.; Gleyzes, P.; Boccara, A. C. Opt. Lett. 1995, 20, 1924. (14) Alvarez, L.; Righi, A.; Guillard, T.; Rols, S.; Anglaret, E.; Laplaze, D.; Sauvajol, J. L. Chem. Phys. Lett. 2000, 316, 186. (15) Brown, S. D. M.; Jorio, A.; Corio, P.; Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Kneipp, K. Phys. ReV. B 2001, 63, 155414. (16) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555.

Nano Lett., Vol. 6, No. 6, 2006

(17) Boresi, A. P.; Sidebottom, O. M. AdVanced Mechanics of Materials; John Wiley & Sons: New York, 1985. (18) Popov, V. N.; Van Doren, V. E.; Balkanski, M. Solid State Commun. 2000, 114, 395. (19) Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Goldberg, B.; U ¨ nlu¨, M. S.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R. Phys. ReV. B 2002, 65, 55412. (20) Thomsen, C.; Reich, S.; Gon˜i, A. R.; Jantoljak, H.; Rafailov, P. M.; Loa, I.; Syassen, K.; Journet, C. Bernier, P. Phys. Status Solidi B 1999, 215, 435. (21) Venkateswaran, B. E. A.; Schlecht, U.; Rao, A. M.; Richter, E.; Loa, I.; Syassen, K.; Eklund, P. C. Phys. Status Solidi B 2001, 223, 225. (22) Schlecht, U.; Venkateswaran, U. D.; Richter, E.; Chen, J.; Haddon, R. C.; Eklund, P. C.; Rao, A. M. J. Nanosci. Nanotechnol. 2003, 3, 139. (23) Venkateswaran, U. D.; Masica, D. L.; Sumanasekera, G. U.; Furtado, C. A.; Kim, U. J.; Eklund, P. C. Phys. ReV. B 2003, 68, 241406. (24) Lammert, P. E.; Zhang, P.; Crespi, V. H. Phys. ReV. Lett. 2000, 84, 2453. (25) Maiti, A.; Svezhenko, A.; Anantram, M. P. Phys. ReV. Lett. 2002, 88, 12680501. (26) Gulseren, O.; Yildirim, T.; Ciraci, S.; Kilic, C. Phys. ReV. B 2002, 65, 155410.

Nano Lett., Vol. 6, No. 6, 2006

(27) Kazaoui, S.; Minami, N.; Yamawaki, H.; Aoki, K.; Kataura, H.; Achiba, Y. Phys. ReV. B 2000, 62, 1643. (28) Wu, J.; Walukiewicz, W.; Shan, W.; Bourret-Courchesne, E.; Ager, J. W., III; Yu, K. M.; Haller, E. E.; Kissell, K.; Bachilo, S. M.; Weisman, R. B.; Smalley, R. E. Phys. ReV. Lett. 2004, 93, 017404. (29) Maruyama, Y.; Futamata, M. J. Raman Spectrosc. 2005, 36, 581. (30) Corio, P.; Brown, S. D. M.; Marucci, A.; Pimenta, M. A.; Kneipp, K.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B 2000, 61, 13202. (31) Krug, T., II; Sanchez, E. J.; Xie, X. S. J. Chem. Phys. 2002, 116, 10895. (32) Martin, Y. C.; Hamann, H. F.; Wickramasinghe, H. K. J. Appl. Phys. 2001, 89, 5774. (33) Peters, M.; McNeil, L. E.; Lu, J. P.; Kahn, D. Phys. ReV. B 2000, 61, 5939. (34) Tang, J.; Qin, L. C.; Sasaki, T.; Yudasaka, M.; Matsushita, A.; Iijima, S. Phys. ReV. Lett. 2000, 85, 1887. (35) Lopez, M. J.; Rubio, A.; Alonso, J. A.; Qin, L. C.; Iijima, S. Phys. ReV. Lett. 2001, 86, 3056.

NL060108Y

1273