- Email: [email protected]
Polarization measurements in tip-enhanced Raman spectroscopy applied to single-walled carbon nanotubes
Chemical Physics Letters 410 (2005) 136–141 www.elsevier.com/locate/cplett
Polarization measurements in tip-enhanced Raman spectroscopy applied to single-walled carbon nanotubes Y. Saito a, N. Hayazawa a, H. Kataura b, T. Murakami a, K. Tsukagoshi a, Y. Inouye a,c,*, S. Kawata a,d b
a RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa,Wako, Saitama 351-0198, Japan National Institute of Advanced Science and Technology (AIST), Nanotechnology Research Institute, C-4,1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan c Graduate School of Frontier Biosciences, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan d Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 27 April 2005 Available online 13 June 2005
Abstract We investigate the polarization property in tip-enhanced Raman spectroscopy by employing a radial plate consisting of four divided half-wave plates each with a different orientation of the slow axis. The radial plate provides both longitudinal (parallel to the tip axis) and lateral (perpendicular to the tip axis) polarization of the electric field on the sample plane by selecting the proper polarization of the incident field. Single-walled carbon nanotubes, which have strong polarization dependence, are investigated with this polarization control. The radial breathing mode and the G-band, which belong to different vibrational symmetries, show opposite polarization dependences. 2005 Elsevier B.V. All rights reserved.
1. Introduction Raman scattering from molecules adsorbed on metallic nanostructures is strongly enhanced due to excitation of localized surface plasmon polaritons. This phenomenon is known as surface enhanced Raman scattering (SERS) and enhancement factor of SERS reaches up to 106 [1]. Even a single metallic nanostructure, e.g., metallic needle tip [2–4], can induce SERS at its apex, which is called tip-enhanced near-field Raman scattering [5–13]. Recently, molecular nanoimaging of a single carbon nanotube [14] and double-stranded DNA network structures [15] has been achieved using this technique. Furthermore, a specific Raman band was observed to be shifted to higher wavenumber by applying the pertur-
*
Corresponding author. Fax: +81 6 6879 7330. E-mail address: [email protected] (Y. Inouye).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.003
bative force to molecules with atomic force microscopy (AFM) [16]. The efficiency of the tip-enhancement has strong polarization dependence that is characterized by two axes, parallel (p-polarization) and perpendicular (s-polarization) direction to the tip axis [17–19]. A p-polarized plane wave illumination onto the conically shaped tip can efficiently induce tip-enhancement while the s-polarized light cannot [20]. In case of plane wave illumination, the illuminated area is much larger than the tip size. This increases the background signal from the illuminated area. In most of the recent works, linearly polarized light has been tightly focused by an inverted oil-immersion microscope objective lens with high N.A. in transmission configurations to reduce background signal [13,15,16]. Furthermore, Novotony et al. have suggested the use of two divided half-wave plates with which optic axis orthogonal to each other to boost p-polarization component under tight focusing
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
geometry [21]. It is generally accepted that tip-enhancements requires inducing p-polarization. However, the enhancement of Raman signal also depends on the direction of transition moment of molecules. Some vibrational modes couple better with s-polarization than p. For example, to excite the Raman mode such as the G-band of carbon nanotubes, s-polarization component is needed. In this Letter, we propose a method to control polarization of light field on a sample plane either p or s, by employing a radial plate consisting of four divided halfwave plates each with a different orientation of slow axis. We investigate polarization properties of tipenhanced near field Raman scattering of single-walled carbon nanotubes for the radial breathing mode (RBM) and G-band using both p- and s-polarization.
2. Experimental 2.1. Tip-enhanced Raman spectroscopy The schematic of our apparatus is shown in Fig. 1a. Laser excitation was provided by a CW YAG laser (532 nm, JDS Uniphase, 0.1 mW at the sample position). The beam was expanded 20 times with a beam expander and an evanescent mask was inserted in the
137
beam path to realize evanescent illumination [22]. A cover slip, on which the sample was put, set on an inverted oil-immersion microscope objective lens (Nikon Co., 60·, NA = 1.4). The Raman signal was efficiently collected by the same objective lens. The signal was guided to a triple spectrometer (slit width 100 lm, Photon Design PPDT3-640) and detected using a liquid nitrogen-cooled CCD camera (Princeton Instruments). For polarization measurements, the combination of a half-wave plate, polarizer and a radial plate was aligned in the incident beam path. Provided that linearly polarized light is focused on the sample plane by using only an objective lens with high N.A., both of the p-polarization (perpendicular to the sample plane) and s-polarization (parallel to the sample plane) components exist in the focused spot field at a sample plane. Then, by introducing the special radial plate at the pupil plane, focused spot consisting of either p- or s-polarization is available on the sample plane. The radial plate consists of four divided half-wave plates of which slow axes are shown in Fig. 1b. By rotating the incident polarization, this configuration could generate both radially and azimuthally polarized light as shown in Fig. 1c. Then, the polarized light was introduced into a microscope through a non-polarizing cube beamsplitter producing p- and s-polarization on the sample plane.
Fig. 1. (a) Schematic diagram of near-field Raman spectroscopy system: BS, non-polarizing cube beamsplitter; W, radial wave plate; Pol, polarizer; k/2, half-wave plate. (b) Optical axis of the wave plate. (c) The usage of the radial wave plate.
138
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
Fig. 2a and b shows the calculated total electric field intensity distributions at a tightly focused spot using p-and s-polarization. We chose a particular position of the focused spot, indicated by arrows, in order to efficiently excite a nanolight-source, either longitudinally or laterally. In the p-polarization configuration, the electric field intensity in the longitudinal direction is the strongest at the center. In the s-polarization configuration, the lateral field intensity is the strongest on the circumference. Note that both x-polarization dominant
and y-polarization dominant areas are formed in the case of the s-polarization configuration. In the p-polarization configuration, the tip was on the center of the spot, which was the most efficient for tip enhancement in the longitudinal direction, while in the s-polarization, the tip was set at the position where the polarization direction was along the tube axis. The nanotube was moved to the probe position using a PZT-controlled sample stage. Fig. 2c shows an AFM image of the carbon nanotube bundle we investigated in this experiment. The cross indicates the position of the probe during the measurements. 2.2. Preparation of silver coated probes A single nanosphere of which size is much smaller than wavelength is considered as a dipole when light field is incident on the sphere. The dipole excites electric field that is perpendicular to the incident field as well as electric field parallel to it. Amplitude of the former field, e.g., s-polarization field, is just half of that of the latter field, e.g., p-polarization field [23]. This means that the s-polarized field can be excited by using a tip having a metal nanosphere at the apex. Even though, it may be rather inefficient compared to p-polarization, we also utilize the s-polarization, since the coupling of the incident field with the Raman transition moment of molecules is also important for tip enhancement. We consider that the image dipole effect is not dominant in the case of a glass substrate compared to a metal surface [24]. To attain such a near-field probe, silver coated tips were prepared as the following process; at first, gold palladium alloy (AuPd) was deposited on a silicon cantilever tip to modify the surface of the silicon tip with a thickness of 5 nm. Then, silver was evaporated with ˚ /s. Due to the thickness of 40 nm at the rate of 0.5 A the pre-coated AuPd, a silver sphere was formed at the tip apex. Fig. 3 shows a photograph of a silver-coated tip taken with a scanning electron microscope
Fig. 2. Calculated total field intensity distributions: (a) for p-polarization; (b) for s-polarization, the probe positions and incident light polarizations are shown, the z-axis shows the relative field intensity; (c) AFM image of single-walled carbon nanotube bundle we measured, 2 · 2 lm, 0.4 Hz, the cross indicates the probe position.
Fig. 3. SEM image of a near-field probe coated with silver nano particles, 5 kV, 7 lA, 100 000·.
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
(HITACHI 4800, 5 kV, 7 lA, 100 000·). A particle of around 40 nm diameter can be seen at the tip apex. 2.3. Sample preparation Single-walled carbon nanotubes were prepared by laser ablation technique (NiCo, 1250 C, 532 nm) [25]. The sample was further purified with a hydrogen peroxide solution for 4 h. The quantity of 0.002 wt% of nanotubes was dissolved in 2,2,3,3-tetrafluoro-1-propanol and sonicated for 2 h. The solution was spin coated on a cover slip and the solvent was dried at 60 C. The Raman signal ratio of G-band to D-band for this sample is 100. Raman peaks from impurities were negligible.
3. Results and discussions We investigated polarization dependence of near-field Raman scattering from single-walled carbon nanotubes. Fig. 4 shows a comparison of the near-field Raman signal intensities of the single-walled carbon nanotubes measured under the p- and s-polarization conditions. ÔNear-field spectraÕ here means the subtraction of the spectra obtained without a tip from those obtained with a silver tip in contact with the sample. Black lines indicate the spectra measured under the p-polarization condition and gray lines indicate those measured under the s-polarization conditions. Under the p-polarization condition, the RBM is efficiently enhanced but the G-band exhibits less enhancement. In contrast, under the s-polarization condition, the G-band exhibits higher enhancement while the RBM does not. The RBM and G-band exhibited opposite enhancement behaviors between two polarizations. This figure indicates the selective enhancement of a particular vibrational mode by the near-field tip and these enhancements can be explained as follows. The explanation is based on an assumption that the tip apex coated with silver particles excited by the inci-
Fig. 4. Near-field Raman spectra of single-walled carbon nanotubes measured under p- and s-polarization conditions. RBM and G-band show different polarization dependences. Excitation: 532 nm; power: 0.1 mW; accumulation time: 60 s.
139
dent laser field act as a single dipole and that the dipole provides the sufficient field enhancement in either p- or s-polarization according to the incident polarization configurations. Under each polarization, the tipenhanced field has either a longitudinal or a lateral polarization, which couples with the different electric resonance of the nanotubes. The vibrational transitions of carbon nanotubes of this exciting wavelength couple with electron transitions either parallel or perpendicular to the tube axis [26]. Since the G-band contains a vibrational transition moment along the tube axis, it is effectively excited by s-polarization. On the other hand the vibrational transition moment of the RBM is located radially around the tube axis and is more efficiently excited by the p than the s-polarization, since the entire excitation field in the p-polarization is perpendicular to the tube axis. We also investigate the polarization selectivity of the tip-enhanced efficiency within the G-band. It has been reported that the G-band consists of several different symmetry components, A1, E1, E2 [27–29]. These symmetry species are expected to exhibit different polarization dependences. Fig. 5a shows the Lorentzian fitting curves of the experimental G-band spectra measured under the p-polarization condition and Fig. 5b shows those obtained under the s-polarization condition. The solid lines show the spectra obtained with a silver tip in contact with the sample and the dotted lines show the spectra obtained without a tip. Raman spectrum of the G-band was decomposed into five Lorentzian functions conditionally, as referred to the previous work [28]. As neither clear band shift nor band broadening was observed, peak positions and bandwidths were fixed in each curve fitting procedure. The fitting data and enhancement efficiencies are summarized in Table 1. Under the p-polarization condition, no marked difference in enhancement values is observed. Under the s-polarization condition, the Raman band at 1592 cm 1 shows a significant enhancement (indicated by arrows in Fig. 5b). Hence, this Raman band is related to symmetry E1 with a polarization that is along the tube axis. On the other hand, the Raman band at 1600 cm 1 is not enhanced at all under the s-polarization condition, but exhibits a significant enhancement under the p-polarization condition. Judging from the result, this Raman band is deduced to be associated with E2 or A1 of which transition moments are tangential to the nanotube. From Table 1, we also find that Raman bands at the 1592 and the 1566 cm 1 exhibit similar behavior, which deduces that Raman band at 1566 cm 1 is attributed to E1 mode. These results qualitatively agree with the report of DresselhousÕs group, which means that this method is available for symmetry assignments of the G-band at nanometric scale [28]. The localized field illumination provided by the metallic tip has an additional advantage besides spatial
140
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
Fig. 5. Lorentzian fitting curves of the experimental results measured: (a) under p-polarization condition; (b) under s-polarization condition, arrows indicate species of anomalous behavior. Intensity normalized (c) far-field Raman spectra; (d) near-field Raman spectra.
Table 1 Lorentzian fitting results of G-band and tip enhancement efficiency of the corresponding peaks Center cm
1553 1566 1572 1592 1600
1
Width cm (HWHM) 22 13 10 15 26
1
Enhancement p-Polarization
s-Polarization
1.4 1.4 1.3 1.3 1.6
1.5 2.4 1.9 2.5 1.1
ÔEnhancementÕ in the column represents the Lorentzian height ratio of the spectra obtained with silver tip divided by the spectra without tip.
since the sample is illuminated over a large area, the polarization information is averaged. So, for detailed polarization measurements, we have to prepare wellaligned carbon nanotubes as a sample [30,31]. By using the silver tip as a local illumination source, we can impose well-regulated p- and s-polarizations for a nanometer scale area. This result indicates that tip-enhanced near-field illumination provides us capability of polarization measurement having spatial resolution of 15 nm [14].
4. Conclusions resolution and enhancement. Fig. 5c and d shows the far-field and near-field G-band spectra, both of which are normalized by intensity maxima. The solid lines show the spectra measured under the p-polarization condition and the dashed lines show those under the spolarization condition. There is no particular difference among the shape of the spectra obtained under the pand s-polarization of Fig. 5c while there exists clear difference among that of Fig. 5d. In far-field spectroscopy,
We investigated polarization properties of field enhancements in tip-enhanced Raman spectroscopy. For polarization control, we employed a radial plate, in which the incident laser polarization was controlled to produce p- and s-polarizations on the sample plane. Single-walled carbon nanotubes were used as a sample. The RBM and the G-band exhibited opposite polarization dependences. Assuming that the metallized tip can provide both p- and s-polarization according to the
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
incident polarization directions, the tip-enhanced field coupled with the electric resonance either parallel or perpendicular to the tube axis. The results also indicate that several symmetry species in the G-band have different enhancement efficiencies, and that these can be used to make symmetry assignments. In this research, we demonstrated that the silver coated probe selectively enhanced the particular vibrational symmetry species.
References [1] R.K. Chang, T.E. Furtak, Surface Enhanced Raman Scattering, Plenum Press, New York, 1981. [2] Y. Inouye, S. Kawata, Opt. Lett. 19 (1994) 159. [3] A. Lahrech, R. Bachelot, P. Gleyzes, A.C. Boccara, Opt. Lett. 21 (1996) 1315. [4] B. Knoll, F. Keilmann, Nature 399 (1999) 134. [5] Y. Inouye, N. Hayazawa, K. Hayashi, Z. Sekkat, S. Kawata, Proc. SPIE 3791 (1999) 40. [6] N. Hayazawa, Y. Inouye, Z. Sekkat, S. Kawata, Opt. Commun. 183 (2000) 333. [7] M.S. Anderson, Appl. Phys. Lett. 76 (2000) 3130. [8] R.M. Sto¨ckle, Y.D. Suh, V. Deckert, R. Zenobi, Chem. Phys. Lett. 318 (2000) 131. [9] N. Hayazawa, Y. Inouye, Z. Sekkat, S. Kawata, Chem. Phys. Lett. 335 (2001) 369. [10] J.J. Wang, D.A. Smith, D.N. Batchelder, Y. Saito, J. Kirkham, C. Robinson, K. Baldwin, G. Li, B. Bennett, J. Microsc. 210 (2002) 330. [11] D.S. Bulgarevich, M. Futamata, Appl. Spectrosc. 58 (2004) 757. [12] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, Phys. Rev. Lett. 92 (2004) 096101.
141
[13] N. Hayazawa, T. Yano, H. Watanabe, Y. Inouye, S. Kawata, Chem. Phys. Lett. 376 (2003). [14] A. Hartschuh, E.J. Sa´nchez, X.S. Xie, L. Novotny, Phys. Rev. Lett. 90 (2003) 095503. [15] T. Ichimura, N. Hayazawa, M. Hashimoto, Y. Inouye, S. Kawata, Phys. Rev. Lett. 92 (2004) 220801. [16] H. Watanabe, Y. Ishida, N. Hayazawa, Y. Inouye, S. Kawata, Phys. Rev. B 69 (2004) 155418. [17] O.J.F. Martin, C. Girard, Appl. Phys. Lett. 70 (1997) 705. [18] L. Novotony, R.X. Bian, X.S. Xie, Phys. Rev. Lett. 79 (1997) 645. [19] L. Vaccaro, L. Aeschimann, U. Staufer, H.P. Herzig, R. Dandliker, Appl. Phys. Lett. 83 (2003) 584. [20] H. Furukawa, S. Kawata, Opt. Commun. 148 (1998) 221. [21] L. Novotony, J. Opt. Soc. Am. 14 (1997) 91. [22] N. Hayazawa, Y. Inouye, S. Kawata, J. Microsc. 194 (1999) 472. [23] E. David, Z. Physik 114 (1939) 389. [24] S. Kawata (Ed.), Near-Field Optics and Surface Plasmon Polaritons, Springer, Berlin, 2001. [25] H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki, Y. Achiba, Carbon 38 (2000) 1691. [26] A. Gru¨neis, R. Saito, J. Jiang, G.G. Samsonidze, M.A. Pimenta, A. Jorio, A.G.S. Filho, G. Dresselhaus, M.S. Dresselhaus, Chem. Phys. Lett. 387 (2004) 301. [27] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhous, M.S. Dresselhouse, Science 275 (1997) 187. [28] A. Jorio, G. Dresselhous, M.S. Dresselhous, M. Souza, M.S.S. Dantas, M.A. Pimenta, A.M. Rao, R. Saito, C. Liu, H.M. Cheng, Phys. Rev. Lett. 85 (2000) 2617. [29] R. Saito, G. Dresselhous, M.S. Dresselhous, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [30] H.D. Sun, Z.K. Tang, J. Chen, G. Li, Solid Sate Commun. 109 (1999) 365. [31] Z.H. Yu, L.J. Brus, Phys. Chem. B 105 (2001) 1123.
Polarization measurements in tip-enhanced Raman spectroscopy applied to single-walled carbon nanotubes Y. Saito a, N. Hayazawa a, H. Kataura b, T. Murakami a, K. Tsukagoshi a, Y. Inouye a,c,*, S. Kawata a,d b
a RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa,Wako, Saitama 351-0198, Japan National Institute of Advanced Science and Technology (AIST), Nanotechnology Research Institute, C-4,1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan c Graduate School of Frontier Biosciences, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan d Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 27 April 2005 Available online 13 June 2005
Abstract We investigate the polarization property in tip-enhanced Raman spectroscopy by employing a radial plate consisting of four divided half-wave plates each with a different orientation of the slow axis. The radial plate provides both longitudinal (parallel to the tip axis) and lateral (perpendicular to the tip axis) polarization of the electric field on the sample plane by selecting the proper polarization of the incident field. Single-walled carbon nanotubes, which have strong polarization dependence, are investigated with this polarization control. The radial breathing mode and the G-band, which belong to different vibrational symmetries, show opposite polarization dependences. 2005 Elsevier B.V. All rights reserved.
1. Introduction Raman scattering from molecules adsorbed on metallic nanostructures is strongly enhanced due to excitation of localized surface plasmon polaritons. This phenomenon is known as surface enhanced Raman scattering (SERS) and enhancement factor of SERS reaches up to 106 [1]. Even a single metallic nanostructure, e.g., metallic needle tip [2–4], can induce SERS at its apex, which is called tip-enhanced near-field Raman scattering [5–13]. Recently, molecular nanoimaging of a single carbon nanotube [14] and double-stranded DNA network structures [15] has been achieved using this technique. Furthermore, a specific Raman band was observed to be shifted to higher wavenumber by applying the pertur-
*
Corresponding author. Fax: +81 6 6879 7330. E-mail address: [email protected] (Y. Inouye).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.003
bative force to molecules with atomic force microscopy (AFM) [16]. The efficiency of the tip-enhancement has strong polarization dependence that is characterized by two axes, parallel (p-polarization) and perpendicular (s-polarization) direction to the tip axis [17–19]. A p-polarized plane wave illumination onto the conically shaped tip can efficiently induce tip-enhancement while the s-polarized light cannot [20]. In case of plane wave illumination, the illuminated area is much larger than the tip size. This increases the background signal from the illuminated area. In most of the recent works, linearly polarized light has been tightly focused by an inverted oil-immersion microscope objective lens with high N.A. in transmission configurations to reduce background signal [13,15,16]. Furthermore, Novotony et al. have suggested the use of two divided half-wave plates with which optic axis orthogonal to each other to boost p-polarization component under tight focusing
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
geometry [21]. It is generally accepted that tip-enhancements requires inducing p-polarization. However, the enhancement of Raman signal also depends on the direction of transition moment of molecules. Some vibrational modes couple better with s-polarization than p. For example, to excite the Raman mode such as the G-band of carbon nanotubes, s-polarization component is needed. In this Letter, we propose a method to control polarization of light field on a sample plane either p or s, by employing a radial plate consisting of four divided halfwave plates each with a different orientation of slow axis. We investigate polarization properties of tipenhanced near field Raman scattering of single-walled carbon nanotubes for the radial breathing mode (RBM) and G-band using both p- and s-polarization.
2. Experimental 2.1. Tip-enhanced Raman spectroscopy The schematic of our apparatus is shown in Fig. 1a. Laser excitation was provided by a CW YAG laser (532 nm, JDS Uniphase, 0.1 mW at the sample position). The beam was expanded 20 times with a beam expander and an evanescent mask was inserted in the
137
beam path to realize evanescent illumination [22]. A cover slip, on which the sample was put, set on an inverted oil-immersion microscope objective lens (Nikon Co., 60·, NA = 1.4). The Raman signal was efficiently collected by the same objective lens. The signal was guided to a triple spectrometer (slit width 100 lm, Photon Design PPDT3-640) and detected using a liquid nitrogen-cooled CCD camera (Princeton Instruments). For polarization measurements, the combination of a half-wave plate, polarizer and a radial plate was aligned in the incident beam path. Provided that linearly polarized light is focused on the sample plane by using only an objective lens with high N.A., both of the p-polarization (perpendicular to the sample plane) and s-polarization (parallel to the sample plane) components exist in the focused spot field at a sample plane. Then, by introducing the special radial plate at the pupil plane, focused spot consisting of either p- or s-polarization is available on the sample plane. The radial plate consists of four divided half-wave plates of which slow axes are shown in Fig. 1b. By rotating the incident polarization, this configuration could generate both radially and azimuthally polarized light as shown in Fig. 1c. Then, the polarized light was introduced into a microscope through a non-polarizing cube beamsplitter producing p- and s-polarization on the sample plane.
Fig. 1. (a) Schematic diagram of near-field Raman spectroscopy system: BS, non-polarizing cube beamsplitter; W, radial wave plate; Pol, polarizer; k/2, half-wave plate. (b) Optical axis of the wave plate. (c) The usage of the radial wave plate.
138
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
Fig. 2a and b shows the calculated total electric field intensity distributions at a tightly focused spot using p-and s-polarization. We chose a particular position of the focused spot, indicated by arrows, in order to efficiently excite a nanolight-source, either longitudinally or laterally. In the p-polarization configuration, the electric field intensity in the longitudinal direction is the strongest at the center. In the s-polarization configuration, the lateral field intensity is the strongest on the circumference. Note that both x-polarization dominant
and y-polarization dominant areas are formed in the case of the s-polarization configuration. In the p-polarization configuration, the tip was on the center of the spot, which was the most efficient for tip enhancement in the longitudinal direction, while in the s-polarization, the tip was set at the position where the polarization direction was along the tube axis. The nanotube was moved to the probe position using a PZT-controlled sample stage. Fig. 2c shows an AFM image of the carbon nanotube bundle we investigated in this experiment. The cross indicates the position of the probe during the measurements. 2.2. Preparation of silver coated probes A single nanosphere of which size is much smaller than wavelength is considered as a dipole when light field is incident on the sphere. The dipole excites electric field that is perpendicular to the incident field as well as electric field parallel to it. Amplitude of the former field, e.g., s-polarization field, is just half of that of the latter field, e.g., p-polarization field [23]. This means that the s-polarized field can be excited by using a tip having a metal nanosphere at the apex. Even though, it may be rather inefficient compared to p-polarization, we also utilize the s-polarization, since the coupling of the incident field with the Raman transition moment of molecules is also important for tip enhancement. We consider that the image dipole effect is not dominant in the case of a glass substrate compared to a metal surface [24]. To attain such a near-field probe, silver coated tips were prepared as the following process; at first, gold palladium alloy (AuPd) was deposited on a silicon cantilever tip to modify the surface of the silicon tip with a thickness of 5 nm. Then, silver was evaporated with ˚ /s. Due to the thickness of 40 nm at the rate of 0.5 A the pre-coated AuPd, a silver sphere was formed at the tip apex. Fig. 3 shows a photograph of a silver-coated tip taken with a scanning electron microscope
Fig. 2. Calculated total field intensity distributions: (a) for p-polarization; (b) for s-polarization, the probe positions and incident light polarizations are shown, the z-axis shows the relative field intensity; (c) AFM image of single-walled carbon nanotube bundle we measured, 2 · 2 lm, 0.4 Hz, the cross indicates the probe position.
Fig. 3. SEM image of a near-field probe coated with silver nano particles, 5 kV, 7 lA, 100 000·.
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
(HITACHI 4800, 5 kV, 7 lA, 100 000·). A particle of around 40 nm diameter can be seen at the tip apex. 2.3. Sample preparation Single-walled carbon nanotubes were prepared by laser ablation technique (NiCo, 1250 C, 532 nm) [25]. The sample was further purified with a hydrogen peroxide solution for 4 h. The quantity of 0.002 wt% of nanotubes was dissolved in 2,2,3,3-tetrafluoro-1-propanol and sonicated for 2 h. The solution was spin coated on a cover slip and the solvent was dried at 60 C. The Raman signal ratio of G-band to D-band for this sample is 100. Raman peaks from impurities were negligible.
3. Results and discussions We investigated polarization dependence of near-field Raman scattering from single-walled carbon nanotubes. Fig. 4 shows a comparison of the near-field Raman signal intensities of the single-walled carbon nanotubes measured under the p- and s-polarization conditions. ÔNear-field spectraÕ here means the subtraction of the spectra obtained without a tip from those obtained with a silver tip in contact with the sample. Black lines indicate the spectra measured under the p-polarization condition and gray lines indicate those measured under the s-polarization conditions. Under the p-polarization condition, the RBM is efficiently enhanced but the G-band exhibits less enhancement. In contrast, under the s-polarization condition, the G-band exhibits higher enhancement while the RBM does not. The RBM and G-band exhibited opposite enhancement behaviors between two polarizations. This figure indicates the selective enhancement of a particular vibrational mode by the near-field tip and these enhancements can be explained as follows. The explanation is based on an assumption that the tip apex coated with silver particles excited by the inci-
Fig. 4. Near-field Raman spectra of single-walled carbon nanotubes measured under p- and s-polarization conditions. RBM and G-band show different polarization dependences. Excitation: 532 nm; power: 0.1 mW; accumulation time: 60 s.
139
dent laser field act as a single dipole and that the dipole provides the sufficient field enhancement in either p- or s-polarization according to the incident polarization configurations. Under each polarization, the tipenhanced field has either a longitudinal or a lateral polarization, which couples with the different electric resonance of the nanotubes. The vibrational transitions of carbon nanotubes of this exciting wavelength couple with electron transitions either parallel or perpendicular to the tube axis [26]. Since the G-band contains a vibrational transition moment along the tube axis, it is effectively excited by s-polarization. On the other hand the vibrational transition moment of the RBM is located radially around the tube axis and is more efficiently excited by the p than the s-polarization, since the entire excitation field in the p-polarization is perpendicular to the tube axis. We also investigate the polarization selectivity of the tip-enhanced efficiency within the G-band. It has been reported that the G-band consists of several different symmetry components, A1, E1, E2 [27–29]. These symmetry species are expected to exhibit different polarization dependences. Fig. 5a shows the Lorentzian fitting curves of the experimental G-band spectra measured under the p-polarization condition and Fig. 5b shows those obtained under the s-polarization condition. The solid lines show the spectra obtained with a silver tip in contact with the sample and the dotted lines show the spectra obtained without a tip. Raman spectrum of the G-band was decomposed into five Lorentzian functions conditionally, as referred to the previous work [28]. As neither clear band shift nor band broadening was observed, peak positions and bandwidths were fixed in each curve fitting procedure. The fitting data and enhancement efficiencies are summarized in Table 1. Under the p-polarization condition, no marked difference in enhancement values is observed. Under the s-polarization condition, the Raman band at 1592 cm 1 shows a significant enhancement (indicated by arrows in Fig. 5b). Hence, this Raman band is related to symmetry E1 with a polarization that is along the tube axis. On the other hand, the Raman band at 1600 cm 1 is not enhanced at all under the s-polarization condition, but exhibits a significant enhancement under the p-polarization condition. Judging from the result, this Raman band is deduced to be associated with E2 or A1 of which transition moments are tangential to the nanotube. From Table 1, we also find that Raman bands at the 1592 and the 1566 cm 1 exhibit similar behavior, which deduces that Raman band at 1566 cm 1 is attributed to E1 mode. These results qualitatively agree with the report of DresselhousÕs group, which means that this method is available for symmetry assignments of the G-band at nanometric scale [28]. The localized field illumination provided by the metallic tip has an additional advantage besides spatial
140
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
Fig. 5. Lorentzian fitting curves of the experimental results measured: (a) under p-polarization condition; (b) under s-polarization condition, arrows indicate species of anomalous behavior. Intensity normalized (c) far-field Raman spectra; (d) near-field Raman spectra.
Table 1 Lorentzian fitting results of G-band and tip enhancement efficiency of the corresponding peaks Center cm
1553 1566 1572 1592 1600
1
Width cm (HWHM) 22 13 10 15 26
1
Enhancement p-Polarization
s-Polarization
1.4 1.4 1.3 1.3 1.6
1.5 2.4 1.9 2.5 1.1
ÔEnhancementÕ in the column represents the Lorentzian height ratio of the spectra obtained with silver tip divided by the spectra without tip.
since the sample is illuminated over a large area, the polarization information is averaged. So, for detailed polarization measurements, we have to prepare wellaligned carbon nanotubes as a sample [30,31]. By using the silver tip as a local illumination source, we can impose well-regulated p- and s-polarizations for a nanometer scale area. This result indicates that tip-enhanced near-field illumination provides us capability of polarization measurement having spatial resolution of 15 nm [14].
4. Conclusions resolution and enhancement. Fig. 5c and d shows the far-field and near-field G-band spectra, both of which are normalized by intensity maxima. The solid lines show the spectra measured under the p-polarization condition and the dashed lines show those under the spolarization condition. There is no particular difference among the shape of the spectra obtained under the pand s-polarization of Fig. 5c while there exists clear difference among that of Fig. 5d. In far-field spectroscopy,
We investigated polarization properties of field enhancements in tip-enhanced Raman spectroscopy. For polarization control, we employed a radial plate, in which the incident laser polarization was controlled to produce p- and s-polarizations on the sample plane. Single-walled carbon nanotubes were used as a sample. The RBM and the G-band exhibited opposite polarization dependences. Assuming that the metallized tip can provide both p- and s-polarization according to the
Y. Saito et al. / Chemical Physics Letters 410 (2005) 136–141
incident polarization directions, the tip-enhanced field coupled with the electric resonance either parallel or perpendicular to the tube axis. The results also indicate that several symmetry species in the G-band have different enhancement efficiencies, and that these can be used to make symmetry assignments. In this research, we demonstrated that the silver coated probe selectively enhanced the particular vibrational symmetry species.
References [1] R.K. Chang, T.E. Furtak, Surface Enhanced Raman Scattering, Plenum Press, New York, 1981. [2] Y. Inouye, S. Kawata, Opt. Lett. 19 (1994) 159. [3] A. Lahrech, R. Bachelot, P. Gleyzes, A.C. Boccara, Opt. Lett. 21 (1996) 1315. [4] B. Knoll, F. Keilmann, Nature 399 (1999) 134. [5] Y. Inouye, N. Hayazawa, K. Hayashi, Z. Sekkat, S. Kawata, Proc. SPIE 3791 (1999) 40. [6] N. Hayazawa, Y. Inouye, Z. Sekkat, S. Kawata, Opt. Commun. 183 (2000) 333. [7] M.S. Anderson, Appl. Phys. Lett. 76 (2000) 3130. [8] R.M. Sto¨ckle, Y.D. Suh, V. Deckert, R. Zenobi, Chem. Phys. Lett. 318 (2000) 131. [9] N. Hayazawa, Y. Inouye, Z. Sekkat, S. Kawata, Chem. Phys. Lett. 335 (2001) 369. [10] J.J. Wang, D.A. Smith, D.N. Batchelder, Y. Saito, J. Kirkham, C. Robinson, K. Baldwin, G. Li, B. Bennett, J. Microsc. 210 (2002) 330. [11] D.S. Bulgarevich, M. Futamata, Appl. Spectrosc. 58 (2004) 757. [12] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, Phys. Rev. Lett. 92 (2004) 096101.
141
[13] N. Hayazawa, T. Yano, H. Watanabe, Y. Inouye, S. Kawata, Chem. Phys. Lett. 376 (2003). [14] A. Hartschuh, E.J. Sa´nchez, X.S. Xie, L. Novotny, Phys. Rev. Lett. 90 (2003) 095503. [15] T. Ichimura, N. Hayazawa, M. Hashimoto, Y. Inouye, S. Kawata, Phys. Rev. Lett. 92 (2004) 220801. [16] H. Watanabe, Y. Ishida, N. Hayazawa, Y. Inouye, S. Kawata, Phys. Rev. B 69 (2004) 155418. [17] O.J.F. Martin, C. Girard, Appl. Phys. Lett. 70 (1997) 705. [18] L. Novotony, R.X. Bian, X.S. Xie, Phys. Rev. Lett. 79 (1997) 645. [19] L. Vaccaro, L. Aeschimann, U. Staufer, H.P. Herzig, R. Dandliker, Appl. Phys. Lett. 83 (2003) 584. [20] H. Furukawa, S. Kawata, Opt. Commun. 148 (1998) 221. [21] L. Novotony, J. Opt. Soc. Am. 14 (1997) 91. [22] N. Hayazawa, Y. Inouye, S. Kawata, J. Microsc. 194 (1999) 472. [23] E. David, Z. Physik 114 (1939) 389. [24] S. Kawata (Ed.), Near-Field Optics and Surface Plasmon Polaritons, Springer, Berlin, 2001. [25] H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki, Y. Achiba, Carbon 38 (2000) 1691. [26] A. Gru¨neis, R. Saito, J. Jiang, G.G. Samsonidze, M.A. Pimenta, A. Jorio, A.G.S. Filho, G. Dresselhaus, M.S. Dresselhaus, Chem. Phys. Lett. 387 (2004) 301. [27] A.M. Rao, E. Richter, S. Bandow, B. Chase, P.C. Eklund, K.A. Williams, S. Fang, K.R. Subbaswamy, M. Menon, A. Thess, R.E. Smalley, G. Dresselhous, M.S. Dresselhouse, Science 275 (1997) 187. [28] A. Jorio, G. Dresselhous, M.S. Dresselhous, M. Souza, M.S.S. Dantas, M.A. Pimenta, A.M. Rao, R. Saito, C. Liu, H.M. Cheng, Phys. Rev. Lett. 85 (2000) 2617. [29] R. Saito, G. Dresselhous, M.S. Dresselhous, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [30] H.D. Sun, Z.K. Tang, J. Chen, G. Li, Solid Sate Commun. 109 (1999) 365. [31] Z.H. Yu, L.J. Brus, Phys. Chem. B 105 (2001) 1123.