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Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy
Chemical Physics Letters 376 (2003) 174–180 www.elsevier.com/locate/cplett
Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy Norihiko Hayazawa a,b,*, Takaaki Yano a, Hiroyuki Watanabe a, Yasushi Inouye a,b, Satoshi Kawata a,b,c a
Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan b CREST, Japan Corporation of Science and Technology, Japan c RIKEN, Wako, Saitama 351-0198, Japan Received 29 January 2003; in final form 14 April 2003 Published online: 1 July 2003
Abstract A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band. Ó 2003 Elsevier Science B.V. All rights reserved.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is happened by the strict limitation of spatial resolution due to the wave nature of light [1]. There have been attempts to overcome the diffraction limit by using nonlinear response of materials [2,3]. Near-field scanning optical microscopy (NSOM) could provide better detecting accuracy [4–6]. The metallic probe tip has been used to enhance the optical field only in
*
Corresponding author. Fax: +81-6-6879-7330. E-mail address: [email protected] (N. Hayazawa).
the vicinity of probe tip [7–11]. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS) [12], while in this case a single metallic tip works for the field enhancement in nanometer scale [13–19]. We have previously demonstrated detection of 30-nm-scale near-field Raman scattering and Raman spectral shift imaging of organic molecules [16,18] using a metallized apertureless tip. Furthermore, we have reported that the near-field Raman spectra involving anomalous enhancement and large frequency shifts of the Raman bands are obtained for polycrystalline Rhodamine6G dyes employing the tip-enhanced Raman NSOM [17]. These spectral
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00883-2
N. Hayazawa et al. / Chemical Physics Letters 376 (2003) 174–180
changes were interpreted to occur as a result of chemical interactions between the dye molecule and the metallic tip [20]. One of the most promising applications of Raman scattering could be the identification of SWNTs where spectroscopic techniques with sub-nanometer spatial resolution are necessary to obtain ultra-sensitive single-molecular chemical analysis. In this Letter, we show, at first, the concept of our developed tip-enhanced near-field Raman spectroscopy, and demonstrate how the metallic probe tip enhances the weak Raman signal from a nanometer region by using DNA-base adenine molecules, which cannot be detected by conventional micro-Raman configuration (far-field Raman). Then, we apply tip-enhanced near-field Raman spectroscopy to the structural analysis of SWNTs where we deduced the possible chirality of the observed SWNT from the experimental results of both radial breathing mode and G-band feature. Fig. 1a shows a tip-enhanced near-field Raman spectrum of a single nano-crystal of adenine molecules detected with our developed tip-enhanced near-field Raman microscope with a silver tip. Adenine nano-crystals are spread out on a coverslip by casting adenine molecules dissolved in ethanol. The crystal size is laterally 7 nm 20 nm wide and 15 nm high. In comparison, the far-field
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Raman spectrum of the same sample without a metallic probe tip is also shown. The body of the metallic tip is a silicon cantilever coated by a 40 nm-thick silver film (Fig. 1b). In the tip-enhanced Raman spectrum, several characteristic Raman peaks of adenine molecules are enhanced and become visible such as peaks at 736 and 1330 cm1 , while no Raman peak at these positions is observed when the probe is withdrawn from the near-field region (far-field Raman spectrum). The experimental condition was exactly the same for both experiments shown in Fig. 1a except the tipsample distance. The strong Raman peaks at 736 and 1330 cm1 are assigned to ring breathing mode of a whole molecule and ring breathing mode of a diazole shown in Fig. 2a, respectively. Other Raman peaks are also assigned to the normal modes of adenine molecules on the basis of density functional (B3LYP) calculation in combination with the 6-311 + G** basis set (Fig. 2b) [21–24]. Observed tip-enhanced near-field Raman peaks (black line in Fig. 2b) are shifted slightly from the far-field ones obtained from powder bulk sample (gray line in Fig. 2b). For example, the ring breathing mode at 720 cm1 in far-field Raman spectrum of bulk sample is shifted to 736 cm1 in the tip-enhanced near-field Raman spectrum while the ring breathing mode of a diazole is not shifted.
Fig. 1. (a) Tip-enhanced near-field Raman (black line) and far-field Raman (gray line) spectra of adenine nano-crystals. To form an evanescent spot on the sample plane, a beam from a 2.5 mW Nd:YVO4 laser frequency-doubled at 532 nm was very tightly focused with an objective lens with numerical aperture (NA) of 1.4. through an annular aperture [10]. Exposure time was for both 1 min. As the metallic tip is moved closer to the focused spot, a localized enhanced electric field is generated at the tip apex. The Raman scattering signal is efficiently collected by the same objective lens and is directed to the spectrophotometer (focul length: 600 mm) equipped with a liquid nitrogen cooled CCD camera [18]. The peak at 923 cm1 is from the glass substrate, and shows the same intensity both in the tipenhanced Raman and the far-field Raman spectra. (b) Scanning electron micrograph image of a metallized cantilever tip that is a silicon cantilever coated by a 40 nm-thick silver layer by a thermal evaporation process. The silver-coated tip diameter is around 30 nm.
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Fig. 2. (a) Assigned vibrational modes at 736 and 1330 cm1 , which are representing the ring breathing mode of a whole molecule and the ring stretching of a diazole. The arrows in the figure represent the amplitude of atoms. (b) Comparison between tip-enhanced nearfield Raman spectrum of an adenine nano-crystal (black line) and far-field Raman spectrum of adenine bulk powders (gray line). Each vibrational mode indicated by an alphabet in the figure is also calculated and assigned to characteristic vibration of adenine molecules.
These phenomena in the spectral shifts are in good agreement with SERS spectra of adenine molecules [25], and ensure that the metallic probe tip works as a surface enhancer for SERS effect. Assuming that the enhanced electric field is 30 nm / corresponding to the tip diameter and the focused light spot is 400 nm /, the enhancement factor for the ring breathing mode of a whole molecule is 2700. Furthermore, the factor for the ring breathing mode of a diazole is uncountable because far-field Raman signal without a metallic probe tip is too weak to be detected. We have demonstrated the applicability of tipenhanced near-field Raman spectroscopy to the analysis of local vibrational modes of an individual SWNT. Recently, far-field Raman spectroscopy has been recognized as a very powerful tool for SWNTs and achieved an individual SWNT detection [26,27] because much structural infor-
mation including chirality and diameter can be deduced from the vibration modes [28]. To further detail the specific structure-dependent Ôsingle moleculeÕ property, a single-molecule addressing capability at nanometer scale is needed, which is now provided by the tip-enhanced near-field microscope. For these measurements, we sparsely spread out SWNTs on a coverslip, and focus a laser beam at a tube (Fig. 3d). SWNTs were purchased from Carbolex (AP-Grade Nanotubes, purity 60%) and was not treated with any further purification. The SWNTs are spincoated onto a coverslip from a sonicated 2.2.3.3 tetrafluorolpropanol suspension with 3000 rpm for 1 min and dried out in an oven with 110 °C for 4 h. Generally, our SWNTs consist of bundles of SWNTs, however, after spincoating, some individual SWNTs are separated out from aggregated bundles of SWNTs. Fig. 3a shows a tip-enhanced
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Fig. 3. (a) Tip-enhanced near-field Raman spectra of an individual SWNT, which is obtained schematically at the tip position of ÔAÕ. Remarkable tangential G-band peaks of SWNT and a small D-band peak at 1331 cm1 are observed. The G-band peaks are well fitted by Lorentzian line shape at the peak positions of 1593, 1570, and 1558 cm1 (gray lines). The fitting was carried out for the subtracted spectrum between tip-enhanced (Fig. 3a) and far-field (Fig. 3b) Raman spectra because the subtracted intensity corresponds to the purely near-field contribution due to the enhanced electric field at the tip [16]. (b) Far-field Raman spectrum of SWNT including impurities (amorphous carbon) without a tip (the tip is 500 nm far away from the sample surface shown in ÔBÕ). (c) Tip-enhanced nearfield Raman spectrum of an amorphous carbon, which is obtained schematically at the probe position of ÔCÕ. (d) Topographic atomic force microscope image and the cross-section (white line in the image) of the isolated SWNTs dispersed on a coverslip. Far-field focused spot was indicated by the mask grid pattern in the image. Some isolated SWNTs indicated by downwards arrows and amorphous carbon particles indicated by upwards arrows can be seen in the image. The dimension of the image is 2 lm 2 lm consisting of 200 200 pixels.
near-field Raman spectrum of an individual SWNT, obtained schematically at the probe position of ÔAÕ. Remarkable tangential G-band peaks, representing graphite mode, of a SWNT that is split into three peaks and fitted well to the Lorentzian line shape at 1593, 1570, and 1558 cm1 were observed (gray lines in the figure). These Lorentzian line shapes feature semiconducting SWNTs, excluding the Breit–Wigner–Fano line shape [29] that is observed in the resonant Raman spectra of metallic SWNTs. The fitted linewidth at
1593 cm1 is 18 cm1 . Taking into account that the FWHM of the excitation laser detected by our single spectrometer with an entrance slit width of 250 lm has a linewidth of 10 cm1 , the deduced linewidth at 1593 cm1 is 8 cm1 . This narrow linewidth supports that the observed isolated SWNTs consist of only one or a few SWNTs [30]. A small D-band peak, representing defect mode, at 1331 cm1 was also detected. Due to the strong coupling between electrons and phonons in the resonant Raman effect, the
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unique one-dimensional electronic density of states (EDOS) of SWNTs play an important role in the exceptionally strong resonant Raman spectra of SWNTs associated with the interband optical transitions between two van Hove singularities [31]. According to Reference [32], while the 532 nm wavelength of our light source, carrying photon energy of 2.33 eV, can resonantly excite electrons contained in semiconducting SWNTs whose diameter is from 1.35 to 1.55 nm, the optical absorption spectrum does not show the corresponding strong absorption [32]. However, tipenhancement effect compensates for these less efficiency of the resonant Raman effect. Furthermore, the high resolution far-field Raman spectrum detected with a double-grating spectrometer [33] at the aggregated bulk of SWNTs included in the sample can give us the diameter distribution of the SWNTs (Fig. 4) [34]. Five apparent peaks are detected as radial breathing mode. By using a simple approximation between mode frequency WRBM (cm1 ) and the diameter d (nm), i.e., WRBM ¼ 224=d [28], the peaks of radial breathing mode at 148, 163, 170.5, 177, and 183.8 cm1 correspond to the diameters of 1.51, 1.37, 1.31, 1.27, and 1.22 nm, respectively. Note that because the numerator of the approximated equation ( ¼ 224) still requires further investigation by considering detailed interactions of SWNTs or a metallic tip, our estimated diameters described above are still one possibility. Even under these conditions, this
Fig. 4. High resolution far-field Raman spectrum of radial breathing mode from aggregated bulk of SWNTs detected with a double-grating spectrometer [32].
diameter feature is also supported by tip-enhanced near-field Raman spectrum of radial breathing mode shown in Fig. 5. The lines of a–c in the figure correspond to tip-enhanced near-field, far-field, and the subtracted Raman spectra, respectively. The metallic probe tip selectively enhanced the SWNT just below the tip so that the enhanced Raman peak at 165.2 cm1 is clearly observed in (a) while other peaks such as 170.8 and 192.6 cm1 which are generated from the far-field focused spot are not enhanced. By using the modified approximation for individual SWNTs, WRBM ¼ 248=d [26], the Raman peak at 165.2 cm1 corresponds to the diameter of 1.50 nm. Topographic image of the sample (Fig. 3d) ensures the height of the SWNT (1.5 nm) and clearly shows the distribution of SWNTs and amorphous carbon. This image was obtained in AFM operation by the tip-enhanced near-field microscope. White lines in the figure indicated by downwards arrows are isolated SWNTs. The height of the observed isolated SWNT is 1.5 nm corresponding to the diameter
Fig. 5. (a) Tip-enhanced near-field and (b) far-field Raman spectra of radial breathing mode of isolated SWNTs. (c) is the subtracted Raman spectra between (a) and (b). The Raman peak at 165.2 cm1 corresponding to the diameter of 1.50 nm is selectively enhanced by the metallic probe tip. The laser power is 4.8 mW and the exposure time is 1 min.
N. Hayazawa et al. / Chemical Physics Letters 376 (2003) 174–180
of an individual SWNT while the other one has the height of 3.5 nm consisting of several number of SWNTs. White spots with a diameter of 100 nm indicated by upwards arrows are the amorphous carbon inherently included in our sample as the impurities of the arc-discharge process of SWNTs. Accordingly, the most possible chirality for the semiconducting SWNT observed in the tip-enhanced Raman spectrum is (17, 4)1.513 nm. Note that the peak at 192.6 cm1 in Fig. 5 is not observable in the high resolution far-field Raman spectra from the aggregated bulk of SWNTs in Fig. 4. This can be attributed to the fact that the number of SWNTs in Fig. 5 is extremely small compared to that of Fig. 4. The averaging effect overwhelmed the characteristic peak at 192.6 cm1 in Fig. 4. Fig. 3b was obtained when the metallic probe tip is 500 nm far away from the sample surface (the tip position of ÔBÕ). This spectrum represents a farfield Raman spectrum, where the featured lines are gone due to the absence of field enhancing metallic probe tip in the near-field except a small protrusion at 1592 cm1 . Since the sample contains not only SWNTs but also impurities (amorphous carbon), this small peak represents an average within a focused spot of laser beam, however, the spectrum dominantly reflects non-enhanced G-band feature of SWNTs. Note that although isolated SWNTs are randomly dispersed and alligned parallel to the substrate, we can observe G-band of isolated SWNTs only at several far-field focused positions by an objective lens, and the observed far-field Raman intensity at each position is different. Because of the polarization effect of both illumination light field and SWNTs and the severe selectivity of the resonant Raman effect [32], the number of observable isolated SWNTs is quite limited. These facts suggest that the observed isolated SWNT consists of only one or a few SWNTs. Fig. 3c shows another tip-enhanced nearfield Raman spectrum from the same sample when the probe enhances field at a different location. A broad D-band representing defect mode of amorphous carbon is dominantly detected around 1335 cm1 [35]. G-band is also seen but wider than that seen in Fig. 3a. This can be schematically attributed to the tip position of ÔCÕ. A small G-band
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peak shown in Fig. 3b (circle in the figure) is recognizable on the top of the broad G-band peak in Fig. 3c (circle in the figure). The broad G-band peak represents the tip-enhanced near-field Raman scattering of amorphous carbon due to the tip, while the small G-band peak is the far-field background signal coming from an isolated SWNT. The sharp tangential G-band feature of an individual semiconducting SWNT (Fig. 3a) and the broad D-band and G-band feature of amorphous carbon (Fig. 3c) are explained on the basis that the enhanced electric field at the metallic probe tip selectively enhances the near-field Raman scattering of the SWNT and amorphous carbon inside of the tightly focused spot by the objective lens. We showed that the combination of both radial breathing mode and G-band observed by tip-enhanced near-field Raman spectroscopy was a very powerful tool for the detail structural analysis of individual SWNTs. Due to the enhancement virtue of a metallic probe tip as a surface enhancer for SERS effect, tip-enhanced near-field Raman spectroscopy allows us to obtain localized molecular vibrational information without averaging the signal inside of the diffraction limited focused spot, and have not only high spatial resolution but also high signal sensitivity compared to far-field Raman spectroscopy that requires much more molecules to compensate for the small Raman scattering cross-section. Acknowledgements We would like to thank Professor H. Harima in Kyoto Institute of Technology for help with radial breathing mode observation using a double-grating monocromater and thank Dr. M. Hashimoto and Dr. N. Smith in Osaka University for fruitful discussion.
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Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy Norihiko Hayazawa a,b,*, Takaaki Yano a, Hiroyuki Watanabe a, Yasushi Inouye a,b, Satoshi Kawata a,b,c a
Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan b CREST, Japan Corporation of Science and Technology, Japan c RIKEN, Wako, Saitama 351-0198, Japan Received 29 January 2003; in final form 14 April 2003 Published online: 1 July 2003
Abstract A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band. Ó 2003 Elsevier Science B.V. All rights reserved.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is happened by the strict limitation of spatial resolution due to the wave nature of light [1]. There have been attempts to overcome the diffraction limit by using nonlinear response of materials [2,3]. Near-field scanning optical microscopy (NSOM) could provide better detecting accuracy [4–6]. The metallic probe tip has been used to enhance the optical field only in
*
Corresponding author. Fax: +81-6-6879-7330. E-mail address: [email protected] (N. Hayazawa).
the vicinity of probe tip [7–11]. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS) [12], while in this case a single metallic tip works for the field enhancement in nanometer scale [13–19]. We have previously demonstrated detection of 30-nm-scale near-field Raman scattering and Raman spectral shift imaging of organic molecules [16,18] using a metallized apertureless tip. Furthermore, we have reported that the near-field Raman spectra involving anomalous enhancement and large frequency shifts of the Raman bands are obtained for polycrystalline Rhodamine6G dyes employing the tip-enhanced Raman NSOM [17]. These spectral
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00883-2
N. Hayazawa et al. / Chemical Physics Letters 376 (2003) 174–180
changes were interpreted to occur as a result of chemical interactions between the dye molecule and the metallic tip [20]. One of the most promising applications of Raman scattering could be the identification of SWNTs where spectroscopic techniques with sub-nanometer spatial resolution are necessary to obtain ultra-sensitive single-molecular chemical analysis. In this Letter, we show, at first, the concept of our developed tip-enhanced near-field Raman spectroscopy, and demonstrate how the metallic probe tip enhances the weak Raman signal from a nanometer region by using DNA-base adenine molecules, which cannot be detected by conventional micro-Raman configuration (far-field Raman). Then, we apply tip-enhanced near-field Raman spectroscopy to the structural analysis of SWNTs where we deduced the possible chirality of the observed SWNT from the experimental results of both radial breathing mode and G-band feature. Fig. 1a shows a tip-enhanced near-field Raman spectrum of a single nano-crystal of adenine molecules detected with our developed tip-enhanced near-field Raman microscope with a silver tip. Adenine nano-crystals are spread out on a coverslip by casting adenine molecules dissolved in ethanol. The crystal size is laterally 7 nm 20 nm wide and 15 nm high. In comparison, the far-field
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Raman spectrum of the same sample without a metallic probe tip is also shown. The body of the metallic tip is a silicon cantilever coated by a 40 nm-thick silver film (Fig. 1b). In the tip-enhanced Raman spectrum, several characteristic Raman peaks of adenine molecules are enhanced and become visible such as peaks at 736 and 1330 cm1 , while no Raman peak at these positions is observed when the probe is withdrawn from the near-field region (far-field Raman spectrum). The experimental condition was exactly the same for both experiments shown in Fig. 1a except the tipsample distance. The strong Raman peaks at 736 and 1330 cm1 are assigned to ring breathing mode of a whole molecule and ring breathing mode of a diazole shown in Fig. 2a, respectively. Other Raman peaks are also assigned to the normal modes of adenine molecules on the basis of density functional (B3LYP) calculation in combination with the 6-311 + G** basis set (Fig. 2b) [21–24]. Observed tip-enhanced near-field Raman peaks (black line in Fig. 2b) are shifted slightly from the far-field ones obtained from powder bulk sample (gray line in Fig. 2b). For example, the ring breathing mode at 720 cm1 in far-field Raman spectrum of bulk sample is shifted to 736 cm1 in the tip-enhanced near-field Raman spectrum while the ring breathing mode of a diazole is not shifted.
Fig. 1. (a) Tip-enhanced near-field Raman (black line) and far-field Raman (gray line) spectra of adenine nano-crystals. To form an evanescent spot on the sample plane, a beam from a 2.5 mW Nd:YVO4 laser frequency-doubled at 532 nm was very tightly focused with an objective lens with numerical aperture (NA) of 1.4. through an annular aperture [10]. Exposure time was for both 1 min. As the metallic tip is moved closer to the focused spot, a localized enhanced electric field is generated at the tip apex. The Raman scattering signal is efficiently collected by the same objective lens and is directed to the spectrophotometer (focul length: 600 mm) equipped with a liquid nitrogen cooled CCD camera [18]. The peak at 923 cm1 is from the glass substrate, and shows the same intensity both in the tipenhanced Raman and the far-field Raman spectra. (b) Scanning electron micrograph image of a metallized cantilever tip that is a silicon cantilever coated by a 40 nm-thick silver layer by a thermal evaporation process. The silver-coated tip diameter is around 30 nm.
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Fig. 2. (a) Assigned vibrational modes at 736 and 1330 cm1 , which are representing the ring breathing mode of a whole molecule and the ring stretching of a diazole. The arrows in the figure represent the amplitude of atoms. (b) Comparison between tip-enhanced nearfield Raman spectrum of an adenine nano-crystal (black line) and far-field Raman spectrum of adenine bulk powders (gray line). Each vibrational mode indicated by an alphabet in the figure is also calculated and assigned to characteristic vibration of adenine molecules.
These phenomena in the spectral shifts are in good agreement with SERS spectra of adenine molecules [25], and ensure that the metallic probe tip works as a surface enhancer for SERS effect. Assuming that the enhanced electric field is 30 nm / corresponding to the tip diameter and the focused light spot is 400 nm /, the enhancement factor for the ring breathing mode of a whole molecule is 2700. Furthermore, the factor for the ring breathing mode of a diazole is uncountable because far-field Raman signal without a metallic probe tip is too weak to be detected. We have demonstrated the applicability of tipenhanced near-field Raman spectroscopy to the analysis of local vibrational modes of an individual SWNT. Recently, far-field Raman spectroscopy has been recognized as a very powerful tool for SWNTs and achieved an individual SWNT detection [26,27] because much structural infor-
mation including chirality and diameter can be deduced from the vibration modes [28]. To further detail the specific structure-dependent Ôsingle moleculeÕ property, a single-molecule addressing capability at nanometer scale is needed, which is now provided by the tip-enhanced near-field microscope. For these measurements, we sparsely spread out SWNTs on a coverslip, and focus a laser beam at a tube (Fig. 3d). SWNTs were purchased from Carbolex (AP-Grade Nanotubes, purity 60%) and was not treated with any further purification. The SWNTs are spincoated onto a coverslip from a sonicated 2.2.3.3 tetrafluorolpropanol suspension with 3000 rpm for 1 min and dried out in an oven with 110 °C for 4 h. Generally, our SWNTs consist of bundles of SWNTs, however, after spincoating, some individual SWNTs are separated out from aggregated bundles of SWNTs. Fig. 3a shows a tip-enhanced
N. Hayazawa et al. / Chemical Physics Letters 376 (2003) 174–180
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Fig. 3. (a) Tip-enhanced near-field Raman spectra of an individual SWNT, which is obtained schematically at the tip position of ÔAÕ. Remarkable tangential G-band peaks of SWNT and a small D-band peak at 1331 cm1 are observed. The G-band peaks are well fitted by Lorentzian line shape at the peak positions of 1593, 1570, and 1558 cm1 (gray lines). The fitting was carried out for the subtracted spectrum between tip-enhanced (Fig. 3a) and far-field (Fig. 3b) Raman spectra because the subtracted intensity corresponds to the purely near-field contribution due to the enhanced electric field at the tip [16]. (b) Far-field Raman spectrum of SWNT including impurities (amorphous carbon) without a tip (the tip is 500 nm far away from the sample surface shown in ÔBÕ). (c) Tip-enhanced nearfield Raman spectrum of an amorphous carbon, which is obtained schematically at the probe position of ÔCÕ. (d) Topographic atomic force microscope image and the cross-section (white line in the image) of the isolated SWNTs dispersed on a coverslip. Far-field focused spot was indicated by the mask grid pattern in the image. Some isolated SWNTs indicated by downwards arrows and amorphous carbon particles indicated by upwards arrows can be seen in the image. The dimension of the image is 2 lm 2 lm consisting of 200 200 pixels.
near-field Raman spectrum of an individual SWNT, obtained schematically at the probe position of ÔAÕ. Remarkable tangential G-band peaks, representing graphite mode, of a SWNT that is split into three peaks and fitted well to the Lorentzian line shape at 1593, 1570, and 1558 cm1 were observed (gray lines in the figure). These Lorentzian line shapes feature semiconducting SWNTs, excluding the Breit–Wigner–Fano line shape [29] that is observed in the resonant Raman spectra of metallic SWNTs. The fitted linewidth at
1593 cm1 is 18 cm1 . Taking into account that the FWHM of the excitation laser detected by our single spectrometer with an entrance slit width of 250 lm has a linewidth of 10 cm1 , the deduced linewidth at 1593 cm1 is 8 cm1 . This narrow linewidth supports that the observed isolated SWNTs consist of only one or a few SWNTs [30]. A small D-band peak, representing defect mode, at 1331 cm1 was also detected. Due to the strong coupling between electrons and phonons in the resonant Raman effect, the
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unique one-dimensional electronic density of states (EDOS) of SWNTs play an important role in the exceptionally strong resonant Raman spectra of SWNTs associated with the interband optical transitions between two van Hove singularities [31]. According to Reference [32], while the 532 nm wavelength of our light source, carrying photon energy of 2.33 eV, can resonantly excite electrons contained in semiconducting SWNTs whose diameter is from 1.35 to 1.55 nm, the optical absorption spectrum does not show the corresponding strong absorption [32]. However, tipenhancement effect compensates for these less efficiency of the resonant Raman effect. Furthermore, the high resolution far-field Raman spectrum detected with a double-grating spectrometer [33] at the aggregated bulk of SWNTs included in the sample can give us the diameter distribution of the SWNTs (Fig. 4) [34]. Five apparent peaks are detected as radial breathing mode. By using a simple approximation between mode frequency WRBM (cm1 ) and the diameter d (nm), i.e., WRBM ¼ 224=d [28], the peaks of radial breathing mode at 148, 163, 170.5, 177, and 183.8 cm1 correspond to the diameters of 1.51, 1.37, 1.31, 1.27, and 1.22 nm, respectively. Note that because the numerator of the approximated equation ( ¼ 224) still requires further investigation by considering detailed interactions of SWNTs or a metallic tip, our estimated diameters described above are still one possibility. Even under these conditions, this
Fig. 4. High resolution far-field Raman spectrum of radial breathing mode from aggregated bulk of SWNTs detected with a double-grating spectrometer [32].
diameter feature is also supported by tip-enhanced near-field Raman spectrum of radial breathing mode shown in Fig. 5. The lines of a–c in the figure correspond to tip-enhanced near-field, far-field, and the subtracted Raman spectra, respectively. The metallic probe tip selectively enhanced the SWNT just below the tip so that the enhanced Raman peak at 165.2 cm1 is clearly observed in (a) while other peaks such as 170.8 and 192.6 cm1 which are generated from the far-field focused spot are not enhanced. By using the modified approximation for individual SWNTs, WRBM ¼ 248=d [26], the Raman peak at 165.2 cm1 corresponds to the diameter of 1.50 nm. Topographic image of the sample (Fig. 3d) ensures the height of the SWNT (1.5 nm) and clearly shows the distribution of SWNTs and amorphous carbon. This image was obtained in AFM operation by the tip-enhanced near-field microscope. White lines in the figure indicated by downwards arrows are isolated SWNTs. The height of the observed isolated SWNT is 1.5 nm corresponding to the diameter
Fig. 5. (a) Tip-enhanced near-field and (b) far-field Raman spectra of radial breathing mode of isolated SWNTs. (c) is the subtracted Raman spectra between (a) and (b). The Raman peak at 165.2 cm1 corresponding to the diameter of 1.50 nm is selectively enhanced by the metallic probe tip. The laser power is 4.8 mW and the exposure time is 1 min.
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of an individual SWNT while the other one has the height of 3.5 nm consisting of several number of SWNTs. White spots with a diameter of 100 nm indicated by upwards arrows are the amorphous carbon inherently included in our sample as the impurities of the arc-discharge process of SWNTs. Accordingly, the most possible chirality for the semiconducting SWNT observed in the tip-enhanced Raman spectrum is (17, 4)1.513 nm. Note that the peak at 192.6 cm1 in Fig. 5 is not observable in the high resolution far-field Raman spectra from the aggregated bulk of SWNTs in Fig. 4. This can be attributed to the fact that the number of SWNTs in Fig. 5 is extremely small compared to that of Fig. 4. The averaging effect overwhelmed the characteristic peak at 192.6 cm1 in Fig. 4. Fig. 3b was obtained when the metallic probe tip is 500 nm far away from the sample surface (the tip position of ÔBÕ). This spectrum represents a farfield Raman spectrum, where the featured lines are gone due to the absence of field enhancing metallic probe tip in the near-field except a small protrusion at 1592 cm1 . Since the sample contains not only SWNTs but also impurities (amorphous carbon), this small peak represents an average within a focused spot of laser beam, however, the spectrum dominantly reflects non-enhanced G-band feature of SWNTs. Note that although isolated SWNTs are randomly dispersed and alligned parallel to the substrate, we can observe G-band of isolated SWNTs only at several far-field focused positions by an objective lens, and the observed far-field Raman intensity at each position is different. Because of the polarization effect of both illumination light field and SWNTs and the severe selectivity of the resonant Raman effect [32], the number of observable isolated SWNTs is quite limited. These facts suggest that the observed isolated SWNT consists of only one or a few SWNTs. Fig. 3c shows another tip-enhanced nearfield Raman spectrum from the same sample when the probe enhances field at a different location. A broad D-band representing defect mode of amorphous carbon is dominantly detected around 1335 cm1 [35]. G-band is also seen but wider than that seen in Fig. 3a. This can be schematically attributed to the tip position of ÔCÕ. A small G-band
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peak shown in Fig. 3b (circle in the figure) is recognizable on the top of the broad G-band peak in Fig. 3c (circle in the figure). The broad G-band peak represents the tip-enhanced near-field Raman scattering of amorphous carbon due to the tip, while the small G-band peak is the far-field background signal coming from an isolated SWNT. The sharp tangential G-band feature of an individual semiconducting SWNT (Fig. 3a) and the broad D-band and G-band feature of amorphous carbon (Fig. 3c) are explained on the basis that the enhanced electric field at the metallic probe tip selectively enhances the near-field Raman scattering of the SWNT and amorphous carbon inside of the tightly focused spot by the objective lens. We showed that the combination of both radial breathing mode and G-band observed by tip-enhanced near-field Raman spectroscopy was a very powerful tool for the detail structural analysis of individual SWNTs. Due to the enhancement virtue of a metallic probe tip as a surface enhancer for SERS effect, tip-enhanced near-field Raman spectroscopy allows us to obtain localized molecular vibrational information without averaging the signal inside of the diffraction limited focused spot, and have not only high spatial resolution but also high signal sensitivity compared to far-field Raman spectroscopy that requires much more molecules to compensate for the small Raman scattering cross-section. Acknowledgements We would like to thank Professor H. Harima in Kyoto Institute of Technology for help with radial breathing mode observation using a double-grating monocromater and thank Dr. M. Hashimoto and Dr. N. Smith in Osaka University for fruitful discussion.
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