Development of apertureless near-field scanning optical microscope tips for tip-enhanced Raman spectroscopy

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1 Journal of Microscopy, Vol. 229, Pt , pp Received 28 September 2006; accepted 19 March 2007 Development of apertureless near-field scanning optical microscope tips for tip-enhanced Raman spectroscopy T. KO DA M A, T. UMEZAWA, S. WATANABE & H. OHTANI Department of Mechanical Engineering, Stanford, California 94305, USA Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama , Japan Key words. Apertureless near-field scanning optical microscope, atomic force microscope, confocal laser scanning microscope, finite-differential time-domain method, nanorod. Summary In this study, we suggested two types of novel metallized tip for the apertureless near-field scanning optical microscope probe. The first is a silver nanorod immobilized tip and the other is a double metallized probe. We calculated the electric field enhancementfactorandtheelectricfielddistributionofasingle sphere, aggregated spheres, an ellipse and a nanorod by the finite-differential time-domain method to improve the silver nanosphere immobilized tip developed in our previous studies. The enhanced field of the nanorod is localized at the external surfaces. The simulation results of the nanorod revealed that the position of the maximum peak is shifted to a longer wavelength and that its electric field enhancement factor increases as the aspect ratio increases. Thus, we developed the silver nanorod immobilized tip, and the tip-enhanced Raman spectrum of rhodamine 6G molecule on the substrate could be measured by the tip though it could not be detected by the previous nanosphere immobilized tip. Further, the finitedifferential time-domain calculation predicted that the double metallized tips considerably enhance the electric field and that its enhancement factor in the longer wavelength region ( nm) does not decrease when the tip is rounded. The results show that the proposed metallized tips were useful for the apertureless near-field scanning optical microscope system. Introduction The apertureless near-field scanning optical microscope (ANSOM) was developed in the 1990s (Zenhausern et al., 1994, 1995; Gleyzes et al., 1995; Kawata et al., 1995). The Correspondence to: H. Ohtani. Tel: ; fax: ; hohtani@bio.titech.ac.jp ANSOM employs a metallized tip instead of an optical fibre probe (Inouye et al., 1999; Anderson 2000; Hamann et al., 2000; Stöckle et al., 2000; Zayats et al., 2000; Hayazawa et al., 2001, 2003; Hartschuh et al., 2003; Jiang et al., 2003; Gerton et al., 2004). It is well known that the performance of the ANSOM strongly depends on the electric field enhancement factor (EFEF) of the metallized tip. The metal-coated atomic force microscope (AFM) tip fabricated by the sputtering method was normally used as an ANSOM probe. However, it is known that the cancellation of the electric field component along the tip axis and the tip rounded effect results in the decrease of the EFEF (Kodama et al., 2006a). Thus, we developed a silver nanoparticles immobilized (SNI) tip to improve the ANSOM probe (Kodama et al., 2006b). By using the SNI tip, we could measure the tip-enhanced Raman scattering (TERS) spectra and the TERS images of carbon onion molecules with high sensitivity (Kodama et al., 2006b). The SNI tip has several advantages: The EFEF is large, thus the TERS measurement can be carried out with high sensitivity. Further, there is no need to polarize the electric field along the tip axis and there is no signal fall-off from tip rounding. However, the SNI tips still have some disadvantages. For one, some tips are noisy because of the surface-enhanced Raman scattering signal from the polymer layer that is used for the immobilization of the nanoparticles. For another, the tips are hard to use on small samples. Figure 1 shows the schematic image of the SNI tip. The incident electric field is enhanced by the aggregated metal nanoparticles attached to the end of the tip. Figure 2(b) is the electric field distribution of the two touching silver nanoparticles calculated by the finite-differential timedomain (FDTD) method (Taflove et al., 2000; Futamata et al., 2003). The results revealed that the enhanced electric field is localized at the contact area of spheres. Therefore, to enhance the Raman spectrum of the sample on the substrate, the height of the sample has to be larger than the radius of the metal Journal compilation C 2008 The Royal Microscopical Society

2 DEVELOPMENT OF ANSOM BIPS FOR TIP-ENHANCED RAMAN SPECTROSCOPY 241 Fig. 1. (a) Schematic view of a SNI tip. (b) the topographic image of the SNI tip observed with a scanning electron microscope. Fig. 2. The electric field distribution around the silver nanostructure obtained by the FDTD simulations. Panels (a), (b), (c) and (d) are the electric field distribution of the single sphere, the two touching silver spheres, the single ellipse 1:2 and the single nanorod 1:5, respectively. The wavelengths of an incident plane wave were 350, 500, 375 and 400 nm, respectively. The polarization of the incident plane wave is shown in each figure. nanoparticles. In our previous studies, we actually attempted to measure the TERS spectra of rhodamine 6G, carbon onion and purple membrane by using the SNI tip. In our experiments, however, the TERS spectra of the rhodamine 6G and the purple membrane (these samples are smaller than the averaged radius of silver nanoparticle) on the substrate surface could not be measured. In this study, to improve the ANSOM probe, two types of novel tips were proposed. The first is the silver nanorod immobilized (SNRI) tip. The EFEF and the electric field distribution of various silver nanoparticles were calculated by the FDTD method to improve the SNI tip, the SNRI tip was developed, and the TERS spectrum of rhodamine 6G molecules was attempted to be measured. The other is the double metallized tip. The practical effectiveness was predicted by the FDTD calculation. Materials and methods Finite-differential time-domain method We determined the electric enhancement factor of the metallized tip by two-dimensional calculations by using the FDTD method (Taflove et al., 2000) with a transverse electric wave set. The details of our calculation method have been described elsewhere (Kodama et al., 2006a). In short, we divided the free space into a square Yee cell with

3 242 T. KO DA M A ET AL. a cell size of 0.5 nm. The metal object was positioned at the centre of the free space and a plane sinusoidal electromagnetic wave was irradiated on the object. We set the intensity of the incident wave to 1, and the maximum electric field around the objects was obtained as a function of the incident wave frequency after it had attained a steady state. Further, the enhancement factor was calculated from the maximum value of the field intensity. To simulate the silver surface plasmon, the permittivity of the silver object was approximated by the Drude model, and it was parameterized by fitting the damping constant, bulk permittivity and plasma frequency to the experimental data of Johnson and Christy (Johnson et al., 1972). We adopted the auxiliary differential equation method, and a perfect matching layer was adopted as the absorption boundary condition (Taflove et al., 2000). Preparation of SNI tip The silver nanosphere and the nanorod were synthesized by the method described below. Silver sphere nanoparticles were prepared by the synthesis method (Lee et al., 1982). The silver nanorod was synthesized by the method described elsewhere (Aslan et al., 2005; Jana et al., 2001). The synthesized silver nanoparticles were immobilized on the surface of an AFM tip by the method described elsewhere (Kodama et al., 2006b). In short, the NH 2 terminus was introduced to the surface of a commercial AFM tip by the silane coupling method. After that, the tip was immersed into the solution that contains Bis[sulfosuccinimidiyl]-suberate (BS3, Pierce, Rockford, IL). After washing it, the tip was immersed into the solution that contains the polyallylamine (PolyA, averaged MW = 17000, Wako, Osaka, Japan). After washing the surface well, the substrate was immersed into the silver nanoparticle solution overnight. TERS measurement setup TERS measurements were performed with our homemade integrated experimental system (Kodama et al., 2004, 2005). This system was composed of an AFM (Smena liquid head, NT-MDT, Moscow, Russia) inserted in the open space above the sample stage of an inverted CLSM (Nanofinder, Tokyo Instruments, Tokyo, Japan). An Ar + laser (488 nm, Nippon Laser, Tokyo, Japan) was used as the excitation light source. Raman spectra of samples were measured with a cooled CCD camera (CCD1, Andor Technology, Belfast, Northern Ireland) equipped with a spectrograph (f = 56 cm, 600 grooves/mm grating). Rhodamine 6G was dissolved in ethanol and the solution was dropped onto a glass substrate. The substrate surface was washed under sonication in piranha solution that contains 70% sulphuric acid and 30% hydrogen peroxide. The substrate was used as the experimental sample after the complete evaporation of the ethanol solution on its surface. Results and discussion In this study, two types of a novel metallized tip were suggested to improve the system performance of the ANSOM. First, the previously proposed SNI tip was improved by both the theoretical and the experimental approaches. Figure 2 is the electric field distribution of the silver nanoparticles. Figure 2(b) shows that the enhanced electric field of the two touching spheres is localized at the contact area. The others show that the enhanced field is localized at the external surfaces. The localized electric fields of the sphere and the ellipse are distributed along the incident electric field. On the other hand, that of the nanorod is localized at the edge. Figure 3 shows the wavelength dependence of the EFEF of the single ellipse and the singlenanorod. Theminoraxisoftheellipsewas50nm, andthe EFEFs of the ellipse whose major axes were 100, 150 and 200 nm (ellipse 1:2, 1:3 and 1:4, respectively) were calculated by the FDTD method. Further, the minor axis of the nanorod was 80 nm, and the EFEFs of the nanorod whose major axes were 80, 160, 240, 320, 400 and 480 nm (nanorod 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively) were calculated. Figures 3(a c) show that the EFEFs of the ellipse and the nanorod are larger than that of the single sphere and that the absorption peak is shifted to a longer wavelength as the aspect ratio increases. Figure 3(d) is the aspect-ratio dependence of the maximum EFEF of the single nanorod. The maximum EFEF of the ellipse decreases as the aspect ratio increases (see Fig. 3a). On the other hand, although the EFEF of the nanorod decreases as the aspect ratio increases up to 3, it increases again when the aspect ratio becomes 4 or more. The intensity of Raman signals is directly proportional to the fourth power of the amplitude of the incident electric field. The EFEF of the nanorod 1:6 is approximately 40. Therefore, its enhancement factor of optical signal may become Furthermore, the enhanced electric field of the nanorod is localized at the external surface (see Fig. 2d). Thus, the individual silver nanorod with its high aspect ratio is considered to enhance the Raman scattering signal by the visible light. Therefore, the silver nanorod was synthesized by the wet chemical method (Aslan et al., 2005; Jana et al., 2001), and the SNRI tip was fabricated by the immobilization of the silver nanorods on the surface of a commercial AFM tip. The topographic images of the substrate surface to which silver nanorods were attached are shown in Fig. 4(a). It is clearly shown that the silver nanorods whose aspect ratio is about 1:5 were observed in the image. Thus, the SNRI tip was fabricated, and the TERS spectrum of rhodamine 6G molecules that adhered on glass substrate was attempted to be measured. The Raman scattering spectra of rhodamine 6G are shown in Fig. 4(b). The solid line represents the TERS spectrum of rhodamine 6G on the glass substrate by the SNRI tip, and the dotted line corresponds to the surface-enhanced Raman scattering spectrum of rhodamine 6G on the silver island substrate surface (it was obtained without the SNRI

4 DEVELOPMENT OF ANSOM BIPS FOR TIP-ENHANCED RAMAN SPECTROSCOPY 243 Fig. 3. The electric field enhancement factor (EFEF) of the single ellipse and the single rod. (a) The wavelength dependence of the EFEF of the single ellipse. Solid squares with the solid line, solid circles with the dotted line and inverted solid triangles with the broken line represent the wavelength dependence of the EFEF of ellipse 1:2, 1:3 and 1:4, respectively. (b) and (c) The wavelength dependence of the EFEF of the single nanorod. Solid squares with the solid line, solid circles with the dotted line and inverted solid triangles with the broken line in (b) represent the wavelength dependence of the EFEF of nanorod 1:1, 1:2 and 1:3, respectively. Solid squares with the solid line, solid circles with the dotted line and inverted solid triangles with the broken line in (c) represent the wavelength dependence of the EFEF of nanorod 1:4, 1:5 and 1:6, respectively. Further, the EFEF of the single sphere whose diameter is 60 nm is represented by open squares in each figure. The vertical and the horizontal axes represent the maximum electric field around the object and the wavelength of the incident plane wave, respectively. A schematic representation of the sample configuration is also inset in this figure. (d) The aspect-ratio dependence of the EFEF of the nanorod. The solid squares represent the EFEF of the nanorod. The vertical and the horizontal axes correspond to the maximum EFEF and the aspect ratio, respectively. tip). Several identical Raman peaks were observed when the comparison of the two spectra was made. From the results, it is confirmed that the TERS spectrum of rhodamine 6G molecules could be detected by the SNRI tip. Further, only the TERS spectrum has a frequency line at 1528 cm 1. This wave number corresponds to the -C = C- vibration mode. As we mentioned in the introduction section, the optical noise from the polymer layer, which is used for the immobilization of nanoparticles, was sometimes observed from 1500 to 1600 cm 1. Therefore, it is considered that the observed frequency line at 1528 cm 1 originates from the optical noise. In our previous study, by using the silver nanospheres immobilized tip, we attempted to measure the TERS spectrum of rhodamine 6G on the glass substrate. However, it has not been detected yet. Therefore, it is concluded that the Raman scattering light of rhodamine 6G on the substrate was enhanced by the localized electric field of the nanorods at the end of the tip. The TERS spectrum could be measured with the input excitation power of 5 μw and exposure time of 1 s. The Raman spectrum could not be detected without the tip in this measurement condition. Our previous FDTD calculation revealed that the EFEF of a single metallized tip decreases when the tip is rounded (Kodama et al., 2006a). Based on the results, another novel metallized tip is suggested by FDTD calculation (see Fig. 5). The two inclined metallized probe contacts at the tip and the electric field are enhanced at the contact point. Figure 5(b) shows the wavelength dependence of the field enhancement of two touching silver tips. These calculation results were obtained when the tip was not rounded. The enhancement factor and the maximum peak wavelength of the two touching silver tips are almost equal to those of a single tip. On the other hand, the enhancement factor in the longer wavelength region ( nm) is very large compared with that of a single tip because of the localized surface plasmon coupling of the two tips. Next, the calculation results of the electric field enhancement of two touching rounded tips are shown in Fig. 6. The rounded tip diameter is 20 nm. Figures 6(a) and (b) show the electric field distribution of the two touching rounded tips and the wavelength dependence of their enhancement factor, respectively. The results revealed that the enhancement factor of the electric field at 400 nm remarkably decreases when the tips are rounded (see Figs 5b and 6b). On the other hand, the enhancement factor in the longer wavelength region ( nm) that originates from the localized surface plasmon coupling does not vary. Also, the tip-diameter dependence of the enhancement factor of electric fields is shown in Fig. 6(c). Although the enhancement factor remarkably decreases at the wavelength of 400 nm as the tip diameter increases, the enhancement factor is independent of the tip diameter at the

5 244 T. KO DA M A ET AL. Fig. 5. FDTDcalculationresultsof a doublemetallized probe. (a) Schematic view of a double metallized probe. (b) Wavelength dependence of the electric field enhancement factor of two touching silver tips. The incident wave was polarized along the tip axis. The vertical and the horizontal axes represent the maximum electric field around the tip and the wavelength of the incident plane wave, respectively. The solid squares represent the enhancement factor of the two touching silver tips. Also, the open circles represent the enhancement factor of a single silver tip. A schematic representation of the sample configuration is also inset in this figure. Fig. 4. Immobilization of the nanorod on the substrate surface for the ANSOM tip. (a) The topographic images of the silver nanorod immobilized substrate. The unit of the height axis is nanometres. The image was measured in air with the contact measurement mode of an AFM. The spring constant of the cantilever was 0.1 N/m. (b) The TERS spectrum of rhodamine 6G molecules. The solid line corresponds to the TERS spectrum of the rhodamine 6G molecules on the glass substrate measured by the silver nanorod immobilized tip. The dotted line corresponds to the surfaceenhanced Raman scattering spectrum of rhodamine 6G on the silver island substrate surface. wavelength of 500 nm. Therefore, it is concluded that the decrease in the enhancement factor because of the rounded tip effect can be prevented by using such twin metallized tips. Further, as shown in Fig. 6(a), the enhanced electric field between the two metallized tips expanded slightly compared with that of two touching spheres (see Fig. 2b). It is considered to be advantageous for the TERS measurement. The fabrication of the double metallized tip may be more difficult than that of the SNI tip. It may be made by touching two metallized tips that have been fabricated individually. Further, it may be also fabricated by making a crack at the end of a metallized tip. In this study, we proposed two types of a novel metallized tip for the ANSOM. First, the EFEF and the electric filed distribution of a single sphere, the two touching spheres, the ellipse and the nanorod were calculated by the FDTD method to improve the SNI tip that was developed previously. The results revealed that the EFEFs of the ellipse and the nanorod are larger than that of the single sphere and that the absorption peak is shifted to a longer wavelength. Although the maximum EFEF of the ellipse decreases as the aspect ratio increases, that of the nanorod does not decrease. Thus, the SNRI tip was developed and its practical effectiveness was verified. As a result, the TERS spectrum of the rhodamine 6G molecules on the glass substrate could be detected by the SNRI tip though it could not be detected in our previous experiment. Further, the double metallized probe was suggested from the FDTD calculation, and its practical effectiveness was shown. The synthesis methods of various metal nanoparticles such as cube (Sun et al., 2002) and triangle (Wei et al., 2004) have been reported yet. For example, it is known that the single triangle nanoparticle can strongly enhance the electric field (Kodama et al., 2006a). If these nanoparticles are introduced

6 DEVELOPMENT OF ANSOM BIPS FOR TIP-ENHANCED RAMAN SPECTROSCOPY 245 microscope. Therefore, inthefuture, varioustypesofmetallized tips will be suggested to improve the performance of the ANSOM. Acknowledgements The authors express their sincere gratitude to Professor Masamichi Fujihira, Tokyo Institute of Technology (T.I.T.), for his encouragement and fruitful discussions through this work. The authors greatly acknowledge the support from the JSPS (Grant-in-Aid for JSPS Fellows, No ). This work was supported in part by grants-in-aid to A.I. from the Japanese Ministry of Education, Science, Culture, and Sports (Scientific Research on Priority Areas B# ) and by Molecular Sensors for Aero-Thermodynamic Research (MOSAIC), the Special Coordination Funds of Ministry of Education, Culture, Sports, Science, and Technology. References Fig. 6. The electric field enhancement factor (EFEF) of two touching rounded silver tips. (a) The electric field distribution around the two touching rounded silver tips. The diameter of the rounded tip is 20 nm. The wavelength of incident plane wave was 500 nm and the incident wave was polarized along the tip axis [shown in (b)]. The vertical and the horizontal axes represent the number of cells, and the cell size is 0.5 nm. (b) Wavelength dependence of the EFEF of two touching rounded silver tips. The incident wave was polarized along the tip axis. The rounded tip diameter is 20 nm. The vertical and the horizontal axes represent the maximum electric field around the tip and the wavelength of the incident plane wave, respectively. The solid squares and the open circles represent the enhancement factor of the two touching rounded silver tips and two touching silver spheres, respectively. A schematic representation of the sample configuration is also inset in this figure. (c) The tip-diameter dependence of the EFEF of the rounded single and two touching silver tips. The wavelength of the incident plane wave was 500 nm. The vertical and the horizontal axes represent the maximum electric field around the tip and the tip diameter, respectively. to the SNI tip, it may be possible to improve its performance. Furthermore, a sharp metal wire is recently used as an ANSOM probe instead of the metallized AFM tip (Billot et al., 2005). The ANSOM is a very promising near-field scanning optical Anderson, M.S. (2000) Locally enhanced Raman spectroscopy with an atomic force microscope. Appl. Phys. Lett. 76, Aslan, K., Leonenko, Z., Lakowicz, J.R. & Geddes, C.D. (2005) Fast and slow deposition of silver nanorods on planar surfaces: Application to metal-enhanced fluorescence. J. Phys. Chem. B. 109, Billot, L., Berguiga, L., de la Chapelle, M.L., Gilbert, Y. & Bachelot, R. (2005) Production of gold tips for tip-enhanced near-field optical microscopy and spectroscopy: Analysis of the etching parameters. Eur. Phys. J. Appl. Phys. 31, Futamata, M., Maruyama, Y. & Ishikawa, M. (2003) Local electric field and scattering cross section of Ag nanoparticles under surface plasmon resonance by finite difference time domain method. J. Phys. Chem. B 107, Gerton, J.M., Wade, L.A., Lessard, G.A., Ma, Z. & Quake, S.R. (2004) Tipenhanced fluorescence microscopy at 10 nanometer resolution. Phys. Rev. Lett. 93, Gleyzes, P., Boccara, A.C. & Bachelot, R. (1995) Near-field optical microscopy using a metallic vibrating tip. Ultramicroscopy 57, Hamann, H.F., Gallagher, A. & Nesbitt, D.J. (2000) Near-field fluorescence imaging by localized field enhancement near a sharp probe tip. Appl. Phys. Lett. 76, Hartschuh, A., Sanchez, E.J., Xie, X.S. & Novotny, L. (2003) Highresolution near-field Raman microscopy of single-walled carbon nanotubes. Phys. Rev. Lett., 90, Hayazawa, N., Inouye, Y., Sekkat, Z. & Kawata, S. (2001) Near-field Raman scatteringenhancedbyametallizedtip.chem.phys.lett.335, Hayazawa, N., Yano, T., Watanabe, H., Inouye, Y. & Kawata, S. (2003) Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy. Chem. Phys. Lett. 376, Inouye, Y., Hayazawa, N., Hayashi, K., Sekkat, Z. & Kawata, S. (1999) Near-field scanning optical microscope using a metallized cantilever tip for nanospectroscopy. Proc. SPIE. 3791, Jana, N.R., Gearheart, L. & Murphy, C.J. (2001) Wet chemical synthesis of silver nanorods and nanowires of controllable aspect ratio. Chem. Commun. 7, Johnson, P.B. & Christy, R.W. (1992) Optical constant of the nobel metals. Phys. Rev. 12,

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