ruthenium(II) on Silver Nanoparticles-Coated Substra - American

Sep 6, 2008 - more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from ...
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Langmuir 2008, 24, 12054-12061

Cantilever Tip Near-Field Surface-Enhanced Raman Imaging of Tris(bipyridine)ruthenium(II) on Silver Nanoparticles-Coated Substrates Yang Jiang, An Wang, Bin Ren, and Zhong-Qun Tian* State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, China ReceiVed May 3, 2008. ReVised Manuscript ReceiVed July 17, 2008 The near-field surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) images of tris(bipyridine)ruthenium(II) adsorbed on a silver nanoparticles-coated substrate were obtained with a scanning nearfield optical microscope (SNOM, or near-field scanning optical microscopy, NSOM) using a cantilever tip. In comparison with the most widely used fiber tip for SNOM, the cantilever tip has higher optical throughput and better thermal stability, making it more suitable for detecting the extremely low Raman signal in the near-field spectroscopic investigations. Our preliminary results show that the near-field SERS with the higher spatial resolution can provide richer fingerprint information than the far-field SERS. A comparison of the two types of images shows that there are more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from the far-field SERS spectra obtained on the same sample in the band frequency and relative intensities of some major Raman bands, and some IR-active bands were observed with the near-field mode. These results are explained mainly by the electric field gradient effect and heterogeneous polarization character that operate only in the near-field SERS.

Introduction The structure and dynamics of solid/gas and solid/liquid interfaces play an increasingly important role in materials science, catalysis, electrochemistry, and many industrial processes. Laser spectroscopic methods are powerful surface analysis techniques which have been widely employed to probe into the chemisorption processes involved in the interface. In the application of these techniques, it is desirable to improve the spatial resolution as high as possible. Scanning near-field optical microscopy (SNOM, or near-field scanning optical microscopy, NSOM) is capable of providing optical information beyond the diffraction limit of ca. λ/2 ruled by the Rayleigh criterion.1 As an optical tool, spectral investigations such as near-field fluorescence single-molecule detection have been performed on SNOM.2 Because of the specific chemical fingerprint information which can be provided by Raman spectroscopy detected in the near-field, great efforts have been devoted to aperture3-7 and apertureless SNOM8 since the 1990s. Near-field Raman spectroscopy based on the aperture SNOM is less sample-invasive compared with the apertureless SNOM.9 The latter interacts with the substrate through the use of a metallic scattering tip. This is because the energy of the highly localized electromagnetic (EM) field at the end of the apertureless tip is much stronger than the illumination energy coming through the * Corresponding author. E-mail: [email protected]. (1) Rayleigh, L. Philos. Mag. 1879, 8, 261. (2) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422. (3) Tsai, D. P.; Othonos, A.; Moskovits, M.; Uttamchandani, D. Appl. Phys. Lett. 1994, 64, 1768. (4) Jahncke, C. L.; Paesler, M. A.; Hallen, H. D. Appl. Phys. Lett. 1995, 67, 2483. (5) Emory, S. R.; Nie, S. Anal. Chem. 1997, 69, 2631. (6) Deckert, V.; Zeisel, D.; Zenobi, R.; Vo-Dinh, T. Anal. Chem. 1998, 70, 2646. (7) Zhang, P.; Smith, S.; Rumbles, G.; Himmel, M. E. Langmuir 2005, 21, 520. (8) Stockle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Chem. Phys. Lett. 2000, 318, 131. (9) Zavalin, A.; Cricenti, A.; Generosi, R.; Luce, M.; Morgan, S.; Piston, D. Appl. Phys. Lett. 2006, 88, 133126–1.

aperture tip. The strong EM field on the apertureless tip has the potential problem of bringing out tip-induced artificial hot sites between the tip and the sample because of the lightning rod effect. Near-field Raman spectroscopy based on aperture SNOM has found applications in various fields including the provision of vibrational information on nanoscale,3,5,10,11 near-field Raman imaging,4,9,12 the probing of molecule interaction at liquid-liquid interface,13,14 residual stress of silicon in submicrometer resolution,15,16 mechanistic studies of catalysts,17 the investigation of electric field gradient effects in the near-field,18,19 mechanistic studies of surface-enhanced Raman scattering (SERS),20 the distribution of the SERS hot spots,6,7,17,21 etc. All the above applications are based on fiber tip SNOM which is the mainstream technique employed in aperture SNOM since its invention in 1984.22,23 However, the near-field Raman study based on fiber tip SNOM is not always easy to realize in real practice. Raman scattering is a process of low quantum efficiency, i.e., usually only 107-1010 incident photons can be converted (10) Smith, D. A.; Webster, S.; Ayad, M.; Evans, S. D.; Fogherty, D.; Batchelder, D. Ultramicroscopy 1995, 61, 247. (11) Zeisel, D.; Dutoit, B.; Deckert, V.; Roth, T.; Zenobi, R. Anal. Chem. 1997, 69, 749. (12) Gucciardi, P. G.; Trusso, S.; Vasi, C.; Patane, S.; Allegrini, M. Appl. Opt. 2003, 42, 2724. (13) De Serio, M.; Bader, A. N.; Heule, M.; Zenobi, R.; Deckert, V. Chem. Phys. Lett. 2003, 380, 47. (14) De Serio, M.; Mohapatra, H.; Zenobi, R.; Deckert, V. Chem. Phys. Lett. 2006, 417, 452. (15) Webster, S.; Batchelder, D. N.; Smith, D. A. Appl. Phys. Lett. 1998, 72, 1478. (16) Webster, S.; Smith, D. A.; Batchelder, D. N.; Karlin, S. Synth. Met. 1999, 102, 1425. (17) Fokas, C.; Deckert, V. Appl. Spectrosc. 2002, 56, 192. (18) Ayars, E. J.; Hallen, H. D.; Jahncke, C. L. Phys. ReV. Lett. 2000, 85, 4180. (19) Hallen, H. D.; Jahncke, C. L. J. Raman Spectrosc. 2003, 34, 655. (20) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173. (21) Stockle, R. M.; Deckert, V.; Fokas, C.; Zeisel, D.; Zenobi, R. Vib. Spectrosc. 2000, 22, 39. (22) Lewis, A.; Isaacson, M.; Harootunian, A.; Muray, A. Ultramicroscopy 1984, 13, 227. (23) Pohl, D. W.; Denk, W.; Lanz, M. Appl. Phys. Lett. 1984, 44, 651.

10.1021/la801376p CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

SERS and SEF of Tris(bipyridine)ruthenium(II)

to one Raman photon. Efforts aiming at enhancing the near-field Raman intensity have been made by using resonance Raman10 and/or SERS.5-7,11,20,24 However, the spectral intensity in these techniques is still too weak to extract quantitative information because of limitations associated with the fiber tip SNOM, such as the low throughput of the probe and the heating breakage at the probe apex. For the fiber probe, the typical throughput of a chemicaletched probe is 10-3-10-4, whereas that of the heating-pulling probe is only 10-6.25 This difference is due to the fact that the heating-pulling probe has a narrower cone angle which constrains more lights inside. On the other hand, the cone angle is related to the temperature on the apex of the probe. Staehelin et al. reported26 that the temperature coefficients varied from 20 K/mW for tips with a large cone angle to 60 K/mW for narrow long tips. Most of the light was reflected by the coated metal film on the end of the tip, and this process can heat up the tip considerably due to light absorption of the metal coating.26 As a result, the input of excessively high incident power into the fiber would melt and blunt the tip.27 When a 9.5 mW laser coupled into the fiber, the temperature at the end of tip can reach as high as 470 °C, and the tip would be destroyed.26 The aperture of the fiber tip which defines the resolution of SNOM usually cannot be smaller than 30-50 nm due to the energy accumulation on the apex with an intensity-dependent expansion of 10-20 nm/mW.11 This thermal effect limits the incident laser power and restrains the transmitted energy to a very low level.28 Consequently, the maximum power output is tens of nanowatts for a typical fiber probe tip.29 Therefore, most of the near-field Raman spectroscopic and imaging studies based on the fiber tip SNOM have to extend the acquisition time and to reduce image pixels in order to make the measurement. In order to obtain higher Raman intensity, it is desirable to apply SNOM tips which have stronger laser output in addition to the application of surface-enhanced and/or resonance Raman effects. Another type of the mainstream aperture SNOM tip is silicon cantilever tip. A sketch comparing the working modes of the cantilever and fiber tips is shown in Figure 1a. The cantilever tip, also called the pyramid tip, is formed by a complicated process with multiple steps of etching and coating.30,31 The pyramidshaped probe is formed by four crossing Si(111) faces during the chemical etching process. The wide ca. 70° cone angle permits more light passing through the aperture with a typical throughput of 10-2.31,32 Furthermore, the thick SiO2 wall of ca. 600 nm in addition to the nearly 200 nm thick layer of deposited chromium and aluminum30,31 strengthen the tip so much that more incident laser power can be endured and the thermal effect could be ignored. These features make the cantilever tip a promising alternative probe in near-field Raman research. To our knowledge, so far there has been only one report by Sakai et al. on the use of this kind of tip in a near-field Raman spectroscopic study of a ferroelectrics sample.33 They utilized the cantilever tip as both (24) Anger, P.; Feltz, A.; Berghaus, T.; Meixner, A. J. J. Microsc. 2003, 209, 162. (25) Kim, J.; Song, K. B. Micron 2007, 38, 409. (26) Staehelin, M.; Bopp, M. A.; Tarrach, G.; Meixner, A. J.; ZschokkeGraenacher, I. Appl. Phys. Lett. 1996, 68, 2603. (27) Kaupp, G.; Herrmann, A.; Haak, M. J. Phys. Org. Chem. 1999, 12, 797. (28) La Rosa, A. H.; Yakobson, B. I.; Hallen, H. D. Appl. Phys. Lett. 1995, 67, 2597. (29) Harris, T. D.; Grober, R. D.; Trautman, J. K.; Betzig, E. Appl. Spectrosc. 1994, 48, 14A. (30) Minh, P. N.; Ono, T.; Esashi, M. Appl. Phys. Lett. 1999, 75, 4076. (31) Minh, P. N.; Ono, T.; Esashi, M. ReV. Sci. Instrum. 2000, 71, 3111. (32) Song, K.-B.; Kim, E.-K.; Lee, S.-Q.; Kim, J.; Park, K.-H. Jpn. J. Appl. Phys., Part 1 2003, 42, 4353. (33) Sakai, A.; Sasaki, N.; Tamate, T.; Ninomiya, T. Ferroelectrics 2003, 284, 15.

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Figure 1. (a) Sketch of the working modes of the cantilever tip (left) and the fiber tip (right); (b) a magnified view around the cantilever tip apex.

an illumination source and a collector for the scattered light. By subtracting the reference background signal of the probe materials, Raman signal was observed. One can further infer from these results that better Raman signal could be obtained if the SERS and resonance Raman effects are combined with the cantilever tip working in the transmission mode. The near-field Raman spectroscopy is a powerful tool to investigate the SERS phenomena of various nanostructures as it can clearly reveal that the excited EM field is highly localized.7,20,34 Through these near-field SERS studies the estimated Raman enhancement factor can be as high as 108-1013.7,20,34 The aperture tip near-field SERS has been applied in the studies of the hot spot distribution and the EM mechanism. The first SERS imaging work was reported by Deckert et al. in 1998 on the adsorption of dye-labeled DNA on silver evaporated Teflon nanospheres.6 Zhang et al. found that the hot spots exist not only in the junctions but also in particleabsent positions.7 Very recently, Imura et al. proved that the near-field SERS-active spots between two nanoparticles exhibit the polarization-dependent behavior.20 However, the exact reason for the dramatic difference between the near-field and far-field SERS spectroscopy, e.g., their differences in the enhancement efficiency and polarization dependence, remains unclear, which is worthy of further investigation. In comparison to the surface-enhanced spectroscopic technique of SERS, much less attention has been paid to surface-enhanced fluorescence (SEF) so far. This is because the enhancement factor of SEF35 is generally less than 100, whereas on the other hand the intrinsic fluorescence signals in most applications are already strong enough for detection. Moreover, the probe dye molecules (34) Zeisel, D.; Deckert, V.; Zenobi, R.; Vo-Dinh, T. Chem. Phys. Lett. 1998, 283, 381. (35) Goulet, P. J. G.; Aroca, R. F. Surface-enhancement of fluorescence near noble metal nanostructures. In RadiatiVe Decay Engineering; Geddes, C. D., Lakowicz, J., Eds.; Springer: New York, 2005; p 223–247.

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need to be separated from the metal substrate by 7-10 nm in order to prevent the fluorescence quenching process so that the strongest fluorescence-enhanced effect can be realized.36 This requirement is in contrast to that of SERS, where close contact between the probe molecules and the metallic surface within a distance below 2 nm is necessary. These opposite properties could be utilized to study the SERS or SEF effect of dye molecules by simply adjusting the molecule-surface distance.37,38 It would be of special interest if one could apply a SNOM imaging method to distinguish the highest active areas as the so-called hot spots of SERS and SEF. In the present SNOM study we utilized the cantilever tip for the optimal laser output and the monodisperse silver nanoparticles to optimize the surface-enhanced spectral activity. This enabled us to detect the near-field Raman and fluorescence signals in a short collecting time and also to complete the near-field Raman and fluorescence mapping process in a reasonable time scale. On the basis of these techniques, we were able to obtain the SERS and SEF images through the SNOM mode. Tris(bipyridine)ruthenium(II) was used as the probe molecule for the near-field SERS and SEF studies. As a polypyridine complex of divalent ruthenium with unusual excited-state properties, this molecule has attracted broad interests because of its potential utility in solar energy conversion schemes.39 More importantly, this molecule shows quite strong SERS activity with good thermal stability and has no obvious light-bleaching comparing with the classical SERS probe molecule of rhodamine 6G (R6G). These properties make it an ideal probe molecule in the near-field spectroscopic study. The conventional confocal Raman microscopic study was also performed in order to correlate the SNOM results and to analyze the spectral differences between spectra recorded in near-field and far-field modes.

Experimental Section Tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate ([Ru(bpy)3]2+) was purchased from Aldrich and used as received. The silver nanoparticles with high SERS activity were prepared according to the literatures on which single-molecule detections were achieved and the enhancement factor can reach as high as 1014.40-42 The sphere silver nanoparticles with diameter of ca. 100 nm were chemically synthesized by Lee’s method.43 After being centrifuged and rinsed, the nanoparticles were mixed with a 10-6 M [Ru(bpy)3]2+ aqueous solution, then dropped on a microscope coverglass and dried in vacuum. Millipore water of 18.2 MΩ · cm was used throughout the experiment. A transmission mode SNOM (R-SNOM of WITec) was used and worked with a silicon cantilever tip (purchased from WITec). The continuous-wave laser excitation was powered by an Nd:YAG laser working at the wavelength of 532 nm. A laser power of ca. 1 mW was coupled into a Carl Zeiss microscope with an optical fiber and focused by an 8× objective onto the backside of pyramid tip with aperture size of less than 100 nm. The near-field laser spot on the tip apex is used as excitation source for near-field spectroscopic detection. The transmitted signal was collected by a 60× Nikon objective with the NA of 0.80. After being filtered through a 532 nm notch filter, the signal was coupled into a multimode optical (36) Lakowicz, J. R.; Geddes, C. D.; Gryczynski, I.; Malicka, J.; Gryczynski, Z.; Aslan, K.; Lukomska, J.; Matveeva, E.; Zhang, J. A.; Badugu, R.; Huang, J. J. Fluoresc. 2004, 14, 425. (37) Sokolov, K.; Chumanov, G.; Cotton, T. M. Anal. Chem. 1998, 70, 3898. (38) Lakowicz, J. R. Anal. Biochem. 2001, 298, 1. (39) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (40) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (41) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102. (42) Xu, H. X.; Bjerneld, E. J.; Kall, M.; Borjesson, L. Phys. ReV. Lett. 1999, 83, 4357. (43) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.

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Figure 2. (a) Absorption spectrum and (b) emission spectrum of the [Ru(bpy)3]2+ aqueous solution, (c) emission spectrum of the [Ru(bpy)3]2+ solid sample, and (d) far-field SERS spectrum of [Ru(bpy)3]2+ on silver nanoparticles-coated substrate. The excitation line is 532 nm.

fiber and delivered to an Acton Spectrapro 2300i spectrometer which spectral resolution is ca. 4 cm-1. A liquid nitrogen cooled CCD working at -130 °C from Roper Scientific was used for spectroscopic detection. The near-field SERS image was obtained while the SNOM tip scanned in AFM mode over an area of 8 × 8 µm2 with 100 × 100 pixels. The spectrum collection time at each point was 3 s. When the regular SNOM image and near-field Raman image are both demanded, usually two scanning processes6,12 or a branch beam path4,7 are necessary. In the present setup with a notch filter, the laser light with wavelength of 532 nm is not filtered completely. So the intensity of the transmitted light with the same wavelength of the laser could be used as the signal for obtaining the regular transmission SNOM image. There are some benefits of obtaining near-field Raman image with a multichannel detector such as a CCD rather than a single-channel detector like a PMT or APD, and these benefits are listed as follows: (i) The information of elastic scattering and Raman scattering can be recorded simultaneously. In comparison with the branch beam path strategy which has been widely applied in previous near-field Raman studies for the provision of regular transmission SNOM images, in the new method the signal intensity will not be lost since the light does not need to pass through the beam splitter. (ii) The sample position can be pointed precisely according to the images of near-field transmission, SERS and SEF. (iii) When using CCD to probe Raman intensity distribution of molecules with strong fluorescence, the influence caused by fluorescence background can be prevented by background subtraction. As will be illustrated below, the spectra recorded by CCD can clearly distinguish the signal from the SERS or SEF hot spots.

Results and Discussion 1. UV-vis Spectrum and Far-Field SERS Spectrum of [Ru(bpy)3]2+ The UV-vis absorption spectrum of [Ru(bpy)3]2+ in aqueous solution is shown in Figure 2a. The spectrum is mainly composed by two resonance peaks of the near-UV π f π* transition at ca. 280 nm and the metal-to-ligand charge-transfer (MLCT) transition at 459.7 nm as previously reported.44 Since the MLCT resonant peak has a long tail up to ca. 560 nm, we tried to use a 532 nm laser as the excitation source and a confocal Raman microscope to record the preresonance Raman spectra of [Ru(bpy)3]2+, both in aqueous solution and solid samples. This, however, resulted only in some strong and broad fluorescence bands as shown in Figure 2, parts b and c. It also can be seen in Figure 2, parts a and b, that there exists a mirror symmetry distribution between the absorption and (44) Bradley, P. G.; Kress, N.; Hornberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 7441.

SERS and SEF of Tris(bipyridine)ruthenium(II)

emission spectra of [Ru(bpy)3]2+ in aqueous solution. This typical mirror symmetry indicates that, for this molecule, the distributions of the vibration levels are similar for the ground states and excited states.45 According to the literature, the Raman spectra of [Ru(bpy)3]2+ in aqueous solution can be obtained using the excitation line at 457.9 nm, which is at the center of the MLCT resonant band,46-48 and also other excitation lines such as 350.6 nm49 and 1064 nm.47 The MLCT resonance Raman studies are mainly concerned with the excited-state MLCT electron density. They further proposed that the MLCT electron density in the excited state is localized on one of the three bipyridine ligands on the vibrational time scale.44,47-51 The Raman band assignment of [Ru(bpy)3]2+ has been made clearly based on their experimental results46,52 and the calculation.51 In the present study it is likely that the fluorescence channel works much more efficiently than the Raman channel at the wavelength around 532 nm. The very weak Raman signal could be buried as background noise in the huge fluorescence signal. The SERS result of [Ru(bpy)3]2+ on silver nanoparticles (Figure 2d) shows dramatically the difference with the Raman spectra of its solution and solid states. The silver nanoparticles with the optimal size and aggregation form can bring out strong surface plasmon resonance, leading to a strong local EM field for the SERS effect. With the use of the confocal Raman microscope, a 532 nm laser of about 2 µW brought out the SERS intensity of about 100 000 counts/s. This extremely high SERS activity indicates that this probe molecule could very well be adapted in our near-field experiments. It can be clearly seen from Figure 2d that the intensity of fluorescence signal has decreased significantly. This demonstrates a strong advantage of SERS in that the highly SERS-active substrate can quench the fluorescence signal remarkably, leading to significantly enhanced Raman signal. The most probably explanation for the phenomenon is that the substrate could have induced the effective transfer of energy from the adsorbed molecule to the metal.36 Such energy transfer process blocks the fluorescence channel but opens up the SERS or surface-enhanced resonance Raman scattering (SERRS) channel. It is important to point out that although both SEF and SERS share the common EM enhancement mechanism,35 the SERS and SEF effects actually have opposite distance dependency on the nanostructured surface. SERS and SERRS require the molecule to contact with or very close to the metal surface, whereas SEF needs a certain separation in distance from the surface in order to prevent signal quenching.36,38 2. Near-Field SERS and SEF Spectra of [Ru(bpy)3]2+ After obtaining the very strong and stable SERS signal of [Ru(bpy)3]2+ in the far-field mode, we then tried to detect its SNOM-SERS signal with the cantilever tip which has the higher throughput compared with the fiber tip. As the tip delivers the laser in the near-field mode (Figure 1b), the spectroscopic information can be obtained with the high spatial resolution of about 80 nm from specific local areas. (45) Turro, N. J.; Ramamurthy, V.; Scaiano, J. Modern Molecular Photochemistry of Organic Molecules; Benjamin/Cummings Publishing Company: Menlo Park, CA, 2006; p 628. (46) Basu, A.; Gafney, H. D.; Strekas, T. C. Inorg. Chem. 1982, 21, 2231. (47) Srnova-Sloufova, I.; Vlckova, B.; Snoeck, T. L.; Stufkens, D. J.; Matejka, P. Inorg. Chem. 2000, 39, 3551. (48) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1979, 101, 4391. (49) Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981, 81, 42. (50) Orman, L. K.; Hopkins, J. B. Chem. Phys. Lett. 1988, 149, 375. (51) Strommen, D. P.; Mallick, P. K.; Danzer, G. D.; Lumpkin, R. S.; Kincaid, J. R. J. Phys. Chem. 1990, 94, 1357. (52) Mallick, P. K.; Danzer, G. D.; Strommen, D. P.; Kincaid, J. R. J. Phys. Chem. 1988, 92, 5628.

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Figure 3. Near-field SERS spectra obtained in various spots on the sample surface show three different near-field spectra of (a) SERS, (b) SEF, and (c) SERS combined with SEF.

Depending on the probed spot of the surface, we could obtain three different near-field spectra, i.e., SERS, SEF, and SERS combined with SEF, from various spots as presented in Figure 3a-c, respectively. It illustrates that depending on the specific spot probed, the Raman and fluorescence processes can be enhanced individually or simultaneously, and therefore the distribution of SERS and SEF hot spots are not totally correlated. It also demonstrates the advantage of near-field SERS over the far-field SERS with the higher spatial resolution, which can reveal the heterogeneous property of the SERS and SEF activities of the surface on the nanometer scale. With the improved spatial resolution, the near-field spectra are capable of reflecting the very local chemical information. It should be noted that the physical enhancements of SERS and SEF are both contributed by the EM mechanism,35 but the chemical influence on the SERS and SEF effect is dramatically different. As a consequence, the two effects are contributed by two kinds of molecules in different local environment, namely, SERS is interacting with the metal surface, whereas SEF should be separated from the surface. The near-field SERS signals are generally contributed by the molecules bonded on the metal surface, whereas the strong fluorescence of [Ru(bpy)3]2+ is mostly quenched by the silver nanoparticles and the aluminum coating at the end of tip working in the contact mode. The SEF signals might have come from two sources. One is the small amount of the physically adsorbed molecules which are separated from the nanoparticle surface by the first layer molecules. The second is from the molecules on the bare glass substrate but close to the particles as proposed by Anger et al. in the study on near-field SERS of R6G.24 When the two situations are satisfied in specific areas, both of the SERS and SEF signals became observable as shown in Figure 3c. In near-field SERS experiments, the fluorescence can be quenched not only by the metallic SERS substrate but also by the metallic-coated SNOM tip. For SNOM, both the fiber tip and silicon cantilever tip are coated with an aluminum layer at end of the tip to constrain light and define the aperture size. The aluminum layer could quench the excited states of the fluorescence molecules, making the fluorescence lifetime shorter and thus the intensity lower. This has been proved by the fact that quenching effect caused by the aluminum coating at smaller diameter aperture tips has been used to prevent further size reduction in fluorescence

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single-molecule detection.53 Evidence has also reported that the fluorescence lifetime is longer at the center of the probe tip than on the edge54 since the aperture is located in the center. Overall, the quenching effect is caused by both the metal surface and the metal coating on tip end making it difficult to observe the fluorescence signal. 3. Near-Field SERS and SEF Images of [Ru(bpy)3]2+ Keeping in mind that the three different spectra are from different spots of the substrate, it is worthy to further apply the near-field imaging method for an overview of the SERS and SEF activities of the whole substrate. During the SNOM image measurement, the scanning cantilever probe actually performs dual functions. It acts first as a contact mode AFM to obtain the topographic image with a feedback control for the near-field measurement, and then second it provides near-field excitation source for acquiring the near-field spectral data with the transmission mode. The original 10 000 near-field spectral data are acquired in an 8 × 8 µm2 region composed of 100 × 100 points with the collection time of 3 s on each point. Through data manipulation, the near-field spectral data can be further separated into three different near-field spectroscopic images, i.e., transmission, SERS, and SEF images from the SNOM spectrum collected in each pixel of the CCD detector. This is an important advantage since the three spectroscopic images together with the topographic image can be obtained simultaneously within the same scanning process. It is especially helpful to correlate the three kinds of spectral features of each spot so that a detailed comparison and analysis can be made. In the present study we derived the spectral data collected from 10 000 pixels to the transmission image based on the peak which has the same wavelength with the incident laser which is composed by transmission laser and the elastic Rayleigh scattering, the SERS image from the Raman band at 1488 cm-1, and the SEF one from the fluorescence band ranged from 610-680 nm (Figure 4 b-d). In Figure 4e, the bands selected from spectra to derive the corresponding near-field images shown in Figure 4b-d are presented in different colors of green, yellow, and red. As the SNOM tip is in large cone angle of ca. 70°, the tip shape cannot be very sharp. As a result, the particles shown in the topography appear larger than the real size. However, the aperture diameter of the tip is considerably smaller. As a consequence, the resolution of images with the optical transmission mode (Figure 4b-d), excited by the output laser from the aperture of the tip, show the better spatial resolution. In the correlated topographic image (Figure 4a), the nanoparticles are absent in the dark areas, indicating that the Ag particles coated surface is not uniform at the nanometer scale. It was from these dark areas that the elastic photons can go through the glass substrate to the CCD to record the spectra. This makes these areas brighter in the transmission image, as shown in Figure 4b. The brightest area in the topographic image corresponds to the particle-aggregated area, where the transmitted signals are severely blocked by the dense aggregated particles. Therefore, the correlated areas in the near-field SERS and SEF images are very dark, as shown in Figure 4, parts c and d. It demonstrates the importance of nanoparticles coating since only with an adequate thickness of nanoparticle coating, e.g., one or submonolayer, the SERS and SEF images with the transition mode can be well recorded. (53) Trautmann, J.; Ambrose, W. P. Near-field optical imaging and spectroscopy of single molecules. In Single-Molecule Optical Detection, Imaging and Spectroscopy; Basche´, T., Moerner, W. E., Orrit, M., Wild, U. P., Eds.; VCH: Weinheim, 1997; p 191. (54) Xie, X. S.; Trautman, J. K. Annu. ReV. Phys. Chem. 1998, 49, 441.

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Figure 4. (a) Topographic image, (b) near-field transmission image, (c) near-field SERS image, and (d) near-field SEF image of [Ru(bpy)3]2+ on silver nanoparticles-coated substrate acquired in an 8 × 8 µm2 region composed of 100 × 100 points. (e) Four spectra obtained at various positions as marked by circle i, ii, iii, and iv, respectively.

It is important to ensure that, for each type of transmission spectrum, its image should be representative individually of the laser-wavelength light and Raman or fluorescence intensity distribution in the scanned area. Accordingly, for the near-field SEF image (Figure 4d), the fluorescence signal was collected in the range of 610-680 nm (corresponding to ca. 2400-4100 cm-1). As is confirmed in Figure 3, there was no obvious Raman band in this range so that the SERS signal does not make significant contribution to the SEF image, whereas for the nearfield SERS image (Figure 4c), the Raman intensity is subtracted from the fluorescence background in order to eliminate fluorescence contribution. To demonstrate that the contrast in the spectra-derived images results from signal intensity, three spectra are selected from three color contrast places in Figure 4c, marked individually with circles i, ii, and iii. As can be clearly seen from Figure 4e, the spectrum acquired from the brightest spot (circle i) is featured as the strongest SERS signal, the mildly bright spot (circle ii) corresponds to the common level SERS signals, and the dark spot (circle iii) represents the absence of SERS signal. The marked circle iv is an SEF hot spot. The corresponding spectrum shows an efficient enhanced fluorescence band, but no obvious Raman band is found. This is another evidence for the lack of correlation between SERS and SEF, although in many areas both SERS and SEF hot spots appear to show up together.

SERS and SEF of Tris(bipyridine)ruthenium(II)

As can be seen from the near-field SERS and SEF images, and a comparison among the topographic image and these images (Figure 4, parts a, c, and d), both the Raman and fluorescence intensities are much stronger in the areas coated with one or two monolayers of nanoparticles as compared to areas without nanoparticles. This indicates a strong enhancement of the Raman signal, along with a slight enhancement of the fluorescence signal by the silver nanoparticles. Since the fluorescence signal was only observed in the particles area, it proves that this signal is surface-enhanced. A closer look at the SERS image reveals that the SERS hot spots with the highest intensity are not round but anomalistic in shape. More interestingly, these hot spots are located not only in junctions between nanoparticles but also at some edges of the aggregated nanoparticles. The portion of the brightest spots at the edges is even higher than the aggregate center. By using aperture near-field SERS technique, Deckert and co-workers have observed that the SERS activity decreases slightly on top of the silver particles compared with their borders21 and the Raman enhancement only exists in the vicinity of metal island.17 Zhang et al. have proposed that the observed near-field SERS hot spots are not necessarily at the junctions between nanoparticles.7 In previously reported normal SNOM studies without molecule adsorption, the authors also found that only a fraction of the nanoparticles illustrate enhanced near-field optical signal. The signal intensities critically depend on the shape, size, and even position of the nanoparticles, since these are the important parameters which effectively boost the plasmon resonances with the incident light.55 Accordingly, the SERS hot spots are distributed randomly in the image. Moreover, one has to consider that, in the transmission mode, the scattered light could be partially blocked by the nanoparticles, especially those at the middle position of aggregates. The hot spots at the edge of nanoparticle aggregates, however, have relatively higher transmission efficiency through the glass substrate to the CCD detector. As has been shown in Figure 4, parts c and d, and also discussed in the last section and shown in Figure 3, the SERS and SEF signal distribution are not overlapped. Furthermore, it can be seen that there are more SERS than SEF hot spots. These phenomena can be explained by the better enhancement efficiency for SERS and the quench effects by both the metallic substrate and the coated tip for the SEF signal.56,57 Furthermore, since the enhanced fluorescence intensity is proportional to the square of the EM field intensity and the enhanced Raman intensity is to the fourth power of the local excitation field intensity,58 the Raman signals should be enhanced much more significantly than fluorescence signals under the same physical environment. Moreover, the majority of the molecules are in direct contact with the silver particle surface with the release of SERS signals. This makes it easier to observe the SERS hot spots because the fluorescence intensity is significantly reduced. 4. Differences of Near-Field and Far-Field SERS Spectra. In order to better understand the near-field SERS of [Ru(bpy)3]2+, we performed the vibrational analysis by comparing the spectra obtained in the near-field and far-field modes. Figures 5 and 6 clearly show that the near-field spectra differ significantly from the far-field spectra in both the frequency and the relative intensity of some major bands. (55) Wiederrecht, G. P. Appl. Phys. 2004, 28, 3. (56) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Science 1994, 265, 364. (57) Zhou, Y.; Chen, D.; Xia, A.; Huang, W. Wuli 2000, 29, 657. (58) Xu, H.; Wang, X.-H.; Persson, M. P.; Xu, H. Q.; Kall, M.; Johansson, P. Phys. ReV. Lett. 2004, 93, 243002–1.

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Figure 5. (a) Far-field SERS spectrum and (b-g) six near-field SERS spectra of [Ru(bpy)3]2+ on silver nanoparticles-coated substrate collected in different positions.

The first difference we observed is that the peak wavenumber in the near-field spectra is not as stable as those of the far-field spectra when several spectra were recorded in sequence. The six far-field spectra of [Ru(bpy)3]2+ collected in different spots are identical as shown in Figure 5a. Contrary to this the six nearfield spectra show random shift within ca. 5 cm-1 for all major bands as illustrated in Figure 5b-g. This interesting phenomenon could be explained as the presence of chemical heterogeneity in the nanometer scale. The large sample surface usually consists of many tiny areas with heterogeneous properties, but they can be ignored in the farfield measurement as the probe spot is much larger than the nanoscale. In the far-field Raman measurement, the depth and diameter of the focus cross section is in the order of several micrometers. Thus, the collected signal is in fact averaged chemical information over a large area. However, the surface heterogeneity can induce different spectral features in the nanoscale, which can be probed by the nearfield mode because of its higher spatial resolution. As illustrated in Figure 1b, the size of the excitation spot of the near-field Raman is about tens of nanometers, and thus the different molecular adsorption states within such small but irregular junctions could be the ones responsible for the random peak shift. To confirm that the random peak shift is a common phenomenon in the present system, we have carried out a statistical analysis of many spectra. The near-field SERS image (not shown) was scanned in an 8 × 8 µm2 region composed of 100 × 100 spectra with the collection time of 3 s for each one. Figure 6a presents the contour plot for a detailed analysis of the peak wavenumber discrepancy, which is composed of 500 spectra selected from 5 scanning lines in the middle of the near-field SERS image. The spectra are standing side by side along the y-axis and looked at from above as shown in Figure 6a. Its x-axis is the wavenumber. A corresponding averaged spectrum of total 10 000 spectra is shown in Figure 6b. For comparison, the farfield SERS image (not shown) was scanned in a 64 × 64 µm2 region composed of 64 × 64 spectra with the exposure time of 0.01 s for each spectrum. The contour plot presented in Figure 6c is selected from 5 scanning lines in the middle of the far-field SERS image composed of 320 spectra. And the averaged spectrum of all these far-field spectra is presented in Figure 6d. It can be seen clearly that the peak shift behavior is observable only in the near-field plot (Figure 6a). Moreover, the peak width of the averaged near-field SERS spectrum of 10 000 individual near-

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Figure 6. Near-field SERS image (not shown) was scanned in an 8 × 8 µm2 region composed of 100 × 100 spectra with the collection time of 3 s for each one. Panel a is contour plot composed of 500 near-field spectra (5 lines × 100 spectra/line) taken from 5 lines in the middle of the mentioned near-field SERS image. Panel b is the averaged near-field spectrum of all the spectra in the near-field image; for comparison, the far-field SERS image (not shown) was scanned in a 64 × 64 µm2 region composed of 64 × 64 spectra with the exposure time of 0.01 s for each spectrum. Panel c is contour plot composed of 320 far-field spectra (5 lines × 64 spectra/line) taken from 5 lines in the middle of the mentioned far-field SERS image. Panel d is the averaged far-field spectrum of all the spectra in the far-field image.

field spectra is considerably wider than that of the far-field, as illustrated in Figure 6, parts b and d. It provides another evidence for the peak shift of the near-field spectra. A comparison of the averaged spectra between the far-field and near-field reveals another difference, namely, the relative intensity of the 1609 cm-1 peak (right-most in wavenumber) to the nearest peak at around 1564 cm-1. These two peaks are assigned to the C-C stretching and C-N stretching, respectively.46,52 For the near-field spectrum, the intensity of the 1609 cm-1 peak is higher than the 1564 cm-1 peak, whereas the opposite is true for the far-field spectrum. It should be noted that for the averaged far-field SERS spectrum, the peak wavenumber and relative intensity are in good agreement with the previously reported resonance Raman spectrum46,48,52,59 and SERRS spectrum,47 with 457.9 nm excitation as the most effective wavelength for the MLCT process. It indicates that the far-field SERS spectrum presents a typical resonance enhancement Raman spectrum because the excitation line at 532 nm in our experiment is still within the tail of the MLCT resonant band. Surprisingly, for the averaged near-field SERS spectrum (Figure 6b), the relative intensity of the 1609 cm-1 peak accords well with the nonresonance Raman spectrum of [Ru(59) Poizat, O.; Sourisseau, C. J. Phys. Chem. 1984, 88, 3007.

(bpy)3]2+.47,59 A possible explanation could be that the nearfield mode carries some special optical characteristics such as the electric field gradient effect and the heterogeneous polarization of the output laser from the tip aperture. The light goes through a subwavelength aperture and forms a wavefront that is distinctively different from the plane wave applied in the farfield. On the basis of the Huygens’ principle, each of the point on the wavefront can be considered as a source of a new wavefront. Therefore, the polarization direction of the near-field light source is heterogeneous. This special excitation source in the near-field could excite some specific vibrational modes much more efficiently, leading to an abnormal change in the relative intensity. This assumption finds its support from the spectra shown in Figure 5b-g, in which the relative intensity of each spectrum is slightly different from the others, whereas for the far-field measurement, there is mainly the in-plane polarization of the incident light, which can enhance primarily vibrational modes.19 On the other hand, the randomly separated nanoparticles could derive a complicated and inhomogeneous local EM field distribution. This in turn could influence the Raman selection rule locally and thus the molecular vibrational property. To search for better explanations, a more systematic study on well-ordered and defined nanostructures and with well-controlled tip-sample distance would be needed.

SERS and SEF of Tris(bipyridine)ruthenium(II)

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enhancement, i.e., the charge-transfer mechanism, is applied to this system, the electron can be excited and then transferred from the silver nanoparticle to one of the ligands, thus forming a bpy- anion. This process is similar to the transfer of one electron from Ru2+ to bpy to form a bpy- by MLCT.51 Then, the Raman spectrum of bpy- would show the 1367 and 1426 cm-1 bands in selected spectra as we observed.

Conclusion

Figure 7. Comparison of the far-field SERS spectrum, near-field SERS spectrum, and IR spectrum of [Ru(bpy)3]2+ on silver nanoparticles-coated substrate.

The last issue we would like to address is the appearance of some infrared-active bands in many of the near-field SERS spectra. In the typical far-field SERRS spectrum of [Ru(bpy)3]2+, no additional peak exists between the two bands of 1326 and 1495 cm-1. But in the near-field spectrum, three extra peaks are observed, including two IR-active peaks at 1426 and 1441 cm-1, which correlate with the IR spectrum in ref 59 and a non-IR-active peak at 1367 cm-1 as shown in Figure 7. The 1367 cm-1 peak is assigned to C-C stretching vibration of the cross rings, and the 1426 and 1441 cm-1 peaks are both assigned to C-H in-plane bending vibration.51 Both physical and chemical pictures are employed to explain this phenomenon. About 13 years ago, Hallen and co-workers observed some IRactive peaks in a near-field Raman spectrum of KTiOPO4 and explained this phenomenon by electric field gradient effects.4,18,19 On the basis of the Bethe-Bouwkamp model calculations,2 the z-component of electric field is only detectable in the near-field by SNOM probe due to its heterogeneous polarization character. In the system, the IR-active mode is not Raman-allowed in farfield Raman measurement because it requires light polarized in the z-direction to excite, and such an electric field is only available when the probing light source is extremely close to the surface.19 This physical picture on the electric field gradient effect in the near-field can also be applied to our results. The two IR-active bands could be activated with the z-component electric field, which is only generated in the near-field, and be satisfied with the Raman selection rule. Only when the probe is very close to the surface, where the dye molecules are excited as they fall into the range of the electric field polarized in the z-direction, could then the IRactive mode vibration signal be observed in the near-field Raman spectra. However, the strength of this activation is strongly dependent on the tip-sample distance,18 and that is why these two IR-active peaks do not appear in all the near-field SERS spectra. Another possible explanation based on the chemical picture is that the 1367 and 1426 cm-1 bands belong to the excited-state Raman spectrum of 2,2′-bipyridine- (bpy-) in [Ru(bpy)3]+. In the previous 3MLCT excited-state study of [Ru(bpy)3]2+ based on timeresolved resonance Raman, two bands at 1365 and 1427 cm-1 are well correlated with our observation.51 When the chemical

In order to overcome the problem of the extremely weak surface Raman signal in the near-field mode, we have developed in this study the cantilever SNOM tip which has much higher output of laser power than the conventional SNOM tip. Together with the optimization of experimental conditions including the application of silver nanoparticles-coated substrate to enhance SERS activity, we were able to obtain high-quality near-field SERS spectra and images of [Ru(bpy)3]2+. On the basis of the developed technique, we have carried out a systematic study to obtain traditional nearfield transmission, SERS, and SEF images simultaneously and to correlate directly the near-field SERS and SEF hot spots. On the basis of the comparison of near-field spectra obtained in different points, and the comparison between the near-field SERS and SEF images, it can be concluded that the distribution of SERS and SEF hot spots are not totally correlated. We observed that, in the same scanned area, there are more SERS than SEF hot spots. This phenomenon could be explained by a combination of several factors including the different enhancement efficiencies existing for SERS and SEF, the quenching effects for fluorescence by the metal substrate and metal-coated tip, and the presence of different numbers of probing molecules in SERS and SEF. It is of interest that the near-field and far-field SERS spectra demonstrate several distinctively different spectral features. The peak wavenumber in the near-field spectra is not as stable as those of the far-field spectra. This could be mainly due to the different molecular adsorption states within the small but irregular nanoparticle junctions. When excited by a 532 nm laser, the far-field SERS spectrum represents a typical resonance Raman spectrum, whereas the near-field SERS spectra behaves like the far-field nonresonance Raman spectrum. We tried to figure out this abnormal phenomenon by the electric field gradient effect in the near-field and the heterogeneous polarization of the output laser from the tip aperture. The observation of some extra bands including the IR-active bands in the near-field SERS was accounted for also by the electric field gradient effect and the existence of excited-state bpy- anion. It is necessary to point out that our near-field SERS study based on the aperture SNOM was rather difficult and preliminary as the vibrational spectroscopic measurement down to 100 nm makes the synergetic optimization of many experimental factors be essentially crucial. However, these fingerprint spectra coming strictly from the near-field region can provide some new and rich vibrational information of the interfacial structure at the nanoscale, which was difficult to achieve before. The approach along this avenue could be helpful for revealing the complicated near-field nature on many aspects which have been far beyond our present knowledge. Acknowledgment. Financial support from the Natural Science Foundation of China (20433040) and Ministry of Science and Technology (973 Program Nos. 2007CB815303 and 2007CB935603) is gratefully acknowledged. The authors thank Professor Andreas Otto and De-Yin Wu for helpful discussion. LA801376P