© Copyright 2002 American Chemical Society
APRIL 16, 2002 VOLUME 18, NUMBER 8
Letters A Simple Chemical Method for the Preparation of Silver Surfaces for Efficient SERS Y. Saito, J. J. Wang, D. A. Smith,* and D. N. Batchelder Department of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, U.K. Received October 15, 2001. In Final Form: February 5, 2002 The relatively well-known mirror reaction method of forming silver films has been used to produce substrates for surface enhanced Raman scattering (SERS). The simple and convenient method produces a thin metal film on silicon that shows an order of magnitude superior surface enhancement properties when compared to a conventional SERS substrate made by vacuum evaporation. Of particular interest is that the method is ideal for coating atomic force microscope probes for apertureless scanning near-field spectroscopy which is usually made difficult by the damage caused by evaporative coating and annealing.
Silver is one of the best materials for making surface enhanced Raman scattering (SERS) active surfaces since the behavior of its dielectric constant near the Fro¨hlich frequency gives rise to an intense surface plasmon absorption in the visible wavelength region. Consequently, a great deal of effort has been put into perfecting methods of forming silver surfaces to obtain strong SERS signals.1-8 Vacuum evaporation of silver is widely used to form SERS active surfaces on glass and silicon, but the metal islands formed by this method are small even after using a high
evaporation rate and a subsequent annealing process is needed to form the large (∼100-200 nm diameter) round islands that have been shown to produce the best SERS enhancement. Recently there has been considerable interest in the use of near-field probes to perform spectroscopy with very high spatial resolution;9-15 apertureless metal probes have found favor because of their relative ease of fabrication, high spatial resolution only limited by the size of the tip apex, and strong near-field effects.16-20 Preparation of such
* To whom correspondence should be addressed: phy6das@ phys-irc.novell.leeds.ac.uk; Fax +44 113 233 3900.
(9) Jahncke, C. L.; Paesler, M. A.; Hallen, H. D. Appl. Phys. Lett. 1995, 67, 2483-2485. (10) Sa´nchez, E. J.; Novotny, L.; Holtom, G. R.; Xie, X. S. J. Phys. Chem. A 1997, 101, 7019-7023. (11) Goetz, M.; Drews, D.; Zahn, D. R. T.; Wannemacher, R. J. Lumin. 1998, 76, 306-309. (12) Webster, S.; Smith, D. A.; Batchelder, D. N. Vib. Spectrosc. 1998, 18, 51-59. (13) Vickery, S. A., Dunn, R. C. Biophys. J. 1999, 76, 1812-1818. (14) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Curr. Sci. 1999, 77, 915-924 (15) Kwak, E. S.; Kang, T. J.; Bout, D. A. V. Anal. Chem. 2001, 73, 3257-3262. (16) Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. Appl. Phys. Lett. 1998, 73, 1469-1471. (17) Sa´nchez, E. J.; Novotny, L.; Xie, X. S. Phys. Rev. Lett. 1999, 82, 4014-4017. (18) Hayazawa, N.; Inoue, Y.; Sekkat, Z.; Kawata, S. Opt. Commun. 2000, 183, 333-336.
(1) Tsang, J. C.; Demuth, J. E.; Sanda, P. N.; Kirtley, J. R. Chem. Phys. Lett. 1980, 76, 54-57. (2) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553-1558. (3) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1632. (4) Vogel, E.; Kiefer, W.; Deckert, V.; Zeisel, D. J. Raman Spectrosc. 1998, 29, 693-702. (5) Zhang, J.; Zhao, J.; He, H.; Zhang, H.; Li, H.; Liu, Z. Langmuir 1998, 14, 5521-5525. (6) Sto¨ckle, R. M.; Deckert, V.; Fokas, C.; Zenobi, R. Appl. Spectrosc. 2000, 54, 1577-1583. (7) Maoz, R.; Frydman, E.; Cohen, R.; Sagiv, J. Adv. Mater. 2000, 12, 424. (8) Kobayashi, Y.; Salgueirin˜o-Maceira, V.; M.Liz-Marza´n, L. Chem. Matter 2001, 13, 1630-1633.
10.1021/la011554y CCC: $22.00 © 2002 American Chemical Society Published on Web 03/19/2002
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Figure 1. (a) Tapping mode AFM image of the silver surface made by evaporation. (b) Cross section showing dimensions of the surface features along the line indicated in the AFM image.
probes typically involves vacuum evaporation of the metal of interest onto a standard atomic force microscope (AFM) cantilever and tip. This method is not ideal since the cantilevers often deform due to heating during evaporation and cannot be annealed to improve the quality of the metal films without damage. The method we present here results in a coating comprising large colloid-like metal particles suitable for SERS without the need for an annealing process. We believe it will be very useful for the preparation of SERS substrates and near-field tips for local probe enhanced spectroscopies. A thin silver film may be formed on glass by a process known as the mirror reaction. Formation of a silver surface on silicon, essential if AFM probes are to be coated, is more difficult, but under the correct experimental conditions a uniform metal surface can also be formed on this substrate. Formation of a silver surface on silicon proceeded as follows. To 3 mL of 2 wt % silver nitrate aqueous solution was added 3.2 wt % KOH solution until a fine brown precipitate of Ag2O was formed. To this mixture 30 wt % ammonia solution was added drop by drop until the precipitate completely dissolved to form [Ag(NH3)2]+. Then 6 wt % silver nitrate solution was added until the solution became pale brown/yellow. After 1 drop of 6 wt % ammonium was added, the solution became transparent again. This [Ag(NH3)2]+ solution was mixed with 1 mL of 35 wt % glucose solution and 0.5 mL of methanol. The solution went black. A clean silicon wafer was immersed in this solution and left for approximately 1 h under 35 °C, after which it was removed and rinsed several times with distilled water. Plasma cleaning was done afterward. (19) Anderson, M. S. Appl. Phys. Lett. 2000, 76, 3130-3132. (20) Sto¨ckle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Chem. Phys. Lett. 2000, 318, 131-136.
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Figure 2. (a) Tapping mode AFM image of the silver surface made by the mirror reaction. (b) Cross section showing dimensions of the surface features along the line indicated in the AFM image.
The silicon wafer was cleaned in an acid solution (3 parts of concentrated H2SO4 was added to 1 part of a 30 wt % H2O2 solution in water). The morphology of the resulting silver surface and the SERS signal from rhodamine 6G were compared with those of a silver surface prepared by vacuum evaporation, with and without subsequent annealing. Evaporation was performed at a pressure of 1 × 10-5 mbar at a rate of 0.1 nm/s. The thickness of the evaporated silver layer was 100 nm as measured by a crystal film balance. Annealing was carried out at 200 °C under atmospheric pressure for 1 h. Tapping mode AFM imaging was performed on a Digital Instruments Multimode AFM controlled by Nanoscope IIIa electronics (cantilever resonant frequency ∼300 kHz, 1 Hz scan rate, 3 µm × 3 µm image, Olympus OTESP tips). Raman spectra were taken using a Renishaw Raman microscope system 2000 using 488 and 780 nm excitation, a 50×/0.75 NA objective lens, 1 mW laser power at the sample, and 10 s accumulation time. The rhodamine 6G sample was prepared from a solution in methanol by placing a drop on each of the metal film samples and allowing the solvent to evaporate under ambient conditions. Figure 1a. shows the morphology of the evaporated silver surface; the roughness is highlighted in the cross section in Figure 1b. The silver islands have typical diameters of about 30 nm and heights of around 5 nm. Annealing increased the size of the silver islands to typically 60 nm in diameter and 10 nm high (data not shown). Figure 2a. shows an AFM image of the silver surface prepared by the mirror reaction; Figure 2b is a cross section of this surface. In contrast with the evaporated film, here the metal islands are about 100-200 nm both in lateral size and in height.
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Figure 3. SERRS spectra of rhodamine 6G on the silver surfaces prepared by evaporation and the mirror reaction. The spectra have been shifted on the intensity axis for clarity: (a) 10-6 mol/L; (b) 10-9 mol/L.
Figure 3a shows typical SERRS (surface enhanced resonance Raman scattering) spectra of rhodamine 6G in 10-6 mol/L concentration from the three different substrates with 488 nm excitation. A SERRS signal was observed in each case, but the vacuum evaporated silver surface gave a very small enhancement compared with the surface made by the mirror reaction which was over an order of magnitude more efficient. The annealed surface prepared by evaporation gave a better enhancement than the nonannealed substrate but was still very small compared to the surface prepared by mirror reaction. Figure 3b shows SERRS spectra of rhodamine 6G in 10-9 mol/L concentration with 488 nm excitation. We can see Raman bands from the surface made by the mirror reaction, but no clear Raman band has been observed from vacuum evaporated silver surface. With 780 nm excitation, the surface made by the mirror reaction still showed some enhancement, whereas no SERS signal could be detected from the surface made by evaporation.
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The principal reason for the large enhancement of the Raman signal in the case of the silver substrate prepared by the mirror reaction is the size of the silver particles formed. When a molecule is placed near the surface of a metal particle, the incident and scattered field strength experienced by the molecule can be dramatically altered by a coupling of excitations in the metallic surface such as surface plasmons. This is one of the dominant mechanisms of SERS. As the size of the metal particles become smaller compared with the mean free path of the electrons in the metal, electrons experience additional losses due to surface scattering. This leads to an increase of the imaginary part of the dielectric function and thereby the plasmon absorption peak is broadened and decreased.21 On the other hand, as the particle size increases toward the bulk, higher order surface modes become dominant and this causes a shift of the absorption maximum toward lower energies, a broadening of the absorption band, and a decrease in the peak intensity. Consequently, there should be an optimum particle radius, which is a function of the excitation wavelength, to obtain a strong, sharp plasmon absorption peak.21 Experimental results have suggested that the optimum particle size is somewhat larger than that predicted by theory. According to the work of Nie and Emory, the colloidal particles which provide the greatest SERS enhancement in the region wavelength around 500 nm were of the order 100 nm in diameter.22 As Figure 2b shows, the particle size created by the mirror reaction process is between 100 and 200 nm and therefore highly suitable for visible SERS. It should be noted that the shape of the metal particle also has a strong effect on the SERS mechanism, which at present is not a parameter that can be controlled in the mirror reaction process, but it may be possible to tailor the shape with further work. Brus et al. showed that a particularly strong SERS enhancement was created between two metal colloids, a so-called “hot spot”.23,24 Localized plasmon modes, created by strong electromagnetic coupling between touching metallic objects, are known to dominate the SERS response.25 This suggests that the collective morphology of the metal surface, possibly including factors such as an alignment of particles or a curvature of the colloidal islands, is also important. The surface prepared by the mirror reaction is a collection of colloidal particles, a morphology which should provide a high number of good hot spots for surface enhancement of Raman scattered light. In summary, we have presented a simple chemical method to prepare silver surfaces which show significantly greater surface enhanced Raman scattering signals than the surface made by the conventional method. This method is particularly suitable for the preparation of metalized nanoscale probes for near-field spectroscopic imaging. LA011554Y (21) Bohren, C. F. Absorption and Scattering of Light by Small Particles; Wiley: 1983; Chapter 4, Chapter 12. (22) Nie, S.; Emory, R. S. Science 1997, 275, 1102-1106. (23) Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965-11971. (24) Gersten, J. I.; Nitzan, A. Surf. Sci. 1985, 158, 165-189. (25) Garcı´a-Vidal, F. J.; Pendry, J. B. Phys. Rev. Lett. 1996, 77, 11631166.
