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Simple Chemical Method for Forming Silver Surfaces with Controlled Grain Sizes for Surface Plasmon Experiments Y. Saito, J. J. Wang, D. N. Batchelder, and D. A. Smith* Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom Received March 26, 2003. In Final Form: June 4, 2003 We report on how the mirror reaction method can be used to prepare silver films with well controlled grain sizes in the range that is known to be ideal for surface plasmon experiments in the visible region of the spectrum. We characterize the morphology of the silver films by atomic force microscopy and their optical properties by UV-vis absorption spectroscopy, comparing these results with the predictions of classical electromagnetism. The colloid particle size can easily be controlled to lie in the range 50-200 nm, and their shape is approximately spherical with aspect ratios (lateral dimensions to height) in the range 1:1 to 1:2. These dimensions and morphology are ideal for surface plasmon based techniques such as surface enhanced Raman spectroscopy (SERS) and surface plasmon resonance (SPR). The method also has the added advantage that it can be used in applications where conventional coating by evaporation can damage the substrate due to heating. We give one such examplesthe coating of an atomic force microscope probe for apertureless scanning near-field optical microscopy. We also report a simple, gentle, and highly effective method for cleaning these metal surfaces to completely eliminate the background contaminant signal that is typically present in SERS spectra from such surfaces.
Introduction Recently there has been considerable interest in making use of surface plasmon absorption of metals for sensitive techniques such as surface enhanced Raman spectroscopy (SERS)1 and apertureless near-field optical microscopy (ASNOM).2-8 The SERS technique in part uses the enhancement of the optical electric field at the surface of a metal colloidal substrate to enhance the Raman scattered signal. The efficiency of the SERS enhancement is known to be very sensitive to the morphology of the metal film. Indeed, with some silver surfaces, it has been possible to record single molecule SERS spectra.9 In ASNOM, a metalcoated solid probe, such as an atomic force microscope (AFM) tip, is used to scatter light from the near-field of a sample and thus overcome the diffraction limited resolution of far-field optics. The apertureless approach to near-field imaging avoids the difficult process of fabricating an aperture at the apex of an extruded optical fiber that is typically used in SNOM. ASNOM also provides the potential for surface plasmon enhancement of the electric field at the tip and therefore an improvement in the signal-to-noise ratio. Silver colloidal surfaces are ideal for surface plasmon generation, since the behavior of the dielectric constant near the Fro¨hlich frequency gives rise to an intense * To whom correspondence should be addressed. E-mail:
[email protected]. Fax +44 113 343 3900. (1) Schatz, G. C.; Van Duyne, R. P. Handbook of vibrational spectroscopy; 2002. (2) Lavers, C. R.; Harris, R. D.; Hao, S.; Wilkinson, J. S.; O’Dwyer, K.; Brust, M.; Schiffrin, D. J. J. Electroanal. Chem. 1995, 387, 11-22. (3) Andersen, P. C.; Rowlen, K. L. Appl. Spectrosc. 2002, 56, 124A135A. (4) Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. Appl. Phys. Lett. 1998, 73, 1469-1471. (5) Sa´nchez, E. J.; Novotny, L.; Xie, X. S. Phys. Rev. Lett. 1999, 82, 4014-4017. (6) Hayazawa, N.; Inoue, Y.; Sekkat, Z.; Kawata, S. Opt. Commun. 2000, 183, 333-336. (7) Anderson, M. S. Appl. Phys. Lett. 2000, 76, 3130-3132. (8) Sto¨ckle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Chem. Phys. Lett. 2000, 318, 131-136. (9) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. P.; Feld, M. S. Curr. Sci. 1999, 77, 915-925.
absorption in the visible region of the spectrum. An ideal silver surface is a stable and well packed film of metal particles in which the size and shape of the silver particles can be readily controlled. Achieving this control is, however, not straightforward, and consequently, a great deal of effort has been put into developing metal films for these applications in recent years.10-28 Vacuum evaporation is widely used to form metal surfaces, but the metal islands that are produced by this method are small (of the order tens of nanometers in diameter), even if a high evaporation rate is used. It has been shown that larger islands of the order 100 nm in diameter produce the best surface plasmon absorption at visible wavelengths.10,19,22 To achieve islands of this size (10) Schlegel, V. L.; Cotton, T. M. Anal. Chem. 1991, 63, 241-247. (11) Lu, Y.; Xue, G.; Dong, J. Appl. Surf. Sci. 1993, 68, 485-489. (12) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553-1558. (13) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-753. (14) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A. Langmuir 1996, 12, 1172-1179. (15) Weissenbacher, N.; Gobel, R.; Kellner, R. Vib. Spectrosc. 1996, 12, 189-195. (16) Norrod, K. L.; Sudnik, L. M.; Rousell, D.; Rowlen, K. L. Appl. Spectrosc. 1997, 51, 994-1001. (17) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695-5701. (18) Vogel, E.; Kiefer, W.; Deckert, V.; Zeisel, D. J. Raman Spectrosc. 1998, 29, 693-702. (19) Zhang, J.; Zhao, J.; He, H.; Zhang, H.; Li, H.; Liu, Z. Langmuir 1998, 14, 5521-5525. (20) Sauer, G.; Nickel, U.; Schneider, S. J. Raman Spectrosc. 2000, 31, 359-363. (21) Maoz, R.; Frydman, E.; Cohen, R.; Sajiv, J. Adv. Mater. 2000, 12, 424. (22) Sto¨ckle, R. M.; Deckert, V.; Fokas, C.; Zenobi, R. Appl. Spectrosc. 2000, 54, 1577-1583. (23) Kobayashi, Y.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, M. L. Chem. Mater 2001, 13, 1630-1633. (24) Nie, C. S.; Feng, Z. R. Appl. Spectrosc. 2002, 56, 300-305. (25) Saito, Y.; Wang, J. J.; Smith, D. A.; Batchelder, D. N. Langmuir 2002, 18, 2959-2961. (26) Hu, J. W.; Zhao, B.; Xu, W. Q.; Fan, Y. G.; Li, B. F.; Ozaki Y. J. Phys. Chem. B 2002, 106, 6500-6506. (27) Leverette, C. L.; Shubert, V. A.; Wade, T. L.; Varazo, K.; Dluhy, R. A. J. Phys. Chem. B 2002, 106, 8747-8755. (28) Nie, S.; Emory, S. Science 1997, 275, 1102-1106.
10.1021/la0301240 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/10/2003
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in evaporated films, a subsequent annealing process is required. The annealing process does not provide the control over the final particle size that is important to optimize the surface plasmon effect. In addition, evaporation and annealing can damage the substrate. For example, when a metal film is prepared on an AFM probe for ASNOM, scanning capacitance, or scanning electrochemical measurements, the AFM cantilever often deforms due to heating or surface tension in the metal coating. Sputtering is also a popular method of forming silver surfaces, but this confines grain sizes to a rather small rangesusually
