0 Copyright 1986 American Chemical Society
NOVEMBER/DECEMBER, 1986 VOLUME 2, NUMBER 6
Articles Distance Dependence of SERS Enhancement Factor from Langmuir-Blodgett Monolayers on Metal Island Films: Evidence for the Electromagnetic Mechanism G. J. Kovacs,* R. 0. Loutfy, and P. S. Vincett Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K 2L1
Carol Jennings and Ricardo Aroca Department of Chemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4 Received March 31, 1986. I n Final Form: July 2, 1986 Langmuir-Blodgett (LB) films of arachidic acid have been used as spacer layers to control the separation distance between evaporated In and Ag metal island films and LB monolayers of a lightly substituted phthalocyanine. From relative intensity measurements, the distance dependence of the surface-enhanced Raman scattering (SERS) enhancement factor for the phthalocyanine was determined and found to be long range. In addition the magnitude of the enhancement was determined by measurement of the normal Raman intensity of phthalocyanine monolayers deposited directly onto unmetalized substrates. The results are compared with calculations of the enhancement factor based on an electromagnetic field enhancement mechanism. Good quantitative agreement is found between calculation and experiment for both the distance dependence and magnitude of the enhancement. We conclude that our SERS from island films is due to the electromagnetic mechanism and rule out short-range chemical mechanisms. 1. Introduction air, porous films evaporated onto cold substrates in UHV5 (in situ studies), films evaporated onto roughened surfaces After the first detection of a Raman signal from an in tunnel junctions,6 and colloidal ~uspensions.~In adadsorbate on a silver electrode,’ it was subsequently found dition to Ag surfaces, SERS has been observed from adthat the Raman scattering cross section of the adsorbed sorbates on C U ,A~ U ,In,loJ1 ~ Li,12 Na,13 and All4 surfaces. molecules could be enhanced, by up to several orders of magnitude, over that of the same molecules in s o l u t i ~ n . ~ , ~ Since then, a great deal of both experimental and theo(4) Chang, R. K., Furtak, T. E., Eds. Surface Enhanced Raman retical effort has been invested in studying this e f f e ~ t , ~ Scattering; Plenum: New York, 1982. known as surface-enhanced Raman scattering (SERS). In ( 5 ) Pockrand, I. Surface Enhanced Raman Vibrational Studies at SolidlGas Interfaces; Springer: Berlin, 1984. addition to electrochemically cycled surfaces, SERS has ( 6 ) Tsang, J. C.; Kirtley, J. R. Solid State Commun. 1979, 30, 617. been observed from adsorbates on silver surfaces (7) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., “roughened” in a number of other ways and in various Faraday Trans. 2 1979, 75, 790. (8) Pettinger, B.; Wetzel, H . Ber. Bunsenges. Phys. Chem. 1981,85, environments: These include evaporated island films in (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys.%ett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) Van Duyne, R. P. J . Phys. (Les Ulis, Paris) Suppl. 1977, C5, 239.
473. (9) Loo, B. H. J. Electroanal. Chem. 1982, 131, 381. (10) Jennings, C.; Aroca, R.; Hor, A. M.; Loutfy, R. 0. Anal. Chem. 1984, 56, 2033. (11)Aroca, R.; Jennings, C.; Kovacs, G. J.; Loutfy, R. 0.;Vincett, P. S . J . Phys. Chem. 1985,89, 4051. (12) Moskovits, M.; DiLella, D. P. in ref 4, p 243.
0 1986 American Chemical Society
690 Langmuir, Vol. 2, No. 6, 1986
These metals are also those which support surface plasmon excitation with visible light.’j SERS has also been reported from adsorbates on Cd,16Hg,” Ni,l* Pd,19 and Pt.20 The list of adsorbates21ranges from volatile gases such as O2 and N2,through small organic molecules such as pyridine and benzene, to large organic molecules such as phthalocyanine. The SERS phenomenon has attracted widespread attention; however, despite intensive interdisciplinary study, the fundamental mechanism of the enhancement remains very controversial?*5In general the proposed enhancement mechanisms are of two types: electromagnetic and chemical. The electromagnetic mechanisms are all based on electric field enhancements of the exciting laser light and Raman scattered light at rough metal The chemical mechanisms are varied and involve such phenomena as direct charge transfer between the adsorbate and electronic excitation of the metal with energy transfer to the adsorbate via special adatoms or defect and chemisorption-induced resonance Raman ~ c a t t e r i n g . The ~ ~ chemical mechanisms all require direct contact between the metal and adsorbate and are therefore necessarily short range. The electromagnetic mechanisms, on the other hand, can be relatively long range depending on the geometry of the system; in addition quantitative predictions of the enhancement factor as a function of the distance of the molecule from the metal surface are possible. It would therefore appear that the best way to distinguish between the two mechanisms and to determine which is operating in a particular system is to measure the distance dependence of the enhancement factor. Previous experimental work on the distance dependence of the SERS enhancement by Murray et a1.35-37was done
(13) Watanabe, T.; Pettinger, B. Chem. Phys. Lett. 1982, 89, 501. (14) Lopez-Rios, T.; Pettenkofer, C.; Pockrand, I.; Otto, A. Surf. Sci. 1982, 121, L541. (15) Kovacs, G. J. Thin Solid Films 1979, 60, 33. (16) Loo, B. H. J . Chem. Phys. 1981, 75, 5955. (17) Naaman, R.; Buelow, S. J.; Cheshnovsky, 0.;Herschbach, D. R. J . Phys. Chem. 1980,84, 2692. (18) Yamada, H.; Yamamoto, Y. Chem. Phys. Lett. 1980, 77, 520. (19) Yamada, H.; Yamamoto, Y.: Tani, T. Chem. Phys. Lett. 1982,86, 397. (20) Krasser, W.; Renouprez, A. J. Solid State Commun. 1982,41,231. (21) Seki, H. J. Electron. Spectrosc. Relat. Phenom. 1983, 30, 287. (22) Moskovits, M. J . Chem. Phys. 1978, 69, 4159. (23) McCall, S. L.; Platzman, P. M.: Wolff, P. A. Phys. Lett. 1980, 77A, 381. (24) Bergman, J. G.; Chemla, D. S.: Liao, P. F.; Glass, A. M.; Pinczuk, A,; Hart, R. M.; Olson, D. H. Opt. Lett. 1981, 6 , 33. (25) Laor, U.; Schatz, G. C. J . Chem. Phys. 1982, 76, 2888. (26) (a) Gersten, J.; Nitzan, A. J. Chem. Phys. 1980, 73, 3023. (b) Gersten, J.; Nitzan, A. Surf. Sci. 1985, 158, 165. (27) Arya, K.; Zeyher, R.; Maradudin, A. A. Solid State Commun. 1982, 42, 461. (28) Metiu, H.; Das, P. Annu. Reu. Phys. Chem. 1984, 35, 507. (29) Aussenegg, F. R.; Lippitch, M. E. Chem. Phys. Lett. 1978,59,214. (30) Ueba, H. Surf. Sci. 1983, 131, 347. (31) Irish, D. E.; Guzonas, D.; Atkinson, G. F. Surf. Sci. 1985,158, 314. (32) Ertiirk, U.; Pockrand, I.: Otto, A. Surf. Sci. 1983, 131, 367. (33) Burstein, E.; Chen, Y. C.; Chen, C. Y.: Lundquist, S.; Tosatti, E. Solid State Commun. 1979, 29, 565. (34) Philpott, M. R. J. Chem. Phys. 1975, 62, 1812. (35) Murray, C. A.; Allara, D. L.: Rhinewine, M. Phys. Reu. Lett. 1981, 46, 57. (36) Murray, C. A.; Allara, D. L. J . Chem. Phys. 1982, 76, 1290. (37) Murray, C. A,; Allara, D. L.; Hebard, A. F.; Padden, F. J.,Jr. Surf. Sci. 1982, 119, 449.
Kovacs et al. by using a rather complex geometry. They used evaporated CaF2 roughening layers beneath evaporated Al-A1203 supporting layers for dip-coated adsorbate and spin-coated spacer layers. The Ag enhancing layer was then vacuumdeposited as the final layer. The highly convoluted valleys produced by the CaF2underlayers resulted% in rather large variations in the separation distance between the Ag surface and the Raman scattering adsorbate. The extremely complex morphology of the Ag films was thoughts6 to produce variations in the enhancements of individual adsorbate molecules, depending on their position at the top or sides of a Ag protusion, by a factor of -lo4 (the “local hot spot” model). In addition, since the Ag layer was deposited last, it was believed36that there was variation in the Ag film morphology (on which the enhancement factor is strongly dependent) from sample to sample as the spacer layer thickness was changed. Moreover, since the adsorbates were deposited on an A1 film, there is also the possibility of some SERS enhancement from the A1 layer.14 Finally, the SERS enhancement factor was not measured directly but was estimated, as are virtually all reported SERS enhancement factors. This procedure involved measuring SERS and solution or bulk Raman intensities, allowing for various intensit,y losses in the multilayer system, estimating the number of molecules involved in the SERS and bulk scattering with the attendant geometrical differences in the two set-ups, and normalizing to produce an absolute enhancement factor. Therefore, with the above limitations only a qualitative interpretation of the data was attempted,35 with the conclusion that the major part of the enhancement was electromagnetic in nature. While this work was very valuable and demonstrated the importance of the electromagnetic mechanism, we have attempted to overcome many of its shortcomings and difficulties in interpretation by studying an inherently simpler system. Our system consists of island films of Ag and In of highly reproducible structure deposited on glass substrates. These substrates are then coated with Langm ~ i r - B l o d g e t t ~(LB) ~ spacer layers of arachidic acid. These consist of an integral number of highly ordered monomolecular layers of arachidic acid with well-defined and controlled thickness. Finally, an LB monolayer of tetra-tert-butyl metal-free p h t h a l ~ c y a n i n e(, ~ ~- B U ) ~ H ~ P C , is deposited on top of the spacer layer. The Raman spectrum of the (t-Bu),H2Pc monolayer is then measured and the intensity determined with 0, 1 , 3 and 5 monolayer spacers of arachidic acid between it and the island film; the LB technique allows us to make the entire series of samples on a single glass slide, if desired. The unenhanced spectrum of one or more (t-Bu),H2Pc monolayer(s) on a bare glass slide is also measured under identical experimental conditions; this allows direct determination of the absolute enhancement factor. By this method we therefore seek to make a direct determination of the distance dependence of the enhancement factor, to make a quantitative comparison with the predictions of electromagnetic theory for a realistic model of our more tractable geometry, and thereby to determine whether electromagnetic or chemical mechanisms are appropriate for our island film system. An appropriate model for our island film system is that of a prolate spheroid on a conducting substrate, as developed by Gersten and Nitzan.26a Good quantitative agreement is found between experiment and electromag(38) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (39) Kovacs, G. J.;Vincett, P. S.; Sharp, J. H. Can. J . Phys. 1985, 63, 346.
Distance Dependence of SERS Enhancement Factor
Langmuir, Vol. 2, No. 6, 1986 691
Metal Island Film
/
,LB
hK
Film(sI
Reflected Seam
Lase. Beam
Figure 1. Schematic of experimental geometry used to measure Raman spectra.
netic theory, and no evidence is found for any short-range chemical enhancement mechanism in our system. (Recently, Gersten and NitzanZsbhave extended their earlier theory% of the SERS enhancement to include fluorescence cross sections, light scattering and emission yields, energy transfer between adsorbed molecules, and photochemical processes.) It should be noted that in an earlier paper" we reported SERS from an LB monolayer of (t-Bu),H,Pc. Raman scattering (RS)of multilayer LB films had previously been demonstrated resonance Raman scattering (RRS) of LB films (7 layers) of specially grafted prophyrins had been observed by a total internal reflection method,'O and RS of LB films of cadmium arachidate (28 monolayers) on gratings and rough metal surfaces had also been reported?I4 SERS of phthalocyanine (Pc) has also been studied in a number of other systems. These include evaporated metal-free Pc films45on evaporated Ag island films, evaporated copper and zinc phthalocyanine filmslo on evaporated Ag and In island films, metal-free,ms cobalt,'6" iron,'74g and coppers0 tetrasulfonated P c on Ag electrodes, iron Pc and iron tetrasulfonated Pc on Cu and Au electrodes,51 copper P c a t a Ag electrode,5, and evaporated copper Pc on evaporated Ag film^.^^,^^ Recently, surface-enhanced resonance Raman scattering (SERRS) of synthetic dyes and photosynthetic pigments in monolayer and multilayer assemblies was reporteds5 on Ag island (40) Vandewer, M.: Ruaudel-Teider, A.; Brehamet, L.; Lutz, M. Thin Solid Fdms 1983, 99, 41. (41) Knoll. W.;Rabe, J.; Philpott. M. R.;Swalen. J. D. Thin Solid F i l m 1983, 99, 173. (42) Philpott, M. R.; Girlando, A.; Golden, W. G.; Knoll, W.:Swalen, J. D.Mol. Cryst. Liq. Cryst. 1983, 96,335. (43) Knoll, W.; Philpott, M. R.: Golden. W. G. J. Chem. Phys. 1982,
..,
7, 91.3 1 . 1 .
(44) Knoll, W.; Philpott, M.R.: Swalen, J. D.: Girlando, A. J . Chem. Phys. 1982, 77.2254. 145) A m a , R.; Loutfy, R. 0. J . Roman Speetrose. 1982. 12. 262. (46) Simie-Glavaski,B.; Zeeevie, S.; Yeager. E. J . Phys. Chem. 1983, I77 -.. (47) Simic-Glavaski,B.; Zeeevie, S.; Yeager, E.J . Eleetroonol. Chem. d
