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Enhanced Raman Spectroscopy at a Nonmetallic Surface. 1. Spacer Layers of Alkyl Mercaptans on Silver Island Films Hem6 Bercegolty* and F. J. Boerio*$t Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012,and Saint-Gobain Recherche, BP 135-39quai Lucien Lefranc, 93303 Aubervilliers Ckdex, France Received April 25, 1994. I n Final Form: August 8, 1994@ Self-assembled monolayers of alkyl mercaptans were used as spacer layers on silver island films. N-Alkylmercaptans(€412,and 16C) yielded low-energy,hydrophobic upper surfacesbut carboxyl-terminated undecyl mercaptan gave a hydrophilic upper surface. Surface-enhanced Raman scattering (SERS) was observed from two different molecules deposited on the alkyl mercaptanslisland film system. One was p-nitrobenzoicacid (PNBA),a small molecule able to chemisorbon silver. The other, cobalt phthalocyanine (CoPC),was bigger and only physisorbed on the substrates. For every spacerlscatterer pair, special care was taken of the exact location of the molecule. The small molecule (PNBA)allowed characterization of the coverage of both types of spacer layers, hydrophobic and hydrophilic. PNBA could always penetrate into the alkyl chains, whatever the spacer. Substrates with hydrophobic and hydrophilic upper surfaces gave very different spectra after deposition of the bigger molecule (CoPC). When CoPC was located at about 15-20 A from silver, the SERS signal was about 10 times lower than when CoPC was adsorbed directly onto the metal. These results constitute a first measurement of the spatial extension of SERS with strong evidence that the scattering molecule was not on the metallic surface.
Introduction Ever since its discovery, surface-enhanced Raman scattering (SERS) has been the subject of a great deal of experimental and theoretical in~estigation.'-~Although some questions remain, it is now generally accepted that there are two mechanisms responsible for the enhancement. The first mechanism is related to coupling between the light wave and localized plasmons in asperities on the roughened surface of a metal. The electromagnetic field and, thus, the Raman scattering intensity are increased a t or near the surface by this purely electromagnetic process. The second mechanism is referred to as chemical enhancement and requires direct adsorption of a molecule onto the metallic surface. The process involved is probably a modification of the energy levels of the molecule due to charge transfer to or from the conduction electrons of the metal. Although the existence of chemical enhancement is not a matter of controversy any longer, the exact physical scheme is not clear. Nor is it clear how much the chemical mechanism contributes to the total enhancement. Only the electromagnetic term of the enhancement remains when a molecule is located away from the surface. Considering the roughness that is observed in TEM micrographs of metal island films, one expects the enhancement to decrease by a factor of about 10 in 50 A. However,the electromagnetic term can also show a quicker decrease from the surface, if one considers roughness a t a smaller scale than that observed with electron microscopes. The chemical contribution to the enhancement introduces a first layer effect or discontinuity between the enhancement of the first adsorbed monolayer and the
* Author to whom all correspondence should be addressed. + University of Cincinnati. Saint-Gobain Recherche. Abstract published in Advance A C S Abstracts, September 15,
*
following layers. However, the presence of a first-layer effect is only consistent with but does not imply the detection of a chemical enhancement. Several interesting applications of SERS have been reported. For example, Boerio et aL69' used SERS to investigate metal-organic interfaces relevant to adhesion. SERS is also of interest for catalysis8 and analytical ~ h e m i s t r y .In ~ all of these applications, SERS has been used to determine adsorption of molecular species onto the surface of a metal, such as silver, gold, or copper, which supports SERS. Recently we have been interested in using SERS to characterize interfaces between organic compounds and nonmetallic substrates. In order to carry out such investigations, a metallic substrate which supports SERS must be completely covered with a very thin film of the nonmetallic material, and the organic compound must be adsorbed onto the film of a nonmetallic material. Crucial questions then arise concerning the extent to which the metallic substrate is covered by the nonmetal and the way in which the SERS signal from the adsorbed molecules decays as a function of thickness of the nonmetallic film. Several experiments relevant to the coverage of SERSactive substrates and to the dependence of the enhancement on distance from the substrate have been reported. Murray et a1.I0 determined the intensity of Raman scattering byp-nitrobenzoic acid (PNBA) as a function of distance from a rough silver substrate by placing a poly(methyl methacrylate) (PMMA) spacer layer of varying thickness between PNBA and the substrate. They found a decrease of the enhancement by a factor of 10 in 30-50 A and interpreted their results in terms of the electromagnetic mechanism. However, it is now known that Murray and co-workers did not succeed in separating PNBA from the silver.
@
1994. (1)Seki, H. J . Electron. Spectrosc. Relat. Phenom. 1986, 39, 289. ( 2 )Cotton, T. M. In Spectroscopy ofSurfaces;Clark, R. J . H., Hester, R. E., Eds.; Wiley: New York, 1988; Chapter 3, p 91. (3) Creighton,J.A. In Spectroscopy ofSurfaces; Clark, R. J . H., Hester, R. E., Eds.; Wiley: New York, 1988; Chapter 2, p 37. (4) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (5) Otto,A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys Condens. Mat. 1992, 4 , 1143.
(6)Young, J.-T.; Tsai, W.-H.; Boerio, F. J. Macromolecules 1992,25, 887. (7)Young, J.-T.; Cave, N. G.; Boerio, F. J. J . Adhes. 1992,37, 143. (8)Stencel, J . M. Raman Spectroscopy for Catalysis; Van Nostrand Rheinhold: New York, 1990; Chapter 5 . (9) Garrell, R. L. Anal. Chem. 1989, 61, 401A. (10)Murray, C. A,; Allara,D. L.; Rhinewine, M.Phys. Rev. Lett. 1981, 46, 57.
0743-746319412410-3684$04.50/00 1994 American Chemical Society
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Figure 1. SERS spectra ofp-nitrobenzoic acid (PNBA) on bare silver island films collected at (A, Top) a low laser power, about 0.25 mW at the sample, and (B, Bottom) a high laser power, about 40 mW at the sample. In part A, a shoulder is visible at 1390 cm-l characteristicof carboxylate species made by chemisorptionof acid on silver. The 1355 cm-l peak is assigned to the symmetric stretching of nitro groups. It tends to disappear with the photolytic silver-catalyzedreaction of PNBA illustrated by part B. From A to B, new peaks appear. Among them, the strongest is the one at 1460 cm-'. PNBA adsorbs on silver as a carboxylate ion. The existence of the carboxylate is easily detected in the SERS spectrum by the presence of a shoulder near 1390 cm-l (see Figure 1A). If the peak is present, the scattering molecules are mostly on the surface of silver. If the peak is absent, the SERS signal probably comes from molecules that are not in contact with silver. PNBA also undergoes a chemical reaction when adsorbed onto silver and illuminated with intense laser light at a wavelength of about 500 nm.11J2 The nitro group of PNBA disappears, forms by and a product which is probably a~odibenzoatel~ the combination of two PNBA molecules. At a low laser power, this reaction is very slow and it is easy to obtain (11)Sun, S.;Birke, R. L.; Lombardi, J. R.; Leung, K. P.; Genack,A. Z.J . Phys. Chem. 1988,92,5965. (12)Roth, P. G.; Venkatachalam, R. S.;Boerio, F. J. J . Chem. Phys. 1986,85,1150. (13) Bercegol, H.; Boerio, F. J. To be published.
a spectrum of the unreacted molecule adsorbed on silver (Figure 1A). At higher laser power (Figure lB), bands characteristic ofthe nitro group, including an intense band near 1355 cm-', decrease in intensity and several new bands appear, including an intense band near 1460 cm-', which are characteristic of azo groups formed by the laserinduced, silver-catalyzed reduction of PNBA (see Figure 1B). To our knowledge, this reaction, which also occurs on smooth silver surfaces, does not occur on substrates other than ~ i 1 v e r . lSince ~ Murray et al. observed a strong band near 1460 cm-', it must be concluded that there was direct contact between PNBA and silver in their experiments. Several groups performed similar experiments a t room temperature and atmospheric pressure, using sputtered silica14or Langm~ir-Blodgettl~ films as spacer layers on silver island films in the configuration shown in Figure
3686 Langmuir, Vol. 10, No. 10, 1994
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Silver island pacer layer
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Figure 2. Representation of the layer geometry. (A) The geometry aimed at by distance dependence experiments for SERS (refs 10,14-16).The spacer layer must cover the metal perfectly. Contact between the scatterer and the metal must be avoided (ref 10 has a slighly different configuration, with calcium fluoride roughened silver and silver on the top). (B) Coverage of a silver island by alkyl mercaptans, the goal of this work. Contrary to other coverage methods, the spacer is attracted to silver and chemically bound to it.
2. They came to the same conclusion as Murray et al. and excluded a chemical contribution to the enhancement. However, in both cases, the conclusion does not hold after a careful examination. In ref 14,PNBA was used as a scatterer and evidence for the reduction of the nitro groups to form azo speciescan be observed in the reported spectra, implying direct contact of PNBA with silver. As for Langmuir-Blodgett spacer layers, the authors showed no evidence of good coverage of silver by the 0 r g a n i ~ s . l ~ It is well-known that LB layers show in-plane structures with regions of low and high density. Moreover, it is difficult to envision that such rough surfaces as silver island films can be perfectly matched by this deposition technique which, basically, matches one plane with another plane. The total surface area of silver island films is about 2 or 3 times that of the underlying substrate. Recently, Otto and colleagues16published a number of spacer experiments, carried out under ultra-high vacuum Torr) with substrates held a t low temperature (30 K). Low pressures allow the deposition of a known number of molecules which will stick to the cold substrate. A low temperature also prevents diffusion of the scatterer through the spacer. All of their experiments were consistent with a first-layer effect. Some other evidence was presented to support a chemical explanation of this first-layer effect. This effect amounts to a factor of approximately 100, on top of a longer range contribution which is explained by the electromagnetic theory. The (14) Walls, D. J.; Bohn, P. W. J. Chem. Phys. 1989,93, 2976. (15)Kovacs, G. J.; Loutfy, R. 0.;Vincett, P. S.; Jenning, C.; Aroca, R.Langmuir 1986,2, 689. (16) Mrozek, I.; Otto, A. Appl. Phys A 1989,49, 389. See also ref 5 and references therein.
magnitude of both enhancements varies from one type of substrate to the other but remains in the same range. Although there is no obvious flaw in their work, it seems that they cannot prove perfect coverage of silver. Moreover, it is difficult to calculate the distance between the scatterer and the surface. These high-vacuum, lowtemperature experiments are feasible but difficult and not easily interpretable in terms of distance. In refs 10 and 14,PNBA shows a chemical reaction characteristic of a contact with silver, canceling any conclusion. In ref 16, the authors claim that no reaction was observed, but their spectrum is also too weak to know if PNBA was chemisorbed onto silver or physisorbed onto the spacer. All of the spacer layers used in those investigations were deposited by physical means. Nothing could ensure a priori obtaining of a good coverage except for randomization of the deposition in the case of lowtemperature experiments.16 In a distance dependence experiment, it is critical to locate the scattering molecule with accuracy. This can be done by using a molecule whose Raman signal provides evidence about the location of the molecule. In that way, it is possible to obtain an in situ probe of the distance which is available during the illumination. Molecules able to chemisorb on silver are practical realizations of this concept. Their spectra can give a binary answer about the contact between the molecule and the metal. Up to now, investigators have used measurements of mean thicknesses to characterize the films.1°J4 They also inferred the quality of the coverage from macroscopic considerations and measurements performed before or after the SERS experiment^.^^*^^>^^ We consider that this
Spacer Layers of Alkyl Mercaptans question needs to be treated on a molecular scale and, if possible, a n answer to the coverage question provided while the SERS spectrum is being obtained. The question of direct adsorption of the scatterer onto the metal is critical because the SERS signal is likely to decrease very quickly from the metallic surface. A small percentage of directly adsorbed molecules may yield the major part of the signal. The purpose of this paper is to describe the results of some experiments we have carried out to determine the SERS enhancement as a function of thickness of a spacer layer and to determine the coverage of SERS-active silver island films by the spacer molecules. Agood spacer is one which has a strong tendency to cover previously uncovered regions of the metal substrate. After coverage, no part of the silver surface should be accessible to other molecules. This leads easily to the use of self-assembled monolayers (SAMs). SAMs of n-alkyl mercaptans on silver" and gold1* have been thoroughly studied. These compounds have the general formula
CH, -(CHJ,
-SH
and are known to chemisorb on noble metals through Ag-S bonds to form densely packed layers of hydrocarbon chains. The resulting surfaces are low-energy and hydrophobic. o-Terminated alkyl mercaptans have also been studied. With hydroxyl or carboxyl groups in the w position, they yield hydrophilic surface^.'^ Our results show that alkylmercaptans can be used as spacers to probe the distance dependence of the SERS signal. Three different n-alkyl mercaptans (8,12, and 16 C atoms) and carboxyl-terminated undecyl mercaptan (11 C) were investigated. These mercaptans are referred to as OCM (octyl mercaptan, 8 C), DDM (dodecyl mercaptan, 12 C), HDM (hexadecyl mercaptan, 16 C) and MUA (11mercaptoundecanoic acid, 10 C carboxyl group). Silver island films covered with S A M s of alkyl mercaptans were first characterized with PNBA as a scatterer, or probe. A dye molecule, cobalt phthalocyanine (CoPC), was then used to counter the drawbacks of PNBA. A SERS signal was obtained for CoPC, with strong evidence that the molecule was away from the surface of silver, at about 15-20 A from the metal.
+
Experimental Section p-Nitrobenzoic acid (PNBA, 98% purity) was purchased from Eastman. Cobalt phthalocyanine (CoPC, 97% purity) was purchased from Aldrich. Absolute ethanol and toluene were obtained from Midwest Grain and from Aldrich, respectively. n-Alkyl mercaptans octyl mercaptan (OCM, 8 C, 98%), dodecyl mercaptan (DDM, 12 C, 98%),and hexadecyl mercaptan (HDM, 16 C, 92%) were purchased from Aldrich. Mercaptoundecanoic acid (MUA, 11C) was synthesized from bromoundecanoic acid (Aldrich), with sodium metal and thiolacetic acid (Aldrich), following a procedure available in the literature.20 Silver Island Films. SERS substrates were prepared by evaporating silver island films onto pieces of glass microscope slides, about 1cm x 1 cm. "he glass was cleaned in an ultrasonic bath first with 0.1 M NaOH, then with 0.1 M HC1, and finally with deionized water. Then the substrates were dried in a nitrogen stream and placed in an oil-free vacuum system which was evacuated to a pressure ofless than Torr using sorption, sublimation, and ion pumps. Silver island films were deposited (17) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D.J. Am. Chem. SOC.1991,113,2370. (18) For example: Porter,M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C . E. D.J . Am. Chem. SOC.1987,109,3559. (19) Bain, C. D.; Evall,J.;Whitesides, G. M. J.Am. Chem. SOC.1989, 111, 7155. (20) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4 , 365.
Langmuir, Vol. 10, No. 10, 1994 3687 onto the glass by wrapping a silver wire (99.99%) around a tungsten filament which was resistive1 heated to evaporate the silver. A mass thickness of about 46 was deposited at a rate of about 1k s . Silver island films were fragde and sensitive to environmental attack. For example, soaking in a solvent decreased the SERS capacity of island films, probably by rinsing away some of the aggregates. Moreover, exposure to the atmosphere induced contamination of the surface of silver. Adsorption on silver was modified by the presence of the contaminant layer. For these reasons, silver island films were used for adsorption or Raman analysis immediately after being removed from the vacuum system. Raman Analysis. The Raman spectrometer was equipped with a Spex 1401 double monochromator and a Coherent Radiation argon ion laser (5145 A). A low power, 1 mW at the sample, was used to avoid the photolytic reduction of PNBA and the desorption of molecules. Powers 20 mW or higher were also used. The SERS signal was collected through a Hamamatsu R-943-02 photomultiplier tube and analyzed with a Stanford Research Model 400 photon-counting apparatus. The spectra were stored in a computer and smoothed to eliminate noise. All the spectra were taken with the same slit width, which gave approximately a 15cm-l resolution, except for the spectra shown in Figure 6 where better resolution (7.5 cm-l) was needed. To compare Raman scattering intensities, the same power and slit width were always used. Only the scan speed was modified. Self-Assembled Monolayers of Mercaptans. Self-assembled monolayers (SAMs)of alkyl mercaptans were adsorbed onto silver island films from 10-2 M solutions of the mercaptans in absolute ethanol. The adsorption lasted from a few minutes to several hours with very few changes in the spectra. After adsorption, the samples were rinsed with ethanol and dried with a stream of inert gas. SERS spectra were taken within the same day. S A M s of mercaptans are known to provide quality layers on the smooth surfaces of noble metals. However, this now wellknown result does not immediately apply in our case. The roughness of silver island films could induce some coverage defects. In fact, our results suggest that some areas of silver remained available for the chemisorption of small molecules such as PNBA. S A M s could perhaps be more effective in preventing adsorption of small molecules like PNBA on smooth silver, but smooth metal surfaces do not support SERS. SAMs on silver island films gave sufliciently good coverage to prevent adsorption of large molecules like CoPC. Spin Coating. PNBA and CoPC were deposited on bare and mercaptan-coated silver island films by spin coating from solutions at 2000 rpm for about 30 s. PNBA was deposited from M solutions in ethanol. On high-energy substrates, this concentration was enough to yield a layer several molecules in thickness but the film was thin enough to give no measurable normal (bulk) Raman signal using the usual experimental M solution in conditions. CoPC was deposited from a 3 x toluene. The SERS signal of these layers was very reproducible on uncoated and mercaptan-coated silver island fdms. Increasing the concentration did not increase the Raman signal that was detected from the layers.
K
Results and Discussion Macroscopic Observations. Before adsorption of the mercaptans, the silver films were orange-pink and easily wettable. After adsorption of the mercaptans, the films turned dark purple. The color was about the same for n-alkyl mercaptans (OCM, DDM, HDM) and the carboxylterminated mercaptan (MUA). A significant difference in wetting properties was seen between those two types of samples. n-Alkyl mercaptans yielded surfaces that were not easily wetted by ethanol and toluene. The expected higher surface energy was obtained with silver island films coated by MUA. These were easily wetted by both solvents. W-visible absorption spectra were obtained. The adsorption of mercaptans on silver films shifted the light absorption maximum to the red part ofthe spectrum. However, the absorption at the laser wavelength was not appreciably modified due to the large width of the absorption band.
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Figure 3. SERS spectra obtained from alkyl mercaptan spacer layers on silver at a high power (20 mW at the sample). (A, Top) Hexadecyl mercaptan (HDM)afier 1h adsorptiontime. Note the flatness ofthe background and the absence of so-called “cathedral” peaks around 1380 and 1580 cm-l. This is a sign of exclusion of hydrocarbon contaminants from the surface and hence of good coverage of silver by the mercaptans. (B, Bottom)Mercaptoundecanoicacid (MUA) after 15 min adsorption time. The background is slightly rougher than in part A but much less than that of SERS of bare silver. This is explained by the hydrophilic character of the upper surface aRer coverage by MUA.
Spectra and Interpretation. Figure 3A shows the
SERS spectrum of HDM adsorbed onto a silver island film during immersion for 24 h in a n ethanol solution. All of the observed bands were characteristic of HDM.21 The strongest band near 715 cm-’ was related to the C-S stretching vibration. That position is characteristic of mercaptans chemisorbed on silver. Bands near 1100 and 1140 cm-l were assigned to C-C streching vibrations. Two features near 1295 and 1440 cm-I were probably due to CH2 twisting and deformation vibrations. It is noteworthy that this spectrum provided no evidence of the hydrocarbon degradation that is catalyzed by silver although it was taken in air a t a high loser power. Under the same conditions, a bare or poorly covered silver island (21)Garoff, S.; Sandroff, C. J. J.Phys. Colloq. 1983, C10,44,483.
film would give two broad bands near 1380and 1580cm-’. The absence of these bands was a sign of good coverage. The SERS spectra of OCM and DDM were similar, except for a slight occurrence of hydrocarbon degradation, particularly for the former. The case of MUA must be considered separately. This molecule could chemisorb on silver through the carboxyl group and through the sulfur atom. It also yields a hydrophilic surface. However, the spectrum shown in Figure 3B was similar to that of HDM shown in Figure 3A. A strong C-S stretching vibration was observed near 710 cm-l, similar to that of n-alkyl mercaptans on silver. Small bands were visible near 950 and 1400 cm-’, indicating some carboxylate formation. However, these bands were much weaker than those obtained when longchain carboxylicacids such as palmitic acid were adsorbed
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Figure 4. SERS spectrum of PNBA spin-coated onto a silver surface previously covered by hexadecyl mercaptan (HDM). The spectrum of HDM has been subtracted. The nitro and carboxylate peaks at 1346 and 1380 cm-l are shifted from Figure 1A and also better resolved. Power = 20 mW. Resolution = 15 cm-l. on silver island films. Thus, it seemed that a very small percentage of the molecules adsorbed onto silver through the carboxylic group, which was consistent with previous results.lg Several other differenceswere observed between the spectra shown in Figure 3A and 3B. Broad bands characteristic of graphitic species were observed near 1580 and 1380 cm-' and the relative intensity of the C-C stretching peaks at 1100 and 1140cm-l. The broad bands related to degradation indicated that MUA could not prevent adsorption of hydrocarbon contaminants from the atmosphere onto silver as well as HDM. The change in the C-C stretching intensity was indicative of a different chain structure of the alkyl. These differences were primarily due to the presence ofthe carboxylicgroup. They do not imply that MUA is a worse spacer than n-alkyl mercaptans. SERS spectra were obtained after various adsorption times. This permitted the decrease of the efficiencyof the SERS substrates due to soaking in the solution to be monitored. For n-alkyl mercaptans, the spectrum changed very little when the adsorption time increased from 10 min to 2 days.22 The decrease ofthe SERS intensity after 2 days was of the order of 25%. For MUA, the bands attributed to adsorbed carboxyl groups tended to decrease when the adsorption time was increased. Apart from that, there was no variation in the relative intensity ofthe peaks. Soaking in MUA solutions reduced the SERS signal by about 50% in 1 day. Thus, for this study, the SERS intensity measurements were performed after a few hours of adsorption, whatever the solution. PNBA as a Probe of the Coverage by the Spacer Layers. PNBA was spin-coated on silver island films that were previously coated by n-alkyl mercaptans (OCM, DDM, and HDM). The SERS spectra of these samples were all very similar to one another, a t least for low laser powers. At a low power, about 1mW at the sample, PNBA on HDM, for example, gave the spectrum shown in Figure 4. The spectra obtained for PNBA on DDM and OCM were identical but less intense. PNBA on OCM, DDM, and HDM gave SERS signals that were, respectively, 10, (22) Only the position of the peaks varies by a few cm-'.
lp
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Figure 5. RelativeSERS enhancementof PNBA on silver films covered by alkyl mercaptans. Ratio of intensities to the signal intensityof bare silverfilms. Spacersare ranked on the abscissa according to their number of carbon atoms. The n-alkyl mercaptans are represented by squares and the w-terminated mercaptan (MUA) by crosses. 15, and 25 times weaker than PNBA on bare silver island films (see Figure 5). At a higher power, for OCM and DDM, PNBA underwent a chemical reactionl1-l3 a t about the same rate for both mercaptans, but much slower than for PNBA on bare silver islands. However, for PNBA on HDM-coated island films, no reaction was detected, whatever the laser power. PNBA was spin-coated onto MUA-covered silver island films. When illuminated at a low laser power, the SERS spectrum was similar to that of PNBA on n-alkyl mercaptans. The signal was about the same as the signal of PNBA on DDM-coated silver islands, i.e., roughly 15times lower than that of PNBA on bare silver films. When a higher laser power was used, the disappearance of nitro groups was detected, at a rate greater than that for n-alkyl mercaptans but less than that for bare silver substrates. The spectrum in Figure 4 can be compared to the SERS spectrum of PNBA on bare silver island films (see Figure 1). The two spectra were very similar except for the shape and position of the bands near 1350 and 1390 cm-'. The
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Figure 6. Carboxylate and nitro peaks in SERS spectra of PNBA (-) spectrum of PNBA on bare silver; (- - -) spectrum of PNBA adsorbed onto a silver island film coated with adsorbed octadecyl mercaptan; (- - -) difference spectrum. This subtraction shows that PNBA films adsorbed on bare silver are composed of two species, only one of which is present when PNBA adsorbs through an alkyl mercaptan layer. Resolution = 7.5 cm-l.
band due to the nitro group of PNBA shifted from 1355 cm-l for bare substrates to 1346 cm-l for silver substrates coated with alkylmercaptans. The new peak near 1346 cm-I was sharper. In fact, it seemed that a feature was already present a t about this position for PNBA on bare silver (Figure 6). Figure 4 also shows a peak near 1380 cm-I which was probably related to the shoulder at 1390 cm-I for PNBA on bare silver. The 1380 cm-1 band could also be seen in the spectrum of PNBA on bare silver, as shown in Figure 6. This band was assigned to carboxylate ions formed by the acid in contact with silver.12J3 The curve fitting in Figure 6 can be interpreted as follows. SERS on bare substrates showed two types of PNBA molecules or two types of scattering configurations, both chemisorbed through a carboxylate ion. Only one type of PNBA was found when silver was previously coated by alkyl mercaptans. The apparent broadening of the nitro and carboxylate bands can be described as a concentrationdependent effect. The SERS intensity and band positions can vary with the surface concentration of adsorbates, as was demonstrated by numerous s t ~ d i e s . ~ ~The - ~ 'nonlinear variation of the intensity was explained by a n electromagnetic depolarization p h e n o m e n ~ n .The ~ ~ ,shift ~~ of the bands reveals the existence of various adsorption sites.24 Other mechanisms for carboxylate formation were considered. Carboxylate ions could have formed with sodium ions coming from the glass substrate. This possibility was ruled out by using silica as a substrate instead of soda lime glass slides. Another result made it very likely that the signal comes from PNBA chemisorbed on silver. When a PNBMmercaptadsilver island film sample was rinsed with ethanol, PNBA was not removed, and the SERS spectrum remained identical. Upon rinsing, only the chemisorbed molecules should have remained. (23) Zeman, E.J.;Carron, K. T.; Schatz, G. C.; Van D u p e , R. P. J . Chem. Phys. 1987,87,4189. (24) Murray, C. A.;Bodoff, S. Phys. Rev. B 1986,32,671. (25)Aroca, R.;Battisti, D. Langmuir 1990,6, 250. (26) Kim, J.-H.; Cotton, T. M.; Uphaus, R. A.; Mobius, D. J . Phys. Chem. 1989,93,3713. (27) Wolkow, R. A,; Moskovits, M. J . Chem. Phys. 1992,96,3966.
Thus, we consider that the entire signal came from PNBA chemisorbed onto silver for island films coated by n-alkyl mercaptans or by the w-terminated mercaptan. As discussed above, one could use the photolytic reaction as a test of molecule location, as Otto et al. did for PNBA on low temperature spacer layers.l6 Yet, we now know that the scattering molecules were chemisorbed onto silver. The presence of the photolytic reaction confirmed this conclusion in the case of OCM, DDM, and MUA. Absence of the reaction for PNBA on HDM did not contradict previous evidence of chemisorption. This absence, however, provides some important evidence regarding the photolytic reaction itself. The reaction is bimolecular, and the adsorption sites of PNBA on silver island films covered by S A M s of HDM are is01ated.I~ With the accuracy available for the detection, it seemed that the signal was measured from PNBA molecules chemisorbed on silver only. In the case of hydrophobic upper surfaces (n-alkyl mercaptans), this result was likely since little adsorption should occur on the low-energy methyl surface. For higher energy upper surfaces (MUA adsorbed onto silver island films), we expected several physisorbed layers of PNBA on the carboxylic surface. Our results mean that the signal of the molecules physisorbed on the upper surface was of the order of 1/100 of that of PNBA on bare silver islands, or even lower. A 10-15A S A M of OCM decreased the signal ofchemisorbed PNBA by a factor of 10. Although we still obtained a signal from chemisorbed PNBA, this signal was small. As said above, there is a nonlinear relation between the surface concentration of scatterer and the measured ~ i g n a l . ~However, ~ - ~ ~ the relation is monotonic for nonresonant Raman. Moreover, the decrease in the coverage is bigger than the decrease in the signal. For example, a decrease of the signal by a factor of 10indicates that less than 10%of silver is available for chemisorption of PNBA. Thus, the coverage of silver could be considered good. DDM and MUA have approximately the same length. They also gave rise to about the same SERS spectrum of PNBA. This proved that the coverage of silver island films was as good with MUA as with n-alkyl mercaptans. Thanks to the use of PNBA as a probe, we could determine the quality of the coverage by alkyl mercaptans. However, PNBA was not adapted to the distance dependence experiment with alkyl mercaptan spacer layers, a t least with spin coating as a deposition technique. Due to its chemisorption on silver and probably to its relatively small size, PNBA could penetrate into the alkyl layer. In the case of MUA, the signal from chemisorbed PNBA hid the signal from PNBA physisorbed on top of the alkyl layer. CoPC as a Probe of the Spatial Extensionof SERS. Another scatterer was tried in a n effort to remedy this problem. Cobalt phthalocyanine (CoPC) is a nonfluorescent dye molecule which absorbs li h t in the red with a n absorption maximum around 6500 , At this wavelength, CoPC shows the phenomenon of SERRS (surface-enhanced resonance Raman scattering) with a very intense Although the signal is more intense with red excitation light, we preferred using the green light a t 5145 A. At this wavelength, the observed enhancement phenomenon was SERS (pre-resonant excitation light). Green light was preferred because SERS of CoPC with 6500 A light showed a signal which was highly nonmonotonic when the surface concentration of CoPC was varied.23Moreover, dramatic changes in the relative intensity of the bands could also be observed a t 6500 Asz3 CoPC is a large molecule with no chemisorption onto silver. SERS on bare silver islands was obtained from physisorbed molecules (see Figure 7A) that were easily removed by rinsing with toluene (see also ref 23). For
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Figure 7. SERS of cobalt phthalocyanine (CoPC) (A, top) on bare silver, low laser power, and (B, bottom) on silver with MUA coating. Only one peak of MUA is visible as a shoulder of the peak at 690 cm-l, and the spectrum was collected at a higher power than A. There are slight differences in relative intensities between A and B. Especially, the intensity of the peak at 1544 cm-I has grown from A to B compared t o the peak at 690 cm-'.
MUA-coated silver films, CoPC was also expected to physisorb on top of the hydrophilic surface. With green light, the SERS spectrum of CoPC on bare silver films (Figure 7A) was similar to the normal Raman spectrum. The strongest band was near 1545 cm-'. By a comparison of signals from layers on silver island films and on smooth Ag or AI substrates, the enhancement of CoPC on silver substrates was determined to be about lo3 with 5145 A excitation. CoPC on Hydrophobic Upper Surfaces. n-Alkyl mercaptadCoPC bilayers were deposited on silver island films by solution adsorption of the mercaptan and spin coating of CoPC from a toluene solution. The strongest band in SERS spectra of CoPC (Figure 7B) was that near 1545 cm-'. The region of this peak had thus to be scanned in the case of AgMDWCoPC samples. The smallest detectable signal would be about 1000 times less than the signal from SERS on bare silver island films, when the
same laser power and resolution are maintained (only the scan speed was modified). When n-alkyl mercaptans were adsorbed on newly deposited silver island films and CoPC spin-coated on those, no signal could be detected. This showed that the signal from CoPC decreased more than 1000 times compared to the case of deposition on bare silver. This result was reproduced with island films that were covered by OCM, DDM, or HDM. It was interesting to notice that when HDM was coated on silver films that had aged for several days, it was possible to obtain a signal from CoPC. The signal was then of the order of 100 times smaller than the signal from SERS on bare silver islands. This was explained by the contamination ofthe surface of silver, which prevented adsorption of mercaptans and hence good coverage. When coverage by the mercaptans was poor, CoPC molecules could be deposited directly on silver. These results indicate that the coverage of silver island
3692 Langmuir, Vol. 10, No. 10, 1994 films by n-alkyl mercaptans was excellent when CoPC was used as a probe of coverage and spin coating as a deposition technique. Let us consider that the SERS signal of CoPC on silver depends linearly on the surface concentration of the first CoPC layer (we know a t least that the variation is monotonic). After the surface is covered with n-alkyl mercaptans, less than 0.1% of the rough surface of silver remained available for CoPc to adsorb on it. These results also indicate that little CoPC was deposited on top of the alkyl layer. This was in agreement with the known low energy of the upper surface of n-alkyl mercaptan SAMs. We can now use MUA-coated silver island films, which display a high-energy surface, as determined earlier. The use ofPNBA as a probe showed that MUA covers silver island films a s well a s DDM and better than OCM. CoPC on HydrophilicUpper Surfaces. When CoPC was coated on MUA-covered island films, a strong SERS signal was still obtained from the dye molecule (see Figure 7B). Most of the bands in the spectra in Figure 7B belonged to the dye, except for the shoulder near 710 cm-l, which was due to the C-S stretching mode of chemisorbed MUA. We can compare the intensity of the 1544 cm-l band. The signal obtained from CoPC was 9.5 f0.5times lower than that of CoPC physisorbed directly on bare island films. We know that CoPC could not penetrate in the alkyl layer of OCM, DDM, or HDM. Moreover, the coverage by MUA was as good as that by DDM and better than that by OCM. Thus, CoPC could very probably not penetrate in the alkyl layer of MUA coatings. For CoPC, the scattering molecules were very probably physisorbed on the hydrophilic upper layer of MUA-coated silver islands. The case of hydrophilic coatings can also be compared to silver island films poorly covered by n-alkyl mercaptans. These low energy substrates gave a signal of CoPC 100 times smaller than that for bare silver island films (see above). Thus, on hydrophilic, higher energy coatings, a mere coverage deficiency would likely yield a signal smaller by a factor of 10, if there was no signal from the upper physisorbed layer. The signal on MUA-coated silver was not due to poor coverage. The measured signal was dominated by the scattering of molecules that were about 15 A from silver. The Raman signal of CoPC on silver island films was enhanced about lo3 times. The signal on coated films was enhanced about lo2 times. Since CoPC is a large molecule and does not chemisorb on silver, we consider to a first approximation that CoPC shows only a n electromagnetic enhancement. An equation which has been widely used to interpret distance dependence experiments relates the spatial extension of the electromagnetic enhancement G to the radius of curvature r of roughness features that are involved in the enhancement.1°J4-16 The enhancement a t a distance d from the surface is
+
[G(d)/G(d= 0)l = [e/(@ d)I1' With d = 15-20 A for MUA layers, we obtained = 5875 A. This value was rather far from those cited in the literature, observed or calculated. This discrepancy may be due to a wrong hypothesis, upon the nature of the enhancement, for example. But it could also mean that
Bercegol and Boerio the important scale of the roughness is not that of the aggregates. However, a scale as small as 50 A brings a new problem. At this scale of roughness, localized plasmons in the metal should undergo much greater losses. This problem certainly requires more experimental and theoretical attention.
Conclusions Self-assembled monolayers of n-alkyl mercaptans (8, 12, and 16 C) and carboxyl-terminated alkyl mercaptans were successfully used as spacer layers on silver island films. p-Nitrobenzoic acid (PNBA) and cobalt phthalocyanine (CoPC) were used as scatterers and as probes of the spacer coverage efficiency. We took special care of the quality of the coverage of silver by the mercaptans. Good coverage had to be obtained to avoid difficulties that other investigators encountered. For each combination of spacer and scatterer that we investigated, we determined the location of the scattering molecules. The samples were first characterized with PNBA, a molecule that chemisorbs on silver and also undergoes a photolytic reaction catalyzed by silver. This molecule helped prove the good coverage of silver films by mercaptans. However,whatever the spacer layer, we obtained SERS of PNBA chemisorbed on silver only. Very little PNBA was expected to adsorb on the hydrophobic upper surface ofn-alkyl mercaptans. But, no signal was detected from the moleculesthat had physisorbed on the hydrophilic upper surface of the carboxyl-terminated alkyl mercaptan. The signal from these molecules was estimated to be lower by a factor of 100 or more than that of PNBA on bare silver island films. No signal was detected from CoPC spin-coated on silver films that were previously covered withn-alkyl mercaptans. Thus, CoPC could not penetrate in the alkyl layer and could not adsorb on silver. The low energy of the upper surface ensured that very little CoPC was deposited on top of the layer. CoPC was not expected to penetrate among the chains of the carboxyl-terminated mercaptan (MUA). However, a signalwas measured from CoPC deposited on MUA-covered silver films. The SERS signal of CoPC was lower by a factor of 9.5 f0.5 when the molecule was located about 15-20 A from the surface. This study proved the feasibility of experiments designed to measure the spatial extension of SERS with rather simple means. The simplicity of the materials and methods authorized the utilization of various spacers and various scatterers. The results owed alot to the possibility of obtaining similar coatings that display different upper surfaces, with low and high surface energy. The use of two different scatterers was also essential to characterize the quality of coverage. The distance dependences were different for PNBA and CoPC. PNBA had a decrease by a factor of 100 or more, whereas the signal of CoPC was divided by about 10. To our knowledge, this is the first time that a n enhanced Raman signal has been obtained from a molecule (CoPC), with strong evidence that this molecule is away from the surface of the metal (silver). This realization should make possible the development of enhanced Raman spectroscopy a t nonmetallic surfaces. Acknowledgment. This research was supported in part by a grant from St. Gobain Recherche. The authors thank Dominique Frugier, Pascal Chartier, and Jin-Ter Young for very fruitful discussions.
