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Surface enhanced Raman scattering of molecules within small cavities in evaporated silver films
890
Applied Surface Science 33/34 (1988) 890-897 North-Holland, Amsterdam
SURFACE ENHANCED RAMAN SCATI'ERING OF MOLECULES WITHIN SMALL CAVITIES IN EVAPORATED SILVER FILMS M. O S A W A , S. Y A M A M O T O a n d W. S U i ~ T A K A Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan
Received 23 August 1987; accepted for publication 15 October 1987
Surface enhanced Raman scattering (SERS) of organic molecules on evaporated silver films of 20-30 nm in mass thickness has been investigated using the Kretschmann ATR configuration. The metal films were continuous but had numerous defects such as pores and crevices. The SERS on these surfaces was short-ranged even in the case where the chemical contribution is ruled out. We found that there is an intimate correlation between the SERS intensity and the electric field strength within the metal layer calculated with the Fresnel formula. These results can be well explained by assuming that the molecules within the pores and crevices exhibit such enhancement.
1. Introduction Surface e n h a n c e d R a m a n scattering (SERS) is the p h e n o m e n o n that R a m a n scattering from molecules on metal surfaces is e n o r m o u s l y (]0 4 106 times) e n h a n c e d a n d it has been applied to the investigations of various chemical reactions on metal surfaces [l]. The origin of the e n h a n c e m e n t is considered to be the result of a n u m b e r of m e c h a n i s m s [2]. These m e c h a n i s m s are divided i n t o two classes: electromagnetic (EM) a n d chemical. In the EM mechanisms, the e n h a n c e m e n t arises from the amplification of the i n c i d e n t a n d the R a m a n scattered E M fields o n rough metal surfaces via r e s o n a n t excitations of the surface p l a s m o n p o l a r i t o n (SPP) of bulk metals or the collective electron r e s o n a n c e of small metal particles. The chemical m e c h a n i s m is a kind of r e s o n a n c e R a m a n scattering which occurs via charge-transfer between adsorbates a n d the metal. It is well k n o w n that surface roughness is necessary for the EM m e c h a n i s m s [2]. The top of the p r o t r u s i o n s or b u m p s o n the metal surface have been believed to be the major SERS active sites [3]. Recently it was proposed that extremely small cavities or pores present on surfaces, such as coldly deposited Ag, are a n o t h e r i m p o r t a n t type of SERS active site [4,5]. However this "cavity site m o d e l " has not been completely accepted yet. In the present study we report the SERS of thin organic films of p - n i t r o b e n z o i c acid ( P N B A ) a n d c o p p e r - p h t h a l o c y a n i n e (CuPc) o n evaporated Ag films which strongly supports the cavity site model. 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
3.t. Osawa et al. / SERS of organic molecules in evaporated Ar
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2. Experimental R a m a n measurements were carried out using the Kretschmann attenuated total reflection (ATR) configuration [6], in which Ag was vacuum evaporated onto the reflecting plane of a hemicylindrical A T R prism (LaSF15 glass, refractive index n = 1.88 at 514.5 nm). The optical arrangement, details of which have been described elsewhere [7], is schematically shown in the inset of fig. 1. R a m a n intensity (peak height from the base line) was measured as functions of the incidence angle (0) and polarization (s and p) of the 514.5 nm line from an Ar ÷ laser (output power of 100 mW). The X, Y and Z axes are defined on the reflecting plane of the prism as shown in the figure for further discussion. The mass thickness ( d ) of the Ag films was monitored with a quartz microbalance and the evaporation rate was kept at 1 n m / m i n . According to field emission scanning electron microscopic observations [7], the Ag films of d = 20-50 nm were continuous but not smooth. Numerous bumps of a few tens of nm in size were observed on the surfaces. Numerous pores and crevices of a few tens of n m in diameter and width, respectively, were also observed on the films of d = 20-30 nm, which were missing on the film of d = 50 nm. Thin PNBA films were formed by dropping its acetone solution on the Ag surfaces and then evaporating the solvent. The CuPc films were deposited on the Ag surfaces by sublimation in vacuum.
3. Results and discussion Fig. 1 shows a SER spectrum of a thin PNBA film of 5 nm in thickness deposited on a Ag film of d = 20 nm, measured with the p-polarized laser b e a m at 0 = 35 o. This spectrum is the same as that observed on Ag island films [8,9]. All the bands observed here, except those denoted by asterisk and an arrow, are attributed to the p-nitrobenzoate ( P N B A - ) ion produced via strong chemisorption through the carboxylate group to the Ag surface [8,9]. The 345 c m - 1 band denoted by the arrow may be assigned to the A g - O bond. The 1460, 1140 and 935 cm -1 bands denoted by asterisk were attributed to azodibenzoate formed by the reductive coupling of PNBA molecules during larger irradiation [9]. The R a m a n intensity of the P N B A - ion was almost independent of the thickness of the P N B A film. In addition, no C--O stretching band of PNBA (observed at 1635 cm -1 in the normal R a m a n spectrum of the bulk crystal) was observed even for thicker PNBA films. These results indicate that the SERS of PNBA is restricted to the first monolayer adsorbed directly on the Ag surface. This is one of the characteristic properties of the SERS originating from the chemical mechanism [2]. However it must be noted that the contribu-
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M. Osawa et aL / S E R S of organic molecules in evaporated A r
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Fig. 1. S u r f a c e e n h a n c e d R a m a n spectrum from a thin P N B A film deposited on a n e v a p o r a t e d A g film of 20 n m in thickness. The inset shows the Kretschmann A T R configuration used in the present study.
tion of the EM mechanism is also important. The enhancement factor resulting from the EM mechanism on the Ag films of d = 20 nm is estimated to be 103 as described below. Since the total enhancement factor of the SERS of the PNBA ion was estimated to be 104-105 from the comparison of the intensity of the 1355 cm -1 band in the SER spectrum shown in fig. 1 and the normal Raman spectrum of the sodium salt of P N B A - , the enhancement factor resulting from the chemical contribution can be estimated to be
101_102" In fig. 2 the relative Raman intensity of some Raman bands of the PNBA ion, normalized to the intensity of the 1355 cm-~ band, is plotted versus their depolarization ratios (Pt) [10] both for the SER spectrum and the normal zo z ~ z
o 1.0
°
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I 0_1
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~xx=~yy=azz DEPOLARIZATION
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Fig. 2. R e l a t i v e R a m a n intensity of the als modes of P N B A adsorbed on a A g film of d = 20 nm (©) a n d in a basic solution (O) as a function of the depolarization ratio. The intensity is n o r m a l i z e d to that of the 1350 c m - 1 b a n d . T h e numbers in the parentheses are the p e a k frequencies of the R a m a n b a n d s .
M. Osawa et aL / SERS of organic molecules in evaporated Ar
893
R a m a n spectrum of the P N B A - ion in a basic solution [10]. According to the normal coordinate analysis by Ernstbrunner et al. [10], all these bands are assigned to alg modes in C2v symmetry. As can be seen in the figure, the relative Raman intensity of the 1600 cm -1 band increases selectively in the SERS. The change in the relative Raman intensity can be related to the molecular orientation [11,12]. The Raman tensor of the alg modes is diagonal and has a lae ranging from 0 to 3 / 4 [13]. If one of the diagonal elements is very much larger than the others, p c = l / 3 . Since the 1600 cm -1 band is assigned to an in-plane stretching mode of the phenyl ring accompanied mainly with a dipole change in the z-axis [10] and its Pe (0.38) is close to 1/3, txzz >> axx, ayy (where the molecular axis z is coincident with the C 2 axis). With decreasing Pc, the relative contribution of the azz element to the Raman scattering decreases as shown in fig. 2. Considering that the adsorbed P N B A - ion is oriented with its C 2 axis almost normal to the metal surface [14], the selective increase in the relative intensity of the 1600 c m - ~ band indicates that the EM field responsible for the SERS is parallel to the C 2 axis of the adsorbed P N B A - ion and also normal to the metal surface [11,12]. The same conclusion can be drawn on the basis of the chemical mechanism because the charge-transfer between the metal and the adsorbates should be excited by the EM field normal to the surface and hence the Raman tensor element azz should be increased. Copper-phthalocyanine (CuPc) is a suitable organic compound to investigate the EM contribution to the SERS free from the chemical contribution [15]. In fig. 3 the intensity of the 1530 cm -1 Raman band of CuPc on Ag films of d = 20 (a) and 30 nm (b) is plotted versus the thickness (t) of the CuPc film. The intensity was normalized to that of the normal Raman scattering
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Fig. 3. Change in the Raman intensity at 1530 cm-~ with increasing thickness of the CuPc film deposited on Ag films of d = 20 (a), 30 (b) and 50 nm (c).
894
M. Osawa et al. / S E R S of organic molecules in evaporated Ar
from a CuPc film of t = 1 nm deposited directly on the A T R prism (without the metal). Therefore, the relative Raman intensity at t = 1 nm is equal to the enhancement factor. The measurements were carried out with a p-polarized laser beam at optimum incidence angles (0 = 35 ° - 4 0 o, cf. fig. 4). The experimental results obtained using the Ag film of d = 50 nm (c) is also shown for comparison. The Raman intensity increases linearly with increasing t on the Ag film of d = 50 nm, which is well explained by the long-range SPP field [7]. This long-ranged enhancement rules out the contribution of the chemical mechanism in the SERS of CuPc. On the Ag films of d = 20-30 nm, however, the Raman intensity greatly increases at t less than 1 nm and then slightly decreases (for d = 20 nm) or increases (for d = 30 nm), indicating the enhancement on these Ag films is short-ranged. Therefore, it is concluded that certain EM fields localized near the surface are responsible for the SERS on the Ag films of d = 20-30 nm. It has been theoretically shown that the local EM fields on top of protrusions or bumps on rough surfaces are very strong [3]. The EM field within depressions on the surface (space between the bumps) also should be strong [16]. These EM fields are short-ranged, corresponding to the observed shortranged enhancement. However, the strongest Raman signal is not located on these sites in the SERS observed in the present study because such surface roughness was observed both on Ag films of d = 20-30 nm and d = 40-50 nm but the effective spatial range of the enhancement for these two groups of Ag films is clearly different as shown in fig. 3. In fig. 4 the SERS intensity of the 1355 cm ~ band of the PNBA ion on the Ag film of d = 20 nm (a) and the 1530 cm -1 band of CuPc on the Ag film of d = 30 nm (b) are plotted as a function of 0 both for p- and s-polarization (open and closed circles, respectively). For the p-polarized laser beam the intensity is very weak around the critical angle of total reflection 0 ( = s i n - T ( 1 / n ) = 32.1 °) and a maximum is observed at an angle slightly greater than 0c. For s-polarization, on the other hand, the 0 dependence is less remarkable and a maximum is observed around 0c. The solid curves in the figure are the 0 dependence of the average electric field strength I E 12 ( E is the amplitude) within the Ag layers of d = 20 nm (a) and 30 nm (b) for the incident laser beam calculated with the Fresnel formula. The subscripts to E in the figure denote the polarization state of the incident laser beam (s, p) and oscillating direction of the field (X, Y, Z). Details of the calculation have been described previously [7]. Briefly, the metal layer is assumed to be homogeneous. In order to take the porous structure of the actual Ag films into account, the dielectric constant (%ff) of a hypothetical metal-void (filled with molecules and ions) composite layer was used in the calculation in stead of that of bulk metal. The %fr's used here are rather arbitrarily chosen but very close to those of evaporated thin Ag films mea-
M. Osawa et al. / SERS of organic molecules in euaporated Ar
895
sured experimentally by Philip and Trompett [17] with h = 508.6 nm light. The volume fraction of the metal (q) of the composite layers is assumed to be 0.9 for d = 20 nm and 0.95 for d = 30 nm [17]. The calculation shows that the Z component of the average field is very weak and the field is essentially in the plane of the film, the X-Y plane, regardless of the polarization of the incident laser beam. The agreement between the electric field strength of the X and Y components ( I Epx 12 and I E~yI 2) and the SERS intensity measured with p- and s-polarized laser beams, respectively, is remarkable. Considering that there exist numerous pores and crevices in the Ag films of d = 20-30 nm and that the enhancement is short-ranged on these surfaces, this agreement strongly suggests that the molecules and ions within these pores or crevices exhibit such enhancement. The electric field calculated here is not strong enough to explain the observed enhancement of the Raman scattering. One of the possible explanations of this problem is that the EM field within the cavities such as pores or crevices can be increased and hence the Raman scattering by the molecules inside the cavities are enhanced [18-21]. Since this local EM field is considered, to first order, to be proportional to the average field calculated in the present study [18], the SERS observed using the evaporated Ag films of d = 20-30 nm is well explained by this model. This model is suitable also for the interpretation of the experimental results shown in fig. 2. As mentioned above the EM field responsible for the SERS of P N B A - is normal to the metal
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Fig. 4. 0 dependence of the R a m a n intensity from molecules on Ag films measured with p- and s-polarized laser beams (© and O, respectively): (a) the 1350 cm -1 band of P N B A - adsorbed on a Ag film of d = 20 nm; (b) the 1530 cm -1 band of CuPc deposited on a Ag film of d = 30 nm. The solid curves show the calculated average electric field strength within hypothetical metal-void composite layers having the effective dielectric constants of (a) - 5.72 + 0.96i and (b) - 8.99 + 0.6i. The subscripts to E show the polarization of the incident laser beam (s, p) and the oscillating direction of the field (X, Y, Z). E 0 is the amplitude of the incident laser beam.
896
M. Osawa et al. / S E R S of organic molecules in evaporated Ar
surface a n d also s h o u l d be in the X - Y p l a n e as shown in fig. 4. This i n d i c a t e s that the molecules which exhibit the e n h a n c e m e n t m u s t be a d s o r b e d on the m e t a l surface n o r m a l to the X~ Y plane. Such a surface is c o n c e i v a b l e inside the p o r e s a n d crevices. A c c o r d i n g to the cavity site model, the R a m a n intensity n o t o n l y of P N B A - ions b u t also of P N B A molecules inside the cavities s h o u l d be e n h a n c e d . H o w e v e r , no evidence of this p r e d i c t i o n c o u l d be o b t a i n e d b e c a u s e the c h e m i c a l m e c h a n i s m c o n t r i b u t e s to the e n h a n c e m e n t only for P N B A ions, which m a k e s it difficult to d e t e c t weaker R a m a n signals from P N B A molecules. F i n a l l y , we n o t e that the intensity of the E M field within the p o r e s in the m e t a l films d e p e n d s on the w a v e l e n g t h of the light. Recently, we showed that the e n h a n c e d i n f r a r e d ( I R ) a b s o r p t i o n b y molecules a d s o r b e d on e v a p o r a t e d thin m e t a l films [22,23] also can be e x p l a i n e d b y the cavity site m o d e l [24,25]. A c c o r d i n g to the m o d e l o f B e r g m a n a n d N i t z a n [20], however, the E M field within the p o r e s is n o t so strong in the I R region. F u r t h e r m o r e , we o b t a i n e d e x p e r i m e n t a l results which suggest that the local E M fields on r o u g h m e t a l surfaces are also i m p o r t a n t for the e n h a n c e m e n t of I R a b s o r p t i o n [25]. A l t h o u g h such local E M fields are n o t p r e d o m i n a n t l y i m p o r t a n t in the S E R S o b s e r v e d in the p r e s e n t study, further investigation of these local E M fields is desirable.
Acknowledgement O n e of the a u t h o r s (M.O.) is grateful to the M i n i s t r y of E d u c a t i o n , Science a n d C u l t u r e of J a p a n for financial s u p p o r t through a G r a n t - i n - A i d for E n c o u r a g e m e n t of Y o u n g Scientists (No. 61750708).
References [1] H. Seki, J. Electron Spectrosc. Related Phenomena 39 (1986) 289, and references therein. [2] For example, R.K. Chang and T.E. Furtak, Eds., Surface Enhanced Raman Scattering (Plenum, New York, 1982). [3] J. Gersten and A. Nitzan, J. Chem. Phys. 73 (1980) 3023. [4] E.V. Albano, S. Daiser, G. Ertl, R. Miranda, K. Wandelt and N. Garci&, Phys. Rev. Letters 51 (1983) 2314. [5] H. Seki and T.J. Chuang, Chem. Phys. Letters 100 (1983) 393. [6] E. Kretschmann, Z. Phys. 241 (1971) 313. [7] M. Osawa and W. Su~taka, Surface Sci. 186 (1987) 583. [8] R. Dornhaus, R.E. Benner, R.K. Chang and I. Chabay, Surface Sci. 101 (1980) 367. [9] P.G. Roth, R.S. Venkatachalam and F.J. Boerio, J. Chem. Phys. 85 (1986) 1150. [10] E.E. Ernstbrunner, R.B. Girling and R.E. Hester, J. Chem. Soc. Faraday Trans. II, 74 (1978) 1540.
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[11] [12] [13] [14] [15] [16]
M. Moskovits, J. Chem. Phys. 77 (1982) 4408. J.A. Creighton, Surface Sci. 124 (1983) 209. For example, D.A. Long, Raman Spectroscopy (McGraw-Hill, New York, 1977). J.T. Hall and P.K. Hansma, Surface Sci. 71 (1978) 1. S. Hayashi and M. Samejima, Surface Sci. 137 (1984) 442. N. Liver, A. Nitzan and J.I. Gersten, Chem. Phys. Letters 111 (1984) 449; J. Gersten and A. Nitzan, Surface Sci. 158 (1985) 165. [17] R. Philip and J. Trompett, Compt. Rend. (Paris) 241 (1955) 627. [18] H. Chew and M. Kerker, J. Opt. Soc. Am. B 2 (1985) 1025. [19] H. Seki, T.J. Chuang, J.F. Escobar, H. Morawitz and E.V. Albano, Surface Sci. 158 (1985) 254.
[20] [21] [22] [23] [24] [25]
D.J. Bergman and A. Nitzan, Chem. Phys. Letters 88 (1982) 409. D.E. Aspnes, Phys. Rev. Letters 48 (1982) 1629. A. Hartstein, J.R. Kirtley and J.T. Tsang, Phys. Rev. Letters 45 (1980) 201. A. Hatta, Y. Suzuki and W. SuStaka, Appl. Phys. A 35 (1984) 135. M. Osawa, M. Kuramitsu, A. Hatta, W. Su8taka and H. Seki, Surface Sci. 175 (1986) L787. Y. Suzuki, M. Osawa, A. Hatta and W. SuEtaka, Appl. Surface Sci. 33/34 (1988) 875.
Applied Surface Science 33/34 (1988) 890-897 North-Holland, Amsterdam
SURFACE ENHANCED RAMAN SCATI'ERING OF MOLECULES WITHIN SMALL CAVITIES IN EVAPORATED SILVER FILMS M. O S A W A , S. Y A M A M O T O a n d W. S U i ~ T A K A Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan
Received 23 August 1987; accepted for publication 15 October 1987
Surface enhanced Raman scattering (SERS) of organic molecules on evaporated silver films of 20-30 nm in mass thickness has been investigated using the Kretschmann ATR configuration. The metal films were continuous but had numerous defects such as pores and crevices. The SERS on these surfaces was short-ranged even in the case where the chemical contribution is ruled out. We found that there is an intimate correlation between the SERS intensity and the electric field strength within the metal layer calculated with the Fresnel formula. These results can be well explained by assuming that the molecules within the pores and crevices exhibit such enhancement.
1. Introduction Surface e n h a n c e d R a m a n scattering (SERS) is the p h e n o m e n o n that R a m a n scattering from molecules on metal surfaces is e n o r m o u s l y (]0 4 106 times) e n h a n c e d a n d it has been applied to the investigations of various chemical reactions on metal surfaces [l]. The origin of the e n h a n c e m e n t is considered to be the result of a n u m b e r of m e c h a n i s m s [2]. These m e c h a n i s m s are divided i n t o two classes: electromagnetic (EM) a n d chemical. In the EM mechanisms, the e n h a n c e m e n t arises from the amplification of the i n c i d e n t a n d the R a m a n scattered E M fields o n rough metal surfaces via r e s o n a n t excitations of the surface p l a s m o n p o l a r i t o n (SPP) of bulk metals or the collective electron r e s o n a n c e of small metal particles. The chemical m e c h a n i s m is a kind of r e s o n a n c e R a m a n scattering which occurs via charge-transfer between adsorbates a n d the metal. It is well k n o w n that surface roughness is necessary for the EM m e c h a n i s m s [2]. The top of the p r o t r u s i o n s or b u m p s o n the metal surface have been believed to be the major SERS active sites [3]. Recently it was proposed that extremely small cavities or pores present on surfaces, such as coldly deposited Ag, are a n o t h e r i m p o r t a n t type of SERS active site [4,5]. However this "cavity site m o d e l " has not been completely accepted yet. In the present study we report the SERS of thin organic films of p - n i t r o b e n z o i c acid ( P N B A ) a n d c o p p e r - p h t h a l o c y a n i n e (CuPc) o n evaporated Ag films which strongly supports the cavity site model. 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
3.t. Osawa et al. / SERS of organic molecules in evaporated Ar
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2. Experimental R a m a n measurements were carried out using the Kretschmann attenuated total reflection (ATR) configuration [6], in which Ag was vacuum evaporated onto the reflecting plane of a hemicylindrical A T R prism (LaSF15 glass, refractive index n = 1.88 at 514.5 nm). The optical arrangement, details of which have been described elsewhere [7], is schematically shown in the inset of fig. 1. R a m a n intensity (peak height from the base line) was measured as functions of the incidence angle (0) and polarization (s and p) of the 514.5 nm line from an Ar ÷ laser (output power of 100 mW). The X, Y and Z axes are defined on the reflecting plane of the prism as shown in the figure for further discussion. The mass thickness ( d ) of the Ag films was monitored with a quartz microbalance and the evaporation rate was kept at 1 n m / m i n . According to field emission scanning electron microscopic observations [7], the Ag films of d = 20-50 nm were continuous but not smooth. Numerous bumps of a few tens of nm in size were observed on the surfaces. Numerous pores and crevices of a few tens of n m in diameter and width, respectively, were also observed on the films of d = 20-30 nm, which were missing on the film of d = 50 nm. Thin PNBA films were formed by dropping its acetone solution on the Ag surfaces and then evaporating the solvent. The CuPc films were deposited on the Ag surfaces by sublimation in vacuum.
3. Results and discussion Fig. 1 shows a SER spectrum of a thin PNBA film of 5 nm in thickness deposited on a Ag film of d = 20 nm, measured with the p-polarized laser b e a m at 0 = 35 o. This spectrum is the same as that observed on Ag island films [8,9]. All the bands observed here, except those denoted by asterisk and an arrow, are attributed to the p-nitrobenzoate ( P N B A - ) ion produced via strong chemisorption through the carboxylate group to the Ag surface [8,9]. The 345 c m - 1 band denoted by the arrow may be assigned to the A g - O bond. The 1460, 1140 and 935 cm -1 bands denoted by asterisk were attributed to azodibenzoate formed by the reductive coupling of PNBA molecules during larger irradiation [9]. The R a m a n intensity of the P N B A - ion was almost independent of the thickness of the P N B A film. In addition, no C--O stretching band of PNBA (observed at 1635 cm -1 in the normal R a m a n spectrum of the bulk crystal) was observed even for thicker PNBA films. These results indicate that the SERS of PNBA is restricted to the first monolayer adsorbed directly on the Ag surface. This is one of the characteristic properties of the SERS originating from the chemical mechanism [2]. However it must be noted that the contribu-
892
M. Osawa et aL / S E R S of organic molecules in evaporated A r
Z ~ ,~ Raman ', J / scattering org. film . ', J /
k
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Fig. 1. S u r f a c e e n h a n c e d R a m a n spectrum from a thin P N B A film deposited on a n e v a p o r a t e d A g film of 20 n m in thickness. The inset shows the Kretschmann A T R configuration used in the present study.
tion of the EM mechanism is also important. The enhancement factor resulting from the EM mechanism on the Ag films of d = 20 nm is estimated to be 103 as described below. Since the total enhancement factor of the SERS of the PNBA ion was estimated to be 104-105 from the comparison of the intensity of the 1355 cm -1 band in the SER spectrum shown in fig. 1 and the normal Raman spectrum of the sodium salt of P N B A - , the enhancement factor resulting from the chemical contribution can be estimated to be
101_102" In fig. 2 the relative Raman intensity of some Raman bands of the PNBA ion, normalized to the intensity of the 1355 cm-~ band, is plotted versus their depolarization ratios (Pt) [10] both for the SER spectrum and the normal zo z ~ z
o 1.0
°
z
l
n-" 0.5 IaJ ~'
P~
n-'
0
I 0_1
gtzz: axx >>atyy C~zz>>~xx, C~yy 0.2
0.3
0.4
~xx=~yy=azz DEPOLARIZATION
RATIO
Fig. 2. R e l a t i v e R a m a n intensity of the als modes of P N B A adsorbed on a A g film of d = 20 nm (©) a n d in a basic solution (O) as a function of the depolarization ratio. The intensity is n o r m a l i z e d to that of the 1350 c m - 1 b a n d . T h e numbers in the parentheses are the p e a k frequencies of the R a m a n b a n d s .
M. Osawa et aL / SERS of organic molecules in evaporated Ar
893
R a m a n spectrum of the P N B A - ion in a basic solution [10]. According to the normal coordinate analysis by Ernstbrunner et al. [10], all these bands are assigned to alg modes in C2v symmetry. As can be seen in the figure, the relative Raman intensity of the 1600 cm -1 band increases selectively in the SERS. The change in the relative Raman intensity can be related to the molecular orientation [11,12]. The Raman tensor of the alg modes is diagonal and has a lae ranging from 0 to 3 / 4 [13]. If one of the diagonal elements is very much larger than the others, p c = l / 3 . Since the 1600 cm -1 band is assigned to an in-plane stretching mode of the phenyl ring accompanied mainly with a dipole change in the z-axis [10] and its Pe (0.38) is close to 1/3, txzz >> axx, ayy (where the molecular axis z is coincident with the C 2 axis). With decreasing Pc, the relative contribution of the azz element to the Raman scattering decreases as shown in fig. 2. Considering that the adsorbed P N B A - ion is oriented with its C 2 axis almost normal to the metal surface [14], the selective increase in the relative intensity of the 1600 c m - ~ band indicates that the EM field responsible for the SERS is parallel to the C 2 axis of the adsorbed P N B A - ion and also normal to the metal surface [11,12]. The same conclusion can be drawn on the basis of the chemical mechanism because the charge-transfer between the metal and the adsorbates should be excited by the EM field normal to the surface and hence the Raman tensor element azz should be increased. Copper-phthalocyanine (CuPc) is a suitable organic compound to investigate the EM contribution to the SERS free from the chemical contribution [15]. In fig. 3 the intensity of the 1530 cm -1 Raman band of CuPc on Ag films of d = 20 (a) and 30 nm (b) is plotted versus the thickness (t) of the CuPc film. The intensity was normalized to that of the normal Raman scattering
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800 ILl I-Z Z .<
:E .< n-
b
o 400 w N < rr0 Z I
I
10
I
20
THICKNESS OF CuPc FILM / n m
Fig. 3. Change in the Raman intensity at 1530 cm-~ with increasing thickness of the CuPc film deposited on Ag films of d = 20 (a), 30 (b) and 50 nm (c).
894
M. Osawa et al. / S E R S of organic molecules in evaporated Ar
from a CuPc film of t = 1 nm deposited directly on the A T R prism (without the metal). Therefore, the relative Raman intensity at t = 1 nm is equal to the enhancement factor. The measurements were carried out with a p-polarized laser beam at optimum incidence angles (0 = 35 ° - 4 0 o, cf. fig. 4). The experimental results obtained using the Ag film of d = 50 nm (c) is also shown for comparison. The Raman intensity increases linearly with increasing t on the Ag film of d = 50 nm, which is well explained by the long-range SPP field [7]. This long-ranged enhancement rules out the contribution of the chemical mechanism in the SERS of CuPc. On the Ag films of d = 20-30 nm, however, the Raman intensity greatly increases at t less than 1 nm and then slightly decreases (for d = 20 nm) or increases (for d = 30 nm), indicating the enhancement on these Ag films is short-ranged. Therefore, it is concluded that certain EM fields localized near the surface are responsible for the SERS on the Ag films of d = 20-30 nm. It has been theoretically shown that the local EM fields on top of protrusions or bumps on rough surfaces are very strong [3]. The EM field within depressions on the surface (space between the bumps) also should be strong [16]. These EM fields are short-ranged, corresponding to the observed shortranged enhancement. However, the strongest Raman signal is not located on these sites in the SERS observed in the present study because such surface roughness was observed both on Ag films of d = 20-30 nm and d = 40-50 nm but the effective spatial range of the enhancement for these two groups of Ag films is clearly different as shown in fig. 3. In fig. 4 the SERS intensity of the 1355 cm ~ band of the PNBA ion on the Ag film of d = 20 nm (a) and the 1530 cm -1 band of CuPc on the Ag film of d = 30 nm (b) are plotted as a function of 0 both for p- and s-polarization (open and closed circles, respectively). For the p-polarized laser beam the intensity is very weak around the critical angle of total reflection 0 ( = s i n - T ( 1 / n ) = 32.1 °) and a maximum is observed at an angle slightly greater than 0c. For s-polarization, on the other hand, the 0 dependence is less remarkable and a maximum is observed around 0c. The solid curves in the figure are the 0 dependence of the average electric field strength I E 12 ( E is the amplitude) within the Ag layers of d = 20 nm (a) and 30 nm (b) for the incident laser beam calculated with the Fresnel formula. The subscripts to E in the figure denote the polarization state of the incident laser beam (s, p) and oscillating direction of the field (X, Y, Z). Details of the calculation have been described previously [7]. Briefly, the metal layer is assumed to be homogeneous. In order to take the porous structure of the actual Ag films into account, the dielectric constant (%ff) of a hypothetical metal-void (filled with molecules and ions) composite layer was used in the calculation in stead of that of bulk metal. The %fr's used here are rather arbitrarily chosen but very close to those of evaporated thin Ag films mea-
M. Osawa et al. / SERS of organic molecules in euaporated Ar
895
sured experimentally by Philip and Trompett [17] with h = 508.6 nm light. The volume fraction of the metal (q) of the composite layers is assumed to be 0.9 for d = 20 nm and 0.95 for d = 30 nm [17]. The calculation shows that the Z component of the average field is very weak and the field is essentially in the plane of the film, the X-Y plane, regardless of the polarization of the incident laser beam. The agreement between the electric field strength of the X and Y components ( I Epx 12 and I E~yI 2) and the SERS intensity measured with p- and s-polarized laser beams, respectively, is remarkable. Considering that there exist numerous pores and crevices in the Ag films of d = 20-30 nm and that the enhancement is short-ranged on these surfaces, this agreement strongly suggests that the molecules and ions within these pores or crevices exhibit such enhancement. The electric field calculated here is not strong enough to explain the observed enhancement of the Raman scattering. One of the possible explanations of this problem is that the EM field within the cavities such as pores or crevices can be increased and hence the Raman scattering by the molecules inside the cavities are enhanced [18-21]. Since this local EM field is considered, to first order, to be proportional to the average field calculated in the present study [18], the SERS observed using the evaporated Ag films of d = 20-30 nm is well explained by this model. This model is suitable also for the interpretation of the experimental results shown in fig. 2. As mentioned above the EM field responsible for the SERS of P N B A - is normal to the metal
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2
21
0
Esy
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5
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o
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,7,
o
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0
30
0 / deg.
60
90
0
60
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Fig. 4. 0 dependence of the R a m a n intensity from molecules on Ag films measured with p- and s-polarized laser beams (© and O, respectively): (a) the 1350 cm -1 band of P N B A - adsorbed on a Ag film of d = 20 nm; (b) the 1530 cm -1 band of CuPc deposited on a Ag film of d = 30 nm. The solid curves show the calculated average electric field strength within hypothetical metal-void composite layers having the effective dielectric constants of (a) - 5.72 + 0.96i and (b) - 8.99 + 0.6i. The subscripts to E show the polarization of the incident laser beam (s, p) and the oscillating direction of the field (X, Y, Z). E 0 is the amplitude of the incident laser beam.
896
M. Osawa et al. / S E R S of organic molecules in evaporated Ar
surface a n d also s h o u l d be in the X - Y p l a n e as shown in fig. 4. This i n d i c a t e s that the molecules which exhibit the e n h a n c e m e n t m u s t be a d s o r b e d on the m e t a l surface n o r m a l to the X~ Y plane. Such a surface is c o n c e i v a b l e inside the p o r e s a n d crevices. A c c o r d i n g to the cavity site model, the R a m a n intensity n o t o n l y of P N B A - ions b u t also of P N B A molecules inside the cavities s h o u l d be e n h a n c e d . H o w e v e r , no evidence of this p r e d i c t i o n c o u l d be o b t a i n e d b e c a u s e the c h e m i c a l m e c h a n i s m c o n t r i b u t e s to the e n h a n c e m e n t only for P N B A ions, which m a k e s it difficult to d e t e c t weaker R a m a n signals from P N B A molecules. F i n a l l y , we n o t e that the intensity of the E M field within the p o r e s in the m e t a l films d e p e n d s on the w a v e l e n g t h of the light. Recently, we showed that the e n h a n c e d i n f r a r e d ( I R ) a b s o r p t i o n b y molecules a d s o r b e d on e v a p o r a t e d thin m e t a l films [22,23] also can be e x p l a i n e d b y the cavity site m o d e l [24,25]. A c c o r d i n g to the m o d e l o f B e r g m a n a n d N i t z a n [20], however, the E M field within the p o r e s is n o t so strong in the I R region. F u r t h e r m o r e , we o b t a i n e d e x p e r i m e n t a l results which suggest that the local E M fields on r o u g h m e t a l surfaces are also i m p o r t a n t for the e n h a n c e m e n t of I R a b s o r p t i o n [25]. A l t h o u g h such local E M fields are n o t p r e d o m i n a n t l y i m p o r t a n t in the S E R S o b s e r v e d in the p r e s e n t study, further investigation of these local E M fields is desirable.
Acknowledgement O n e of the a u t h o r s (M.O.) is grateful to the M i n i s t r y of E d u c a t i o n , Science a n d C u l t u r e of J a p a n for financial s u p p o r t through a G r a n t - i n - A i d for E n c o u r a g e m e n t of Y o u n g Scientists (No. 61750708).
References [1] H. Seki, J. Electron Spectrosc. Related Phenomena 39 (1986) 289, and references therein. [2] For example, R.K. Chang and T.E. Furtak, Eds., Surface Enhanced Raman Scattering (Plenum, New York, 1982). [3] J. Gersten and A. Nitzan, J. Chem. Phys. 73 (1980) 3023. [4] E.V. Albano, S. Daiser, G. Ertl, R. Miranda, K. Wandelt and N. Garci&, Phys. Rev. Letters 51 (1983) 2314. [5] H. Seki and T.J. Chuang, Chem. Phys. Letters 100 (1983) 393. [6] E. Kretschmann, Z. Phys. 241 (1971) 313. [7] M. Osawa and W. Su~taka, Surface Sci. 186 (1987) 583. [8] R. Dornhaus, R.E. Benner, R.K. Chang and I. Chabay, Surface Sci. 101 (1980) 367. [9] P.G. Roth, R.S. Venkatachalam and F.J. Boerio, J. Chem. Phys. 85 (1986) 1150. [10] E.E. Ernstbrunner, R.B. Girling and R.E. Hester, J. Chem. Soc. Faraday Trans. II, 74 (1978) 1540.
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[11] [12] [13] [14] [15] [16]
M. Moskovits, J. Chem. Phys. 77 (1982) 4408. J.A. Creighton, Surface Sci. 124 (1983) 209. For example, D.A. Long, Raman Spectroscopy (McGraw-Hill, New York, 1977). J.T. Hall and P.K. Hansma, Surface Sci. 71 (1978) 1. S. Hayashi and M. Samejima, Surface Sci. 137 (1984) 442. N. Liver, A. Nitzan and J.I. Gersten, Chem. Phys. Letters 111 (1984) 449; J. Gersten and A. Nitzan, Surface Sci. 158 (1985) 165. [17] R. Philip and J. Trompett, Compt. Rend. (Paris) 241 (1955) 627. [18] H. Chew and M. Kerker, J. Opt. Soc. Am. B 2 (1985) 1025. [19] H. Seki, T.J. Chuang, J.F. Escobar, H. Morawitz and E.V. Albano, Surface Sci. 158 (1985) 254.
[20] [21] [22] [23] [24] [25]
D.J. Bergman and A. Nitzan, Chem. Phys. Letters 88 (1982) 409. D.E. Aspnes, Phys. Rev. Letters 48 (1982) 1629. A. Hartstein, J.R. Kirtley and J.T. Tsang, Phys. Rev. Letters 45 (1980) 201. A. Hatta, Y. Suzuki and W. SuStaka, Appl. Phys. A 35 (1984) 135. M. Osawa, M. Kuramitsu, A. Hatta, W. Su8taka and H. Seki, Surface Sci. 175 (1986) L787. Y. Suzuki, M. Osawa, A. Hatta and W. SuEtaka, Appl. Surface Sci. 33/34 (1988) 875.