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Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size
Surface Science Letters 250 (1991) L383-L388 North-Holl~d
SurfaceScience Letters
Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size P. Dawson *, J.W. Haas III b, K.B. Alexander a Department of Pure and Applied Physics, Queen’s University, ’ Health and Safety Division, Oak Ridge N~~ona~ Laboratory,
b, J. Thompson
b and T.L. Ferrell
b
Belfbst, BT7 INN, UK Oak Ridge, TN 378313, USA
Received 25 February 1991; accepted for publication 27 March 1991
The intensity of surface enhanced Raman scattering from berm&c acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal gram size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate gram size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate., it is argued that the contrast in Raman scattering is due to differences in elastic gram boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.
1. Introduction In a recent examination [l] of surface enhanced Raman scattering (SERS) from benzoic acid on CaF,-roughened Ag films the signal intensity was found to depend significantly on the rate at which the Ag had been deposited; slowly deposited (typically 0.1 nm s- ‘) Ag films gave a superior performance to quickly deposited (typically 2 nm s-‘) films. Such mildly roughened surfaces are not outst~din~y SERS active but were chosen because of their amenability to characterisation by both scanning tunnelling microscopy (STM) and tr~s~ssion electron microscopy (TEM). Also the enhancement mechanism is unambiguously connected with the excitation of surface plasmon polaritons (SPPs) [l-3] which makes for more straightforward interpretation of the results than might otherwise be the case. The most significant physical distinction between the two types of film was in the metal grain size. It was argued that the superior SERS signals from the large grained, slowly deposited films was due to relatively less SPP scattering at intergrain boundaries and a consequent, relative decrease in internal damping of 0039~6028/91/$03.50
SPPs excited on the metal surface; the argument is briefly recapped below. In this Letter we report the extension of our initial work [l] to various benzoic acid derivatives (amino-, iodo-, and nitro-benzoic acid) and to metal films of intermediate grain size. Importantly the physical trends observed in experiments on benzoic acid on Ag surfaces are confirmed and further specified. A more general experimental basis for understanding the influence of metal grain size on SERS is thus established. So far as we are aware this matter has not previously been studied in the context of either roughened surfaces (and delocalised SPPs) or of metal island arrays (and localised SPP modes) [4]. Indeed, even in the area of elastic SPP-photon scattering the influence of metal grain size has received only very limited attention [5,6].
2. Experiment and results The samples were prepared by first e-beam evaporating a 126 nm thick layer of CaF, (average rate 0.6 nm s-l) at normal incidence onto a clean
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
P. Dawson et al. / Surfaceenhanced Raman
scatteringfrom mildly roughened surfaces 1B
r.AMINOI BENZOIC ACID
IODO-BENZOIC
ACID
800
-600 'i VI if z400 s v = 24f 5; z z z - 200
160
tzc 800
1000 WAVE
1200 NUMBER
,
1400
1600
J
(cm-l)
WAVE
700/ -CNITRO-
40(
120
8C
I
1
800
I
I
1000 1200 1400 WAVE NUMBER (cm-')
1
1600
1
NUMBER
(cmq)
P. Danvan et al. / Surface enhanced Raman scattering frommildly toughened surfaces
glass substrate at ambient temperature, at a base pressure of 4 X 10F4 Pa. Subsequently 48 nm of Ag was deposited over the CaF, layer by thermal evaporation at a pressure of 2 X 10V4 Pa, again at ambient temperature. For the three sets of samples yielding the results presented here, three distinctly different Ag deposition rates were used 0.095 nm s-l, 0.48 nm s-l and 1.72 nm s-l. One ~1 of lo-’ molar solution of each benzoic acid derivative in methanol ws in turn spotted onto a freshly prepared Ag substrate of each type. All derivatives were para-substituted in order to minimize intramolecular interactions between the substituents. The SERS spectra were obtained immediately after sample preparation using light of 647.1 nm wavelength and 78 mW power from a krypton laser, incident on the Ag surface at an angle of - 45 O. Typical spectra for amino-, iodoand nitro-benzoic acid on all three types of Ag film are presented in fig. 1. All samples share a common CaF, substrate; in addition ah the spectra labelled (i) were taken from different portions of the same Ag substrate and likewise for the spectra labelled (ii) and (iii). For a given chemical species it is clear that in progressing from fast- to slow-deposited films (from spectra (i) to (iii) in each case) the SERS signals improve significantly; the general background level of scattered light also increases, though to a lesser extent. These features are further detailed in fig. 2 which presents the photon count rates for the SERS signals and corresponding background as a function of wave number. While the background count rates are not always unambiguously ascertainable and the intensities plotted may therefore be subject to some error, the general trend of the data is both obvious and noteworthy. Two further points in relation to the SERS data should be noted. Firstly, films with no deliberate roughening, irrespective of deposition rate, yielded no measurable SERS signals. Secondly the results show no dramatic dependence on the polarisation
Fig. 2. Photon count rates for SERS signals and corresponding backSrounds as function of wave number. Data represented as circles for amino-benzoic acid, triangles for iodo-benaoic acid and squares for nitro-benzoic acid. Filled, centre-dotted and empty symbols refer to species on Ag films deposited at fast, intermediate and slow rates respectively.
state of the input beam. The SERS signals obtamed with p-polarised input are approximately 30% stronger than those obtained with a spolarised input which is very much in line with previous results on CaFs-roughened Ag films 131. Detailed information on the analysis by STM and TEM of CaF, roughened Ag films is given in our previous work [l] and is not repeated here; only the principal features are summarised. Data pertaining to the Ag films used here are very
Fig. 1. SERS spectra of benzoic acid derivatives on CaFs-roughened Ag films, (A) Aminc+beuaoic acid, (B) iodo-benaoic acid, (C) nitro-benzoic acid. Spectra were taken for Ag films deposited at different rates, (i) fast (1.72 nm s-t), (ii) intermediate (0.48 mu s-l) and (iii) slow (0.095 nm s-l). Note that the large peak at - 1095 cm-‘, marked with an “ X ” in all cases, is due to background radiation and is therefore spurious.
I? Dawson et al. / Surface enhanced Raman scattering from mirdry roughened surfaces
similar to those of ref. [l] in every respect. The principal contrast between the differently prepared Ag films is the metal gram size; the average lateral grain dimensions were 40 nm (fast deposition, 1.72 nm s-l), - 90 nm (intermediate deposition, 0.48 nm s-i) and - 150 nm (slow deposition, 0.095 nm s-l). A point of lesser contrast is the root mean square roughness height, 6, as determined using STM. Here it was found that the larger values of S (typically 2.0-2.2 nm) were associated with the small grained (fast-deposited) films and the smaller values (typically 1.4-1.5 nm) with the larger grained (slow-deposited) films. No data on the intermediate case were taken. The reason for this difference is possibly that the faster deposition replicates better the roughness of the underlying CaEi, layer. Very little distinction, if any, can be made between the different Ag films with regard to the average lateral scale of roughness. It is thus interesting to note that the metal grains do not make a noticeable ~n~bution to the lateral characteristics of the surface roughness; the influence of the rough CaF, underlayer is predo~nant.
3. Discussionand conclusions From figs. 1 and 2 it is clear that for any given type of Ag film there are differences in the absolute values of the SERS signals depending on which benzoic acid derivative is being examined. Details of the chemistry and adsorption process presumably differ from case to case. However the salient feature of physical significant is that, for a given species, all the SERS signals increase significantly in intensity in progressing from small grained to large grained Ag films. This is true for all lines recorded for a given benzoic acid derivative and for each derivative, as well as for benzoic acid itself [f]. It is this trend and the scattering mechanism to which we direct our attention. An explanation of these observations is based on the premise that for the type of surface examined here, the SERS is brought about by optical field amplification due to the excitation of de~o&~~is~d SPPs; there is strong evidence to this effect in other work [3]. Also, comparison of the
modest polarisation dependence of the SERS signal intensities with that of radiative, elastic scattering of delocalised SPPs on CaFz-roughened Ag metal films (see e.g. ref. [7]) and with that of the contrasting case of local&d SPPs on metal island arrays [8], implies that localised SPP resonances do not play an import~t role here. Furthermore, the surfaces used in this work do not exhibit the rugged, very small scale (< 10 nm) features associated with cold-deposited Ag films [9] and electrochemically etched Ag electrodes [lO,ll] where the short-range or chemical enhancement mechanism is important [12,13]. (Refs. 19-111 report STM analysis of SERS active surfaces but present no accompanying SERS data.) Thus, the most important enhancement mechanisms on many SERS active substrates, optical field enhancement due to localised SPPs and the short range enhancement mechanism, appear to play only a minor role, if any, on the mildly roughened Ag films examined here. The increase in SERS activity with metal grain size cannot be explained in terms of variation in SPP-photon coupling due to differing surface roughness characteristics. Indeed, since the coupling efficiency varies as a2 [3,14] and linearly as the surface roughness power spectral density [14], the small grained films should offer a superior SERS activity. (They have a power spectral density little different from that of the large grained films but a larger value of 6.) It is therefore argued that the superior SERS activity of the large grained films is accounted for in terms of elastic grain boundary scattering of the SPPs as follows [l]. In large grained films the elastic scattering mean free path is greater than in small grained films thus allowing the SPPs greater opportunity to scatter radiatively from surface inhomogeneities. The radiative scattering mean free path is thus relatively shorter and, since the internal damping per unit path length is constant, internal damping is relatively less. Conversely, with increased elastic scattering (small grained films) the radiative scattering path length increased and SPP decay is more likely to proceed by means of internal damping than through radiative scattering. This applies whether the radiative scattering itself is elastic or inelastic; there is a direct experi-
P. Dawson et al. / Surface enhanced Raman scattering from mildly roughened surfaces
mental connection between elastic SPP-photon coupling efficiency and the intensity of SERS signals from CaF, roughened Ag films [3]. In summary, a metal film of large grain size will produce stronger SERS signals than an equally rough film of small grain size. In progressing frorh small to large grained samples it is clear that for ah lines the SERS signal always increases more than proportionately to the background and by considerably more in certain cases e.g. an average x10 increase in the SERS signal for the carbon ring stretch mode at - 1600 cm-',common to all the derivatives, compared with only a X 1.3-2.7 increase in the background. However, there is no obvious, simple relationship between the intensities of the background and SERS signals from one Ag film to another. Is there any physical basis to suppose that there might be? It may be envisaged that the Raman energy shift is effected in the radiative scattering event or on the coupling of light to a SPP in the Ag film. In the process of SPP creation or armihilation the conditions obtain for the necessary exchange of energy and momentum between the adsorbed molecule and the metal film, i.e. enhancement of the Raman signal occurs in both input and output channels. In addition the possibility of inelastic SPP-SPP roughness scattering may exist provided there is availability of appropriately d~ension~ surface roughness features; the inelastically scattered SPP must of course radiative subsequently in order to contribute to the Raman signal. In all these scattering events parallel channels exist for the elastic scattering of light which, if scattered through the spectrometer, gives rise to a background signal of SPP origin. For given surface roughness characteristics this background should increase in direct proportion to the SERS signals as the metal grain size increases. Conformity of the experimentally observed intensities to simple linear relationship may be achieved if it is assumed that SPP elastically re-emitted radiation accounts for only a portion of the recorded background. The remainder of the background is taken to be due to scattering of the primary beam through the filtration-single spectrometer system used in this work. Carrying out appropriate background adjustment for the 1600
L387 1
cm -* line, for example, leads to a linear relationship to within rt 10% on average for each of the benzoic acid derivatives. The procedure is rather arbitrary but seems physically reasonable pending the acquisition of more detailed data. The results presented here have been corroborated in recent, ~dependent follow-up experiments [15] on elastic SPP-photon scattering on very similarly prepared Ag surfaces. In this work the scattering of SPPs to light (at 632.8 nm) was found to be more efficient (by a factor of 2-3) on films with large (120 nm} metal grains. The connection between large metal grain size and more efficient SPP to photon decay also holds true in the context of light emitting metal-oxide-metal tunnel junctions [6] where the emission is due to the radiative decay of SPPs excited by noise fluctuations in the tunnel current [16]. A consistent view of the importance of metal grain size in SPP-photon scattering efficiency, whether elastic or inelastic, is thus emerging. Here, it has been demonstrated for benzoic acid derivatives, that with increasing Ag grain size the efficiency in SERS increases quite subst~ti~ly.
Acknowledgements One of us (P.D.) would like to acknowledge the support of the Oak Ridge National Laboratory during the period this work was carried out and funding for travel from grant GR/F 12242 from the UK Science and Engineering Research Council.
References PI P. Dawson, K.B. Alexander, J. Thompson, J.W. Haas III and T.L. Fezrell, Phys. Rev. B 15, in press. Tsang, J.R. Kirtley and J.A. Bradley, Phys. Rev. Lett. 43 (1979) 772. [31 S. Hayashi, Surf. Sci. 158 (1985) 229. [41 A. Wokaun, Mol. Phys. 56 (1985) 1. PI D.G. Hall and A.J. Braundmeier, Phys. Rev. B 17 (1978) 1557. PI A.J.L. Ferguson, D.G. Walmsley, H.P. Hagan, R.J. Turner and P. Dawson, 3. Phys. (Condensed Matter) 1 (1989) 7931.
121J.C.
P, Lawson et al. / Surface enhanced Raman scattering from mildly roughened surfaces
H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, VoI. 3 of Springer Tracts in Modern Physics (Springer, Berlin, 1988). PI M.J. Bloemer, T.L. Ferrell, MC. Buncick and R.J. Warmack, Phys. Rev. B 37 (1988) 8015. [91 J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. PO1 O.A. Atksipetrov, S.I. Vasil’ev and V.I. Panov, JETP Lett. 47 (1988) 1010. Pll K. Sakamaki, K. Itoh and A. Fujihisma, J. Vat. Sci. Technol. A 8 (1990) 525.
[12] T.E. Furtak and D. Roy, Surf. Sci. 158 (1985) 126. (131 A. Otto, T. Bomemann, 0. Erttirk, I. Mrozek and C. Pettenkofer, Surf. Sci. 210 (1989) 363. [14] D.L. Mills, Phys. Rev. B 12 (1975) 4036; A.A. Maradudin, in: Surface Polaritons, Eds. V.M. Agvanovich and D.L. Mifls (North-HoB~d, Amsterdam, 1982) p. 405. [15] A.J.L. Ferguson, P. Dawson and D.G. Walmsley, in preparation. [16] D. Hone, B. Muhlschlegel and D.J. Scaiapino, Appl. Phys. Lett. 33 (1978) 203.
SurfaceScience Letters
Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size P. Dawson *, J.W. Haas III b, K.B. Alexander a Department of Pure and Applied Physics, Queen’s University, ’ Health and Safety Division, Oak Ridge N~~ona~ Laboratory,
b, J. Thompson
b and T.L. Ferrell
b
Belfbst, BT7 INN, UK Oak Ridge, TN 378313, USA
Received 25 February 1991; accepted for publication 27 March 1991
The intensity of surface enhanced Raman scattering from berm&c acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal gram size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate gram size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate., it is argued that the contrast in Raman scattering is due to differences in elastic gram boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.
1. Introduction In a recent examination [l] of surface enhanced Raman scattering (SERS) from benzoic acid on CaF,-roughened Ag films the signal intensity was found to depend significantly on the rate at which the Ag had been deposited; slowly deposited (typically 0.1 nm s- ‘) Ag films gave a superior performance to quickly deposited (typically 2 nm s-‘) films. Such mildly roughened surfaces are not outst~din~y SERS active but were chosen because of their amenability to characterisation by both scanning tunnelling microscopy (STM) and tr~s~ssion electron microscopy (TEM). Also the enhancement mechanism is unambiguously connected with the excitation of surface plasmon polaritons (SPPs) [l-3] which makes for more straightforward interpretation of the results than might otherwise be the case. The most significant physical distinction between the two types of film was in the metal grain size. It was argued that the superior SERS signals from the large grained, slowly deposited films was due to relatively less SPP scattering at intergrain boundaries and a consequent, relative decrease in internal damping of 0039~6028/91/$03.50
SPPs excited on the metal surface; the argument is briefly recapped below. In this Letter we report the extension of our initial work [l] to various benzoic acid derivatives (amino-, iodo-, and nitro-benzoic acid) and to metal films of intermediate grain size. Importantly the physical trends observed in experiments on benzoic acid on Ag surfaces are confirmed and further specified. A more general experimental basis for understanding the influence of metal grain size on SERS is thus established. So far as we are aware this matter has not previously been studied in the context of either roughened surfaces (and delocalised SPPs) or of metal island arrays (and localised SPP modes) [4]. Indeed, even in the area of elastic SPP-photon scattering the influence of metal grain size has received only very limited attention [5,6].
2. Experiment and results The samples were prepared by first e-beam evaporating a 126 nm thick layer of CaF, (average rate 0.6 nm s-l) at normal incidence onto a clean
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
P. Dawson et al. / Surfaceenhanced Raman
scatteringfrom mildly roughened surfaces 1B
r.AMINOI BENZOIC ACID
IODO-BENZOIC
ACID
800
-600 'i VI if z400 s v = 24f 5; z z z - 200
160
tzc 800
1000 WAVE
1200 NUMBER
,
1400
1600
J
(cm-l)
WAVE
700/ -CNITRO-
40(
120
8C
I
1
800
I
I
1000 1200 1400 WAVE NUMBER (cm-')
1
1600
1
NUMBER
(cmq)
P. Danvan et al. / Surface enhanced Raman scattering frommildly toughened surfaces
glass substrate at ambient temperature, at a base pressure of 4 X 10F4 Pa. Subsequently 48 nm of Ag was deposited over the CaF, layer by thermal evaporation at a pressure of 2 X 10V4 Pa, again at ambient temperature. For the three sets of samples yielding the results presented here, three distinctly different Ag deposition rates were used 0.095 nm s-l, 0.48 nm s-l and 1.72 nm s-l. One ~1 of lo-’ molar solution of each benzoic acid derivative in methanol ws in turn spotted onto a freshly prepared Ag substrate of each type. All derivatives were para-substituted in order to minimize intramolecular interactions between the substituents. The SERS spectra were obtained immediately after sample preparation using light of 647.1 nm wavelength and 78 mW power from a krypton laser, incident on the Ag surface at an angle of - 45 O. Typical spectra for amino-, iodoand nitro-benzoic acid on all three types of Ag film are presented in fig. 1. All samples share a common CaF, substrate; in addition ah the spectra labelled (i) were taken from different portions of the same Ag substrate and likewise for the spectra labelled (ii) and (iii). For a given chemical species it is clear that in progressing from fast- to slow-deposited films (from spectra (i) to (iii) in each case) the SERS signals improve significantly; the general background level of scattered light also increases, though to a lesser extent. These features are further detailed in fig. 2 which presents the photon count rates for the SERS signals and corresponding background as a function of wave number. While the background count rates are not always unambiguously ascertainable and the intensities plotted may therefore be subject to some error, the general trend of the data is both obvious and noteworthy. Two further points in relation to the SERS data should be noted. Firstly, films with no deliberate roughening, irrespective of deposition rate, yielded no measurable SERS signals. Secondly the results show no dramatic dependence on the polarisation
Fig. 2. Photon count rates for SERS signals and corresponding backSrounds as function of wave number. Data represented as circles for amino-benzoic acid, triangles for iodo-benaoic acid and squares for nitro-benzoic acid. Filled, centre-dotted and empty symbols refer to species on Ag films deposited at fast, intermediate and slow rates respectively.
state of the input beam. The SERS signals obtamed with p-polarised input are approximately 30% stronger than those obtained with a spolarised input which is very much in line with previous results on CaFs-roughened Ag films 131. Detailed information on the analysis by STM and TEM of CaF, roughened Ag films is given in our previous work [l] and is not repeated here; only the principal features are summarised. Data pertaining to the Ag films used here are very
Fig. 1. SERS spectra of benzoic acid derivatives on CaFs-roughened Ag films, (A) Aminc+beuaoic acid, (B) iodo-benaoic acid, (C) nitro-benzoic acid. Spectra were taken for Ag films deposited at different rates, (i) fast (1.72 nm s-t), (ii) intermediate (0.48 mu s-l) and (iii) slow (0.095 nm s-l). Note that the large peak at - 1095 cm-‘, marked with an “ X ” in all cases, is due to background radiation and is therefore spurious.
I? Dawson et al. / Surface enhanced Raman scattering from mirdry roughened surfaces
similar to those of ref. [l] in every respect. The principal contrast between the differently prepared Ag films is the metal gram size; the average lateral grain dimensions were 40 nm (fast deposition, 1.72 nm s-l), - 90 nm (intermediate deposition, 0.48 nm s-i) and - 150 nm (slow deposition, 0.095 nm s-l). A point of lesser contrast is the root mean square roughness height, 6, as determined using STM. Here it was found that the larger values of S (typically 2.0-2.2 nm) were associated with the small grained (fast-deposited) films and the smaller values (typically 1.4-1.5 nm) with the larger grained (slow-deposited) films. No data on the intermediate case were taken. The reason for this difference is possibly that the faster deposition replicates better the roughness of the underlying CaEi, layer. Very little distinction, if any, can be made between the different Ag films with regard to the average lateral scale of roughness. It is thus interesting to note that the metal grains do not make a noticeable ~n~bution to the lateral characteristics of the surface roughness; the influence of the rough CaF, underlayer is predo~nant.
3. Discussionand conclusions From figs. 1 and 2 it is clear that for any given type of Ag film there are differences in the absolute values of the SERS signals depending on which benzoic acid derivative is being examined. Details of the chemistry and adsorption process presumably differ from case to case. However the salient feature of physical significant is that, for a given species, all the SERS signals increase significantly in intensity in progressing from small grained to large grained Ag films. This is true for all lines recorded for a given benzoic acid derivative and for each derivative, as well as for benzoic acid itself [f]. It is this trend and the scattering mechanism to which we direct our attention. An explanation of these observations is based on the premise that for the type of surface examined here, the SERS is brought about by optical field amplification due to the excitation of de~o&~~is~d SPPs; there is strong evidence to this effect in other work [3]. Also, comparison of the
modest polarisation dependence of the SERS signal intensities with that of radiative, elastic scattering of delocalised SPPs on CaFz-roughened Ag metal films (see e.g. ref. [7]) and with that of the contrasting case of local&d SPPs on metal island arrays [8], implies that localised SPP resonances do not play an import~t role here. Furthermore, the surfaces used in this work do not exhibit the rugged, very small scale (< 10 nm) features associated with cold-deposited Ag films [9] and electrochemically etched Ag electrodes [lO,ll] where the short-range or chemical enhancement mechanism is important [12,13]. (Refs. 19-111 report STM analysis of SERS active surfaces but present no accompanying SERS data.) Thus, the most important enhancement mechanisms on many SERS active substrates, optical field enhancement due to localised SPPs and the short range enhancement mechanism, appear to play only a minor role, if any, on the mildly roughened Ag films examined here. The increase in SERS activity with metal grain size cannot be explained in terms of variation in SPP-photon coupling due to differing surface roughness characteristics. Indeed, since the coupling efficiency varies as a2 [3,14] and linearly as the surface roughness power spectral density [14], the small grained films should offer a superior SERS activity. (They have a power spectral density little different from that of the large grained films but a larger value of 6.) It is therefore argued that the superior SERS activity of the large grained films is accounted for in terms of elastic grain boundary scattering of the SPPs as follows [l]. In large grained films the elastic scattering mean free path is greater than in small grained films thus allowing the SPPs greater opportunity to scatter radiatively from surface inhomogeneities. The radiative scattering mean free path is thus relatively shorter and, since the internal damping per unit path length is constant, internal damping is relatively less. Conversely, with increased elastic scattering (small grained films) the radiative scattering path length increased and SPP decay is more likely to proceed by means of internal damping than through radiative scattering. This applies whether the radiative scattering itself is elastic or inelastic; there is a direct experi-
P. Dawson et al. / Surface enhanced Raman scattering from mildly roughened surfaces
mental connection between elastic SPP-photon coupling efficiency and the intensity of SERS signals from CaF, roughened Ag films [3]. In summary, a metal film of large grain size will produce stronger SERS signals than an equally rough film of small grain size. In progressing frorh small to large grained samples it is clear that for ah lines the SERS signal always increases more than proportionately to the background and by considerably more in certain cases e.g. an average x10 increase in the SERS signal for the carbon ring stretch mode at - 1600 cm-',common to all the derivatives, compared with only a X 1.3-2.7 increase in the background. However, there is no obvious, simple relationship between the intensities of the background and SERS signals from one Ag film to another. Is there any physical basis to suppose that there might be? It may be envisaged that the Raman energy shift is effected in the radiative scattering event or on the coupling of light to a SPP in the Ag film. In the process of SPP creation or armihilation the conditions obtain for the necessary exchange of energy and momentum between the adsorbed molecule and the metal film, i.e. enhancement of the Raman signal occurs in both input and output channels. In addition the possibility of inelastic SPP-SPP roughness scattering may exist provided there is availability of appropriately d~ension~ surface roughness features; the inelastically scattered SPP must of course radiative subsequently in order to contribute to the Raman signal. In all these scattering events parallel channels exist for the elastic scattering of light which, if scattered through the spectrometer, gives rise to a background signal of SPP origin. For given surface roughness characteristics this background should increase in direct proportion to the SERS signals as the metal grain size increases. Conformity of the experimentally observed intensities to simple linear relationship may be achieved if it is assumed that SPP elastically re-emitted radiation accounts for only a portion of the recorded background. The remainder of the background is taken to be due to scattering of the primary beam through the filtration-single spectrometer system used in this work. Carrying out appropriate background adjustment for the 1600
L387 1
cm -* line, for example, leads to a linear relationship to within rt 10% on average for each of the benzoic acid derivatives. The procedure is rather arbitrary but seems physically reasonable pending the acquisition of more detailed data. The results presented here have been corroborated in recent, ~dependent follow-up experiments [15] on elastic SPP-photon scattering on very similarly prepared Ag surfaces. In this work the scattering of SPPs to light (at 632.8 nm) was found to be more efficient (by a factor of 2-3) on films with large (120 nm} metal grains. The connection between large metal grain size and more efficient SPP to photon decay also holds true in the context of light emitting metal-oxide-metal tunnel junctions [6] where the emission is due to the radiative decay of SPPs excited by noise fluctuations in the tunnel current [16]. A consistent view of the importance of metal grain size in SPP-photon scattering efficiency, whether elastic or inelastic, is thus emerging. Here, it has been demonstrated for benzoic acid derivatives, that with increasing Ag grain size the efficiency in SERS increases quite subst~ti~ly.
Acknowledgements One of us (P.D.) would like to acknowledge the support of the Oak Ridge National Laboratory during the period this work was carried out and funding for travel from grant GR/F 12242 from the UK Science and Engineering Research Council.
References PI P. Dawson, K.B. Alexander, J. Thompson, J.W. Haas III and T.L. Fezrell, Phys. Rev. B 15, in press. Tsang, J.R. Kirtley and J.A. Bradley, Phys. Rev. Lett. 43 (1979) 772. [31 S. Hayashi, Surf. Sci. 158 (1985) 229. [41 A. Wokaun, Mol. Phys. 56 (1985) 1. PI D.G. Hall and A.J. Braundmeier, Phys. Rev. B 17 (1978) 1557. PI A.J.L. Ferguson, D.G. Walmsley, H.P. Hagan, R.J. Turner and P. Dawson, 3. Phys. (Condensed Matter) 1 (1989) 7931.
121J.C.
P, Lawson et al. / Surface enhanced Raman scattering from mildly roughened surfaces
H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, VoI. 3 of Springer Tracts in Modern Physics (Springer, Berlin, 1988). PI M.J. Bloemer, T.L. Ferrell, MC. Buncick and R.J. Warmack, Phys. Rev. B 37 (1988) 8015. [91 J.K. Gimzewski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. Lett. 55 (1985) 951. PO1 O.A. Atksipetrov, S.I. Vasil’ev and V.I. Panov, JETP Lett. 47 (1988) 1010. Pll K. Sakamaki, K. Itoh and A. Fujihisma, J. Vat. Sci. Technol. A 8 (1990) 525.
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