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IV. WHAT IS "THE" SERS MECHANISM?
Surface-Enhanced Raman Scattering (SERS)
10. How does the SERS depend on the "solvent," especially
as regards the excitation spectrum?
11. What is the dependence of SERS of one molecule on the
concentration of other chemical species present?
12. What should be the effect of temperature annealing, and
how does it correlate with irreversible behavior under
electric potential changes?
13. Regarding the dependence of SERS on the Raman shift,
how does the SERS of the various modes in the same
14. How is the dependence of the excitation maximum on
the electric potential explained, especially as regards the
magnitude of the shift?
Most of these points were addressed in this review, both from the
experimental side and the theoretical aspect.
There are several other criteria which we have not mentioned
here, or discussed in the main body of this review, such as the
polarization dependence, angular dependence, selection rules, the
appearance of a continuum, and the dependence on intensity. These
topics were not discussed here due to space limitations. It seems
that the polarization characteristics, the angular dependence, and
the selection rules are less model sensitive and depend more strongly
on the metal dielectric properties and the adsorption formation
than on the specific enhancement mechanism. The continuum background, which appears in most SERS studies, is perhaps more
indicative of the enhancement mechanism, or parallel processes at
Another topic which was not discussed in this review is the
role of graphitic layers in the promotion of SERS. A short exposition
will be given in Section V.
This is an appropriate place to acknowledge the contributions
of the large number of researchers to the advancement of the
understanding of SERS, and to apologize to those whose work has
not been adequately covered in this review, as it certainly deserves.
2. "The" SERS Mechanism
This section is left for the reader to fill in!
The experimental facts (and myths) were critically reviewed
and exposed here. The theoretical models were discussed.
The present author can only reiterate his conclusion, stated in
the Introduction, based on the evaluation of theory and experiment
as given above: "There is no 'one' mechanism at the root of SERS;
however, there is a mechanism which, in the large majority of
systems, is the main contributor to the surface enhancement effect.
That mechanism is a resonance mechanism. It is felt there is not
enough evidence, yet, to determine which of the mechanisms
belonging to this group is the important one, or which can be ruled
out. The LFE mechanism certainly has a role, but a more minor
one. Note, however, that a minor factor in SERS is a factor of a
100 or so, which may be the difference between a detectable and
a nondetectable signal!"
V. SERS—A USEFUL TOOL
It is appropriate to begin this last section by quoting Furtak,138 in
one of the early SERS papers: 'The long sought for experimental
tool for detailed chemical characterization of the solid-aqueous
electrolyte interface may have at last been found." This was the
general feeling or at least the general wish. Has it been realized?
What does the future hold for it?
As was shown in this review, the major effort in SERS studies
was directed toward understanding this phenomenon, and generally
not toward its application and utilization. Nevertheless, some very
interesting information was acquired along the way, regarding
interfaces and molecules at interfaces, as can be seen from the
reports reviewed through this review. An increasing number of
studies is carried out now to use SERS as a tool and the proportion
can be expected to increase further.
In this section a brief summary of representative studies, which
have used SERS for surface studies, is given.
SERS is not the only method to apply Raman techniques to
surfaces. There are several other ways of increasing the sensitivity
of the Raman technique, and all work together to offer a versatile
and powerful instrument. Nowadays, Raman measurements can be
performed with the high sensitivity of OMA techniques, which pose
no special restrictions on the systems investigated (no specific metal
or surface preparation).28110'428
Surface-Enhanced Raman Scattering (SERS)
An alternative is using metal grating surfaces.110'170'175'281'429'430
Such surfaces can exhibit enhancements up to two orders of magnitude. Also ATR configurations can be used on a variety of surfaces.94'177'431"435 Interference techniques can be applied 436 as well
as waveguide techniques in 100-nm films.437"439 New geometries for
surface enhancement were suggested, for instance by Aravind et
An interesting possibility is inducing SERS activity, in a nonSERS-active substrate, by depositing submonolayer quantities of
silver on its surface. Van Duyne and Haushalter225 used this method
to measure Raman scattering from a GaAs semiconductor interface.
There was also an experiment to use a silver underlayer to induce
SERS in a layer covering it.256
A very important feature of SERS is that it works in "dirty"
rough-surface systems, and is relatively insensitive to the phase
adjacent to the metal. This allows its use in "real" systems of
interest, for instance, to the study of absorption in catalysis.
Another interesting characteristic of SERS is that, as was shown
above, in many systems special sites are most efficient in promoting
it. Thus, generally, SERS does not "see" all the molecules, but
mainly those adsorbed near surface defects. But these same defects
and sites may be the center of the chemical activity of the surface.
It would be of extreme importance if such a connection could be
made, and future research in this direction may prove very fruitful.
Knoll et al156 have shown that IR and SERS are sensitive to
molecules in different environments on the same system. Also,
Yamada et al.277 found for CO on silver that the IR exhibited a
band'at 1940 cm"1, while in SERS a band at 2112 cm"1 was observed.
Consequently, these two methods are complementary also on the
SERS provides a means to study adsorbed molecules through
their vibrational structure. However, a simple comparison is, alas,
not straightforward in most cases. Besides the chemical changes
which can occur, we still do not know in what way the enhancement
mechanism affects the various vibrational bands, shifts them, and
changes their relative intensities. It is established that the selection
rules on the surface are different than those which pertain to a bulk
situation. Hexter and Albrecht89 have analyzed the selection rules
by assuming that the metal surface is approximately a perfect mirror,
which adds a symmetry element, imposing some restrictions on
the appearance of vibrational bands. However, real (SERSactive) metals at optical frequencies are not perfect conductors
and one would not expect a complete disappearance of modes
which are parallel to the surface. Furthermore, in Raman scattering the direction of change of the polarizability is the important direction and not that of the vibrational transition dipole,
which complicates the analysis even further. Erdheim et al51 have
found that the SERS of pyrazine exhibited asymmetric bands,
which in solution are Raman forbidden. Kunz et al120 have demonstrated that azide ion adsorbed on silver exhibits SERS of IRallowed-Raman-forbidden bands. Moskovits and DiLella105'441
find similar behavior. Moskovits et al442444 analyze this in
terms of the existence of electric field gradients at the surface.
They show that the bands that appear have the proper symmetry
to be explained by this assumption. The possibility of the lowering of the symmetry upon adsorption in a site of a specific symmetry, was also mentioned by them and further discussed by
The vibrational analysis of the SERS spectrum can be further
complicated by the appearance of overtones.445 The net result is
that extreme care should be exercised when trying to use the
vibrational shifts and relative intensities to infer the identity of a
surface species and its immediate environment. Nevertheless, such
analyses were carried out in the past, and will continue to be
performed also in the future, until we have a better understanding
of the surface contributions involved.
Allen and Van Duyne446 have correlated the Raman intensity
for the cyano group in 2-, 3-, and 4-cyanopyridine to its direction
with respect to the surface. They found that the molecule for which
this group is parallel to the surface exhibits the weakest signal.
Creighton447 discussed the possibility of determining adsorbate
orientation from SERS relative intensities. The conclusion is that
it is feasible, unless chemical changes are involved.
SERS was used to determine the structure of adsorbed
molecules. Many examples were cited in the various sections of
this review, such as the study of adsorbed water, and will not be
repeated here. We give here only a few additional examples, most
of them very recent.
Surface-Enhanced Raman Scattering (SERS)
Fleischmann et a/.209'448 discuss the structure of water in the
double layer and adsorbed on a rough silver electrode. They find
several bands indicating different forms of water. These are associated with solvated cations. The effect of anions is also studied and
it was found that they are coadsorbed with the water. Fleischmann
et al.,449 in a separate sutdy, have inferred from the SERS spectra
that quinoline and isoquinoline ions adsorbed as ion pairs when
adsorbed from an acidic medium. Bunding and Bell450 infer selective
hydration of pyridine carboxaldehydes producing carbinols, for the
para and ortho derivatives but not for the meta. Pockrand et al451
identify two forms of acetylene on a silver film. They cannot assign
the bands seen to any specific form. Sandroff et al452 use tetrathiafulvalene to probe the charge transfer to silver and gold surfaces,
monitored by the position and intensity of the vibrational bands.
The possibility of detecting a vibrational band associated with
a surface-molecule bond was already discussed. In this context we
mention the silver-thiourea bond seen by Macomber and Furtak57
and the copper-N vibrations seen on copper colloids.36
Several molecules were investigated because of the applicability in a system of interest. Silver oxide was investigated by Kotz
and Yeager453 for the significance of the silver/silver oxide electrode
as a cathode in batteries and as a catalyst for oxygen reduction.
Mercaptobenzothiazole was studied454 for its role as a corrosion
inhibitor for several metals. Von Raben et a/.455'456 could follow
the adsorption of sulphates and nitrates on a silver powder in the
context of catalysis. Sandroff and Herschbach56 showed that several
disulfides dissociate to the sulfides when adsorbed on silver. This
was discussed in relation to lubrication problems. Sandroff et al457
investigated the surface conformations of hexadecane thiol as a
function of the solvent in contact with silver. This molecule is an
amphiphile, of interest in wettability problems.
Reactions were also monitored by SERS. Yamada et al45S
discussed the reaction of rose bengal adsorbed on a ZnO electrode.
This is an example of the use of the resonance enhancement to
gain enough sensitivity. Billmann et al92 followed the chemisorption
of cyanide on silver, and detected the formation of the various
cyanides on the surface. Similarly, Loo190 investigated halide complexes on a gold electrode. Pemberton and Buck214 used SERS to
see the adsorption of diphenylthiocarbazone and its oxidation into
a disulfide. Itabashi et al222 looked at the dissociation of porphyrin into its monomers and silver incorporation into the ring.
Fleischmann et al459 investigated the electropolymerization of
phenol on silver.
SERS is important also in the study of catalysis. Here silver
itself is of interest such as for the ethylene epoxidation reaction.460
Moskovits et a/.46-50-106 showed the usefulness of SERS for the study
of the adsorption of simple alkenes on silver. Dorain et al461 found,
using SERS, catalytic formation of sulfites from sulfur dioxide on
silver powder, and followed the oxidation by oxygen to sulfates.
Pettenkofer et al462 report the detection of peroxides and superoxides on silver.
SERS has been used also under high-pressure conditions by
Podini and Schnur463 who discuss the reliability of the measured
parameters. Sandroff et al464 have used high-pressure chambers,
Suh et al97 demonstrated that SERS could be used to detect
"two-dimensional" phase equilibrium between a gaslike phase
and a solidlike phase, for /7-aminobenzoic acid adsorbed on
Competition over surface sites and surface displacement of
adsorbates was observed by Garrell et al,192 who detected replacement of chloride by bromide on a silver colloid. Owen et al465
report similar displacements on a silver electrode. Bachackashvilli
et ai49>229>230 have monitored competition of pyridine and several
azo dyes over the surface of a silver colloid.
Kinetics of adsorption were investigated by Pemberton and
Buck,466 who followed the adsorption of dithizone on a rotating-disk
silver electrode, on the time scale of a few seconds. Dendramis et
al461 reported adsorption of cetyltriammonium on copper in the
minute time scale.
An interesting application of SERS was reported by Farquharson et al46S They determined the reversible redox potential of
osmium ions by monitoring their relative surface concentrations as
a function of electric potential.
The investigation of dye molecules or other highly fluorescing
molecules can be facilitated by the quenching of the fluorescence
by the surface, as noted by Nimmo et al241 A good example can
be seen in References 59 and 227.
Surface-Enhanced Raman Scattering (SERS)
SERS has been used for the study of molecules of biological
importance, such as nucleic acid components.469'470 The adsorption
on a silver electrode was considered in some way similar to the
adsorption to the charged membranes in biological systems. The
adsorption of a nucleic acid itself was also studied with SERS.471
We have already discussed the work of Cotton et a/.218'219'221
who studied Cytochrome C and myoglobin. SERS requires only
small quantities of material, which is very suitable for biological
Special mention must be made of SERS studies of carbonates,
carbon, and graphitic layers on surfaces. Cooney et al131'412'415
have reported the appearance of Raman vibrational structures
associated with the presence of carbon species on the SERS-active
surfaces, in electrochemical systems. They even suggest that a
graphitic layer formed on the surface is responsible for the SERS
phenomenon itself. Tsang et al416 also discuss the broad bands
seen in SERS at 1350 and 1550 cm"1 in terms of amorphous carbon.
In the UHV systems, Pockrand and Otto69 report carbonate
impurities exhibiting enhanced Raman scattering. They find that
these impurities are incorporated into the sample below the surface.
DelPriore et al.166 find by XPS several layers of carbon and oxygen
on SERS-active vapor-deposited surfaces in the UHV.
Very recently, Efrima477 suggested a combination of SERS and
optical-activity measurements. Provided large field gradients are
present near the metal surfaces, one should measure very large
effects. This may provide an easy way to extract chiral information.
Besides the direct utility of SERS and the impact it had had
on the way we think about metal interfaces, SERS and the SERS
mechanisms have suggested many other effects, some of which have
already been put to trial. This review would not be complete without
Nitzan and Brus478'479 have proposed that photochemical reactions on SERS-active substrates may be also enhanced. Goncher
and Harris480 reported photofragmentation of several molecules
adsorbed on a silver surface, with an incident wavelength of
363.8 nm(!). They attribute the reaction to enhanced singlet-triplet
transitions or to multiphoton processes. Chen and Osgood481 reported enhanced photodeposition on several metals, but not on gold,
at 257 nm.
Garoff et al226 report contrary behavior. They noticed slower
rates of photofragmentation on silver islands as compared to an
oxide substrate. This is probably due to the efficient dissipative
channel for transfer of excess energy of the molecule to the metal,
which arises upon adsorption.
Weitz et al240 also noted an effect of SERS-active systems on
the fluorescence of molecules. For the molecules they studied, a
low quantum yield in the bulk was associated with higher fluorescence on the surface, while efficient emitters exhibited a quenched
Chuang482 and Chuang and Seki483'484 reported enhanced
desorption rates of pyridine from silver when illuminated in the IR
near a pyridine vibrational frequency. The rate was nonlinear in
the laser intensity and exhibited a definite resonance behavior
around the breathing mode of pyridine. Pyridine on KC1 gave
essentially similar results. It seems that local heating due to the
absorbance of the pyridine is important here.
Another phenomenon related to SERS and supposed to be
affected by the same mechanisms, is second harmonic generation
(SHG). This was seen a long time ago by Lee et a/.485 In a series
of papers, Chen et a/.95-486-489 reported enhanced SHG with
enhancement factors ranging from 100 to 104. These results have
an immediate impact on the validity of the LFE mechanism.
However, the role of field gradients has not beeen considered. SHG
was used to monitor changes on a silver electrode during electric
Also coherent anti-Stokes Raman scattering (CARS) was tried
in SERS-active systems. Schneider491 measured the CARS of benzene on a silver film. The degree of enhancement (if any) is not
given. Chew et al492 have presented a theoretical discussion of the
CARS on colloids. Schneider491 reported that CARS was not seen
on colloids, due to a small interaction zone.
Finally, Glass et al493 investigated two-photon fluorescence of
molecules adsorbed on silver surfaces, and found an enhancement
factor of -150.
Examples of several of the ways SERS has been utilized were
quoted here in order to show the directions of research and application. The validity of the various applications and their results are
still an open question, and hopefully will be investigated in future
Surface-Enhanced Raman Scattering (SERS)
As a final note, it is useful to quote Furtak138 once again: "We
have shown in this report, given the state of understanding as it
now exists, that some... systems can already be studied in detail
using enhanced Raman scattering. If we can develop the technique
to its potential, as Auger spectroscopy has been developed in
surface-vacuum characterization, understanding of the metal-electrolyte interface will be propelled by a genuine breakthrough." This
holds true today, five years later. The if is perhaps smaller, but it
is still there!
I wish to thank Mr. Hai Cohen for his invaluable help in assembling
and cataloguing the extensive literature.
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