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IV. WHAT IS "THE" SERS MECHANISM?

IV. WHAT IS "THE" SERS MECHANISM?

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Surface-Enhanced Raman Scattering (SERS)



349



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

molecule compare?

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

the surface.

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.



350



S. Efrima



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)



351



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

al440



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

surface.

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,



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S. Efrima



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

Dornhaus.55

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)



353



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



354



S. Efrima



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,

too.

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

silver.

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)



355



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

studies.

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

mentioning them.

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.



356



S. Efrima



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

fluorescence.

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

potential cycling.490

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

experiments.



Surface-Enhanced Raman Scattering (SERS)



357



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!

ACKNOWLEDGMENTS



I wish to thank Mr. Hai Cohen for his invaluable help in assembling

and cataloguing the extensive literature.

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