SERS mechanisms

3 Basics of Surface-Enhanced Raman Scattering (SERS) 

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The cyanide ion was one of the first adsorbed ions which was found to be surface enhanced (8,9) on silver electrodes. In this first stage of SERS investigation of the cyanide ion, it was assumed to be linearly bonded to Ag, through the carbon atom, perpendicular to the metal surface (9). However faced with the pressure of explaining the SERS mechanism, alternative structures were proposed. With the conception of the ad-atom model for SERS, Otto conjectured that the main SERS band was due... 

A third way (Creighton, 1988) in which the surface may modify the intensity of Raman scattering by molecules adsorbed at the surface is by perturbing the molecules as a result of the bonding interaction with the surface. However, it turned out that this chemical effect on the Raman spectra is relatively small, and it is not normally considered to be a part of SERS, though it is inseparable from it. Several other SERS mechanisms have also been proposed (see for instance the discussion by Moskovits, 1985) which by now are widely thought to make at most only minor contributions to the enhancements observed in SERS. 

There are two types of SERS mechanisms, which are responsible for the observation of the SERS enhancement [8] one type is the long-range EM effect and the other is the short-range CHEM effect. The EM effect is believed to be the result of localized surface plasmon resonance electric fields (hot spot) set up onto the roughened metallic surfaces [9, 10, 31]. The probe molecules residing within these hot spots will be strongly excited and subsequently emit amplified Raman... 

Fig. 11.4 Schemes demonstrating two SERS mechanisms, (a) Local field concentration near the gap between two particles [19]. The white spheres and polygons are silver, and the color scale bar indicates the electro-magnetic field enhancement. The separation distance is 1 and 5.5 nm for the first and second columns, respectively. The wavelength of the incident field is 514.5 nm with vertical polarization, and (b) Possible charge transfer excitations between molecule and metal [22]. The occupied and unoccupied molecular orbitals are broadened by the molecule-metal interactions. Metal-to-molecule and molecule-to-metal charge transfers are indicated by arrows...

The collective excitation of the electron cloud of a conductor is known as a plasmon, if the excitation is confined to the surface of the conductor it is known as a surface plasmon. For the excitation of surface plasmons by light, surface roughness or curvature is required. The electromagnetic field of the light at the surface can be greatly increased when the surface plasmon is excited. This results in the amplification of both the incident and scattered and this is the basis of the electromagnetic SERS mechanism [15]. 

The reversible dependence cannot be explained on the basis of adsorption and desorption or chemical reactions alone but must reflect also some intrinsic dependence of the SERS mechanism on voltage. 

In a unique experiment Wetzel et determined the dependence of SERS of pyridine on a silver sol on the electric potential, which was maintained by a reversible redox system (europium +3, +2) added to the suspension. The relative concentration of the europium ions was maintained by means of an electrode whose potential was controlled with standard electrochemical equipment. They found that the ratio between the two main bands behaved similarly on an electrode and on a sol. Also frequency shifts were seen. They interpreted these results as indicating that the same SERS mechanisms are operative on the sol and in electrochemical systems. 

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. 

D. R. Mullins and A. Campion, Unenhanced Raman-Scattering From Pyridine Chemisorbed on a Stepped Silver Surface -Implications for Proposed SERS Mechanisms, Chem. Phys. Lett. 110, 565 (1984)... 

A number of SERS mechanisms have been proposed to explain the above experimental characteristics however, no one mechanism can explain all of the observed evidence. There are at least two major mechanisms that contribute jointly... 

The possibility of a photon-assisted charge transfer SERS mechanism involving an electronic resonance between states of the metal and the molecule was first suggested by a quantum mechanical treatment from our laboratory and, independently, by other groups for several types of charge transfer mechanisms.This theory, which was further developed,predicts a background continuum, which is observed experimentally. Subsequent... 

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