Introduction

Normal Raman scattering is an inherently weak effect, and was not widely used in laboratories until the availability of inexpensive lasers, CCDs and holographic notch filters. Raman signals can typically be increased by using shorter wavelength sources, due to the \nu^{4} dependence of the Raman intensity. Resonance Raman scattering, due to excitation of an electronic resonance, produces a significant increase in Raman signal – as much as 103. Surface enhanced Raman scattering (SERS), which occurs when molecules are adsorbed to metal substrates (usually Ag or Au), can produce enhancements as high as 1014, and has been used for single molecule detection.

SERS was first observed in 1977 by Fleischmann’s group when they recorded a large Raman signal for pyridine adsorbed to a silver electrode. The following is a brief review of the mechanisms that produce SERS signals. Readers who wish to know more are directed to excellent review articles:

  1. Campion and Kambhampati, Chemical Society Reviews 27, 241 – 250 (1998)
  2. Moskovits, Rev. Mod. Phys. 57, 783 (1985).

Enhancement Mechanisms

There are two generally accepted mechanisms that produce the large increases in Raman scattering observed in SERS experiments. Chemical enhancement is generally due to the chemical interactions between adsorbed molecules and the metal substrates. Electromagnetic enhancement is due to electromagnetic interactions at (or near to) the surfaces of metals.

Chemical Enhancement

Chemical enhancement occurs as a result of charge transfer excitations from the metal to the molecule or the molecule to the metal (as shown in the following figure).

Chemical Enhancement
When this occurs charge transfer excitations become resonant in the visible region (in which most SERS experiments are performed). Such resonance enhancement is similar to what is observed in resonance Raman scattering (see for example Lombardi et al., J. Chem. Phys. 84, 4174 (1986)).

Electromagnetic Enhancement

The electromagnetic contribution to SERS enhancement is generally accepted as a result of localized surface plasmon resonance. A plasmon is a collective excitation of the electron gas of a conducting material, and may be travelling or localized, the later being the case for metal nanoparticles used in our SERS experiments.

When the size of a metal nanoparticle is much smaller than the wavelength of the incident radiation (as shown in the figure below), the magnitude of the electromagnetic field at the surface can be greatly enhanced.
Electromagnetic enhancement
For SERS, it is important to note that both the incident electric field, and the scattered fields are enhanced, and that there is a frequency dependence for the induced electric field

E_{induced} = \frac{\epsilon_{1}(\nu) - \epsilon_{2}}{\epsilon_{1}(\nu) + 2\epsilon_{2}}E_{laser}

\epsilon_{1}(\nu) is the complex, frequency dependent dielectric function of the metal, \epsilon_{2} is the relative permittivity of the medium. Einduced is the magnitude of the induced electric field and Elaser is the magnitude of the laser’s electric field.

Resonance occurs when Re[\epsilon_{1}] = -2\epsilon_{2}, which for the noble metals (Au, Ag and Cu) occurs in the visible. One of the interesting results of the frequency dependence of this resonance is that lower frequency molecular vibrations are enhanced more than higher frequency vibrations. SERS signals are also typically independent of the wavelength of the incident laser’s frequency – there is no \nu^{4} dependence for SERS.