ABSTRACT
Raman spectroscopy is a powerful analytical technique for chemical characterization due to the wealth of information on molecular structures, surface processes, and interface reactions that can be extracted from experimental data. e spectral selectivity associated with the narrow emission lines and the molecular specic vibrational bands of Raman labels make it an ideal tool for molecular genotyping. However, a limitation of Raman techniques for trace detection is the very weak Raman cross section. However, Raman spectroscopy has gained increasing interest as an analytical tool with the advent of the SERS eect, which can produce signicant enhancement of the Raman signal. It is believed that the origin of the enormous Raman enhancement is produced by at least two main mechanisms that contribute to the SERS eect: (1) an electromagnetic eect occurring near metal surface structures associated with large local elds caused by electromagnetic resonances, oen referred to as surface plasmons, and (2) a chemical eect involving a scattering process associated with chemical interactions between the molecule and the metal surface. Plasmons are quanta associated with longitudinal waves propagating in matter through the collective motion of large numbers of electrons. According to classical electromagnetic theory, molecules on or near metal nanostructures experience enhanced elds relative to that of the incident radiation. When a metallic nanostructured surface is irradiated by an incident electromagnetic eld (e.g., a laser beam), conduction electrons are displaced into frequency oscillations equal to those of the incident light. ese oscillating electrons, called surface plasmons, produce a secondary electric eld, which adds to the incident eld. ese elds can be quite large (106-to 107-fold,even up to 1015-fold enhancement at hot spots). When these oscillating electrons become spatially conned, as is the case for isolated metallic nanospheres or otherwise roughened metallic surfaces (nanostructures), there is a characteristic frequency (the plasmon frequency) at which there is a resonant response of the collective oscillations to the incident eld. is condition yields intense localized elds that can interact with molecules in contact with or near the metal surface [1-4]. In an eect analogous to a lightning rod eect, secondary elds can become concentrated at high curvature points on the roughened metal surface.
