Electromagnetic Mechanism of SERS

Surface-enhanced Raman spectroscopy (SERS) was originally discovered in the 1970s [1, 2] where it was found that submonolayers of small nonresonant organic molecules, such as pyridine, when adsorbed onto the surface of silver nanoparticles (and a few other metals such as copper and gold) would exhibit greatly enhanced Raman intensities (enhancements of 10). These results have always held the promise for using this technique to observe very low concentrations of molecules on nanoparticles and nanostructured surfaces, but only recently has this promise started to be fulfilled in a predictable way. Thanks to exciting advances in techniques for making nanoparticles (such as nanosphere and e-beam lithography), to characterizing the surfaces using electron and scanning probe microscopies, to functionalizing the surfaces of the particles using self-assembled monolayers with attached chemical receptors, and to the laser and optics technology associated with measuring the Raman spectra, a number of important applications have been reported recently [3]. Included in these results have been the determination of SERS excitation spectra for benzenethiol on lithographically fabricated silver surfaces (periodic particle arrays fabricated using nanosphere lihtography) and the first observation of glucose using SERS [4, 5]. Another area of great interest in the SERS community concerns the observation of single-molecule SERS (SMSERS) [6,7,8]. This technique, which was originally developed in 1997 [6, 7], has proven to be of fundamental interest due to the nominal enhancement factor of > 10 required for the observation of any signal, but it has so far proven to be elusive in providing widespread applicability, due to the limited numbers of molecules and substrates for which successful observations have been made. In particular, almost all of the observations to date have been with molecules that are resonant Raman scatterers (such as rhodamine 6G), which typically have larger Raman intensities for non-SERS applications than nonresonant molecules like pyridine by factors of 10 or more. In addition, the range of substrates that yield SMSERS has been limited mostly to colloidal aggregate structures, which presumably are efficient at producing particle junction structures (i.e., dimers and small clus-