Surface-Enhanced Raman Spectroscopy and its Progeny

Abstract

Some thirty years ago surface-enhanced Raman (SERS) was discovered. In a nutshell, molecules positioned near roughened silver and gold surfaces were found to produce Raman spectra some 6 orders more intense than what an equivalent number of solution-phase molecules did. A large number of mechanisms were proposed to account for this spectacular effect, among which the one that seems to account for most of the observations essentially ascribes SERS to the concentration of the optical field in appropriately structured, interacting nanoscale features which operate both on the incident and Raman-scattered light. This concentration is to be appreciable only for features in which strong and narrow localized surface plasmons were excited. This “plasmonic” model not only accounted for many of SERS seminal features but also gave birth to the research fields of plasmonics and so-called metamaterials most of which achieve the necessary conditions governing the electrical permittivity and magnetic susceptibility of metamaterials in wavelength regions where plasmons are excited. SERS was again in the news approximately 10 years ago when a number of groups pointed out that SERS from individual molecules could be observed leading some to speculate that this observation challenged the plasmonic origin of SERS. The discovery of single-molecule SERS, coincident with the intense interest of the research community in nanoscience and technology, produced a renaissance of interest in SERS that is still with us. The work of the past half dozen years reaffirmed SERS as ultimately a plasmonic effect wherein most SERS-active systems are actually rather heterogeneous with most of the enhancement originating from “hot spots” where the enhancement could top 10 orders of magnitude averaged over territory where the enhancement is rather low. The major current challenge in the field is to devise nanostructures where the hot spots dominate, leading to systems with an inordinate ability to focus electromagnetic fields so as to produce not only extraordinarily intense SERS (presumably as a super-sensitive chemical analysis tool) but also as loci where other unusual photo-induced physical and chemical processes occur when the system is illuminated with rather banal light sources. The talk will illustrate some of the most recent advances in this field.