SERS and the Single Molecule

Abstract

Surface Enhanced Raman spectroscopy (SERS) was discovered in 1978 and has grown to become a significant surface diagnostic and analytical technique. It has also launched a wide variety of investigations into the electromagnetic, and especially the optical, properties of nanostructured disordered materials. A number of phenomena contribute to SERS including adsorbate resonances as well as new resonances (such as metal to molecule charge transfer transitions) that result form the formation of the adsorbate-to-surface bonds, or other adsorbate-metal interactions. Chief among the contributions to SERS, however, is the enhancement of the optical fields in the vicinity of the nanoparticles constituting the SERS-active system. The field enhancement is especially high when highly localizable resonances such as surface plasmons are excited. Aggregates and assemblies of nanoparticles (of appropriate materials) can, in turn, manifest unusually enhanced SERS by virtue of particle-particle interactions. For example, while the SERS enhancement in the vicinity of single silver nanoparticles rarely exceed 10⁴, the Raman spectrum of molecules located in the interstitial volume between two closely-spaced nanoparticles can be enhanced some 10 orders of magnitude when the two particles approach each another to within molecular dimensions and the system is excited at an appropriate wavelength. Other aggregates can Show similar levels of enhancement at special locations within the aggregate. Large fractal aggregates form a special class of enhancing aggregates. Illuminating such aggregates, in general, results in a highly inhomogeneous distribution of enhancement over the body of the aggregate with electromagnetic hot Spots where the Raman enhancement can reach or slightly exceed 10 orders of magnitude. Moreover, such hot Spots can be excited with a broad range of wavelengths (although the pattern of hot spots is critically wavelength dependent). Recently, reports have been published suggesting SERS enhancements upwards of 10¹⁴, sufficient for single molecule SERS detection. Although the cause of such huge enhancements was at first mysterious, we suggest that these observations result from the aforementioned electromagnetic effects in aggregates combined with either intramolecular or metal-to-molecule (or molecule-tometal) resonances. We also Show that the purported optical pumping of vibrationally excited states by such intense SERS transitions is spurious.