Surface Enhanced Raman Spectroscopy
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Surface Enhanced Raman Spectroscopy (also known as SERS) was pioneered by Richard P. Van Duyne [1] and popularized by Tuan Vo-dinh. In SERS, the Raman effect is found to be greatly enhanced when it is close to a rough metal surface. The enhancement can be huge (1014 or so), and enables Raman spectroscopy to be a sensitive technique. It is now known that Surface Enhanced Raman Scattering is observed for molecules found close to silver or gold nanoparticles because of surface plasmon resonance. Other metals may be used, but with a reduction in enhancement. The mechanism by which the enhancement of the Raman signal is provided is from a local electromagnetic field enhancement provided by an optically active nanoparticle. Because the theoretically predicted enhancements fall short of the experimental results, another mechanism was proposed called a chemical enhancement. The chemical enhancement is from a charge transfer complex or bond formation of the metal and adsorbate which increases the molecular polarizability.
In the early days of SERS, there was no consensus about the mechanism, and there still appear to be papers published in the literature in which the authors themselves are rather confused about what is going on. Current understanding suggests that the enhanced optical activity results from the excitation of local surface plasmon modes that are excited by focusing laser light onto the nanoparticle.
SERS gives all the information usually found in Raman spectra; it is a sensitive vibrational spectroscopy that gives structural information on the molecule and its local interactions.
