The nanogap is possibly the single most important physical entity in surface-enhanced Raman scattering. Nanogaps between noble metal nanostructures deliver extremely high electric field-enhancement, resulting in an extraordinary amplification of both the excitation rate and the emission rate of Raman active molecules situated in the gap. In some cases, the resulting surface-enhancement in the gap can be so high that Raman spectra from single molecules can be measured. Here, we briefly review some important concepts and experimental results on nanoscale gaps for SERS applications.
