Single-molecule spectroscopy has been significantly focused recently due to both large potentials of applications in life-sciences and fundamental aspects of physics and chemistry in the statistical limit of single molecules. Surface-enhanced Raman scattering (SERS) is an effective method to achieve the single-molecule limit, when the molecule is located in the so-called “hot spots” in metal nanostructures with the enhancement factor more than ten orders. The SERS enhancement stems from the electromagnetic enhancement, which is the result of the enhanced local electromagnetic field in the excitation channel and the antenna effect in the emission channel. The chemical enhancement may also contribute to SERS in some special cases, but it is believed to be much weaker than the electromagnetic enhancement. In this presentation, we will talk about single molecule SERS on complex metal nanostructures. We demonstrated, experimentally as well as theoretically, aggregations of metallic nanoparticles are essential for single molecule SERS. The experimental observation of dimer structures, where two Ag particles are bridged by a single hemoglobin molecule, reveal the simplest nanoparticle system that can amplify Raman scattering to the extent that vibrational spectra of single molecules can be recorded. The theoretical calculations show that the “hot spots” located in the interstitial sites of metal dimer nanostructures can give the enhancement up to 10^12 for bare metal nanoparticles and 10^14 for metallodielectric core/shell nanoparticles. A unified treatment of Raman scattering and fluorescence processes near metal surfaces in the scope of quantum optics can give deep insights of single molecule SERS. Additionally, the optimization and controllability of single molecule SERS, and single molecule trapping are also discussed.
