Single-Molecule Raman Spectroscopy: A Probe of Surface Dynamics and Plasmonic Fields

摘要

Sningle-molecule spectroscopy has opened exciting new realmsnof research, allowing the exploration of molecular dynam-nics within heterogeneous media, from live cells to chemical cat-nalysts. Raman spectroscopy of individual molecules is particularlynuseful because it may provide more detailed information than isnavailable in the typically broad fluorescent spectrum. To over-ncome the problem of small Raman cross sections, however,nenhancement by surface plasmon excitation is necessary. Thisnenhancement is particularly strong in the gaps between noblenmetal nanoparticles; indeed, it is strong enough for the obser-nvation of Raman signals from single molecules.nThe electromagnetic fields generated by surface plasmonsndepend quite intricately on the shape of the nanoparticles, theirnspatial arrangement, and their environment. Single molecules cannserve as the ultimate local probes for the plasmonic fields. Such a “mapping expedition” requires accurate molecular posi-ntioning abilities on one hand, and nanoparticle cluster engineering methods on the other hand. This Account describes ournfirst steps toward achieving these goals. It is shown that a molecule can indeed be judiciously positioned within the gap ofna nanoparticle dimer and that it can report on the effect of particle size on the plasmon resonance spectrum. When a thirdnparticle is added, breaking the dimer symmetry, the electromagnetic field at the gap changes significantly, as manifestednby dramatic polarization effects. A combination of electron microscopy, Raman spectroscopy, and theoretical calculations isnused to fully understand symmetry breaking in nanoparticle trimers.