Single-molecule Raman spectroscopy: a probe of surface dynamics and plasmonic fields.

Single-molecule spectroscopy has opened exciting new realms of research, allowing the exploration of molecular dynamics within heterogeneous media, from live cells to chemical catalysts. Raman spectroscopy of individual molecules is particularly useful because it may provide more detailed information than is available in the typically broad fluorescent spectrum. To overcome the problem of small Raman cross sections, however, enhancement by surface plasmon excitation is necessary. This enhancement is particularly strong in the gaps between noble metal nanoparticles; indeed, it is strong enough for the observation of Raman signals from single molecules. The electromagnetic fields generated by surface plasmons depend quite intricately on the shape of the nanoparticles, their spatial arrangement, and their environment. Single molecules can serve as the ultimate local probes for the plasmonic fields. Such a "mapping expedition" requires accurate molecular positioning abilities on one hand, and nanoparticle cluster engineering methods on the other hand. This Account describes our first steps toward achieving these goals. It is shown that a molecule can indeed be judiciously positioned within the gap of a nanoparticle dimer and that it can report on the effect of particle size on the plasmon resonance spectrum. When a third particle is added, breaking the dimer symmetry, the electromagnetic field at the gap changes significantly, as manifested by dramatic polarization effects. A combination of electron microscopy, Raman spectroscopy, and theoretical calculations is used to fully understand symmetry breaking in nanoparticle trimers. As is well-known, the strong interaction of molecules with metallic surfaces may lead to modulation of their excited state energies and even to charge transfer to or from the surface. The impact of charge transfer on surface-enhanced Raman scattering has been debated for many years. Single-molecule spectroscopy offers new opportunities for probing this phenomenology. Charge-transfer excitations may enhance Raman scattering, sometimes also modulating the Raman spectrum in a manner reminiscent of the molecular resonance effect. Two approaches for looking into this effect are described in the Account. First, the observation of spectral dynamics driven by molecular motion provides indirect evidence for the importance of molecule-surface electronic coupling. More direct evidence is offered by single-molecule Raman spectroscopy studies within an electrochemical cell. The surface potential is systematically modulated, and the effect on Raman spectra is studied. It is found that the charge transfer interaction increases the signals by at least 3 orders of magnitude, but it also changes dramatically Raman spectral shapes. A mechanism for this complex behavior is proposed based on the theory of charge-transfer resonance-Raman scattering.