The Willets_lab is interested in using spectroscopic techniques to characterize materials and materials properties at the nanoscale. We use a variety of techniques in our work including single molecule fluorescence, surface-enhanced Raman scattering (SERS), localized surface plasmon resonance spectroscopy, and atomic force microscopy.

Silver nanoparticles (~50-150 nm in size) can enhance local electromagnetic fields by 10^2 - 10^14, yet measuring these local field enhancements is an experimental challenge because the diffraction limit prevents resolution of objects under ~250 nm in size. Current techniques do not allow local electromagnetic fields to be visualized as a function of nanoparticle shape, and offer no insight into how these local fields can affect the spectroscopic properties of a single molecule. We are using a variety of sub-diffraction limited techniques to characterize the local EM field enhancements of metal nanoparticles. We have two different experimental approaches. The first is based on single molecule fluorescence in which we attach fluorescent dyes to a metal nanoparticle and measure both the position and fluorescence intensity of the dye. The second involves a combined AFM and optical microscope to get structural and spectral information about the metal nanoparticles.

Organic fluorescent dyes are extremely useful for biological labeling experiments because of their small size and ability for one-to-one stoichiometric labeling. However, these dyes suffer from photobleaching, in which they become irreversibly non-emissive after some number of excitation cycles.