| This dissertation describes a systematic study of the size-dependent optical properties of single metallic nanoparticles. Previous studies have measured the ensemble-averaged properties of these materials, but statistical averaging masks their intrinsic or fundamental properties. A phenomenon exhibited by such metal (e.g. Ag, Au, and Cu) nanoparticles is surface-enhanced Raman scattering (SERS), by which the Raman scattering cross sections of adsorbed molecules are enhanced by 6 to 8 orders of magnitude on average. However, we report single particle studies that reveal intrinsic enhancement factors as large as 1015. This enormous enhancement effect allows the detection and spectroscopy of a single molecule at room temperature.; The physical nature of these highly SERS-active nanoparticles and nanoaggregates is investigated. Their optical and physical properties are correlated by using an integrated optical and atomic force microscope. The relationship between nanoparticle-size and optimum excitation wavelength is determined to be approximately linear over the size range examined. The enrichment of these SERS-active nanoparticles is demonstrated using a size-selective filtration scheme. During this study, it was discovered that the nanoparticles (Ag and Au) emit Stokes- shifted photons intermittently. This "on and off" behavior is very similar to that observed for single semiconductor quantum dots and single fluorescent molecules. To study this behavior, an avalanche photodiode is used to count the Stokes-shifted photons from a narrow spectral region. Photon counting data indicates that this process occurs on the millisecond to second time scale and is thermally activated. The exact nature of this behavior has yet to be determined.; The study of single nanostructures shows that new information can be obtained by removing statistical averaging. This work should provide new insights into the size-dependent properties of nanomaterials as well as the fundamental mechanisms of SERS. |
