Thursday, March 10, 2011
1:00pm (334 JFB)
Title: Optical structure-property relations in metal &semiconductor nanoparticles
The optoelectronic properties of nanoscale metal and semiconductor material systems are notably sensitive to their corresponding physical structure. Contemporary synthesis techniques enable careful control of nanoparticle configurations and therefore provide a wide array of systems where the effects of physical morphology on the interaction between nanoscale materials and light can be carefully probed. The investigated properties are immediately relevant to light-harvesting and ultra-sensitive trace-analysis and sensing applications.
The structure-property relationships of both individual semiconductor nanocrystal heterostructures and aggregates of plasmonic silver nanoparticles in rough metal films are probed. The semiconductor heterostructures behave as model light-harvesting systems where optical energy absorbed by one portion of the structure is funneled, on the nanoscale, to a model light-harvesting center, in analogy to photosynthesis. In the plasmonic silver nanostructures, collective optical excitation of the conduction electrons-plasmons- confines electromagnetic radiation to well beyond the traditional diffraction limit of light in nanoscale regions called “hot spots.” Within these hot pots, light-matter interactions are greatly enhance and thus enable trace-sensing applications such as Raman scattering from a single molecule. Thorough application of relatively simple single particle spectroscopy techniques is combined with high resolution electron microscopy to elucidate the subtle details on how physical structure controls the optical properties of both material systems.
There are four main results of this work. (1) The linear and nonlinear optical response of rough silver films is shown to be enhanced by the excitation of surface Plasmon polaritons. (2) The enhanced nonlinear response of rough metal films is conjectured to originate from metal clusters, and the observation of stark fluctuations in their efficiency of second-harmonic generation is reported for the first time. (3) The presence of and enhanced emission from silver clusters of only a few atoms plays an important role in the intrinsic optical response of the silver films with considerable implications for surface-enhanced Raman scattering. (4) The effects of physical anisotropy on the electronic states of semiconductor nanocrystals are explicitly identified through correlated optical and electron microscopy of single particles. These effects are shown to have important ramifications in the internal energy-transfer process of single nanocrystals.