(321e) Dark-Field Microscopy for Sensing the Shape and Orientation of Isolated Gold Nanostructures

We demonstrate a robust and straightforward method to determine the shape and orientation of different gold nanostructures using dark-field microscopy (DFM). By simply analyzing the dark-field scattering images of the gold nanostructures with incidence polarized light in a bench-top DFM, the shapes and orientations of the underlying nanostructures can be accurately determined. Depending on the size, shape, and orientation of plasmonic nanoparticles, which dictate the plasmonic behaviors on a dielectric substrate, the nanoparticles appear as a distribution of color as captured by a commercial, color CCD camera. Extracting the red, green, and blue channels of the scattering signal from a CCD imaging detector as a function of azimuthal angle provides a polar plot of color intensity values that is characteristic of the shape of the underlying nanostructure. Using this technique, we have been able to sense the shape and orientation of different gold nanostructures, including nanorods and nanowires, spherical nanoparticles, nanoparticle dimers, and nanotriangles. We demonstrate that the polarization-dependent scattering can be used to quickly identify the shape of individual objects as well as the orientations of vastly populated objects on a substrate. We anticipate that this technique will provide a rapid and inexpensive complement to the typical structural analysis of nanoparticles that is achieved by electron microscopy.