PNNL

Abstract

Tip-enhanced Raman (TER) spectral images of 4-thiobenzonitrile-coated Au nanorods map the spatial profiles and trace the resonances of dipolar and multipolar plasmonic modes that are characteristic of the imaged particles. For any particular rod, we observe sequential transitions between high-order modes at low frequency shifts and lower-order modes at higher frequencies. We also notice that higher order modes (up to m=4) are generally observed for long rods as compared to their shorter analogues, where longitudinal dipolar resonances (m=1) are observable. In effect, this work adds a new dimension to local optical field mapping via TERS, which we have previously explored. Not only can the magnitudes, vector components, local/non-local characters of local optical fields be imaged through molecular TERS, but spatially varying local optical resonances are direct observables. AB, CFW, BTO, and SW were supported by the US Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research (OBER) through the bioimaging technology development program. PZE was supported by the US DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the OBER and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the US DOE under contract number DE-AC05-76RL1830.