Imaging Electric Fields in SERS and TERS Using the Vibrational Stark Effect.

Electric fields associated with Raman enhancements are typically inferred from changes in the observed scattering intensity. Here we use the vibrational Stark effect from a nitrile reporter to determine the electric field dependent frequency shift of cyanide (CN) on a gold (Au) surface. Electroplated Au surfaces with surface enhanced Raman (SERS) activity exhibit larger Stark shifts near the edge and in areas with large roughness. The Stark shift is observed to correlate with intensity of a co-adsorbed thiophenol molecule. Gap-mode Tip enhanced Raman scattering (TERS), using a Au nanoparticle tip, show dramatic shifts in the CN stretch that correlate to enhancement factors of 1013 in the gap region. The observed peak widths indicate the largest fields are highly localized. Changes in the nitrile stretch frequency provide a direct measurement of the electric fields in SERS and TERS experiments.