Imaging Nanoscale Electric Fields at Au(111) Step Edges by Tip-Enhanced Raman Scattering

Tip-enhanced Raman scattering (TERS) can be used to image plasmon-enhanced local electric fields on the nanoscale. Our observed two-dimensional TERS images map electric fields localized at Au(111) step edges using silver atomic force microscope tips coated with 4-mercaptobenzonitrile molecules. We establish that such measurements are not only sensitive to spatial variations in the enhanced electric fields but also to their vector components. We also experimentally demonstrate that single nanometer precision is attainable in TERS nanoscopy using tips with radii on the order of 100-200 nm. Our results underscore the importance of considering molecular orientation and the tensorial nature of Raman scattering in interpreting TERS imaging measurements. To the best of our knowledge, these are the first TERS measurements of the vector components of plasmon-enhanced local electric fields. We anticipate that ensuing correlated electric field mapping and chemical imaging measurements, using TERS, will lead to a better understanding and broader applicability of this powerful technique. Overall, we illustrate the concept of electric field imaging via TERS and establish the connections between our observations and conventional TERS chemical imaging measurements.