Imaging the Optical Fields of Functionalized Silver Nanowires through Molecular TERS.

We image 4-mercaptobenzonitrile-functionalized silver nanowires (~20 nm diameter) through tip-enhanced Raman scattering (TERS). The enhanced local optical field-molecular interactions that govern the recorded hyperspectral TERS images are dissected through hybrid finite-difference time-domain-density functional theory simulations. Our forward simulations illustrate that the recorded spatio-spectral profiles of the chemically functionalized nanowires may be reproduced by accounting for the interaction between orientationally averaged molecular polarizability derivative tensors and enhanced incident/scattered local fields polarized along the tip axis. In effect, we directly map the enhanced optical fields of the nanowire in real space through TERS. The simultaneously recorded atomic force microscopy (AFM) images allow a direct comparison between our attainable spatial resolution in topographic (13 nm) and TERS (5 nm) imaging measurements performed under ambient conditions. Overall, our described protocol enables local electric-field imaging with few nm precision through molecular TERS, and it is therefore generally applicable to a variety of plasmonic nano-structures.