Gold bowtie nanoantennas for surface-enhanced Raman scattering under controlled electrochemical potential

Single gold bowtie nanoantennas are used for surface-enhanced Raman scattering under controlled electrochemical potential. Raman spectra of para-mercaptoaniline molecules chemisorbed to newly-prepared bowties at low laser power show only stable a1 or a2 symmetric modes. Surprisingly, electrochemical cycling induces a reproducible switching-on of b2 symmetric modes. The changes are related to electrochemically-induced surface roughening which alters the relative orientation between adsorbate and local electromagnetic fields. Finite-difference time-domain calculations along with symmetry-based arguments allow determination of the initial orientation of the adsorbed molecules. Our findings underscore the necessity of well-characterized substrates for a meaningful analysis of surface-enhanced Raman spectra.