  Noble metal nanoparticles show specific optical properties due to the
excitation of localized surface plasmons that make them attractive candidates
for highly sensitive bionanosensors. The underlying physical principle is
either an analyte-induced modification of the dielectric properties of the
medium surrounding the nanoparticle or an increase of the excitation and
emission rates of an optically active analyte by the resonantly enhanced
plasmon field. Either way, besides the nanoparticle geometry the dielectric
properties of the metal and nanoscale surface roughness play an important role
for the sensing performance. As the underlying principles are however not yet
well understood, we aim here at an improved understanding by analyzing the
optical characteristics of lithographically fabricated nanoparticles with
different crystallinity and roughness parameters. We vary these parameters by
thermal annealing and apply a thin gold film as a model system to retrieve
modifications in the dielectric function. We investigate, on one hand,
extinction spectra that reflect the far-field properties of the plasmonic
excitation and, on the other hand, surface-enhanced Raman spectra that serve as
a near-field probe. Our results provide improved insight into localized surface
plasmons and their application in bionanosensing.
