Scanning‐Tip and Confocal Raman Characterization of Gold Nanoparticles as Plasmonic Structures for Molecular Sensing

Plasmonic nanoparticles, and nanoantennas in general, are important in all sorts of applications. Their functioning is based on the interaction of alternating electric fields with conduction electrons, resulting in concentration of the electromagnetic energy of pump light into so‐called hotspots that are only nanometers in size. Detailed measurements of the hotspots are interesting and important, but made extremely difficult by their sub‐diffraction‐limit size. We use Raman scattering to study the field enhancement by plasmonic structures. In one experiment, we coat the structures with a layer of Raman‐active molecules and perform confocal Raman micro‐spectroscopy. In another measurement, we apply a sharp silicon tip, typically used for atomic force microscopy, as a nanoscale probe to investigate local electromagnetic fields in various plasmonic structures. We use the Raman shift, characteristic of silicon, in order to distinguish photons coming from the probe from the overwhelming number of linearly scattered photons. We demonstrate that our methods allow us to map local field variations and hotspots with nanometer spatial resolution. With that we study two types of samples, both of interest in the context of surface‐enhanced Raman scattering (SERS). One consists of simple gold nanospheres and their particle assemblies and aggregates. The other sample contains gold nanostars that are becoming increasingly popular in SERS. Compared to nanospheres, we find that nanostar structures produce large, easily quantifiable plasmonic field enhancement under 660‐ excitation in both micro‐ and nano‐spectroscopy detection, which is also in agreement with other studies. This overall approach offers an effective way to probe various types of plasmonic antennas and nanosystems.