Understanding the spatial distribution of SERS enhancement across AuFON substrates to control reactivity

Densely packed nanoparticles, such as gold film over nanosphere (AuFON) substrates, have multimodal plasmonic resonances that arise from short- and long-range coupling between nanostructures. When excited, the plasmonic resonance generates an intense, localized electromagnetic field that is useful in a variety of applications, including optical waveguides, nanophotonics, biological sensing, and driving chemical reactions. Our work investigates the relationship between the EM field generated from different plasmonic resonances and the substrate’s geometry. Understanding this relationship can be used to control the spatial extent of reactivity by selectively exciting specific regions on the substrate. I used confocal Raman microscopy to excite specific plasmon resonances with different wavelengths and then mapped the SERS intensity of an absorbed molecule across the substrate. The Raman microscopy images showed that the spatial distribution of SERS intensity across the AuFON varies with laser wavelength. This result indicates that different wavelengths of light can enhance specific regions on the plasmonic substrate and could be used to spatially control chemical reactions. My experimental work has been supplemented with 3D simulations in COMSOL to produce a theoretical localized surface plasmon resonance and investigate the electromagnetic field distribution around the AuFON substrate. The simulations show that certain wavelengths produce EM fields at the tops of the nanostructure while other wavelengths have localized EM fields between the crevices. My work aids in uncovering the connection between substrate geometry and plasmonic response, which helps advance the logical design of plasmonic substrates.