Engineering Substrate-Mediated Localized Surface Plasmons in Gold Nanodiscs

Abstract

A variety of nanostructures capable of generating strong local electromagnetic fields (hot spots) in interaction with radiation have been under intensive investigation towards plasmonic applications in surface-enhanced Raman scattering (SERS), biosensing, broadband absorbers, thermophotovoltaics, photocatalysis, etc. In many cases, these nanostructures are formed on a surface or embedded into a near-surface layer of the dielectric substrate, making some part of the field dissipate into the bulk and not contribute to the desired plasmonic functionality. To reduce such losses, the interface between the metallic nanostructures and the dielectric environment should be engineered. In the current work, Au nanodiscs are fabricated on Si posts of very small diameter (pin-shaped structures), enabling them to decouple the strong optical near fields localized at the nanodiscs from the bulk Si substrate. The Si post diameter is optimized by adjusting the gas flow rates in reactive-ion etching, resulting in a minimum post diameter of 20 nm at the nanodisc interface. The effect of this diameter on the localized surface plasmon resonance of the nanodisc is investigated with linear optical spectroscopic measurements, where a significant spectral blue shift of the resonance band is noticed compared to similar discs formed on bulk Si surface. The experimental results are compared with modelling where a 3-fold increase in the electric field enhancement is demonstrated. The fabricated pin-shaped nanostructures are tested in SERS measurements showing a significant increase in the enhancement factor in the order of 106. Thus, this work suggests a way of engineering 3D morphology to tune the substrate influence on the plasmonic properties of nanostructures and to develop efficient nanofabrication technologies.