Large-area Low-cost Fabrication of Complex Plasmonic Nanostructures for Sensing Applications

In this thesis, we introduce hole-mask colloidal lithography and nanosphere lithography techniques for low-cost nanofabrication of large-area (about 1 cm) plasmonic nanostructures with different complex shapes. For the first one, we use thin film PMMA-gold hole-masks, which are first prepared with polystyrene colloids, combined with following tilted-angle-rotation evaporation to fabricate large-area randomly deposited plasmonic nanostructures. For the second one, we use hexagonal close-packed polystyrene nanosphere monolayers directly as evaporation masks to fabricate large-area periodic plasmonic nanostructures. We describe the fabrication process step by step, and manufacture a variety of different plasmonic nanostructures for different sensing applications. For example, we use split-ring-resonators for antenna-assisted surface-enhanced infrared absorption measurements to detect monolayer molecules with an up to 20000-fold enhancement factor. We also utilize asymmetric double split-ringresonators for localized surface plasmon resonance sensing with experimental sensitivities of up to 520 nm/RIU and figures of merit up to 2.9. Furthermore, we investigate plasmonic oligomers consisting of touching triangular building blocks, which show fundamental modes, higher-order modes, as well as Fano resonances due to coupling between bright and dark modes within the same complex structures. Large-area low-cost direct contact Au-Pd hydrogen sensors are demonstrated, which show much improved spectral shifts as large as 30 nm upon hydrogen exposure. Additionally, we improve hole-mask colloidal lithography for three-dimensional and multishape fabrication. With multiple repetitions of holemask lithography, single-layer metasurfaces with complex, multi-shape plasmonic nanostructures can be created that exhibit desired optical functionalities. Largearea and low-cost fabrication of different samples with independently tunable resonances is demonstrated. These single-layer metasurfaces could find possible applications as bifunctional surface-enhanced infrared absorption and surfaceenhanced Raman spectroscopy, multi-line, as well as broadband substrates. The fabrication method is particularly suited for the creation of large-area, singlelayer C3-symmetric chiral metasurfaces, and this approach circumvents common problems with elliptical birefringence and can be utilized for interaction with chiral substances.