Design and Fabrication of Plasmonic Nanostructures for Multifunctional Applications
Creators: Ahmed, Haseeb
Publisher: name: University of Alberta Library
Year: 2023

<p>Plasmonic nanostructures have received increasing attention due to their unique ability to mediate the conversion of light into different forms of energy. This opens pathways for numerous applications from ultrasensitive surface-enhanced Raman scattering (SERS) materials characterization to heterogeneous photocatalysis and green energy harvesting. However, plasmonic nanostructures should meet a number of requirements for their potential to be realized. In addition to nanoscale dimensions, a high uniformity and compatibility with existing microelectronics and microfluidics settings are required. Direct-write nanofabrication techniques, <br>
such as electron beam lithography (EBL) and more recently, focused ion beam (FIB) processes offer unmatched control over nanoscale geometries and also high flexibility to allow for various designs. However, careful co-optimization of the process conditions is required to fabricate periodic nanopatterns efficiently. Use of dielectric substrates, which are often required for nanoplasmonic designs, is particularly challenging for EBL fabrication due to the accumulation of <br>
charge during EBL exposures. In this work, a 10 keV EBL process was optimized to fabricate periodic arrays of 50 nm pitch holes in positive-tone EBL resist (polymethyl methacrylate or PMMA) on fused silica (FS) supports. The patterned PMMA was used as a lift off mask to create<br>
50 nm pitch arrays of Au dots on FS. In order to verify the performance of these Au/FS structures, SERS biodetection experiments were performed. For this purpose, the samples were biofunctionalized with thiolated DNA aptamers that bind specifically to an important biomarker, protein interleukin 6 (IL-6). The samples were loaded with IL-6 from a solution and characterized by SERS. The results suggest that the fabricated Au/FS plasmonic nanostructures are efficient for SERS enhancement, and potentially also for broader applications. In order to test multifunctional applications, Au/FS plasmonic substrates created by physical vapour deposition (PVD) deposition of Au were additionally employed to characterize liquid-liquid phase separation in a solution of microtubule-associated protein tau, which plays an important role in Alzheimer’s, Parkinson’s, and several other neurodegenerative diseases. The results suggest that plasmonic substrates may <br>
both facilitate liquid-liquid phase separation in solutions of proteins and enhance the SERS effect, which makes possible in-situ characterization of proteinaceous phase-condensates. Furthermore, 10 keV EBL was employed to fabricate plasmonic Ag nanostructures on indium-titanium oxide (ITO) supports in various designs. Currently, the Ag/ITO plasmonic nanostructures are undergoing characterization in a collaborator’s group. An alternative nanopatterning technique, He-FIB milling, was also tested. A superb positional control was achieved; however, the He-FIB technique was found to be slower than EBL for the fabrication of large arrays of plasmonic nanostructures. To summarize, the results demonstrate that plasmonic nanostructures consisting of nanostructured noble metals such as Au or Ag on dielectric supports could potentially serve for a broad variety of applications. The challenges relate primarily to the efficiency and cost of the direct-write processes such as EBL or FIB. Further optimization would address, in the first place, improvements of the sensitivity of <br>
EBL and/or FIB nanopatterning.</p>