Fabrication, characterisation, and analysis of plasmonic noble metal nanostructures

Abstract

Plasmonic nanostructures have potential applications across a diverse range of fields, from biosensing to solar energy. The coupling of electric fields to oscillating free- electrons on the surface of a material create plasmonic resonances, which are the key to concentrating and utilising light beyond the diffraction limit. The ability to tailor the plasmonic response of metallic nanostructures, based on many, easily tuneable, properties, is the foundation for much of the current research, as this will allow for the efficient integration of plasmonic components into a wide range of existing photonic and laser technologies.

In this work, opportunities to chemically alter the dielectric, and thus plasmonic, properties of Ag nanoparticles were investigated through boundary element simulations. It was found that the three most cited compounds of Ag corrosion in the literature, Ag2O, AgCl, and Ag2S, have very different effects on the plasmonic response of Ag nanostructures, when they form a thin shell on the nanostructure surface. While a thin shell of Ag2O and AgCl were seen to red-shift the surface plasmon modes, Ag2S was observed to completely quench the plasmonic response. This is the case even for very thin shells of 1.5nm thickness. This chemically-induced red-shift can move the Ag nanoparticle plasmon resonances from the UV region of the electromagnetic spectrum in to the visible region, opening up opportunities for more facile photonic integration. There is huge scope to take this investigation to the next step, and create chemically altered nanoparticles for experimental analysis to compare with the results and discussion presented herein.

Ag is known to corrode rapidly under atmospheric conditions, and this corrosion is known to be detrimental to its plasmonic properties. However, there is still some debate in the literature as to the exact mechanism and composition of this corrosion. Experimental electron energy loss spectroscopy (EELS) was used to verify that Ag2S is the only Ag corrosion compound observed on atmospherically exposed Ag nanoparticles, the build-up of which was seen to be inhomogeneous. Energy loss near edge structure (ELNES) of the EELS spectra was used to determine whether the Ag atoms in a nanostructure sample were chemically bonded to other elements present on the sample, or if these were merely physisorbed on the surface. This particular ELNES analysis of the Ag M4,5 edge has not been seen elsewhere in the literature, and grants a hugely important insight into the chemical makeup, and thus dielectric properties, of an Ag sample. In addition, standard samples of Ag oxides, AgO and Ag2O, were analysed for comparison with Ag nanoparticles which had been stored in a citrate stabiliser and had oxygen signal present in their EELS spectra. A 9 eV shift in the O K edge was found depending on whether the O was chemically bonded to the Ag, or present as part of some organic contamination. This is discussed as a potential reason for the misidentification of Ag oxide as an atmospheric corrosion product in past studies.

The focus of the work then shifted to fabricating plasmonic nanostructures using focused electron beam induced deposition (FEBID). This is a relatively new technique when it comes to plasmonic structures, which allows for precise fabrication of nanoscale structures in both two and three dimensions. In this work, the goal was to see if the purity of the final products could be improved by injecting a reactive agent, in this case H2O, into the deposition chamber during the deposition, to remove un- wanted by-products. In addition, a post-deposition annealing step was investigated, again with a view to increasing the purity of the Au nanostructures. From the results outlined here, and from recent literature, both methods showed some potential promise for the development of suitable plasmonic samples. There is excellent opportunity for extending this research further, particularly to optimising the H2O injection mechanism to increase the local pressure, and, as discussed at length, to integrating an annealing step during the deposition process, to aid in flushing out unwanted by products before they become part of the final structure.

Thesis embargoed until 31 December 2026.

Date of Award	Dec 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	University of Glasgow
Supervisor	Donald MacLaren (Supervisor) & Fumin Huang (Supervisor)