Fabrication and Characterisation of an Adaptable Plasmonic Nanorod Array

This thesis focused on the fabrication and characterisation of an adaptable plasmonic platform consisting of a regular array of vertically aligned gold nanorods, supported on a gold substrate. The research aimed to understand the optical properties of the array, with particular emphasis on determining the coupling effects between the plasmonic nanorods. Characterisation was performed, both in the near and far-field, by means of optical spectroscopy, finite element modelling (FEM) and electron energy loss spectroscopy (EELS). The work subsequently aimed to use the knowledge gained from characterisation to optimise the absorption of visible light and enhancement of the electric-fields surrounding the plasmonic nanoparticles by controlling the geometrical factors of the array. Lastly, the nanorod arrays were then utilised for photocatalytic applications following their coating in a semiconductor material. During this research, gold nanorod arrays were successfully fabricated by means of electrodeposition in anodic aluminium oxide (AAO) membranes, with accurate control over the geometrical factors. UV-Vis measurements revealed that coupling within the array resulted in the higher wavelength longitudinal mode of the nanorods blue-shifting considerably to occur within the visible spectrum. For short aspect ratio (AR) nanorods, this mode overlapped with that of the transverse mode, however for longer ARs the mode could be tuned throughout the visible spectrum. This was in agreement with FEM results, however, it was additionally revealed that strongly coupled nanorod arrays undergo a redistribution in their electric field from localisation at the end of the nanorods, to one within the middle or base of the nanorods for unsupported and supported arrays respectively. It was further found that the presence of the substrate led to the nanorods coupling with their substrate image leading to greatly red-shifted resonances. Through experimental EELS measurements it was confirmed that this red-shift is due to coupling with the substrate, in addition to further analysing the particle coupling effects for both dimers, and larger arrays of nanorods. It was found that for dimers with strong coupling the longitudinal mode splits into both a bright mode with symmetrically aligned dipoles, and a dark mode with anti-symmetrically aligned dipoles, thereby agreeing with plasmon hybridisation theory. Furthermore, as the number of particles within the array increases, the modes split to form hybridised bright and dark modes consisting of elements of each. An initial attempt at photocatalysis was also performed based on the degradation of methylene blue by injection of hot-electrons into TiO2. No significant increase in activity was found, attributed to the semiconductor layer completely covering the nanorods thereby not allowing available sites for the oxidation reaction to replenish lost electrons in the metal.