Metal nanostructures can favorably change the properties of fluorescent molecules, increasing quantum yield, excitation and emission rates in a process known as plasmon enhanced fluorescence (PEF). Interactions between the nanostructures and fluorescent molecules can be described by three PEF mechanisms; near-field enhancement (NFE), resonance energy transfer (RET) and radiative decay engineering (RDE). The effect of these mechanisms on fluorescence is distance dependent, with enhancement occurring for distances greater the ~5nm and quenching of the signal when the fluorescent molecule is in close proximity to the nanostructure. This work focusses on the near-field enhancements associated with a gold nanorod array surface to determine a suitable setup for PEF applications. Using a finite element method (FEM) model, various nanorod array setups were simulated and the resonance and maximum field enhancements, E/E0 determined for each. Field enhancements occurred at different wavelengths than resonance as the enhancement was dominated by the existence of hot-spots. The maximum field enhancement of 7.88 occurred for an array of nanorods with 50nm diameter, 150nm height and center-to-center spacing of 60nm. The enhancement was due to hot-spots within the narrow gaps between nanorods, therefore this setup was not experimentally as fluorescent molecules would be unable to fit into the gaps. Nanorods with 50nm diameter and 100nm height in an array with 100nm periodicity provided an alternative setup, with maximum field enhancement of 6.37 due to a hot-spot at the top of the nanorod. Analysis showed that the field enhancement decreased rapidly with distance from the surface, but remained sufficiently strong for PEF applications.
The plasmonic properties of a gold nanorod array surface can be tuned through modification of the surface parameters. To experimentally fabricate and investigate would be both resource and time expensive. This work utilises a finite element method (FEM) model to investigate the effect of varying parameters on the optical properties of the surface. Near-field coupling effects are considered within the nanorod array and between the array and gold underlayer. Increased coupling and blue-shifted resonance peaks occur for reduced array spacing and increased underlayer thicknesses red-shift resonance positions and increase overall extinction values. Nanorod geometry simulations show that larger diameters significantly blue-shift resonance peaks and increase local field enhancements throughout the array; whereas increasing height increases the extinction spectra and causes red-shift of resonance peaks. The results obtained for these investigations aid understanding of the electromagnetic interactions associated with the nanorod array which will benefit practical applications of the surface. Current experimental nanorod array geometries were investigated for plasmon-enhanced fluorescence (PEF) applications, with the maximum plasmonic signal and field enhancements occurring for 25x200nm array with 60nm spacing at fluorescent absorption and emission wavelengths. However, these significant field enhancements are localised to the surface of the nanorods rather than throughout the array so fluorescent molecules would have to be in contact with the surface to experience these enhancements.
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