Extended Arrays of Aluminium Nanostructures: Fabrication, Properties, and Applications

A process for the fabrication of ordered arrays of aluminium nanoparticles using interference lithography was developed. A photoresist comprised of self-assembled monolayers of 16-phosphonohexadecanoid acid was exposed to 244 nm UV laser light in a Lloyd’s mirror interferometer. Aluminium oxide thickening under UV exposure was observed, resulting in an aluminium oxide film resistive to etching. This thickening was circumvented by employing a two-stage etching process, first of a chromium trioxide based etch to selectively remove thickened alumina, then sodium hydroxide to remove the aluminium film. The plasmonic properties of the array structures fabricated by this procedure were investigated in details, and characterised by their size and geometry. The use of titanium as a stabilisation layer for the aluminium structures was analysed by ellipsometry. A model was developed to explain the ellipsometric spectra of the aluminium arrays and used to determine the thickness of titanium ad-layers, from which the decay length of the array plasmons was determined to be 12.52 nm. The stability of the arrays immersed in acid, base, and buffer solutions was investigated. It was found that 2.5 nm of titanium will effectively stabilise the arrays for at least 24 hours in solutions that dissolve the unprotected arrays within minutes. A predictive model for the sensitivity of the refractive index shift to protein adsorption for different thickness of titanium dioxide was developed. This was used to fabricate a reusable refractive index biosensor based on the adsorption of streptavidin onto titanium dioxide coated arrays. The sensor showed an average shift of 21.9 nm ± 4.5 nm after the adsorption of a monolayer of streptavidin, which matched well the prediction. Protein was adsorbed to this sensor then cleaned for 20 cycles, after which no drift in the plasmon shift was observed. The arrays were investigated for use as substrates for the surface enhanced Raman scattering of phthalocyanine and a maximum enhancement factor of 86.9 was observed.