Production of optically functional nanoscale structures by chemical de-alloying of metallic precursor alloys

Publication Type:
Thesis
Issue Date:
2019
Full metadata record
Nanostructured thin films have diverse optical, structural, electrical and/or magnetic properties. There are comparatively few methods to produce refractory nanostructures, so in this thesis we investigated the use of cosputtering for high temperature optical applications. Precursor alloys were produced via deposition of aluminium paired with a less reactive metal (gold initially and then with refractory metals). A specific gold alloy, AuAl₂ was selected due to its unique optical properties. Colour changes were observed by annealing through in situ heated ellipsometry and when optical constants were compared to simulation results, gold and aluminium vacancies were the cause of the shifts in colour and optical properties in the alloy. A popular technique in the formation of nanostructured materials is the selective removal of a less reactive metal called de-alloying. Sodium hydroxide was selected as the etchant to remove the aluminium in the alloys. This was employed to produce nanoporous gold and refractory metal. Though there is difficulty in knowing the complete dissolution of the aluminium within the thin films. Therefore, a simple and inexpensive process utilising optical transmittance was developed to optimise de-alloying times and understand the mechanisms behind the removal of aluminium. Three stages were apparent during the in situ optical analysis, initial depassivation, bulk dissolution and then delamination. Dissolution rates were linear with hydroxide concentration, and exponential with temperature, with an activation energy of approximately 0.5 eV. Further characterisation of these sponges yielded interesting optical properties. In situ heated ellipsometry measurements indicated that gold sponges began to alter at temperatures below 50°C. At these temperatures coarsening of gold sponges was evident, with further coarsening occurring at approximately 150°C. Refractory metals substituted the gold and sponge variants were producible, though optically they resembled the resulting metal oxide. Extended de-alloying times and cracking of the films proved difficult, though structural examination demonstrated various morphologies were possible between refractory metals. Another type of morphology was fabricated using DC magnetron sputtering and de-alloying of the resulting nanostructures to form ‘nano-fins’. The removal of the aluminium through selective dissolution enables the nanostructure array to transmit light. The polarization spans 500 to 1100 nm and the extinction ratio significantly increases to >100. The as-deposited nano-fins have high surface area with capabilities of limited charge storage and supercapacitor properties. When produced with vanadium, it can be oxidised to form VO₂ possessing a metal-insulation transition with the opposite effect to typical VO₂.
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