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DNA directed self-assembly of gold nanoparticle structures using templated substrates
thesisposted on 2017-02-21, 00:17 authored by Ng, Soon Hock
The construction and manipulation of nanoscale objects is the hallmark of nanotechnology. However, it is becoming increasingly difficult to control the fabrication of nanostructures with precision and repeatability, due to the need for ever decreasing feature size. Using traditional top-down techniques, production costs increase with decreasing feature size, while bottom-up techniques usually lack reproducibility. Combining top-down with bottom-up methods can play to their strengths and mitigate their weaknesses. The objective of this thesis was to develop a system which can create 2D gold nanoparticle (AuNP) assemblies based on lithographically defined templates in a cyclic manner: a nanofactory. This would be achieved by immobilising the AuNP building blocks onto a templated substrate using DNA directed self-assembly, then cross-linking and desorbing them from the substrate, thereby regenerating it for subsequent cycle. This opens up a range possibilities for the fast fabrication of particle assemblies based on theoretical models and for fabrication of unique structures which cannot be made any other way. Citrate stabilised AuNPs were functionalised with thiolated DNA to prepare them for self-assembly and to act as stabilising ligands. To promote hexagonal close packing of the AuNPs during assembly, HS-PEG-SO₃ˉ (TPS) was added after DNA functionalisation. Zeta-potential and size of the AuNPs increased appreciably after DNA functionalisation while the addition of TPS did not change zeta-potential or size significantly. This suggested that the DNA shell was responsible for both the final size and zeta-potential of the particles. Substrates were fabricated using a combination of e-beam evaporation, plasma enhanced chemical vapour deposition (PECVD), electron beam lithography (EBL) and reactive ion etching (RIE). These techniques were chosen due to their high resolution, anisotropic etching capability and conformal coating ability, all used in together to create recessed gold patterns in SiO₂. Increasing the writing speed of EBL was needed for eventual scaling up. Reducing pattern generator overhead was evaluated as a means to achieve this, resulting in a 7% increase. This was attributed to the simple shapes used. Simply increasing beam current was shown to quadruple speed, while showing minimal resolution degradation. A RIE recipe was developed with Ar and CHF₃ gas to etch SiO₂ at 10 nm/min, without stripping the PMMA mask too quickly. Polymer deposition was found to occur, resulting in patterns debonding during self-assembly. To counter this, a sandwich layer of evaporated Ti-Au-Ti then PECVD SiO₂ was deposited onto a silicon wafer before EBL. Early assemblies exhibited little particle adsorption to the patterns, which was solved by evaporating a thin layer of gold after etching. The final design yielded consistent particle assembly over multiple substrates with no pattern debonding. Alongside this work, initial cross-linking and desorption studies show promising results, attaining 93% desorption of particles on non-patterned substrates. A nanofactory lays the groundwork for the fast and cheap production of customisable assemblies, able to take advantage of a library of nanoparticle materials and shapes.