Self-assembly of vertically aligned gold nanorod arrays
thesisposted on 28.02.2017, 05:07 by Thai, Thibaut
The main objective of this thesis was to develop a fabrication method combining bottom-up and top-down approaches to self-assemble anisotropic building blocks into advanced nanostructures on patterned substrates. Building blocks (gold nanorods) were synthesised following a seed-mediated protocol and subsequently purified by a fractionated precipitation strategy to remove nanoparticulate byproducts formed during the chemical synthesis. Gold nanorods (GNRs) were self-assembled into discrete vertically aligned arrays based on capillary and convective forces into templates fabricated by means of lithography processes. Patterned substrates were fabricated by a series of cleanroom processes to provide different templates for the self-assembly of GNRs. The role of the patterned surface was to guide and confine the fabrication of vertically aligned GNR arrays by providing a chemical and geometrical template. Recessed gold features were produced on silica-coated silicon wafers with a variety of shapes depending on the application envisioned for the GNR arrays. Square templates were fabricated by photolithography while rectangular patterns were produced by electron beam lithography. Surface treatments were carried out to endow the patterned substrates with a wettability contrast, required for the GNR self-assembly. The hydrophilicity of the gold surface was increased by a UV-ozone treatment and the silica surface was passivated with a PEG-silane functionalisation making it hydrophobic. The seed-mediated synthesis is a well-known method to produce GNRs but it also inevitably yields nanoparticulate byproducts. In a typical synthesis of GNRs with an aspect ratio of 3, three types of impurities can be identified: (1) large spherical nanoparticles, (2) nanoplates and (3) high aspect ratio nanorods. A size- and shape-selective purification strategy was developed to remove these three nanoparticulate byproducts from GNR solutions. The purification method exploits the sharp size-dependent colloidal stability threshold exhibited by gold nanoparticles functionalised with thiol-PEG-carboxyl. The ligand provides gold nanoparticles an excellent colloidal stability due to electrostatic interparticle repulsion. These repulsions forces can be attenuated to induce nanoparticle precipitation beyond specific thresholds of ionic strength or ethanol concentrations. Based on this concept, a two-step protocol enabled the separation of GNR from nanoparticulate byproducts increasing the purity of the as-synthesised GNR solution from 88.6% to 98.1%. The fabrication of vertically aligned GNR arrays was achieved by capillary and convective assembly on patterned substrates. The wettability contrast directed the self-assembly of GNR arrays onto predefined areas with an unprecedented accuracy. Two main factors have shown to play crucial roles in the self-assembly process. The temperature controlled the confinement of GNR arrays inside the template whereas the GNR concentration influenced the quality of the hexagonal close-packed (hcp) ordering of standing GNRs. When the self-assembly was performed at 45˚C with a GNR concentration of 9 nM, the nanostructures comprised three layers of vertically aligned nanorods with an interparticle distance of 5 nm and an excellent hcp ordering over a long range (substrate scale). The surface-enhanced Raman scattering (SERS) activity of these nanostructures has exhibited a sensitivity up to 36 times, when compared to a commercial SERS substrate.