Towards Functional Nanostructures using Self-organising Building Blocks

Abstract

This thesis reports on the formation of functional nanostructures using self-organising building blocks. Specifically, the formation and characterisation of thin-film nanostructures directly targeted toward the development of either magnetic or electric devices are presented. Using the Langmuir-Blodgett (LB) technique, aligned assembly in a 2-D gel of a water- soluble peptide (Ac-IKHLSVN-NH2) was investigated. In the presence of an appropriate electrolyte as the sub-phase, stable and ordered 2-D monolayers of a water-soluble peptide could be formed and transferred as thin films onto a solid support. The strong effects of the electrolyte and surface-pressure cycling as an “annealing” technique on the assembly and quality of the LB films were explored. Well-structured films comprising aligned self- assembled rods were obtained. Furthermore, as a follow-on study, the effect of sidechain modifications on the formation of cross-fibre linkages was explored. The templated self-assembly approach to organising magnetic particles was explored. This is an approach targeted toward assembling magnetic particles into a regular 2-D array that can support magnetic spin waves in magnonic devices – one of the currently preferred routes to reducing the power consumption of computation. Specifically, the use of the core of a redox- active protein peroxiredoxin (Prx) ring, and cationic surfactant for magnetic particle synthesis and assembly as 2-D LB films were explored. An unusual packing arrangement was noted in the surfactant-assisted assembly, which is interpreted in terms of geometrical frustration – the occurrence of packing arrangements of very similar energy when spheres are confined within a space of thickness comparable to the sphere diameter. The ‘annealing’ effect by surface- pressure oscillation was again demonstrated. Lastly, the electrical conductivity in networks of nanofibres formed from a semiconducting dye (Thiophene-diketopyrrolopyrrole, TDPP-dye) that is attached to a self-assembling peptide (HEFISTAH) was explored. Although similar systems have been proposed as organic semiconductors, in this case, no electronic conductivity was observed. Instead, the fibres behaved as ionic (probably proton) conductors as a consequence of adsorbed water. A strong dependence of electrical conductivity on relative humidity was demonstrated. The system of nanofibers bridging gold electrodes behaved as an electrolytic cell, with oxygen reduction as a limiting electrode reaction.