Solution syntheses of silicon and germanium nanowires, nanorods and axial heterostructures using low solubility catalysts

This thesis describes the synthesis of Si, Ge and Si-Ge axial heterostructure nanowires and nanorods through the use of two solution synthesis based approaches. These nanostructures are catalysed by low solubility catalysts via both the vapour-liquid-solid and solution-liquid-solid nanowire growth mechanisms. Chapter 1 gives an introduction to Si and Ge nanowires detailing the different growth mechanisms and synthetic techniques allowing for their production. The different types of catalysts available for the synthesis of these nanowires are also outlined and discussed. The general synthetic strategies and characterisation techniques used for the synthesis and analysis of the nanostructures produced within this work are detailed in Chapter 2. Chapter 3 investigates the synthesis of Si-Ge axial heterostructure nanowires catalysed by Sn seeds. These structures were initially found to grow in high density in the vapour phase of an organic solvent, via the vapour-liquid-solid growth mechanism. Through the variation of the synthetic approach, it was also possible to synthesise these nanowires via the solution-liquid-solid mechanism, in the liquid phase of the organic solvent. The electrochemical potential of these nanowires as anodes in a Li-ion battery was then examined. The modification of the synthetic protocol for the growth of Sn seeded multi-segment Si-Ge axial heterostructure nanowires is described in Chapter 4. The ability to reverse the configuration of the previously synthesised Si-Ge heterostructure nanowires to form Ge-Si heterostructure nanowires was first investigated. This then iv led to the synthesis of more complex, multi-segment heterostructures of up to at least six alternating segments of Si and Ge. The interfacial abruptness of the Ge-Si interface was determined through the use of atomic resolution electron energy loss spectroscopy analysis. The synthesis of Pb seeded Ge nanowires is described in Chapter 5, catalysed by both an evaporated Pb layer and Pb nanoparticles. These nanowires were synthesised by both the vapour-liquid-solid and solution-liquid-solid growth mechanisms, simultaneously. Pb nanoparticles were then attached to the as-synthesised Pb-Ge nanowires, from which branched Ge nanowires were grown, in order to increase the overall density of the Ge nanowires on the substrate. Both the single crystal and branched nanowires were investigated for their use as Li-ion battery anodes. Chapter 6 describes the low temperature, solution synthesis of Si, Ge and Si-Ge axial heterostructure nanowires and nanorods through the solution-liquid-solid growth mechanism, exclusively. These structures were synthesised from Sn catalysts which were formed in situ from an evaporated layer of Sn on stainless steel or from Sn nanoparticles. The synthesis of these nanowires/nanorods at temperatures ranging from 200 °C to 300 °C was enabled through the addition of a reducing agent to the reaction, with control over the lengths of the resulting nanostructures achieved through the variation of the reaction temperature. Chapter 7 offers conclusions for each chapter and recommendations for further study pertaining to the work completed in this thesis.