Nanowires, nanorods and axial heterostructures of silicon and germanium synthesised in a solvent vapour system

This thesis describes the formation of Si and Ge nanostructures using two growth systems. Chapter 3 details the controlled growth of Ge nanowires and nanorods using a surfactant-free hotplate based growth method. Control over the aspect ratio of the Ge nanostructures is achieved by varying the concentration and reactivity of the organometallic precursor. This route used Cu3Ge catalyst seeds which were formed in situ from bulk copper foil. This approach was significantly expanded in Chapter 4 to allow the formation of Ge nanowires over centimetre squared areas. The use of a thin thermally evaporated Cu layer as catalyst source led to high density nanowire growth on substrates which were ideally suited to direct incorporation as Li-ion battery anodes. The optimised nanowire covered substrates showed excellent capacities of 1040 mAhrg-1 after 500 charge/discharge cycles. Focus then shifted to the use of a glassware-based solvent vapour growth system. Here, dense Ge and Si nanowire mats were formed directly on stainless steel using an abundant catalyst material, Sn (Chapter 5). These nanowires are promising candidates for Li-ion storage applications as both the nanowire and catalyst add to device capacity. Capacity figures of 1078 mAhrg-1 for Si and 1000 mAhrg-1 for Ge were noted after 50 cycles (with both the masses of the Li-active Sn seed and nanowire segment taken into consideration). Finally, Sn was shown to be a suitable catalyst for the formation of compositionally abrupt Si/Ge heterostructure nanowires (Chapter 6) using the same solvent vapour growth system detailed in Chapter 5. Compositional abruptness was facilitated by the extremely low solubilities of Si and Ge in the Sn catalyst material while control over the length of the Si and Ge segments was achieved by controlling the reactivity of the respective precursors and associated reaction conditions.