Synthesis and characterisation of self-seeded germanium and germanium tin nanowires

Abstract

Li-ion batteries are important energy storage devices today, powering a range of electrical equipment from mobile phones to electric vehicles. Most commercial Li-ion battery anodes are made from carbon-based materials which limit their improvement of total battery capacity, energy density and cycle life performance because of the relatively low specific charge capacity of carbon. Group IV semiconductors, particularly Ge and GeSn, are a promising alternative to conventional carbon-based electrodes: especially in niche energy storage applications like small high-tech devices. In this thesis, I describe a cost-effective and simple method for synthesising Ge and GeSn nanowires, with potential application as anode materials in Li-ion batteries. A single-pot, solvothermal-like synthetic method, employing supercritical solvent conditions, was used to grow self-seeded group IV nanowires, without the need for traditional metal catalysts or templating agents. My approach permitted the growth of thin (~ 10 nm) and highly crystalline nanowires of carbon-coated cubic Ge (C-Ge), metastable tetragonal ST12 Ge and GeSn, from commercially available precursors at moderate reaction temperatures, between 330 and 490 °C. Photoluminescence studies carried out on tetragonal ST12 Ge nanowires suggested they possessed a fundamental direct bandgap of ~ 0.64 eV. Both C-Ge and GeSn nanowires were directly grown onto Ti current collectors and evaluated as potential anodes for Li-ion batteries. C-Ge nanowires demonstrated exceptional performance, displaying a high specific charge value of > 1200 mA h g-1 after 500 cycles at 0.2 C, while GeSn alloy nanowires also displayed an impressive specific charge capacity of 1127 mA h g-1 after 150 cycles at 0.2 C. The growth mechanisms for each set of nanowires are also described in detail in the thesis. Similarities in growth mechanisms were observed between nanowires grown using the single-step approach and previously reported solution-phase studies.