Low-temperature Electrodeposition of Crystalline Semiconductor Nano/Microstructures & Application of As-prepared Materials in Li-ion Batteries

Abstract

Crystalline group IV semiconductor materials (i.e. Si and Ge) are essential components in many electronic and optoelectronic devices. Based on their alloying reactions with Li ion, Si and Ge are also potential rechargeable battery anode materials. In these and other applications, nanostructured forms of crystalline Group IV semiconductors particularly show enhanced and desirable properties, providing impetus for their development. The current synthetic methods for nano/microstructured group IV semiconductors usually demand high energy input, involve complicated instrumentation or require highly-refined precursors. The aim of this thesis is to develop non-energy-intensive synthetic methods, namely electrochemical liquid-liquid-solid (ec-LLS) processes, for the synthesis of nano/microstructured group IV semiconductor materials using simple instruments and common chemicals. Furthermore, this thesis also demonstrates that the ec-LLS grown materials can directly be used in energy storage devices and shows high performance. The essence of the ec-LLS process is the utilization of liquid metal (e.g. Ga (l), eGaIn (l), Hg (l)) electrodes for the electrodeposition of group IV semiconductors. The liquid metal serves as the cathode on which precursors are electrochemically reduced and the solvent phase for crystallization. This dissertation summarizes results from several hypotheses that address the fundamental aspects and practical utility of ec-LLS for the preparation of crystalline Ge and Si. Chapter 2 examines the hypothesis that the size of the liquid metal electrode determines the diameter of ec-LLS grown Ge nanowires. The change in the morphology of Ge deposits when Ga reaches micrometer size is also shown. Chapter 3 focuses on the hypothesis that crystalline Si nanowires can be grown via ec-LLS process at low temperatures. The morphology and crystallinity of ec-LLS grown Si nanowires as a function of substrate orientation, temperature and liquid metal identity is also discussed. Chapter 4 investigates the hypothesis that ec-LLS grown Ge microwires are electrochemically active to be used as Li-ion battery anode. The effect of Ga impurities on the performance of Ge anode is also discussed. Chapter 5 describes work illustrating that Si microwires can be prepared by ec-LLS with liquid metal electrodes showing high solubility for Si. Preliminary results are summarized that suggest further strategies to improve the prospects for this ec-LLS process. Overall, this dissertation will also serve as a foundation for future advancements in the ec-LLS preparation of other materials of interest.