Silicon and germanium nanostructures : synthesis and in situ TEM study




Lu, Xiaotang

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A variety of chemical routes exist for a wide range of nanomaterials with tunable size, shape, composition and surface chemistry. Of these materials, silicon (Si) and germanium (Ge) nanomaterials have been some of the most challenging to synthesize. Solution-liquid-solid (SLS) growth of Si was studied using tin (Sn) as the seeding metal. Si nanorods with narrow diameters can be grown by the decomposition of trisilane in hot squalane in the presence of Sn nanocrystals. Photoluminescence could be obtained from the Si nanorods by thermal hydrosilylation passivation. This colloidal synthesis could be further simplified to a single-step reaction procedure by the in situ formation of Sn seed particles. In addition to trisilane as a Si source, isotetrasilane, neopentasilane and cyclohexasilane were studied for Si nanorod growth: all three reactants enabled nanorod formation at lower growth temperatures. A monophenylsilane (MPS) enhanced growth was discovered for supercritical fluid-liquid-solid (SFLS) growth of Ge nanowires that enables the Ge conversion of ~100%. A variety of metalorganic compounds were studied for replacing pre-synthesized metal nanoparticles to induce Ge nanowire growth. Si and Ge nanowires are some of the most promising anode materials in lithium ion batteries (LIBs) because of their high lithium storage capacity. However, the significant chemical and physical changes that occur during cycling hamper their practical uses. In situ transmission electron microscopy (TEM) techniques were conducted to observe and understand structural and interfacial changes of the Si and Ge nanowires during electrochemical cycling; and, therefore, resolving the problems with current anodes by materials modification. The in situ TEM experiments showed that the incorporation of Sn into Si nanowires can enhance their rate capability. But the enhanced Li diffusion leads to the premature pore formation in Si nanowires. Ge nanowires has been discovered the potential as sodium ion battery anodes after an initial activation with a lithiation step to amorphize the nanowires.



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