The Impact of Surface Chemistry on Stable Semiconductor Nanowire Growth
Sivaram, Saujan Venkat
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The vapor-liquid-solid (VLS) mechanism – whereby a liquid eutectic “catalyst” droplet collects precursor molecules (or atoms) from the vapor and directs crystallization of the solid nanowire – is a ubiquitous method for bottom-up nanowire synthesis. In this thesis, we use in situ infrared absorption spectroscopy to identify the previously unknown, yet critical, role of reactive surface intermediates on semiconductor nanowire synthesis. We quantitatively determine the surface coverage of hydrogen atoms by coupling operando measurements with a novel in situ surface titration and show these adsorbates are vital for stable Ge nanowire growth. In the second part of the thesis, we use in situ spectroscopy to explore the interplay between the supercooled AuGe catalyst state and surface chemistry. We find a strong correlation between loss of surface hydrogen and catalyst solidification. To unambiguously identify the influence of surface chemistry on the supercooled AuGe catalyst, we deliver atomic hydrogen to the nanowire sidewall, which prevents Au migration from the supercooled catalyst and preserves the liquid catalyst state in the absence of Ge2H6 flow. We conclude that solidification likely occurs via heterogeneous nucleation in the presence of solid particles near the trijunction region and present general strategies to maintain the supercooled catalyst state in other material systems. Our experiments identify a key chemical mechanism underlying nanowire growth via chemical vapor deposition and demonstrate that changes to surface bonding are critical to understand nanowire synthesis. The fundamental insights shown promise unprecedented control of nanowire structure and function by providing a chemical foundation for rational synthetic design.