Crystal Growth on One-Dimensional Substrates: Plateau-Rayleigh Crystal Growth and Other Opportunities for Core/Shell Nanowire Synthesis

Abstract

Nanowires hold significant promise for both fundamental studies and technological applications ranging from energy conversion to electronics to biological sensing. The detailed understanding of nanowire synthesis and the realization of new synthetic approaches have enabled precise control over their size, morphology, and composition, and, consequently, their material properties. While much of the work on synthesis in the literature relates to axial nanowire growth, where growth proceeds in the direction of its long axis, this thesis has focused on probing the unique opportunities of shell growth, where material deposits radially around a nanowire core. To this end, I will show, first, that faceted Si core/shell nanowires can be synthesized with embedded pn junctions and that these structures can function as efficient photovoltaic devices with enhanced light absorption properties distinct from bulk Si devices. Second, through choice of reactants and reaction conditions used for shell growth, we demonstrate fine control over the size and morphology of these nanowires, which, in turn, drastically enhances their light absorption at particular wavelengths. Finally, we report for the first time a growth phenomenon that is unique to one-dimensional materials and which combines the underlying physics of the Plateau-Rayleigh instability with crystal growth. By exploiting this phenomenon, which we term Plateau-Rayleigh crystal growth, we demonstrate the growth of periodic shells on one-dimensional substrates. Specifically, we show that for conditions near the Plateau-Rayleigh instability the deposition of Si onto uniform-diameter Si cores, Ge onto Ge cores, and Ge onto Si cores can generate diameter-modulated core/shell nanowires. Rational control of deposition conditions enabled tuning of distinct morphological features, including diameter-modulation periodicity, amplitude and cross-sectional anisotropy. More generally, Plateau-Rayleigh crystal growth highlights the opportunities in understanding the thermodynamics and kinetics unique to crystal growth on nanowires and other low dimensional systems.