A combined program of multi-scale modeling and experiments will be conducted to understand the most significant aspects of the growth of crystalline zinc oxide (ZnO) in the form of nanowires. Arrays of these structures are grown from supersaturated liquid phases and are of particular interest for the fabrication of nanowire-based, dye-sensitized solar cells. These low-cost, photovoltaic devices are especially attractive due to their potential for very low cost and good efficiency. The quality, microstructure, and dimensions of the ZnO nanowire array determine the solar cell's performance, yet quantitative knowledge of the effects of process-level variables on the fabrication of these structures is lacking. Key to improving these devices is a more fundamental understanding of the mechanisms by which the crystalline nanowires grow from liquid solution.
The overall objectives of the proposed work are the development and validation of fundamental, mechanistic models describing the growth of crystals from the liquid phase and the application of these models to better understand the growth of ZnO nanowire crystals. Such knowledge is needed to link growth conditions to microscopic properties of crystalline structural perfection and composition, as well as characteristics of crystal shape, i.e., growth habit and size, that affect the density of the nanowire arrays. Multi-scale, theoretical models, based on the phase-field approach, will be developed to simulate nano-scale growth spirals on crystal surfaces in a supersaturated liquid. Experiments will be conducted on the solution growth of ZnO that utilize novel nano-indentation techniques to selectively place dislocations through an array of seed nanocrystals. Growth kinetics will be measured for nanowires both without and with growth spirals that have evolved from those dislocations. Theory and experiment will be applied to test the hypothesis that long-aspect-ratio nanowires in this system arise primarily from kinetic factors associated with a growth spiral on one face. The synergy between model and experiment will enable advances not possible by theory or experimentation alone.
Intellectual merit: The work addresses fundamental scientific issues of crystal growth together with a clear goal toward improving a practical application. The coupling of bulk transport with step growth kinetics, via the phase-field approach, will result in multi-scale models for solution crystal growth of new rigor and relevance. Such models will enable a fundamental exploration of the coupled factors of fluid flow, mass transfer, and interfacial kinetics in solution crystal growth processes. A specific outcome of this work will be a greater understanding of the liquid-phase growth of crystalline, ZnO nanowires.
Broader impacts: Results obtained by this work will increase the fundamental understanding of how crystals grow from liquid solutions and specifically advance nanowire-based, dye sensitized solar cells. Other applications involving solution crystal growth are also likely to be affected by the understanding provided by this research. For example, solution crystallization is the most commonly used unit operation in the chemical and pharmaceutical industries for the purification and separation of chemical products that are solids at room temperature and pressure. Solution growth is also applied for the production of many inorganic crystals, ranging from the growth of large-scale optical materials to the growth of epitaxial layers. Broader activities include the education of graduate students in multi-scale modeling and nanotechnology, as well as an outreach program for the general public involving the Science Museum of Minnesota.
