Surface chemistry and material integration of metal oxide nanocrystals

Lakhanpal, Vikram Shri
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Metal oxide nanocrystals have a variety of chemical, electronic, and optical properties unique not only to their material composition but also to their size and shape. Control and tunability over these physical parameters can be achieved through colloidal synthesis. In this process, small nuclei form in a heated mixture and long organic molecules known as ligands, which are typically amines or carboxylates, regulate their growth. These ligands also provide long term stability to the nanocrystals when dispersed in solution, as their bulky chains prevent the particles from aggregating. However, many of the properties that make nanocrystals so intriguing involve interaction with their surfaces, which necessitates the removal of the ligands. A variety of methods for ligand stripping have been explored, all with the goal of obtaining ligand-free nanocrystals that retain their properties in dispersions that are stable long enough to combine with other materials, such as polymers, or to process into a functional device, such as a film or coating. In this dissertation, I relate my work during my PhD in various methods of ligand stripping and nanocrystal processing. In particular, I focus on nanocrystalline cerium oxide, a rare-earth metal oxide with a highly reactive surface, and cerium-doped indium oxide, an optically transparent conductor. Cerium oxide nanocrystals are ligand stripped with an organic salt and transferred into dimethylformamide, a polar organic solvent, after which they are mixed with poly(ethylene oxide) to make composite thin films. Protons are generated from water vapor by the cerium oxide surface, turning the films into a proton conducting electrolyte. Cerium-doped indium oxide nanocrystals are stripped with potassium hydroxide in order to transfer them into water, where it is mixed with the conductive polymer PEDOT:PSS in order to create conductive films with enhanced transparency over more opaque films containing just the polymer. Following the mixed results of these projects, I developed and adapted the potassium hydroxide ligand stripping process to a broader range of metal oxide nanocrystals, providing a general method for ligand stripping and transfer into aqueous media.