Interaction between structural and electronic phase changes of metal oxide semiconductor nanocrystals
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Semiconducting metal oxides have emerged as a core class of materials in functional electronic devices because of their versatile compositions and tunable electronic and optical properties. Applying a charge to metal oxides can modulate carrier properties and induce structural changes from charge-compensating defects. However, charge-mediated transformations are contingent upon efficient transport of carriers, compensating species, or field biases into the bulk. Nanostructured materials, including colloidal metal oxide nanocrystals, can accommodate efficient charge transport across the semiconductor interface, and exhibit sensitive optical and electronic properties that arise from their nanoscale geometry. This dissertation studies the relationship between charge-mediated electronic and structural phase changes in metal oxide nanocrystals, and correlates these transformations with their nanoscale geometry and interfacial environment. The first investigation studies anatase TiO₂ nanocrystals during electrochemical charging. TiO₂ nanocrystal films can undergo two independent charging processes within a Li-ion electrolyte: surface capacitance, which raises the Fermi level upon reduction and induces Drude-like infrared localized surface plasmon resonance without affecting structure, and intercalative charging caused by the insertion of Li⁺ into the nanocrystal lattice. These two charging processes create independent dual-spectrum visible (Li-ion intercalation) and infrared (plasmon resonance) optical responses to applied bias, with applications for versatile electrochromic smart windows. The optical and electrochemical properties of both charging mechanisms are isolated and studied independently to examine the role of structure and interfacial environments on these transformations. The second part of this dissertation explores charge-mediated transformations in nanocrystalline VO₂, which has a highly non-ideal, charge-correlated electronic structure. A charge-mediated electrochemical insulator to metal transformation in VO₂ is found to be highly sensitive to nanoscale grain size, leading to a secondary metal-insulator transformation for sufficiently confined particles. The results of these studies establish general principles to control the interplay between defect-mediated structural transformations, ideal semiconductor gating behavior and interfacial environments in metal oxide nanocrystals.