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Date of Award
Stephanie L. Brock
ABSTRACT ASSEMBLY OF QUANTUM DOTS INTO POROUS ARCHITECTURES: METHODS, MECHANISMS, AND APPLICATIONS by KARUNAMUNI LAKMINI SAMANTHA SILVA May 2022 Advisor: Prof. Stephanie L. Brock Major: Chemistry Degree: Doctor of Philosophy The dissertation research is focused on achieving two main goals: (1) identifying the chemical parameters that govern the oxidative assembly method of semiconductor metal chalcogenide nanocrystals (NCs) to tune the properties of resultant gel structures of multicomponent systems, and (2) developing new assembly techniques to improve properties associated with their quantum-confined structures. NC assembly plays a key role in determining the performance of solid-state devices and the oxidative assembly method provides a porous three-dimensional architecture. To enable the fabrication of multi-component composites with control over the degree of mixing the dissertation research evaluate the kinetics of assembly in single-component systems as a function of cation solubility and facet energy. To evaluate the role of metal cation solubility we composed CdS and ZnS of two polymorphs (cubic and hexagonal). ZnS NCs have faster aggregation relative to CdS. These data correlate with the relative solubilities of the nanoparticles, as probed by free-cation concentration in solution, confirming our hypothesis. To evaluate the rate of facet energy/shape on the aggregation rate different aspect ratios of CdS nanorods were created, altering the relative ratio of polar end-facets vs. non-polar side walls. Upon oxidant addition, highly polar tip facets of nanorods assembled to make highly porous structures with the rate scaling with the side of the polar end facets. In contrast, nanorods with no polar end facets (nanorice) and cubic polymorph nanoplatelets with polar faces slow to assemble and form compact structures. As a complementary method to the chemical oxidative assembly that can enable direct deposition on electrode surfaces, electrogelation methods were established using the electrooxidation method. Gels of CdS could be deposited on a Pt electrode by applying the potential. The mechanism of electrogel formation was identified by employing linear sweep voltammograms (LSV) in which potentials involved in the oxidation of ligand removal and dichalcogenide bond formation were identified at 0.8 and 1.2, respectively for thioglycolate capped CdS NCs. The developed method is amenable to other NC formulations (ZnS and CdSe) but is redox-sensitive and is reversed under reducing conditions, limiting the applications. Thus, another electrochemical method was introduced to connect discrete NCs employing in-situ generated metal ions using a Ni electrode. The low oxidation potential of Ni enables the release of Ni2+ ions to the solution producing assemblies by cross-linking of surface carboxylate functionalities on NCs. The versatility of the electrochemical assembly methods provides the ability to control the production of NC assemblies directly onto functional architectures as demonstrated by the production of a chemo resistive NO2 gas sensor.
Silva, Karunamuni Lakmini Samantha, "Assembly Of Quantum Dots Into Porous Architectures: Methods, Mechanisms, And Applications" (2022). Wayne State University Dissertations. 3583.