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Anisotropic Nanocrystals from Supramolecular Templates

Abstract

The study of nanocrystals has proven to be a rapidly burgeoning field in the last couple of decades. Advances in characterization techniques and a rapidly maturing electronics industry has spurned continued interest in the development of materials with unique electrical and optical properties. The phenomena encountered at the nanoscale, such as quantum confinement and localized surface plasmon resonances, represent powerful tools to be utilized in next-generation optoelectronic devices. Accordingly, there has been a significant push towards understanding fundamental processes at the nanoscale, such as self-assembly, crystalline nucleation and growth, and surface chemical interactions.

This dissertation discusses a method of producing supramolecular assemblies for the purposes of obtaining highly anisotropic and shaped nanocrystals from confined nucleation and growth. This method is noteworthy in that it differs from “traditional” nanocrystal synthesis. Traditional nanocrystal growth is accomplished via solvothermal reaction of anions and cations, or reduction of cations in the case of a metal, in the presence of chelating ligands or polymer in methods broadly termed colloidal synthesis. In that case, the resulting shape of the nanocrystal generated is dictated by the chemisorption or physisorption of coordinating chemical groups to specific crystalline facets, according to their surface energy. Here, we react metal with thiol to form metal alkanethiolate complexes, which adopt a lamellar structure; moreover, this lamellar structure is subject to structural mesomorphism, where it may adopt pseudo-liquid crystalline phases with heating. Lastly, these complexes represent single-source precursors for metal or metal sulfide nanocrystals upon heating in a process termed solventless thermolysis.

The scope of this work has several components. Firstly, to understand the self-assembly of metal alkanethiolate complexes, and the mechanism of solventless thermolysis to metal or metal sulfide nanocrystal. Next, we seek to understand the chemical interactions which give rise to the thermal properties such as thermolysis temperature and structural transition temperatures. By understanding the chemical interactions which determine thermal properties, we seek to attain chemical control over thermal properties such that there is some tunability of the decomposition temperature or structural phase during thermolysis. In doing so, we leverage control over nanocrystal nucleation and growth within specific structural phases which serve as molecular templates and thus dictate nanocrystal growth in a format quite different from colloidal synthesis. We show that these supramolecular templates can facilitate nucleation and growth of nanocrystals within ordered, pseudo-liquid crystalline melts which have the capacity to impart extreme anisotropy to the nanocrystal morphology. Herein, we report the synthesis and characterization of a host of metal alkanethiolates and some metal alkaneselenolates, detail their thermal properties, and utilize them as platforms to synthesize an array of shaped metal, metal sulfide, and metal selenide nanocrystals.

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