Synthesis and characterization of structured one and two component clusters
Abstract
Nanometer size metal clusters exhibit unique physical and chemical properties which are a strong function of their size and atomic structure. If practical applications of these ultrafine particles are to be realized, it is vital that ways be developed to control not only their size but also their atomic structure. An inert gas aggregation source known as the multiple expansion cluster source (MECS), capable of producing clusters of controlled mean size and composition with a narrow size and composition distribution, has been developed at Purdue. This thesis presents research directed at understanding and modifying the atomic structure of nanometer size clusters synthesized in the MECS. Structural transformations were achieved by gas phase annealing. The atomic structures of the clusters were analyzed by transmission electron microscopy. An aim of these studies was to determine the lowest energy or most stable structures of nanometer size clusters. The lowest energy structure of Au clusters with diameters greater than 2 nm is a fcc crystal with a roughly spherical shape. As the diameter of these clusters is increased they become faceted, assuming the shape of truncated octahedra. Incomplete annealing of Au clusters produces multiply twinned particles (MTPs). Gas phase annealing of clusters composed of two immiscible atomic species produces phase segregation. Metal/silicon and metal/metal clusters were studied. These clusters exhibit two types of lowest energy structure: (1) a fish-eye structure with one species forming a spherical core and the other species forming an outer skin and (2) a bicrystal structure with two single species phases segregating side by side. Exposure of these nanometer size clusters to air transforms them into metal/silica or metal/metal oxide clusters. The lowest energy structure of the metal/silica clusters was found to be the fish-eye structure. The lowest energy structure of metal/metal oxide clusters was found to be the bicrystal structure.
Degree
Ph.D.
Advisors
Andres, Purdue University.
Subject Area
Chemical engineering
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