Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Characterization and Catalytic Applications of Hydrothermally Synthesized Poyoxotungstate Clusters Containing Organic-Inorganic Hybrid Materials(Izmir Institute of Technology, 2019) Ece, Özlem; Eanes, Mehtap; Çağır, AliMany scientists have been interested in hydrothermal synthesis and characterization of polyoxometalate clusters containing organic-inorganic hybrid materials recently because of having different applications in lots of fields, especially, using as catalysis in chemical reactions. In this study, three novel polyoxotungstate clusters were synthesized using hydrothermal method and catalytic activities of these three novel polyoxometalates and in addition two crystals previously synthesized in our group were studied. The first compound, [(4,4’-bpyH2)3][PCuW11O39] yellow crystals, crystallize in the space group P2(1)/n of the monoclinic system with four formula units in a cell. The crystal is a Keggin polyoxometalate and includes free 4,4’-bipyridine groups between the clusters. The second compound, (4,4’-bpyH2)[H2PW12O40]2.H2O colorless crystals, crystallize in the space group Ia-3 of the cubic system with twelve formula units in a cell. The crystal is a Keggin polyoxometalate and consists of two polyoxotungstate clusters, a free 4,4’-bipyridine group and a water molecule. The third and the last novel compound, (4,4’-bpyH2)4[H2P2W18O62]2 dark yellow crystals, crystallize in the space group P-1 of the triclinic system with four formula units in a cell. The crystal is a Wells-Dawson polyoxometalate and contains two polyoxotungstate clusters and free 4,4’-bipyridine groups between clusters. The catalytic study has been carried out using starch hydrolysis reactions by using hydrothermal synthesis and five different crystals were tried and conversion results were obtained. Starch samples converted to D-glucose with glucose yields above 90 wt.%. Catalyst reusability was performed for each crystal. No appreciable loss was observed in activity after five reaction cycles for each crystal.Doctoral Thesis Molecular Beam Epitaxy Growth and Characterization of Cdte Heterostructures on Gaas-Effect of Interface, Growth, and Annealing Conditions To Crystal Quality(Izmir Institute of Technology, 2017) Arı, Ozan; Öztürk, OrhanHighly crystalline CdTe structures are desired for solar cells, x-ray detectors, electro-optical modulators, and especially in Hg1-xCdxTe infra-red detectors. Epitaxial growth of Hg1-xCdxTe infra-red layers are usually performed on lattice matched bulk CdZnTe substrates. But, limited size and fragile nature of the CdZnTe have led to a push for alternative substrates such as GaAs. The large lattice mismatch between Hg1-xCdxTe and GaAs requires an implantation of a buffer layer such as CdTe. In addition (211)B orientation is preferred due to high sticking coefficient of Hg on this orientation and suppression of twin formation. In the first part of this study, the effect of the thermal deoxidation of GaAs(211)B surface on which CdTe layers grown was investigated by various in situ and ex situ experimental techniques. The changes in the surface chemical structure and morphology of GaAs(211)B substrates with As4 and In assisted deoxidation under various conditions were presented. Secondly, the effect of the growth conditions on CdTe epilayers by using molecular beam epitaxy were investigated in two parts; (1) the initiation of the CdTe growth and (2) the equilibrium growth conditions. The correlations between the structural defects, twins, point defects, and dislocations with the growth conditions are determined. Thirdly, the effect of the cyclic annealing to the crystal and surface quality of the CdTe epilayers were investigated by using different temperatures during the annealing. Finally, the effect of the temperature uniformity during the production of the CdTe layers was investigated by the two substrate heater geometries consisting of rotational symmetric and tilted at the edge. A new approach to study the dislocations with different types of cores proposed by Ayers is applied to the zinc blende (211) crystal orientation. It has been shown that the dislocations having two different cores responded differently to both growth and annealing conditions. The results of the experimental techniques probing the dislocation density in CdTe layers are not well correlated with each other due to dual origin of these dislocations. The compressive and biaxial stresses building in the CdTe layers due to growth and annealing conditions were resolved with the investigation of the optical properties of the layers.