Browsing by Subject "Materials Chemistry"

Now showing 1 - 3 of 3
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    An ATR-FTIR study of semiconductor-semiconductor and semiconductor-dielectric interfaces in model organic electronic devices.
    (2009-08) Mills, Travis
    Organic electronics offer many benefits to inorganic electronics such as the promise of cheap, large-scale processing on flexible substrates and incorporation into many household devices. Organic photovoltaic (OPV) devices and organic field effect transistors (OFETs) offer low-cost implementation which might compete in some applications with their inorganic counterparts. However, fundamental work is necessary to uncover the physics governing the operation of OPVs and OFETs, in order to improve the efficiency of the devices. Much of the fundamental understanding developed in this work occurs at buried interfaces, such as the donor acceptor interface in OPVs or the semiconductor dielectric interface in OFETs. This thesis first introduces the reader to the device physics and state of the art in the development of OPVs and OFETs. After describing the experimental techniques used, a discussion of interfacial electric fields in bulk heterojunction polymer/small molecular solar cells will follow. It was found using the vibrational Stark effect, that donor acceptor interfacial electric fields could be measured and related to previous experiments. The interfacial field hinders the dissociation of excitons but also prevents geminate pair recombination. In OFET devices, the semiconductor dielectric interface was studied and the rate limiting steps to device performance in polymer electrolyte gated OFETs were determined. The interfaces studied provide insight into the fundamental operation of both OPVs and OFETs, which should help produce more efficient and controllable production of organic electronic devices.
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    Chemical Vapor Deposition Growth of Two-Dimensional Transition Metal Dichalcogenides and Related Heterostructures
    (2018-09) DeGregorio, Zachary
    Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are atomically thin, layered materials with unique physical and electronic properties relative to their bulk forms. Due to these properties, 2D TMDCs show promise for many applications, including catalysis, nanoelectronics, optoelectronics, and spin- and valleytronics. To utilize TMDCs for these applications, they must first be reproducibly isolated. Much previous work in this area has resulted in material batches with low yield, small crystal sizes, and little control over the crystal morphology and orientation. Here, I present the reproducible chemical vapor deposition (CVD) growth of a wide array of 2D TMDCs, including MoS2, WS2, MoTe2, NbS2, and WSe2. Control of the growth of these materials is achieved through the optimization of many parameters, including substrate surface chemistry and synthetic growth parameters. Through the optimization of these parameters, I demonstrate control over the resulting material thickness, phase, and morphology. These high-quality TMDCs are subsequently used to grow many relevant heterostructures, including MoS2/WS2 lateral and vertical heterostructures, MoO2/MoS2 core/shell plates, 2H-1T´ MoTe2 few-layer homojunctions, and WS2/NbS2 lateral heterostructures, and the utility of these heterostructures is assessed. MoS2/WS2 heterostructures show promise as a semiconductor-semiconductor heterostructure in which the nature of the alignment is controlled by the initial MoS2 seed crystal. MoO2/MoS2 core/shell plates are freestanding and show epitaxial alignment with the underlying crystal substrate, with potential applications in catalysis. 2H-1T´ MoTe2 few-layer homojunctions are grown using a patternable phase engineering procedure, and devices fabricated from these homojunctions show reduced contact resistance relative to 2H MoTe2 devices with noble metal contacts. Finally, WS2/NbS2 lateral heterostructures show promise as an alternative metal-semiconductor heterostructure system for creating 2D TMDC devices with low contact resistance. The controlled CVD growth of these materials and heterostructures bolsters their future use for relevant applications.
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    Design, Synthesis, And Characterization Of Aluminum(Iii) Porphyrin Assemblies For Use In Photochemical Cells
    (2020-05) Boe, Benjamin
    A series of axially-coordinated aluminum(III) porphyrins were synthesized and evaluated as potential photosensitizers of a ruthenium-based water oxidation catalyst. The porphyrins themselves are 5,10,15,20-tetraphenylporphyrins, and differ in the degree of fluorination on the peripheral phenyl groups. These aluminum(III) porphyrins readily assemble into catalytic dyads through formation of a covalent ester linkage between the central aluminum atom of the porphyrin and a terminal carboxyl group on the ruthenium catalyst. The aluminum center is also able to act as a Lewis acid, forming the final triad by way of a coordinate bond with a suitable Lewis base. Catalytic dyads were successfully synthesized from all three porphyrins in the series; a set of control compounds were also prepared. The dyads and reference molecules were then characterized, with molecular structure and successful formation of the dyads being confirmed with proton NMR spectroscopy, optical properties assessed with respect to UV-Vis absorption and fluorescence spectroscopy, and redox potentials being assessed by both cyclic and differential pulse voltammetry. Formation of the final triad was achieved by titration of the catalytic dyads with a C60 fullerene functionalized to act as a Lewis base; absorption and fluorescence spectra were monitored during titration, allowing for confirmation of the triad formation, as well as calculation of binding constants. The characterization data were used to construct energy level diagrams, laying the groundwork for a theoretical abstraction of these molecule’s functioning. the catalytic systems as synthesized, as well as how they might function in a prototypical photochemical cell. ii Analysis of the results reveal these materials to be promising candidates as photoactivated water oxidation catalysts. The absorption and electrochemical data demonstrate that, when the catalytic dyads are formed, the electronic structure of the constituent parts is preserved. The fluorescence spectra of the dyads show significant quenching relative to the reference porphyrins. Control studies allowed for the exclusion of intermolecular processes as being the source of this quenching, and therefore the optically excited porphyrin must be able to interact with the attached ruthenium catalyst, either through energy or electron transfer. Based on the negligible overlap of the spectra of the catalyst with that of the porphyrins, energy transfer is unlikely. The most likely source of the fluorescence quenching is therefore electron transfer across the ester-bond. The formation and persistence of such a radical ion pair is a fundamental prerequisite for the material to function as a water oxidation catalyst, as it is on this charge-separated species that water oxidation proceeds. Coordination of the dyads with a fullerene ligand was similarly demonstrated, with the resulting triad exhibiting complete fluorescence quenching. The fullerene ligand itself was chosen specifically for its suitability as an electron acceptor, and once again the most likely cause of this quenching is intramolecular electron transfer.