http://hdl.handle.net/1721.1/7594 2017-06-10T13:25:56Z http://hdl.handle.net/1721.1/109674 Controlling nanomaterial self-assembly for next generation optoelectronic applications Weidman, Mark Clayton Semiconductor nanocrystals, also known as quantum dots, are an exciting class of materials because their band gap can be tuned according to the nanocrystal size. In this way, the material band gap can be largely decoupled from its atomic composition - a property unique to this system. The potential applications for semiconductor nanocrystals are wide ranging and include: LEDs, photovoltaics, photon downconversion, photon upconversion, and thermoelectrics. However, their size-dependent band gap can also be a hindrance, as any size variation in the ensemble of nanocrystals introduces energetic disorder and spatial disorder in films. While synthesized as a colloid, for most applications the nanocrystals are deposited as a thin film. The rate of energy transfer between nanocrystals in the film, dictated by the arrangement and distance between neighbors, is therefore a critical parameter affecting device efficiency. As a result, controlling the nanocrystal physical arrangement is crucial to the success of these materials. Despite this, there is a lack of understanding of how to observe and control these processes at the nanoscale. This thesis begins by improving the synthesis of lead sulfide (PbS) nanocrystals to produce narrow size dispersity ensembles with tunable average size by ensuring the reaction is diffusion-limited. We then experimentally determine what parameters (ligand coverage, solvent, size dispersity) most affect the ability of these nanocrystals to self-assemble into highly ordered superlattice structures. We show that superlattices can be produced with a wide variety of surface ligands of differing lengths, either directly from a colloidal suspension or post-deposition and we thoroughly characterize the interparticle spacing as a function of ligand species. Next, we demonstrate an in situ X-ray scattering technique which enables the real-time visualization of nanocrystal self-assembly, with details unprecedented by any other experimental method. This technique led to a better understanding of the colloid to superlattice transition, including the observation of intermediate states and the ability to compare kinetics of different self-assembly aspects. Finally, we present experimental measurements demonstrating that nanocrystal size dispersity and selfassembly are critical to efficient energy transfer in films and that as energetic disorder is minimized through improved synthetic methods, spatial disorder becomes an increasingly important parameter to control. In the final experimental chapter of this thesis, we apply this knowledge to a different material system perovskite nanoplatelets, which have the potential to be useful as an inexpensive, solution-processable emission material. For these 2D materials, we optimize the thickness homogeneity and study the selfassembly of the nanoplatelets into stacked superstructures. We highlight the incredible tunability of this material system accessible through thickness and compositional tuning, which allows absorption and emission to be shifted across the entire visible range. Lastly, we demonstrate the potential of this system for next generation LEDs. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February 2017.; "September 28th, 2016." Cataloged from PDF version of thesis.; Includes bibliographical references (pages 127-133). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109673 Organometallic redox-interfaces for selective electrochemical separations Su, Xiao, Ph. D. Massachusetts Institute of Technology Electrochemical separation methods are promising due to their modularity, fast kinetics and potential integration with renewable sources. However, they are still limited in application due to high energetic costs and lack of chemical selectivity. This work explores redox-electrodes as a platform for targeting aqueous and organic contaminants with high separation factors, in the contexts of environmental water remediation, chemical product purification in organic synthesis, metal-recovery and bio-separations. The design of selective stimuli-responsive interfaces is a crucial challenge for advanced electrochemical processes. Whereas redox-electrodes are well known in sensing, catalysis and energy storage, here we focus on their unique potential for selective ion removal - cases in which one dilute compound is targeted in the presence of large excess of competing electrolyte. In particular, organometallics and associated metalcomplexes offer an attractive material platform, due to their flexible metal-ligand design and as a consequence, extensive control allowed of their electronic properties. The first major thrust is the molecular design of various organometallic species for specific interactions with charged compounds in solution. We developed a series of heterogeneous, nano-structured metallocene interfaces to control the selective sorption and release of anions, cations, and even proteins, based on electrochemical potential. In parallel, through a combination of electronic structure calculations and spectroscopy, we unraveled the unique binding mechanism between ferrocenium and organic ions demonstrating an unusual redox-mediated hydrogen-bonding between cyclopentadienyl and carboxylates; and utilize this knowledge to further tune our redox-systems to enhance chemical selectivity. We expanded our organometallic set to various bi-pyridines and functionalized metallocenes, and studied various problems ranging from reactive separations to catalytic remediation of contaminants of emerging concern. A second major thrust consists in utilizing asymmetric pseudo capacitors as the next generation configuration for electrochemical separation devices. Asymmetric systems were shown to have much higher energy storage capabilities as well as separation efficiencies. We focused on counter-electrode design, in which the redox reaction at the cathode works in tandem with the anode, thus maintaining the water chemistry by suppressing parasitic reactions which otherwise lower current efficiency. From a fundamental perspective, the novel interaction mechanisms explored in this thesis were shown to have broader implications in deionization, sensing, catalysis and energy storage. For chemical engineering, this work demonstrated redox-based electrochemical methods as an energy-efficient and sustainable route to process intensification, and paved their way for practical implementation in industry. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February 2017.; "October 2016." Cataloged from PDF version of thesis.; Includes bibliographical references (pages 255-295). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109672 Modern control methods for chemical process systems Paulson, Joel Anthony Strong trends in chemical engineering have led to increased complexity in plant design and operation, which has driven the demand for improved control techniques and methodologies. Improved control directly leads to smaller usage of resources, increased productivity, improved safety, and reduced pollution. Model predictive control (MPC) is the most advanced control technology widely practiced in industry. This technology, initially developed in the chemical engineering field in the 1970s, was a major advance over earlier multivariable control methods due to its ability to seamlessly handle constraints. However, limitations in industrial MPC technology spurred significant research over the past two to three decades in the search of increased capability. For these advancements to be widely implemented in industry, they must adequately address all of the issues associated with control design while meeting all of the control system requirements including: -- The controller must be insensitive to uncertainties including disturbances and unknown parameter values. -- The controlled system must perform well under input, actuator, and state constraints. -- The controller should be able to handle a large number of interacting variables efficiently as well as nonlinear process dynamics. -- The controlled system must be safe, reliable, and easy to maintain in the presence of system failures/faults. This thesis presents a framework for addressing these problems in a unified manner. Uncertainties and constraints are handled by extending current state-of-the-art MPC methods to handle probabilistic uncertainty descriptions for the unknown parameters and disturbances. Sensor and actuator failures (at the regulatory layer) are handled using a specific internal model control structure that allows for the regulatory control layer to perform optimally whenever one or more controllers is taken offline due to failures. Non-obvious faults, that may lead to catastrophic system failure if not detected early, are handled using a model-based active fault diagnosis method, which is also able to cope with constraints and uncertainties. These approaches are demonstrated on industrially relevant examples including crystallization and bioreactor processes. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 301-322). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109671 Catalytic, low temperature oxidation of methane into methanol over copper-exchanged zeolites Narsimhan, Karthik As production of shale gas has increased greatly in the United States, the amount of stranded shale gas that is flared as carbon dioxide has become significant enough to be considered an environmental hazard and a wasted resource. The conversion of methane, the primary component of natural gas, into methanol, an easily stored liquid, is of practical interest. However, shale wells are generally inaccessible to reforming facilities, and construction of on-site, conventional methanol synthesis plants is cost prohibitive. Capital costs could be reduced by the direct conversion of methane into methanol at low temperature. Existing strategies for the partial oxidation of methane require harsh solvents, need exotic oxidizing agents, or deactivate easily. Copper-exchanged zeolites have emerged as candidates for methanol production due to high methanol selectivity (> 99%), utilization of oxygen, and low reaction temperature (423-473 K). Despite these advantages, three significant shortcomings exist: 1) the location of surface intermediates on the zeolite is not well understood; 2) methane oxidation is stoichiometric, not catalytic; 3) there are few active sites and methanol yield is low. This work addresses all three shortcomings. First, a new reaction pathway is identified for methane oxidation in copper-exchanged mordenite zeolites using tandem methane oxidation and Koch carbonylation reactions. Methoxy species migrate away from the copper active sites and adsorb onto Bronsted acid sites, signifying spillover on the zeolite surface. Second, a process is developed as the first instance of the catalytic oxidation of methane into methanol at low temperature, in the vapor phase, and using oxygen as the oxidant. A variety of commercially available copper-exchanged zeolites are shown to exhibit stable methanol production with high methanol selectivity. Third, catalytic methanol production rates and methane conversion are further improved 100- fold through the synthetic control of copper speciation in chabazite zeolites. Isolated monocopper species, directed through the one-pot synthesis of copper-exchanged chabazite zeolites, correlates with methane oxidation activity and is likely the precursor to the catalytic site. Together, these synthetic methods provide guidelines for catalyst design and further improvements in catalytic activity. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 135-147). 2017-01-01T00:00:00Z