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Thesis advisor: Vidya Madhavan / We present Scanning Tunneling Microscopy (STM) studies on several systems in which spin-orbit coupling leads to new and interesting physics, and where tuning by doping and/or strain can significantly modify the electronic properties, either inducing a phase transition or by sharply influencing the electronic structure locally. In the perovskite Iridate insulator Sr3Ir2O7, we investigate the parent compound, determining the band gap and its evolution in response to point defects which we identify as apical oxygen vacancies. We investigate the effects of doping the parent compound with La (in place of Sr) and Ru (in place of Ir). In both cases a metal-insulator transition (MIT) results: at x ~ 38% with Ru, and x ~ 5% with La. In the La-doped samples we find nanoscale phase separation at dopings just below the MIT, with metallic spectra associated with clusters of La atoms. Further, we find resonances near the Fermi energy associated with individual La atoms, suggesting an uneven distribution of dopants among the layers of the parent compound. Bi2Se3 is a topological insulator which hosts linearly dispersing Dirac surface states. Doping with In (in place of Bismuth) brings about topological phase transition, achieving a trivial insulator at x ~ 4%. We use high-magnetic field Landau level spectroscopy to study the surface state’s properties approaching the phase transition and find, by a careful analysis of the peak positions find behavior consistent with strong surface-state Zeeman effects: g~50. This interpretation implies, however, a relabeling of the Landau levels previously observed in pristine Bi2Se3, which we justify through ab initio calculations. The overall picture is of a g-factor which steadily decreases as In is added up to the topological phase transition. Finally, we examine the effects of strain on the surface states of (001) thin films of the topological crystalline insulator SnTe. When these films are grown on closely-related substrates—in this case PbSe(001)—a rich pattern of surface strain emerges. We use phase-sensitive analysis of atomic-resolution STM topographs to measure the strain locally, and spatially-resolved quasiparticle interference imaging to compare the Dirac point positions in regions with different types of strain, quantifying for the first time the effect of anisotropic strain on the surface states of a topological crystalline insulator. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Our group has developed a strategy for the synthesis of benzofused carbocycles and heterocycles through tandem palladium-catalyzed reactions of gem-dibromoolefins. In these syntheses, one bromide undergoes a Pd-catalyzed cyclization reaction, and the other participates in an orthogonal inter- or intramolecular Pd-catalyzed reaction to functionalize or annulate that ring, respectively. Chapter 1 describes the pairing of an intramolecular C–N bond forming reaction (the Buchwald-Hartwig coupling) with an intramolecular direct arylation for the synthesis of fused indole derivatives. A range of previously unknown heterocycles were synthesized through this method. Chapter 2 describes the synthesis of benzothiophenes through the combination of a Pd-catalyzed C–S coupling with an orthogonal Suzuki, Heck, or Sonogashira reaction. This represents the first example of the incorporation of Pd-catalyzed C–S coupling into a tandem reaction. In Chapter 3, a tandem intramolecular Heck / intermolecular Suzuki reaction is described for the synthesis of methyleneindenes. Studies on this reaction have provided advanced understanding of the mechanism, including how variation of the ligand controls the regioselectivity of the reaction.

My research is focused on two main objectives, the study of catalytic efficiency and mechanism of palladium nanoparticles stabilized by poly(vinylpyrrolidone) (PVP) for carbon-carbon coupling reactions, and to rationally synthesize metal nanoparticles stabilized by metal-carbon bonds and apply them to catalyze carbon-carbon coupling reactions.<p> In the first project, Pd nanoparticles stabilized by PVP were used to catalyze carbon-coupling reactions, specifically the Stille and Suzuki reactions. The mechanism of carbon-carbon coupling reactions was studied. The uncertainty of whether nanoparticles or Pd salts are the catalyst was also examined using the same experimental procedure with Pd salts to examine their catalytic activity in carbon-carbon coupling reactions. Results show that the presence of O2 is crucial to the Stille reaction with the Pd nanoparticles, which are nearly completely inert under N2, while the K2PdCl4 precursor is itself quite active for the Stille reaction. However, the Pd nanoparticles were found to be active for the Suzuki reaction with high yields in the absence of O2. The yields for 4-chlorobenzoic acid are higher than 4-bromobenzoic acid and occur for un-catalyzed reactions, for reasons that are still unknown. Finally Au nanoparticles have been tested by the same experimental procedure and have no catalytic activity for these two reactions.<p> In the second project, the synthesis of Au and Pd monolayer protected clusters (MPCs) with metal carbon covalent linkages was examined, and the stability of the resulting MPCs was tested. UV-Vis spectra and TEM images show the formation of Au and Pd nanoparticles and 1H NMR was used to characterize the ligands attached to the surface of the nanoparticles. The decylphenyl-stabilized Pd MPCs were synthesized successfully and quite stable in air, while decylphenyl-stabilized Au MPCs prepared with the same protocol have less stability and are easily decomposed. XPS spectra indicate the composition of decylphenyl-stabilized Pd MPCs is a combination of Pd0 and Pd2+ species with the Pd2+ species in excess. In addition, alkylphenyl-stabilized Pd nanoparticles were shown to be effective catalysts for carbon-carbon coupling reactions such as Suzuki and Stille reactions as well as hydrogenation reactions. Finally, it was noted that Pd-C bonds could be easily reduced by H2 when performing hydrogenation reactions resulting in nanoparticle aggregation and precipitation under hydrogenation conditions.

A new interfacial code was developed to couple the reactor physics code PARCS/AGREE to the systems level code MELCOR, with a goal of enabling state- of-art transient event analysis for high temperature gas reactor designs. Following the completion of this new code, it was then demonstrated by running two different coupled simulations, one of which was a transient event. The resultant code is capable of coupling spatial power profiles, point kinetics information and transient reactivity values from PARCS/AGREE to MELCOR by means of input/output file manipulation. The coupling demonstrations were between PBMR400 models that were designed to have an equivalent core region nodalization to that which was used in the OECD/NEA PBMR400 benchmark, thus allowing for comparisons. The accessible coupled simulation output results as extracted from MELCOR appeared to be overly generalized. Even so, the axial profiles from the coupled steady-state demonstration were in good agreement with the axial profiles of other OECD/NEA participants. Conversely, the coupled transient simulations showed a suspect, maximum average nodal component temperature rise of approximately 0.4K from a 3+$ reactivity insertion.

Our group has developed a strategy for the synthesis of benzofused carbocycles and heterocycles through tandem palladium-catalyzed reactions of gem-dibromoolefins. In these syntheses, one bromide undergoes a Pd-catalyzed cyclization reaction, and the other participates in an orthogonal inter- or intramolecular Pd-catalyzed reaction to functionalize or annulate that ring, respectively. Chapter 1 describes the pairing of an intramolecular C–N bond forming reaction (the Buchwald-Hartwig coupling) with an intramolecular direct arylation for the synthesis of fused indole derivatives. A range of previously unknown heterocycles were synthesized through this method. Chapter 2 describes the synthesis of benzothiophenes through the combination of a Pd-catalyzed C–S coupling with an orthogonal Suzuki, Heck, or Sonogashira reaction. This represents the first example of the incorporation of Pd-catalyzed C–S coupling into a tandem reaction. In Chapter 3, a tandem intramolecular Heck / intermolecular Suzuki reaction is described for the synthesis of methyleneindenes. Studies on this reaction have provided advanced understanding of the mechanism, including how variation of the ligand controls the regioselectivity of the reaction.

My research is focused on two main objectives, the study of catalytic efficiency and mechanism of palladium nanoparticles stabilized by poly(vinylpyrrolidone) (PVP) for carbon-carbon coupling reactions, and to rationally synthesize metal nanoparticles stabilized by metal-carbon bonds and apply them to catalyze carbon-carbon coupling reactions.<p> In the first project, Pd nanoparticles stabilized by PVP were used to catalyze carbon-coupling reactions, specifically the Stille and Suzuki reactions. The mechanism of carbon-carbon coupling reactions was studied. The uncertainty of whether nanoparticles or Pd salts are the catalyst was also examined using the same experimental procedure with Pd salts to examine their catalytic activity in carbon-carbon coupling reactions. Results show that the presence of O2 is crucial to the Stille reaction with the Pd nanoparticles, which are nearly completely inert under N2, while the K2PdCl4 precursor is itself quite active for the Stille reaction. However, the Pd nanoparticles were found to be active for the Suzuki reaction with high yields in the absence of O2. The yields for 4-chlorobenzoic acid are higher than 4-bromobenzoic acid and occur for un-catalyzed reactions, for reasons that are still unknown. Finally Au nanoparticles have been tested by the same experimental procedure and have no catalytic activity for these two reactions.<p> In the second project, the synthesis of Au and Pd monolayer protected clusters (MPCs) with metal carbon covalent linkages was examined, and the stability of the resulting MPCs was tested. UV-Vis spectra and TEM images show the formation of Au and Pd nanoparticles and 1H NMR was used to characterize the ligands attached to the surface of the nanoparticles. The decylphenyl-stabilized Pd MPCs were synthesized successfully and quite stable in air, while decylphenyl-stabilized Au MPCs prepared with the same protocol have less stability and are easily decomposed. XPS spectra indicate the composition of decylphenyl-stabilized Pd MPCs is a combination of Pd0 and Pd2+ species with the Pd2+ species in excess. In addition, alkylphenyl-stabilized Pd nanoparticles were shown to be effective catalysts for carbon-carbon coupling reactions such as Suzuki and Stille reactions as well as hydrogenation reactions. Finally, it was noted that Pd-C bonds could be easily reduced by H2 when performing hydrogenation reactions resulting in nanoparticle aggregation and precipitation under hydrogenation conditions.

Thick composites (~ 3 mm in thickness) of polypyrrole with electrically insulating porous (polystyrene) and nonporous (polymethyl methacrylate) substrates were prepared using a two-step batch method. In the two-step method, impregnation of volatile (iodine) or nonvolatile (ferric chloride) oxidant in the substrate is followed by in-situ polymerization of pyrrole. Conductivities as high as 10-1 S/cm were obtained in this work in the case of composites of polypyrrole and porous, crosslinked polystyre. Use of the nonvolatile oxidant (ferric chloride) resulted in higher conducting polymer yield, as well as composites having a higher conductivity, thermal stability, and mechanical strength. However, the volatile oxidant (iodine) could be transported to the substrate using supercritical carbon dioxide as the solvent. As a result, partitioning of the oxidant between the solvent phase and the polymer substrate, and hence the distribution of the oxidant in the substrate, could be controlled by manipulation of the pressure. The two-step batch method in which supercritical carbon dioxide is used to facilitate transport and as a solvent for the oxidant was found to be an effective method for the production of thick composites with uniform conductivity, thermal stability, and mechanical strength. Such composites are desired in important practical applications such as rechargeable battery electrodes and electromagnetic interference shielding materials.

During the past decade, the demand for clean renewable energy continues to rise drastically in Europe, the US, and other countries. Wind energy in the ocean can possibly be one of those future renewable clean energy sources as long it is economically feasible and technologically manageable. So far, most of the offshore wind farm research has been limited to fixed platforms in shallow-water areas. In the water depth deeper than 30m, however, floating-type wind farms tend to be more feasible. Then, the overall design and engineering becomes more complicated than fixed platforms including the coupled dynamics of platforms, mooring lines, and blades. In the present study, a numerical time-domain model has been developed for the fully coupled dynamic analysis of an offshore floating wind turbine system including blade-rotor dynamics and platform motions. As a test case, the TLP-type floater system with 3 blades of 70-m diameter designed by the National Renewable Energy Laboratory (NREL) is selected to analyze the dynamic coupling effects among floating system, mooring lines, and wind turbine. The performance of the selected system in a typical wind-wave-current condition has been simulated and analyzed. A similar study for the floater and rotor coupled dynamic analysis was conducted by MIT and NREL. However, in the present case, the dynamic coupling between platform and mooring lines are also considered in addition to the rotor-floater dynamic coupling. It is seen that the rotor-floater coupling effects increase with wind velocity and blade size. The increased coupling effects tend to increase the dynamic tension of TLP tethers. The developed technology and numerical tool are applicable to the new offshore floating wind farms planned in the future.

This dissertation presents the development of a novel hybrid wave model, comprised of the irrotational, 1-D horizontal Boussinesq and 2-D vertical turbulence-closed Reynolds Averaged Navier-Stokes (RANS) wave models. The two constituents are two-way coupled with the interface placed at a location where turbulence is relatively small. Boundary conditions on the interfacing side of each model is provided by its counterpart model through data exchange, requiring certain transformation due to the difference in physical variables employed in both models. The model is intended for large-scale wave simulation, accurate in both the nonbreaking and breaking zones with relatively coarser grid in the former and finer in latter, and yet efficient. Hybrid model tests against idealized solitary and standing wave motions and wave-overtopping on structure exhibit satisfactory to very good agreement. Compared with pure RANS simulations, the hybrid model saves computational time by a factor proportional to the reduction in the size of the RANS domain. Also, a large-scale tsunami simulation is provided for a numerical setup that is practically unapproachable using RANS alone; not only does the hybrid model offer more rapid simulation of relatively small-scale problems, it provides an opportunity to examine very large total domains with the fine resolution typical of RANS simulations. To allow for implementation on even larger domain with affordable CPU time, the hybrid model is parallelized to run on distributed memory machine. This is done by parallelizing the RANS model while leaving the Boussinesq model serial. One of the processors is responsible for both the sub-RANS and Boussinesq calculations. ICCG(0) for solving the pressure equation is parallelized using the nonoverlappingdecomposition technique, requiring more iterations than the serial one. Standing wave and hypothetical tsunami simulations with 960×66 and 1000×100 grids, and using 8 processors confirm model validity and computational efficiency of 82% and 65%. Finally, the 2-D Boussinesq model is parallelized using domain decomposition technique. The solution to the tridiagonal system arising in the model is calculated as the sum of the homogeneous and particular solutions. Parallel model tests using up to 32 processors exhibit model accuracy and efficiency of 80% for simulation with 500×500–2000×2000 grids.