Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (p. 45). / The application of rippled gold nanoparticles with bi-ligand surface morphology as a catalyst was tested. The hydrolysis of 2,4-dinitrophenyl acetate (DNPA) served as the catalytic reaction being analyzed and the bi-ligand composition used was 16-mercaptohexadecanoic acid to imidazole thiol (MHA to IT). The influence of temperature on catalytic reaction of DNPA with the MHA: IT system was tested for ligand rations of 2:1, 1:2, and 1:1 by monitoring the catalytic system on a UV-VIS spectrometer. Catalytic rate constants were obtained and found to increase with increased temperature. The measured catalytic rate constants were greatest overall for the 1:1 system, followed by the 1:2 system, and lastly the 2:1 system. The activation energy for each ligand-ratio system was measured and found to be 22.17 kJ/mol for the 2:1 system, 14.7 kJ/mol for the 1:2 system, and 26.52 for the 1:1 system. The 2:1 and 1:2 systems followed the trend of lower activation energy values for systems with faster rates; however the 1:1 system did not fit this trend as it resulted in the highest activation energy value as well as the fastest reaction rates. / by Cristina F. Stefanescu. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student submitted PDF version of thesis. / Includes bibliographical references. / Ligand-stabilized nanoparticles (NPs) assembled into long-range ordered arrays, also known as "nanoparticle supracrystals (NPSCs)", are expected to provide a powerful general platform for designing new types of solids. In particular, the NPs are themselves self-assembled structures consisting of a core and a self-assembled monolayer of ligand molecules surrounding it. The self-assembled structure of the NPs themselves determines the structure of the self-assembled supracrystals. Ligands are of special interest in this respect, because it is an important component for the NP system which play a major role in the design of self-assembly of the complex matter and also provide a powerful entry into the supracrystal engineering. The increasing ability to control the way in which ligand molecules associate gives means for the designed generation of supraparticle architectures in the self-assembly. In spite of this, elucidation of how the ligands play a role in affecting the structural behavior of NPSCs remains largely unrevealed. In this thesis, the effect of ligands for the two dimensional (2D) self-assembled NPSCs structure was investigated. The key materials advancement that enables this work is that we have been able to synthesize monodisperse gold NPs of same core size but different ligand molecules. Additionally, a new method for monolayer film processing has been developed to prepare the 2D NPSCs, based on a Langmuir assembly through successive compression cycles. Importantly, as there is little effect exhibited by solvent interactions in the NPs structure obtained from this approach, the corresponding NPs structural variation in this work is truly driven by the different ligand interactions in NPSCs. Specifically, we show that such ligand interactions have direct consequences on the ordering and symmetry of the assembled NPSCs structures. Here, we report on a set of NPSC arrays in which small changes in either the NP ligand environment or the ligand configuration geometry induce significant variations in the order parameters of the crystal. First, we show that the packing organization of a 2D NPSC array of hydrophobic alkanethiol ligands varies with subtle chemical changes in the system, leading to a transition between long-range to short-range (almost glassy) ordered phases. The balance between long and short-range order is driven by small differences in intermolecular interpenetration of the ligand molecules, that can be related to ligand conformational and that can be rigorously the experimentally measured. Second, we show the first 2D NPSC structures to have unique anisotropic symmetry due to the interaction between amphiphilic NP ligand shells. It is understood that the ligand interactions on NPs through their unique molecular configuration of amphiphilic ligands may provide the anisotropic feature in the orientational alignment of NPSC symmetry. / by Jin Young Kim. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 115-121). / AlGaN/GaN high electron mobility transistors (HEMTs) constitute a new generation of transistors with excellent electrical characteristics and great potential to replace silicon technology in the future, especially in high power and high frequency applications. However, the poor long term reliability of these devices is an important bottleneck for their wide market insertion and limits their advanced development. This thesis tackles this problem by focusing on understanding the physics behind various degradation modes and providing new quantitative models to explain these mechanisms. The first part of the thesis, Chapters 2 and 3, reports studies of the origin of permanent structural and electrical degradation in AlGaN/GaN HEMTs. Hydroxyl groups (OH-) from the environment and/or adsorbed water on the III-N surface are found to play an important role in the formation of surface pits during the OFF-state electrical stress. The mechanism of this water-related structural degradation is explained by an electrochemical cell formed at the gate edge where gate metal, the II-N surface and the passivation layer meet. Moreover, the permanent decrease of the drain current is directly linked with the formation of the surface pits, while the permanent increase of the gate current is found to be uncorrelated with the structural degradation. The second part of the thesis, Chapters 4 and 5, identifies water-related redox couples in ambient air as important sources of dynamic on-resistance and drain current collapse in AlGaN/GaN HEMTs. Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species is found at the AlGaN surface at room temperature. It is also found that these species, as well as the current collapse, can be thermally removed above 200 °C in vacuum conditions. An electron trapping mechanism based on H₂O/H₂ and H₂O/O₂ redox couples is proposed to explain the 0.5 eV energy level commonly attributed to surface trapping states. Moreover, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface. Finally, fluorocarbon, a highly hydrophobic material, is proven to be an excellent passivation to overcome transient degradation mechanisms in AlGaN/GaN HEMTs. / by Feng Gao. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Vita. / Includes bibliographical references (p. 167-170). / by Krassimir P. Nenov. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 159-165). / Phase transitions are commonly observed in ion storage compounds when being used in rechargeable batteries and thus, the phase behavior of ion storage compounds as electrode active materials has significant impact on battery performance. This thesis aims to understand the interplay between materials structure, phase behavior and battery performance. The effects of operating conditions, especially overpotential and temperature, on phase behavior and battery performance are also investigated. Using olivine-type phosphates (i.e. phospho-olivines) with varying composition and particle size as model system, strain accommodation mechanism within single nanoparticles (Chapter 2 to 3) and mesoscale kinetics of nanoparticle aggregates (Chapter 4 to 5) during electrochemically-induced phase transition have been systematically investigated. In the first part, phospho-olivines with varying transformation strain, from 0 - 3vol% for LiMnyFe1-yPO4 (LMFP, y<0.5), 5vol% for LiFePO4 (LFP), to 17vol% for NaFePO4 (NFP), have been studied using operando Powder X-ray Diffraction (PXD), among other methods. While small transformation strain as in LMFP is accommodated and even avoided by formation of metastable solid solution, large transformation strain as in NFP is mitigated by formation and dissolution of intermediate amorphous phase. This novel mechanism to accommodate large transformation strain may pave the way of utilizing battery materials that deem not working otherwise. In the second part, potentiostatic studies are conducted and a model modified from Avrami model is developed to quantitatively describe phase transformation progresses. The phase transition of LMFP and LFP nanoparticle aggregates is found to follow a nucleation and growth process while the growth is governed by lithium ion diffusion. Based on analysis using the modified Avrami model, more instantaneous nucleation and facile growth tend to occur when transformation strain is small (intermediate Mn content and/or small particle size), overpotential is high and/or temperature is high. And instantaneous nucleation and facile growth improve the rate capability of batteries. The relationship between phase behavior and material structure as well as operating conditions is attributed to: 1) decreasing transformation strain reduces energy barrier for both nucleation and growth; 2) increasing overpotential and temperature boost the electrochemical driving force for phase transition and promote more instantaneous nucleation and facile growth. / by Kai Xiang. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged student-submitted from PDF version of thesis. / Includes bibliographical references (pages 171-187). / To minimize risks associated with climate change, we must rapidly reduce greenhouse gas emissions worldwide by shifting reliance away from fossil fuels. Solar photovoltaic (PV) modules are well suited for reducing emissions; however, manufacturing and capital costs must continue to decline for rapid, worldwide PV adoption. Low-cost and Earth-abundant "thin film" materials offer potential in spurring PV growth, but their development is often hampered by the presence of defects, which degrade solar cell efficiency due to short charge-carrier lifetimes. In this thesis, such defects and their impact on lifetime in early-stage PV materials are investigated, focusing on experimental methods to assess lifetime connected to theoretical concepts about both defects and lifetime measurements themselves. First, time-resolved photoluminescence is performed, and both analytical and numerical modeling are used to determine lifetimes exceeding 1 nanosecond in six materials predicted to be "defect tolerant." Two-photon spectroscopy is then employed to decrease the effect of surface recombination, enabling more representative estimates of "bulk" lifetime. Second, the role of impurities is explored by intentionally contaminating lead halide perovskites with iron. Synchrotron-based X-ray techniques are also utilized to investigate the distribution and charge state of incorporated iron, and perovskite solar cells are found to tolerate approximately 100 times more iron in the feedstock than comparable p-type silicon solar cells. In addition, improved methods for extracting lifetime from solar cell devices are explored. Quantum efficiency measurements are performed and modeled on tin monosulfide solar cells to verify that very short lifetimes (30-100 picoseconds) limit device performance. Furthermore, temperature- and illumination-dependent current-voltage measurements are performed and modeled in iron-contaminated silicon solar cells- and analyzed with the help of a Bayesian inference algorithm-to estimate the defect parameters that directly relate to lifetime. Collectively, these studies serve to provide a more robust framework for assessing and mitigating the presence of defects in early-stage PV materials, streamlining efforts to better optimize their photovoltaic performance. / by Jeremy Roger Poindexter. / Ph. D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references (leaves 32-33). / by Kelly Stengel. / B.S.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / "September 2007." / Includes bibliographical references (p. 67-69). / Ion channels represent an important category of drug targets. They play a significant role in numerous physiological functions, from membrane excitation and signaling to fluid absorption and secretion. An ion-channel assay system using optical nanosensors has recently been developed. This high-throughput, high-content system improves on the existing patch clamp and fluorescent dye technologies that presently dominate the ion-channel screening market. This paper introduces the nanosensor technology, reviews the current market for ion-channel assays, assesses the costs associated with the nanosensors, and evaluates their commercialization potential. / by Hannah Yun. / M.Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 164-192). / Oxygen defects are essential building blocks for properties and functionalities of oxides, including electrical conductivity, magnetism, ferroelectricity as well as catalytic and electrocatalytic activity. Therefore, fundamental understanding of how to tune the oxygen defect chemistry is essential for advancing applications based on these defect sensitive properties. This thesis investigated pathways to controlling the concentration and structure of oxygen defects on selected case studies with model oxide systems. Three novel effects were assessed and shown to be operative for obtaining a large impact on the oxygen defect chemistry equilibria. These are heterogeneous chemical doping of the surface for improving surface electrocatalytic activity and stability, electrochemical bias to control phase with drastic changes obtained in electronic and phonon transport properties, as well as strain engineering to alter the oxygen interstitial capacity and oxygen exchange kinetics. Surface chemical modifications were applied to the near-surface regions of Lao.8Sro.2CoO 3 (LSC) by replacing the Co cations locally with less reducible cations, such as Hf and Ti. This strategy was shown to effectively stabilize the LSC surfaces and suppress surface segregation of Sr at elevated temperatures. This introduced surface stability by local chemical doping greatly enhanced the long-term electrochemical performance of LSC electrode, which provides a new route for improving the efficiency of solid oxide fuel and electrolysis cells. Applying electrical bias was investigated as another effective method to tune the oxygen stoichiometry, exemplified by the case studies on SrCoOx (SCO). In situ X-ray diffraction was used to investigate the topotactic phase transition between brownmillerite phase SrCoO2.5 (BM-SCO) and perovskite phase SrCoO 3 6 (P-SCO) triggered electrochemically at elevated temperatures. An electrical bias of merely 30 mV was shown sufficient to trigger the BM-->P phase transition. This is much more feasible than chemically induced phase transition, which requires high pressure (> 1 bar) and specialized pressurized apparatus. Moreover, the evolution of electronic structure during the BM4P phase transition was probed in operando by using ambient-pressure X-ray photoelectron and absorption spectroscopy (AP-XPS/XAS). The similar experimental scheme, which combines in operando surface characterizations and electrochemical controlling of oxygen stoichiometry, was extended to oxide systems beyond perovskites. This allows us to investigate the defect chemistry of oxides in a much broader range of effective oxygen partial pressure than what conventional methods can achieve. Firstly, we showed that the surface defect chemistry equilibrium of fluoritestructured Pro.iCeo.902-6 (PCO) strongly deviated from the bulk counterpart, due to the possibly enhanced defect-defect interactions or lattice strain effect at surfaces. Secondly, we found a novel metal-insulator transition triggered electrochemically in VO, by changing the phase between the metallic dioxide VO2 and the insulating pentoxide V2O5 Lastly, we lowered the operation temperature of this electrochemical control of oxygen stoichiometry down to room temperature by using ionic liquid or ion gels as the electrolyte. We achieved tuning of thermal conductivity in SrCoOx with a range of more than one order of magnitude, by using electrochemically triggered phase transitions at room temperature. We also investigated the effect of lattice strain on oxygen defect formation energy in Ruddlesden-Popper (RP) phase oxide Nd2NiO4+6 (NNO). We found that tensile strain along the c-axis of NNO lattice effectively reduced the formation enthalpy of oxygen interstitials, which can provide a new route for designing the defect chemistry of RP phase oxide for electrocatalytic applications.. / by Qiyang Lu. / Ph. D.

Thesis: M.S., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1996. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Advanced hybrid microelectronics require high performance packaging materials and components. Alumina, the most widely used ceramic material in microelectronics, is being replaced by new high performance materials such as aluminum nitride (AlN) and beryllia (BeO). AlN's thermal conductivity is an order of magnitude greater than alumina and it has a thermal expansion coefficient very close to silicon. Metallization is used to generate areas for wire and die bonding and to form corrosion barriers for underlying metals. Plating is one metallization process which immerses the component to be coated into a chemical medium. The plating baths usually operate at temperature greater than 70°C and a pH greater than 12. Alumina is stable in this environment but AlN corrodes. An electroless gold gel plating process was developed which selectively plates specific areas of a surface without exposing the component to a corrosive environment. The developed process exhibits several advantages: selective area metallization, variable metal thickness on the same component, rework of defectively plated parts, reduction of waste produced during operation, optimal usage of gold, higher plating rates, and elimination of variation in the chemical composition during operation. An electroless plating gel was formulated using a modified commercial bath and a polymeric thickening agent. The relationship between gel processing and the resulting chemical and physical properties are discussed. The gel was transferred to the substrate in a defined pattern by a screen printer and then deposited in a chemical reactor. Several chemical reactors were fabricated to reduce the time required to obtain the operating temperature and decrease the drying rate during plating. The final reactor prototype increased the heating rate over five fold and decreased the drying rate by a factor of 4. Metallic impurity contamination was discovered in residual gel samples after plating. This caused non-uniform coatings and decreased the plating rate. A chelating agent was added to the electroless gel which complexed with the metal ions preventing them from interfering in the plating reactions. Scanning electron microscopy (SEM) showed the resulting gold plate was dense and uniform.. Gold conversion efficiencies, the amount of gold plated from the gel onto the substrate surface, were determined by DCP. Conversion efficiencies were typically greater than 90% with reproducible values greater than 95%. / by Kenneth C. Arndt. / M.S.