Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / 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 153-163). / Photovoltaics (PV) offer a promising route to combat climate change. However, the growth rate of the dominant commercial photovoltaic (PV) technology is limited by large capital expenditure requirements. This motivates fundamental research into thin-film materials, such as lead sulfide (PbS) quantum dots (QDs), that are composed of earth-abundant elements, can be produced through low-cost deposition techniques, and are stable under operating conditions. In this thesis, a device architecture that combines a zinc oxide (ZnO) nanowire ordered bulk heterojunction (OBHJ) architecture with band alignment engineering of the PbS QD film to enhance charge extraction is demonstrated. This approach results in PV devices with photocurrent density greater than 30 mA/cm2, which represents a 15% improvement compared to planar devices and enables solar cells with power conversion efficiency up to 9.6%. This photocurrent density is the highest achieved for QDs with a 1.3 eV band gap, which is the optimal band gap in the detailed balance limit. The enhanced photocurrent in the nanowire devices is shown to be a result of both improved light harvesting due to improved in-coupling of light after the addition of the ZnO nanowire array and improved carrier collection due to the bulk heterojunction effect. Furthermore, electron beam-induced current (EBIC) was used to study charge transport in PbS QD films. It is shown that holes are the minority carrier in PbS QD films treated with tetrabutylammonium iodide (TBAI). This finding indicates that the thickness of OBHJ devices composed of a PbS-TBAI film paired with an n-type nanowire array are constrained by minority carrier transport. Moreover, quantitative EBIC was applied for the first time on PbS QD diodes to measure the bulk minority carrier diffusion length (Lbulk). Lbulk was extrapolated by comparing the effective diffusion length measured at different beam energies. EBIC injection leads to high-level injection conditions, therefore a lower bound for the hole diffusion length in PbS-TBAI QD films is established, with Lbulk e 110 nm. This provides a critical design parameter for OBHJ solar cells. This thesis motivates further work on optimization of ZnO nanowire arrays for PbS QD OBHJ solar cells through array patterning, acceptor-doping, and passivation of the nanowire surface. Furthermore, the EBIC technique developed in this work can be applied to quantitatively measure nanoscale carrier diffusion lengths in other thin-film PV materials. / by Paul Harlan Rekemeyer. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1995. / Includes bibliographical references (leaves 109-114). / by Ariel Javier Sebastián Traiber. / Ph.D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 107-116). / This thesis demonstrates and evaluates the potential application of luminescent quantum dot/polymer solutions on crystalline silicon photovoltaics. After spin coating the QD/polymer onto silicon photodiodes, an increase of 3% in current density was observed. This performance improvement was used to determine the impact application would have on the crystalline silicon photovoltaic supply chain. Supply chain costs were modeled to estimate the segment costs for Sharp's NUU230F3 230W module. The benefits realized by use of cells coated with the QD/polymer solution were then estimated at both the module and the cell segments. Finally, an installation cost model for the residential market was built to determine the impact an increase in efficiency had on total system costs. / by Garrett Bruer. / M.Eng.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1984. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Soon-Ju Kwon. / Ph.D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / 35 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 65-67). / Recent advances in material processing are presenting groundbreaking opportunities for biomedical engineers. Projection-micro-stereolithography, or PuSL, is an additive manufacturing technique in which complex parts are built out of UV-curable resins using ultraviolet light. The primary strength of PuSL is its capacity to translate CAD files into three-dimensional parts with unusually small feature sizes (~0.5 microns). It is an ideal candidate, therefore, for making tissue scaffolds with sophisticated microscopic architecture. Nearly all multicellular biological tissues display a hierarchy of scale. In human tissues, this means that the mechanics and function of an organ are defined by structural organization on multiple levels. Macroscopically, a branching blood supply creates a patent network for nutrient delivery and gas exchange. Microscopically, these vessels spread into capillary beds shaped in an organ-specific orientation and organization, helping to define the functional unit of a given tissue. On a nano-scale, the walls of these capillaries have a tissue-specific structure that selectively mediates the diffusion of nutrients and proteins. To craft a histologically accurate tissue, each of these length scales must be considered and mimicked in a space-filling fashion. In this project, I sought to generate a cellular, degradable tissue scaffolds that mimicked native extracellular matrix across length scales. The research described here lays the groundwork for the generation of degradable, vascularized cell scaffolds that might be used to build architecturally complex multi-cellular tissues suitable for both pharmacological modeling and regenerative medicine. / by Micha Sam Brickman Raredon. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 32). / Growing evidence supports that the unique mechanical behavior of mussel byssal threads, such as high toughness and self-healing, rely on an intricate balance of permanent covalent and reversible metal coordination bonds. Inspired by this material crosslink chemistry balance, we synthesized polyethylene glycol (PEG) hydrogels with two crosslinked networks; a primary permanent network composed of covalently crosslinked 4-arm PEG and a secondary network composed of 4-arm PEG functionalized with histidine on each arm. The histidine decorated PEG forms a mechanically reversible network via metal ion coordinated crosslinks. Using rheometry, we study the contribution of the metal-coordinate network to the bulk gels mechanics and find that we can control both the amplitude and the frequency of peak mechanical dissipation with the histidine: metal ion ratio and the choice of metal ion, respectively. Furthermore, we can control the mechanical contribution of metal coordinate bonds by changes in pH. These simple bio-inspired gels promise to serve as a new model system for further study of opto-mechanical coupling of metal-coordinate soft materials. / by Robert Learsch. / S.B.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references. / This thesis explores the commercialization of germanium-based nanocrystal memories. Demand for smaller and faster electronics and embedded systems supports the development of high-density, low-power non-volatile electronic memory devices. Flash memory cells designed for ten years of data retention require the use of a thick tunneling oxide. This compromises writing and reading speed as well as endurance. A smaller device size can be achieved and speed and can be improved by decreasing the oxide thickness. However, significant charge leakage will occur if the oxide is too thin, which will reduce the data retention time dramatically. This imposes a limit to the amount by which the oxide thickness can be decreased in conventional devices. Research has shown that by incorporating nanocrystals in the tunnel oxide, charge traps are created which reduce charge leakage and improve endurance through charge-storage redundancy. By replacing the conventional floating gate memory with one using Si or Ge nanocrystals, the nonvolatile memory exhibits high programming speed with low programming voltage and superior retention time, and yet is compatible with conventional silicon technology. This thesis provides an analysis of competing technologies, an intellectual property analysis, costs modeling as well as ways to improve nanocrystal memories in order to compete with other forms of emerging technologies to replace conventional Flash memories. / by Kian Chiew Seow. / M.Eng.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 29). / Raman spectroscopy is used to probe the structural change of Lanthanum Strontium Cobaltite (La1.xSrxCoO 3 -8) thin films across change in composition (0%-60% strontium) and temperature (30*C-520°C). Raman shift peaks were identified and correlated with specific vibrational modes. Results were consistent with relevant data, but no transition to the high spin state was observed above 200°C. Compositions were compared to oxygen catalytic data to investigate success in high temperature electrochemical applications. No structural phase changes were found in the research of this thesis, interesting effects in the surface regime were observed and possible explanations are offered. Future research should focus on resolving the surface regime via altered experimental set up. Keywords: LSC, LCO, Raman, in silu. / by Justin Daniel Breucop. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 107-128). / Domain walls are regions of spatially non-uniform magnetizations in magnetic materials. They form the boundaries between two or more uniformly magnetized regions called domains. Skyrmions are circular magnetic domains with chiral domain walls that are interesting due to their stability and potential for fast motion. These spin structures can be used to encode Os and Is in spintronic memory. Chiral domain walls and skyrmions have been seen in magnetic thin films sandwiched between non-identical non-magnetic materials which have high spin-orbit coupling and Dzyaloshinskii-Moriya interaction. An optimization of the different physical interactions involved in magnetic thin films can result in stripe and labyrinth domain patterns which can then be transformed into skyrmion lattices. In this thesis, we present a detailed understanding of single- and multi-layer magnetic thin films along with all the relevant physical interactions. We show that inplane magnetic fields stabilize domain walls in thin films with Dzyaloshinskii-Moriya interaction. The application of in-plane magnetic fields is shown to create multi-domain patterns in films where the ground state is uniform magnetization. Next, we study the formation of stripe and labyrinth domain patterns in magnetic films. The domain widths obtained are compared with the predictions of several theoretical models developed over the last 50 years. The appropriate model that works for thin films with strong Dzyaloshinskii-Moriya interaction is identified with the help of micromagnetic simulations. The appropriate model includes effects of finite domain wall width and volume charges inside Neel domain walls. This model is then used to measure the Dzyaloshinskii-Moriya interaction in experimentally grown magnetic thin films. Thereafter, we highlight the role of other design variables such as the thickness of magnetic and non-magnetic layers, the choice of materials, and the role of geometrical confinement in controlling the length scale of the domain patterns. This work generates the necessary knowledge and develops techniques to engineer chiral spin textures in single- and multi-layer magnetic thin films. / by Parnika Agrawal. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 32). / The effects of oxygen on Fe-catalyzed carbon nanotube (CNT) growth on Ta substrates was studied. CNTs were grown on Fe thin-film catalysts deposited on silicon substrates via exposure to C₂H₄ in a thermal chemical vapor deposition (CVD) furnace. Heating for CVD growth causes the Fe film to dewet to form catalyst particles. During CVD, the sample was exposed to gas mixtures of Ar, Ar/O₂, H₂, and C₂H₄. Experiments were performed with varying amounts of oxygen from mixing of the Ar and Ar/O₂ carrier gas, as well as pre-annealing samples in oxygen or hydrogen-rich environments. Samples were characterized via scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was found that when an optimum amount of oxygen was introduced, taller CNT carpets were observed. Pre-annealing samples in an oxygenrich environment shows additional benefits in carpet growth. In contrast, pre-annealing in a hydrogen-rich environment counteracts the benefits of introducing oxygen during the growth phase. Coarsening of the catalyst particles was suspected as a reason for the difference in growth patterns, and pre-annealed sample morphologies were characterized without C₂H₄ flow. AFM scans show apparent coarsening in samples exposed to hydrogen-rich environments, and reduced coarsening in the samples exposed to oxygen-rich environments. / by Ryan K. Bonaparte. / S.B.