Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / "June 1996." / Includes bibliographical references (leaves 84-85). / by Wai-Kit Chang. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 137-143). / III-V semiconductor epitaxial nanowires have gained significant attention in recent years, as they showcase an opportunity to combine III-V material properties with a non-planar morphology. To date, semiconductor devices have been continuously engineered to realize optoelectronic devices with ever smaller size, higher efficiency, and lower power consumption. However, many device improvements have reached fundamental physical limitations. One way to address these challenges is to adapt a non-planar device structure. Nanowires are onedimensional structures that can be grown on a substrate with epitaxial relationship using traditional vapor deposition techniques such as metal-organic chemical vapor deposition (MOCVD). Therefore, the novelty of non-planar morphology can be achieved using industrial scale high throughput deposition techniques. To realize the full potential of nanowires as building blocks in a range of different devices, growth of nanowire arrays with controlled density, morphology, composition and alignment is necessary. In this thesis, we demonstrate controlled growth of self-seeded III-V binary and ternary nanowires by MOCVD. First, self-seeded binary III-V nanowire growth is demonstrated for gallium (Ga)-seeded gallium arsenide (GaAs) nanowires. High yield of vertical nanowires are grown reproducibly by a two-step approach: in situ deposition of Ga seed particles at high temperatures (500°C - 600°C), followed by GaAs nanowire growth at lower temperatures (420°C - 435°C). The fabricated GaAs nanowires show a single crystalline structure at the base and occasional twin planes along the nanowire growth direction. We develop a growth model based on incorporation and extraction of Ga from seed particle to explain the observed tapering of nanowires. Second, control over the density and diameter of nanowire arrays is achieved by controlling seed deposition conditions. We demonstrate that higher seed deposition temperatures or changing the GaAs substrate orientation from (11 )A to (110) and (11 1)B yield reduced areal density and larger nanowire diameters. Seed deposition temperature affects the surface diffusion of Ga adatoms, whereas substrate orientation affects the nucleation of seed particles due to varied chemical potential of Ga adatoms and surface energies on different surface orientations. Lastly, controlled self-seeded ternary III-V nanowire growth is realized in the case of Ga-seeded GaAs1-xPx nanowire growth on GaAs substrates. Composition control for x = 0 - 0.3 and growth of GaP nanowires are demonstrated by varying group-V precursor percentage. It was found that strain due to lattice mismatch between GaAs substrate and GaAsP nanowires can be released due to nanowire geometry. Cathodoluminescence measurements have shown emission of light in GaAsP band gap energies, confirming the successful growth of nanowires in this ternary material system. The methods developed for self-seeded growth of GaAs and GaAsP nanowires, as well as density and diameter control of self-seeded growth are extendable to other self-seeded III-V nanowire material systems. / by Sema Ermez. / Ph. D.

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 107-119). / Quantum Monte Carlo (QMC) is an electronic structure calculation method that is capable of calculating incredibly accurate solutions of Schrödinger equation of quantum mechanics for real systems. However, QMC is computationally very expensive compared to density functional theory (DFT) method, such that its application has been limited. In addition, QMC is a stochastic (Monte Carlo) method, meaning that the way calculations are initialized, where a lot of user effort is invested, is crucial for getting accurate results. Computational expense can be justified if the data would be used repeatedly, however the lack of automatization is a severe problem, if QMC would be used in materials discovery. In Chapter 4, we show our automated calculation strategy for formation energy of periodic materials using QMC. We show that our method performs almost by an order of a magnitude more accurate, compared to high throughput DFT strategies having empirical corrections. Nevertheless, it would be beneficial to understand when DFT methods fail such that QMC is used only when the computational expense is justified. A single DFT functional rarely performs uniformly accurate accross different materials and properties due to nonsystematic errors. In Chapter 5, we investigate one specific example: dihydroazulene ring opening photoisomerization, where different substitutions on the ring opening moiety introduce isomerization enthalpy errors up to 0.8 eV. We show that GGA exchange is the main reason for failure in B3LYP, PBE and TPSSH functionals. However, performing a test, similar to the Chapter 5, on each chemical reaction can be an intimidating task where the benchmark set must be carefully devised by an expert in the field. In the absence of experiments, the DFT functional choice is still often done in heuristic way. In Chapter 6, we demonstrate how we can systematically analyze benchmark sets using machine learning to provide highly accurate reaction energies and provide DFT functional selection for different classes of materials when high accuracy calculations or experiments are not available. Our approach provides probabilities of getting accurate results for a reaction that is investigated using each DFT functional. / by Kayahan Saritas. / Ph. D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1992. / Includes bibliographical references (leaf 50) / by Michelle M. Hou. / M.S.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1990. / Vita. / Includes bibliographical references (leaves 156-162). / by Richard James Higgins. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (p. 199-206). / This thesis focuses on silicon-based high index contrast (HIC) photonics. In addition to mature fiber optics or low index contrast (LIC) platform, which is often referred to as Planar Lightwave Cirrcuit (PLC) or Silica Optical Bench (SiOB), the use of HIC platform has been attracting considerable attention recently for the purpose of dense integration of optical components on chip. There are two ultimate solutions to mold of the flow of light. One is high index contrast HIC optics, where the index difference ([delta]n) of core and cladding is more than 0.5 and light is strongly confined in the core, which enables us to integrate optical circuits in m order. Another technique is the introduction of photonic crystal, with which the flow of light is controlled by its photonc bandgap (PBG) and the defect. The concept of photonic crystal can be applied to optical wavgeuides by placing the defect, which is surrounded with photonic crystal structures. In addition to wavgeuide applications, there are lots of unexplored attractive applications for photonic crystal, especially for high index contrast photonic crystal (HIC-PC or HIC-PBG), such as Si/SiO₂ or Si/Si₃N₄ materials systems, due to the wide stop-band. In this thesis, the various applications based on HIC-PBG platform are proposed and investigated. All of the works in this thesis are based on Silicon CMOS-compatible techniques for practical applications. In first three chapters (chapter 2,3 and 4), waveguide applications are mainly focused based on HIC or HIC-PBG platform. In the latter chapters (chapter 5, 6 and 7), the applications of HIC-PBG are explored such as visible-light reflector, semiconductor saturable absorber (SESAM) and thermophotovoltaic (TPV) applications. / by Shoji Akiyama. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. / Includes bibliographical references. / Molten salts are important heat transfer fluids used in nuclear, solar and other high temperature engineering systems. Dispersing nanoparticles in molten salts can enhance the heat transfer capabilities of the fluid. High temperature and high ionicity of the medium make it difficult to make a colloidally stable dispersion of nanoparticles in molten salts. The aggregation and sedimentation kinetics of different nanoparticles dispersed in molten salts is studied, and trends of settling rates with system parameters like particle size, temperature and concentration are observed. Finally, a hypothesis based on ultra low values of Hamaker coefficient is suggested in order to achieve long term colloidal stability in molten salts medium. / by Vaibhav Somani. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (leaf 73). / The potential for land-based turbine buckets material rejuvenation presents a significant commercial and scientific interest. Ni-based superalloy GTD111 is used at a number of GE-manufactured power generation turbines. The outstanding creep resistance features of Ni-based superalloys can be attributed to a large extent, to the gamma prime ([gamma]') precipitates found within the FCC [gamma]-matrix. Service-induced material degradation mainly involves coarsening and shape transformation of [gamma]'-phase precipitates; therefore, any bucket repair attempt should primarily address the restoration of [gamma]' precipitates to the original configuration. In the present study a quantitative metallographic analysis of GTD111 alloy under different conditions was performed. Several micrographs were taken and analysed using image analysis software. Gamma prime precipitate size was measured and compared between the different alloy conditions, leading to useful conclusions concerning material degradation as a result of high-temperature service exposure. In addition, microstructural transformations observed as a result of different heat treatments, formed the basis for investigation of procedures that can potentially restore the alloy microstructure in the original condition. High temperature solution and aging heat treatments sequence is considered as potentially sufficient for restoring the GTD111 microstructure. Finally, the measurements were examined for correlation with existing [gamma]' particle coarsening theory, by calculating and evaluating the metal service temperature during service. A satisfactory correlation exists. / by Apostolos Kountras. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (leaves 33-35). / The performance of a biological scaffold formed by the self-assembling peptide RADA16 is comparable to the most commonly used synthetic materials employed in the culture of neural stem cells. Furthermore, improvements in the performance of RADA16 have recently been made by appending the self-assembling peptide sequence with various functional motifs from naturally occurring proteins. The focus of this work is to further analyze the performance of these functionalized self-assembling peptide scaffolds when used for the culture of neural stem cells, and to characterize these newly developed materials for comparison with RADA16. The effect of the functional motifs on the structure of the peptide scaffold was evaluated with circular dichroism and scanning electron microscopy, and the mechanical properties of the peptide scaffolds were examined through theological analysis. The functionalized peptides were found to have lower percentages of beta-sheet structure as well as reduced storage moduli in comparison with RADA16. SEM images confirmed the ability of the functionalized peptides to form three-dimensional nanofiber scaffolds capable of encompassing, neural stem cells. Three-dimensional cell culture techniques were used to evaluate the ability of the functionalized peptide scaffolds to promote neural stem cell proliferation, and a scaffold formed by the combination of different functionalized peptides was found to increase the proliferation of neural stem cells in comparison to non-functionalized RADA 16. / by Angus M. Hucknall. / S.M.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references. / Block copolymer photonic gels are a simple and easily processed material which responds rapidly to environmental stimuli through a color change. The diblock copolymer that forms the gel self-assembles into a lamellar structure that has the potential to reflect light over a broad range of wavelengths, from the IR to the UV. Application of a stimulus causes a change in the periodicity and/or index of refraction of layers that result in a shift of the stop band. The types of stimuli include temperature, pressure, pH, electric field, salt concentration, and humidity. Due to the high level of tunability of the polymers, it is possible to tailor the response of the gel to achieve a desired effect. This thesis is an assessment of the commercial applications of the photonic gel technology. First a cost model was developed for the polymerization of the block copolymer, polystyrene-b-poly(2-vinyl pyridine). The results indicate that it is cost effective to invest in a small scale production facility at large production volumes. Next, an evaluation of three potential markets was conducted. The anti-counterfeit market is most promising because of large profit margins and the opportunity for future company growth through R&D of new anti-counterfeit measures. The other two markets in color cosmetics and food preservation present potential opportunities for licensing. / by Sally S. Lou. / M.Eng.