Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (leaves 85-87). / Visible range Raman spectra of graphene are generated based on the double resonant process employing a full two-dimensional numerical calculation applying second-order perturbation theory. Tight binding expressions for both the TO phonon dispersion and the [pi] - [pi]* electronic bands are used, which are then fit to experimental or ab-initio results. We are able to reproduce the single-peak D mode of graphene at ~ 1380 cm-1 that is identical to experiment. A near linear shift in the D mode peak with changing incoming laser energy of 33 cm-1/eV is calculated. Our shift marginally underestimates the experimental shifts as most of the literature features specimens that contain a few or more layers of graphene through to graphite that ought to subtly alter their electronic and phonon dispersions. However, our approach is readily applicable to such homologous forms of graphene once we have available their electronic band structure and phonon dispersions. / by Rohit Narula. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (leaves 27-28). / Thermoelectrics are gaining significant amounts of attention considering their relevance today in the areas of sustainable energy generation and energy efficiency. In this thesis, the thermoelectric properties of bulk Silicon were modeled using ab initio density functional theory methods to determine the Si band structure. Specifically, three different models for determining the Seebeck coefficient - Parabolic Bands, Boltzmann's theory, and the 'Pudding Mold' approximation to Boltzmann's theory - were studied in depth and compared with experimental values. Here we show first principles calculations to yield Seebeck coefficients for n-type Silicon to be on the order of 300 gtV/K at -300 K, and -500 gtV/K at 300 K for the Parabolic Bands and Boltzmann approach, respectively. While the 'Pudding Mold' Theory failed in its approximations of the Seebeck coefficients, the calculations using the other two theories were found to agree closely with experimentally determined Seebeck coefficients. / by Saahil Mehra. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Includes bibliographical references (leaves [71-73]). / Semiconducting metal-oxides such as SnO₂, TiO₂, ZnO and WO₃ are commonly used for gas sensing in the form of thin film resistors (TFRs) given their high sensitivity to many vapor species, simple construction and capability for miniaturization. Furthermore, they are generally more stable than polymer-based gas sensors. However, unlike polymers, metal oxide gas sensors must typically be operated between 200-400°C to insure rapid kinetics. Another problem impacting TFR performance and reproducibility is related to poorly understood substrate-semiconductor film interactions. Space charges at this heterojunction are believed to influence chemisorption on the semiconductor-gas interface, but unfortunately, in an unpredictable manner. In this study, the feasibility of employing illumination and the thin film transistor (TFT) platform as a means of reducing operation temperature was investigated on ZnO based TFTs for gas sensors applications. Response to NO₂ is observed at significantly reduced temperature. Photoconductivity measurements, performed as a function of temperature on ZnO based TFRs, indicate that this results in a photon-induced desorption process. Also, transient changes in TFT channel conductance and transistor threshold voltage are obtained with application of gate bias, suggesting that TFTs offer additional control over chemisorption at the semiconductor-gas interface. / by Yoonsil Jin. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 25). / Striped gold nanoparticles are unique in several of their characteristics and applications. Recent experiments have determined a new medium with which contain the nanoparticles is that of a chemical gel. The nanoparticles for use in these studies do not require a polymer base in order to form a gel phase. However, a concrete analysis of the transition temperature between the gel and liquid phases had yet to be performed. The work performed in this experiment has determined a portion of the phase transition curve for different concentrations of ethanol and water as a solvent in this nanoparticle gel. The results of this project showed that, as expected, with an increased concentration of dissolved gold nanoparticles, the gel to liquid transition temperature increased. / by Thomas Gerard Schaefer. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013. / 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 (p. 211-238). / Carbon nanotubes have been actively investigated for integration in a wide variety of applications since their discovery over 20 years ago. Their myriad desirable material properties including exceptional mechanical strength, high thermal conductivities, large surface-to-volume ratios, and considerable electrical conductivities, which are attributable to a quantum mechanical ability to conduct electrons ballistically, have continued to motivate interest in this material system. While a variety of synthesis techniques exist, carbon nanotubes and nanofibers are most often conveniently synthesized using chemical vapor deposition (CVD), which involves their catalyzed growth from transition metal nanoparticles. Vertically-aligned nanotube and nanofiber carpets produced using CVD have been utilized in a variety of applications including those related to energy storage. Li-air (Li-O₂) batteries have received much interest recently because of their very high theoretical energy densities (3200 Wh/kgLi2O₂), which make them ideal candidates for energy storage devices for future fully-electric vehicles. During operation of a Li-air battery O₂ is reduced on the surface a porous air cathode, reacting with Li-ions to form lithium peroxide (Li₂O₂). Unlike the intercalation reactions of Li-ion batteries, discharge in a Li-air cell is analogous to an electrodeposition process involving the nucleation and growth of the depositing species on a foreign substrate. Carbon nanofiber electrodes were synthesized on porous substrates using a chemical vapor deposition process and then assembled into Li-O₂ cells. The large surface to volume ratio and low density of carbon nanofiber electrodes were found to yield a very high gravimetric energy density in Li-O₂ cells, approaching 75% of the theoretical energy density for Li₂O₂. Further, the carbon nanofiber electrodes were found to be excellent platforms for conducting ex situ electron microscopy investigations of the deposition Li₂O₂ phase, which was found to have unique disc and toroid morphologies. Subsequent studies were conducted using freestanding carpets of multi-walled CNT arrays, which were synthesized using a modified CVD process. The freestanding CNT arrays were used as a platform for studying the morphological evolution of Li₂O₂ discharge product as a function of rate and electrode capacity. SEM imaging investigations found that the Li₂O₂ particles underwent a shape evolution from discs to toroids as their size increased. TEM imaging and diffraction studies showed that the microscale Li₂O₂ particles are composed of stacks of thin Li₂O₂ crystallites and that splaying of the stacked crystallite array drives the observed disc to toroid transition. Modeling was performed to gain insights into the nucleation and growth processes involved during discharge in Li-O₂ cells. The modeling study suggests that poor electronic conductivity of the depositing phase limits the rate capability obtainable in Li-O₂ cells. Modeling can provide substantial insights into paths toward electrode optimization. Understanding the size and shape evolution of Li₂O₂ particles and engineering improved electrode architectures is critical to efficiently filling the electrode void volume during discharge thereby improving the volumetric energy density of Li-O₂ batteries. / by Robert Revell Mitchell III. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1991. / Vita. / Includes bibliographical references (leaves 256-261). / by Tuck Chow Choo (Roland). / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (p. 199-205). / In this work, we examine fundamental materials processes in the growth of indium gallium phosphide (InGaP) via metalorganic chemical vapor deposition (MOCVD). In particular, we realize improvements in the epitaxial integration of high-quality InGaP device materials on non-standard platforms, such as GeSi graded buffers orSi substrates, and InGaP or indium aluminum gallium phosphide (InAlGaP) graded buffers on GaP substrates. We apply these improvements to the design and implementation of strained-InGaP quantum-well light emitting diodes (LEDs) operating in the yellow-green region of the visible spectrum. The innovative use of these traditional materials is intended to provide a solution for bright green solid-state light emitters. Initial modes of InGaP lattice-matched epitaxy on GeSi were studied. Three- dimensional growth was observed over a wide range of deposition temperatures and V/III ratios. Pre-growth thermal cycling in a H2 plus PH3 ambient led to a large increase in surface roughness and the formation of surface mesas. Thermodynamic simulations suggest that these mesas may be P clusters or GeP solid complexes. They may also be surface oxides formed in conjunction with water vapor in the deposition chamber. Such surface degradation prior to the initiation of epitaxy is unfavorable for monolayer growth. The development and evolution of defect microstructures in relaxed, compositionally graded InGaP buffers deposited on GaP were examined. In particular, the properties of branch defects in InGaP graded buffers were examined for a large number of growth and annealing conditions. / (cont.) These studies confirm that branch defect formation is driven by surface, not bulk, processes. Branch defects in the bulk arise from surface features that are metastably "frozen" in place by subsequent deposition and propagate through the thickness of the sample. We conclude that branch defects comprise a local compositional fluctuation resulting from the clustering of In atoms. This identification is supported by the suppression of branch defect formation under conditions of reduced adatom mobility, including low growth temperature and high V/III ratio. In addition, we demonstrate that dislocations gliding in the [110] direction are -preferentially blocked by strain fields arising from nearly-[110]-oriented branch defects. This is further evidence for the link between branch defects and In clustering. A relaxed InAlGaP graded buffer platform was utilized in the design and fabrication of a novel strained-InGaP quantum-well epitaxial-transparent-substrate LED (ETS-LED). The best devices exhibited yellow-green emission with a primary wavelength of 590 nm and a secondary wavelength of 560 nm. These devices had [rho]TD = 7 x 106 cm2 and an In 0.32 Ga 0.68 P quantum well active region, and operated at 0.18 [mu]W per facet at 20 mA, corresponding to a luminous efficacy of approximately 0.01 1m/W. Transmission electron diffraction indicates that the observed spectral lineshape is the result of emission from ordered and disordered domains in the quantum well. Devices with [rho]TD = 5 x 107 cm-2 and an In 0.32 Ga 0.68P ... / by Lisa Megan McGill. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 101-104). / by Ching-Te Lin. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / 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 133-150). / Alloys based on face-centered cubic (FCC) elements such as nickel (Ni) are among the most resistant to fracture. However, when embrittled by impurities, they lose their toughness and crack along grain boundaries. Though long known, this phenomenon remains poorly understood. In this thesis, we use large-scale molecular dynamics (MD) simulations to study the effects of grain boundaries (GBs) on various aspects of fracture properties in Ni, including intergranular fracture mechanisms, fracture toughness as well as crack healing. By performing statistical analysis on crack tip processes for fracture along different GBs, we revealed three distinct crack propagation mechanisms. For fracture along coherent twin boundary with the crack front along the [112] direction, no bond breaking is observed and crack advance is solely attributed to the slip of atoms at its tip due to the emission of dislocations. The dislocation process leads to the blunting of the crack tip. For fracture along [Sigma]265(100) symmetrical tilt GB, we discovered a new crack propagation mechanism, decohesion restrained by emission of dislocations (DRED). In it, bursts of brittle fracture initiate emission of dislocations, which pre- vent cracks from propagating more than a few nanometers in a single burst. For fracture along coherent twin boundary with the crack front along the [110] direction, crack propagates by brittle decohesion, which initiates dislocation emission in a similar way as DRED. However, the dislocation process does not arrest the crack due to the local hardening mechanism, which constraints the motion of dislocations. Using the method developed to calculate the critical energy release rate Gc from atomistic simulations, we also compared the toughness of fractures by these three mechanisms. In the course of investigating intergranular fracture, we discovered a new mechanism for crack healing in a 2D model. This mechanism relies on the generation of disclination dipoles due to GB migration, which can interact with the crack, causing it to advance or heal. We also demonstrate the healing of nanocracks in realistic 3D microstructures. / by Guoqiang Xu. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1995. / Includes bibliographical references (p. 211-217). / by Orhun Kamil Muratoğlu. / Ph.D.