Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1983. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by Tseng-Hua Tsiang. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references. / Monolayer Protected Metal Nanoparticles have recently found widespread use in and are the focus of intensive study in many areas of scientific research ranging from biology to physics to medicine. Consisting of a nanoscale, crystalline, metallic core surrounded by a self-assembled monolayer of ligands (a 3-D SAM or ligand shell), their appeal and utility stem from their numerous unique properties-many of which arise and are modulated by the intimate spatial and electronic contact between core and shell. The ligand shell controls the particle's interactions with its environment (e.g. sensing, assembly, and electron transfer ability). Furthermore, the ability to manipulate and assemble such nanomaterials through the ligand shell is paramount to their incorporation into and the development of new nanoparticle based materials and devices. However, little is known of the exact composition and packing arrangements of molecules within the ligand shell, and even less so on how to control the resulting nanostructuring. In this thesis we present a Scanning Tunneling Microscopy investigation of the ligand shells of homo- and mixed-ligand metal nanoparticles. / (cont.) We develop an understanding and model of the ligand arrangements around the nanoparticle core, showing that the multifaceted, high curvature, and topologically spherical nature of the core results in a 3-D SAM that has many differences from its 2-D SAM counterparts. We show that the core curvature (and correspondingly, the changing facet to edge ratio on the core surface) of the particles is the dominant driver for the packing and behavior of the ligand shell. Most interestingly, we find that when certain two-component, mixed SAMs are assembled around the particle core, the ligands phase-separate into ordered, ribbon-like domains, only a few molecules in width-a behavior never before seen on flat surfaces. We show that both the domain morphology and width can be controlled through the ligand shell composition and the particle core size, and that the observed phase-separation is a general phenomenon across nanoparticle compositions. We present these studies as a first step towards developing a complete model of and control over the ligand shell structure of nanoparticles. / by Alicia M. Jackson. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 27-28). / Controlling cell behavior has been a primary goal for scientists, and physical interactions, specifically cell-surface interactions, have the potential to be a robust system for cell control. Much research has been conducted on the effect of substrata stiffness on cell behavior, but there has been no systematic study of the effect of varying substrata thickness, and its correlation to a substratum's effective stiffness that a cell feels. Furthermore, there have been differing views on what the critical thickness of a substrate is, above which there will be no difference in cell behavior. An experimental study was carried out to determine the effects of substratum thickness on the behavior of cells adhering to polyacrylamide thin film gels functionalized with gelatin. Relatively compliant thin film gels, with an elastic modulus E ~ 5 kPa, were varied in thickness on stiff glass supports from ~75 nm to 60 microns. 3T3 fibroblast cells were seeded onto the gels to observe differences in behavior. Observed cell behaviors were the projected area of the cells on the surface due to adhesive spreading and the rate of reduction of Alamar Blue dye, which correlates to the proliferation, or growth rate, of the cells. It was found that the cell area had a fairly welldefined power-law dependence on substrate thickness, while the gel thickness did not have a detectable effect on the rate of proliferation of the cells. Additionally, a theoretical model for thin film deflection was fit to the cell area data, and it described the areathickness relationship well. / (cont.) By using the theoretical model, a critical thickness of 2.3 tm was identified over which average cell area would not change significantly. This critical thickness was found to be on the order of the reported length scale of focal adhesions in the cells, not the lateral dimensions of the cell. These results are useful in establishing a practical lower limit of'film thickness for normal cell behavior. Additionally, this relationship could be exploited as a way to control stem cell differentiation, cell size, cell motility, cell ligand density, and other cell behaviors. / by Christopher M. Bruce. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997. / Includes bibliographical references (leaves 210-218). / by Bizhong Zhu. / Ph.D.

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 121-127). / In 1947, the first transistor was made of germanium, but soon silicon became the core material of computer chips because of its processability. However, as the typical dimensions of transistors are getting closer to the atomic size, the traditional approach of scaling down transistors to improve performance is reaching its limits, and other elements need to be used in conjunction with silicon. Germanium is one of the key materials to empower silicon based devices because it possesses electronic and optoelectronic properties complementary to those of silicon, among them higher carrier mobilities and a direct band gap (G-valley) at 1.55 [mu]m (the telecom C-band, therefore adding new capabilities to silicon integrated microphotonics). Furthermore, good quality Ge layers can be grown epitaxially on a Si substrate, allowing a monolithic integration of devices. However, compared to silicon, little is known about the point defects in germanium. The goal of the present doctoral work is to remedy this gap. To this end, we have used radiation (gamma rays, alpha particles, and neutrons) to controllably introduce point defects in crystalline germanium, which were then characterized by Deep-Level Transient Spectroscopy (DLTS), a technique that allows the determination of the activation energy, capture cross-section, and concentration of the said defects. By studying their electronic properties, annealing kinetics, and introduction rates, we were able to separate vacancy-containing from interstitial-containing defects and gain insight on their physical nature and formation process. We especially identified a di-interstitial defect and a tri-interstitial defect. In addition, we proved that in the case of alpha particles and neutron irradiation, the fact that defects are generated in a collision cascade influences their carrier capture rates and annealing behaviors. We have also characterized the impact of radiation on commercial germanium-on-silicon photodetectors, and showed that point defects associate with dislocations in epitaxial Ge-on-Si layers. Finally, we have investigated the passivation of midgap states by implanting germanium with fluorine, and showed how the interaction between the halogen element, the amorphous/crystalline interface during the solid phase epitaxy, and the implantation damage is key in obtaining a high performance material / by Corentin Monmeyran. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Thermoelectric (TE) materials, which can convert unused waste heat into useful electricity or vice versa, could play an important role in solving the current global energy challenge of providing sustainable and clean energy. Nevertheless, thermoelectrics have long been too inefficient to be utilized due to the relatively low energy conversion efficiency of present thermoelectrics. One way to obtain improved efficiency is to optimize the so-called TE figure of merit, ZT = S2[sigma]/[kappa], which is determined by the transport properties of the active layer material. To this end, higher-efficiency thermoelectrics will be enabled by a deep understanding of the key TE properties, such as thermal and charge transport and the impact of structural and chemical changes on these properties, in turn providing new design strategies for improved performance. To discover new classes of thermoelectric materials, computational materials design is applied to the field of thermoelectrics. This thesis presents a theoretical investigation of the influence of chemical modifications on thermal and charge transport in carbon-based materials (e.g., graphene and crystalline C60 ), with the goal of providing insight into design rules for efficient carbon-based thermoelectric materials. We carried out a detailed atomistic study of thermal and charge transport in carbon-based materials using several theoretical and computational approaches - equilibrium molecular dynamics (EMD), lattice dynamics (LD), density functional theory (DFT), and the semi-classical Boltzmann theory. We first investigated thermal transport in graphene with atomic-scale classical simulations, which has been shown that the use of two-dimensional (2D) periodic patterns on graphene substantially reduces the room-temperature thermal conductivity compared to that of the pristine monolayer. This reduction is shown to be due to a combination of boundary effects induced from the sharp interface between sp 2 and sp 3 carbon as well as clamping effects induced from the additional mass and steric packing of the functional groups. Using lattice dynamics calculations, we elucidate the correlation between this large reduction in thermal conductivity and the dynamical properties of the main heat carrying phonon modes. We have also explored an understanding of the impact of chemical functionalization on charge transport in graphene. Using quantum mechanical calculations, we predict that suitable chemical functionalization of graphene can enhance the room-temperature power factor of a factor of two compared to pristine graphene. Based on the understanding on both transport studies we have gained here, we propose the possibility of highly efficient graphene-based thermoelectric materials, reaching a maximum ZT ~ 3 at room temperature. We showed here that it is possible to independently control charge transport and thermal transport of graphene, achieving reduced thermal conductivity and enhanced power factor simultaneously. In addition, we discuss here the broader potential and understanding of the key thermoelectric properties in 2D materials, which could provide new design strategies for high efficient TE materials. Transport properties of crystalline C60 are investigated, and the results demonstrate that these properties can be broadly modified with metal atom intercalation in crystalline C60. In contrast to the case of graphene, where chemical modifications induce structural changes in graphene lattice (from sp 2 C to sp3 C), intercalating metal atoms only modify van der Waals interactions between C60 molecules, but still having a huge impact on both thermal and charge transport. Taken both transport studies together, we suggest that the metal atom intercalation in crystalline C60 could be a highly appealing approach to improve both transports in solid C60, and with appropriate optimization of TE figure of merit, ZT value as large as 1 at room-temperature can be achieved. This dissertation consists of five chapters. Chapter 1 contains a brief review of thermoelectric materials. Chapter 2 introduces the theoretical approaches for computing both thermal (with molecular dynamics and lattice dynamics) and charge transport (with density functional theory and semi-classical Boltzmann approach) in materials. In Chapter 3, our study of thermal transport in functionalized graphene is presented. Chapter 4 describes our results on charge carrier transport in functionalized graphene. Combining these two works, we predict the full ZT values of functionalized graphene. Chapter 5 describes how to optimize ZT value in metal atom intercalated crystalline C60 / by Jeong Yun Kim. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 111-129). / Recent trends in renewable energy made silicon based photovoltaics the undisputed leader. Therefore, technologies that enhance, instead of compete with, silicon based solar cells are desirable. One such technology is the use of organic semiconductors and noncrystalline semiconductors for photon up- and down-conversion. However, the understanding of energy transfer in these hybrid systems required to effectively engineer devices is missing. In this thesis, I explore and explain the mechanism of energy transfer between noncrystalline semiconductors and organic semiconductors. Using a combination of density functional calculations, molecular dynamics, and kinetic theory, I have explored the geometry, morphology, electronic structure, and coarse grained kinetics of these system. The result is improved understanding of the transfer mechanism, rate, and the device structure needed for efficient devices. I have also looked at machine learning inspired algorithm for acceleration of density functional theory methods. By training machine learning models on DFT data, a much improved initial guess can be made, greatly accelerating DFT optimizations. Generating and examining this data set also revealed a remarkable degree of structure, that perhaps can be further exploited in the future. / by Nadav Geva. / Ph. D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (p. 104-105). / Lithium Ion (Li-ion) battery technology has the potential to compete with the more matured Nickel Metal Hydride (NiMH) battery technology in the Hybrid Electric Vehicle (HEV) energy storage market as it has higher specific energy and energy. However, in order to improve Li-ion battery technology to fulfill the' HEV energy storage requirements, a very high specific power characteristic is needed to boost its commercial attractiveness. The high specific power characteristic will in turn lead to better a vehicle performances, reduced fuel consumption and emissions. In this thesis, we quantify the fuel savings benefits from HEV, and the marginal value of each W/kg improvement in this battery technology. From the analysis, we conclude that the marginal value of regenerative braking, acceleration, social cost and fuel economy are $13.83, $22.64, $0.9959 and 0.0987 MPG per W/kg per each HEV lifespan respectively. Besides, a variety of start-up companies in various stages of commercialization of these technologies as well as the related intellectual property strategies are also discussed. Finally, suggestion of potential business strategies for licensing and commercializing Li-ion battery technology with respect to HEV energy storage market is presented. / by Seh Kiat Teo. / M.Eng.

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 (p. 173-184). / Chemomechanical coupling in single eukaryotic animal cells is investigated in the con- text of the attached (substratum-adhered) and the suspended (free-floating) states. These dichotomous configurations determine behavioral differences and commonalities relevant to therapeutic reimplantation of stem cells and to our general under- standing of the cell as an animate material. Analytical, simulation, and experimental techniques are applied to key questions including: (1) How deep can mechanosensitive attached cells "feel" into the adjacent environment? (2) In what manner do suspended cells deform, absent the prominent actomyosin stress fibers that arise upon attachment to a rigid substratum? (3) What explains the remarkable mechanical heterogeney among single cells within a population? (4) Can we leverage putative mechanical markers of useful stem cells to sort them before reimplantation in tissue generation therapies? Attached cells are found to barely detect an underlying rigid base more than 10 micrometers below the surface of a compliant coating. This conclusion, based on ex- tensions to the Boussinesq problem of elasticity theory, is validated by observations of cell morphology on compliant polyacrylamide coatings in a range of thicknesses. Analytical equations are developed for estimating the effective stiffness sensed by a cell atop a compliant layer. We also identify and consider conceptualizations of a "critical thickness," representing the minimum suitable thickness for a specific application. This parameter depends on the cell behavior of interest; the particular case of stem cell culture for paracrine extraction is presented as a case study. Suspended cells are found to exhibit no single characteristic time scale during de- formation; rather, they behave as power-law (or "soft glassy") materials. Here, optical stretching is used as a non-contact technique to show that stress fibers and probe-cell contact are not critical in enabling power-law rheological behavior of cells. Further- more, suspended cell fluidity, as characterized by both the hysteresivity of complex modulus and the power-law exponent of creep compliance, is found to be unaffected by adenosine triphosphate (ATP) depletion, showing that ATP hydrolysis is not the origin of fluidity in cells during deformation. However, ATP depletion does reduce the natural variation in hysteresivity values among cells. This finding, and the finding that changes in the power-law exponent and stiffness of single cells are correlated upon repeated loading, motivates study of how and why these parameters are coupled. To further explore this coupling, chemomechanical cues are applied to cell populations to elucidate the origin of the wide, right-skewed distribution of stiffness values that is consistently observed. The distribution and width are found to be not detectably dependent on cell-probe contact, cell lineage, cell cycle, mechanical perturbation, or fixation by chemical crosslinking. However, ATP depletion again reduces heterogeneity, now in the case of cell stiffness values. It is further found analytically that a postulated Gaussian distribution of power-law exponent values leads naturally to the log-normal distribution of cell stiffness values that is widely observed. Based on these connections, a framework is presented to improve our understanding of the appearance of mechanical heterogeneity in successively more complex assemblies of cell components. Two case studies are described to explore the implications of unavoidable intrinsic variation of cell stiffness in diagnostic and therapeutic applications. Finally, all the single-cell mechanical parameters studied so far (stiffness during creep and recovery, stiffness heterogeneity among cells, and power-law exponents in creep and recovery) are characterized in mesenchymal stem cells during twenty population doublings with the aim of developing a high-throughput sorting tool. How- ever, mechanical and structural changes that are observed in the attached state during this culture time are not observed after cell detachment from the substratum. The absence in the suspended state of these alterations indicates that they manifest themselves through stress fiber arrangement rather than cortical network arrangement. While optical stretching under the present approach does not detect mechanical markers of extended passaging that are correlated with decreased differentiation propensity, the technique is nevertheless found capable of investigating another structural transition: mechanical stiffening over tens of minutes after adherent cells are suspended. This previously unquantified transition is correlated with membrane resorption and reattachment to the cortex as the cell "remodels" after substratum detachment. Together, these quantitative studies and models of attached and suspended cells de- fine the extremes of the extracellular environment while probing mechanisms that con- tribute to cellular chemomechanical response. An integration of the results described above shows that no one existing model can describe cell chemomechanics. However, the cell can be usefully described as a material -- one in which animate mechanisms such as active contraction will generally, but not invariably, need to be considered as augmenting existing viscoelastic theories of inanimate matter. / by John Mapes Maloney. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (p. 135-143). / Controlling the diameter and chirality of carbon nanotubes to fine tune their electronic band gap will no longer be enough to satisfy the growing list of characteristics that future carbon nanotube applications are starting to require. Controlling their band gap, wall reactivity and mechanical properties is imperative to make them functional. The solution to these challenges is likely to lie in smart defect engineering. Defects of every kind can induce significant changes on the intrinsic properties of carbon nanotubes. In this context, this thesis analyzes the effects of doping single and double walled carbon nanotubes with nitrogen and boron. We describe the synthesis of N-doped single-walled carbon nanotubes (N-SWNTs), that agglomerate in bundles and form long strands (<10cm), via the thermal decomposition of ferrocene/ethanol/benzylamine (FEB) solutions in an Ar atmosphere at 950°C. Using Raman spectroscopy, we noted that as the N content is increased in the starting FEB solution, the growth of large diameter tubes is inhibited. We observed that the relative electrical conductivity of the strands increases with increasing nitrogen concentration. Thermogravimetric analysis (TGA) showed novel features for highly doped tubes, that are related to chemical reactions on N sites. / (cont.) We also carried out resonance Raman studies of the coalescence process of double walled carbon nanotubes in conjunction with high resolution transmission electron microscope (HRTEM) experiments on the same samples, heat treated to a variety of temperatures and either undoped or Boron doped. As the heat treatment temperatures are increased (to 1300°C) a Raman mode related to carbon-carbon chains (w = 1855cm-1) is observed before DWNT coalescence occurs. These chains are expected to be 3-5 atoms long and they are established covalently between adjacent DWNTs. The sp carbon chains trigger nanotube coalescence via a zipper mechanism and the chains disappear once the tubes merge. Other features of the Raman spectra were analyzed as a function of heat treatment with special emphasis on the metallic or semiconducting nature of the layers constituting the DWNTs. DWNTs whose outer wall is metallic tend to interact more with the dopant and their outer tubes are the predominant contributors to the line shape of the G and G' bands. / (cont.) The metallic or semiconducting nature of the layers of the DWNTs does not affect their coalescence temperature. All the experiments and analysis presented in this thesis are the result of a collaborative effort between Professor Dresselhaus' group at MIT and its international collaborators, including Professor Endo's group at Shinshu University, Nagano, Japan, Professors Pimenta's and Jorio's group at the Federal University of Minas Gerais, Belo Horizonte, Brazil, and Professors M. and H. Terrones' group at IPICYT, San Luis Potosi, Mexico. / by Federico Villalpando Paéz. / S.M.