Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Includes bibliographical references (leaves 201-211). / Adhesion of many cell types to the extracellular matrix or to synthetic bioactive surfaces is mediated by transmembrane integrin receptors. Integrin clustering is believed to be closely associated with focal contact formation and signaling, as assessed by the behavior of cells on surfaces presenting relatively uniform ligand distributions. It has therefore been hypothesized that controlled clustering of 2, 3.....n integrins might be achieved by controlling the spatial distribution of adhesion ligands on biomaterial surfaces. Substrates were prepared on which cell-surface interactions are controlled by modifying non-adhesive poly(ethylene oxide) (PEO) hydrogels with the minimal cell-adhesion peptide sequence GRGDY (RGD). The peptide is tethered to the hydrogel surfaces via star PEO molecules, producing surfaces on which the ligands are presented to cells in "clusters", or domains of high concentration. The substrates are compared with others on which the RGD peptide is uniformly distributed. Control of the RGD cluster size was achieved by varying the relative concentrations of reactants in solution. The binding of RGD-modified stars to surfaces was found to be a non-linear function of its concentration in solution and degree of modification, and is reasonably explained by a Langmuir model of competitive adsorption. Quantitative techniques for visualizing the ligand distribution on the surface were developed, and indicated that surfaces to which ligands had been tethered via star molecules showed a significant deviation from normal, random distribution. Thus, control of the ligand spatial distribution was achieved. In addition, preliminary biological testing suggests that substrates on which adhesion ligands are presented to cells in a clustered format produces more physiological behaviour than those on which ligands are uniformly distributed at the same average ligand density. Thus, we have fabricated surfaces which, because of their resistance to non-specific cell interactions and the control of specific interactions at the molecular level, can serve as a model for artificial matrix development and can be used for fundamental in vitro studies. / by Gillian L. Brown. / Ph.D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 108-110). / The technical and commercial prospects of polymer solar cells were evaluated. Polymer solar cells are an attractive approach to fabricate and deploy roll-to-roll processed solar cells that are reasonably efficient (total PV system efficiency>10%), scalable and inexpensive to make and install (<100 $/m2). At a cost of less than 1$/Wp, PV systems will be able to generate electricity in most geographical locations at costs competitive to coal's electricity (at 5-6 cents/KWh) and will make electricity available to more people around the world (-20% of the world population is without electricity). In this chapter, we explore organic polymer solar cell technology. The first chapter discusses the potential impact of solar cells on electricity markets and the developing world and its promise as a sustainable scalable low carbon energy technology. The second chapter discusses some of the complexity in designing polymer solar cells from new materials and the physics involved in some detail. I also discuss the need to develop new solution processed transparent conductors, cost effective encapsulation and long life flexible substrates. The third chapter discusses polymer solar cells cost estimates and how innovative designs for new modules could reduce installation costs. In the final chapter I discussed the prospects for commercialization of polymer solar cells in several niche markets and in grid electricity markets; the commiseration prospects are dim especially with the uncertainty in the potential improvement in polymer solar cell stability. / by Burhan K. Saif Addin. / 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. 169-176). / Silicon is the most abundant semiconductor on earth and benefits from decades of technological development driven by the integrated circuit industry. Furthermore, silicon allows for facile n-type and p-type doping, has a naturally passivating surface oxide, and long minority carrier lifetimes. The major drawback of silicon is that it has an indirect band gap at 1.1 eV. It is therefore a poor light absorber and is naturally transparent to light in the infrared. Femtosecond (fs) laser irradiation offers multiple unique approaches to improving the optoelectronic properties of silicon, enabling both thin-film silicon photovoltaics and silicon-based IR photodetectors. In this thesis I study the structure-property-processing relationships related to the fs-laser irradiation of silicon in the context of both surface texturing and optical hyperdoping. Fs-laser surface texturing enables the use of thinner silicon wafers through efficient light trapping at the surface, but laser induced damage can degrade the performance of optoelectronic devices. The first part of this thesis investigates the relevant mechanisms of plastic deformation during surface texturing with fs-laser irradiation. Through a combination of Raman spectroscopy and TEM, I show that pressure-induced silicon polymorphs (amorphous silicon, Si-XII, Si-III) form beneath the surface during fs-laser irradiation. Combining characterization of the surface morphology using scanning electron microscopy, Raman investigations of the formation of Si- XII and Si-III, and TEM investigations of the spatial distribution of the amorphous silicon, we report that pressure-induced phase transformations are closely coupled to micron-scale surface texturing. Next, I identify the pressure generation mechanisms responsible for the pressure-induced phase transformations through a systematic investigation into the relationship between irradiation conditions and silicon polymorphs formation. Beginning with the observation that rastering the Gaussian laser beam drastically increases the amount of Si-XII formed, I use Raman spectroscopy to investigate silicon polymorph formation and residual lattice strains following irradiation at constant fluence and irradiation under modulated fluence. A strong increase in Si- XII formation is reported in laser spots that received a combination of high-fluence and lowfluence irradiation, as is generated by rastering the Gaussian laser beam across the surface. TEM investigations confirm that low-fluence irradiation increases the melt depth and that the spatial distribution of silicon polymorphs is correlated with melting and resolidification on roughened surfaces. Based on these investigations, it is concluded that resolidification-induced stresses are responsible for the observed pressure-induced plastic deformation. Optical hyperdoping, the subject of the second half of this thesis, refers to the use of pulsed laser irradiation to drive supersaturated concentrations of dopants into a semiconductor. Fs-laser hyperdoping of silicon with chalcogens has been shown to extend the responsivity of silicon photodiodes into the near-infrared and increase absorption in the visible and infrared. First, hyperdoping from a thin-film dopant precursor is investigated through the comparative structural (SEM and TEM), electronic (p-n diode formation), and optical (UV-VIS-NIR spectrophotometry) characterization of silicon irradiated with fs-laser pulses following the deposition of a selenium thin film on the surface, silicon irradiated in the presence of a gaseous dopant precursor, and silicon irradiated without dopant present. The use of a thin-film dopant precursor is found to have significant consequences on the resulting microstructure and dopant distribution compared to fs-laser doping from a gaseous precursor; producing large, discontinuous volumes of polycrystalline hyperdoped material. The observed microstructure and dopant distribution can account for the increased sub-band gap absorptance and poor diode rectification exhibited by thin-film hyperdoped surfaces. Next, advanced structural investigations into the selenium distribution with annealing show significant selenium segregation and precipitation. With this information, previous investigations into the optical deactivation of selenium with annealing are revisited and shown to be consistent with a kinetic model for optical deactivation by precipitation. To improve the dopant distribution achieved by thin-film fs-laser doping, the dopant incorporation process is elucidated by monitoring the surface structure (SEM) and dopant distribution (TEM) with variedMJS laser fluence and number of laser pulses. From very early stages of irradiation, the crystallization of hyperdoped material is found to be closely coupled to the surface structuring process, likely due to the effects that surface roughness has on local energy deposition and heat dissipation. The large, polycrystalline peaks are shown to form through a novel regime of crystallization-driven growth, which transitions into ablationdominated surface structuring after many laser pulses. Finally, the suppression of localized recrystallization is achieved by irradiation with many pulses (100) at very low fluences (1.2-1.4 kJ/m2), resulting in a thin, continuous layer of hyperdoped material. The investigations presented in this thesis present progress towards controllable and optimized implementation of fs-laser irradiation as a platform for improving the optoelectronic properties of silicon through both surface texturing and optical hyperdoping. / by Matthew John Smith. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references (leaves 128-138). / by Chenting Lin. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (leaves 49-50). / Electrohydraulic forming was used to biaxially stretch commercial Aluminum 5052 sheet metal workpieces at a high strain rate. Annealed and unannealed workpieces were formed. Specimens were taken from unformed metal and from the formed workpieces. Microstructures were examined with optical microscopy and pole figures were generated from X-ray diffraction data. Microstructures and crystalline textures were compared between formed and unformed and annealed and unannealed metal specimens, and strains were measured from the formed workpieces. / by Isaac Benjamin Feitler. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references. / Lightweight nano-network structures in polymers have been fabricated and investigated for their mechanical properties. Fabrication techniques via holographic interference lithography and phase mask lithography were implemented for periodic and quasiperiodic bicontinuous polymer-air structures on the submicrometer length scale. For 3D quasiperiodically nanostructured materials, quasicrystalline phase mask lithography utilizing 2D quasiperiodic phase mask was successfully employed. 2D hexagonal arrays of air cylinders in SU8 polymer films and 3D four-beam connected (3- R3m ) and octagonal quasicrystalline SU8 films were fabricated and analyzed in this thesis. For investigating the mechanical properties of various nano-network structures, three different methods of mechanical characterization were applied. Atomic force microscopy with its nanometer scale resolution was adopted to conduct force measurements to probe local elastic properties of the sample. Templated by the light intensity distribution from three-beam interference, the spatial distribution of elastic modulus was observed in the pattern of 2D hexagonal air-cylinder and a uniform SU8 polymer film by AFM nanoindentation. A second method for mechanical characterization, the microtensile tester enabled us to evaluate a symmetry effect on the elastic and plastic properties of the polymer fibers and thin films. Large plastic deformation of 200nm-diameter struts comprising the 3D periodic and quasiperiodic microframes of the normal brittle bulk polymer was discovered and is an example of length-scale dependent mechanical behavior. Crack propagation and energy absorption were guided along the symmetry directions in the periodic structures. However, there was found no preferred direction of crack propagation in quasicrystalline nanostructures due to the absence of translational symmetry. / (cont.) The third method, Brillouin light scattering (BLS) allowed estimation of the phonon properties in the structured films and the associated mechanical properties. The BLS measurements also confirmed the isotropy of modulus with the symmetry of the structures. The length scale dependence, the effect of structural symmetry and the processing dependence of the mechanical behavior of the various nanostructures in SU8 polymer films were observed. The hundred-nanometer length scale of 3D nanostructures induces plastic deformation of struts under an applied force, which makes the film tougher and energy absorbing. The symmetry of the structured films determines the preferred direction of crack propagation and following fracture behavior. Octagonal-patterned (8mm) quasicrystalline films via quasicrystalline phase mask lithography (QCPML) exhibit higher specific toughness and fracture strength with the unit mass than uniform solid films. / by Taeyi Choi. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (leaves 35-37). / An investigation was carried out of the electronic characteristics of drop-cast films comprised of phenylated ligand-capped gold nanoparticles. In homoligand-type films, the dominant mechanism of charge transfer was expected to involve orbital overlap and end group-effected wave function displacement, whereas heteroligand-type films were expected to conduct through less efficient hopping mechanisms. Films utilizing the former mechanism are expected to have great applicability within microelectronics and rapid-prototyping technologies due to the small scale (2-6nm) of functionalized nanoparticles and the structural flexibility of interdigitation as a form of inter-particle bonding. The comparative conductances of the cast films reveal a strong correlation with the ligand Hammaker constant (effectively a measure of the work function of the conjugated bond with the gold core of the nanoparticle and the charge displacement effected by the electronegativity or polarity of the ligand end group). The conductance was also greatly affected by the size of ligand end groups - a rough measure of the close-packing ability of a given ligand both within the ligand shell and amongst the shells of adjacent nanoparticles. The following experiments illustrate these correlations, as well as the effects of ligand spacing and shell composition on the dominant charge transfer mechanism. / by Thomas C. Schilling. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 40-42). / Multifunctionality can be imparted to protein-based fibers and coatings via either synthetic or biological approaches. Here, we demonstrate potent antimicrobial functionality of genetically engineered, phage-based fibers and fiber coatings, processed at room temperature. Facile genetic engineering of the M13 virus (bacteriophage) genome leverages the well-known antibacterial properties of silver ions to kill bacteria. Predominant expression of negatively-charged glutamic acid (E3) peptides on the pVIII major coat proteins of M13 bacteriophage (or phage) enables solution-based, electrostatic binding of silver ions and subsequent reduction to metallic silver along the phage length. Antibacterial fibers of micrometer-scale diameters are constructed from such E3-modified phage, via wet-spinning and glutaraldehyde-crosslinking of the E3-modified phage. Silverization of the free-standing fibers is confirmed via energy-dispersive spectroscopy (EDS) and inductively-coupled plasma atomic emission spectroscopy (ICP-AES), showing -0.61,pg/cm of silver on E3-Ag fibers. This degree of silverization is threefold greater than that attainable for the unmodified M13-Ag fibers. Conferred bactericidal functionality is determined via live-dead staining and a modified disk-diffusion (Kirby-Bauer) measure of zone of inhibition (Zol) against Staphylococcus epidermidis and Escherichia coli bacterial strains. Live-dead staining and Zol distance measurements indicate increased bactericidal activity in the genetically engineered virus fibers attached to silver. / (cont.) Coating of Kevlar fibers with E3 viruses exhibits antibacterial effects, as well, with relatively smaller ZoIs attributable to the lower degree of silver loading attainable in these coatings. Such antimicrobial functionality is amenable to rapid incorporation within fiber-based textiles to reduce risks of infection, biofilm formation, or odor-based detection, with the potential to exploit the additional electronic and thermal conductivity of fully silverized fibers and coatings. / by Joan Y. Mao. / S.M.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999. / Vita. / Includes bibliographical references (leaves 90-93). / The Electron Beam Solid Freeform Fabrication process involves the use of an electron beam to make near-net-shape metal parts without the need for tooling. Material in wire form is fed into a melt pool maintained on the surface of the part by the electron beam and a positioning system causes the deposition to occur in a line-by-line, layer-by-layer fashion. Solidification occurs at a high rate, forming a fine dendritic microstructure and fine dispersion of primary carbides. This structure is believed to be optimal for the manufacture and safe use of certain nickel-base superalloy parts, notably turbine disks. The growth of carbide particles from the liquid during EBSFF processing of Alloy 718 has been modeled assuming diffusion control and isolated spherical carbides. The driving force for growth is assumed to increase in a linear manner throughout the temperature range of carbide precipitation. The model predicts the maximum carbide size as a function of EBSFF operating parameters and the alloy niobium and carbon levels. For the material and conditions used experimentally in this work, the model predicts a maximum diameter of approximately I .0 [mu]m. The maximum carbide size will become an important determining factor for turbine disk performance when oxide and nitride inclusions have been eliminated through improved melt practices. To illustrate this, the low-cycle fatigue life as a function of carbide size for a standard specimen geometry was calculated. Extraction replica transmission electron microscopy of EBSFF samples identified carbides in the 300-600 nm range, consistent with a population having the predicted maximum size. Another dispersion of carbides larger than 3 [mu]m was also observed in the EBSFF samples. These are believed to be original carbides that survived the EBSFF thermal cycle without completely dissolving. More thorough dissolution can probably be obtained with EBSFF process modifications. Control material from a conventional vacuum arc remelted ingot with similar composition was also examined and plate-like carbides up to 40 [mu]m in length were noted. This is an indication of the enormous potential of the EBSFF process to refine the carbide morphology and size distribution without the need for a reduction in carbon content. / by John Edward Matz. / Sc.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 33-34). / One of the key limitations in the wide-scale adoption of mature renewable energy technologies is the lack of grid-level energy storage solutions. One important figure of merit in these battery systems is a high rate capability to match fluctuating demands for electricity. Molten salt batteries are an attractive option for stationary storage due to fast kinetics and good cycling capability, but high temperatures (>300 °C) limit available materials. In this thesis, the molten NaCl-AlCl3-SnCl2 electrolyte and liquid Sn electrode couple at 250 °C is investigated as part of the potential cell Na I NaCl-AlCl 3-SnCl2 I Sn for a lower temperature molten salt battery. An electrochemical study of the kinetics in the molten salt electrolyte and at the liquid Sn electrode-electrolyte interface is conducted using cyclic voltammetry and the galvanostatic pulse method. The liquid metal electrode is found to have suitably fast kinetics with an exchange current density of 92 mA/cm2. Parameters for a new Na+ conducting membrane are proposed, requiring an ionic conductivity of 0.056 S/cm, which would allow for a hypothetical Na I NaCl-AlC 3-SnCl2 I Sn battery to operate with an energy efficiency of 70%. / by Raku Watari. / S.B.