Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 41-46). / Carbon related systems have attracted a large amount of attention of the science and technology community during the last few decades. In particular, graphene and carbon nanotubes have remarkable properties that have inspired applications in several fields of science and engineering. Despite these properties, creating structurally perfect samples is a difficult objective to achieve. Defects are usually seen as imperfections that degrade the properties of materials. However, defects can also be exploited to create novel materials and devices. The main topic of this thesis is studying the effect of isotope doping on the phonon properties of graphene. The advantage of the isotope enrichment technique is that only phonon frequencies or thermal properties can be modified without changing the electrical or chemical properties. We calculated the values of the phonon lifetimes due to isotope impurity scattering for all values of isotopic fractions, isotopic masses and for all wave-vectors using second order perturbation theory. We found that for natural concentrations of 13C, the contribution of isotopic scattering of optical modes is negligible when compared to the contribution from the electron-phonon interaction. Nevertheless, for atomic concentrations of 13C as high as [rho] = 0.5 both the isotopic and electron-phonon contributions become comparable. Our results are compared with recent experimental results and we find good agreement both in the 13C atomic density dependence of the lifetime as well as in the calculated spectral width of the G-band. Due to phonon scattering by 13C isotopes, some graphene phonon wave-functions become localized in real space. Numerical calculations show that phonon localized states exist in the high-energy optical phonon modes and in regions of flat phonon dispersion. In particular, for the case of in-plane optical phonon modes, a typical localization length is on the order of 3 nm for 13C atomic concentrations of [rho] ~~ 0.5. Optical excitation of phonon modes may provide a way to experimentally observe localization effects for phonons in graphene. / by Joaquin F. Rodriguez-Nieva. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 197-205). / by Patrick David Tepesch. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (p. 84-87). / by John S. Guzek. / M.S.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 61-62). / Chemosensors hold many vital applications in today's world, particularly as detectors for explosives. There is still vast room for improvement as other technologies-particularly those of explosives-continue to evolve and expand. Herein, we develop a novel device structure with the potential for much higher sensitivity. The lateral bilayer photoconductor is comprised of an exciton generation layer (EGL) and a charge transport layer (CTL). This separates the functionality of chemical sensing from the charge transport, allowing each film to be independently optimized. As a further improvement on this structure, we introduce a spacer layer to separate charge carriers in the EGL and the CTL, reducing bimolecular recombination at the interface. As a proof of concept, we fabricate and characterize lateral multilayer photoconductors composed of small molecule organic films. It is experimentally demonstrated that the utilization of a spacer layer can produce an order of magnitude enhancement in quantum efficiency over the of a spacer layer can produce an order of magnitude enhancement in quantum efficiency over the The work reported here provides encouraging results in the fields of chemosensors and organic optoelectronics. / by Jill A. Rowehl. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 214-217). / The manipulation and control of phonons is extremely important from both a fundamental scientific and applied technological standpoint, providing applications ranging from sound insulation to heat management. Phononic crystals and metamaterials are artificially structured materials (at certain length scales) that provide promise in controlling the propagation of phonons in solids. However, the vector nature of the phonon makes the development of a governing framework with which to guide the design of these phononic metamaterials complicated and no coherent framework exists for the design of phononic structures. In this thesis, we utilize a combination of global symmetry principles, adopted from group theory and the theory of representations, together with conservation principles and broken symmetry concepts to formulate our generalized design framework. This framework allows us to exactly treat the vector nature of phonons and control their propagation, unifying the design of phononic crystals, metamaterials, waveguides and numerous other structures, both infinite and finite. In particular, utilizing only this general framework which we develop, we are able to explain the choice of a particular physical topography for a desired phononic propagation behavior in a coherent fashion. In addition, we show how we may explicitly control the dispersion relations of a phononic metamaterial in order to obtain a desired final band structure. Some of our demonstrations include a new polychromatic phononic metamaterial which possesses multiple complete in-plane spectral gaps totaling over 100% in normalized gap size to a phononic metamaterial which exhibits a single complete in-plane spectral gap of 102% and a complete spectral gap of 88%, both significant advancements over the state of the art. This thesis also removes the artificial distinction between the phononic crystal and metamaterial classifications by unifying their behavior within the same generalized framework. As a result, we show that only a few governing principles are required to design the complex band dispersion relations of phononic metamaterials. The generality of our framework allows extension to other vector and scalar waves, such as photonic, plasmonic and magnonic structures and provides a promising route forward to the development of integrated structured material platforms that allow for the rational manipulation and interactions of phonons with other waves, such as phonons and spin waves. / by Cheong Yang (Henry) Koh. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998. / Includes bibliographical references (p. 77-79). / The interlayer dielectric plays an important role in multilevel integration. Material choice, processing, and contamination greatly impact the performance of the layer. In this study, particle generation, deposition, and adhesion mechanisms are reviewed. In particular, four important sources of interlayer dielectric particle contamination were investigated: the cleanroom environment, improper wafer handling, the backside of the wafer, and microarcing during process. / by Elaine D. Haberer. / S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 55-61). / Semiconductor nanostructures exhibit distinct properties by virtue of nano-scale dimensionality, resulting in recent interest in semiconducting nanowires for electronic, photonic, and energy applications. Along with nanowires, quantum dots are solution-processable nanocrystals with tunable band gap energies as a function of their size. Based on all of these promising properties that nanostructures exhibit, nanowires and quantum dots are excellent candidates for next-generation optoelectronic devices, including solar cells and light-emitting diodes. However, the realization of nanostructured materials for solar cell device applications is limited by the fundamental trade-off between light absorption and photocarrier collection. Vertically aligned ZnO nanowire arrays can decouple absorption and collection by acting as highly-conductive channels for extracting photogenerated electrons from deep within the film. This thesis illustrates a scheme for the development of ordered bulk heterojunction photovoltaic devices incorporating solution-based n-type doped ZnO nanowires and PbS quantum dots. In order to improve the electrical properties of ZnO nanowires, Al doping of hydrothermally synthesized ZnO nanowires is studied along with the optimization of doping concentration. The morphology of ZnO nanowire arrays is also studied as a function of the doping concentration in the growth solution. Finally, photovoltaic devices are fabricated and the effect of Al-doping of ZnO nanowires is investigated by device characterization techniques. / by Hyoungwon Park. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003. / Includes bibliographical references (p. 245-255). / A study was made that is logically divided into two parts, both involving radiation damage effects. The first is a study of the effects of neutron and gamma radiation on the dimensions of two borosilicate glasses, Pyrex® and Hova SD-2®. These two glasses are commonly used as substrates for silicon microelectromechanical (MEMS) devices, and radiation-induced compaction in a substra.te can have deleterious effects on device performance. Results are presented for density changes induced in both glasses by neutron irradiation. Pyrex was shown to compact at a rate of (in [delta]p[rho]/p[rho] per n/cm2̂) 8.14 x 10-̂20 (thermal) and 1.79 x 10-̂20 (fast). The corresponding results for Hoya SD-2 were 2.21 x 10-̂21 and 1.71 x 10-̂21, respectively. On a displacement per atom (dpa) basis, the compaction of the Pyrex was an order of magnitude greater than that of the Hoya SD-2. Our results are the first reported measurement of irridiation-induced densification in Hoya SD-2. The compaction of Pyrex agreed with a previous study. Our results for gamma irradiations were unexpected. Silicon MEMS strain gauges mounted on glass wafers were gamma-irradiated to hundreds of Mrad. Based on expectations from the literature, the Pyrex was supposed to compact to a level easily measurable by the MEMS strain gauges. Almost no substrate compaction registered in the strain gauges, however. It is hypothesized that the anodic bonding process (by which a silicon wafer was bonded to the glass before etching to create the MEMS strain gauges) was responsible for either 1) changing the bulk radiation response of the glass or 2) creating a layer near the bond interface which somehow prevented the MEMS strain gauges from registering the compaction that was occurring in the glass substrate. While not yet understood, this null result for apparent substrate compaction is of great importance to the problem of mechanically rad-hard MEMS, since it indicates that the response of an anodically bonded Si-glass system to radiation is not simply the sum of the effects on the unbonded materials. To investigate this further, glass samples were prepared in various stages of the anodic bonding process (which involves heating in the presence of an electric field), then irradiated with neutrons. No difference in bulk compaction was noted among the / (cont.) treated samples or the untreated glass, but this result may have been influenced by the high temperature at which the glass was irradiated; however, temperature alone could not have annealed away all the effects of treatment. We conclude that the unexpected results of the MEMS strain gauge experiment were caused by surface layer phenomena at the bonding interface, though we do not currently understand the exact mechanism for this. The second major topic of this study is the effect of neutron irradiation on the Young's modulus of silicon, the constancy of which is key to the operation of many MEMS devices. The elastic constants of defected and amorphous silicon simulation cells were calculated using EDIP. Simulation cells included some containing randomly generated defect distributions, as well as several that were completely amorphous and one containing a small amorphous region. An extensive and careful characterization of point defects was made ... / by Clark L. Allred. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references. / by Ding-Kuo Ding. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 117-128). / The majority of the neural engineering efforts in the past decade have focused on brain interfaces. The searching of tools for recording and modulation of neural activity in the spinal cord limits fundamental understanding of neural dynamics in this organ. Spinal cord poses a challenge to probe design due to its fibrous structure, repeated deformation, low elastic modulus, and sensitivity to implantation procedures. This work addresses the elastic modulus mismatch between spinal cord tissue and synthetic devices by designing flexible multifunctional neural probes capable of conforming to the spinal cord geometry and mechanical properties, while providing functions for optical stimulation and neural recording. In this thesis, fiber drawing techniques are applied to produce flexible and stretchable probes. The utility of the devices for recording and optical stimulation is demonstrated in the spinal cord of transgenic mice expressing the light sensitive protein channelrhodopsin 2 (ChR2). Furthermore, it is shown that the optical stimulation of the spinal cord with the polymer fiber probes induces on-demand limb movements. Finally, the modest dimensions and high flexibility of the devices permitting chronic implantation into the mouse spinal cord with minimal damage to the neural tissue are demonstrated. The findings of this thesis are anticipated to aid the studies of the spinal cord circuits and pave way to new directions in flexible fiber-based optoelectronic devices. / by Chi Lu. / Ph. D.