Browsing by Subject "Material science and engineering"

Now showing 1 - 8 of 8
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    Complexity at cobaltite interfaces: the interplay between strain, stoichiometry, magnetism and transport
    (2014-12) Bose, Shameek
    Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells and gas membranes. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial "dead layers". Working with the prototypical SrTiO3 (001)/La1-xSrxCoO3 (LSCO) system, our group recently discovered that this degradation in the magnetism and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the interplay between the strain state and the ordering of oxygen vacancies. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure. We first demonstrate a synthesis technique that utilizes a unique high pressure oxygen plasma to sputter LSCO thin films over a wide doping range 0.05  x  0.80. Then, using reciprocal space mapping and transmission electron microscopy, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). We then provide cross sectional electron energy loss spectroscopy data to show that this strain and orientation control preserves both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface. SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we use scanning tunneling microscopy to provide direct real space images of the magneto-electronic phase separation in ultrathin LSCO on SrTiO3(001). Our work thus demonstrates the ability to utilize oxygen vacancy ordering as a tunable control parameter to tailor interfacial electronic and magnetic properties, with profound implications for the myriad other systems that exhibit unique properties due to such ordering.
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    Cryo-SEM Study of Nanostructure Development of latex dispersions and block copolymer solutions
    (2008-11) Gong, Xiaobo
    High resolution cryogenic scanning electron microscopy (cryo-SEM) was used to study the physics of latex film formation. Fast freezing, controlled freeze-drying and annealing under vacuum, followed by room-temperature and cryogenic SEM demonstrated that van der Waals force alone can compact a latex coating under conditions devoid of surface tension and capillary forces. Rewetting tests of the annealed coatings shed light on distinguishing elastic and viscoelastic deformation. Key factors affecting the freeze-thaw (F/T) stability of polymer latexes were studied. The nanostructural changes during freeze-thaw cycles were visualized by cryo-SEM. Reducing Tg and modulus of the polymer, latex particle size, amount of protective functional groups, molecular weight and addition of coalescent all lead to reduced F/T stability. Both the freezing and thawing rates have strong impact on F/T stability. Both functional acid monomer type and degree of neutralization in pre-emulsion greatly influence the ability of the latex and titanium dioxide (TiO2) particles to interact with each other which prevents TiO2 particle aggregation. Latexes incorporated with vinylphosphonic or itaconic acid show better TiO2 efficiency than latexes with acrylic acid or methacrylic acid. For acid monomers with high water solubility, higher degree of neutralization in pre-emulsion yields in general lower TiO2 efficiency. Cryo-SEM was employed to further understand the nature of nanostructure deduced by small angle x-ray scattering (SAXS) for poly(butadiene-b-ethylene oxide) diblock copolymers solutions, as a function of copolymer concentration and block copolymer composition. The SAXS measurements and cryo-SEM images reveal a new type of network morphology, comprised of a random arrangement of interconnected cylinders, in addition to the other classical structures.
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    A DFT+U study of a Ni-Mn-Ga magnetic shape.
    (2011-03) Katukuri, Vamshi Mohan
    We have studied the structural, electronic and magnetic properties of the magnetic shape memory alloy Ni2MnGa using Hubbard-rooted DFT+U functional and recent extensions. Both austenite and martensite phases corresponding to high temperature and low temperature phases respectively have been investigated. The obtained results are compared with the results of standard DFT (GGA) calculations and available experiments. DFT+U method is particularly useful in predicting the energy landscape of the tetragonally distorted martensite from which the total energy minimum for c/a = 1.23, that has been obtained with GGA functionals, but not observed in experiments, disappears. Our results indicate that, the better description of the energitics of the tetragonal phase can be attributed to a longer extent of localization of electrons due to the Hubbard correction. Phonon dispersion study of austenite show a soft mode along the [110] direction indicating an instability of austenite at low temperatures.
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    Diblock copolymer stabilized nanoparticles for drug delivery via flash nanoprecipitation
    (2014-10) Han, Jing
    Cancer is one of the most challenge diseases to treat around the world. Drug delivery system, as one of the chemotherapeutic treatments has received enorrmous attention from researchers. This thesis is to develop amphiphilic diblock copolymer protected nanoparticles loaded with anti-cancer drug, with small size and high drug loading, to achieve selective drug delivery using EPR effect. Chapter 1 briefly describes the motivation and novelties of this research pursuit. Chapter 2 introduces a modified confined impingement jets mixer with dilution (CIJ-D mixer), using flash nanoprecipitation to produce nanoparticles made of hydrophobic drugs. The CIJ-D mixer was evaluated by the sizes of β-carotene nanoparticles at varied flow conditions compared to these made by multi-inlet vortex mixer. The CIJ-D mixer provides higher efficiency and easiness of handling for nanoparticle preparation. That is why CIJ-D mixer was used for all the work presented in the following chapters. In Chapter 3, we made the first attempt to produce PEG-b-PLGA protected paclitaxel loaded nanoparticles but failed, because paclitaxel is too hydrophilic to be captured in particles. Thus, a series of silicate ester derivatized paclitaxel were synthesized by Hoye research group and successfully encapsulated into nanoparticles. Several nanoparticle post-treatments, such as filtration, hollow fiber diafiltration, and ultracentrifugation were used and assessed, in order to purify nanoparticles. Lyophilization was found to induce nanoparticle aggregation due to the freezing process. The addition of sucrose as cryoprotectant was studied to prevent aggregation and recover nanoparticle. Chapter 4 focuses on developing in vitro drug release protocols, for more accurate quantification of highly hydrophobic paclitaxel prodrugs. Different dialysis devices were used such as dialysis tubes, dialysis cassettes, and dialysis mini capsules. Infinite sink and limited sink conditions were compared as well to provide sufficient concentration gradient across dialysis semi-permeable membrane. At last, a reverse drug release experimental protocol was customized to determine the remaining drug left in dialysis mini capsules while the sink condition was maintained by frequently refreshing buffer solution during in vitro drug release study. Chapter 5 mainly presents the pharmacokinetics of paclitaxel prodrug nanoparticles loaded with different silicate ester derivatives, at different pH, both inside nanoparticles and in buffer solution. Chapter 6 includes a series of Cryo-TEM images of nanoparticles collected at different time, such as fresh nanoparticles immediately after being prepared by CIJ-D mixer, nanoparticles after ultracentrifugation, after lyophilization, 0hr, and 24 hr during drug release study. These images not only showed a reverse liner relation of average particle size and hydrophobicity of the loaded drug, but also displayed a core-shell internal structure of nanoparticles prepared via flash nanoprecipitation and potential particle disassembly after 24hr drug release. Finally, Chapter 7 summarizes the key results and conclusions obtained from previous chapters, lessons learned from mistakes and failures, and future directions for this project, in order to prepare nanoparticles with better controlled size and drug release kinetics and to understand deeply on nanoparticle formation and release mechanisms.
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    Inelastic scattering in STEM for studying structural and electronic properties of chalcogenide-based semiconductor nanocrystals
    (2013-09) Gunawan, Aloysius Andhika
    Transmission electron microscopy (TEM) relies upon elastic and inelastic scattering signals to perform imaging and analysis of materials. TEM images typically contain contributions from both types of scattering. The ability to separate the contributions from elastic and inelastic processes individually through energy filter or electron energy loss spectroscopy (EELS) allows unique analysis that is otherwise unachievable. Two prominent types of inelastic scattering probed by EELS, namely plasmon and core-loss excitations, are useful for elucidating structural and electronic properties of chalcogenide-based semiconductor nanocrystals. The elastic scattering, however, is still a critical part of the analysis and used in conjunction with the separated inelastic scattering signals. The capability of TEM operated in scanning mode (STEM) to perform localized atomic length scale analysis also permits the understanding of the nanocrystals unattainable by other techniques. Despite the pivotal role of inelastic scatterings, their contributions for STEM imaging, particularly high-angle annular dark field STEM (HAADF-STEM), are not completely understood. This is not surprising since it is currently impossible to experimentally separate the inelastic signals contributing to HAADF-STEM images although images obtained under bright-field TEM mode can be analyzed separately from their scattering contributions using energy-filtering devices. In order to circumvent such problem, analysis based on simulation was done. The existing TEM image simulation algorithm called Multislice method, however, only accounts for elastic scattering. The existing Multislice algorithm was modified to incorporate (bulk or volume) plasmon inelastic scattering. The results were verified based on data from convergent-beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), and HAADF-STEM imaging as well as comparison to experimental data. Dopant atoms are crucial factors which control optical, electronic, and also magnetic properties of semiconductors. Their location inside the materials has become more important with the miniaturization of devices. The precise determination of the position, however, poses a great challenge. Imaging using HAADF-STEM has proven adequate for locating heavy dopant atoms buried in relatively light matrix, particularly using aberration-corrected microscopes. The imaging method has been unsuccessful in detecting dopant atoms with similar atomic number as the matrix. Inelastic core-loss or inner-shell electronic excitations using EELS offer a unique solution when simultaneous imaging and EELS acquisitions are performed. The dopant atoms that are invisible in the images due to the small atomic number differences can be detected via spatial correlation with EELS core-loss data. Three types of samples with varying concentration of Mn dopant atoms in ZnSe nanocrystals were used to confirm such method. Precise locations of the dopant atoms on planes perpendicular to electron beam propagation could be determined although not all of the dopant atoms were detected due to limitations in experimental conditions.Another important type of chalcogenide-based nanocrystals is PbSe which is useful for solar cells. Colloidal method commonly used to synthesize the nanocrystals leave oleic acid capping ligands as surface passivation and size stabilizer. These ligands have critical roles in controlling electrical and optical properties of an individual nanocrystal and their assembly. Deemed insulating due to long chains of carbons, oleic acid is typically treated with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals. Despite its apparent insulating behavior, oleic acid was shown to exhibit surface plasmon coupling under certain circumstances. The geometric arrangement of the ligands was first investigated by HAADF-STEM imaging. Under air exposure, PbSe nanocyrstals easily oxidize to form oxide shells that are responsible for p-type doping by introducing surface acceptor states. At early oxidation stage (partial oxidation), prior to the formation of uniform oxide shells, the nanocrystals appear to form links between neighbors. Localized EELS analysis shows that these links are made of carbon based materials, most likely modified form of oleic acid ligands consisting of conjugated double bonds. Such modification occurred through oxidative dehydrogenation of the oleic acid ligands that is facilitated by the growing oxide shells on the surface of nanocrystals.
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    Investigation of secondary hardening in Co-35Ni-20Cr-10Mo alloy using analytical scanning transmission electron microscopy
    (2014-08) Sorensen, Daniel David
    The mechanism of secondary hardening in MP35N (Co-35Ni-20Cr-10Mo) alloy due to exposures at elevated temperatures has been studied. It was observed that short exposure to elevated temperatures increased the ultimate tensile strength and yield stress while decreasing the elongation of MP35N wires. Upon aging at temperatures from 300 to 900°C the elastic modulus increased although no changes in crystallographic orientation or microstructure were observed. No proposed model for this apparent increase in elastic modulus is suggested as yet. The grain size and major texture components were unchanged following aging. Analytical scanning transmission electron microscope investigation showed that MP35N is hardened by preferential segregation of molybdenum to stacking faults and deformation twins. It also revealed that the concentration of molybdenum segregation was proportional to the amount of initial cold work before aging.
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    Self-assembly of block copolymers in thin films
    (2013-08) Kim, Sangwon
    The self-assembly of block copolymers in thin films has been a subject of recent studies from both academic and industrial perspectives. One of its potential applications is nanolithography; block copolymers can function as novel mask materials intended for fabrication of small features, not easily realizable by current optical lithography. This dissertation addresses several fundamental issues associated with thin-film block copolymers. The bulk and interfacial wetting properties of partially epoxidized poly(styrene-b-isoprene) diblock copolymers, denoted as PS-PEI, were studied while varying the degree of the chemical modification. The incorporation of the random copolymer architecture induced decoupling between the bulk and the thin-film thermodynamics. The tunable surface wetting, a consequence of the partial modification, permitted control over the orientation of the domains in thin films. The morphologies of thin-film block copolymers were investigated using two different boundary conditions that involve one neutral interface and one preferential interface. The neutralities at the free surface and the underlying substrate were attained independently by the partial epoxidation in PS-PEI and the composition adjustment of random copolymer mats, respectively. For both boundary conditions, thin-film block copolymers formed an island/hole motif, characterized by 0.5 L0 step heights (L0: bulk lamellar periodicity). The thin-film behavior of PS-PEI block copolymers with random copolymer architecture was examined as the segregation strength (χN) was adjusted systematically across the order-disorder transition. Unlike in the bulk, the random copolymer architecture did not generate abnormal behavior in thin-film thermodynamics compared to plain linear architecture. With decreasing segregation strength, the thin-film system exhibited fluctuation-pervaded morphologies prior to reaching a disordered state. An agreement was found between the order-disorder transition temperatures in three dimensions (bulk) and in two dimensions (thin film). Lastly, the bulk properties and the thin-film structures of lamellae-forming poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) triblock copolymers were studied. The thin-film morphology exhibited the dependence on the size of the poly(isoprene) (PI) middle block. While perpendicular lamellae were observed for the thin-film SIM block copolymer with a small PI volume fraction, complex behavior was observed for the sample with a large PI volume fraction.
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    Stress and microstructure development in particle-based coatings
    (2014-09) Price, Kyle Kirk-Arthur
    Particle-based coatings have a wide range of uses and applications in everyday life. Stress development during the drying process has the potential to impact the performance of the coating. Stress development can be monitored in-situ using a cantilever deflection technique with a laser-photodiode combination. Stress development in the film is directly related to the development of the coating microstructure during drying. Cryogenic scanning electron microscopy (cryoSEM) is a powerful characterization method capable of visualizing the microstructure of the coating during the intermediate stages of drying. Using this method, the coating is frozen to arrest microstructure development and solidify the sample so that it can survive the high-vacuum environment of the SEM. This thesis explores the connections between stress and microstructure development in particle-based coatings during drying. Characterization is often complicated by lateral drying, a common phenomenon in particle-based coatings. To avoid these complications, walled substrates were developed which are used to suppress lateral drying and promote drying uniformity. CryoSEM revealed that latex coatings dried on substrates (with photoresist walls) exhibit a greater degree of drying uniformity. Silicon cantilevers with poly(dimethyl siloxane) (PDMS) walls along the perimeter were used to suppress the effects of lateral drying during stress measurement. The walled cantilevers were used to characterize stress development in ceramic particle coatings and latex films. For the ceramic particle coatings, stress measurements were combined with cryoSEM revealing the origins of stress development in hard particle coatings. Stress development was correlated with the extent of drying and the degree of saturation in the coating. Stress development in latex particle coatings was influenced by the composition and morphology of the latex particles. Additionally, the influence of coalescing aids on stress development was also investigated. The film formation behavior was studied using a variety of techniques including AFM, cryoSEM, and minimum film formation temperature (MFFT) measurements.