The research of this dissertation has involved the nanoscale quantitative characterization of patterned magnetic nanostructures and devices using off-axis electron holography and Lorentz microscopy. The investigation focused on different materials of interest, including monolayer Co nanorings, multilayer Co/Cu/Py (Permalloy, Ni81Fe19) spin-valve nanorings, and notched Py nanowires, which were fabricated via a standard electron-beam lithography (EBL) and lift-off process. Magnetization configurations and reversal processes of Co nanorings, with and without slots, were observed. Vortex-controlled switching behavior with stepped hysteresis loops was identified, with clearly defined onion states, vortex states, flux-closure (FC) states, and Omega states. Two distinct switching mechanisms for the slotted nanorings, depending on applied field directions relative to the slot orientations, were attributed to the vortex chirality and shape anisotropy. Micromagnetic simulations were in good agreement with electron holography observations of the Co nanorings, also confirming the switching field of 700-800 Oe. Co/Cu/Py spin-valve slotted nanorings exhibited different remanent states and switching behavior as a function of the different directions of the applied field relative to the slots. At remanent state, the magnetizations of Co and Py layers were preferentially aligned in antiparallel coupled configuration, with predominant configurations in FC or onion states. Two-step and three-step hysteresis loops were quantitatively determined for nanorings with slots perpendicular, or parallel to the applied field direction, respectively, due to the intrinsic coercivity difference and interlayer magnetic coupling between Co and Py layers. The field to reverse both layers was on the order of ~800 Oe. Domain-wall (DW) motion within Py nanowires (NWs) driven by an in situ magnetic field was visualized and quantified. Different aspects of DW behavior, including nucleation, injection, pinning, depinning, relaxation, and annihilation, occurred depending on applied field strength. A unique asymmetrical DW pinning behavior was recognized, depending on DW chirality relative to the sense of rotation around the notch. The transverse DWs relaxed into vortex DWs, followed by annihilation in a reversed field, which was in agreement with micromagnetic simulations. Overall, the success of these studies demonstrated the capability of off-axis electron holography to provide valuable insights for understanding magnetic behavior on the nanoscale.

Miedema's plot is used to select the Cu/metal barrier for Cu metallization.The Cu/metal barrier system selected should have positive heat of formation (Hf) so that there is no intermixing between the two layers. In this case, Ru is chosen as a potential candidate, and then the barrier properties of sputtered Cu/Ru thin films on thermally grown SiO2 substrates are investigated by Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and electrical resistivity measurement. The Cu/Ru/SiO2 samples are analyzed prior to and after vacuum annealing at various temperatures of 400, 500, and 600 oC and at different interval of times of 0.5, 1 and 2 hrs for each temperature. Backscattering analysis indicate that both the copper and ruthenium thin films are thermally stable at high temperature of 600 oC, without any interdiffusion and chemical reaction between Cu and Ru thin films. No new phase formation is observed in any of the Cu/Ru/SiO2 samples. The XRD data indicate no new phase formation in any of the annealed Cu/Ru/SiO2 samples and confirmed excellent thermal stability of Cu on Ru layer. The electrical resistivity measurement indicated that the electrical resistivity value of the copper thin films annealed at 400, 500, and 600 oC is essentially constant and the copper films are thermally stable on Ru, no reaction occurs between copper films and Ru the layer. Cu/Ru/SiO2 multilayered thin film samples have been shown to possess good mechanical strength and adhesion between the Cu and Ru layers compared to the Cu/SiO2 thin film samples. The strength evaluation is carried out under static loading conditions such as nanoindentation testing. In this study, evaluation and comparison is donebased on the dynamic deformation behavior of Cu/Ru/SiO2 and Cu/SiO2 samples under scratch loading condition as a measure of tribological properties. Finally, the deformation behavior under static and dynamic loading conditions is understood using the scanning electron microscope (SEM) and the focused ionbeam imaging microscope (FIB) for topographical and cross-sectional imaging respectively.

This study analyzes the thermoelectric phenomena of nanoparticle suspensions, which are composed of liquid and solid nanoparticles that show a relatively stable Seebeck coefficient as bulk solids near room temperature. The approach is to explore the thermoelectric character of the nanoparticle suspensions, predict the outcome of the experiment and compare the experimental data with anticipated results. In the experiment, the nanoparticle suspension is contained in a 15cm*2.5cm*2.5cm glass container, the temperature gradient ranges from 20 °C to 60 °C, and room temperature fluctuates from 20 °C to 23°C. The measured nanoparticles include multiwall carbon nanotubes, aluminum dioxide and bismuth telluride. A temperature gradient from 20 °C to 60 °C is imposed along the length of the container, and the resulting voltage (if any) is measured. Both heating and cooling processes are measured. With three different nanoparticle suspensions (carbon nano tubes, Al2O3 nanoparticles and Bi2Te3 nanoparticles), the correlation between temperature gradient and voltage is correspondingly 8%, 38% and 96%. A comparison of results calculated from the bulk Seebeck coefficients with our measured results indicate that the Seebeck coefficient measured for each suspension is much more than anticipated, which indicates that the thermophoresis effect could have enhanced the voltage. Further research with a closed-loop system might be able to affirm the results of this study.

In this work a newly fabricated organic solar cell based on a composite of fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and regioregular poly (3-hexylthiophene) (P3HT) with an added interfacial layer of AgOx in between the PEDOT:PSS layer and the ITO layer is investigated. Previous equivalent circuit models are discussed and an equivalent circuit model is proposed for the fabricated device. Incorporation of the AgOx interfacial layer shows an increase in fill factor (by 33%) and power conversion efficiency (by 28%). Moreover proper correlation has been achieved between the experimental and simulated I-V plots. The simulation shows that device characteristics can be explained with accuracy by the proposed model.

GaAs single-junction solar cells have been studied extensively in recent years, and have reached over 28 % efficiency. Further improvement requires an optically thick but physically thin absorber to provide both large short-circuit current and high open-circuit voltage. By detailed simulation, it is concluded that ultra-thin GaAs cells with hundreds of nanometers thickness and reflective back scattering can potentially offer efficiencies greater than 30 %. The 300 nm GaAs solar cell with AlInP/Au reflective back scattering is carefully designed and demonstrates an efficiency of 19.1 %. The device performance is analyzed using the semi-analytical model with Phong distribution implemented to account for non-Lambertian scattering. A Phong exponent m of ~12, a non-radiative lifetime of 130 ns, and a specific series resistivity of 1.2 Ω·cm2 are determined.

Thin-film CdTe solar cells have also attracted lots of attention due to the continuous improvements in their device performance. To address the issue of the lower efficiency record compared to detailed-balance limit, the single-crystalline Cd(Zn)Te/MgCdTe double heterostructures (DH) grown on InSb (100) substrates by molecular beam epitaxy (MBE) are carefully studied. The Cd0.9946Zn0.0054Te alloy lattice-matched to InSb has been demonstrated with a carrier lifetime of 0.34 µs observed in a 3 µm thick Cd0.9946Zn0.0054Te/MgCdTe DH sample. The substantial improvement of lifetime is due to the reduction in misfit dislocation density. The recombination lifetime and interface recombination velocity (IRV) of CdTe/MgxCd1-xTe DHs are investigated. The IRV is found to be dependent on both the MgCdTe barrier height and width due to the thermionic emission and tunneling processes. A record-long carrier lifetime of 2.7 µs and a record-low IRV of close to zero have been confirmed experimentally.

The MgCdTe/Si tandem solar cell is proposed to address the issue of high manufacturing costs and poor performance of thin-film solar cells. The MBE grown MgxCd1-xTe/MgyCd1-yTe DHs have demonstrated the required bandgap energy of 1.7 eV, a carrier lifetime of 11 ns, and an effective IRV of (1.869 ± 0.007) × 103 cm/s. The large IRV is attributed to thermionic-emission induced interface recombination. These understandings can be applied to fabricating the high-efficiency low-cost MgCdTe/Si tandem solar cell.

The initial microstructure of oxide fuel pellets can play a key role in their performance. At low burnups, the transport of fission products has a strong dependence on oxygen content, grain size distribution, porosity and grain boundary (GB) characteristics (crystallography, geometry and topology), all of which, in turn depend on processing conditions. These microstructural features can also affect the fuel densification, thermal conductivity and microstructure evolution inside the reactor. Understanding these effects can provide insight into microstructure evolution of fuels in-pile. In this work, mechanical and ion beam serial sectioning techniques were developed to obtain Electron Backscatter Diffraction (EBSD) data, both in 2-D and 3-D, for depleted UO2+X pellets manufactured under different conditions. The EBSD maps were used to relate processing conditions to microstructural features, with emphasis on special GBs according to the Coincident Site Lattice (CSL) model, as well as correlations between pore size and location in the microstructure. Furthermore, larger grains (at least 2.5 times the average grain size) were observed in all the samples and studied. Results indicate that larger grains, in samples manufactured under different conditions, dominate the overall crystallographic texture and have a fairly strong GB texture. Moreover, it seems that the preferential misorientation axis for these GBs, regardless of the O/M, is {001}. These results might be related to GB energy and structure and, suggest that the mechanism that controls grain growth seems to be independent of both processing conditions and stoichiometry. Additionally, a sample was heat treated to relate grain growth and crystallography. The results indicate that at least two mechanisms were involved. Lengthening of GBs was observed for larger grains. Another mechanism of grain growth was observed, in this case, grains rotate to match a neighboring grain forming a larger grain. In the new grain, the misorientation between the two neighboring grains decreases to less than 5 degrees, forming a new larger grain. The results presented in this work indicate that detailed studies of the initial microstructure of the fuel, with emphasis on the crystallography of grains and GBs could help to give insights on the in-pile microstructural evolution of the fuel.

Organic optoelectronic devices have remained a research topic of great interest over the past two decades, particularly in the development of efficient organic photovoltaics (OPV) and organic light emitting diodes (OLED). In order to improve the efficiency, stability, and materials variety for organic optoelectronic devices a number of emitting materials, absorbing materials, and charge transport materials were developed and employed in a device setting. Optical, electrical, and photophysical studies of the organic materials and their corresponding devices were thoroughly carried out. Two major approaches were taken to enhance the efficiency of small molecule based OPVs: developing material with higher open circuit voltages or improved device structures which increased short circuit current. To explore the factors affecting the open circuit voltage (VOC) in OPVs, molecular structures were modified to bring VOC closer to the effective bandgap, ∆EDA, which allowed the achievement of 1V VOC for a heterojunction of a select Ir complex with estimated exciton energy of only 1.55eV. Furthermore, the development of anode interfacial layer for exciton blocking and molecular templating provide a general approach for enhancing the short circuit current. Ultimately, a 5.8% PCE was achieved in a single heterojunction of C60 and a ZnPc material prepared in a simple, one step, solvent free, synthesis. OLEDs employing newly developed deep blue emitters based on cyclometalated complexes were demonstrated. Ultimately, a peak EQE of 24.8% and nearly perfect blue emission of (0.148,0.079) was achieved from PtON7dtb, which approaches the maximum attainable performance from a blue OLED. Furthermore, utilizing the excimer formation properties of square-planar Pt complexes, highly efficient and stable white devices employing a single emissive material were demonstrated. A peak EQE of over 20% for pure white color (0.33,0.33) and 80 CRI was achieved with the tridentate Pt complex, Pt-16. Furthermore, the development of a series of tetradentate Pt complexes yielded highly efficient and stable single doped white devices due to their halogen free tetradentate design. In addition to these benchmark achievements, the systematic molecular modification of both emissive and absorbing materials provides valuable structure-property relationship information that should help guide further developments in the field.

Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and modeling research. Discrepancies between experimental observations and modeling results have led to various proposals for HE mechanisms. Therefore, to gain insights into HE mechanisms in iron, this dissertation aims to investigate several key issues involving HE such as: a) the incipient crack tip events; b) the cohesive strength of grain boundaries (GBs); c) the dislocation-GB interactions and d) the dislocation mobility.

The crack tip, which presents a preferential trap site for hydrogen segregation, was examined using atomistic methods and the continuum based Rice-Thompson criterion as sufficient concentration of hydrogen can alter the crack tip deformation mechanism. Results suggest that there is a plausible co-existence of the adsorption induced dislocation emission and hydrogen enhanced decohesion mechanisms. In the case of GB-hydrogen interaction, we observed that the segregation of hydrogen along the interface leads to a reduction in cohesive strength resulting in intergranular failure. A methodology was further developed to quantify the role of the GB structure on this behavior.

GBs play a fundamental role in determining the strengthening mechanisms acting as an impediment to the dislocation motion; however, the presence of an unsurmountable barrier for a dislocation can generate slip localization that could further lead to intergranular crack initiation. It was found that the presence of hydrogen increases the strain energy stored within the GB which could lead to a transition in failure mode. Finally, in the case of body centered cubic metals, understanding the complex screw dislocation motion is critical to the development of an accurate continuum description of the plastic behavior. Further, the presence of hydrogen has been shown to drastically alter the plastic deformation, but the precise role of hydrogen is still unclear. Thus, the role of hydrogen on the dislocation mobility was examined using density functional theory and atomistic simulations. Overall, this dissertation provides a novel atomic-scale understanding of the HE mechanism and development of multiscale tools for future endeavors.

The inexorable upsurge in world’s energy demand has steered the search for newer renewable energy sources and photovoltaics seemed to be one of the best alternatives for energy production. Among the various photovoltaic technologies that emerged, organic/polymer photovoltaics based on solution processed bulk-heterojunctions (BHJ) of semiconducting polymers has gained serious attention owing to the use of inexpensive light-weight materials, exhibiting high mechanical flexibility and compatibility with low temperature roll-to-roll manufacturing techniques on flexible substrates. The most widely studied material to date is the blend of regioregular P3HT and PC61BM used as donor and acceptor materials. The object of this study was to investigate and improve the performance/stability of the organic solar cells by use of inexpensive materials. In an attempt to enhance the efficiency of organic solar cells, we have demonstrated the use of hexamethyldisilazane (HMDS) modified indium tin oxide (ITO) electrode in bulk heterojunction solar cell structure The device studies showed a significant enhancement in the short-circuit current as well as in the shunt resistance on use of the hexamethyldisilazane (HMDS) layer. In another approach a p-type CuI hole-transport layer was utilized that could possibly replace the acidic PEDOT:PSS layer in the fabrication of high-efficiency solar cells. The device optimization was done by varying the concentration of CuI in the precursor solution which played an important role in the efficiency of the solar cell devices. Recently a substantial amount of research has been focused on identifying suitable interfacial layers in organic solar cells which has efficient charge transport properties. It was illustrated that a thin layer of silver oxide interfacial layer showed a 28% increase in power conversion efficiency in comparison to that of the control cell. The optoelectronic properties and morphological features of indium-free ZnO/Ag/MoOx electrodes was also studied. Organic solar cells on these composite electrodes revealed good optical and electrical properties, making them a promising alternative indium free and PEDOT:PSS-free organic solar cells. Lastly, inverted solar cells utilizing zinc oxide and yttrium doped zinc oxide electron transport was also created and their device properties revealed that optimum annealing conditions and yttrium doping was essential to obtain high efficiency solar cells.

Environmentally responsive hydrogels are one interesting class of soft materials. Due to their remarkable responsiveness to stimuli such as temperature, pH, or light, they have attracted widespread attention in many fields. However, certain functionality of these materials alone is often limited in comparison to other materials such as silicon; thus, there is a need to integrate soft and hard materials for the advancement of environmental-ly responsive materials.

Conventional hydrogels lack good mechanical properties and have inherently slow response time, important characteristics which must be improved before the hydrogels can be integrated with silicon. In the present dissertation work, both these important attrib-utes of a temperature responsive hydrogel, poly(N-isopropylacrylamide) (PNIPAAm), were improved by adopting a low temperature polymerization process and adding a sili-cate compound, tetramethyl orthosilicate. Furthermore, the transition temperature was modulated by adjusting the media quality in which the hydrogels were equilibrated, e.g. by adding a co-solvent (methanol) or an anionic surfactant (sodium dodecyl sulfate). In-terestingly, the results revealed that, based on the hydrogels’ porosity, there were appre-ciable differences when the PNIPAAm hydrogels interacted with the media molecules.

Next, an adhesion mechanism was developed in order to transfer silicon thin film onto the hydrogel surface. This integration provided a means of mechanical buckling of the thin silicon film due to changes in environmental stimuli (e.g., temperature, pH). We also investigated how novel transfer printing techniques could be used to generate pat-terned deformation of silicon thin film when integrated on a planar hydrogel substrate. Furthermore, we explore multilayer hybrid hydrogel structures formed by the integration of different types of hydrogels that have tunable curvatures under the influence of differ-ent stimuli. Silicon thin film integration on such tunable curvature substrates reveal char-acteristic reversible buckling of the thin film in the presence of multiple stimuli.

Finally, different approaches of incorporating visible light response in PNIPAAm are discussed. Specifically, a chemical chromophore- spirobenzopyran was synthesized and integrated through chemical cross-linking into the PNIPAAm hydrogels. Further, methods of improving the light response and mechanical properties were also demonstrat-ed. Interestingly, such a system was shown to have potential application as light modulated topography altering system