abstract: The energy crisis in the past decades has greatly boosted the search for alternatives to traditional fossil foils, and solar energy stands out as an important candidate due to its cleanness and abundance. However, the relatively low conversion efficiency and energy density strongly hinder the utilization of solar energy in wider applications. This thesis focuses on employing metamaterials and metafilms to enhance the conversion efficiency of solar thermal, solar thermophotovoltaic (STPV) and photovoltaic systems. A selective metamaterial solar absorber is designed in this thesis to maximize the absorbed solar energy and minimize heat dissipation through thermal radiation. The theoretically designed metamaterial solar absorber exhibits absorptance higher than 95% in the solar spectrum but shows emittance less than 4% in the IR regime. This metamaterial solar absorber is further experimentally fabricated and optically characterized. Moreover, a metafilm selective absorber with stability up to 600oC is introduced, which exhibits solar absorptance higher than 90% and IR emittance less than 10%. Solar thermophotovoltaic energy conversion enhanced by metamaterial absorbers and emitters is theoretically investigated in this thesis. The STPV system employing selective metamaterial absorber and emitter is investigated in this work, showing its conversion efficiency between 8% and 10% with concentration factor varying between 20 and 200. This conversion efficiency is remarkably enhanced compared with the conversion efficiency for STPV system employing black surfaces (<2.5%). Moreover, plasmonic light trapping in ultra-thin solar cells employing concave grating nanostructures is discussed in this thesis. The plasmonic light trapping inside an ultrathin GaAs layer in the film-coupled metamaterial structure is numerically demonstrated. By exciting plasmonic resonances inside this structure, the short-circuit current density for the film-coupled metamaterial solar cell is three times the short-circuit current for a free-standing GaAs layer. The dissertation is concluded by discussing about the future work on selective solar thermal absorbers, STPV/TPV systems and light trapping structures. Possibilities to design and fabricate solar thermal absorber with better thermal stability will be discussed, the experimental work of TPV system will be conducted, and the light trapping in organic and perovskite solar cells will be looked into. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2016

abstract: DNA nanotechnology has been a rapidly growing research field in the recent decades, and there have been extensive efforts to construct various types of highly programmable and robust DNA nanostructures. Due to the advantage that DNA nanostructure can be used to organize biochemical molecules with precisely controlled spatial resolution, herein we used DNA nanostructure as a scaffold for biological applications. Targeted cell-cell interaction was reconstituted through a DNA scaffolded multivalent bispecific aptamer, which may lead to promising potentials in tumor therapeutics. In addition a synthetic vaccine was constructed using DNA nanostructure as a platform to assemble both model antigen and immunoadjuvant together, and strong antibody response was demonstrated in vivo, highlighting the potential of DNA nanostructures to serve as a new platform for vaccine construction, and therefore a DNA scaffolded hapten vaccine is further constructed and tested for its antibody response. Taken together, my research demonstrated the potential of DNA nanostructure to serve as a general platform for immunological applications. / Dissertation/Thesis / Movie 4.1-a / Movie 4.1-b / Movie 4.1-c / Ph.D. Biochemistry 2012

The presence of and interactions between structural defects, fluids, and trace elements during deformation play a vital role in the manner in which materials respond to an applied stress. Although the links between crystal defects and trace element mobility have been lying at the frontier of research in Earth sciences, the role of fluids and the underlying physico-chemical processes linking them remain poorly understood. Investigation of these nanometer scale processes requires a correlative approach combining high-spatial resolution analytical techniques. This thesis integrates novel 2D and 3D structural and geochemical mapping methods such as electron channeling contrast imaging, electron backscatter diffraction, scanning transmission electron microscopy (STEM) and atom probe tomography (APT) to interrogate the atomic structure and composition of geomaterials in an attempt to better understand long-standing questions in Earth sciences and build bridges between materials science and geoscience. The processes investigated in this thesis include: 1) the underlying diffusion processes that mobilize trace elements into deformation-induced nanostructures; 2) the mechanisms of trace element segregation associated with fluid inclusions; 3) the influence of fluid inclusions on the mobility of structural defects and trace element mobility; and 4) the initial stages of bubble nucleation in the presence of nanoscale chemical heterogeneities. Ultimately, this research interrogates the feedbacks between deformation and trace element diffusion processes, fundamentally investigating their impact on rheology. More specifically, the thesis investigates the influence of deformation and associated nanostructures on the remobilization of trace elements and, in turn, the influence of trace elements on the nucleation and mobility of nanostructures. The combined work successfully identified two diffusion mechanisms for deformation-induced trace element mobility, characterized fluid-inclusions in APT data, documented two processes that led to proposing a new fluid inclusion-induced hardening model, and documented direct evidence of bubble nucleation on the surface of nanoscale chemical heterogeneities. This work not only pushes the limits of high-spatial resolution analytical techniques including STEM and APT, but the results have significant transdisciplinary implications in the fields of geoscience, materials science, engineering, and analytical microscopy.

The aim of this work is to fabricate nanostructured membranes from polysulfone-based block copolymers through self-assembly and non-solvent induced phase separation. Block copolymers containing polysulfone are novel materials for this purpose providing better mechanical and thermal stability to membranes than polystyrene-based copolymers, which have been exclusively used now. Firstly, we synthesized a triblock copolymer, poly(tert-butyl acrylate)-b-polsulfone-b-poly(tert-butyl acrylate) through polycondensation and reversible addition-fragmentation chain-transfer polymerization. The obtained membrane has a highly porous interconnected skin layer composed of elongated micelles with a flower-like arrangement, on top of the graded finger-like macrovoids. Membrane surface hydrolysis was carried out in a combination with metal complexation to obtain metal-chelated membranes. The copper-containing membrane showed improved antibacterial capability. Secondly, a poly(acrylic acid)-b-polysulfone-b-poly(acrylic acid) triblock copolymer obtained by hydrolyzing poly(tert-butyl acrylate)-b-polsulfone-b-poly(tert-butyl acrylate) formed a thin film with cylindrical poly(acrylic acid) microdomains in polysulfone matrix through thermal annealing. A phase inversion membrane was prepared from the same polymer via self-assembly and chelation-assisted non-solvent induced phase separation. The spherical micelles pre-formed in a selective solvent mixture packed into an ordered lattice in aid of metal-poly(acrylic acid) complexation. The space between micelles was filled with poly(acrylic acid)-metal complexes acting as potential water channels. The silver0 nanoparticle-decorated membrane was obtained by surface reduction, having three distinct layers with different particle sizes. Other amphiphilic copolymers containing polysulfone and water-soluble segments such as poly(ethylene glycol) and poly(N-isopropylacrylamide) were also synthesized through coupling reaction and copper0-mediated reversible-deactivation radical polymerization. Finally, phase inversion membranes were prepared from polytriazole-polysulfone random copolymers, which were obtained by “clicking” 1,2,3-triazole ring substituents bearing OH groups onto the polysulfone backbone via copperI-catalyzed azide-alkyne cycloaddition. The increased hydrophilicity of membranes imparted the higher water permeability and fouling resistance to the ultrafiltration membranes. Polytriazole-b-polysulfone-b-polytriazole triblock copolymer was synthesized by RAFT and post-polymerization click modification. Hydrogen bond-mediated self-assembly induced the formation of a nanostructured polytriazole-b-polysulfone-b-polytriazole / poly(acrylic acid)-b-polysulfone-b-poly(acrylic acid) blend membrane with a 1: 1 stoichiometric ratio of triazole and acid. String-like fused micelles with polytriazole/poly(acrylic acid) corona were present on the membrane surface, after immersion in a coagulation bath of copper2+ aqueous solution.

GaAs nanowires (NW) have been grown on Si (111) substrates by the self-assisted vapor-liquid-solid (VLS) mechanism using molecular beam epitaxy (MBE). Substrates were prepared with nano-patterned oxide templates using electron beam lithography (EBL) in order to achieve position controlled NW growth. Early experiments uncovered several key issues with regards to the patterning process. Cross-sectional lamella prepared using the focused-ion beam (FIB) technique were used to study the NW-substrate interface using transmission electron microscopy (TEM). Undesirable growth outcomes were found to be caused in part by an unintended residual layer of oxide. Uniform NW dimensions were then obtained by improving the pattern transfer method. The effects of deposition parameters on the growth results were then explored in further experiments. The first systematic study of the axial and radial growth rates of vertical NWs in the positioned array was conducted. It was proposed that the observed expansion of the Ga droplet in Ga-rich growth conditions results in a slight inverse tapered morphology, promoting significant radial growth. While the growth rates were shown to be approximately constant in time, their measured values were found to increase with increasing pattern pitch and decrease with increasing hole diameter. A phenomenological model was then developed based on the principle of mass conservation. A fit to the experimental data was obtained by calculating the collection of growth material supplied by a secondary flux of both gallium and arsenic species desorbing from the oxide surface between the NWs, subsequently impinging on the liquid droplet and NW sidewalls. The reduction of this contribution due to shading of the incident and scattered flux by neighboring NWs in the array was able to account for the differences in final NW morphologies observed with increasing pattern pitch. This model demonstrates the significant impact of secondary adsorption in patterned self-assisted NW growth. / Thesis / Doctor of Science (PhD)

No description available.

Thermoelectric materials can convert thermal energy into electrical energy and vice-versa. They are widely used in energy harvesters, thermal sensors, and cooling systems. However, the low efficiency and high cost of the known material compositions limit their widespread utilization in electricity generation applications. Therefore, there is a strong interest in identifying new thermoelectric materials with high figure of merit. In response to this need, this dissertation works on the synthesis, structure, doping mechanism, and thermoelectric properties of zinc oxide (ZnO) and lead tellurium (PbTe). The main focus is on ZnO based materials and in improving their performance. The influences of micro- or nano-structures on thermal conductivity, as well as the correlation between the electrical property and synthesis conditions, have been systematically investigated. ZnO is a likely candidate for thermoelectric applications, because of its good Seebeck coefficient, high stability at high temperature, non-toxicity and abundance. Its main drawbacks are the high thermal conductivity (κ) and low electrical conductivity (σ). To decrease κ, two novel structures—namely, precipitate system and layered-and-correlated grain microstructure—have been proposed and synthesized in ZnO. The mechanisms iii governing the nature of thermal behavior in these structures have been explored and quantified. Due to strong phonon scattering, the nano-precipitates can reduce the thermal conductivity of ZnO by 73%. The ZnO with layered-and-correlated grains can further reduce κ by about 52%, which compares favorably with the dense ZnO with nanoprecipitates. The figure of merit of this ZnO based structure was 0.14×10⁻³ K⁻¹ at 573 K. In order to understand the electrical behavior in nanostructured ZnO, the impact of Al doping and chemical defects in ZnO under different synthesis conditions were studied. Under varying sintering temperatures, atmospheres and initial physical conditions, ZnO exhibited very distinct σ. High temperature, lack of oxygen, vacuum condition, and chemically synthesized powder can increase the carrier concentration and σ of ZnO. A promising alloy system, PbTe-PbS, undergoes natural phase separation by nucleation and growth, and spinodal decomposition depending on the thermal treatment. The correlation between the thermal treatment, structure, and the thermoelectric properties of Pb0.9S0.1Te has been studied. The nano-precipitates were incorporated in the annealed alloy resulting in a 40% decrease in κ. The PbS precipitation was shown to enhance the carrier concentration and improves the Seebeck coefficient. These concomitant effects result in a maximum ZT of 0.76 at 573 K. Throughout the thesis, the emphasis was on understanding the impact of the microstructures on thermal conductivity and the effect of the synthesis condition on thermal and electrical properties. The process and control variables identified in this study provide practical ways to optimize the figure of merit of ZnO and PbTe materials for thermoelectric applications. / Ph. D.

High temperature thermal energy harvesting has attracted much attention recently. In order to achieve stable operation at high temperatures there is emerging need to develop efficient and oxidation-resistant materials. Most of the well-known materials with high dimensionless figure of merit (ZT) values such as Bi2Te3, PbTe, skutterudites, and half-Heusler alloys, are not thermally stable at temperatures approaching 500°C or higher, due to the presence of volatile elements. Oxide thermoelectric materials are considered to be potential candidates for high temperature applications due to their robust thermal and chemical stability in oxidizing atmosphere along with the reduced toxicity, relatively simpler fabrication, and cost. In this dissertation, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. In order to tailor the electrical and thermal properties, Lu and Ga co-doping was investigated in Ca3Co4O9 system. The effect of co-doping at Ca and Co sites on the thermoelectric properties was quantified and the anisotropic behavior was investigated. Because of the effective scattering of phonons by doping-induced defects, lower thermal conductivity and higher ZT were achieved. The layered structure of Ca3Co4O9 has strong anisotropy in the transport properties. For this reason, the thermoelectric measurements were conducted for the samples along both vertical and horizontal directions. The ZT value along the vertical direction was found to be 3 to 4 times higher than that along the horizontal direction. Metallic inclusions along with ionic doping were also utilized in order to enhance the ZT of Ca3Co4O9. The texturing occurring in the nanostructured Ca3Co4O9 through ion doping and Ag inclusions was studied using microscopy and diffraction analysis. Multi-length scale inclusions and heavier ion doping in Ca3Co4O9 resulted in higher electrical conductivity and reduced thermal conductivity. The maximum ZT of 0.25 at 670°C was found in the spark plasma sintered Ca2.95Ag0.05Co4O9 sample. In literature, limited number of studies have been conducted on understanding the anisotropic thermoelectric performance of Ca3Co4O9, which often results in erroneous estimation of ZT. This study addresses this limitation and provides systematic evaluation of the anisotropic response with respect to platelet microstructure. Textured Ca3Co4O9/Ag nanocomposites were fabricated using spark plasma sintering (SPS) technique and utilized for understanding the role of microstructure towards anisotropic thermoelectric properties. The thermoelectric response was measured along both vertical and horizontal direction with respect to the SPS pressure axis. In order to achieve enhanced degree of texturing and increase electrical conductivity along ab planes, a two-step SPS method was developed. Ag nanoinclusions was found to increase the overall electrical conductivity and the thermoelectric power factor because of improved electrical connections among the grains. Through two-step SPS method, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C in Ca3Co4O9/Ag nanocomposites was achieved. Lastly, this dissertation provides significant progress towards understanding the effect of synthesis method on thermoelectric properties and evolution of textured microstructure. The anisotropy resulting from the crystal structure and microstructural features is systematically quantified. Results reported in this study will assist the continued progress in developing Ca3Co4O9 materials for practical thermoelectric applications. / PHD / Among the wide range of renewable energy sources, wasted thermal energy has attracted worldwide interest as it is freely available from most of the industrial and natural processes. Among various choices for converting thermal energy into electricity, thermoelectric devices are attractive as they are solid state, noiseless, no moving parts, and can be easily integrated with most of the heat sources. Thus, there has been significant efforts to develop high efficiency thermoelectric energy harvesting devices. However, currently available thermoelectric materials are not thermally stable in oxidizing environments because of heavy metals’ evaporation and reactivity. In order to overcome this limitation of thermoelectric materials, in this dissertation, the focus is on developing calcium cobalt oxide (Ca₃Co₄O₉) materials through innovation in the processing, composition design, and modulation of the thermal transport mechanism by exploiting the anisotropy. Ca₃Co₄O₉ is promising candidate for high temperature thermoelectric applications due to its thermal and chemical stability in oxidizing atmosphere, reduced toxicity, easy fabrication, and low cost. Its main disadvantages are the high thermal conductivity and low electrical conductivity. In order to tailor the electrical and thermal properties, Lu and Ga co-doped Ca₃Co₄O₉ were synthesized and characterized. The thermoelectric measurements were conducted along both vertical and horizontal directions with respect to pressure axis during spark plasma sintering. Layered structure of Ca₃Co₄O₉ induces strong anisotropy in the transport properties which indicates that textured microstructure will result in better properties. Texturing and interface engineering were employed to control the grain orientation and thereby improve the electrical and thermal properties. In textured and nanostructured Ca₃Co₄O₉, Ag inclusions along with ionic doping was utilized to enhance the thermoelectric performance. In literature, the importance of the anisotropy in Ca₃Co₄O₉ has been less emphasized, which has restricted accurate thermoelectric evaluation of this material for practical application. In order to address this issue, first textured Ca₃Co₄O₉/Ag nanocomposites were fabricated using spark plasma sintering (SPS) techniques and next detailed investigation was conducted on correlation between microstructure and anisotropic thermoelectric properties. The power factor of the Ca₃Co₄O₉/Ag nanocomposites at high temperatures was almost 50% enhanced, as compared to the pure Ca₃Co₄O₉, which resulted in 50% improvement in ZT both horizontal and vertical directions. The samples with texturing along the vertical direction were used to perform the long-term durability test and almost same value of resistivity was maintained after a long-term heating. Two-step SPS method was developed to improve the in-plane electrical conductivity. Through this newly proposed synthesis process, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C was obtained in Ca₃Co₄O₉/Ag nanocomposites. Using a wide range of composition and synthesis process, the anisotropy and microstructural effects clarified in this study provides promising pathway towards enhance the thermoelectric performance of Ca₃Co₄O₉ materials.

La spintronique dans les semiconducteurs vise à utiliser le spin de l’électron comme degré de liberté supplémentaire (en plus de la charge électrique) afin de véhiculer l’information, ce qui permettrait la mise au point de composants intégrant de nouvelles fonctionnalités. Ce travail de thèse porte sur deux étapes importantes qui doivent être maîtrisées : l’injection électrique de porteurs polarisés en spin dans les semiconducteurs III-V, et la manipulation du spin de l’électron (par champ magnétique) dans ces matériaux optimisés. Dans un premier temps, la grande efficacité des injecteurs de spin à base de CoFeB/MgO/GaAs est démontrée dans des dispositifs de type Diodes Electroluminescentes polarisées en spin (SpinLEDs). La comparaison entre des injecteurs comprenant une barrière tunnel fabriquée soit par pulvérisation cathodique, soit par épitaxie par jets moléculaires (MBE), permet de montrer que ces deux techniques donnent des résultats comparables. Dans les deux cas, l’efficacité de l’injection est améliorée par un recuit de l’échantillon autour de 300−350◦C. Le recuit induit une amélioration de la qualité de l’interface CoFeB/MgO. De plus, l’efficacité de l’injection de spin n’est stable en fonction du courant injecté que lorsque la barrière tunnel est fabriquée par pulvérisation cathodique. Ceci est dˆu aux caractéristiques de l’interface MgO/GaAs qui diffèrent selon la technique de croissance de la barrière. Dans un deuxième temps, l’injection de spin en l’absence de champ magnétique externe appliqué est réaliséegrâce à un nouveau type d’injecteur constitué d’une électrode de CoFeB ultrafine présentant une aimantation rémanente de la couche le long de l’axe de croissance de l’échantillon. Pour la première fois des taux de polarisation circulaire de l’électroluminescence de l’ordre de 20% sont mesurés à 25 K à champ magnétique nul. Ensuite, la problématique de la relaxation de spin des porteurs injectés dans les vallées L de haute énergie dans GaAs (phénomène non négligeable sous injection électrique) est également traitée. Nous observons qu’une fraction de la mémoire du spin photogénéré en L est conservée lorsque les électrons sont diffusés vers la vallée Γ, malgré une relaxation d’énergie de plusieurs centaines de meV. Le temps de relaxation de spin dans les vallées L est estimé autour de 200 fs. Enfin, nous avons exploré le matériau GaAsBi dilué (x ∼ 2.2%) dont la perturbation de la matrice par l’élément Bi permet d’attendre des propriétés électroniques et de spin fortement modifiées. Des mesures de photoluminescence ont mis en évidence une diminution de l’énergie de bande interdite de l’ordre de 85meV/%Bi. De plus, par la mesure directe des battements quantiques de la polarisation de photoluminescence nous avons déterminé un facteur de Landé des électrons de conduction de l’ordre de deux fois supérieur à celui de GaAs. Ces résultats témoignent de la forte perturbation des états de valences et de l’augmentation de l’interaction spin-orbite / Spintronics of semiconductors aims at using carrier spins as supplementary means of information transport. Thiswould lead to components showing extended functionalities. This thesis work is dedicated to the study of injectionand manipulation of electron spin in semiconductors, which are the basis of any spintronic application. In a first stepwe demonstrate the high efficiency of CoFeB/MgO/GaAs - based spin injectors. Circular polarization degrees of electroluminescence over 20% are measured on spin polarized LEDs (SpinLEDs) at 0.8 T and 25 K. Comparison betweensputtering- and MBE- grown spin injectors has shown similar results. In both case, spin injection efficiency is increasedby thermal annealing of the sample, in the range 300 − 350◦C. Indeed, annealing improves the quality of CoFeB/MgOinterface, and induces the crystallization of CoFeB above 300◦C. A higher stability of spin injection with current injectionis found when the tunnel barrier is grown by sputtering. This is due to the MgO/GaAs interface characteristicswhich is related to the growth technique. In a second step, we demonstrate spin injection without external appliedmagnetic field, through an ultra-thin (a few atomic layers) CoFeB electrode, taking advantage of the perpendicular magnetic anisotropy of the layer which leads to a remanant magnetization along the growth axis. For the first time in this configuration, circular polarization degrees of electroluminescence of about 20% are measured at 25 K at zero magnetic field. In a third step, due to the crucial role it may play in electrical injection, electron spin dynamics in high energy L-valleys is investigated. Using polarization resolved excitation photoluminescence in the range 2.8-3.4 eV, we observe that a fraction of photogenerated spin polarization is preserved when electrons are scattered hundreds of meV down to Γ valley. Spin relaxation time in L valleys is estimated to 200 fs. Finally we investigate electron and spin properties of GaAsBi dilute bismide alloy. We observe that the bandgap energy is reduced by 85meV/%Bi when Bi element is introduced into GaAs matrix. Moreover, the electron Land´e factor is about twice the one in GaAs for a 2.2% Bi composition. These features are evidence of the strong perturbation of host states and spin-orbit interaction enhancement

Les nanostructures ont des propriétés mécaniques qui diffèrent de celles des matériaux massifs. La compréhension des propriétés mécaniques aux échelles nanométriques requièrent la mise en place d’essais mécaniques combinés à des observations structurales. Au cours de cette thèse nous avons développé un microscope à force atomique (AFM) dédié permettant de solliciter mécaniquement un nano-objet unique sur une ligne de lumière synchrotron. Les possibilités offertes par cette nouvelle approche expérimentale sont démontrées sur deux exemples de sollicitation mécanique in situ: (i) la nanoindentation in situ de cristaux d’or combinée à la diffraction cohérente des rayons X; (ii) la flexion trois points de nanofils d’or associée à la micro-diffraction de Laue. Ces expériences permettent d'accéder au comportement élastique ainsi qu’au comportement plastique du nanomatériau et permettent de déterminer la limite d'élasticité et le type de défauts induits par le chargement mécanique. / Nanostructures were found to exhibit different mechanical properties compared to their bulk counterpart. For obtaining further insight into the mechanical behaviour on the nanoscale, mechanical tests are combined with observation techniques allowing for monitoring the structural evolution. Within this thesis a special atomic force microscope has been developed which is compatible with different X-ray diffraction techniques at synchrotron sources for in situ mechanical testing on single nano-objects. The great potential of the new experimental approach is demonstrated on two kinds of in situ mechanical tests: (i) in situ nano-indentation on Au crystals with coherent X-ray diffraction. (ii) In situ three point bending tests on Au nanowires with μLaue diffraction. These experiments give access to the elastic as well as the plastic behavior of the nanomaterial and allows for determining the elastic limit and the type of defects induced by the mechanical loading.