The terahertz (THz) and mid-infrared (MIR) spectral regions have many potential applications in the industrial, biomedical, and military sectors. Yet, a wide portion of this region of the electromagnetic spectrum (particularly the THz range) is still relatively unexplored, due mainly to the absence of suitable sources and photodetectors, related to the lack of practical semiconductor materials with adequately small band gap energies. Intersubband transitions (ISBTs) between quantized energy states in quantum heterostructures provide tunable wavelengths over a broad spectral range including the THz region, by choosing appropriate layer thicknesses and compositions. This work focuses on the development of THz and MIR Quantum Well Infrared Photodetectors (QWIPs) based on ISBTs in GaN/AlGaN and Si/SiGe heterostructures. Due to their large optical phonon energies, GaN materials allow extending the spectral reach of existing far-infrared photodetectors based on GaAs, and may enable higher-temperature operation. In the area of MIR optoelectronic devices, I have focused on developing QWIPs based on ISBTs in Si/SiGe heterostructures in the form of on strain-engineered nanomembranes. Due to their non-polar nature, these materials are free from reststrahlen absorption and ultrafast resonant electron/phonon scattering, unlike traditional III-V semiconductors. Therefore, Si/SiGe quantum wells (QWs) are also promising candidates for high-temperature high-performance ISB device operation (particularly in the THz region), with the additional advantage of direct integration with CMOS technology. In this thesis work, numerical modeling is used to design the active region of the proposed devices, followed by sample fabrication and characterization based on lock-in step-scan Fourier transform infrared spectroscopy. Three specific QWIP devices have been developed. The first is a III-nitride THz QWIP based on a novel double-step QW design in order to alleviate the material limitations provided by the intrinsic electric fields of GaN/AlGaN heterostructures. Next, I have developed a THz GaN/AlGaN QWIP grown on semi-polar (202 ̅1 ̅) GaN, where the detrimental effects of the internal fields are almost completely eliminated. Finally, I have demonstrated a Si/SiGe MIR QWIP based on a novel fabrication approach, where nanomembrane strain engineering is used to address the materials quality issues normally found in SiGe QWs. Promising photodetector performance is obtained in all cases. / 2017-06-21T00:00:00Z

La photonique silicium connait un essor très important, porté notamment par la réalisation de câbles optiques actifs permettant de transférer optiquement des données à haut débit dans des environnements de type “High performance computing” ou “data center”. L'intégration de cette source laser est un enjeu très important pour la photonique silicium. Actuellement, ces sources sont obtenues avec des semi-conducteurs de type III-V sur substrats GaAs ou InP. Leur intégration dans une filière silicium est délicate et surtout ne permet pas de tirer pleinement parti de l'environnement de fabrication CMOS de la microélectronique.L'intégration d'une source optique monolithique représente donc un enjeu considérable. Les éléments de la colonne IV (Si, Ge) sont des semi-conducteurs à bande interdite indirecte, avec une faible efficacité de recombinaison radiative, et ne sont donc pas a priori de bons candidats. Un changement de paradigme est cependant en cours avec la récente démonstration qu'il était possible de manipuler la structure de bande des semi-conducteurs à base de germanium pour les rendre à bande interdite directe, i.e. les transformer en émetteurs efficaces. Cette ingénierie peut être réalisée soit en utilisant des tenseurs externes comme le nitrure de silicium soit en réalisant des alliages avec de l'étain (GeSn), ou en combinant les deux. Cette thèse porte donc sur l'étude de ces semi-conducteurs à bande interdite directe, avec pour objectif de faire la démonstration d'un laser avec ce nouveau type de matériaux. / Silicon photonics is experiencing a very important development. The laser source integration is a very important issue in silicon photonics. Currently, these sources are obtained with type III-V semiconductors on GaAs or InP substrates. Their integration in a silicon industry is delicate and above all does not allow to take full advantage of a CMOS environment.The integration of a monolithically optical source represents an important challenge. The elements of column IV (Si, Ge) are indirect bandgap semiconductors, with low radiative recombination efficiency, and therefore are not good candidates. However, a paradigm shift is underway with the recent demonstration that it was possible to manipulate the band structure of germanium-based semiconductors to direct bandgap, i.e. transform them into efficient transmitters. This engineering can be achieved either by using external tensors such as silicon nitride or by making alloys with tin (GeSn), or by combining both. This thesis deals with the study of these direct bandgap semiconductors. The goal will be to demonstrate a laser with this new type of materials.

Monolayer molybdenum disulfide (MoS2) is a three-atom-thick direct band gap semiconductor, which has received considerable attention for use as a channel material in atomically thin transistors, photodetectors, excitonic LED’s, and many other potential applications. It is also a mechanically exceptional material with a large stiffness and flexibility, and can withstand very large strains (11%) before rupture. In this dissertation we investigated the mechanics of the stiffness and adhesion forces in atomically thin MoS2 membranes, and how biaxial strains can be used to induce large modulations in the band structure of the material. First, we used chemical vapor deposition (CVD) to grow MoS2 crystals that are highly impermeable to gas, and used a pressure difference across suspended membranes to induce large biaxial strains. We demonstrated the continuous and reversible tuning of the optical band gap of suspended monolayer membranes by as much as 500 meV, and induced strains of as much as 5.6% before rupture. We observed the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) and Raman spectra and found their linear strain tuning rates, then report evidence for the strain tuning of higher level optical transitions. Second, we determined the Young’s modulus and works of separation and adhesion of MoS2 membranes, and found that adhesion hysteresis is an important effect in determining the behavior of our systems. Finally, we investigated the use of atomically thin materials as nanofiltration membranes, by perforating the material with nanopores which selectively permit the transport of smaller molecules while rejecting larger ones. We studied ion transport through nanopores in graphene membranes and demonstrate that in-situ atomic force microscope measurements in liquid are a powerful way to reveal occlusions and contaminants around the pores - work which will aid future researchers in further unveiling the properties of these fascinating systems.

Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films, and strain-mediated magnetoelectric (ME) nanocomposites, have the potential to fulfill a number of functional requirements in actual applications—principally, direct control of magnetization by the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. Pulsed laser deposition (PLD) was used in this study to fabricate diverse epitaxial thin film heterostructures on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were also studied using neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were evaluated and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-30PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for the nanograted PMN-30PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 single crystal substrates. A thickness dependent incommensurate/commensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial thin films were prepared using PLD, whose crystal structures and multiferroic properties were related through the strain. Accordingly, properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future. / Doctor of Philosophy / As a general definition, the class of materials known as multiferroics possess more than one ferroic order parameter. Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films and strain-mediated magnetoelectric (ME) nanocomposites show significant potential for use in next-generation devices due to the fact that one can control magnetic properties via the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. In this study, diverse epitaxial thin film heterostructures were fabricated on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were studied using powerful neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were discussed and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) technique was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for nanograted PMN-PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 substrates. A thickness dependent incommensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial perovskite thin films were prepared, whose crystal structures and multiferroic properties were related through the strain. Accordingly, the properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future.

Owing to the advantages of high power density, fast charge/discharge rates as well as long lifetime, micro-supercapacitor (MSC) has drawn much attention for its potential application in miniaturized electronics. Many efforts have been devoted to the design and fabrication of high-performance MSCs. On the other hand, the integration of MSCs with multiple functional materials and devices has emerged with the development of portable and wearable microelectronics. To date, the biggest challenge in research is to develop a reliable and smart fabrication technology/strategy, which can integrate diverse objective materials into compact devices. Rolled-up nanotechnology is a unique approach to self-assemble 2D nanomembranes into 3D structures by using strain engineering. This self-assembly process smartly combines top-down and bottom-up methods to pattern functional nanomaterials into ordered 3D micro- and nanostructure arrays. One promising advantage of this approach is that such a self-assembled structure can endow micro-devices with functionality and high performance under a limited footprint area. The first part of this thesis focuses on the fabrication of planar interdigital MSCs with thermo-responsive function. Based on this work, the second part involves the research on novel tubular MSC which was fabricated by employing shapeable materials and strain engineering. A polymeric framework consisting of swelling hydrogel and polyimide layers ensures excellent ion transport between electrodes and provides efficient self-protection of the tubular MSC against external compression. Such tubular device also exhibits excellent areal capacitance, and an improved cycling stability compared to that of planar MSCs. The third part introduces the step-by-step experiments towards the fabrication and optimization of inorganic strained layer-based tubular MSC. Al2O3/Ni/Cr/Al2O3 strained nanomembrane is designed and can successfully drive the rolling up of MnO2 electrodes with a high yield under magnetic fields.:Chapter 1. Introduction 1 1.1. General background 1 1.2. Motivation of this work 2 1.2.1. Integration of micro-supercapacitors 2 1.2.2. Thermo-responsible micro-supercapacitors 2 1.2.3. 3D tubular functional micro-supercapacitors 2 1.3. Dissertation structure 3 Chapter 2. Overview of micro-supercapacitors 5 2.1. Introduction to MSCs 5 2.1.1. Capacitor 5 2.1.2. Electric double-layer capacitor 5 2.1.3. Pseudocapacitor 7 2.2. MSC configuration 8 2.3. Fabrication strategies of interdigital MSCs 9 2.4. Fabrication methods of active materials 12 2.5. Functionalization of supercapacitors 15 2.5.1. Tribo/piezoelectric driven self-charging function 15 2.5.2. Solar cell driven self-charging function 16 2.5.3. Electrochromic function 18 2.5.4. Self-healing function 19 2.5.5. Sensing function 20 2.5.6. Stretchable function 21 2.5.7. Thermo-responsive function 22 2.5.8. Photo-switchable function 23 2.6. Conclusion and outlook 23 Chapter 3. Overview of rolled-up technology 27 3.1. 3D self-assembly of the inorganic nanomembrane 27 3.1.1. Introduction 27 3.1.2. Rolled-up nanomembranes for capacitors 28 3.1.2. Rolled-up nanomembranes for Li-ion batteries 30 3.2. 3D self-assembly of the polymeric layers 32 3.2.1. Introduction 32 3.2.2. Self-assembled polymeric layers for microelectronics 35 Chapter 4. Experimental methods 39 4.1. Deposition methods 39 4.1.1. Photolithography 39 4.1.2. Electron beam evaporation 39 4.1.3. Atomic layer deposition 40 4.1.4. Electrochemical deposition 41 4.2. Characterization methods 43 4.2.1. Scanning electron microscopy and focused ion beam milling 43 4.2.2. Electrochemical characterization 43 Chapter 5. An integrated MSC with thermo-responsible function 47 5.1. Introduction 47 5.2. Fabrication and characterization of thermo-responsible MSCs 47 5.2.1. Single thermo-responsible MSCs 47 5.2.2. The array of thermo-responsible MSC 51 5.3. Conclusion 53 Chapter 6. Self-assembly of 3D tubular MSCs 55 6.1. Introduction 55 6.2. Fabrication of tubular MSCs 57 6.2.1. Diagram of processing flow 57 6.2.2. Polymeric layer stack 58 6.2.3. Microelectrodes, self-assembly and capsulation 59 6.3. Results and discussion 60 6.3.1. On-chip and free-standing sample morphology 60 6.3.2. Electrochemical characterization of tubular MSCs 64 6.3.3. Self-protection function of tubular structures 72 6.3.4. Assembly of tubular structures in series/parallel 76 6.4. Conclusion 80 Chapter 7. Tubular nanomembranes for MSCs 81 7.1. Introduction 81 7.2. Self-assembly of Al2O3/Ti/Cr/Al2O3 strained nanomembranes 82 7.2.1. Fabrication method 82 7.2.2. Results and discussion 83 7.3. Self-assembly of Al2O3/Ni/Cr/Al2O3 strained nanomembranes 87 7.3.1. Fabrication method 87 7.3.2. Results and discussion 88 7.4. Conclusion 92 Chapter 8. Summary and outlook 93 8.1. Summary 93 8.2. Outlook 94 Bibliography 95 List of Figures 109 List of Tables 117 Theses 119 Acknowledgment 121 Publications and presentations 123 Curriculum Vita 125

The combination of strain engineering, lithography, thin film deposition and etching techniques is an elegant approach to create single microtubes and well-defined arrays of magnetic microtubes. In this work we have successfully shown that strain engineering techniques developed for rolled-up nanomembranes can be applied to magnetic materials and material combinations. To obtain sufficiently strained nanomembranes, different substrates and sacrificial layers in combination with the magnetic layers were used. Careful tuning of the etching parameters ensured a controlled roll-up process without damage or oxidation of the magnetic layer. Additionally, rolled-up nanomembranes were further integrated in a highly parallel fashion on chip, by development and application of multi-step fabrication procedures. Based on the prepared rolled-up magnetic structures and their planar counterparts we have performed a comprehensive study of their magnetic properties, mainly under the influence of magnetic field, strain and temperature. The role of the special cylindrical or curved geometry and their impact on the magnetic properties was outlined and explained based on our understanding. Moreover, the magnetic properties were also discussed in relationship to other influencing material parameters, e.g. composition, crystallographic structure, and surface effects. The first experimental magnetization study on rolled-up InGaAs/Fe3Si heterostructures was presented. It was demonstrated for tube arrays that the change in the geometry from a planar film to the cylindrical shape has a significant effect on the magnetization behavior. A deeper study provided insight into the magnetic switching behavior of single tubes and arrays. Rolled-up Au/Co/Au tubes and showed that in addition to shape anisotropy, magnetostrictive anisotropy due to the anisotropic stress release can inverse the magnetization direction. Exchange coupling at ferromagnetic/antiferromagnetic interfaces due to partial oxidation of Co was observed at low temperatures. The results suggest possibilities to tune magnetic properties by controlling the tube dimensions and careful control of thin film growth parameters. The cylindrical shape, the layer thickness the number of rotations and the type of magnetic material are proven to have a strong influence on the magnetic domain patterns and magnetization behavior. Therefore, Ni/Fe tubes have been studied by means of magneto optical Kerr effect. It was found that the magnetization reversal in rolled-up tubes with 1.2 and 2.5 windings occurs via nucleation and propagation of magnetic domain walls. On the other hand, we have demonstrated for rolled-up Au/Co tubes that a certain magnetic layer thickness is required to observe magnetic stripe domains. In another experiment performed with magnetic force microscopy, rolled-up Co/Pt nanomembranes with magnetic domains radially aligned due to perpendicular anisotropy, which behaves as radially polarized cylindrical magnets, were achieved. Moreover, we have demonstrated an elegant approach to create compact MR devices based on rolled-up Co/Cu-ML nanomembranes. We have shown the magnetization behavior and the MR magnitude in comparison to the corresponding planar structures. The influence of number of Co/Cu bilayers, non-magnetic spacer layer, interface roughness and multiple windings on MR was discussed. Our fabrication method can be applied to the most common magnetic materials. Certainly, further optimization of MR towards application as magnetic sensor or magneto-fluidic sensors can be achieved by change of Co/Cu-layer thickness, increase of rolling length and reduced spacer layer thickness. Finally, we have shown a fabrication route to realize freestanding tubes based on Ni-Mn-Ga alloys grown by molecular beam epitaxy on GaAs substrates. The evolution of structural and magnetic properties induced by roll-up was investigated in detail and showed a pronounced influence of crystallographic orientation and strain state of the Ni-Mn-Ga alloys. These insights are fundamental in order to realize thin nanomembranes and freestanding three-dimensional FSMA structures with defined composition for smart applications as compact actuators and microsensors. Consequently, rolled-up magnetic nanomembranes offer a great chance in reducing the size of electronic components and can bring several functionalities to the device. These facts make rolled-up tubes highly attractive for the detection, stimulation and manipulation of small objects, such as ions, molecules, cells and particles. It is expected in the future, that magnetic lab-in-a-tube systems will further account in analysis of microfluidic systems. On the other hand, rolled-up structures significantly contribute to the field of shapeable magnetoelectronics.

Le germanium dopé n et contraint en tension est un candidat potentiel pour démontrer un laser sur silicium compatible avec un environnement CMOS. Dans ce travail de thèse, j’ai d’abord développé un formalisme qui permet de calculer le gain optique en fonction de la déformation en tension, du dopage n et de l’injection des porteurs. Une technique de transfert de déformation via le dépôt de couche contrainte de SiN a été optimisée. J’ai réalisé plusieurs types de cavités germanium contraintes sous forme de guides d’onde et de microdisques. Le transfert de déformation a été optimisé par sous-gravure et par une méthode de bi-encapsulation qui a permis d’aboutir à une déformation biaxiale homogène et élevée de l’ordre de 1.5%. L’évaluation des déformations a été confrontée à des simulations par éléments finis, photoluminescence et spectroscopie Raman. L’étude expérimentale et théorique des guides d’onde a montré l’avantage de la direction <100> par rapport à la direction <110> permettant une injection plus efficace de porteurs en centre de zone. L’étude expérimentale des microdisques a permis d’observer des modes de galerie avec un facteur de qualité Q = 1540 à λ = 1940 nm. D’autre part, j’ai mis en évidence par photoluminescence la présence d’un fort dopage de 4×10¹⁹ cm⁻³ dans des couches germanium sur silicium épitaxiées par épitaxie par jets moléculaires utilisant une technique de co-dopage. Une modélisation du gain modal a permis de mettre en exergue l’effet du gradient de déformation dans le volume de la cavité. L’élargissement homogène a été introduit dans la modélisation du gain optique afin de prendre en compte l’impact d’un dopage élevé. / Tensile strained and n-doped germanium is a potential candidate to demonstrate a laser on silicon in a CMOS-compatible environment. In this thesis, I developed a formalism to calculate the optical gain as a function of tensile strain, n-doping and carrier injection. A tensile strain transfer technique via strained SiN layer deposition has been optimized. I realized several types of strained germanium cavities. Tensile strain transfer was optimized by under-etching and a bi-encapsulation technique which allowed to achieve a high and uniform biaxial strain up to 1.5%. The evaluation of strain level was faced with finite elements modeling, photoluminescence and Raman spectroscopy. The experimental and theoretical study of the waveguides showed the advantage of the <100> direction as compared with the <110> direction for more efficient carrier injection at zone center. The experimental study of microdisks allowed us to observe gallery modes with quality factor up to Q = 1540 at λ= 1940 nm. On the other hand, photoluminescence enhancement has shown the presence of a heavy doping of 4×10¹⁹ cm⁻³ in germanium on silicon layers grown by molecular beam epitaxy and using a co-doping technique. Modeling the modal gain helped to emphasize the effect of the strain gradient in the cavity volume. The homogeneous broadening was introduced in the optical gain modeling to take into account the impact of a high doping.

De par ses extraordinaires propriétés physiques, on s'attend à ce que le graphène devienne un matériau de nouvelle génération, susceptible de compléter les semi-conducteurs traditionnels dans les technologies de dispositifs électroniques. Depuis sa découverte expérimentale en 2004, de nombreux travaux ont cherché à en évaluer les potentialités. Toutefois, en vue d'applications en électronique, le graphène souffre d'un inconvénient majeur : l'absence de bande interdite dans sa structure de bandes. Ainsi, il est très difficile de moduler et couper le courant dans un transistor de graphène, ce qui restreint considérablement son champ d'applications. Du point de vue des propriétés thermoélectriques, l'absence de bande interdite empêche la séparation des contributions opposées des électrons et des trous au coefficient Seebeck, qui reste donc faible dans le graphène parfait. Aussi, l'ouverture d'une bande interdite (gap) dans le graphène est une nécessité pour contourner les inconvénients de ce matériau et bénéficier pleinement de ses excellentes propriétés de conduction. Il a été montré que de nombreuses approches de nanostructuration peuvent être utilisées dans ce but : découpage de nanorubans de graphène, bicouche de graphène avec application d'un champ électrique transverse, percement d'un réseau périodique de nano-trous (nanomesh), structures mixtes de graphène et de nitrure de bore, dopage du graphène à l'azote. Cependant, toutes ces approches ont leurs propres difficultés de fabrication et/ou restent encore à confirmer expérimentalement. Dans ce travail, je me suis focalisée sur une autre approche : l'ingénierie de contrainte, qui offre un large éventail de possibilités pour moduler les propriétés électroniques des nanostructures de graphène. Pour ce travail théorique, tous les calculs ont été faits en utilisant essentiellement deux méthodes : un modèle atomistique de Hamiltonien de liaisons fortes pour décrire les propriétés électroniques du matériau et l'approche des fonctions de Green hors-équilibre pour le calcul du transport quantique. Après une introduction du contexte général de ce travail et des techniques de calcul développées dans ce but, j'ai d'abord analysé les effets de contrainte. En fait, une contrainte d'amplitude supérieure à 23% est nécessaire pour ouvrir un gap dans la structure de bande du graphène. Mais je montre qu'avec une contrainte de quelques pourcents, le décalage du point de dirac induit par la contrainte peut suffire à ouvrir un gap de conduction très significatif (500 meV ou plus) dans des hétérostructures de graphène constituées de jonctions graphène contraint/graphène non contraint, alors que chacun des matériaux reste semi-métallique. Après l'analyse détaillée de cette propriété en fonction de l'amplitude de la contrainte, de sa direction et de la direction du transport, j'exploite cet effet dans des jonctions appropriées pour améliore le comportement et les performances de différents types de dispositifs. En particulier, je montre qu'avec une contrainte de seulement 5% il est possible de couper efficacement le courant dans les transistors, de sorte que le rapport ON/OFF peut atteindre 100000, ce qui constitue une très forte amélioration par rapport aux transistors de graphène pristine où ce rapport ne peut pas excéder 10. Puis, nous montrons qu'en combinant ingénieries de contrainte et de dopage dans de telles jonctions, le coefficient Seebeck peut atteindre des valeurs aussi fortes que 1.4 mV/K, ce qui est 17 fois plus élevé que dans le graphène sans gap. Cela peut contribuer à faire du graphène un excellent matériau thermoélectrique. Enfin, j'ai étudié l'effet de conductance différentielle négative (CDE) dans des diodes de graphène, constituées soit d'une simple-barrière contrainte contrôlée par une grille, soit d'une jonction PN. Je montre qu'une ingénierie de contrainte appropriée peut induire de forts effets de CDE, avec un rapport pic/vallée de quelques centaines à température ambiante. / Due to its outstanding physical properties, graphene is expected to become a new generation material, able to replace or complement traditional semiconductors in device technology. Hence, many studies have been led to explore the potential of this material immediately after the successful fabrication of a single layer of graphene in 2004. However, applications of graphene in electronic devices are still questionable due to the gapless character of this material. In particular, regarding electronic applications, the absence of energy bandgap in the band structure makes it difficult to switch off the current in graphene devices like transistors. Regarding thermoelectric properties, the gapless character is also a strong drawback since it prevents the separation of the opposite contributions of electrons and holes to the Seebeck coefficient. Thus, a sizable band gap in graphene is a requirement to overcome the disadvantages of graphene and to fully benefit from its excellent conduction properties. It has been shown that many Nano structuring techniques can be used to open such a bandgap in graphene, e.g., graphene nanoribbons, graphene bilayer with a perpendicular electric field, graphene nanotech lattices, channels based on vertical stack of graphene layers, mixed graphene/hexagonal boron nitride structures, nitrogen doped graphene, and so on. However, each of these methods has its own fabrication issues and/or need to be further confirmed by experiments. In this work, we focus on strain engineering, which offers a wide range of opportunities for modulating the electronic properties of graphene nanostructures. For this theoretical work, all calculations were performed using essentially two main methods, i.e., an atomistic tight binding Hamiltonian model to describe the electronic structure and the non-equilibrium Green's function approach of quantum transport. The main aim is to analyze in details the strain effects in graphene and to provide strategies of strain engineering to improve the performance of both electronic (transistors and diodes) and thermoelectric devices. After introducing the general context if this work and the numerical techniques developed for this purpose, we first analyze the only effect of strain. Actually, if uniformly applied, a strain of large amplitude (> 23%) is required to open a bandgap in the band structure of graphene. However, we show that with a strain of only a few percent, the strain-induced shift of the Dirac point in k-space may be enough to open a sizable conduction gap (500 meV or more) in graphene heterojunctions made of unstrained/strained junctions, though the strained material remains gapless. After analyzing in details this property according the amplitude and direction of strain and the direction of transport, we exploit this effect using appropriate strain junctions to improve the behavior and performance of several types of devices. In particular, we show that with a strain of only 5%, it is possible to switch-off transistors efficiently, so that the ON/OFF current ratio can reach 100000, which is a strong improvement with respect to pristine graphene transistors where this ratio cannot exceed 10. Then we show that by combining strain and doping engineering in such strain junctions the Seebeck coefficient can reach values higher than 1.4 mV/K, which is 17 times higher than in gapless pristine graphene. It can contribute to make graphene an excellent thermoelectric material. Finally, we study the effect of negative differential conductance (NDC) in graphene diodes made of either as single gate-induced strained barrier or a p-n junction. We show that appropriate strain engineering in these devices can lead to very strong NDC effects with peak-to-valley ratios of a few hundred at room temperature.

In this thesis, we focus on the generation of entangled photon pair from III-V quantum dots. The achievements mainly consists of two aspects: one is the wavelength tunability of these entangled photon pairs, which is enabled by on-chip strain engineering; the other is the brightness enhancement with an optical-broadband antenna to boost the extraction of entangled photons from the material matrix where quantum dots are embedded in.

<p>  Non-model microorganisms offer a wealth of biotechnological potential that may be leveraged to address a variety of global grand challenges. These include challenges in carrying out complex or altogether new chemistries, discovery and production of bioactive molecules, sustainable production of biochemicals and bioproducts from renewable feedstocks, and improving agricultural practices for responsible management of carbon. Specifically, using renewable plant biomass as a substrate for production of fuels and or chemicals offers a near ubiquitous supply that does not compete with food or petrochemicals. Alternatively, identifying new natural products will be essential to addressing the ever-increasing occurrence of antibiotic resistance. Non-model organisms may provide elegant solutions to many of these challenges, whether by possessing new or more efficient strategies to depolymerize lignocellulose, by encoding enzymes with increased stabilities and or specific activities, or perhaps by containing rich biosynthetic capabilities for production of previously unidentified natural products, among others. Yet efforts to leverage non-model microorganisms for their diverse biotechnological potential remain limited to a variety of often difficult, yet not insurmountable challenges.</p> <p>     In this work, I propose anaerobic gut fungi (Neocallimastigomycota) as a robust microbial system that may be leveraged to efficiently depolymerize crude lignocellulose, increase animal nutrition, or identify novel natural products. To this end, I detail the first chromosomally resolved genome assembly of anaerobic fungi (<em>Piromyces communis </em>var. <em>indianae</em> UH3-1). I investigate the genome organization of this isolate and describe how acquisition of Carbohydrate Active EnZymes (CAZymes) contribute to the robust lignocellulolytic activity of gut fungi. I then detail efforts to build a nascent genetic engineering toolbox for these anaerobic organisms. With the acquisition of the first chromosomally resolved genome assemblies, I identify a basic set of genetic parts needed for a genetic engineering toolkit. I show these parts are functional and detail methods to enable higher throughput testing in vivo. I subsequently detail efforts to construct the first preliminary CRISPR tools for anaerobic fungi as these will be essential to establish precise DNA targeting in future strain engineering efforts. I then describe the role of epigenetics in anaerobic fungi, detailing the extent to which it may be leveraged to control gene expression. Finally, I provide a discussion of this work and describe how it may guide future efforts to domesticate these organisms. Collectively, this work provides the first chromosomally resolved genome assembly as a resource for the community, along with genetic tools and techniques to begin domesticating these non-model organisms. Importantly, this work reveals that despite the challenges associated with anaerobic microbes of relatively high complexity, they are not insurmountable, and thus efforts to domesticate them are feasible.</p>