abstract: Fluctuation Electron Microscopy (FEM) has become an effective materials' structure characterization technique, capable of probing medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy between the computed kinematical variance and the experimental variance, which previously was attributed to source incoherence. Although high-brightness, high coherence, electron guns are now routinely available in modern electron microscopes, they have not eliminated this discrepancy between theory and experiment. The main objective of this thesis was to explore, and to reveal, the reasons behind this conundrum. The study was started with an analysis of the speckle statistics of tilted dark-field TEM images obtained from an amorphous carbon sample, which confirmed that the structural ordering is sensitively detected by FEM. This analysis also revealed the inconsistency between predictions of the source incoherence model and the experimentally observed variance. FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples was carried out in an attempt to explore the conundrum. Electron probe and sample parameters were varied to observe the scattering intensity variance behavior. Results were compared to models of probe incoherence, diffuse scattering, atom displacement damage, energy loss events and multiple scattering. Models of displacement decoherence matched the experimental results best. Decoherence was also explored by an interferometric diffraction method using bilayer amorphous samples, and results are consistent with strong displacement decoherence in addition to temporal decoherence arising from the electron source energy spread and energy loss events in thick samples. It is clear that decoherence plays an important role in the long-standing discrepancy between experimental FEM and its theoretical predictions. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

abstract: In mesoscopic physics, conductance fluctuations are a quantum interference phenomenon that comes from the phase interference of electron wave functions scattered by the impurity disorder. During the past few decades, conductance fluctuations have been studied in various materials including metals, semiconductors and graphene. Since the patterns of conductance fluctuations is related to the distributions and configurations of the impurity scatterers, each sample has its unique pattern of fluctuations, which is considered as a sample fingerprint. Thus, research on conductance fluctuations attracts attention worldwide for its importance in both fundamental physics and potential technical applications. Since early experimental measurements of conductance fluctuations showed that the amplitudes of the fluctuations are on order of a universal value (e2/h), theorists proposed the hypothesis of ergodicity, e.g. the amplitudes of the conductance fluctuations by varying impurity configurations is the same as that from varying the Fermi energy or varying the magnetic field. They also proposed the principle of universality; e.g., that the observed fluctuations would appear the same in all materials. Recently, transport experiments in graphene reveal a deviation of fluctuation amplitudes from those expected from ergodicity. Thus, in my thesis work, I have carried out numerical research on the conductance fluctuations in GaAs nanowires and graphene nanoribbons in order to examine whether or not the theoretical principles of universality and ergodicity hold. Finite difference methods are employed to study the conductance fluctuations in GaAs nanowires, but an atomic basis tight-binding model is used in calculations of graphene nanoribbons. Both short-range disorder and long-range disorder are considered in the simulations of graphene. A stabilized recursive scattering matrix technique is used to calculate the conductance. In particular, the dependence of the observed fluctuations on the amplitude of the disorder has been investigated. Finally, the root-mean-square values of the amplitude of conductance fluctuations are calculated as a basis with which to draw the appropriate conclusions. The results for Fermi energy sweeps and magnetic field sweeps are compared and effects of magnetic fields on the conductance fluctuations of Fermi energy sweeps are discussed for both GaAs nanowires and graphene nanoribbons. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

abstract: A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin polarization and interface geometry is presented. The 3D model gives the same intrinsic spin polarization and superconducting gap dependence as the 1D model. This demonstrates that the 1D model can be use to t 3D data. Using this model, a Heusler alloy is investigated. Andreev reflection measurements show that the spin polarization is 80% in samples sputtered on unheated MgO(100) substrates and annealed at high temperatures. However, the spin polarization is considerably smaller in samples deposited on heated substrates. Ferromagnetic FexSi􀀀x alloys have been proposed as potential spin injectors into silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35 alloys. The amorphous phase has a significantly higher spin polarization than that of the crystalline phase. In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3. Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors can be used in ARS. Based on the behavior of the differential conductance for highly spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc is not a triplet superconductor. Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been utilized as a characteristic feature to reveal many novel physics. Complexities at a normal metal/superconducting interface often cause nonessential ZBA-like features, which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA, which is due to the contact resistance, cannot be suppressed by a highly spin-polarized current while a nonessential ZBA cannot be affected the contact resistance. Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only do the GMR structures have a large enhancement of resistance, but by applying an external magnetic eld it is shown that, unlike most materials, the spin polarization can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

abstract: Multifunctional oxide thin-films grown on silicon and several oxide substrates have been characterized using High Resolution (Scanning) Transmission Electron Microscopy (HRTEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Electron Energy-Loss Spectroscopy (EELS). Oxide thin films grown on SrTiO3/Si pseudo-substrate showed the presence of amorphised SrTiO3 (STO) at the STO/Si interface. Oxide/oxide interfaces were observed to be atomically clean with very few defects. Al-doped SrTiO3 thin films grown on Si were of high crystalline quality. The Ti/O ratio estimated from EELS line scans revealed that substitution of Ti by Al created associated O vacancies. The strength of the crystal field in STO was measured using EELS, and decreased by ~1.0 eV as Ti4+ was substituted by Al3+. The damping of O-K EELS peaks confirmed the rise in oxygen vacancies. For Co-substituted STO films grown on Si, the EDS and EELS spectra across samples showed Co doping was quite random. The substitution of Ti4+ with Co3+ or Co2+ created associated oxygen vacancies for charge balance. Presence of oxygen vacancies was also confirmed by shift of Ti-L EELS peaks towards lower energy by ~0.4 eV. The crystal-field strength decreased by ~0.6 eV as Ti4+ was partially substituted by Co3+ or Co2+. Spinel Co3O4 thin films grown on MgAl2O4 (110) were observed to have excellent crystalline quality. The structure of the Co3O4/MgAl2O4 interface was determined using HRTEM and image simulations. It was found that MgAl2O4 substrate is terminated with Al and oxygen. Stacking faults and associated strain fields in spinel Co3O4 were found along [111], [001], and [113] using Geometrical Phase Analysis. NbO2 films on STO (111) were observed to be tetragonal with lattice parameter of 13.8 Å and NbO films on LSAT (111) were observed to be cubic with lattice parameter of 4.26 Å. HRTEM showed formation of high quality NbOx films and excellent coherent interface. HRTEM of SrAl4 on LAO (001) confirmed an island growth mode. The SrAl4 islands were highly crystalline with excellent epitaxial registry with LAO. By comparing HRTEM images with image simulations, the interface structure was determined to consist of Sr-terminated SrAl4 (001) on AlO2-terminated LAO (001). / Dissertation/Thesis / Doctoral Dissertation Physics 2015

abstract: Conductance fluctuations associated with quantum transport through quantumdot systems are currently understood to depend on the nature of the corresponding classical dynamics, i.e., integrable or chaotic. There are a couple of interesting phenomena about conductance fluctuation and quantum tunneling related to geometrical shapes of graphene systems. Firstly, in graphene quantum-dot systems, when a magnetic field is present, as the Fermi energy or the magnetic flux is varied, both regular oscillations and random fluctuations in the conductance can occur, with alternating transitions between the two. Secondly, a scheme based on geometrical rotation of rectangular devices to effectively modulate the conductance fluctuations is presented. Thirdly, when graphene is placed on a substrate of heavy metal, Rashba spin-orbit interaction of substantial strength can occur. In an open system such as a quantum dot, the interaction can induce spin polarization. Finally, a problem using graphene systems with electron-electron interactions described by the Hubbard Hamiltonian in the setting of resonant tunneling is investigated. Another interesting problem in quantum transport is the effect of disorder or random impurities since it is inevitable in real experiments. At first, for a twodimensional Dirac ring, as the disorder density is systematically increased, the persistent current decreases slowly initially and then plateaus at a finite nonzero value, indicating remarkable robustness of the persistent currents, which cannot be discovered in normal metal and semiconductor rings. In addition, in a Floquet system with a ribbon structure, the conductance can be remarkably enhanced by onsite disorder. Recent years have witnessed significant interest in nanoscale physical systems, such as semiconductor supperlattices and optomechanical systems, which can exhibit distinct collective dynamical behaviors. Firstly, a system of two optically coupled optomechanical cavities is considered and the phenomenon of synchronization transition associated with quantum entanglement transition is discovered. Another useful issue is nonlinear dynamics in semiconductor superlattices caused by its key potential application lies in generating radiation sources, amplifiers and detectors in the spectral range of terahertz. In such a system, transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

abstract: In recent years, there has been increased interest in the Indium Gallium Nitride (InGaN) material system for photovoltaic (PV) applications. The InGaN alloy system has demonstrated high performance for high frequency power devices, as well as for optical light emitters. This material system is also promising for photovoltaic applications due to broad range of bandgaps of InxGa1-xN alloys from 0.65 eV (InN) to 3.42 eV (GaN), which covers most of the electromagnetic spectrum from ultraviolet to infrared wavelengths. InGaN’s high absorption coefficient, radiation resistance and thermal stability (operating with temperature > 450 ℃) makes it a suitable PV candidate for hybrid concentrating solar thermal systems as well as other high temperature applications. This work proposed a high efficiency InGaN-based 2J tandem cell for high temperature (450 ℃) and concentration (200 X) hybrid concentrated solar thermal (CSP) application via numerical simulation. In order to address the polarization and band-offset issues for GaN/InGaN hetero-solar cells, band-engineering techniques are adopted and a simple interlayer is proposed at the hetero-interface rather than an Indium composition grading layer which is not practical in fabrication. The base absorber thickness and doping has been optimized for 1J cell performance and current matching has been achieved for 2J tandem cell design. The simulations also suggest that the issue of crystalline quality (i.e. short SRH lifetime) of the nitride material system to date is a crucial factor limiting the performance of the designed 2J cell at high temperature. Three pathways to achieve ~25% efficiency have been proposed under 450 ℃ and 200 X. An anti-reflection coating (ARC) for the InGaN solar cell optical management has been designed. Finally, effective mobility model for quantum well solar cells has been developed for efficient quasi-bulk simulation. / Dissertation/Thesis / Doctoral Dissertation Physics 2017

Multislice HAADF - STEM image simulations of SrTiO 3 are performed at 300 K.The procedure of these simulations and the used techniques are briefly ex-plained and reasoned. The results are presented and discussed in a conciseway and in an attached paper a comparison to experimental images is made.The paper proofs that the electron optical setup developed in Dresden is indeed capable of producing atomic-sized EVBs, a precondition for measuring EMCD with atomic resolution.

Cette thèse expérimentale explore les propriétés supraconductrices du silicium très fortement dopé, en particulier au bore, ainsi que les propriétés physiques anormale observées à plus hautes températures. La supraconductivité de Si:B est obtenue sous 1K, pour des dopages en bore supérieurs à la limite de solubilité du bore dans le silicium. Le Si:B est métallique à ces taux de dopage. Dans une première partie, nous exposons les différentes techniques expérimentales exploitées au cours de cette thèse. Nous expliquons les différentes techniques de dopage hors équilibre identifiées pour doper du silicium au-delà de la limite de solubilité, puis les techniques de caractérisation pour contrôler la qualité des couches dopées obtenues, ainsi que les méthodes de mesures aux très basses températures. Dans une deuxième partie, nous exposons les résultats obtenus sur la supraconductivité de Si:B en faisant varier dans un premier temps le taux de dopage en bore, puis en renouvelant l'étude à différentes épaisseurs de couche dopée. Nous montrons notamment que l'évolution de la Tc avec le couplage électron-phonon $lambda$ ne suit pas une loi de McMillan classique, mais plutôt une loi de puissance comme celle observée dans le cas du diamant supraconducteur. Nous montrons que ce résultat peut être expliqué dans le cadre d'un modèle d'un supraconducteur à deux couches de $lambda$ différents. En étudiant la dépendance en température et angulaire de Hc2, nous montrons que Si:B est un supraconducteur intrinsèquement de type I, mais qui devient de type II sous effet d'impuretés, et que la supraconductivité est à caractère bidimensionnel. Dans une troisième partie, nous présentons des comportements anormaux de certaines caractéristiques physiques mesurées dans certaines séries de Si:B, à partir de 50K et qui persistent jusqu'à au moins 400K. Nous présentons des mesures de magnétotransport, d'effet Hall et de mesures thermoélectriques qui présentent toutes des caractéristiques hautement non linéaires, et donc anormales pour un métal. L'origine de ces anomalies est toujours ouverte. Enfin, nous présentons quelques perspectives de travail, en particulier les premières mesures sur un échantillon avec une géométrie de type SQUID. / This experimental PhD thesis explore the superconductivity of heavily boron doped epilayers as well as some unusual properties observed at high temperatures. The superconductivity of Si:B is observed below 1K and triggered by boron content exceeding the solubility limit of boron into silicon. For such high boron contents, the silicon layers are metallic. In a first part, we develop the various experimental techniques used. We explain the principles of the out-of-equilibrium doping techniques required to doped beyond the solubility limit. We develop also on the characterisation techniques used to control the quality of the samples, as well as the low temperatures measurement techniques. In a second part, we show the results obtained on the superconductivity of Si:B, obtained forst by varying the boron content at a given layer thickness, and then as a function of the layer thickness. We show that the evolution of Tc with the electron-phonon coupling constant lambda doesn't follow the classical McMillan law, but rather a power of law as it was reported for superconducting diamond. We show that this result can be explained by a double layer model with dislocations resulting in two different lambda values for each sublayer. The study of the temperature and angular dependency of the Hc2 also show that Si:B is an intrinsic type I superconductor turned into type II with defect effects, and that the superconductivity is bidimensionnal. In a third part, we present the anomalous high temperature behaviour of some Si:B epilayers, starting from 50K and observed at least up to 400K. We present magnetotransport, Hall effect and thermoelectric measurements that all show a highly non linear behaviour, unusual for a metal. The origin of these anomalies is still an open question. We finally present some future perspectives, including the first measurements on a Si:B SQUID-like geometry.

Molybdenum disulfide (MoS2) has been demonstrated to be a potential catalyst in the hydrogen evolution reaction (HER). Due to its highly active edge site, abundance, and low cost, it rivals Pt. However, the potential activity of the MoS2 basal plane has largely been ignored. The physical characteristics of MoS2 and its corresponding band structure change significantly with decreasing thickness, especially at the monolayer limit. Thus, an investigation on the thickness dependence may provide important insights into the MoS2 basal plane activity. In this thesis, the layer dependent electrocatalytic performance is investigated with mono-, bi- and multilayer MoS2 based individual nanodevices. Three conclusions were reached. (1) Monolayers showed exchange current densities more than one order of magnitude higher than that of the multilayers, 0.12 mA/cm2 and 8.7 mA/cm2, respectively. Furthermore, the onset potential of the monolayer was several hundred millivolts lower than that of the multilayer, about 0.2 V vs RHE for the monolayer versus 0.5 V vs RHE for the multilayer. The Tafel slope of 100-200 mV/dec revealed that the rate limiting step was the adsorption of hydrogen. (2) Interestingly, the bilayer sample exhibited an increase in its exchange current density from 0.3 mA/cm2 to 8 mA/cm2 when cycled extensively. This is suspected to be caused by intercalation of hydrogen between the atomic layers. (3) Additionally, the back-gate voltage is applied to tune the Fermi level of the material and the catalytic performance. It was found that the back-gate voltage induces an irreversible change in all samples, increasing the exchange current density by an order of magnitude. The superior basal plane performance of the monolayers to that of the multilayers reveals a new way to optimize the performance of MoS2 as a HER catalyst. In addition, the results above illuminate the yellow brick road to potential improvements in other layered materials as well.

abstract: Layer structured two dimensional (2D) semiconductors have gained much interest due to their intriguing optical and electronic properties induced by the unique van der Waals bonding between layers. The extraordinary success for graphene and transition metal dichalcogenides (TMDCs) has triggered a constant search for novel 2D semiconductors beyond them. Gallium chalcogenides, belonging to the group III-VI compounds, are a new class of 2D semiconductors that carry a variety of interesting properties including wide spectrum coverage of their bandgaps and thus are promising candidates for next generation electronic and optoelectronic devices. Pushing these materials toward applications requires more controllable synthesis methods and facile routes for engineering their properties on demand. In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition. Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018