This thesis reports a passive method for Fermi level regulation in quantum dot assemblies through ground state transfer between QDs. Here, ZnTe/CdS, and PbSe/CdSe core/shell QDs were used as valence band electron donors, while Cu containing CdS or ZnSe acts as electron acceptor QDs. Prior to study of ground state charge transfer process, this report discusses the synthesis of ZnTe/CdS, and PbSe/CdSe core shell QDs, which are later used to study charge transfer. Since ZnTe QDs are unstable and prone to oxidation, a CdS coated ZnTe QDs were used. Growing a CdS shell on ZnTe core is difficult because high reduction potential of Te. To overcome this problem, partially reduced sulphur is used for the synthesis of ZnTe/CdS. The peculiar optical properties exhibited by ZnTe/CdS also have been discussed. Even though the synthesis of Lead chalcogenide nanoparticles has been investigated previously, certain inconsistencies between the behavior expected from known mechanisms and empirical observations. An anion exchange mechanism is proposed and demonstrated to be involved in PbSe formation. Both ZnTe and PbSe based QDs are extensively used to study hole injection and copper containing QDs were used as acceptors. The charge transfer has been studied using optical spectroscopy. The structure and composition of the assemblies was identified using powder crystallography, electron-microscopy and composition analysis. The unique physical and chemical properties of these materials are exciting both fundamentally as well as from the point of view of applications.

Nós calculamos o efeito da interação elétron-fonons longitudinais óticos (LO) sobre a energia de transição ls &#8594 2p+ entre os níveis de uma impureza doadora, localizada em um poço quântico de GaAs-AlxGa1-xAs. Nossos resultados para a energia de transição em função do campo magnético aplicado mostram claramente, que a saturação da energia de transição (efeito pinning) ocorre na energia dos fônons LO, em boa concordância com recentes dados experimentais. Obtemos também a massa de cíclotron de polarons confinados em fios quânticos quase-unidimensionais, com potencial de confinamento parabólico. Observamos que o comportamento da massa é diferente daquele para sistemas bi-dimensionais e que esta diferença é maior quanto maior o potencial de confinamento. Para a heterojunção de GaAs-AlGaAs e GaAs-GaSb, investigamos a importância da interação elétron-fonons interfaciais sobre a massa de cíclotron. Verificamos que a contribuição dos fonons interfaciais é fundamental nas regiões próximas às resonâncias, onde domina o espectro. Finalmente, calculamos a energia de ligação de uma impureza hidrogenóide, localizada no centro de um ponto quântico circular de GaAs-AlGaAs. Na ausência de campo magnético aplicado, obtivemos uma expressão analítica para a função de onda do elétron ligado. Notamos, que a influência do campo magnético sobre a energia de ligação é fraca nas regiões de pequenos raios, devido ao forte potencial de confinamento. / We calculate the effects of the electron-longitudinal optical (LO) phonons interaction on the intra donor ls &#8594 2p+ transition energy in GaAs-AlGaAs quantum wells structures. Our results to the transition energy as a function of the magnetic Field strength, show that the pinning effect occur in the phonon LO energy in good agreement with recent experimental data. The cyclotron mass of polarons confined in quasi.one.dimensional quantum-well wires with parabolic confinement potential, is also obtained. The behavior of electrons effective mass with magnetic field is different, of the two-dimensional systems, and the difference increase when the confinement potential increase. To heterojunctions of GaAs-AlAs and GaAs-GaSb, we investigate the electroninterfacials optical (IO) phonons interactions on the effective cyclotron mass. We find that the electron-IO-phonons interaction is fundamental near the resonances, where they dominate the spectra. Finally, the ground state binding energy of donor impurity, placed in the center of a circular quantum dot is calculated. Without magnetic field, we obtained the analytic expression to the bound electron wave function. The influence of the magnetic field on the donor binding energy is weaker, when the radius of the quantum dot became smaller.

Mankind has come a long way since the invention of wheel to accessing information in the quintillionth of a second. At the heart of every invention ever made, there has been only one objective, to ease the way of living. The progeny of this philosophy automatically came to be known as technology. It was technology that led to the design of the wheel for fast human transportation and the same motivation let him design more sophisticated machines. In mankind’s journey to improve technology, it began to learn efficient or correct ways to utilize and understand resources around it, creating a whole new philosophy called science. Ingeniously, it was science that let humans understand what they were made of: matter, to discovering what matter itself was composed of: atoms and what puts these together: forces. Science and technology has been of tremendous comfort for mankind and has helped it evolve throughout history. However, it is not always that science and technology go hand in hand. Technology has always helped man design devices and instruments which often bring physical comfort. Science on the other hand has made sure that loss in manual labor is compensated by increased inquisitiveness. There were times when technology was more developed than science. This was the time when machines were taking mankind by fire, resulting in the first and second industrial revolutions. During that same time, science was develop-ing slowly by increasing human curiosity to learn the way nature functioned at finer details. This led to the discovery of the electron by Joseph John Thomson, who proved the electron to be a negatively charged particle. Consequently, he was awarded the 1906 Nobel Prize in Physics for his work on electricity conduction in gases. Later, his son, George Paget Thomson, counter-proved that electrons are actually waves. He was also awarded the 1937 Nobel Prize in Physics, along with Clinton Joseph Davisson for their discovery of electron diffraction caused by crystals. Despite the ambiguity, mankind today accepts electrons to have dual properties. It is both a wave and a particle. This duality is not limited to electrons but is applicable to all matter, as proposed by Louis de Broglie and is one of the fundamental principles in science. With the help of well-developed technology, mankind can now design machines that allow controlled flow of electrons establishing the world of electronics, allowing faster human communication. The study of electronic properties and its usage in designing efficient devices is what electronics is all about. Electrons are the protagonist of mankind today. The presence of electrons is unanimously accepted by everyone. All physical and chemical processes are a result of electrons getting transported. Electron transfer processes are ubiquitous in nature, be it in photosynthesis or energy production in mitochondria . It is the fundamental process in all chemical reactions and all physical processes related to electricity. Every piece of hi-tech gadget practically uses the electron, and the whole of humanity is being serviced by it. In fact, a life without utilizing the electrons is abysmally mundane. Electronics has evolved from designing the first millimeter sized point contact transistor to silicon chip processors that contain billions of nanosized transistors. Studying electron transport has also led to the discovery of light emission during conduction popularly known as LED, an abbreviation for light emitting diode. Heating up of devices during electron transport forced mankind to study heat transport and design materials that have highly efficient electron transfer processes. Electron transfer is also the basic principle behind the Scanning Tunneling Microscope (STM), Scanning Electron Microscope (SEM) and the Transmission Electron Microscope (TEM) which replaced the conventional idea of using light (photons) as a source to observe matter at the nanolevel. However, mankind is still in the process of developing a technology which exploits both properties of the electron simultaneously. Today, science and technology work together to overcome this barrier. Indeed, science and technology today have come as far as controlling electron transport up to a single atomic level where quantum effects (discretization and interference of states that make up the system) are very pronounced. This branch can be referred to as quantum electronics or quantronics. It is one of the possible alternatives to conventional silicon based electronics, and is made of three separate fields. The first one that exploits the quantum nature of electron transport in nanoscopic systems, is usually called molecular electronics or moletronics. The second involves ex-ploiting the spin of the electron and is termed as spintronics. The third is the most challenging where neither science nor technology has been able to fully grasp the characteristics, i.e utilizing the heat quanta in designing thermal de-vices at the single atomic level. In general, for ultimate exploitation of both the wave and particle characteristics of the electron, a proper comprehension of the quantum effects during electron transport is necessary to design a quantronic device. Also, in any quantronic device, apart from quantum effects, fluctuations in temperature cause changes in the flow of electrons. Since electron flow is a random process, fluctuations need to be analyzed from a statistical point of view. Moreover, to address issues related to efficiency and power of these quantronic devices, a proper understanding of the thermodynamic aspects is required. The aim of the work in the thesis is to theoretically analyze the fluctuations, quantum effects and thermodynamics, that in principle, affect the basic physics and chemistry during electron and heat transport in a specific class of out of equilibrium quantum systems. This class of quantum systems are prototypes for designing quantronic devices, where both wave and particle nature of the electrons are pronounced. These are called molecular junctions or quantum junctions. It will in turn help the field of quantronics in the long run. However, in this thesis, it is the science that I address and not the technological aspects.

Two dimensional (2D) systems with low carrier density is an outstanding platform for studying a wide spectrum of physics. These include both classical and quantum effects, arising from disorder, Coulomb interactions and even non-trivial topological properties of band-structure. In this thesis, we have explored the physics at low carrier number density in GaAs/AlGaAs heterostructure and bilayer graphene, by investigating in a larger phase space using a variety of electrical measurement tools. A two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure offers an avenue to build a variety of mesoscopic devices, primarily because its surface gates can very effectively control its carrier density profile. In the first half of the thesis, we study the relevance of disorder in two kinds of devices made in a 2DES. A very strong negative gate voltage not only reduces the carrier density of the 2DES, but also drives it to a disordered state. In this state, we explore a new direction in parameter space by increasing in-plane electric field and investigating its magneto-resistance (MR). At sufficiently strong gate voltage and source-drain bias, we discover a remarkably linear MR. Its enormous magnitude and weak temperature dependence indicate that this is a classical effect of disorder. In another study, we examine a specially designed dual-gated device that can induce low number density in a periodic pattern. By applying appropriate gate voltages, we demonstrate the formation of an electrostatically tunable quantum dot lattice and study the impact of disorder on it. This work is important in paving way for solid state based platform for experimental simulations of artificial solids. The most striking property of bilayer graphene is the ability to open its band gap by a perpendicular electric field, giving the prospects of enabling a large set of de-vice applications. However, despite a band gap, a number of transport mechanisms are still active at very low densities that range from hopping transport through bulk to topologically protected 1D transport at the edges or along 1D crystal dislocations. In the second half of the thesis, we have used higher order statistical moment of resistance/conductance fluctuations, namely the variance of the fluctuations, to complement averaged resistance/conductance, and study and infer the dominant transport mechanism at low densities in a gapped bilayer graphene. Our results show possible evidence of percolative transport and topologically protected edge transport at different ranges of low number densities. We also explore the same phase space by studying its mesoscopic conductance fluctuations at very low temperatures. This is the first of its kind systematic experiment in a dual-gated bilayer graphene device. Its conductance fluctuations have several anomalous features suggesting non-universal behaviour which is at odds with conventional disordered systems.

Les boîtes quantiques sont des sources de photons uniques prometteuses pour les réseaux d’information quantique, qui peuvent être intégrées dans des circuits photoniques et s’appuyer sur des technologies de semi-conducteur éprouvées. Dans ce contexte, ce travail se concentre sur les propriétés d’indiscernabilité des photons émis par une boîte quantique semiconductrice sous excitation résonnante. Nous utilisons une configuration particulière où les boîtes sont insérées dans une microcavité planaire permettant de s’affranchir du fond de diffusion parasite du laser d’excitation et d’améliorer la collection du signal d’émission. Nous pouvons ainsi explorer un régime de très basse puissance, où les photons d’excitation sont diffusés élastiquement sur la transition fondamentale de la boîte quantique (régime de diffusion Rayleigh résonnante). Dans ce régime, la cohérence du laser d’excitation est transmise aux photons émis, faisant des boîtes quantiques une source de photons uniques avec une cohérence extrêmement longue.Les propriétés d’indiscernabilité sont étudiées en utilisant les interférences à deux photons (coalescence) dans un interféromètre de Hong–Ou–Mandel. Une étude expérimentale complète de l’indiscernabilité est présentée en fonction de la puissance d’excitation ainsi que du temps de cohérence du laser d’excitation. Elle montre en particulier l’effet de la diffusion élastique dans la limite de basse puissance d’excitation. Il apparaît qu’une nouvelle caractéristique quantitative doit être introduite afin d’estimer l’indiscernabilité en tant que phénomène temporel, un aspect particulièrement important lorsque les émetteurs sont des sources continues de photons. / Quantum dots are good candidates as single photon emitters for quantum information networks, facilitating their integration in photonic circuits based on well known semiconductor technology. In this context, this work focuses on the indistinguishability of the photons emitted by semiconductor quantum dots excited resonantly. We use a peculiar configuration where the quantumdots are embedded in a planar microcavity, allowing for better excitation and collection efficiencies. We are then able to investigate very low excitation power regimes, where the photons are elastically scattered by the fundamental transition of the quantum dot (Resonant Rayleigh Scattering). In this regime, the coherence of the excitation laser is imprinted on the emitted photons, making the quantum dot a source of single photons with a very long coherence.The indistinguishability is investigated by using a Hong–Ou–Mandel interferometer to perform two-photon interference. We carry out a comprehensive experimental study of the excitation power dependence of the indistinguishability as well as its dependence on the excitation laser coherence, which shows the important role of elastic scattering in the low excitation power limit. It appears that a new figure of merit needs to be introduced to assess the indistinguishability as a temporal phenomenon, an aspect which is particularly relevant when dealing with continuous-wave excitation.

Cette thèse décrit l'état de l'art des sondes et nanoparticules fluorescents traditionnels utilisés en imagerie de fluorescence ainsi que le développement de nouveaux nanomatériaux à base de polymère à empreinte moléculaire, aussi dénommé ‘anticorps plastique’, pour le ciblage et la bioimagerie. En biologie et en médecine, il y a un besoin constant de diagnostiquer diverses maladies pour leur éventuel traitement et prévention. Une distribution anormale et un taux élévé de glycosylation (e.g. acides hyaluronique et sialique) à la surface ou dans les cellules sont indicateurs d’une infection ou d’un cancer. Généralement, l’imagerie par fluorescence permet de visualiser, localiser et quantifier les biomarqueurs de pathologie mais à l’heure actuelle, il n’existe pas d’outil analytique fiable pour cibler spécifiquement les molécules de glycosylation car les anticorps et les lectines vendus dans le commerce ont une faible affinité et sélectivité vis-à-vis de ces cibles. Dans ce contexte, les polymères à empreintes moléculaires (MIPs) pourraient apporter une solution. Les MIPs sont des récepteurs synthétiques possédant des affinités et sélectivités comparables à ceux des anticorps, mais exhibant une stabilité physique, thermique et chimique bien plus accrue. De plus, leur fabrication est peu coûteuse et ne nécessite pas de tuer des animaux comme pour l’obtention des anticorps biologiques. Dans cette thèse, nous avons optimisé et synthétisé des MIPs biocompatibles pour leur utilisation en bioimagerie afin de détecter et quantifier l’acide hyaluronique et l’acide sialique sur les cellules et les tissus de peau humaine. L’acide glucuronique, une composante de l’acide hyaluronique et l’acide N-acétylneuraminique, l’acide sialique le plus commun, ont été utilisés comme molécules ‘patron’, générant des MIPs très sélectifs envers leur cible en milieu aqueux. Deux types de nanoparticules de MIPs fluorescents ont été synthétisés: (1) en incorporant un colorant rhodamine polymérisable dans la solution de pré-polymérisation et (2) en encapsulant des boîtes quantiques InP/ZnS générant ainsi des MIPs de type cœur-coquille. Pour cela, nous avons adopté une stratégie innovante qui consiste à synthétiser les coquilles de MIPs directement autour des boîtes quantiques en utilisant l’énergie de l’onde fluorescente émise par l’excitation des points quantiques, pour initier la polymérisation. Un protocole d'immunocoloration standard a ensuite été optimisé afin d’imager des kératinocytes humains fixés et vivants ainsi que des tissus de peau, par microscopie à épifluorescence et confocale. Les résultats étaient similaires à ceux obtenus par la méthode de référence utilisant une protéine biotinylée reconnaissant l'acide hyaluronique. L'imagerie multiplex en combinant deux MIPs couplés à deux couleurs de boîtes quantiques et l’imagerie des cellules cancéreuses ont également été démontrées. Bien que les MIPs n’étaient pas cytotoxiques aux concentrations utilisées pour la bioimagerie, la toxicité des différentes composantes du MIP pourrait être un frein à leur utilisation dans le domaine biomédical. Afin de rendre ces MIPs plus ‘inoffensifs’, nous avons supprimé l’amorceur de polymérisation, une molécule considérée comme toxique. Les MIPs ont été synthétisés en employant des monomères qui s’auto-initient sous l’effet de l’UV ou de la chaleur. La spécificité et la sélectivité des MIPs obtenus étaient similaires à ceux préparés avec des amorceurs. En conclusion, cette thèse décrit la première utilisation des MIPs comme anticorps synthétique pour la bioimagerie de fluorescence. Ce travail ouvre la voie à de nouvelles applications en détection, diagnostique et thérapie par des MIPs. / This thesis describes the state of the art in nanomaterials-based targeted bioimaging and introduces molecularly imprinted polymers, also termed ‘plastic antibodies’ as novel biorecognition agents for labeling and imaging of cells and tissues. In fundamental biology and medical diagnostics, there is a constant need to localize and quantify specific molecular targets. Abnormal glycosylation levels or distributions of hyaluronan or sialic acids on cells are indicators of infection or malignancy. In general, bioimaging with fluorescent probes enables the localization and qualitative or quantitative determination of these pathological biomarkers. However, no reliable tools for the recognition of glycosylation sites on proteins exist, because the commercially available antibodies or lectins have poor affinity and selectivity for these targets. In this context, tailor-made molecularly imprinted polymers (MIPs) are promising synthetic receptor materials since they present a series of advantages over their natural counterparts such as the ease and low cost of preparation and their physical and chemical stability. Thus, MIPs could provide a robust and specific imaging tool for revealing the location/distribution, time of appearance and structure of glycosylation sites on/in cells, which would lead to a better insight of the tremendously diverse biological processes in which these molecules are involved. Herein, we describe the synthesis of water-compatible MIPs for the molecular imaging of hyaluronan and sialylation sites on cells and tissues. Since molecular imprinting of entire biomacromolecules like oligosaccharides is challenging, we opted for what is commonly called the ‘epitope approach’, which was inspired by nature. The monosaccharides, glucuronic acid and N-acetylneuraminic acid were imprinted, and the resulting MIPs were able to bind these molecules when present and accessible on the terminal unit of hyaluronan and sialylation sites. Fluorescent MIPs were synthesized as rhodamine-labeled nanoparticles and as MIP-coated InP/ZnS core-shell quantum dot (QD) particles. For the coating of the QDs, a novel versatile solubilization and functionalization strategy was proposed, which consists of creating polymer shells directly on QDs by photopolymerization using the particles as individual internal light sources. A standard immunostaining protocol was then successfully adapted for the application of the fluorescently labeled MIPs to image fixed and living human keratinocytes and skin tissues, by epifluorescence and confocal fluorescence microscopy. The results were comparable to those obtained with a reference method where staining was done with a biotinylated hyaluronic acid binding protein. Multiplexed and cancer cell imaging were also performed, demonstrating the potential of molecularly imprinted polymers as a versatile biolabeling and bioimaging tool. Although the MIPs were not cytotoxic at the concentrations used for bioimaging, in order to render them generally applicable in biomedicine, where toxicity of the polymerization precursors is a matter of concern, we suppressed the initiator, a toxic chemical. Initiator-free MIPs were thus synthesized by using monomers that can self-initiate under UV irradiation or heat. The specificity and selectivity of the obtained MIPs were as good as the ones prepared with initiators. In conclusion, we have demonstrated for the first time the great potential of MIPs as synthetic antibody mimics for bioimaging. The possibility to associate other functionalities such as QDs and additionally attach drugs to the same material appears rather straightforward due to the synthetic polymeric nature of MIPs, which paves the way to new potential applications in theranostics.

Ce document présente notre travail sur la modélisation en formalisme des fonctions de Green (abrégé formalisme de Green) du transport quantique et de l’intéraction éléctron-photon dans une cellule photovoltaïque composée d’une boîte quantique connectée à deux nanofils semi-infinis, La simulation numérique a été réalisée sur le cluster de calculs MERLIN (IM2NP). Nous présentons le formalisme de Green en général puis appliqué à cellule. Le fonctionnement général de la cellule est déduit de son diagramme de bande qui comporte des contacts sélectifs. Ensuite, nous présentons les résultats obtenus dans l’approximation de bande plate, qui simplifie le contact aux nanofils. Ceux-ci mettent en lumière des effets intriqués du couplage tunnel (couplage entre la boîte et les nanofils) et du couplage optique (couplage avec la lumière). Nous présentons ensuite un calcul analytique effectué dans le régime de fort couplage tunnel et qui explique le fonctionnement contre-intuitif du couplage tunnel dans ce régime. Nous observons également une transition dans le processus de production du courant entre le régime de fort couplage tunnel et le régime de fort couplage optique. Ensuite, nous sortons de l’approximation de bande plate et découvrons que les effets contre-intuitifs sont toujours valides, même si le modèle analytique lui ne l’est plus. Nous présentons le nouvel effet induit par la nouvelle forme du couplage aux réservoirs hors de l’approximation de bande plate: la courbe courant-tension présente une conductance de shunt négative. Cela n’a jamais été observé dans une cellule photovoltaïque auparavant. Enfin, nous présentons une réalisation possible de notre cellule. / This document present our work on the modeling of quantum transport coupled to electron-photon interaction in a solar cell composed of one quantum dot connected to two semi-infinite quantum wires. The proposed cell based on a dot in a wire, is a concept imagined in order to investigate quantum effects inside 1D structures in contact with 0D ones. The numerical simulation powered on the Merlin cluster (IM2NP) relies on Green’s function formalism. The philosophy of Green’s function formalism is introduced and then applied to the photovoltaic cell. An overview of the functioning of the cell is given. Results on the cell are presented in the wide band limit (approximation that simplifies the contact to wires). We observe an interlinked impact of the tunneling coupling (dot-wires coupling) and the optical coupling (to light) on the current. In the strong tunneling regime, an increase of the tunneling coupling decreases the current and similarly in the strong optical coupling regime, an increase of the optical coupling decreases the current. We investigate the counter-intuitive impact of the tunneling coupling in the strong tunneling regime through analytical calculations, considering only the first loop of the numerical code instead of the whole self-consistent process. We observe a transition in the current creation process while switching from the strong tunneling regime to the strong optical coupling regime. Results on the cell beyond the wide band limit approximation are presented in which the system exhibits another atypical response to illumination: I-V curve exhibits a negative shunt conductance! Finally, a realization proposal for the concept cell is described.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.

Colloidal semiconductor quantum dots (QDs) have gained substantial interest as adjustable, bright and spectrally tunable fluorophores in the past decades. Besides their in-depth analyses in the scientific community, first industrial applications as color conversion and color enrichment materials were implemented. However, stability and processability are essential for their successful use in these and further applications. Methods to embed QDs into oxides or polymers can only partially solve this challenge. Recently, our group introduced the embedding of QDs into ionic salts, which holds several advantages in comparison to polymer or oxide-based counterparts. Both gas permeability and environmental-related degradation processes are negligible, making these composites an almost perfect choice of material. To evaluate this new class of QD-salt mixed crystals, a thorough understanding of the formation procedure and the final composites is needed. The present work is focused on embedding both aqueous-based and oil-based metal-chalcogenide QDs into several ionic salts and the investigations of their optical and chemical properties upon incorporation into the mixed crystals. QDs with well-known, reproducible and high-quality synthetic protocols are chosen as emissive species. CdTe QDs were incorporated into NaCl as host matrix by using the straightforward "classical" method. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Besides NaCl, also borax and other salts are used as host matrices. Mercaptopropionic acid stabilized CdTe QDs can easily be co-crystallized with NaCl, while thioglycolic acid as stabilizing agent results in only weakly emitting powder-like mixed crystals. This challenge was overcome by adjusting the pH, the amount of free stabilizer and the type of salt used, demonstrating the reproducible incorporation of highest-quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. A disadvantage of the "classical" mixed crystallization procedure was its long duration which prevents a straightforward transfer of the protocol to less stable QD colloids, e.g., initially oil-based, ligand exchanged QDs. To address this challenge, the "Liquid-liquid-diffusion-assisted-crystallization" (LLDC) method is introduced. By applying the LLDC, a substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process opens the field of incorporating ligand-exchanged Cd-free QDs into NaCl matrices. To overcome the need for a ligand exchange, the LLDC can also be extended towards a two-step approach. In this modified version, the seed-mediated LLDC provides for the first time the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. The ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment. As one of the main results, these matrices provide extraordinary high photo- and chemical stability. It is further demonstrated with absolute measurements of photoluminescence quantum yields (PL-QYs), that the PL-QYs of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix by applying the "classical" crystallization procedure. The achievable PL enhancement factors depend strongly on the PL-QYs of the parent QDs and can be described by the change of the dielectric surrounding as well as the passivation of the QD surface. Studies on CdSe/ZnS in NaCl and CdTe in borax showed a crystal-induced PL-QY increase below the values expected for the respective change of the refractive index, supporting the derived hypothesis of surface defect curing by a CdClx formation as one main factor for PL-QY enhancement. The mixed crystals developed in this work show a high suitability as color conversion materials regarding both their stability and spectral tunability. First proof-of-concept devices provide promising results. However, a combination of the highest figures of merit at the same time is intended. This ambitious goal is reached by implementing a model-experimental feedback approach which ensures the desired high optical performance of the used emitters throughout all intermediate steps. Based on the approach, a white LED combining an incandescent-like warm white with an exceptional high color rendering index and a luminous efficacy of radiation is prepared. It is the first time that a combination of this highly related figures of merit could be reached using QD-based color converters. Furthermore, the idea of embedding QDs into ionic matrices gained considerable interest in the scientific community, resulting in various publications of other research groups based on the results presented here. In summary, the present work provides a profound understanding how this new class of QD-salt mixed crystal composites can be efficiently prepared. Applying the different crystallization methods and by changing the matrix material, mixed crystals emitting from blue to the near infrared region of the electromagnetic spectrum can be fabricated using both Cd-containing and Cd-free QDs. The resulting composites show extraordinary optical properties, combining the QDs spectral tunability with the rigid and tight ionic matrix of the salt. Finally, their utilization as a color conversion material resulted in a high-quality white LED that, for the first time, combines an incandescent-like hue with outstanding optical efficacy and color rendering properties. Besides that, the mixed crystals offer huge potential in other high-quality applications which apply photonic and optoelectronic components.

Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics. In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge. Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented. In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed. In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.:Abstract i Contents iii List of Figures vi List of Tables viii 1 Preface 1 I Background and Experimental 5 2 Principles of ESR 7 2.1 The Resonance Phenomenon . . . . . . . . . . . . . . . . . . . 7 2.2 ESR Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.1 The g -factor . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2.2 Relaxation Times . . . . . . . . . . . . . . . . . . . . . . 12 2.2.3 Lineshape Properties . . . . . . . . . . . . . . . . . . . . 13 2.3 Effective Spin Hamiltonian . . . . . . . . . . . . . . . . . . . . . 15 2.4 Spin-Orbit Coupling . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.5 d-electrons in a Crystal Field . . . . . . . . . . . . . . . . . . . . 17 2.6 Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.6.1 Dipolar Coupling . . . . . . . . . . . . . . . . . . . . . . 23 2.6.2 Exchange Interaction . . . . . . . . . . . . . . . . . . . . 23 2.6.3 Superexchange . . . . . . . . . . . . . . . . . . . . . . . 24 2.6.4 Symmetric Anisotropic Exchange . . . . . . . . . . . . 25 2.6.5 Antisymmetric Anisotropic Exchange . . . . . . . . . . 25 2.6.6 Hyperfine Interaction . . . . . . . . . . . . . . . . . . . 26 3 Experimental 27 3.1 Setup for Experiments at 10GHz . . . . . . . . . . . . . . . . . 27 3.2 Implementation of an EDMR Setup . . . . . . . . . . . . . . . . 29 3.2.1 Basic Characterization . . . . . . . . . . . . . . . . . . . 31 3.3 High Frequency Setup . . . . . . . . . . . . . . . . . . . . . . . . 31 3.3.1 MillimeterWave Vector Network Analyzer . . . . . . . 33 3.3.2 Waveguides and Cryostats . . . . . . . . . . . . . . . . . 34 3.4 Development of the Resonant Cavity Setup . . . . . . . . . . . 35 3.4.1 Mode Propagation . . . . . . . . . . . . . . . . . . . . . 38 3.4.2 Resonant CavityModes . . . . . . . . . . . . . . . . . . 40 3.4.3 Resonant Cavity Design . . . . . . . . . . . . . . . . . . 41 3.4.4 Resonant Cavity Sample Stick . . . . . . . . . . . . . . . 45 3.4.5 Experimental Characterization . . . . . . . . . . . . . . 47 3.4.6 Performing an ESR Experiment . . . . . . . . . . . . . . 53 II Quasi One-Dimensional Spin-Chains 57 4 Motivation 59 5 Quasi One-Dimensional Systems 61 5.1 Magnetic Order and Excitations . . . . . . . . . . . . . . . . . . 63 5.2 Competing Interactions . . . . . . . . . . . . . . . . . . . . . . . 64 5.3 Haldane Spin Chain . . . . . . . . . . . . . . . . . . . . . . . . . 66 6 Linarite 69 6.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.2 Magnetization and ESR . . . . . . . . . . . . . . . . . . . . . . . 71 6.3 NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 6.4 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . 81 6.5 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 7 The Ni-hybrid NiCl3C6H5CH2CH2NH3 83 7.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 7.2 Susceptibility andMagnetization . . . . . . . . . . . . . . . . . 85 7.3 ESR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 7.4 Further Investigations . . . . . . . . . . . . . . . . . . . . . . . . 95 7.5 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . 96 8 Summary 99 III SiGe Nanostructures 101 9 Motivation 103 10 SiGe Semiconductor Nanostructures 107 10.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 10.1.1 Silicon and Germanium . . . . . . . . . . . . . . . . . . 107 10.1.2 Epitaxial Growth of SiGe Heterostructures . . . . . . . 109 10.1.3 Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 10.1.4 Band Deformation . . . . . . . . . . . . . . . . . . . . . 112 10.2 Sample Structure and Characterization . . . . . . . . . . . . . 114 11 Modelling of SiGe/Si Heterostructures 119 11.1 Program Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 120 11.2 Implementation of Si/Ge System . . . . . . . . . . . . . . . . . 121 11.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 11.3.1 Single Quantum Dot . . . . . . . . . . . . . . . . . . . . 123 11.3.2 Multiple Quantum Dots . . . . . . . . . . . . . . . . . . 127 11.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 11.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 12 ESR Experiments on Si/SiGe Quantum Dots 135 12.1 ESR on Si Structures . . . . . . . . . . . . . . . . . . . . . . . . . 135 12.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . 137 12.2.1 Samples grown at 600◦C . . . . . . . . . . . . . . . . . . 138 12.2.2 Samples grown at 700◦C . . . . . . . . . . . . . . . . . . 139 12.2.3 T1-Relaxation Time . . . . . . . . . . . . . . . . . . . . . 143 12.2.4 Effect of Illumination . . . . . . . . . . . . . . . . . . . . 145 12.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 12.3.1 Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . 149 12.3.2 Assignment of ESR Lines . . . . . . . . . . . . . . . . . . 150 12.3.3 Relaxation Times . . . . . . . . . . . . . . . . . . . . . . 153 12.3.4 Donors in Heterostructures . . . . . . . . . . . . . . . . 153 12.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 13 Summary and Outlook 157 Bibliography 163 Acknowledgements 176