Low dimensional semiconductor structures are modeled using techniques from the field of many-body atomic physics. B-splines are used to create a one-particle basis, used to solve the more complex many-body problems. Details on methods such as the Configuration Interaction (CI), Many-Body Perturbation Theory (MBPT) and Coupled Cluster (CC) are discussed. Results from the CC singles and doubles method are compared to other high-precision methods for the circular harmonic oscillator quantum dot. The results show a good agreement for the energy of many-body states of up to 12 electrons. Properties of elliptical quantum dots, circular quantum dots, quantum rings and concentric quantum rings are all reviewed. The effects of tilted external magnetic fields applied to the elliptical dot are discussed, and the energy splitting between the lowest singlet and triplet states is explored for varying geometrical properties. Results are compared to experimental energy splittings for the same system containing 2 electrons.

Deposition of silica shells onto ZnO nanoparticles (NPs) in dimethyl sulfoxide was found to be an efficient tool for terminating the growth of ZnO NPs during thermal treatment and producing stable core–shell ZnO NPs with core sizes of 3.5–5.8 nm. The core–shell ZnO–SiO2 NPs emit two photoluminescence (PL) bands centred at [similar]370 and [similar]550 nm originating from the direct radiative electron–hole recombination and defect-mediated electron–hole recombination, respectively. An increase of the ZnO NP size from 3.5 to 5.8 nm is accompanied by a decrease of the intensity of the defect PL band and growth of its radiative life-time from 0.78 to 1.49 μs. FTIR spectroscopy reveals no size dependence of the FTIR-active spectral features of ZnO–SiO2 NPs in the ZnO core size range of 3.5–5.8 nm, while in the Raman spectra a shift of the LO frequency from 577 cm−1 for the 3.5 nm ZnO core to 573 cm−1 for the 5.8 nm core is observed, which can indicate a larger compressive stress in smaller ZnO cores induced by the SiO2 shell. Simultaneous hydrolysis of zinc(II) acetate and tetraethyl orthosilicate also results in the formation of ZnO–SiO2 NPs with the ZnO core size varying from 3.1 to 3.8 nm. However, unlike the case of the SiO2 shell deposition onto the pre-formed ZnO NPs, individual core–shell NPs are not formed but loosely aggregated constellations of ZnO–SiO2 NPs with a size of 20–30 nm are. The variation of the synthetic procedures in the latter method proposed here allows the size of both the ZnO core and SiO2 host particles to be tuned. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

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CdSe quantum dots produced by organometallic synthesis are useful as tunable emitters for photonic devices and as multi-colored protein markers for biomedical imaging, applications requiring bright and narrow emission. A diffusion-limited model helped monitor growth rates via photoluminescence and absorbance spectroscopy, in order to characterize synthesis kinetics in stearic acid, dodecylamine, and in trioctylphosphine oxide. The nucleation rate increased with Se concentration, while the growth rate followed the Cd concentration. Emission peak widths, emission redshift rates, nanocrystal growth rates, and reactant concentrations all decreased to a minimum when emission reached the critical wavelength, at a reaction completion time, tc. The temperature dependence of 1/tc and of redshift rates followed Arrhenius behavior governed by activation energies, which were tailored by the choice of solvent. Synthesis in solvents, such as stearic acid, with lower activation energies produced faster initial nanocrystal growth and longer critical wavelengths. The highest photoluminescence quantum yield was generally at wavelengths shorter than the critical wavelength, when moderate growth rates enabled surface reconstruction while precursors were still available. / Ph. D.

[pt] s pontos quânticos de grafeno têm potencial para aplicações biológicas devido às suas propriedades ópticas e de tamanho nanométrico. Este estudo investigou por 28 dias (672 h) as interações de GQDs de diferentes precursores (ácido cítrico + ureia e ácido cítrico + tioacetamida) com biomoléculas modelo (albumina sérica humana - HSA) e DNA do timo de bezerro (ctDNA). Os GQDs-ureia mostraram estabilidade no diâmetro hidrodinâmico (12 nm) e carga superficial (-7 mV). Já os GQDs-tioacetamida apresentaram agregação progressiva de 5,0 nm iniciais para 22,7 nm após 28 dias, sem sedimentação devido à compensação de cargas preservando a dispersão coloidal. Ensaios revelaram supressão da fluorescência da HSA com aumentos na concentração dos GQDs. A constante de interação (Ki) GQDs-ureia oscilou inicialmente, estabilizando após 48 h. Para GQDs-tioacetamida houve menor flutuação de Ki ao longo de 672 h, indicando rearranjos conformacionais das biomoléculas com os GQDs antes do equilíbrio. A interação com o DNA, acompanhada por titulação absorciométrica no UV-Vis mostrou biointeração fraca de natureza hidrofóbica/eletrostática para ambos GQDs, com constantes de ligação aparentes (∼105 L mol−1). Ensaio com brometo de etídio revelou alterações na estrutura do DNA sem intercalação dos GQDs.Testes estatísticos confirmam a reprodutibilidade das interações dos GQDs com proteínas (HSA) e DNA no período de 28 dias (95 por cento de confiança). A estabilidade dos parâmetros de quantificação ao longo do tempo sugere a viabilidade dos GQDs como sondas analíticas após longos períodos de bioconjugação. Assim, o estudo apresenta bases metrologicamente sólidas para aplicação segura de GQDs em tecnologias biomédicas, expandindo o entendimento da relação tempo-estrutura-atividade nesses nanossistemas. / [en] Graphene quantum dots have potential for biological applications due to their optical properties and nanometric size. This study investigated for 28 days (672 h) the interactions of GQDs from different precursors (citric acid + urea and citric acid + thioacetamide) with model biomolecules (human serum albumin - HSA) and Calf thymus DNA (ctDNA). The GQDs-urea showed stability in hydrodynamic diameter (12 nm) and surface charge (- 7 mV). In contrast, GQDs-thioacetamide showed progressive aggregation from 5.0 nm initially to 22.7 nm after 28 days, without sedimentation due to charge compensation preserving colloidal dispersion. Tests revealed quenching of HSA fluorescence with increases in GQD concentration. The GQDs-urea interaction constant (Ki) fluctuated initially, stabilizing after 48 h. For GQDs-thioacetamide there was less fluctuation in Ki over 672 h, indicating conformational rearrangements of the biomolecules with the GQDs before equilibrium. Interaction with DNA monitored by UV-Vis photometric absorption titration showed weak bio-interaction of a hydrophobic/electrostatic nature for both GQDs, with apparent binding constants (∼105 L mol−1). Ethidium bromide assay revealed changes in DNA structure without intercalation of the GQDs. Statistical tests confirm the reproducibility of GQDs interactions with proteins (HSA) and DNA over 28 days (95 percent confidence). The stability of the quantification parameters over time suggests the viability of GQDs as analytical probes after long periods of bioconjugation. Thus, the study presents metrologically sound bases for the safe application of GQDs in biomedical technologies, expanding the understanding of the time-structure-activity relationship in these nanosystems.

Optical properties of nanoscopic materials have been intensively perused over last couple of decades due to their tunable optical properties. Recent interests in this field have been mainly focused on the preparation of ordered arrays of nanoscopic materials and study of their optical properties. These interests have been motivated by the usability of such systems for nano photonic devices. Theoretical predictions from such systems reveal complex absorption and emission properties, different from individual ones mainly because of energy transfer between them. These properties can be controlled further by preparing hybrid arrays of nanostructures, including nano crystals of different types. Hybrid arrays with semiconducting quantum dots and metallic nanoparticles are an example of such system. Optical properties of such a system can be tuned by controlling the interaction between excitons and plasmons. This the-sis presents the experimental studies on optical properties of polymer capped polymer nanoparticles, quantum dot arrays and hybrid arrays with semiconducting quantum dot and metal nanoparticles. A brief summary of the experi-mental methods and results have been highlighted below. First chapter deals with the theoretical aspects of confined nanoscopic materials, especially describing the physics of zero dimensional systems and its optical properties. The discussions are mostly focused on two types of nano materials cadmium selenide (CdSe) quantum dot (QDs) and gold nano particles (Au NPs), used for the experimental study. Variation of energy levels of CdSe QDs and its absorption and emission properties under strong confinement regime has been discussed with respect to effective mass approximation (EMA) model. This is followed by the discussion on optical properties of Au NPs, describing absorption properties, based on Mie theory. Size dependent variation of absorption spectra of Au NPs and the modifications based on different models has been discussed. Second part of the chapter describes the physics of QD arrays and theory of exciton plasmon interactions based on the recent literatures. Energy transfer mechanism between semiconducting QDs and metal nanoparticles has been discussed based on numerical method and dipole approximation. Second chapter deals with the discussion on experimental techniques used for the study. Chapter 2 starts with the discussion on the synthesis method for CdSe QDs and Au NPs with different capping ligands. Preparation of QD ar-rays and hybrid arrays using self assembly technique has been discussed in this chapter. Preparation CdSe QD arrays and hybrid arrays with CdSe QDs and Au NPs using block copolymer (BCP) template and Langmuir Blodgett (LB) technique has been the main focus in the discussion. This is followed by the discussion on optical microscopy techniques, confocal, near field scanning microscopy (NSOM), Brewster angle microscopy and electron microscopy techniques, transmission electron microscopy and scanning electron microscopy. Studies on variation of band structure of small polymer capped Au NPs, with respect to the size and grafting density of the capping polymer is discussed in chapter 3. Polymer capped Au NPs with sizes 2-5 nm was used for the study. Dielectric constants of Au NPs were extracted from the absorption spectra by fitting the data using modified Mie theory. Dielectric constants of Au NPs were reproduced using an analytical expression, describing the contribution from different transitions in the optical regions. Results indicate systematic variations of the band structure with respect to the particle size and grafting density. The observations have been interpreted in terms of variation of co ordination number and chemical interaction of capping polymer with the surface atoms. Our new method analysis points to the importance of both quantum and surface effects in determining optical and electronic properties of polymer capped gold nanoparticles. Chapter 4 describes the study on morphology of the CdSe QD arrays prepared using different BCP templates and its correlation with optical properties. Spatially resolved spectra from the thin films of QD arrays were collected in near field and the compared with the spectra collected in far field. Spectra collected in near field mode shows sharp features in the emission spectra, possibly indicating the interaction of optical near field with QD excitation. It has been suggested that such fine structure could be induced by coupling between optical near filed and excitons and this coupling seems to be determined by local heterogeneity in QD density and disorder. Variation of exciton life time with respect to QD density and absorption spectra from the QD -BCP system is also described in chapter 4. Chapter 5 and 6 deals with the experimental studies on exciton -plasmon interaction in hybrid arrays of CdSe QDs and Au NPs. Emission properties hybrid arrays prepared using BCP templates has been the focus of chapter 5. Photoluminescence (PL) and lifetime measurements were performed on hybrid arrays and their variation with respect to the density and dispersion of Au NPs has been described. Optical measurements were performed on two sets of films using two different sizes of CdSe QDs, with the smaller QD emission overlapping with the plasmon resonance of Au NPs, while a red shifted emission peak for larger QDs. PL emission from hybrid arrays with smaller QDs shows en-hancement/quenching with respect to the dispersion of Au NPs, also showing systematic reduction of life time of CdSe QDs with Au NP density. Even though enhancement/quenching of emission properties of hybrid film with large QD shows similar behavior, PL decay measurements from such films shows non monotonic variation of exciton life time with respect to Au NP density. The enhancement/quenching behavior of the PL emission has been explained in terms of two competing mechanism, electromagnetic field enhancement and non radiative energy transfer. However to explain the energy transfer mechanism in hybrid arrays requires more systematic calculations. Chapter 6 describes the optical properties of highly compact hybrid arrays prepared using LB techniques. Hybrid arrays prepared at the air water inter-face were transferred to a glass substrates. The main focus on chapter 6 is to study the emission properties of highly compact hybrid arrays with respect to the spectral overlap between exciton energy of CdSe QDs and plasmon band of Au NPs with respect to their surface density (inter particle distance). Hybrid arrays were prepared with three types of QDs, with smaller QDs emission peak overlapping with plasmon band of Au NPs and clearly separated exciton and plasmon band for largest QDs. The PL emission from hybrid arrays with smaller QDs shows quenching, compared to strong enhancement in the emission from hybrid films with larger QDs. The disagreement of the observed results with respect to the theoretical calculations based on dipole approximation has been highlighted in the chapter. Chapter 7 includes the summary of the experimental results and the future works to be carried out as a continuation of the work presented in this thesis.

Les qubits de spin sont des candidats prometteurs pour le traitement de l’information quantique en raison de leurs longs temps de cohérence. Les deux principaux qubits présents dans un système à trois spins ont été démontré au cours des dernières années dans la boîte quantique latérale triple. Le diagramme des niveaux d’énergie de quelques électrons dans la boîte quantique triple est beaucoup plus complexe que son homologue à deux ou à une boîte. Il en résulte des possibilités de fuites hors des qubits ciblés. Dans ce mémoire, nous présenterons une nouvelles technologie pour améliorer le contrôle des états de spin et augmenter le temps de cohérence des qubits. Nous avons effectué des mesures préliminaires sur des échantillons sur lesquels a été incorporé un microaimant. Ce microaimant crée un champ magnétique non-uniforme au niveau des boîtes quantiques qui sera utilisé pour effectuer une rotation de spin et pour améliorer certains types d’oscillations. Nous avons optimisé la forme des géométries afin de créer des gradients de champ magnétique optimaux spécifiquement pour la boîte quantique triple. Différents problèmes ont été encourus et la stratégie que nous avons adoptée pour les régler sera présentée. De plus, nous avons analysé les phénomènes de fuites entre les états quantiques en étudiant la réponse d’un système à trois spins en fonction de différentes impulsions électriques. Nous présentons deux processus d’interférence jamais répertoriés entre les qubits de la boîte quantique triple. Afin d’identifier l’origine de ces interférences, nous avons utilisé leur dépendance en champ magnétique.

This PhD thesis focuses on the study of mode locked laser diodes based on novel optimized InAs Quantum Dash structures grown on InP substrates. It covers several important modelling aspects, the clean room processing of single and two section shallow ridge waveguide lasers, characterization of the fabricated devices and the evaluation of their performance in different application scenarios. Systematic characterization experiments and subsequent analyses have allowed to gain a much deeper comprehension of the physical mechanisms related to the mode locking regime in these devices, thus far not completely understood. This has allowed to better control most of the main physical phenomena limiting device performance, resulting in first demonstrations of record peak power, sub-picosecond pulse, low radio frequency linewidth and high repetition frequency mode locked lasers grown on InP substrates, opening the way to a vast number of potential applications in the 1.55 µm telecommunication window

We study correlated two-level quantum impurity models coupled to a metallic conduction band in the hope of gaining insight into the physics of nanoscale quantum dot systems. We focus on the possibility of formation of a spin-1 impurity local moment which, on coupling to the band, generates an underscreened (USC) singular Fermi liquid state. By employing physical arguments and the numerical renormalization group (NRG) technique, we analyse such systems in detail examining in particular both the thermodynamic and dynamic properties, including the differential conductance. The quantum phase transitions occurring between the USC phase and a more ordinary Fermi liquid (FL) phase are analysed in detail. They are generically found to be of Kosterlitz-Thouless type; exceptions occur along lines of high symmetry where first-order transitions are found. A `Friedel-Luttinger sum rule' is derived and, together with a generalization of Luttinger's theorem to the USC phase, is used to obtain general results for the $T=0$ zero-bias conductance --- it is expressed solely in terms of the number of electrons present on the impurity and applicable in both the USC and FL phases. Relatedly, dynamical signatures of the quantum phase transition show two broad classes of behaviour corresponding to the collapse of either a resonance or antiresonance in the single-particle density of states. Evidence of both of these behaviours is seen in experimental devices. We study also the effect of a local magnetic field on both single- and two-level quantum impurities. In the former case we attempt to resolve some points of contention that remain in the literature. Specifically we show that the position of the maximum in the spin resolved density of states (and related peaks in the differential conductance) is not linear in the applied field, showing a more complicated form than a simple `Zeeman splitting'. The analytic result for the low-field asymptote is recovered. For two-level impurities we illustrate the manner in which the USC state is destroyed: due to two cancelling effects an abrupt change in the zero-bias conductance does not occur as one might expect. Comparison with experiment is made in both cases and used to interpret experimental findings in a manner contrary to previous suggestions. We find that experiments are very rarely in the limit of strong impurity-host coupling. Further, features in the differential conductance as a function of bias voltage should not be simply interpreted in terms of isolated quantum dot states. The many-body nature of such systems is crucially important to their observed properties.

Die vorliegende Arbeit behandelt die Modellierung einer Quantenpunkt-LED und die Berechnung des elektrischen Stromes und der Lichtproduktion im Landauer-Büttiker-Formalismus. Die Elektron-Photon-Wechselwirkung kann im Landauer-Büttiker-Formalismus behandelt werden, indem wir annehmen, dass jedes Elektron mit einem separaten Photonenfeld interagiert. Dies erlaubt es uns, ein Elektron zusammen mit seinem Photonenfeld als „einzelnes, nicht wechselwirkendes Teilchen“ im Sinne des Landauer-Büttiker-Formalismusses zu betrachten. Wir entwickeln ein Modell einer QP-LED, dessen Elektron-Photon-Wechselwirkung auf dem Jaynes-Cummings-Modell basiert, das die Interaktion eines Quantenpunkts mit einer Mode des elektromagnetischen Feldes beschreibt. Um auch die Energieverteilung der emittierten Photonen analysieren zu können, schlagen wir ein auf einem Pauli-Fierz-Modell basiertes Modell vor. Anstelle einer einzelnen Mode modelliert es Photonen beliebiger Energie, allerdings beschränken wir uns auf den Unterraum mit maximal einem Photon. Wir beweisen eine abstrakte Landauer-Büttiker-Formel, die für alle relativ nuklearen Streusysteme gilt. Sie ist ähnlich zu dem Ergebnis von Aschbacher et al. (2007), unterscheidet sich aber in der Regularisierung des Stroms. Wir wenden das abstrakte Ergebnis auf die Jaynes-Cummings-QP-LED an. Als Startpunkt für die Berechnung der Streumatrix verallgemeinern wir die Darstellung der Streumatrix durch die Weyl-Funktion eines Randwert-Triplets von Behrndt et al. (2010) vom Fall für Störungen endlichen Ranges auf den Fall relativ nuklearer Störungen. Dies deckt insbesondere den Fall der Jaynes-Cummings-QP-LED ab. Die Resolventendifferenz der Pauli-Fierz-QP-LED ist nicht nuklear, weshalb wir eine verallgemeinerte Landauer-Büttiker-Formel für eine gewisse Klasse von Multiplikationsoperatoren beweisen, die in der Faser nuklear sind. Dieses abstrakte Resultat liefert uns auch für die Pauli-Fierz-QP-LED eine Landauer-Büttiker-Formel. / This thesis treats the modeling of a quantum dot LED and the calculation of the electric current and the light production in the Landauer-Büttiker framework. The electron-photon interaction is fitted into the Landauer-Büttiker framework by assuming that every electron interacts with a separate photon field. It allows us to consider an electron together with its photon field as a ''single non-interacting particle'' in the sense of the Landauer-Büttiker formalism. We develop a model of a QD-LED with an electron-photon interaction that is based on the Jaynes-Cummings model, which describes the interaction of a quantum dot with a single mode of the electromagnetic field. To be able to analyze the energy distribution of the emitted photons, we propose a second model of a QD-LED that is based on a one-dimensional Pauli-Fierz model. It models photons of arbitrary positive energy instead of just a single mode, but we restrict it to the subspace of at most one photon. We prove an abstract Landauer-Büttiker formula that applies to all relatively trace class interactions. It is similar to the result by Aschbacher et al. (2007), but differs in the regularization of the flux. We apply this formula to the Jaynes-Cummings QD-LED. Since knowing of the scattering matrix is essential for explicit calculations with the Landauer-Büttiker formula, we generalize a result by Behrndt et al. (2010) on a representation of the scattering matrix in terms of the Weyl function of a boundary triplet from the finite rank case to relatively trace class perturbations, which covers the case of the Jaynes-Cummings QD-LED. The resolvent difference of the Pauli-Fierz QD-LED is not trace class, whence we prove a generalized Landauer-Büttiker formula for a certain multiplication operators that are trace class in the fiber. This abstract result gives us a Landauer-Büttiker formula also for the Pauli-Fierz QD-LED.