Stuttgart, Univ., Fachstudie, 2002.

Lateral semiconductor quantum dot structures have been proposed as an effective quantum bit (qubit) for quantum computation. A single excess electron with the freedom to move between two capacitively coupled quantum dots creates a `pseudo'-spin system with the same qubit behavior as the more natural two level system of a single electron spin. The excess electron in the double dot system is restricted to one of the two dots, thereby creating two separate and distinct states (usually referred to as |L> and |R>). The benefit of these charge qubits lie in the relative ease with which they can be manipulated and created. Experiments have been performed in this area and have shown controllable coherent oscillations and thus efficient single-qubit operations. However, the decoherence rates observed in the experiments is still quite high, making double dot charge qubits not very appealing for large-scale implementations. The following work describes the effect of the electromagnetic (EM) environment of the double quantum dot system on the decoherence of the charge state. Sources of decoherence in similar systems have been extensively investigated before and this paper follows a close theoretical framework to previous work done in the area. The effect of the EM environment can been seen in the calculations discussed below, although it is clear that the decoherence seen in experiments cannot be fully explained by the voltage fluctuations as they are investigated here. The limitations of the calculations are discussed and improvements are suggested.

Les systèmes théranostiques, consistant en un dispositif unique contenant des agents de diagnostic et des principes actifs, suscitent un interêt accru car ils peuvent améliorer le traitement de maladies telles que le cancer en réduisant les effets secondaires des médicaments et en permettant un suivi du traitement. L’objectif de ce travail était d’insérer des Quantum Dots (QDs) à base de ZnO dans des nanoparticules lipidiques pouvant délivrer un principe actif anti-cancéreux. Nous avons d’abord cherché à synthétiser des QDs présentant une structure coeur-coquille ZnO/ZnS pour améliorer leurs propriétés de luminescence. La spectroscopie d’absorption des rayons X, associée à des techniques usuelles de caractérisation, a permis de déterminer les conditions de synthèse conduisant à la formation d’une structure coeur-coquille. Néanmoins, l’émission dans le visible de ces QDs n’était pas satisfaisante. Des QDs dopés par des ions Mg ont donc été synthétisés. L’intensité de leur luminescence passe par un maximum pour une concentration molaire nominale d’ions Mg dans le milieu de réaction égale à 20%. Les QDs Zn0.8Mg0.2O présentent un rendement quantique (QY) six fois plus grand (QY ~64%) que celui des QDs de ZnO non dopés (QY ~ 10%). Les QDs dont la surface a été modifiée par de l’acide oléique (OA) forment une suspension colloidale stable dans le chloroforme et le toluène. Le rendement quantique des QDs OA-Zn0.8Mg0.2O était environ quatre fois plus élevé (Qy ~40%) que celui des QDs OA-ZnO. Les QDs Zn0.8Mg0.2O et OA-Zn0.8Mg0.2O ont été incorporés dans des nanoparticules lipidiques ayant un diamètre hydrodynamique moyen de l’ordre de 100- 220 nm. Les nanoparticules lipidiques solides (SLN) contenant des QDs sont restées stables dans différents milieux biologiques pendant trois heures à 37°C. Des mesures de fluorescence sur des suspensions de macrophages J774 ont montré une faible augmentation de l’intensité de l’émission visible pour les cellules incubées avec 2 mg/mL de SLNs luminescentes pendant 50 min, suggérant une internalisation partielle des nanoparticules par les macrophages. Malheureusement, ces résultats n’ont pas pu être confirmés par vidéo-microscopie et microscopie de fluorescence sur les cellules parce que les conditions expérimentales ( longueurs d’onde d’excitation et d’émission possibles) ne permettaient pas d’observer un signal supérieur à celui de l'auto-fluorescence des cellules. / Theranostic systems consist of a single device containing therapeutic and diagnosis agents and receive increased attention because these devices can improve the therapy of diseases such as cancer, decreasing the toxic side effects and permitting to monitor the treatment. The aim of this work was to develop theranostic systems consisting of lipid based nanocarriers containing ZnO based quantum dots (QDs) as luminescent probes, and allowing to encapsulate a model drug for cancer therapy. Firstly, the synthesis of ZnO/ZnS QDs was studied, aiming to achieve improved luminescent properties. In this step, X-Rays Absorption Spectroscopy, together with other usual characterization techniques, could identify the synthesis condition in which core-shell structures were formed. Nevertheless, the emission of ZnO/ZnS QDs in the visible range was not promising. Therefore, Mg-doped ZnO QDs were synthesized; their luminescence went through a maximum for a 20 mol% nominal concentration of Mg2+ ions in the reaction medium. Zn0.8Mg0.2O QDs presented quantum yield (QY) six times higher (QY = 64%) than undoped ones (QY = 10%). ZnO and Zn0.8Mg0.2O QDs capped by oleic acid (OA) were synthesized and formed stable colloidal dispersions in chloroform and toluene. The QY of OA-Zn0.8Mg0.2O was about 4 times (around 40%) higher than that of the OA-ZnO QDs. Zn0.8Mg0.2O QDs and OA-Zn0.8Mg0.2O QDs could be incorporated into lipid based nanocarriers of average hydrodynamic diameter around 100 – 220 nm. The luminescent solid lipid nanoparticles (SLN) were stable in different media at 37°C during 3 hours. The fluorescence study showed slightly enhanced emission of the J774 macrophage-like cells treated with 2 mg/mL of luminescent SLN during 50 min, suggesting partial internalization of the nanoparticles into the macrophages. However, the internalization studies using fluorescence video-microscopy and microscopy were not successful, because the equipment (wavelengths of excitation and emission) did not allow overcoming the cell auto-fluorescence phenomena.

Quantum dot sensitized solar cells (QDSCs) constitute one of the most promising low-cost solutions that are explored for the world’s needs of clean and renewable energy. Efficient, low-toxic and stable QDSCs for large-scale applications have formed the subject for the solar cell research during recent years. This circumstance also forms the motivation for this thesis, where the results of my studies to improve the efficiency and stability of green QDSCs are presented and discussed. The surface condition of quantum dots (QDs) is always crucial to the performance of QDSCs, since surface ligands can influence the loading amount of QDs, and that the surface defects can induce charge recombination in the solar cells. In this thesis work, a hybrid passivation approach was firstly utilized to improve the photovoltaic performance of CdSe QDs. After hybrid passivation by MPA and iodide ions, the loading efficiency of the QDs was increased with the ligands of MPA, and the surface defects on the QDs were reduced by the iodide ions, both contributing to an enhancement in the efficiency of the CdSe based QDSCs. This hybrid passivation strategy was then employed for low-toxic CuInS2 QDs, and was also demonstrated as an effective way to modify the surface state of the CuInS2 QDs and improve the performance of the QDSCs based on CuInS2 QDs.   To improve the stability of the QDSCs, solid state quantum dot sensitized solar cells (ss-QDSCs) based on CuInS2 QDs were investigated. In addition to the hybrid passivation, increasing the pore size of the TiO2 active layer and changing the composition of the CuInS2 QDs were also found to be useful approaches to improve the performance of the ss-QDSCs based on CuInS2 QDs. Furthermore, for the most used hole transport material- Spiro-OMeTAD- in solid state solar cells, silver bis(trifluoromethanesulfonyl)imide was shown to be an effective p-type dopant to increase its conductivity and to improve the performance of the solar cells based on it. / <p>QC 20160516</p>

In tropical algebras we substitute min or max for the typical addition and then substitute addition for multiplication. A dot product representation of a graph assigns each vertex of the graph a vector such that two edges are adjacent if and only if the dot product of their vectors is greater than some chosen threshold. The resultS of creating dot product representations of graphs using tropical algebras are examined. In particular we examine the tropical dot product dimensions of graphs and establish connections to threshold graphs and the threshold dimension of a graph.

In this thesis we study the role of nonradiative degrees of freedom on quantum optical properties of mesoscopic quantum dots placed in the structured electromagnetic reservoir of a photonic crystal. We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption lineshape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the ``colored" electromagnetic vacuum of a photonic band gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. Phonon sidebands in an ordinary electromagnetic reservoir are recaptured in a simple model of optical phonons using a mean-field factorization of the atomic and lattice displacement operators. Our formalism is then used to treat the non-Markovian dynamics of the same system within the structured electromagnetic density of states of a photonic crystal. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to enhanced lifetime of a photon-atom bound state in a PBG by (i) dephasing and reducing the transition electric dipole moment of the atom and (ii) reducing the quantum mechanical overlap of the state vectors of the excited and ground state (polaronic shift). This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (break-up of phonons into lower energy phonons) and purely nonradiative decay. We demonstrate how these additional damping effects limit the extent of the polaronic (Franck-Condon) shift of the atomic excited state. We also derive the modified polarization decay and dephasing rates in the presence of such damping. This leads to a microscopic, quantum theory of the optical absorption lineshapes. Our model and formalism provide a starting point for describing dephasing and relaxation in the presence of external coherent fields and multiple quantum dot interactions in electromagnetic reservoirs with radiative memory effects.

In this thesis we study the role of nonradiative degrees of freedom on quantum optical properties of mesoscopic quantum dots placed in the structured electromagnetic reservoir of a photonic crystal. We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption lineshape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the ``colored" electromagnetic vacuum of a photonic band gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. Phonon sidebands in an ordinary electromagnetic reservoir are recaptured in a simple model of optical phonons using a mean-field factorization of the atomic and lattice displacement operators. Our formalism is then used to treat the non-Markovian dynamics of the same system within the structured electromagnetic density of states of a photonic crystal. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to enhanced lifetime of a photon-atom bound state in a PBG by (i) dephasing and reducing the transition electric dipole moment of the atom and (ii) reducing the quantum mechanical overlap of the state vectors of the excited and ground state (polaronic shift). This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (break-up of phonons into lower energy phonons) and purely nonradiative decay. We demonstrate how these additional damping effects limit the extent of the polaronic (Franck-Condon) shift of the atomic excited state. We also derive the modified polarization decay and dephasing rates in the presence of such damping. This leads to a microscopic, quantum theory of the optical absorption lineshapes. Our model and formalism provide a starting point for describing dephasing and relaxation in the presence of external coherent fields and multiple quantum dot interactions in electromagnetic reservoirs with radiative memory effects.

Quantum confinement structures are attractive for their unconventional size dependence of the optical and electrical properties. There are still challenges to control the size uniformity for the application. The thesis studies the correlation between the size distribution of CdSe/ZnS quantum dots (QD), and the fluorescence properties to understand the shape and size influences of their fluorescence properties. Results from the transmission electron microscopy (TEM) provide the structure and size distribution of the samples. Excitation dependent fluorescence spectra as well as the PL excitation at various emission wavelength confirm that the inhomogeneous distribution of the samples. The results show that the samples are mostly composed of QDs with quasi-spherical structure (aspect ration between 1.1 and 1.5 ;76%) and spherical structure ( aspect ratio < 1.1; 12.8%). In addition, it exhibits a distribution of the long axis of 5.4nm¡Ó1.3nm. By measuring the fluorescence spectra of individual QDs, we construct the distribution. The peaks of the fluorescence spectra show a Gaussian distribution with center at 615.7 nm and width 13.8 nm. In addition, the spectra exhibit a width of 19.7¡Ó8.0nm. This is consistent with the ensemble measurement of the fluorescence from a solution (peak at 616 nm, and width 25 nm). Results of the fluorescence lifetime on the individual QDs indicate the lifetime distribution of 10.3¡Ó5.6ns. Further analyze the size distribution by constructing the size ¡V fluorescence spectrum relationship. By analysis the distribution of the fluorescence spectra, it results the corresponding size distribution of width 0.7 nm. This is much narrower than the size distribution of the long axis measured by TEM, but is more consistent with the corrected size distribution considering the short axis contribution. We conclude that the deviation results from the non-spherical structures in the samples.

In this work, we studied the optical characteristics of the coupled double DBR structure. We use the conventional transfer matrix simulation to find the intermediate multilayer periods (NC), and control the position of the transmission peak and stop band. Sample is grown by solid source molecular beam epitaxy (MBE) on n+GaAs (001) substrate, and the InGaAs QDs (quantum dots) are grown in the coupled cavity structure. The 23 periods of DBR multilayer, GaAs (91.8 nm) / AlAs (108.1 nm), obtain 99.5% reflectivity in the 1260 nm wavelength by the simulation. After the simulation from the conventional transfer matrix method, we choose NC = 13.5, the position of the transmission peak are at 1177 and 1188 nm, and optical frequency difference = 2.27 THz (£G=11 nm) in this study.From PL spectra, we observed interference between the enhanced light fields of the two cavity modes and the agreement between measurement and simulation. This structure is potential to be a compact terahertz emission device or vertical cavity surface emitting laser in room temperature.

Recent scientific progress has resulted in the development of sophisticated hybrid nanostructures composed of semiconductor and metal nanoparticles. These hybrid structures promise to produce a new generation of nanoscale optoelectronic devices that combine the best attributes of each component material. The optical response of metal nanostructures is dominated by surface plasmon resonances which create large local electromagnetic field enhancements. When coupled to surrounding semiconductor components, the enhanced local fields result in strong absorption/emission, optical gain, and nonlinear effects. Although hybrid nanostructures are poised to be utilized in a variety of applications, serious hurdles for the design of new devices remain. These difficulties largely result from a poor understanding of how the structural components interact at the nanoscale. The interactions strongly depend on the exact composition and geometry of the structure, and therefore, a quantitative comparison between theory and experiment is often difficult to achieve. Colloidal semiconductor quantum dots are strong candidates for integration with metal nanostructures because they have a variety of desirable optical properties, such as tunable emission and long term photostability. However, one potential drawback of colloidal quantum dots is the intermittency in their fluorescence (commonly referred to as “blinking”). Blinking was first observed over a decade ago, yet there is still no complete theory to explain why it occurs. In spite of the lack of a full theoretical explanation, multiple methods have been used to reduce blinking behavior, including modifying quantum dot interfaces and coupling quantum dots with metal nanostructures. This thesis focuses on studying the coupling between colloidal quantum dots and metal nanoparticles in simple model systems. Atomic force microscopy nanomanipulation is used to assemble the hybrid structures with a controlled geometry. The experimental studies report for the first time the modified fluorescence decay, emission intensity, and blinking of a single quantum dot coupled to a single Au nanoparticle. Since the geometry of the structure is known, these studies provide reliable information on the interparticle coupling, and quantitative experimental results are shown to be consistent with classical electrodynamic theories. / text