A ratchet is an asymmetric, non-equilibrated system that can produce a directed current of particles without the need for macroscopic potential gradients. In rocked quantum electron ratchets, tunnelling and wave-reflection can induce reversals in the direction of the net current as a function of system parameters. An asymmetric quantum point contact in a GaAs/GaAlAs heterostructure has been studied experimentally as a realisation of a quantum electron ratchet. A Landauer model predicts reversals in the direction of the net current as a function of temperature, amplitude of the rocking voltage, and Fermi energy. Artifacts such as circuit-induced asymmetry, also known as self-gating, were carefully removed from the experimental data, which showed net current and net differential conductance reversals, as predicted by the model. The model also predicts the existence of a heat current where the net electron current changes sign, as equal numbers of high and low energy electrons are pumped in opposite directions. An idealised quantum electron ratchet is studied analytically as an energy selective electron heat engine and refrigerator. The hypothetical device considered consists of two electron reservoirs with different temperatures and Fermi energies. The reservoirs are linked via a resonant state in a quantum dot, which functions as an idealised energy filter for electrons. The efficiency of the device approaches the Carnot value when the energy transmitted by the filter is tuned to that where the Fermi distributions in the reservoirs are equal. The maximum power regime, where the filter transmits all electrons that contribute positively to the power, is also examined. Analytic expressions are obtained for the power and efficiency of the idealised device as both a heat engine and as a refrigerator in this regime of operation. The expressions depend on the ratio of the voltage to the difference in temperature of the reservoirs, and on the ratio of the reservoir temperatures. The energy selective electron heat engine is shown to be non-endoreversible, and to operate in an analogous manner to the three-level amplifier, a laser based quantum heat engine. Implications for improving the efficiency of thermionic refrigerators and power generators are discussed.

Quantum dots (QDs) are common as luminescing markers for imaging in biological applications because their optical properties seem to be inert against their surrounding solvent. This, together with broad and strong absorption bands and intense, sharp tuneable luminescence bands, makes them interesting candidates for methods utilizing Förster Resonance Energy Transfer (FRET), e. g. for sensitive homogeneous fluoroimmunoassays (FIA). In this work we demonstrate energy transfer from Eu<SUP>3+</SUP>-trisbipyridin (Eu-TBP) donors to CdSe-ZnS-QD acceptors in solutions with and without serum. The QDs are commercially available CdSe-ZnS core-shell particles emitting at 655 nm (QD655). The FRET system was achieved by the binding of the streptavidin conjugated donors with the biotin conjugated acceptors. After excitation of Eu-TBP and as result of the energy transfer, the luminescence of the QD655 acceptors also showed lengthened decay times like the donors. The energy transfer efficiency, as calculated from the decay times of the bound and the unbound components, amounted to 37%. The Förster-radius, estimated from the absorption and emission bands, was ca. 77 Å. The effective binding ratio, which not only depends on the ratio of binding pairs but also on unspecific binding, was obtained from the donor emission dependent on the concentration. As serum promotes unspecific binding, the overall FRET efficiency of the assay was reduced. We conclude that QDs are good substitutes for acceptors in FRET if combined with slow decay donors like Europium. The investigation of the influence of the serum provides guidance towards improving binding properties of QD assays.

Owing to their unique physical, chemical, and mechanical properties, nanoparticles (NPs) have been used, or are being evaluated for use, in many fields (e.g., personal care and cosmetics, pharmaceutical, energy, electronics, food and textile). However, concerns regarding the environmental and biological implications of NPs are raised alongside the booming nanotechnology industry. Numerous studies on the biological effect of NPs have been done in the last decade, and many mechanisms have been proposed. In brief, mechanisms underlying the adverse biological effect caused by NPs can be summarized as: (i) indirect adverse effect induced by reactive oxygen species (ROS) generated by NPs, (ii) indirect adverse effect induced by released toxic ions, and (iii) adverse effect induced by direct interactions of NPs with biological systems. Up to now, most efforts have been focused on the first two mechanisms. In contrast, adverse biological effects induced by direct nano-bio interactions are the least researched. This is largely because of the complexity and lack of suitable techniques for characterizing the nano-bio interface. This dissertation aims at advancing our understanding of the nano-bio interactions leading to the adverse biological effect of NPs. Specifically, it is comprised of three parts. Firstly, because the aggregation of NPs alters particle size and other physicochemical properties of NPs, the property of NPs reaching and interacting with biological cells is very likely different from that of what we feed initially. Consequently, as the first step and an essential prerequisite for understanding the biological effect of NPs, NP aggregation is investigated and models are developed for predicting the stability and the extent of aggregation of NPs. Secondly, interactions between NPs and cell membrane are studied with paramecium as the model cell. Due to the lack of cell wall, the susceptible cell membrane of paramecium is directly exposed to NPs in the medium. The extent and strength of direct nano-cell membrane interaction is evaluated and quantified by calculating the interfacial force/interaction between NPs and cell membrane. A correlation is further established between the nano-cell membrane interaction and the lethal acute toxicity of NPs. We find NPs that have strong association or interaction with the cell membrane tend to induce strong lethal effects. Lastly, we demonstrate systematic experimental approaches based on atomic force microscope (AFM), which allows us to characterize nano-bio interfaces on the single NP and single-molecular level, coupled with modeling approaches to probe the nano-DNA interaction. Using quantum dots (QDs) as a model NP, we have examined, with the novel application of AFM, the NP-to-DNA binding characteristics including binding mechanism, binding kinetics, binding isotherm, and binding specificity. We have further assessed the binding affinity of NPs for DNA by calculating their interaction energy on the basis of the DLVO models. The modeling results of binding affinity are validated by the NP-to-DNA binding images acquired by AFM. The investigation of the relationship between the binding affinity of twelve NPs for DNA with their inhibition effects on DNA replication suggests that strong nano-DNA interactions result in strong adverse genetic effects of NPs. In summary, this dissertation has furthered our understanding of direct nano-bio interactions and their role in the biological effect of NPs. Furthermore, the models developed in this dissertation lay the basis for building an “ultimate” predictive model of biological effects of NPs that takes into account multiple mechanisms and their interactions, which would save a lot of testing costs and time in evaluating the risk of NPs.

Reversible circuits are similar to conventional logic circuits except that they are built from reversible gates. In reversible gates, there is a unique, one-to-one mapping between the inputs and outputs, not the case with conventional logic. Also, reversible gates require constant ancilla inputs for reconfiguration of gate functions and garbage outputs that help in keeping reversibility. Reversible circuits hold promise in futuristic computing technologies like quantum computing, quantum dot cellular automata, DNA computing, optical computing, etc. Thus, it is important to minimize parameters such as ancilla and garbage bits, quantum cost and delay in the design of reversible circuits. The first contribution of this dissertation is the design of a new reversible gate namely the TR gate (Thapliyal-Ranganathan) which has the unique structure that makes it ideal for the realization of arithmetic circuits such as adders, subtractors and comparators, efficient in terms of the parameters such as ancilla and garbage bits, quantum cost and delay. The second contribution is the development of design methodologies and a synthesis framework to synthesize reversible data path functional units, such as binary and BCD adders, subtractors, adder-subtractors and binary comparators. The objective behind the proposed design methodologies is to synthesize arithmetic and logic functional units optimizing key metrics such as ancilla inputs, garbage outputs, quantum cost and delay. A library of reversible gates such as the Fredkin gate, the Toffoli gate, the TR gate, etc. was developed by coding in Verilog for use during synthesis. The third contribution of this dissertation is the set of methodologies for the design of reversible sequential circuits such as reversible latches, flip-flops and shift registers. The reversible designs of asynchronous set/reset D latch and the D flip-flop are attempted for the first time. It is shown that the designs are optimal in terms of number of garbage outputs while exploring the best possible values for quantum cost and delay. The other important contributions of this dissertation are the applications of reversible logic as well as a special class of reversible logic called conservative reversible logic towards concurrent (online) and offline testing of single as well as multiple faults in traditional and reversible nanoscale VLSI circuits, based on emerging nanotechnologies such as QCA, quantum computing, etc. Nanoelectronic devices tend to have high permanent and transient faults and thus are susceptible to high error rates. Specific contributions include (i) concurrently testable sequential circuits for molecular QCA based on reversible logic, (ii) concurrently testable QCA-based FPGA, (iii) design of self checking conservative logic gates for QCA, (iv) concurrent multiple error detection in emerging nanotechnologies using reversible logic, (v) two-vectors, all 0s and all 1s, testable reversible sequential circuits.

A system with two magnetic molecules embedded in a junction between non-magnetic leads was studied. In this system electrons tunnel from the localized energy level in region one to the localized energy level in region two generating a flow of electric charge through the quantum dot system. The current density and thus the conductance changes depending on the molecular spin moment. In this work we studied molecules with either spin "up" or spin "down" and with symmetric coupling strengths. The results indicate that the coupling strength between energy level and molecule together with the tunneling rate through the insulating layer play a major role when switching from parallel to anti-parallel molecular spin, for a specific combination of the coupling strength and tunneling rate we could observe a decrease in the current by 99.7% in the non-gated system and 99.4% in the gated system.

No description available.

Ce mémoire porte sur les mécanismes de relaxation et de fuite des excitons dans des systèmes de boîtes quantiques(BQs) d’InAs/InP. Les systèmes sont composés d’un sub- strat volumique d’InP, appelé matrice (M), d’un puits quantique d’InAs, nommé couche de mouillage (CM), et des familles de BQs d’InAs. La distinction entre les familles est faite par le nombre de monocouche d’épaisseur des boîtes qui sont beaucoup plus larges que hautes. Une revue de littérature retrace les principaux mécanismes de relaxation et de fuite des excitons dans les systèmes. Ensuite, différents modèles portant sur la fuite thermique des excitons des BQs sont comparés. Les types de caractérisations déjà produites et les spécifications des croissances des échantillons sont présentés. L’approche adoptée pour ce mémoire a été de caractériser temporellement la dynamique des BQs avec des mesures d’absorbtion transitoire et de photoluminescence résolue en temps (PLRT) par addition de fréquences. L’expérience d’absorption transitoire n’a pas fait ressortir de résultats très probants, mais elle est expliquée en détails. Les mesures de PLRT ont permis de suivre en température le temps de vie effectif des excitons dans des familles de BQs. Ensuite, avec un modèle de bilan détaillé, qui a été bien explicité, il a été possible d’identifier le rôle de la M et de la CM dans la relaxation et la fuite des excitons dans les BQs. Les ajustements montrent plus précisément que la fuite de porteurs dans les BQs se fait sous la forme de paires d’électrons-trous corrélées. / This thesis focuses on the mechanisms of relaxation and leakage of excitons in systems of quantum dots (QDs) InAs / InP. The systems are composed of a substrate of InP volume, called matrix (M), of a quantum well of InAs, named wetting layer (CM), and of QD families of InAs. The distinction between the families can be explained by the number of monolayer-thick boxes that are wider than high. A literature review highlights the main relaxation mechanisms and leakage of excitons in systems. Then, different models on the thermal leakage of the QD excitons are compared.Then, a presentation of the different types of characterizations already and of the specifications on the samples growths. The approach used for this thesis is to temporarily characterize the dynamic of the QDs with transient absorption and upconversion. The transient absorption experiment’s results are not very convincing, but are minutely explained. PLRT measures were used to follow in temperature the excitons effective lifetime in the QDs families. Then, with a detailed balance model, which has been well explained, it was possible to identify the role of theMand CM in relaxation and leakage of excitons in QDs. As shown by the adjustement, the escape of carriers in the QDs is made in a correlated electron-hole pairs form.

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.

Il est possible de modifier en quelques picosecondes les fréquences de résonance d’une microcavité optique semiconductrice en injectant optiquement des porteurs de charge dans le semiconducteur. Dans cette thèse, nous étudions en détail de tels évènements de commutation tout-optique pour des cavités planaires et des cavités en forme de micropilier à base de GaAs/AlAs, en utilisant l’émission de boîtes quantiques intégrées dans ces cavités comme source interne de lumière pour sonder la fréquence des modes résonnants en fonction du temps. Des décalages en fréquence très conséquents, de l’ordre de 34 fois la largeur du mode considéré, sont obtenus après optimisation. Nous réalisons une commutation différentielle des modes d’un micropilier en injectant les porteurs de manière très localisée, et modélisons les comportements observés en prenant en compte la distribution des porteurs injectés ainsi que leur diffusion et leur recombinaison en fonction du temps. Nous étudions par ailleurs deux applications potentielles importantes de la commutation ultrarapide de cavité. D’une part, nous modélisons le changement de couleur qui est induit sur de la lumière piégée dans un mode de cavité lors d’un évènement de commutation. Nous montrons que pour une cavité planaire optimisée, une telle conversion de fréquence peut être réalisée de façon très efficace. D’autre part, la commutation de cavité peut aussi être employée pour contrôler en temps réel l’émission spontanée d’émetteurs intégrés, et plus généralement tous les effets d’électrodynamique quantique en cavité. Nous présentons la génération d’impulsions de lumière incohérente de quelques picosecondes seulement, en utilisant l’émission spontanée de boîtes quantiques dans un micropilier commuté. Nous montrons aussi par une étude théorique qu’il est possible de donner une forme choisie aux impulsions à un photon émises par une boîte quantique, ce qui ouvre des applications intéressantes dans le domaine des liens optiques quantiques et du traitement quantique photonique de l’information. / The resonance wavelengths of semiconductor optical microcavities can be changed within few picoseconds through the optical injection of free charge carriers. In this PhD thesis, we study in detail such “cavity switching” events for GaAs/AlAs planar and micropillar cavities, using the spontaneous emission of embedded QDs as an internal light source to probe the time-dependent frequencies of the cavity modes. Switching amplitudes as large as 34 mode linewidths are observed for optimized pumping conditions. Differential switching of micropillar modes is achieved by performing a localized injection of charge carriers, and modeled by taking into account their injection profile, diffusion and recombination processes. We investigate two important potential applications of cavity switching in the field of quantum optics. On one hand, we model the frequency conversion of light trapped in a cavity mode, which is induced by a switching event, and show that adiabatic and highly efficient frequency conversion can be achieved in properly designed planar cavities. On the other hand, cavity switching appears as a powerful resource to control in real-time the spontaneous emission of embedded emitters and more generally CQED effects. As a first example, we demonstrate the generation of few picosecond short pulses of incoherent light, using the spontaneous emission of switched QD-micropillars. We also show theoretically that cavity switching can be used to shape the time-envelope of single photon pulses emitted by a single QD, which is highly desirable for quantum-optical links and photonic quantum information processing.

Le contrôle du nombre et de l'état quantique d'électrons individuels est un élément clé pour la construction d'applications innovantes comme les sources à un électron ou les standards métrologiques de courant. La difficulté d'atteindre la précision métrologique pour une source de courant alimente la recherche fondamentale sur le transport individuel d'électrons dans les structures mésoscopiques. Un candidat prometteur combine le concept de quantification de la charge dans un transistor à un électron et la bande interdite de la densité d'états d'électrodes supraconductrices. Le transport corrélé en temps d'électrons entre les électrodes supraconductrices est alors assuré par la densité d'états continue de l'ilot métallique central. Le grand nombre d'états électroniques disponibles dans l'ilot, bien que favorable en termes de couplage tunnel, a néanmoins deux conséquences importantes que sont les fluctuations thermiques et des processus parasites d'ordre supérieur, ce qui limite la performance de ces dispositifs. Dans ce contexte, nous explorons le transport de charges dans un tourniquet à électrons hybride basé sur une boîte quantique en lieu et place de l'ilot métallique. Les dispositifs sont réalisés par l'électromigration contrôlée de constrictions d'Aluminium précédée par le dépôt aléatoire de nano-particules d'or. Ce procédé in-situ (réalisé à 4 K) permet l'obtention de jonctions tunnel entre des électrodes supraconductrices d'aluminium et nano-particules d'or avec un taux de succès de l'ordre de 4%. Nous caractérisons le transport statique et en fréquence dans ces nanostructures par la mesure statique du courant à une température de 100 mK dans un environnement fortement filtré, mais néanmoins compatible avec l'électro-migration, d'un réfrigérateur à dilution. L'analyse des cartes de conductance en fonction des tensions drain-source et de grille révèle une énergie de charge très élevée de l'ordre de 10 meV et un écart entre niveaux discrets d'énergie de l'ordre de 1 meV. Par une étude détaillée de l'élargissement des pics de cohérence au seuil du blocage de Coulomb, nous montrons que le transport électronique est assuré par un niveau unique dans la boîte quantique. Bien que le couplage tunnel soit faible, le temps de vie d'un électron dans un niveau donné est dominé par l'hybridation des états électroniques entre les électrodes et la boîte quantique. En effet, les fluctuations thermiques et les processus inélastiques sont inopérants du fait du grand écart d'énergie entre niveaux et de la bande interdite supraconductrice dans les électrodes. L'observation de résonances sous le seuil imposé par le blocage de Coulomb est décrite par des processus de cotunneling de type paire de Cooper-électron. Lorsqu'un signal radio-fréquence de forme sinusoïdale ou carrée est ajouté à la tension de grille, un fonctionnement de tourniquet à électron est montré. Nous obtenons un courant quantifié jusqu'à une fréquence de 200 MHz, au delà de laquelle la précision se dégrade à cause d'évènements tunnel manqués. Le couplage à un niveau unique dans la boîte quantique est clairement démontré par l'apparition d'effets de transport tunnel inversé à grande tension drain-source ainsi que l'insensibilité à la température jusqu'à environ 300 mK. Enfin, nous observons une suppression systématique du courant uniquement à basse fréquence et avec un signal r.f. sinusoïdal. En accord avec une prédiction théorique, nous montrons que les effets tunnel manqués sont causés par un processus adiabatique au travers l'anti-croisement d'un niveau quantique sur la boîte quantique avec la densité d'états des électrodes supraconductrices. Nos expériences fournissent la première démonstration expérimentale de la répulsion de niveaux entre un niveau discret et un semi-continuum, illustrant ainsi l'évolution cohérente de nos tourniquets hybrides à électron dans un régime adiabatique. / Accurate control over the state and motion of single individual electrons would enable a variety of appealing applications reaching from quantized to quantum coherent electron sources. Realizing the accuracy of quantized current sources required for a metrological standard is however extremely challenging and has naturally fuelled fundamental research into single electron transport through mesoscopic structures. A promising candidate, foreseen to meet the demand, combines the concept of quantized charge in single electron transistors (SETs) and the gapped density of states in superconducting metals (hence called hybrid electron turnstile), to produce a quantized current. The time-correlated electron transport (sub-poissonian) between the superconducting leads is conveyed by the continuous density of states of the central normal island. The large amount of available states at the normal island, although favorable in terms of tunnel coupling, has nevertheless two important ramifications i.e. 1) thermal fluctuations and 2) adverse higher-order processes, which limit the performance of hybrid electron turnstiles. Inspired by this ingenious application and the advances in quantum dot trans- port, we explore the operation of a hybrid electron turnstile embodying a bottom-up quantum dot instead of the usual metallic island. The desired devices are obtained by controlled electromigration of aluminium nano-wires preceded by the deposition of gold nano-particles. This in-situ process (conducted at 4 K) produces pristine tunnel junctions between aluminium leads and gold nano-particles with a yield of about 4%. We characterize the stationary and turnstile operation by direct current measurements at 100 mK, in a heavily filtered, but electromigration compatible, inverse dilution refrigerator. Analysis of the acquired conductance maps under stationary conditions, reveal a large charging energy (> 10 meV) and mean level spacing (> 1 meV). With a detailed study of the coherence peak broadening at the Coulomb blockade (CB) threshold, we show that electron transport through the quantum dot is conveyed by a single quantum level. Although the tunnel coupling is weak, the single level life-time is dominated by the lead - quantum dot hybridization as thermal energy fluctuation and in-elastic scattering are suppressed by the large single level spacing on the quantum dot and the superconducting gap in the leads. The observation of sub-threshold resonances parallel to the CB diamond edges are consistent with earlier predicted higher-order Cooper-pair - electron (CPE) cotunneling processes. Under turnstile operation a periodic modulation signal (sine or square wave) is added to the static gate potential. We demonstrate quantized current up to 200 MHz at which its accuracy starts to worsen due to missed tunnel events. Strong experimental evidence of the single quantum dot level nature of our turnstile device is provided by a sharp onset of backtunneling processes and the temperature-robust operation beyond 300 mK. Finally we observe a systematic current suppression unique to the low frequency sine wave operation. Supported by theoretical work, we show that the underlying missed tunnel events are caused by adiabatic traverses across the avoided crossing of a quantum dot level and superconducting gap edges. These experiments deliver the first experimental observation of the level repulsion between an electronic discrete state and a semi-continuum and demonstrate the quantum coherent evolution of our devices under adiabatic operation conditions.