Дисертації з теми "Verres à quantum dots"
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1
Wang, Zheng. "Synthesis, properties and applications of glasses containing chalcogenide quantum dots." Electronic Thesis or Diss., Université de Rennes (2023-....), 2023. http://www.theses.fr/2023URENS093.
Анотація:
Dans cette thèse, la synthèse, les propriétés et les applications de verres contenant des quantum dots (QDs) de chalcogénure ont été étudiées. Des verres contenant des QDs à base de chalcogénure de plomb (PbSe ou PbS) ont été préparés. Leurs propriétés optiques et leurs applications potentielles ont été explorées en combinaison avec le co-dopage aux ions Tm3+. De plus, sur la base de ces résultats, des verres contenant des QDs de ZnS ou de ZnSe, sans plomb, ont été préparés avec succès. Leurs performances luminescentes ont été encore améliorées par dopage avec des ions de métaux de transition représentés ici par le nickel. Ces résultats jettent les bases pour l’amélioration des propriétés optiques de verres contant des QDs à base de chalcogénure de plomb et aussi pour le développement de verres aux QD sans métaux lourds et donc plus respectueux de l’environnement. Bien que des améliorations futures soient possibles et nécessaires pour des applications réelles, ces verres aux QDs de chalcogénure, développés dans ce travail, présentent un potentiel d'applications dans les domaines des concentrateurs solaires luminescents, de l'anti-contrefaçon optique, de l'éclairage à semi-conducteurs et de la mesure optique de la températureIn this dissertation, the synthesis, properties and applications of glasses containing chalcogenide quantum dots (QDs) have been studied. Multicomponent lead chalcogenide QDs glasses (containing PbSe or PbS QDs) were successfully prepared, and their optical properties and potential applications were explored in combination with rare earth Tm3+ ion doping. In addition, based on the results, lead-free and environmentally friendly chalcogenide QDs glasses (containing ZnS or ZnSe QDs) were successfully prepared, and its luminescent performance was further improved by doping with transition metal nickel ions. These results lay the foundation for the improvement of optical properties of lead-based chalcogenide QDs and for the development of environmentally friendly heavy metal-free chalcogenide QDs glasses. Although future improvements are possible and necessary for practical applications, these chalcogenide QDs glasses developed in this work have application potential in the fields of luminescent solar concentrators, optical anti-counterfeiting, solid-state lighting, and optical temperature sensing
2
Shliahetskiy, A. A. "Quantum dots." Thesis, Sumy State University, 2015. http://essuir.sumdu.edu.ua/handle/123456789/40495.
Анотація:
The investigation of semiconductors quantum dot began in
1981 by Alexei Ekimov. Scientists started interested in quantum dot
after the quantum effects were discovered in spectrum of many
nanocrystals. The term ―Quantum dot‖ appeared in 1988.
3
Wardrop, Matthew Phillip. "Quantum Gates for Quantum Dots." Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/14938.
Анотація:
Since the mid-20th century it has been understood that a general-purpose quan- tum computer would be able to efficiently solve problems that will forever be out-of-reach for conventional computers. Since then, many quantum algorithms have been developed with applications in a wide range of domains including cryptography, simulations, machine learning and data analysis. While this has resulted in substantial attention being paid to the development of quantum com- puters, the best architectures to use in their fabrication is not yet clear. Semiconductor quantum dot devices are a particularly promising candidate for use in quantum computing architectures, as it is anticipated that once the funda- mental building blocks are implemented, they might be massively scalable using the existing lithography techniques of the semiconductor industry. So far, how- ever, it is not yet clear how best to implement the high-fidelity gates required for general-purpose quantum computation. In this thesis, we present and characterise novel theoretical proposals for fast, simple and high-fidelity two-qubit gates using magnetic (exchange) coupling for specific semiconductor quantum dot qubits; namely, the singlet-triplet and resonant-exchange qubits. These two-qubit operations are simple enough that it is feasible for them to be implemented in experiments of the near future. Success- ful implementations would significantly extend the experimentally demonstrable frontier of semi-conductor quantum dot devices as relevant to their use in uni- versal quantum computing architectures. We also develop simple parameter estimation schemes by which it is possible to substantially mitigate the dominant sources of error for our proposed gates; namely, low-frequency charge and magnetic noise. We develop the techniques in the context of pseudo-static magnetic field gradient fluctuations in singlet- triplet qubits, and demonstrate that these techniques lead to a several orders of magnitude improvement in single-qubit coherence times. With minimal effort this could be ported to other qubit architectures.
4
Garrido, Mauricio. "Quantum Optics in Coupled Quantum Dots." Ohio University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1273589966.
5
Chiu, Kuei-Lin. "Transport properties of graphene nanodevices - nanoribbons, quantum dots and double quantum dots." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610526.
6
Chan, Ka Ho Adrian. "Quantum information processing with semiconductor quantum dots." Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648684.
7
Xu, Xiulai. "InAs quantum dots for quantum information processing." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615012.
8
Christ, Henning. "Quantum computation with nuclear spins in quantum dots." München Verl. Dr. Hut, 2008. http://d-nb.info/992162831/04.
9
Erdem, Rengin. "Ag2s/2-mpa Quantum Dots." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614384/index.pdf.
Анотація:
Quantum dots are fluorescent semiconductor nanocrystals that have unique optical properties such as high quantum yield and photostability. These nanoparticles are superior to organic dyes and fluorescent proteins in many aspects and therefore show great potential for both in vivo and in vitro imaging and drug delivery applications. However, cytototoxicity is still one of the major problems associated with their biological applications.
The aim of this study is in vitro characterization and assessment of biological application potential of a novel silver sulfide quantum dot coated with mercaptopropionic acid (2-MPA). In vitro studies reported in this work were conducted on a mouse fibroblast cell line (NIH/3T3) treated with Ag2S/2-MPA quantum dots in 10-600 &mug/mL concentration range for 24 h. Various fluorescence spectroscopy and microscopy methods were used to determine metabolic activity, proliferation rate and apoptotic fraction of QD-treated cells as well as QD internalization efficiency and intracellular localization. Metabolic activity and proliferation rate of the QD treated cells were measured with XTT and CyQUANT®
cell proliferation assays, respectively. Intracellular localization and qualitative uptake studies were conducted using confocal laser scanning microscopy. Apoptosis studies were performed with Annexin V assay. Finally, we also conducted a quantitative uptake assay to determine internalization efficiency of the silver sulfide particles. Correlated metabolic activity and proliferation assay results indicate that Ag2S/2-MPA quantum dots are highly cytocompatible with no significant toxicity up to 600 &mu
g/mL treatment. Optimal cell imaging concentration was determined as 200 &mu
g/mL. Particles displayed a punctuated cytoplasmic distribution indicating to endosomal entrapment. In vitro characterization studies reported in this study indicate that Ag2S/2-MPA quantum dots have great biological application potential due to their excellent spectral and cytocompatibility properties. Near-infrared emission of silver sulfide quantum dots provides a major advantage in imaging since signal interference from the cells (autofluorescence) which is a typical problem in microscopic studies is minimum in this part of the emission spectrum. The results of this study are presented in an article which was accepted by Journal of Materials Chemistry. DOI: 10.1039/C2JM31959D.
10
Korkusinski, Marek. "Correlations in semiconductor quantum dots." Thesis, University of Ottawa (Canada), 2004. http://hdl.handle.net/10393/29128.
Анотація:
In this Thesis, I present a theoretical study of correlation effects in strongly interacting electronic and electron-hole systems confined in semiconductor quantum dots. I focus on three systems: N electrons in a two-dimensional parabolic confinement in the absence and in the presence of a magnetic field, an electron-hole pair confined in a vertically coupled double-quantum-dot molecule, and a charged exciton in a quantum-ring confinement in a magnetic field.
To analyse these systems I use the exact diagonalisation technique in the effective-mass approximation. This approach consists of three steps: construction of a basis set of particle configurations, writing the Hamiltonian in this basis in a matrix form, and numerical diagonalisation of this matrix. Each of these steps is described in detail in the text.
Using the exact diagonalisation technique I identify the properties of the systems due to correlations and formulate predictions of how these properties could be observed experimentally. I confront these predictions with results of recent photoluminescence and transport measurements.
First I treat the system of N electrons in a parabolic confinement in the absence of magnetic field and demonstrate how its properties, such as magnetic moments, can be engineered as a function of the system parameters and the size of the Hilbert space.
Next I analyse the evolution of the ground state of this system as a function of the magnetic field. In the phase diagram of the system I identify the spin-singlet nu = 2 phase and discuss how correlations influence its phase boundaries both as a function of the magnetic field and the number of electrons.
I also demonstrate that in higher magnetic fields electronic correlations lead to the appearance of spin-depolarised phases, whose stability regions separate the weakly correlated phases with higher spin. Further on, I consider electron-hole systems. I show that the Coulomb interaction leads to entanglement of the states of an electron and a hole confined in a pair of vertically coupled quantum dots.
Finally I consider the system of two electrons and one hole (a negatively charged exciton) confined in a quantum ring and in the presence of the magnetic field. I show that the energy of a single electron in the ring geometry exhibits the Aharonov-Bohm oscillations as a function of the magnetic field. In the case of the negatively charged exciton these oscillations are nearly absent due to correlations among particles, and as a result the photoluminescence spectra of the charged complex are dominated by the energy of the final-state electron. The Aharonov-Bohm oscillations of the energy of a single electron are thus observed directly in the optical spectra.
11
Larsson, Mats. "Spectroscopy of semiconductor quantum dots /." Linköping : Linköpings universitet, 2005. http://www.bibl.liu.se/liupubl/disp/disp2005/tek976s.pdf.
12
Foo, Edward. "Ultrafast spectroscopy of quantum dots." Thesis, University of Oxford, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.393775.
13
Lee, Steven Frank. "Photodynamics of single quantum dots." Thesis, University of Sussex, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.494932.
Анотація:
A range of photochemical processes in biologically compatible quantum dots (QDs) at the individual nanocrystal level has been examined. Using objective type total internal reflection fluorescence microscopy we demonstrate the observation of photoluminescence activation (PLA), enhancement (PLE), fluorescence intermittency (Fl), decay and spectral shifting over the photochemical lifetimes of single QDs. The ability to count individual QDs along with their intensities has revealed a measurable nonlinear correlation between the total fluorescence and the QD population that is the result of enhancement and decay in the quantum yield (QY) of individual dots. This allows the differentiation between ensemble photoluminescence enhancement due to the fluorescence activation of a dark, non-emitting fraction of QDs (PLA) and that due to genuine QY modification in individual nanocrystals (PLE).
14
Hill, Richard John Allan. "Tunnelling into InAs quantum dots." Thesis, University of Nottingham, 2003. http://eprints.nottingham.ac.uk/10002/.
Анотація:
This thesis describes an experimental study of the electronic properties of semiconductor heterostructure tunnel devices. InAs self-assembled quantum dots (QDs) are incorporated into the barrier layer of a GaAs/AlAs/GaAs tunnel diode. When a voltage, V, is applied across the device, we observe resonant features in the tunnel current, I, whenever an electron state in one of the qds comes into resonance with an occupied electron state in the emitter. We employ an electron state of a single qd as a spectroscopic probe of a two-dimensional electron system (2DES), from the Fermi energy to the subband edge [1]. For magnetic field B applied parallel to the current, we observe peaks in the I(V) characteristics corresponding to the formation of Landau levels in the 2DES. We obtain quantitative information about the energy dependence of the quasiparticle lifetime, Tqp, of the 2DES. We find that Tqp ~ 2.5 hbar=(Ef - E), in contrast with the expectation for a normal Fermi liquid, but in agreement with predictions for a Fermi liquid state of a disordered 2DES. Close to filling factor nu = 1 we observe directly the exchange enhancement of the g factor. This thesis also describes the design, realisation and measurement of a tunnel diode incorporating InAs QDs and a series of 4 planar electrostatic gates. By applying a bias to the gates, it is possible to selectively inject current into a particular QD. We use magneto-tunnelling spectroscopy to determine the energy levels of the ground and excited state of a single QD, and to map the spatial form of the wave functions of these states [2]. The effect of pressure on the resonant tunnelling of the QDs is also described. [1] P. C. Main et al., Phys. Rev. Lett. 84, 729 (2000) [2] R. J. A. Hill et al., Appl. Phys. Lett. 79, 3275 (2001)
15
Tipton, Denver Leonard John. "Electron correlations in quantum dots." Thesis, King's College London (University of London), 2001. https://kclpure.kcl.ac.uk/portal/en/theses/electron-correlations-in-quantum-dots(b6e1a547-fbb8-41f7-9d2b-f235b68b85be).html.
16
Tewordt, Matthias Ludwig. "Resonant tunnelling in quantum dots." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260415.
17
Yang, Kaiyu, and 楊開宇. "Quantum information processing with quantum dots and Josephson junctions." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B29285835.
18
Curtin, Oliver James. "Quantum criticality and emergent symmetry in coupled quantum dots." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/42499.
Анотація:
We consider strongly correlated regimes which emerge at low temperature in coupled quantum dot (or magnetic impurity) systems. In strongly correlated systems a single particle description fails to explain the observed behaviour, so we resort to many body methods. We describe our system using a 2-impurity Anderson model and develop a numerical renormalisation group procedure which provides non-perturbative insight into the low energy behaviour, through calculation of dynamic quantities. We combine this approach with renormalised perturbation theory, thus acquiring a picture of how the Hamiltonian and interactions change at low energies. These approaches are first used to study the emergence of a Kondo effect with an SU(4) symmetry in capacitively-coupled double quantum dot systems. We classify the 'types' of SU(4) symmetry which can emerge and show how an experimentalist might achieve such emergence through tuning their system. We provide a way of distinguishing between the SU(2) and SU(4) Kondo regimes by considering the conductance. We also study a quantum critical point which occurs in the Heisenberg coupled quantum dot/impurity model. There is an anomalous entropy contributed by the impurities in this regime which is indicative of an uncoupled Majorana Fermion. We calculate dynamic quantities in regimes with different symmetries and establish correspondence with the 2-channel Kondo model. We formulate possible pictures of the underlying mechanisms of the critical point and construct a Majorana fermion model for the case with particle-hole symmetry, which explains the non-Fermi liquid energy levels and degeneracies obtained. We conjecture that a Majorana zero mode is present, and that this is responsible for the anomalous entropy.
19
Goldmann, Eyal. "Studies of quantum dots in the quantum hall regime /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC IP addresses, 1999. http://wwwlib.umi.com/cr/ucsd/fullcit?p9945779.
20
Pegahan, Saeed. "QUANTUM ERROR CORRECTION AND LEAKAGE ELIMINATION FOR QUANTUM DOTS." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1753.
Анотація:
The development of a quantum computer presents one of the greatest challenges in science and engineering to date. The promise of more ecient computing based on entangled quantum states and the superposition principle has led to a worldwide explosion of interest in the elds of quantum information and computation. Decoherence is one of the main problems that gives rise to dierent errors in the quantum system. However, the discovery of quantum error correction and the establishment of the accuracy threshold theorem provide us comprehensive tools to build a quantum computer. This thesis contributes to this eort by investigating a particular class of quantum error correcting codes, called Decoherence free subsystems. The passive approach to error correction taken by these encodings provides an ecient means of protection for symmetrically coupled system-bath interactions. Here I will present methods for determining the subsystem-preserving evolutions for noiseless subsystem encodings and more importantly implementing a Universal quantum computing over three-quantum dots.
21
Schmall, Nicholas Edward. "Fabrication of Binary Quantum Solids From Colloidal Semiconductor Quantum Dots." Bowling Green State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1245257669.
22
Siegert, Jörg. "Carrier dynamics in semiconductor quantum dots." Doctoral thesis, KTH, Mikroelektronik och tillämpad fysik, MAP, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4019.
Анотація:
This thesis presents results of time-resolved photoluminescence experiments conducted on several different self-assembled InGaAs/GaAs and InAs/GaAs semiconductor quantum dot (QD) structures. Depending on the application in mind, different structural, electronic or optical properties have a different weight of importance. Fast carrier capture and relaxation is critical for QD based lasers, for example. In this thesis, the influence of surplus carriers, introduced through modulation-doping, is studied. It is shown that carrier capture is essentially unaffected whereas the intradot relaxation mechanisms, at least at low carrier concentrations, are fundamentally different. The phonon mediated cascade relaxation found in the undoped reference sample is replaced by efficient scattering with the built-in carriers in the case of the doped structures. Moreover, spin relaxation also depends on presence of extra carriers. During energy relaxation via carrier-carrier scattering, the spin polarization is preserved whereas in the undoped sample the strong interaction of relaxing carriers with LO phonons causes spin relaxation. The decay of the ground state spin polarization proceeds at the same rate for doped and undoped structures and is shown to be caused by acoustic phonons, even up to 300 K. While optimizing QD growth for specific applications, it is imperative to evaluate the influence of nonradiative recombination, which is most often detrimental. While misfit dislocations, deliberately introduced in the substrate, lead to the formation of laterally ordered, uniform dots, these samples are found to suffer from strong nonradiative recombination. Structures with different barrier thicknesses and numerical simulations indicate defects in the vicinity of the QDs as main origin of fast carrier trapping. On the other hand, it is shown that direct dot doping, compared to barrier doping or undoped structures, causes only minor degradation of the optical properties. Directly doped dots even exhibit a significantly weaker photoluminescence quenching with temperature, making them prospective for devices operating at room temperature. Finally, the superior proton radiation hardness of QD structures compared to quantum wells is demonstrated, which is due to the three-dimensional confinement. The increase of photoluminescence intensity at low to moderate doses is interpreted as an enhanced carrier transfer into the dots via the defects introduced into the material by the protons.QC 20100920
23
Larsson, Arvid. "Optical spectroscopy of InGaAs quantum dots." Doctoral thesis, Linköpings universitet, Halvledarmaterial, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-64707.
Анотація:
The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+). Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.Arbetet som presenteras i denna avhandling rör studier av kvantprickars optiska egenskaper. En kvantprick är en halvledarkristall som endast är några tiotals nanometer stor. Den ligger oftast inbäddad inuti en större kristall av ett annat halvledarmaterial och pga. den begränsade storleken får en kvantprick mycket speciella egenskaper. Bland annat så kommer elektronerna i en kvantprick endast att kunna anta vissa diskreta energinivåer liknande situationen för elektronerna i en atom. Följaktligen kallas kvantprickar ofta för artificiella atomer. För halvledarmaterial gäller det generellt att det inte endast är fria elektroner i ledningsbandet, som kan leda ström utan även tomma elektrontillstånd i valensbandet, vilka uppträder som positivt laddade partiklar, kan leda ström. Dessa kallas kort och gott för hål. I en kvantprick har hålen såsom elektronerna helt diskreta energinivåer. Precis som är fallet i en atom, så kommer elektroniska övergångar mellan olika energinivåer i en kvantprick att resultera i att ljus emitteras. Energin (dvs. våglängden alt. färgen) för detta ljus bestäms av hur energinivåerna i kvantpricken ligger, för elektronerna och hålen, och genom att analysera ljuset kan man således studera kvantprickens egenskaper. Studierna i den här avhandlingen visar att växelverkan mellan en kvantprick och den omgivande kristallen, som den ligger inbäddad i, har stor inverkan på kvantprickens optiska egenskaper. T.ex. visas att man kan kontrollera antalet elektroner, som kommer att finnas i kvantpricken genom att modifiera hur elektronerna kan röra sig i omgivningen. Dessa rörelser modifieras här genom att variera temperaturen och genom att lägga på ett magnetiskt fält. Ett magnetiskt fält, vinkelrätt mot en elektrons rörelse, kommer att böja av dess bana och dess chans att nå fram till kvantpricken kan således minskas. Elektronen kan då istället fastna i andra potentialgropar i kvantprickens närhet. Genom att öka temperaturen, vilket ger elektronerna större energi, kan deras chans att nå fram till kvantpricken å andra sidan öka. En annan effekt, som studerats, är möjligheten att kontrollera spinnet hos elektronerna i en kvantprick. Även i dessa studier visar det sig att växelverkan med omgivningen spelar stor roll och kan användas till att kontrollera elektronens spin. Mekanismen som föreslås är att om elektronerna hinner före hålen till kvantpricken, så hinner de överföra sitt spin till atomkärnorna i kvantpricken. På detta sätt kan man få atomkärnornas spin polariserat, vilket resulterar i ett inbyggt magnetfält, i storleksordningen 1.5 Tesla, som i sin tur hjälper till att upprätthålla en hög grad av spinpolarisering även hos elektronerna. För att få elektronerna att hinna först, måste deras rörelser i omgivningen kontrolleras. I en ytterligare studie undersöktes den process där en elektronisk övergång i kvantpricken inte enbart resulterar i emission av ljus, utan även i att en annan partikel tar över en del av energin och blir exciterad. Dessa processer avspeglas i att en del av det ljus som emitteras har lägre energi. Detta ljus är också mycket svagt, ca 1000 ggr lägre intensitet, och möjligheten att kunna mäta detta är helt beroende på hur ljusstarka kvantprickarna är. De prover som använts i denna studie består av pyramidstrukturer, ca 7.5 mikrometer stora, med kvantprickar inuti. Denna geometri ger ca 1000 ggr bättre ljusutbyte jämfört med traditionella strukturer, vilket möjliggjort studien.
24
Siegert, Jörg. "Carrier dynamics in semiconductor quantum dots /." Stockholm, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4019.
25
Jain, Manasi Pancholy. "Quantum dots: from cytotoxicity to metalloestrogenicity." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=110346.
Анотація:
The fields of Nanomedicine is rapidly expanding. Already numerous nanoparticles have entered clinical trials. However, certain nanostructures, though, therapeutically and diagnostically promising, can induce toxicity both in vitro and in vivo; cadmium telluride quantum dots fall into this category. Quantum dots (QDs) are highly fluorescent, semi-conducting nanocrystals, that consist of a metallic core. Compared to traditional fluorophores, QDs have superior optical qualities, including resistance to photobleaching, broad spectrum excitation and narrow emission. Cadmium telluride (CdTe) QDs were the first to be synthesized without organic solvents, and thus, considered suitable for biological application. However, early studies demonstrated that QDs induced cytotoxicity and oxidative stress. Although, this QD-toxicity was ascribed to cadmium (Cd) liberation from the QD-core, no empirical evidence was shown. Due to the attractiveness of QDs for biological applications, it was necessary that the mechanisms involved in QD-toxicity be understood so they may be prevented. Preliminary studies from our laboratory indicated that the antioxidant, N-acetlycysteine could prevent QD-toxicity. We, thus, hypothesized that QD-toxicity was not exclusively due to cadmium leaching from the QD. To evaluate this, an assay measuring free Cd was adapted for cellular use, to measure Cd present in both the cellular media and intracellularly. The cytotoxicity of various QDs was evaluated and correlated with the intracellular Cd. Results from this study showed no correlation between QD-toxicity and Cd release from QDs. We next questioned whether QD-toxicity could be explained as the sum of parts of the toxicity associated with core constituting metals, and whether the complexity of the model system used influences the cytotoxicity observed. We employed three model systems of the peripheral nervous system (an immortalized cell line, heterogeneous primary cultures and a three dimensional tissue model) and evaluated the toxicity of Cd, Te and QDs in each. Our findings showed that QDs are not a sum of parts and QD-toxicity is better ascribed to the induction of oxidative stress which is prevented by application of the multi-modal antioxidant, lipoic acid. Further, the model systems did not depict QD-toxicity comparably, stressing the need for standardization in nanotoxicological studies. Finally, it had been shown that Cd association with the estrogen receptor (ER) in ER expressing cells can activate estrogenic signalling. As such, Cd is considered a metalloestrogen. Given that QDs liberate Cd that can be detected in the cell, we investigated whether in ER expressing cells, QDs may act as metalloestrogens and induce estrogenic signalling. Results in vitro showed that QDs exert potent estrogenic signalling, comparable with estradiol, including cell proliferation, AKT phosphorylation, ERK phosphorylation and nuclear ER activation. These effects could be attenuated via pretreatment with the specific ER antagonist, ICI 182780, affirming that QD-induced estrogenic activity was mediated via the ER. To determine whether the estrogenic activity of QDs could also be demonstrated in vivo, ovariectomized mice were treated with QDs for two weeks, prior to being sacrificed. Subsequently, the wet weights of the mice uteruses were measured. In mice treated with QDs and estradiol a comparable 2.5 fold increase in uterine wet weight was observed. Taken together, these results indicate that CdTe QDs are both cytotoxic and endocrine disrupting, metalloestrogens. Though these nanocrystals may have valuable applications in biology the implications of QD-use are dangerous to plants, animals, humans and the environment. Therefore, it is imperative that Cd-free QDs be developed that retain the attractive qualities of QDs while preventing the detrimental side effects. Further, standardized testing of nanoparticles is imperative for the safe use of these novel tools.La nanotechnologie et la nanomédecine sont des domaines en pleine expansion. Certaines nanoparticules sont déjà entrées dans des essais cliniques et sont deja utilisées par les patients. Par contre, certaines nanostructures, bien qu'elles soient prometteuses à des fins cliniques ou diagnostiques, sont capables d'induire de la cytotoxicité in vitro et in vivo; les boîtes quantiques de cadmium telluride (CdTe) constituent un exemple.Les boîtes quantiques (BQ) sont des nanocrystaux semi-conducteurs fluorescents qui contiennent un noyau métallique entouré d'une couche organique. Les BQ ont des propriétés optiques supérieures aux autres fluorophores traditionnelles. Par exemple, ils sont plus résistants au photoblanchiment et sont caractérisés par un spectre d'excitation large et d'émission étroit. Les BQ CdTe, les premiers à être synthétisés sans l'utilisation de solvant organique, ont été prometteurs dans certaines applications biologiques. Cependant, les premières études ont démontré que ces BQ induisent de la cytotoxicité et produisent du stress oxydatif. Le cadmium (Cd) libéré pourrait être à la base de ces effets toxiques, mais cela n'a pas encore été prouvé. Les études préliminaires au sein de notre laboratoire démontrent que le pré-traitement avec un antioxydant, N-acétyl-cystéine, était capable de diminuer le niveau de toxicité associée au BQ. On a avancé l'hypothèse que cette toxicité n'était pas exclusivement liée au Cd qui est libéré des BQ. Nous avons effectué des expériences fluorométriques où nous mesurions les niveaux de Cd libres dans la cellule et dans le milieu extracellulaire. Ces expériences indiquent qu'il n'existe aucune corrélation entre la toxicité associée au BQ et le Cd libéré. Par après, nous nous sommes demandé si les constituants métalliques du noyau des BQ et les modèles dans lesquels les BQ avaient été évalués, étaient impliqués dans la toxicité associée au BQ. Nous avons évalué la toxicité du Cd, tellurium et BQ dans trois modèles du système nerveux périphérique (lignée de cellule immortalisée, cultures primaires hétérogènes et modèle de tissue tridimensionnel). Les résultats démontrent que la toxicité des BQ est principalement attribuée à l'induction du stress oxydatif, qui peut être prévenu en appliquant un antioxydant multimodal, l'acide lipoique.Dans les cellules, Cd peut s'associer avec les récepteurs d'œstrogènes (RE) et activer les voies de signalisation reliées à ce récepteur. Par conséquent, Cd est considéré comme étant un métallo-œstrogène. Nous avons montré que les BQ libèrent du Cd et que celui-ci est internalisé et retenu dans les cellules. Les effets induits par les BQ dépendaient du modèle biologique utilisé. Les études in vitro montrent que les BQ exercent une forte signalisation oestrogénique comparable à celle de estradiol et induisent la prolifération cellulaire, la phosphorylation d'AKT et d'ERK et l'activation du RE nucléaire. Ces effets étaient atténués par un pré-traitement avec un inhibiteur du RE, ICI 182780. Ces résultats affirment, donc, que les BQ exercent leurs activités oestrogéniques via les RE. Dans nos études in vivo nous avons utilisé des souris ovariectomisées qui avaient été traitées avec des BQ ou du estradiol pendant deux semaines, et par ensuite sacrifiées. Les traitements ont fait augmenter de 2.5 fois le poids de l'utérus des souris. Dans l'ensemble, ces résultats montrent que les BQ CdTe exercent à la fois des effets cytotoxiques et métallo-eostrogéniques. Malgré leurs potentiels d'application en imagerie ou dans les procédures diagnostiques, il est clair que les BQ peuvent nuire aux plantes, aux animaux, aux humains et à l'ensemble de l'environnement. Il y a un besoin urgent de développer des BQ sans cadmium qui possèdent des qualités attrayantes de BQ mais qui sont dépourvus d'effets secondaires détrimentaux. Cet objectif pourra être atteint à l'aide d'essais plus élaborés et sophistiqués pour déceler les risques des nanoparticules.
26
Hull, Peter J. "Synthesis and characterisation of quantum dots." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318760.
27
Mulrooney, Ray. "Analyte sensing with luminescent quantum dots." Thesis, Robert Gordon University, 2009. http://hdl.handle.net/10059/452.
Анотація:
Semiconducting nanocrystals otherwise known as Quantum Dots (QDs) have attracted considerable attention over the last number of years due to their unique optical properties and potential applications. Their narrow size-tunable emission spectra, broad absorption spectra, resistance to photobleaching and long fluorescent lifetimes make them ideal for sensing ions and small molecules. This thesis explores the potential of QDs to function as the emissive unit in fluorescent probes. Primarily, the focus of the work is to develop QD-based sensors that operate through an electron transfer mechanism. Chapter 3 discusses the synthesis and characterisation of CdSe and CdSe/ZnS QDs. Three different sized QDs were prepared each with distinct emission wavelengths. The sizes of these nanoparticles were determined by three methods, transmission electron microscopy (TEM), dynamic light scattering (DLS) and by a UV-vis method. Surface functionalisation of these synthesised QDs (chapter 4) with mercaptosuccinic acid rendered them water soluble and were shown to display selectivity for Cu2+ over a number of biologically relevant metal ions. The negatively charged surface of the QDs and the position of copper in the Irving-William series were believed to be responsible for this interaction. Positively charged CdSe/ZnS QDs were also prepared and were shown to detect ATP and to a much lesser extent GTP over the other nucleotides screened. The greater net negative charge of the ATP and GTP when compared to their mono and diphosphate analogues was the likely cause of this discrimination. In chapter 5 the relatively unexplored field of anion sensing with QDs was examined using charge neutral urea and thiourea receptors. Based on a design by Gunnlaugsson et al, a CdSe/ZnS QD with a thiourea receptor anchored to its surface displayed similar PET-mediated fluorescence quenching as an organic dye sensor containing the same receptor. A ferrocenyl urea receptor was also anchored to a QD surface and shown to “switch off” the QD’s fluorescence emission. On addition of fluoride ions the emission was restored, most likely due to a modulation of the ferrocene’s redox activity. In chapter 6 the assembly of Schiff base receptors on the surface of preformed CdSe/ZnS QDs were shown to arrange in such a way to enable the simultaneous detection of Cu2+ and Fe3+. The intriguing aspect of this study was that the receptors themselves displayed no selectivity for any metal ion until they were assembled on the QDs. Recognition was also confirmed by a distinct colour change visible to the naked eye.
28
Dowley, David Gordon. "Modelling optical emission in quantum dots." Thesis, University of Leeds, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.522934.
29
Smith, Christopher Roy. "Quantum dots for antibody based sensors." Thesis, Imperial College London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.528301.
30
Bylsma, Jason Michael. "Multidimensional Spectroscopy of Semiconductor Quantum Dots." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4001.
Анотація:
The coherent properties of semiconductor nanostructures are inherently difficult to measure and one-dimensional spectroscopies are often unable to separate inhomogeneous and homogeneous linewidths. We have refined and improved a method of performing multidimensional Fourier transform spectroscopy based on four-wave
mixing (FWM) experiments in the box geometry. We have modified our system with broadband beamsplitters in all interferometer arms, high-resolution translation stages and the ability to work in reflection geometry. By improving the phase-stability of our setup and scanning pulse delays with sub-optical cycle precision, we are able to
reproduce 2DFT spectra of GaAs multiple quantum wells. With the FWM signal reflected from the sample surface instead of transmitted through, we show that very low pulse powers can be used to generate coherent 2D signals from colloidal PbS quantum dots. Dephasing times are particularly difficult to measure in small colloidal quantum dots due to environmental broadening effects from the colloidal growth. We show that low-temperature pure excitonic dephasing can be measured via time-integrated measurements as well as from the cross-diagonal linewidths of 2DFT spectra. Ultrafast
sub-picosecond dephasing times are measured at 5 K in 3 nm PbS quantum dots, while excitation-density-dependence is investigated in these dots. By retrieving the global phase with an all-optical method, we are able to retrieve the real-part 2D spectra of PbS quantum dots.
31
Chaney, Darren. "Relaxation processes in semiconductor quantum dots." Thesis, University of Leicester, 2004. http://hdl.handle.net/2381/4521.
Анотація:
In this thesis we investigate the relaxation mechanisms that occur in quantum dots (QDs). First we consider energy relaxation in single particle self-assembled QDs by means of an Auger process. For the first time, relaxation rates are compared for dots of a realistic truncated pyramid shape and for the more elementary dot models considered previously. We find that the fast (pico-second) relaxation necessary for quantum dot based optoelectronics applications is made possible by dot electrons scattering with electrons located in the surrounding bulk material. We show that this relaxation mechanism is dominant by two orders of magnitude over the two-dimensional wetting layer scattering mechanism that has been considered in previous calculations. Exact numerical diagonalisation is used to calculate the two-particle selfassembled QD wave functions. The small size of the QD means that the two electrons in the dot are found to be only weakly interacting. We find the relaxation rate for states of total spin 0 to be larger than the spin I rate by a factor of approximately 2. This is due to the double occupancy of the spin 0 spatial states. We also consider the much slower spin flip relaxation in electrostatic QDs. We include the spin-orbit mixing that results from the bulk inversion asymmetry of the crystal lattice in calculating the exact two-particle states. We find that the spin orbit mixing causes anti-crossings to appear in the energy spectrum and deduce a new conservation rule related to this. We find an oscillation capable of slowing the relaxation time from microseconds to tenths of a second. This oscillation results from the vertical finite well confinement of the QD. It is found to depend on both magnetic field and the QD thickness and is of particular interest for quantum information applications where long-lived excited states are desirable.
32
APEL, VICTOR MARCELO. "ELECTRONIC CORRELATION IN QUANTUM DOTS SYSTEMS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2004. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=6583@1.
Анотація:
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICONesta tese investigamos os efeitos das interações elétron- elétron nas propriedades de transporte nanosistemas. Em particular, estudamos sistemas constituídos por dois pontos quânticos conectados a dois contatos, em diferentes topologias. O principal interesse é estudar os efeitos do regime Kondo e da fase eletrônica na condutância. Na configuração onde os dois pontos são inseridos em cada braço de um anel atravessado por um fluxo magnético, denotada por PPL, calculamos as fases das correntes que circulam através de cada braço do anel. Estas fases são determinadas pelo efeito Aharonov-Bohm combinado com a inflência da interação de muitos corpos das cargas nos pontos. Este sistema apresenta ressonância Kondo para um número par de elétrons em concordância com os resultados experimentais1. Outro aspecto interessante da configuração PPL é que, mesmo na ausência de fluxo magnético, pode existir circulação de corrente no anel, dependendo dos parâmetros escolhidos. Consideramos outras duas topologias que envolvem dois pontos quânticos acoplados através de interação de tunelamento. Em uma delas, denotada PAL, os dois pontos estão alinhados com os contatos, e na outra, a configuração PPD, um ponto está inserido nos contatos entanto que o outro interage só com o primeiro. No limite de acoplamento fraco, estas duas configurações apresentam características bem distintas, no só na dependência da condutância com o potencial de porta mas também na correlação de spin dos pontos quânticos. Ambas configurações apresentam ressonância Kondo para um número par de elétrons de diferente natureza. Quando cada ponto está carregado com um elétron, no caso da configuração PAL, os spins dos pontos quânticos estão descorrelacionados enquanto que, na configuração PPD, os spins estão correlacionados ferromagneticamente. No limite do acoplamento forte as propriedades de transporte das dois configurações são similares. Os sistemas discutidos acima são representados por o Hamiltoniano de Anderson de duas impurezas acopladas, o qual é resolvido diagonalizando exatamente um aglomerado que é embebido no resto do sistema. Desta forma obtemos as propriedades de transporte a T = 0. Para estudar a dependência com a temperatura utilizamos o método da equação de movimento (EOM) no limite da repulsão Coulombiana infinita. Aplicamos este método ao caso da topologia PPD, obteniendo resultados para baixas temperaturas consistente com os obtidos com o método do aglomerado.
In this thesis we investigate the effects of the eletron- eletron interaction on the transport properties of nanosystems. In particular, we study systems constituted by two quantum dots conected to leads, in different topologies. Our main interest is to study the effects of the Kondo regime and the electronic phase on the conductance. In the configuration where the two dots are inserted in each arm of a ring threaded by a magnetic flux, denoted by PPL, we calculate the phases of the currents going along each arm of the ring. These phases are determined by the Aharonov-Bohm effect combined with the dots many body charging effects. This system presents the Kondo phenomenon for an even number (two) of electrons in the dots, in agreement with experimental results1. An interesting aspect of PPL configuration is that, even in the absence of magnetic flux there can be a circulating current around the ring, depending on the system parameters. In the two other topologies we consider the two quantum dots coupled through tunneling interaction. In one of them, denoted by PAL, the two dots are aligned with the leads, and in the other, the PPD configuration, one dot is inserted into the leads while the other interacts only with the first. In the weak coupling limit these two configurations present quite different features, not only on the dependence of the conductance on the gate potencials applied to the dots, but also on the dots spin correlation. Both configurations present Kondo resonance for an even number electrons. In the PAL configuration the spins of the charged dots are uncorrelated, while in the PPD configuration they are ferromagnetically correlated. In the strong tunneling coupling limit the transport properties of two interacting dot configurations are very similar. The systems discussed above are represented by an Anderson two- impurity first-neighbor tight-binding Hamiltonian, that is solved by exactly diagonalizing a cluster that is embebed into the rest of the system. In this way we obtain only the properties of the system at T = 0. In order to study temperature dependence phenomena we use the equation of motion method (EOM) in the limit of infinite Coulomb repulsion. We apply it to the dots in the PPD topology. The results for low temperatures are consistent with hose obtained with the cluster method.
33
Mavalankar, Aquila Mukund. "Thermometry and refrigeration using quantum dots." Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708092.
34
Oertel, David C. (David Charles). "Photodetectors based on colloidal quantum dots." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/39737.
Анотація:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.Vita.
Includes bibliographical references (p. 193-212).
Inspired by recent work demonstrating photocurrent enhancement in quantum-dot (QD) solids via post-deposition chemical annealing and by recent successes incorporating single monolayers of QDs in light-emitting devices (QD-LEDs), we set out to develop thin-film, layered photodetectors in which the active layer is a chemically annealed QD solid. This thesis reports initial steps in this development. Chapters 1 and 2 contain introductory material. In Chapter 3, we demonstrate a layered QD photodetector (QD-PD) in which the active layer is a 200-nm thick film of CdSe QDs annealed with n-butylamine. These "thick-slab" devices, active in the visible spectrum, represent a minimal transformation from the transverse channel test structures employed for more fundamental studies to a layered, sandwich geometry. The thick-slab design exhibits a number of desirable characteristics, including near-unity internal quantum efficiency and a 50-kHz bandwidth. More complex QD-PD designs, allowing the successful incorporation of much thinner QD solids, are discussed in Chapter 4, and these devices provide insight into the zero-bias operation of thick-slab QD-PDs.
(cont.) The final three chapters of the thesis focus on PbSe QDs, this group's initial material of choice for accessing the short-wavelength-IR spectral window. Initial study of PbSe QDs as a material for QD-PDs is described in Chapter 5. Chapter 6 discusses an 31P-based NMR investigation of the mechanism of PbSe monomer formation, allowing for rational "doping" of syntheses to increase chronically low yields. Finally, Chapter 7 discusses a brief assessment of the suitability of PbSe QDs for in vivo imaging.
by David C. Oertel.
Ph.D.
35
Shi, Lifang. "Luminescent Quantum dots for Cellular Analysis." ScholarWorks@UNO, 2007. http://scholarworks.uno.edu/td/597.
Анотація:
Luminescent quantum dots have attracted great interest in recent years among biological researchers since they provide solutions to problems associated with use of organic fluorophores in cellular studies. Quantum dots show high photostability, high emission quantum yield, narrow and symmetric emission peaks and size-dependent wavelength tunability. The objective of my PhD studies was to develop CdSe/ZnS quantum dot-based probes and utilize them in cellular assays. The first phase of the work was to develop luminescent quantum dot fluorescence resonance energy transfer (FRET) based probes for protease activity. The probes were based on FRET interactions between quantum dots that serve as donors and rhodamine molecular acceptors that were immobilized to the surface of the quantum dots through peptide linkers, which contained selective enzymatic cleavage sites. Upon enzymatic cleavage of the peptide linkers, the rhodamine molecules no longer provided an efficient energy transfer channel to the quantum dots, which brightened the previously quenched quantum dots. The probes were applied to detect enzyme activity, screen enzyme inhibitors, and discriminate between normal and cancerous cells primarily because of the difference in the proteolytic activity in extracellular matrices. The second phase of my work was to take advantage of FRET and quantum dots to develop pH sensor. First quantum dots were modified with metallothionein (MT) to be water-soluble and biocompatible. The MT-coated quantum dots were labeled with Rhodamine through the formation of amide bonds with å-amine group of lysine in MT peptide to form the probes. FRET efficiency between quantum dots (donor) and rhodamine (acceptor) was pH dependent. The final phase of my studies focused on the first preparation of reversible quantum dot-based cellular probes for labile iron. The MT coated quantum dots was modified with EDTA to form probes. When captured by the EDTA molecules, iron ions quenched the emission of quantum dots. Removal of iron from the quantum dot surface by free EDTA or other iron chelators with higher binding affinity resulted in a rise in the luminescence of quantum dots. The analytical properties of the probes including sensitivity, selectivity, and reversibility were characterized. Intracellular assays in iron-enriched astrocytes will be carried out.
36
Brash, Alistair. "Ultrafast dynamics of single quantum dots." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/16098/.
Анотація:
In this thesis the results of experiments on the optical properties of single InGaAs quantum dots are presented. Using a range of ultrafast spectroscopy techniques the dynamics of single charge carriers confined within these quantum dots are probed. Both the coherent and incoherent driving regimes are explored with a view to applications for quantum information processing. Three key results are described in detail in this thesis. Firstly, by performing resonant coherent driving it is shown that reduction of the quantum dot neutral exciton fine-structure splitting enables fast and high-fidelity initialization of long-lived hole spin quantum bits. Secondly, by detuning the laser from the ground state - neutral exciton transition it is possible to both excite and de-excite exciton population through incoherent driving via the acoustic phonon bath. Finally, by considering quantum dots coupled to optical microcavities it is shown that the driving strength of both resonant coherent and quasi-resonant phonon-assisted excitation may be strongly modulated.
37
Ramanathan, Swati. "Polarization Studies of Coupled Quantum Dots." Ohio University / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1194984001.
38
Schmid, Jörg D. "The Kondo effect in quantum dots." [S.l. : s.n.], 2000. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB8948245.
39
So, Tak Ki. "MBE-grown Fe ferromagnetic quantum dots /." View Abstract or Full-Text, 2003. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202003%20SO.
Анотація:
Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003.Includes bibliographical references (leaves 61-62). Also available in electronic version. Access restricted to campus users.
40
Puebla, Nunez Jorge Luis. "Spin phenomena in semiconductor quantum dots." Thesis, University of Sheffield, 2013. http://etheses.whiterose.ac.uk/4026/.
Анотація:
This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm.
41
Quilter, John Howard. "Coherent spectroscopy of single quantum dots." Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/7711/.
Анотація:
This thesis presents experiments carried out into the coherent properties of charge carriers confined within a single InGaAs/GaAs quantum dot. Specifically the ground state - single exciton qubit system is controlled with ultrafast optical pulses and then the population of the dot is measured by a high-sensitivity photocurrent detection technique. There are two principal results of these experiments, firstly by modulating the bias applied to the quantum dot diode, the detection efficiency of the photocurrent measurement technique is enhanced. The enhancement is possible as the modulated bias allows fast switching of the voltage-dependent carrier tunneling rates. Secondly, when the transition is addressed by an intense, positively detuned laser, the qubit system can be driven to a population inversion. This occurs due to phonon-assisted relaxation of the optical dressed states of the system. Here, the rapid thermalization due to the exciton-phonon interaction actually enables the population inversion, instead of simply being a source of decoherence.
42
Elkin, Mark Dennis. "Ferromagnetically contacted carbon nanotube quantum dots." Thesis, University of Leeds, 2012. http://etheses.whiterose.ac.uk/2852/.
Анотація:
This thesis presents research into spin-transport in Carbon nanotube quantum dots. Sputtered Permalloy electrodes designed with shape anisotropy were used to contact Carbon nanotubes grown by chemicalvapour deposition in lateral spin-valve structures. The magnetoresistance of these spin-valves were measured at low-temperatures and as a function of the charge state of the quantum dots. Two conductance regimes were measured in a Carbon nanotube spinvalve with Permalloy nucleation pads. At high bias outside of the coulomb blockade regime a ~ 10% magnetoresistance was measured that is analogous to giant-magnetoresistance, in that it is due to spindependent scattering at the ferromagnet-Carbon nanotube interfaces. At lower bias the device enters the coulomb blockade regime and the magnetoresistance observed develops a different structure, over a larger field range, together with the development of an offset in conductance between saturations. The maximum value of this MR was MR ~ 245% and it was attributed to changes in the induced charge on the quantum dot. By modifying the design of the Permalloy electrodes, a single domain state at the point of contact of the Carbon nanotube was achieved. A well defined anti-parallel state of the Permalloy electrodes, with associated changes in the conduction of the devices was observed, yet the conductance offset remained, with a maximum MR of ~ 60%. The positions of the coulomb peaks were measured during magnetic reversal of the electrodes, showing the change in induced charge on the quantum dot, with a maximum MR ~ 350%. Predictions of device transport based on the magneto-coulomb effect and spin-dependent interfacial phase shifts were compared to experimental results and found to not fit the observed behaviour. This led to the conclusion that changes in the charge state of the quantum dots must be due to a fixed spin-quantisation axis intrinsic to the Carbon nanotube.
43
Wang, Shidong. "Probing and electron tunneling of quantum dot systems /." View Abstract or Full-Text, 2003. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202003%20WANG.
Анотація:
Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003.Includes bibliographical references (leaves 102-112). Also available in electronic version. Access restricted to campus users.
44
Hsieh, Chang-Yu. "Quantum Circuit Based on Electron Spins in Semiconductor Quantum Dots." Thesis, Université d'Ottawa / University of Ottawa, 2012. http://hdl.handle.net/10393/20738.
Анотація:
In this thesis, I present a microscopic theory of quantum circuits based on interacting electron spins in quantum dot molecules. We use the Linear Combination of Harmonic Orbitals-Configuration Interaction (LCHO-CI) formalism for microscopic calculations. We then derive effective Hubbard, t-J, and Heisenberg models. These models are used to predict the electronic, spin and transport properties of a triple quantum dot molecule (TQDM) as a function of topology, gate configuration, bias and magnetic field.
With these theoretical tools and fully characterized TQDMs, we propose the following applications:
1. Voltage tunable qubit encoded in the chiral states of a half-filled TQDM. We show how to perform single qubit operations by pulsing voltages. We propose the "chirality-to-charge" conversion as the measurement scheme and demonstrate the robustness of the chirality-encoded qubit due to charge fluctuations. We derive an effective qubit-qubit Hamiltonian and demonstrate the two-qubit gate. This provides all the necessary operations for a quantum computer built with chirality-encoded qubits.
2. Berry's phase. We explore the prospect of geometric quantum computing with chirality-encoded qubit. We construct a Herzberg circuit in the voltage space and show the accumulation of Berry's phase.
3. Macroscopic quantum states on a semiconductor chip. We consider a linear chain of TQDMs, each with 4 electrons, obtained by nanostructuring a metallic gate in a field effect transistor. We theoretically show that the low energy spectrum of the chain maps onto that of a spin-1 chain. Hence, we show that macroscopic quantum states, protected by a Haldane gap from the continuum, emerge.
In order to minimize decoherence of electron spin qubits, we consider using electron spins in the p orbitals of the valence band (valence holes) as qubits. We develop a theory of valence hole qubit within the 4-band k.p model. We show that static magnetic fields can be used to perform single qubit operations. We also show that the qubit-qubit interactions are sensitive to the geometry of a quantum dot network. For vertical qubit arrays, we predict that there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.
45
Brunner, Daniel. "Laser spectroscopy of coherent quantum states in single quantum dots." Thesis, Heriot-Watt University, 2010. http://hdl.handle.net/10399/2350.
Анотація:
Laser spectroscopy was used for studying single charge-tunable InAs quantum dots (QD). The spectroscopy system consisted of a high resolution microscope combined with a solid immersion lens, a grating spectrometer and an in-situ detector to study the homodyne signal of the resonant laser and the QD. Low density QD samples were fabricated, which allowed spectral isolation of individual QDs. A modulation technique was used for noise rejection. Resonant absorption spectroscopy was used for directly probing transitions between ground and excited QD states. Lineshapes and signal strength were linked to life and coherence times of QD states. A theoretical model was developed combining coherent and non coherent processes in a master equation. Positively and negatively doped sample structures enabled spectroscopy of negatively, neutral and positively charged excitons. The relaxation time of hole spin ground states in a single QD was probed using resonant excitation in a magnetic eld parallel to the growth direction. Optical selection rules enable control over hole spin orientation. Hole spin relaxation times were studied from zero to ve Tesla, with relaxation times of di erent QDs ranging from 200 s to 1 ms. No signi cant in uence of the external magnetic eld on the hole spin relaxation time was found. A hole spin initialisation delity close to 100 % was achieved. Readout of resonantly created QD states was realised via a new microscope system. This dark eld microscope utilised spatial and polarisation ltering techniques to suppress the excitation laser by up to six orders of magnitude. Both ltering devices were included in the standard microscope, making it a highly practical and versatile system. Collected QD emission exceeded the resonant laser background by a factor of 100 for an unsaturated X1 transition. Pump-probe spectroscopy of the 3-level biexciton system was carried out, with the back scattered signal collected in re ection allowing spectral ltering via a grating spectrometer. The recorded probe spectrum revealed Autler-Townes splittings for high pump laser intensities, demonstrating the coherent superposition of QD exciton states. Swapping the pump probe geometry revealed weak quantum interferences. Spectroscopy of hole spin ground states in an in-plane magnetic eld created a coherent superposition of hole spin ground states via a -system. The resulting quantum interference between hole spin states resulted in the creation of a dark state. This experiment is known in quantum optics as coherent population trapping. The extracted lower bound of the hole spin coherence time was 1 s with greater than 40 % probability, demonstrating the enormous potential of hole spins in QDs for quantum information processing as well as for quantum optical experiments.
46
Liu, Tai-Min. "Electronic Interactions in Semiconductor Quantum Dots and Quantum Point Contacts." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1311773375.
47
Htoon, Han. "Studies on quantum coherence phenomena of self-assembled quantum dots." Access restricted to users with UT Austin EID Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3037502.
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Dewaele, Nicholas. "Quantum Computing With Quantum Dots Using The Heisenberg Exchange Interaction." OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1600.
Анотація:
One of the most promising systems for creating a working quantum computer is the triple quantum dots in a linear semiconductor. One of the biggest advantages is that we are able to perform Heisenberg exchange gates on the physical qubits. These exchanges are both fast and relatively low energy. Which means that they would be excellent for producing fast and accurate operations. In order to prevent leakage errors we use a 3 qubit DFS to encode a logical qubit. Here we determine the theoretical time dependent affects of applying the Heisenberg exchange gates in the DFS basis as well as the effect of applying multiple exchange gates at the same time. we also find that applying two heisenberg exchange gates at the same time is an effective way of implementing a leakage elimination operator.
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Krenner, Hubert Johannes. "Coherent quantum coupling of excitons in single quantum dots and quantum dot molecules /." München : Walter-Schottky-Inst, 2006. http://opac.nebis.ch/cgi-bin/showAbstract.pl?u20=3932749774.
50
Boese, Daniel. "Quantum transport through nanostructures : quantum dots, molecules, and quantum wires = Quantentransport durch Nanostrukturen /." Aachen : Shaker, 2002. http://swbplus.bsz-bw.de/bsz096321318abs.htm.