Literatura académica sobre el tema "Quantum Dots (QD)"

The use of colloidal quantum dots (QDs) for photovoltaic energy conversion is a nascent field that has been dominated for well over a decade by the use of 3-mercaptopropionic acid (3-MPA) capped PbS QDs. These QDs are routinely deposited via an in situ solid state ligand exchange process that displaces the native oleate ligand on the PbS QD surface. This ligand exchange procedure is wasteful of material and has been demonstrated to leave numerous impurities that limit electronic performance of the as-deposited QD devices. Until the last few years there was very little understanding in chemical literature as to many important aspects of QD chemistry for this material pairing outside the framework of a QD solid. In this work, a colloidal suspension of 3-MPA capped PbS QDs in DMSO was formulated and investigated to probe ligand dynamics and optical properties of the suspended colloid. QD bound 3-MPA was found to be in dynamic exchange with "free" ligand in solution by 1H-NMR spectroscopy. Optical properties and colloidal stability were found to be heavily dependent on the presence of a significant excess of free ligand. PbS QDs were also found to be highly photo-catalytic towards oxidative dimerization of 3-MPA to its dimer, dithiodipropionic acid (dTdPA). After an initial colloidal suspension was achieved, attempts were made to directly deposit the colloid as a QD "ink" to form optoelectronic devices. While photo-switchable devices were obtained, ultimately it was determined that DMSO was a largely incompatible solvent choice for solution processing methodologies. Subsequently, 3-MPA capped PbS QD colloids were obtained in volatile organic solvents amenable to solution processing by the addition of a stabilizing ammonium salt. These QD colloids maintained excellently resolved optical properties and were able to form conformal coatings from simple evaporative deposition. The ligand chemistry of this colloid was extensively investigated via NMR and optical spectroscopy. These QDs were also found to be highly photo-catalytic towards conversion of monomer 3-MPA to dTdPA.

The work presented in this thesis was submitted to The University of Manchester for the degree of Doctor of Philosophy in June 2010 by Stuart K Stubbs and is entitled “Photo-physics and applications of colloidal quantum dots”. In this thesis the results of spectroscopic studies on various colloidal quantum dots, particularly related to the measurement and characterisation of multiple exciton generation are presented. Research conducted with Nanoco Technologies Ltd. that involved the design and development of hybrid quantum dot organic light emitting diodes for use in flat panel display technology is also presented. Cadmium selenide (CdSe), indium phosphide (InP), and lead sulphide (PbS) type I and cadmium selenide/cadmium telluride type II colloidal quantum dots were characterised using steady state photoluminescence and absorption spectroscopy. The fluorescence lifetimes of the decay of single excitons was measured in these quantum dots using time correlated single photon counting. An ultrafast transient absorption spectrometer was designed, built, and used to observe the picosecond dynamics of the decay of multiexcitons. These absorption transients were analysed in order to extract the quantum efficiency of producing multiple excitons per absorbed photon. The characteristic signature for multiple exciton generation was first found in CdSe using a time correlated single photon counting set-up. Results from the transient absorption spectrometer demonstrated efficient multiple exciton generation in InP for the first time as well as in PbS, where the efficiency was found to agree with values obtained by other research groups. The absorption transients taken for the type II CdSe/CdTe type II quantum dots demonstrated some novel decay dynamics that could not be attributed to the generation of multiple excitons. Quantum dot organic light emitting diodes were fabricated using Nanoco Technologies high quality cadmium based quantum dots and were shown to demonstrate bright, colour saturated emission originating from the quantum dot layer only. Using quantum dots of different sizes and structures red, green and blue devices were made and shown to be appropriate both in terms of brightness and chromaticity for the use as the red, green and blue pixels of a flat panel display. Because heavy metals like cadmium are restricted or banned from commercial products in many countries, Nanoco Technologies heavy metal free quantum dots, made from InP, were also incorporated in devices. Devices are demonstrated that emit from the quantum dot layer only, albeit at a lower luminance and efficiency than that found in the cadmium containing devices. This was the first demonstration of a heavy metal free, hybrid quantum dot organic light emitting diode.

Luminescent quantum dot (QD) based probes have gained significance in the last decade for optical imaging of cells, tissues and in bioassays as alternatives to conventional organic fluorophores. The main objective of my PhD dissertation was to develop luminescent quantum dot based bioassays for real time monitoring of enzyme activity and simultaneous detection of several biomarkers. The quantum dot based bioassays developed will be potential tools in identification and diagnosis of several ailments that interfere with normal living conditions of human beings. In Chapter 2 new liposome encapsulated quantum dot based fluorescence resonance energy transfer (FRET) probes have been fabricated and characterized for monitoring the enzymatic activity of phospholipase A 2. The probes were able to detect the enzyme activity as low as 0.0075 U/mL (PLA2 = 1500 U/mg) in 30 min. Further these FRET probes were also used to screen the inhibition efficiencies of phospholipase A2 inhibitors. Chapter 3 focuses on the first time synthesis and characterization of liposome encapsulated InP/ZnS quantum dots while preserving the integrity of the liposomes. Results from the experiments to assess photostability and effect of pH on the optical properties of InP/ZnS QD-liposomes showed greater advantages over InP/ZnS quantum dots demonstrating their utility as a potential tool in several biological applications such as bio imaging, bioassays and in immunoassays. Chapter 4 discusses the development of fluorescence based immunoassay for simultaneous detection of the cardiac biomarkers troponin T and troponin I using CdSe/ZnS quantum dots. The assay achieved a detection limit was 0.1 pg/mL for both biomarkers troponin xi T and I. The method was highly specific for the both the biomarkers with no observed cross reactivity. The multiplex assay was able to detect two biomarkers simultaneously that will yield a high throughput diagnostic tool for heart attack. A similar method discussed as above was used in chapter 5 for the simultaneous detection of atherosclerosis biomarkers. The detection limits achieved in this study are comparable to the detection limits of the biomarkers reported so far. Incorporation of QDs in silica beads before conjugation to antibodies might improve detection limits that will also improve risk assessment.

Le travail de recherche effectué durant cette thèse avait pour but de synthétiser des billes de polystyrène de taille sub-micrométriques contenant des points quantiques, aussi appelés " quantum dots " ou QDs. Cette technique est appelée encapsulation. Elle correspond à piéger un ou plusieurs QDs au sein de la matrice formée de polymère. Les QDs émettent de la fluorescence à une longueur d'onde dépendant de la taille du nanocristal et de sa composition chimique. Les QDs employés seront des QDs de CdSe émettant dans la gamme de 500nm à 630nm. Pour la réalisation de ces billes, nous avons opté pour un processus de mini-émulsion qui nous permet de produire des billes de latex en grande quantité et très monodisperses. Les propriétés optiques des QDs lors du processus d'encapsulation doivent alors être conservées. La principale difficulté a consisté à réaliser une dispersion homogène de QDs au sein des billes. Pour cela, il faut obtenir une répartition statistique des QDs dans chaque bille. L'utilité de développer ces billes (en polymères contenant des nanoparticules) de taille sub-micrométrique composites, est de solubiliser les QDs dans l'eau en vue d'application pour la biologie en tant que marqueurs. Ces objets, pourront alors être fonctionnalisés avec le greffage d'un groupement fonctionnel qui présentera une affinité spécifique pour un matériel biologique précis. Cette fonctionnalisation sera assurée par l'intermédiaire du surfactant qui permet de produire la mini-émulsion. Nous avons enfin, essayé de réaliser aussi des auto-assemblages de QDs avec des copolymères blocs chargés. Ce travail consistait également à réaliser des agrégats à la fois magnétiques et fluorescents.

Quantum dots (QDs) are a semiconductor nanostructure in which a small island of one type of semiconductor material is contained within a larger bulk of a different one. These structure are interesting for a wide range of applications, including highly efficient LASERs, high-density novel memory devices, quantum computing and more. In order to understand the nature of QDs, electrical characterisation techniques such as capacitance-voltage (CV) profiling and deep-level transient spectroscopy (DLTS) are used to probe the nature of the carrier capture and emission processes. This is limited, however, by the nature of QD formation which results in a spread of sizes which directly affects the energy structure of the QDs. In this work, I sought to overcome this by using Si substrates patterned with a focused ion beam (FIB) to grow an array of identically-sized Ge dots. Although I was ultimately unsuccessful, I feel this approach has great merit for future applications.In addition, this thesis describes several unusual characteristics observed in InAs QDs in a GaAs bulk (grown by molecular beam epitaxy-MBE). Using conventional and Laplace DLTS, I have been able to isolate a single emission transient. I further show an inverted relation between the emission rate and the temperature under high field (emissions increase at lower temperatures). I attribute this to a rapid capture to and emission from excited states in the QD. In addition, I examine a metastable charging effect that results from the application of a sustained reverse bias and decreases the apparent emission rate from the dots. I believe this to be the result of a GaAs defect with a metastable state which acts as a screen, inhibiting emission from the dots due to an accumulation of charge in the metastable state. These unusual characteristics of QDs require further intensive work to fully understand. In this work I have sought to describe the phenomena fully and to provide hypotheses as to their origin.

In this research, InAs quantum dot structures for mid-infrared emission were self-assembled on InP substrate by using metal-organic chemical vapor deposition. To improve the grown quantum dot’s shape, the dot density and the dot size uniformity, a two-step growth method has been used and investigated. By changing the composition of the InxGa1 – xAs matrix layer of the InAs / InxGa1 – xAs / InP quantum dot structure, emission wavelength of the InAs quantum dot structure has been extended to the longest  2.35 m measured at 77 K. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35383

Le but de ce travail est de développer un processus d’élaboration de boîtes quantiques (QD) de silicium-germanium (SiGe) avec des compositions allant du Si au Ge pur, et permettant d’obtenir des QD semi-conductrices et de tailles suffisamment petites pour l’obtention de confinement quantique. Pour cela, nous avons utilisé une combinaison de différentes techniques : l’épitaxie par jets moléculaires, la lithographie ionique par faisceau d’ions focalisés (FIBL) et le démouillage solide hétérogène. Dans ce contexte, la finalité de cette recherche est d’une part de développer un FIB qui puisse être couplé à un bâti d’épitaxie par jets moléculaires sous ultra-vide et d’autre part de valider le FIB avec deux applications : des nanogravures pour l’auto-organisation des QD et des nano-implantations de Si et de Ge pour la création de défauts locaux émetteurs de lumière. Nous avons utilisé la FIBL avec des sources d’ions d’alliage métallique liquide (LMAIS) filtrées en énergie utilisant des ions non polluants (Si et Ge) dans des substrats issus de la microélectronique tels que des substrats de SiGe sur silicium-sur-isolant (SGOI). Les nano-gravures doivent être totalement dénuées de pollution et aux caractéristiques variables et parfaitement contrôlées (taille, densité, profondeur). La morphologie des nano-gravures obtenues est ensuite caractérisée in-situ par microscopie électronique à balayage (SEM), et la profondeur est déterminée par des caractérisations ex-situ par microscopie de force atomique (AFM). Les nano-gravures réalisées par FIBL ont été comparées d’une part aux gravures plasmas avec He et Ne et d’autre part aux gravures obtenues par lithographie électronique (EBL)<br>The goal of this work is to develop a process for the elaboration of silicon-germanium (SiGe) quantum dots (QDs) with compositions ranging from Si to pure Ge, and allowing to obtain semiconducting QDs with sufficiently small sizes to obtain quantum confinement. For this purpose, we have used a combination of different techniques: molecular beam epitaxy, focused ion beam lithography (FIBL) and heterogeneous solid state dewetting. In this context, the aim of this research is on the one hand to develop a new FIB that can be coupled to the ultra-high vacuum molecular beam epitaxy growth chamber, and on the other hand to realize two applications: (i) nanopatterns for the self-organisation of Si and Ge QDs and (ii) nano-implantations of Si and Ge. We used FIBL with energy-filtered liquid metal alloy ion sources (LMAIS) using non-polluting ions (Si and Ge) for the milling of conventional microelectronic substrates such as SiGe on silicon-on-insulator (SGOI). The nanopatterns must be totally free of pollution and with variable and perfectly controlled characteristics (size, density, depth). The morphology of the nanopatterns is then characterized in-situ by scanning electron microscopy (SEM), and the depth is determined ex-situ by atomic force microscopy (AFM). The nanopatterns made by FIBL were compared on the one hand to plasma etchings with He and Ne and on the other hand to the etchings obtained by electronic lithography (EBL). Nanoimplantations of Si and Ge ions were realised in diamond and in ultra-thin SGOI for the fabrication of local defects

Submitted by GEOVANE MODENA PEREIRA null (geovanemodena@hotmail.com) on 2018-02-10T03:04:38Z No. of bitstreams: 1 Dissertação Geovane - Criptografia de Qubits de Férmions de Majorana por meio de Estados Ligados no Contínuo.pdf: 7524654 bytes, checksum: 0bd9409e8fa9c0c2da9190e44f4cfa33 (MD5)<br>Approved for entry into archive by Ana Paula Santulo Custódio de Medeiros null (asantulo@rc.unesp.br) on 2018-02-14T16:11:32Z (GMT) No. of bitstreams: 1 pereira_gm_me_rcla.pdf: 7420427 bytes, checksum: 0a0aec5beec2ecdd26883e0f4524844f (MD5)<br>Made available in DSpace on 2018-02-14T16:11:32Z (GMT). No. of bitstreams: 1 pereira_gm_me_rcla.pdf: 7420427 bytes, checksum: 0a0aec5beec2ecdd26883e0f4524844f (MD5) Previous issue date: 2017-12-01<br>Nós investigamos teoricamente uma cadeia topológica de Kitaev conectada a dois pontos quânticos (QDs) hibridizados a terminais metálicos. Neste sistema, observamos o surgimento de dois fenômenos marcantes: (i) uma decriptografia do Férmion de Majorana (MF), que é detectado por meio de medições de condutância devido ao estado de vazamento assimétrico do qubit de MFs nos QDs; (ii) criptografia desse qubit em ambos os QDs quando o vazamento é simétrico. Em tal regime, temos portanto a criptografia proposta, uma vez que o qubit de MFs separa-se nos QDs como estados ligados no contínuo (BICs), os quais não são detectáveis em experimentos de condutância.<br>We theoretically investigate a topological Kitaev chain connected to a double quantum-dot (QD) setup hybridized with metallic leads. In this system, we observe the emergence of two striking phenomena: i) a decrypted Majorana Fermion (MF) - qubit recorded over a single QD, which is detectable by means of conductance measurements due to the asymmetrical MF-leaked state into the QDs; ii) an encrypted qubit recorded in both QDs when the leakage is symmetrical. In such a regime, we have a cryptography-like manifestation, since the MF-qubit becomes bound states in the continuum, which is not detectable in conductance experiments.

Lasers find a wide range of applications in many areas including photo-biology, photo-chemistry, materials processing, imaging and telecommunications. However, the practical use of such sources is often limited by the bulky nature of existing systems. By fabricating channel waveguides in solid-state laser-gain materials more compact laser systems can be designed and fabricated, providing user-friendly sources. Other advantages inherent in the use of waveguide gain media include the maintenance of high intensities over extended interaction lengths, reducing laser thresholds. This thesis presents the development of Yb:tungstate lasers operating around 1μm in waveguide geometries. An Yb:KY(WO₄)₂ planar waveguide laser grown by liquid phase epitaxy is demonstrated with output powers up to 190 mW and 76 % slope efficiency. This is similar to the performance from bulk lasers but in a very compact design. Excellent thresholds of only 40 mW absorbed pump power are realised. The propagation loss is found to be less than 0.1 dBcm⁻¹ and Q-switched operation is also demonstrated. Channel waveguides are fabricated in Yb:KGd(WO₄)₂ and Yb:KY(WO₄)₂ using ultrafast laser inscription. Several of these waveguides lase in compact monolithic cavities. A maximum output power of 18.6 mW is observed, with a propagation loss of ~2 dBcm⁻¹. By using a variety of writing conditions the optimum writing pulse energy is identified. Micro-spectroscopy experiments are performed to enable a fuller understanding of the induced crystal modification. Observations include frequency shifts of Raman lines which are attributed to densification of WO₂W bonds in the crystal. Yb:tungstate lasers can generate ultrashort pulses and some preliminary work is done to investigate the use of quantum dot devices as saturable absorbers. These are shown to have reduced saturation fluence compared to quantum well devices, making them particularly suitable for future integration with Yb:tungstate waveguides for the creation of ultrafast, compact and high repetition rate lasers.