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1
Patel, Robin. "Quantum dot lasers." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:47b97874-65c0-41e5-afce-debd778e1fc5.
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Here we present direct investigation of the lasing behaviour by performing gain spectroscopy of solution-based CQDs enabled via in-situ tuning of the feedback wavelength of an open-access hemispherical microcavity. The investigation is performed on two different types of CQDs, namely spherical CdSe/CdS core-shell CQDs and nanopletelets (NPs). The lasing threshold and the differential gain/slope efficiency of the fundamental cavity mode are measured as a function of their spectral position over a spectral range of ∼ 32 nm and of ∼ 42 nm for the spherical CQDs and NPs, respectively. The results of the gain spectroscopy are described using theoretical models, providing insights into the mechanism governing the observed lasing behaviour. Furthermore, the open-access cavity architecture provides a very convenient way of producing in-situ tunable lasing, and single-mode lasing of the fundamental cavity mode over a spectral range of ∼ 25 nm and ∼ 37 nm is demonstrated using spherical CQDs and NPs, respectively. In addition, the stability of laser emission is investigated, with the lasing intensity of the fundamental cavity mode remaining constant over a time period of almost 6 mins. It is hoped that the results will provide a detailed understanding of the lasing behaviour of CQDs. This information can be fed back into the design of CQDs in which the lasing threshold can be reduced to the point where useful devices can be constructed, and in the design of resonant optical feedback structures for which the appropriate wavelength must be carefully selected.
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Kruppa, Suzanne L. "Modeling the quantum dot." Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1997. http://handle.dtic.mil/100.2/ADA333891.
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Hinzer, Karin. "Semiconductor quantum dot lasers." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0003/MQ36702.pdf.
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Martins, Luis. "Quantum dot-cavity systems." Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/18277/.
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This thesis presents experiments carried out on a single InGaAs/GaAs quantum dot coupled with a photonic crystal cavity (H1). The single exciton qubit system is controlled by ultrafast optical pulses. Then, the resonance fluorescence (RF), which is proportional to the population of the quantum dot, is measured by a spectrometer. The two main results of the whole thesis are: i) measurement of the short exciton lifetime (22.7~ps); ii) achievement of full populated quantum dot through a few photons (on average approx. 2.5 photons). To measure such a short lifetime the Two Pulse Resonance Fluorescence technique was developed. This technique enables measurements with high time resolution. This required the development of the Differential Resonance Fluorescence technique. This technique is highly efficient in suppressing the laser scattered light, permitting measurements of the RF of the dot. These two main results are the consequence of cavity enhancement. A Purcell factor of 42 was measured. This is the largest Purcell factor reported so far for the weak coupling regime. This enhancement allows the recovery of the coherence of the QD, permitting the investigation of the quantum dot--cavity system as a near--ideal single photon source on--chip and on--demand. The cavity enhancement also affects the exciton--phonon interaction. The full monotonic phonon side band is here presented for first time. This quantum dot--cavity system also allows the control of the cavity scattered light from the quantum dot. This can be used as an ultrafast switcher.
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Murphy, Helen Marie. "Quantum transport in superlattice and quantum dot structures." Thesis, University of Nottingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364637.
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Woodhouse, Michael. "Quantum dot ensembles as an optical quantum memory." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/11843/.
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In this Ph.D. Thesis we investigate the viability of using quantum dot ensembles as a quantum memory architecture through the use numerical simulations to study population transfer within quantum dots. This is followed by an investigation into the effects of high order wavemixing on the population transfer within two level systems, which was born from effects noted while simulating quantum dots. We study the initialisation of an ensemble of inhomogeneously broadened quantum dots, introducing a novel initialisation method utilising pump field with a slow frequency sweep. We focus on the properties of such an initialisation procedure and conclude that the maximum initialisation fidelities are determined entirely by the Zeeman splittings and decay rates of the quantum dots. We study several possibilities for performing π rotations on the population of an ensemble of quantum dots, and show the RCAP protocol is the most applicable. We study this protocol in the context of quantum dots and give the optimal parameters to use to generate high fidelity π pulses. We then bring together our work on quantum dots population transfer with the work of others covering the write and read procedures on quantum dots to provide a feasibility analysis of the complete quantum memory protocol. The work on wavemixing presented in this thesis uses a novel approach to analyse wavemixing effects which is used to predict the population transferred in two level simulations of wavemixing processes. We provide simulation confirmation of our approach to analyse wavemixing effects and then go on to calculate the disruptive effects of wavemixing caused by high intensity lasers on some simple systems. Finally we show that large orders of wavemixing can, at least in principle, be used for coherent population transfer.
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Lumb, Matthew. "Quantum dot saturable absorber mirrors." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.504906.
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Arango, Alexi Cosmos 1975. "A quantum dot heterojunction photodetector." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/27869.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 113-119).
This thesis presents a new device architecture for photodetectors utilizing colloidally grown quantum dots as the principle photo-active component. We implement a thin film of cadmium selenide (CdSe) quantum dot sensitizers, sandwiched between an electron-transporting titanium dioxide (TiO2) layer and a hole-transporting N,N' diphenyl-N,N' bis(3-ethylphenyl)-(1,1'-biphenyl)- 4,4'-diamine (TPD) organic small molecule layer. The wide band gap TiO2 and TPD layers are found to block charge injection under reverse bias, yet serve as transport layers for photo-excited charge generated in the CdSe. The internal quantum efficiency is approximately 1% at zero bias and saturates at 3% at -1V. Current-voltage sweeps yield low dark current in reverse bias and significant hysteresis under illumination. We speculate that the hysteresis and low quantum efficiency are due to charge accumulation at the TiO2/CdSe interface.
by Alexi Cosmos Arango.
S.M.
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Spencer, Peter David. "Quantum dot bilayer laser diodes." Thesis, Imperial College London, 2008. http://hdl.handle.net/10044/1/1413.
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Optical communication was developed to allow high-speed and long-distance data transmission and is currently a £6bn market. This has also led to the adoption of optical technologies in other areas including the CD, DVD and medical imaging systems. Standardisation of components means that these systems require light sources that operate near the 1310 and 1550 nm telecommunications windows but existing lasers here are expensive due to their high temperature sensitivity. The exploitation of quantum con¯nement has led to the development of \quan- tum dot" (QD) laser material because of predictions of huge gains in performance. Emission wavelengths of InAs/GaAs QD lasers have been extended to the telecom- munications window near 1300 nm by various growth technologies and the first commercial devices have recently been brought to the market. However, progress to longer wavelengths has been stalled for several years as well as the speed and tem- perature sensitivity of these devices falling short of the predictions; partly because QDs are grown by self-assembly resulting in a random distribution of sizes, compo- sitions and strain-states, leading to inhomogeneous broadening which is a departure from the ideal \atom-like" system. This work details the growth, design and development of QD bilayer laser devices, which o®er a unique approach to fixing these shortcomings. When two QD layers are grown close together; the first layer provides a template that allows larger, more uniform QDs to be grown in the second layer, giving greater uniformity and deeper confinement. This has the potential to increase the efficiency and to achieve emission wavelengths out towards the more-commonly used telecommunications window at 1550 nm directly on GaAs substrates. Multiple bilayer laser diodes with inhomge- neous broadening of less than 30meV, lasing at up to 1430 nm and room-temperature photoluminescence at 1515 nm are shown. Despite the vastly reduced inhomogeneous broadening of QD bilayers, it is still found to be a relevant factor due to the change from de-localised geometries of quantum wells to an ensemble of separate QDs. It will be shown that understanding this is essential for describing the observed optical and electrical behaviour of the laser diodes.
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Zhang, Nanlin. "Collodial quantum dot solar cells." Thesis, University of Oxford, 2017. http://ora.ox.ac.uk/objects/uuid:76dd4ff5-abc6-4f47-91d1-8cdc65362b12.
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This thesis presents three specific strategies to enhance colloidal quantum dot (CQD) solar cells' performances, including band alignment engineering, quantum dot passivation, and smart device architecture design. Firstly, by inserting a PbS CQD layer with a relatively smaller band gap as the hole transport layer (HTL), the carrier extraction of the solar cells is much improved, so as the efficiency. The improvement is due to a better interfacial band alignment between the HTLs and the absorber layer, proved by Kelvin probe and cyclic voltammetry results. Small band gap PbS CQDs have deeper Fermi levels because of the easy oxidation. Coupled with a p-type inducing ligand, 1,2-ethanedithiol (EDT), a better valence band alignment is achieved to extract the holes more efficiently. Secondly, alcohol-dissolvable CsI is used as the ligands to passivate QDs' surface with the aim of reducing dangling bonds and defects on the surface. Compared to the commonly used ligand, tetrabutylammonium iodide (TBAI), CsI resulted in better passivation, which was proved by the full ligand exchange, a narrow photoluminescence peak, fewer oxide defects, and the existence of Cs-S bond. A high efficiency of 10% is achieved, which is attributed to that fewer defects and better passivation lead to larger depletion region pushing its optimal thickness and current output higher, as well as the efficiency. Thirdly, a microgroove-structured flexible PbS CQD micro-module solar cell is reported for the first time with a record Voc of 9.2 V. This device was fabricated by automatic dip coating methods, and it avoids the complex recombination layers required in monolithic tandem devices. By e-beam depositing two electrodes on the two walls of the V-shape grooves, devices were connected in series in less than 100 Î1⁄4m width. By using three-dimensional characterisations, the reasons for low efficiency were explained.
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Shortell, Matthew P. "Zinc oxide quantum dot nanostructures." Thesis, Queensland University of Technology, 2014. https://eprints.qut.edu.au/76335/4/Matthew_Shortell_Thesis.pdf.
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Zinc oxide (ZnO) is one of the most intensely studied wide band gap semiconductors due to its many desirable properties. This project established new techniques for investigating the hydrodynamic properties of ZnO nanoparticles, their assembly into useful photonic structures, and their multiphoton absorption coefficients for excitation with visible or infrared light rather than ultraviolet light. The methods developed are also applicable to a wide range of nanoparticle samples.
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Hendrickson, Joshua. "Cavity Quantum Electrodynamics with Quantum Dot - Photonic Crystal Nanocavities." Diss., The University of Arizona, 2010. http://hdl.handle.net/10150/196032.
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High quality factor, small mode volume photonic crystal cavities and single emitter quantum dots are the topic of this dissertation. They are studied as both a combined system with InAs quantum dots grown in the center of a 2D GaAs photonic crystal slab nanocavity as well as individually. The individual studies are concerned with passive 1D silicon photonic crystal nanobeam cavities and deterministic, site-selectively grown arrays of InAs quantum dots.For the combined system, strong light matter coupling in a quantum dot photonic crystal slab nanocavity is discussed. Vacuum Rabi splitting is seen when the interaction strength exceeds the dissipative processes of the coupled system. In order to increase the probability of a spectral matching between cavity modes and quantum dot transitions, a technique for condensing an inert gas onto a sample is used. This can lead to a spectral tuning of up to 4 nm of the cavity mode with minimal change in the cavity quality factor while maintaining cryogenic temperatures down to 4 K. The effect of a large density of quantum dots within a quantum dot photonic crystal slab nanocavity is also addressed. Gain and absorption effects are found to occur, changing the cavity emission linewidth from that of its intrinsic value, as well as lasing with a low number of quantum dots and with high spontaneous emission coupling factors. Additionally, methods for improving the quality factor of GaAs photonic crystal cavities and better understanding different loss mechanisms are discussed.In the individual studies, the site-selective growth of InAs quantum dots on pre-structured GaAs wafers is shown as a promising method for the eventual deterministic fabrication of photonic crystal cavities to single quantum dots. An in-situ annealing step is used to reduce quantum dot density, helping ensure that dots are not grown in unwanted locations.Given silicon's potential for achieving higher quality factors than its GaAs counterpart, a study of 1D passive silicon photonic crystal nanobeam cavities is carried out. Transmission through a coupled microfiber is used to measure quality factors of the cavities and compared with that of a crossed polarized resonant scattering measurement.
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El-Moghraby, Dureid. "Finite difference solutions of quantum wire and quantum dot systems." Thesis, University of Leeds, 2005. http://etheses.whiterose.ac.uk/9441/.
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A new implementation of the finite difference method was developed, and discussed, for solving the time-independent, constant effective mass Schrodinger equation in three dimensions. The motivation behind this approach was to develop a computational technique which is fast to execute and requires a small memory footprint. To demonstrate its validity, this numerical finite difference method was utilised to calculate the electronic eigenenergies of an infinitely deep quantum wire (QWW), where the results were within 0.25 meV of the analytical values. The method was used to calculate energies of a triangular QWW of finite depth that was found in the literature [62]. The calculated energies showed very good agreement with that of Gangopahdhyay [62], with the difference in eigenenergies ranging between 1 and 10 meV. This difference is likely to arise from the simplified constant effective mass Hamiltonian. The case of a pyramidal quantum dot (QD) was then investigated. It was found that the calculated results were within 2 meV of the values found in the literature [5]. However, the advantages o{ this method become apparent as it requires a fraction of the memory needed by the eigenvalue method and the computational times also compare favourably. The effect of the inter-dot separation in a system of vertically aligned pyramidal QDs was then investigated. It was found that when the separation between the QDs was large enough, they behaved as if isolated. As the proximity increased, so did the interaction, which manifests itself as an increase in the peak value of the wave function of the higher energy dot and a reduction in the overall eigenenergies. The method was extended to incorporate the Poisson equation, and used to calculate the eigenenergies of a QD for a varying number of electrons. As would be expected the eigenenergies of the system rose as more electrons were added to the system. The effect of introducing a varying number of electrons into a system of vertically aligned QDs,for a number of inter-dot separations, showed that the eigenenergies for a single electron increased as the inter-dot separation was increased. However, for the case of multiple electrons, it was found that the eigenenergies initially decrease and then increase as the inter-dot separation is increased.
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Kuntz, Matthias. "Modulated InGaAs/GaAs quantum dot lasers." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=978686241.
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Nah, Seungjoo. "Kondo temperature of a quantum dot." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/41137.
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The low-energy properties of quantum dot systems are dominated by the Kondo effect. We study the dependence of the characteristic energy scale of the effect, the Kondo temperature, on the gate voltage, which controls the number of electrons in the strongly blockaded dot. We show that in order to obtain the correct Kondo temperature as a function of the gate voltage, it is crucial to take into account the presence of many energy levels in the dot. The dependence turns out to be very different from that in the conventional single-level Anderson impurity model. Unlike in the latter, the Kondo temperature cannot be characterized by a single parameter, such as the ratio of the tunneling-induced width of the energy levels in the dot and the charging energy.
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Gustafsson, Oscar. "Type-II interband quantum dot photodetectors." Doctoral thesis, KTH, Integrerade komponenter och kretsar, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-122294.
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Photon detectors based on single-crystalline materials are of great interest for high performance imaging applications due to their low noise and fast response. The major detector materials for sensing in the long-wavelength infrared (LWIR) band (8-14 µm) are currently HgCdTe (MCT) and AlGaAs/GaAs quantum wells (QW) used in intraband-based quantum-well infrared photodetectors (QWIPs). These either suffer from compositional variations that are detrimental to the system performance as in the case of MCT, or, have an efficient dark current generation mechanism that limits the operating temperature as for QWIPs. The need for increased on-wafer uniformity and elevated operating temperatures has resulted in the development of various alternative approaches, such as type-II strained-layer superlattice detectors (SLSs) and intraband quantum-dot infrared photodetectors (QDIPs). In this work, we mainly explore two self-assembled quantum-dot (QD) materials for use as the absorber material in photon detectors for the LWIR, with the aim to develop low-dark current devices that can allow for high operating temperatures and high manufacturability. The detection mechanism is here based on type-II interband transitions from bound hole states in the QDs to continuum states in the matrix material. Metal-organic vapor-phase epitaxy (MOVPE) was used to fabricate (Al)GaAs(Sb)/InAs and In(Ga)Sb/InAs QD structures for the development of an LWIR active material. A successive analysis of (Al)GaAs(Sb) QDs using absorption spectroscopy shows strong absorption in the range 6-12 µm interpreted to originate in intra-valence band transitions. Moreover, record-long photoluminescence (PL) wavelength up to 12 µm is demonstrated in InSb- and InGaSb QDs. Mesa-etched single-pixel photodiodes were fabricated in which photoresponse is demonstrated up to 8 µm at 230 K with 10 In0.5Ga0.5Sb QD layers as the active region. The photoresponse is observed to be strongly temperature-dependent which is explained by hole trapping in the QDs. In the current design, the photoresponse is thermally limited at typical LWIR sensor operating temperatures (60-120 K), which is detrimental to the imaging performance. This can potentially be resolved by selecting a matrix material with a smaller barrier for thermionic emission of photo-excited holes. If such an arrangement can be achieved, type-II interband InGaSb QD structures can turn out to be interesting as a high-operating-temperature sensor material for thermal imaging applications.QC 20130521
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Farrow, T. "Quantum dot single-photon emitting diodes." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598951.
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Significant advances have been made in the past five years in realizing photon sources based on various kinds of emitters in the 500-1000 nm spectral region. However, reliable sources emitting at wavelengths compatible with standard telecommunication optical fibre are still lacking. Self-assembled quantum dots offer a way to generate wavelength-tunable photons. They offer also relatively high radiative efficiencies, short spontaneous emission lifetimes (~1 ns) and, importantly, can easily be incorporated into compact semiconductor devices fabricated with established technologies. In this thesis, single photon sources based on InAs/GaAs quantum dots were studied and developed for applications at the technologically important wavelength around 1.3 μm compatible with standard telecommunication optical fibre. A single photon source suitable for use in a quantum key distribution system was demonstrated by optically pumping an InAs/GaAs quantum dot incorporated inside a micropillar optical cavity (chapter 4). Single photon emission driven by short electrical pulses was then realised for the first time at λ ~ 1.3 μm using an InAs/GaAs quantum dot embedded in a planar optical microcavity (chapter 5). Electrically driven single photon emission was also achieved for quantum dot devices based on the micropillar geometry, demonstrating the viability of such compact cavities with small optical mode volumes at both λ ~ 900 nm and 1.3 μm (chapters 6 and 7). The realization of these devices surmounted the difficulty of contacting the small cross-sectional diameters (~ 2 – 2.5 μm) of the micropillars. Positioning quantum dots accurately inside optical microcavities can improve the efficiency of single photon sources. Chapter 3 presents results of a study of semiconductor wafers with intentionally positioned quantum dots, demonstrating the viability of a technique for placing quantum dots reproducibly.
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Amecke-Mönnighoff, Nicole. "Characterization of Single Quantum Dot Blinking." Doctoral thesis, Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-172422.
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This thesis addresses the observed fluorescence intermittency of single semiconductor nanocrystals, so called Quantum Dots (QDs), which is also referred to as blinking. Despite continuous excitation their fluorescence is randomly interrupted by dark periods that can last over several minutes. Especially the extraction of power law dwell time statistics in bright and dark states indicates very complex underlying processes that are not fully understood to date. Here two approaches are followed to reveal the nature of the blinking mechanism.
One addresses the common threshold method for extraction of power law dwell times. Its performance is tested with simulations to a broad range of experimentally determined parameters. Strong deviations are found between input and extracted statistics dependent on input parameters themselves. A comparison with experimental data does not support the assignment of power law statistics for the bright state and indicates the existence of distinct blinking mechanisms.
The second approach directly aims at the nature of the dark state, which is mostly attributed to charges in the QD or trap states in its vicinity. A method is developed to detect charging processes on single QDs with their fluorescence. Electrochemistry is combined with confocal microscopy also allowing evaluations of excited state lifetimes and emission spectra. Reduction and oxidation of the QD bands are successfully observed as a quenching of QD fluorescence. Single QD observations identify two independent blinking mechanisms, that are assigned to positive and negative charging. Positive charging is not only observed after hole injection but also the extraction of excited electrons. Three additional quenching mechanisms are identified, two of which are assigned to trap relaxation. Differences between two substrate electrodes demonstrate the importance of the substrate material.
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McLellan, Luke Jonathan. "Colloidal quantum dot and hybrid lasers." Thesis, University of Strathclyde, 2018. http://digitool.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=29395.
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This thesis presents research on solution-processed semiconductor lasers, with an emphasis on colloidal quantum dot (CQD) lasers, aimed at progressing the state of the art of the technology. A particular focus is placed on demonstrating such lasers under long (> nanosecond) pulse pumping in order to bring this device technology closer to applications. Alloyed-core CdSSe CQDs with a ZnS outer shell that emit in the visible are studied. These were prepared in a solid-state thin-film overcoated with a polymeric cladding (polyvinyl alcohol or PVA) to demonstrate a distributed feedback(DFB) laser having a bilayer waveguide structure. It is shown that the PVA symmetrises the laser structure and reduces the modal losses, in turn enabling the lowest threshold operation in the nanosecond regime for CQD lasers at the time;13.5 μJ/cm2 at 5 ns. The format is also beneficial for the photostability of the laser. The CQDs are also demonstrated as the gain material of a robust, orange-emitting Vertical-Cavity Surface-Emitting Laser (VCSEL). Oscillation threshold comprised between 1 and 20 mJ/cm2 for 5ns pump pulse duration are obtained. For a broad area excitation (250 μm at 1/e2 radius), the operation is highly multimode and characterised by spatio-temporal instabilities. When pumped with a spot size at or below 100μm in radius, the VCSEL is observed to be single mode for low pump energies while additional longitudinal modes, as well as transverse mode,appear as the pumping level is increased. A study of the polarisation of the CQD laser emission for different polarisation of the pump was carried out and the Stokes parameters calculated. Results show that the pump polarisation has no significant effect on the VCSEL polarisation. While the bulk of the research concerns CQDs, two types of organic semiconductors were also applied to the aforementioned DFB laser structure. Poly(paraphenylenevinylene) (PPV) copolymer Super Yellow leading to what is to the best of our knowledge, the only case of completely ambient lasing using Super Yellow. Refractive index sensing with the laser was attempted and results are summarised. The second type in fact consisted of two organic semiconductor materials from the same novel family of autofluorescence molecules respectively named Blue 4 and Blue 6. These are investigated for the first time as laser materials. Both materialsshow signs of random lasing when prepared in a solid-state film. Single modelasing in distributed feedback format is also demonstrated with Blue 4.
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Wood, Vanessa Claire. "All inorganic colloidal quantum dot LEDs." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/40882.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 85-89).
This thesis presents the first colloidal quantum dot light emitting devices (QD-LEDs) with metal oxide charge transport layers. Colloidally synthesized quantum dots (QDs) have shown promise as the active material in optoelectronic devices because of their tunable, narrow band emission. To date, the most efficient QD-LEDs involve a monolayer of closely packed QDs sandwiched between organic charge transport layers. However, these organic materials are subject to degradation due to atmospheric oxygen and water vapor. In contrast, metal-oxide films used in this work are chemically and morphologically stable in air and can withstand numerous organic solvents, which increases the flexibility of device processing. Furthermore, they can sustain higher carrier injection rates needed to realize an electrically pumped colloidal QD laser. This thesis details the characterization techniques, such as Atomic Force Microscopy, photoluminescence spectroscopy, Hall Effect measurements, X-Ray Diffraction, and Ultraviolet Photoelectron Spectroscopy, used to design efficient QD-LEDs. It reviews the steps used to optimize device performance and obtain a transparent device architecture with external quantum efficiency of 0.15% and a peak luminance of 7000 Cd/m2. This manifests a 100-fold improvement in efficiency over any previously reported all inorganic QD-LED structure.
by Vanessa Claire Wood.
S.M.
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Tutu, F. K. K. "InAs/GaAs quantum dot solar cells." Thesis, University College London (University of London), 2014. http://discovery.ucl.ac.uk/1430283/.
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Self-assembled III-V quantum dots (QDs) have been intensely studied for potential applications in solar cell (SC) devices in order to increase power conversion efficiency. Due to their quantum confinement of carriers, QDs have been proposed as a means of implementing the intermediate band solar cell (IBSC). The IBSC concept is characterised by in an increase in photocurrent and a preservation of output voltage, resulting from an enhanced sensitivity to the solar spectrum. The work reported in this thesis is concerned with the development of InAs QDs in GaAs p-i-n solar cell structures, with the aim of realising of an IBSC. The work involves the design, epitaxial growth by molecular beam epitaxy (MBE), device processing and characterisation of the QDSCs. This thesis first investigates InAs/InGaAs dot-in-a-well (DWELL) solar cell structures grown under different conditions. The use of a high-growth-temperature GaAs spacer layers is demonstrated to significantly enhance the performance of the multilayer DWELL solar cells. Threading dislocations were observed for a 30-layer QD structure with GaAs spacer layers grown at a low temperature (510 oC). By growing the GaAs spacer layer at a higher temperature (580 oC), the formation of threading dislocations were suppressed, resulting in enhanced optical properties. The thesis then goes on to address the main challenges facing QD IBSCs, that is, the reduction in open-circuit voltage and the lack of significant increase in short-circuit current. To eliminate the wetting layer and enhance the open-circuit voltage of the QD solar cell, an AlAs cap layer technique was used. This resulted in an enhancement of the open-circuit voltage of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, for the QDSC with AlAs cap layer, the enhancement in the open-circuit voltage was enough to ensure that its efficiency is higher than the QDSC without AlAs cap layers. In an attempt to enhance the short-circuit current, an antimony-mediated growth approach was used to grow high-density QDs. After optimisation of the growth temperature and InAs coverage, a very high in-plane QD density of 1 1011 cm-2 was achieved by applying a few monolayers of antimony prior to QD growth. Compared with a reference QDSC without the incorporation of antimony, the high-density QDSC demonstrates a distinct improvement in short-circuit current from 7.4 mA/cm2 to 8.3 mA/cm2. This result shows that a significant increase in short-circuit current could potentially compensate for the drop in open-circuit voltage observed in InAs/GaAs QD solar cells. Ongoing work on the development of QDSCs with both AlAs capping and antimony-mediated growth have resulted in the simultaneous elimination of the wetting layer and increase in QD absorption in a single device. Overall, the studies in this thesis present important implications for the design and growth of InAs/GaAs QD solar cell structures for the implementation of IBSCs.
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Butkus, Mantas. "Quantum dot based semiconductor disk lasers." Thesis, University of Dundee, 2012. https://discovery.dundee.ac.uk/en/studentTheses/6b17df24-a721-4904-b49f-e35055990c16.
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Since its first successful demonstration more than five decades ago [1], laser technology experienced a huge leap forward in terms of technological innovations and the understanding of underlying physical principles of operation. There were many efforts made by those in both the scientific and commercial communities who envisioned the potential of lasers. As a result, the laser now is a powerful scientific tool in many disciplines. It is widely used not only in physics, but also in chemistry, biology, medicine, engineering, environmental sciences, arts and their interdisciplinary fields. Moreover, it now has a vast number of applications in industry and everyday life whether it is light and matter interaction, communication and IT, healthcare and many other uses of this light source. By the same time, photonics comprises a market of multi-billion EUR value [2].At every stage of development, different laser parameters were engineered to suit those to specific application with some other parameters usually being sacrificed. Together with this, things like compactness and cost were always an issue to consider. A huge impact to the field of photonics was made by the development of semiconductor based structures that could be used as a light amplifying medium. Semiconductor lasers not only allowed the miniaturization of many devices, but also provided new opportunities for laser scientists due to ability to engineer their bandgap properties and to confine the carriers in different dimensions.The development of vertical external cavity surface emitting lasers (VECSELs), which are also known as optically pumped semiconductor lasers (OPSLs) or semiconductor disk lasers (SDLs) realized an important feature in semiconductor based lasers – high multi- Watt output power was combined with diffraction limited output beam profile.This work is devoted to the development of semiconductor disk lasers based on novel quantum dot (QD) structures. QD structures were embedded in this type of laser recently and allowed a number of advantages compared with the widely used quantum well (QW) structure. These included new spectral region coverage at 1-1.3 µm, enhanced wavelength tuneability and ultrafast carrier dynamics, which potentially will improve mode locked operation. QDs were also used as a base for semiconductor saturable absorbers in modelocking experiments.During the time of these studies, QD SDLs at new spectral regions and record output power were demonstrated. Power scaling up to 6 W was achieved for 1040 nm, 2.25 W for 1180 nm and 1.6 W for 1260 nm devices. Excited state transition in QDs was shown to be more efficient for high power QD SDLs as compared with ground state transition. New spectral regions were covered by QD SDLs using frequency doubling into the visible region with green, orange and red light emission with output powers of 2 W, 2.5 W and 0.34 W respectively. The broad gain bandwidth of the quantum dot material was explored and wavelength tuneability up to 60 nm around 1040 nm, 69 nm around 1180 nm, and 25 nm around 1260 nm was demonstrated.A QD based saturable absorber was used to mode-lock the quantum well SDL, resulting in the first such type of laser with sub-picosecond pulse widths. Pulses with duration of 870 fs at a repetition rate of 896 MHz and wavelength of 1028.5 nm were demonstrated. Pulses were 1.14 times Fourier limited and an average output power of 46 mW was achieved. Finally, quantum well based VECSELs with electrical pumping schemes were tested. The devices were first tested in the cw configuration. Highest output powers up to 60 mW were achieved from such devices. Devices were then tested in mode-locking experiments. Pulsed operation was observed and the measurements indicated 270 ps width pulses with 8 mW average output power at 1.9 GHz repetition rate. All devices operated at 980 nm.This thesis consists of six chapters. In the introductory part of this work, QD based SDLs and their development and applications will be reviewed together with their operational principles. Chapter two will describe the growth, fabrication and preparation of SDL samples. Continuous wave and mode-locked operation results will be presented in chapters three and four. Electrically pumped devices will be presented in chapter five along with experimental results. Conclusions and future prospects will be given at the end of this work. The list of publications which were generated during the studies is included at the beginning of this work.The work presented in thesis was done under the FAST-DOT project. This is a European FP 7 project targeted at the development of compact and low-cost novel quantum dot based laser sources for biophotonic applications.
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Masse, Nicholas. "Recombination processes in quantum dot lasers." Thesis, University of Surrey, 2008. http://epubs.surrey.ac.uk/844607/.
Full textAbstract:
The drive for low threshold and temperature-stable semiconductor lasers for telecommunication applications has led to a significant interest in quantum dot (QD) lasers emitting in the 1.3 mum and 1.5 mum wavelength range. The literature shows that although low threshold current densities can be achieved, this is usually at the expense of a poor temperature stability. Low-temperature and high-pressure measurements of the threshold current and its radiative component are performed on undoped and p-doped 1.3 mum InAs/GaAs and 1.5 mum InAs/InP (311)B QD lasers. The results show that despite a fairly temperature-stable radiative current around room temperature, undoped QD lasers suffer from a poor temperature stability of their threshold current. This is because there is a large contribution (70% and 90% of the threshold current at room temperature in 1.3 and 1.5 mum lasers, respectively) from a strongly temperature sensitive non-radiative Auger recombination process. Several pieces of evidence are found to explain the observed decrease of the radiative current, explained by an improvement of the carrier distribution with increasing temperature. We find that in p-doped devices the temperature dependence of the radiative component of the threshold current can be modified by the doping. In these devices the radiative current can decrease with increasing temperature around room temperature while the non-radiative current increases. This results in a small range of temperatures over which the threshold current is constant (from ~ 270 to 300 K). This effect is very sensitive to the doping concentration. If the doping concentration is carefully chosen, this can result in high T0 devices but with larger threshold currents than in comparable undoped lasers. Gain measurements reveal that the differential gain of p-doped lasers is less than that of the undoped devices because of the increased non-radiative current and the non-thermal distribution of the carriers induced by the doping. Finally, a new method is demonstrated to measure the band gap dependence of the Auger coefficient, C, using a combination of high hydrostatic pressure measurements coupled with gain calculations.
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Tongkhundam, Jindee. "Synthesis of quantum-dot doped microparticles." Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686186.
Full textAbstract:
A key factor in the success of using laser scanning confocal microscopy to image molecular systems such as fluorescent microspheres, is the requirement for the microspheres to have a high intensity of florescence whilst having a high photobleaching resistance. In this work, a number of straightforward and reliable synthetic procedures to prepare silica and PMMA microspheres using quantum dots as the fluorescent material are described. Experimental data reveals that quantum dots perform as an excellent fluorophore, showing significantly greater photobleaching resistance as compared to organic dye.
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Wang, Chia-Jean. "Sub-diffraction quantum dot nanophotonic waveguides /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/5879.
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Park, Tyler Drue. "Characterization of InGaAs Quantum Dot Chains." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/3718.
Full textAbstract:
InGaAs quantum dot chains were grown with a low-temperature variation of the Stranski-Krastanov method, the conventional epitaxial method. This new method seeks to reduce indium segregation and intermixing in addition to giving greater control in the growth process. We used photoluminescence spectroscopy techniques to characterize the quality and electronic structure of these samples. We have recently used a transmission electron microscope to show how the quantum dots vary with annealing temperature. Some questions relating to the morphology of the samples cannot be answered by photoluminescence spectroscopy alone. Using transmission electron microscopy, we verified flattening of the quantum dots with annealing temperature and resolved the chemical composition with cross-section cuts and plan view cuts.
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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.
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Bauer, Sven [Verfasser]. "High-Speed 1.55 µm Quantum Dot Lasers with Electronically Coupled Quantum Well - Dot Active Regions / Sven Bauer." Kassel : Universitätsbibliothek Kassel, 2019. http://d-nb.info/1201500753/34.
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Varnava, Christiana. "Quantum dot-based Entangled-Light Emitting Diodes (E-LED) for quantum relays." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/271783.
Full textAbstract:
Sources of entangled pairs of photons can be used for encoding signals in quantum-encrypted communications, allowing a sender, Alice, and a receiver, Bob, to exchange keys without the possibility of eavesdropping. In fact, any quantum information system would require single and entangled photons to serve as qubits. For this purpose, semiconductor quantum dots (QD) have been extensively studied for their ability to produce entangled light and function as single photon sources. The quality of such sources is evaluated based on three criteria: high efficiency, small multi-photon probability, and quantum indistinguishability. In this work, a simple quantum dot-based LED (E-LED) was used as a quantum light source for on-demand emission, indicating the potential for use as quantum information devices. Limitations of the device include the fine-structure splitting of the quantum dot excitons, their coherence lengths and charge carrier interactions in the structure. The quantum dot-based light emitting diode was initially shown to operate in pulsed mode under AC bias frequencies of up to several hundreds of MHz, without compromising the quality of emission. In a Hong-ou-Mandel interference type experiment, the quantum dot photons were shown to interfere with dissimilar photons from a laser, achieving high two-photon interference (TPI) visibilities. Quantum entanglement from a QD photon pair was also measured in pulsed mode, where the QD-based entangled-LED (E-LED) was electrically injected at a frequency of 203 MHz. After verifying indistinguishability and good entanglement properties from the QD photons under the above conditions, a quantum relay over 1km of fibre was demonstrated, using input qubits from a laser source. The average relay fidelity was high enough to allow for error correction for this BB84-type scheme. To improve the properties of the QD emission, an E-LED was developed based on droplet epitaxy (D-E) QDs, using a different QD growth technique. The relevant chapter outlines the process of QD growth and finally demonstration of quantum entanglement from an electrically injected diode, yielding improvements compared to previous E-LED devices. For the same reason, an alternative method of E-LED operation based on resonant two-photon excitation of the QD was explored. Analysis of Rabi oscillations in a quantum dot with a bound exciton state demonstrated coupling of the ground state and the biexciton state by the external oscillating field of a laser, therefore allowing the transition between the two states. The results include a considerable improvement in the coherence length of the QD emission, which is crucial for future quantum network applications. We believe that extending this research can find application in quantum cryptography and in realising the interface of a quantum network, based on semiconductor nanotechnology.
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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.
Full textAbstract:
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.
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Han, Dae-Seob. "Theory of Tunneling-Injection Quantum Dot Lasers." Diss., Virginia Tech, 2009. http://hdl.handle.net/10919/28997.
Full textAbstract:
This work develops a comprehensive theoretical model for a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). The potential of such a tunneling-injection QD laser for temperature-stable and high-power operation is studied under the realistic conditions of out-tunneling leakage of carriers from QDs (and hence parasitic recombination outside QDs) and the presence of the wetting layer (WL). The following topics are included in the dissertation:
1) Characteristic temperature of a tunneling-injection QD laser
The threshold current density jth and the characteristic temperature T0 are mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, the tunneling-injection laser shows the potential for significant improvement of temperature stability of jth â the characteristic temperature T0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.
2) Output power of a tunneling-injection QD laser
Closed-form expressions for the light-current characteristic (LCC) and carrier population across the layered structure are derived. Even in the presence of out-tunneling leakage from QDs, the intensity of parasitic recombination outside QDs is shown to remain restricted with increasing injection current. As a consequence, the LCC of a tunneling-injection QD laser exhibits a remarkable feature â it becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within QDs â in view of the three-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited.
3) Effect of the WL on the output power of a tunneling-injection QD laser
In the Stranski-Krastanow self-assembling growth mode, a two-dimensional WL is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron-hole recombination in the WL, even in a tunneling-injection structure. Since the opposite sides of a tunneling-injection structure are only connected by the current paths through QDs, and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (QWs and optical confinement layer), the parasitic electron-hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a tunneling-injection QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation.Ph. D.
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Mehl, Sebastian Johannes [Verfasser]. "Achieving quantum computation with quantum dot spin qubits / Sebastian Johannes Mehl." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/1065974485/34.
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Harankahage, Dulanjan Padmajith Dharmasena. "Quantum Confinement Beyond the Exciton Bhor Radius in Quantum Dot Nanoshells." Bowling Green State University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1593955468720583.
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Chen, Siming. "Hybrid quantum well/quantum dot structure for broad spectral bandwidth emitters." Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/5592/.
Full textAbstract:
This thesis details a hybrid quantum well (QW)/quantum dot (QD) active element for an application in broadband source. First of all, a literature review on the fundamentals of optical coherence tomography (OCT) and superluminescent light emitting diodes is provided in Chapter 1. Basic principles of QD formation using molecular beam epitaxy and several experimental techniques are reviewed in Chapter 2. The first vertically integrated hybrid QW/QD structure for application in broadband light sources is proposed in Chapter 3. Spontaneous emission from both the QW and the QDs resulted in a full width half maximum (FWHM) of 250nm being demonstrated. In Chapter 4, experimental results on the modal gain and lasing characteristic of hybrid QW/QD laser are described. Due to the contribution from the QD ground state and first excited state, and the lowest energy transition of the QW, the modal gain at room temperature is extended to 300nm. The values for modal gain are further confirmed by simultaneous three state lasing. The first hybrid QW/QD superluminescent diode is discussed in Chapter 5. High order QW transitions are observed at high current densities. As a result, a 3dB emission spectrum of FWHM linewidth of 289nm centered at ~1200nm with a corresponding power of 2.4mW is achieved. The origin of high order QW transitions is discussed in Chapter 6. New device designs utilizing a larger number of QD layers with higher areal density and larger state separation is reported in Chapter 7. Chapter 8 summarizes the whole thesis.
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Hansson, Conny, and Rachavula Krishna Kishore. "Comparative study of infrared photodetectors based on quantum wells (QWIPs) and quantum dots (QDIPs)." Thesis, Halmstad University, School of Information Science, Computer and Electrical Engineering (IDE), 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:hh:diva-234.
Full textAbstract:
This master’s thesis deals with studies of lateral and vertical carrier transport Dot-in-
a-Well (DWELL) Quantum Dot Infrared Photodetectors (QDIPs). During the pro ject,
devices have been developed and tested using a Fourier Transform Infrared (FTIR) spec-
trometer with the purpose to find the processes governing the flow of photocurrent in
the different kinds of detectors, the dark current magnitude in the vertical Quantum Dot
Infrared Photodetector (QDIP) and the Quantum Well Infrared Photodetector (QWIP)
and the light polarization dependences for the vertical QDIP and the QWIP.
The lateral carrier transport DWELL QDIP was found to have poor conduction
in the well mainly due to re-trapping of electrons in this region. The main process gov-
erning the flow of photocurrent for this type of device at 77K is photo-excitation from
the Quantum Dot (QD)s to the excited state in the Quantum Well (QW) and further
thermal excitation. If the electrons are mainly transported in the matrix or the well at
77K is presently not clear.
For the vertical carrier transport DWELL QDIP at 77K, the wavelength response
could be tuned by altering the applied voltage. At higher voltages, the dominant process
was found to be photo-excitation from the QDs to the excited state in the QW followed
by thermal assisted tunneling into the GaAs-matrix. At lower voltages, photo-excitation
from the QDs directly into the the GaAs-matrix was the predominant process. The dark
current level in the vertical QDIPs was found to be 1.5 to 5 orders of magnitude smaller
than for the QWIP measured at 77K. Furthermore, the QDIP was found to be close to
polarization independent. As expected the QWIP had a reduced sensitivity to normal
incident light. The existence of this signal was attributed to interface scattering of light
inside the device.
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Gallardo, D. E. "Dynamic effects in CdTe quantum-dot LEDs." Thesis, Cranfield University, 2006. http://dspace.lib.cranfield.ac.uk/handle/1826/2961.
Full textAbstract:
In this work the electrical and electroluminescence properties CdTe nanocrystal films were analysed. The structure consisted of a multilayer of CdTe nanocrystals deposited by the layer-by-layer technique, sandwiched between an ITO anode and an aluminium cathode. The first part of this work was dedicated to structural and process improvement. Earlier devices, produced through a layer-by-layer (LbL) manual procedure, had an average thickness of 30nm per nanocrystal monolayer, with a roughness near 30% of the overall thickness. Electrical tests showed current densities over 100 mA/cm^2, with a frequent occurrence of pinholes and short-circuits that caused erratic sample behaviour and device rupture. SEM and AFM microscopic analysis showed that the nanoparticles were aggregated in 15-20nm thick clusters bound by polymer. A high porosity and non- uniformity of the multilayer was observed, explaining the formation of short-circuits. Luminescence was obtained from a very small number of samples, and with a very short duration that did not make spectral analyses possible. A robotic arm was programmed to carry out the LbL deposition, in an attempt to reduce the inhomogeneities of the multilayer. A key factor was the introduction of a special routine to remove the samples from the solutions. The sample withdrawal was designed to be in the vertical at a rate of 1.18 mm/s. The idea was to use gravity and the surface tension of the aqueous solutions to remove all the excess liquid from the surface. Additionally, poly(ethylenimine) (PEI) was eliminated from the process to improve homogeneity. These modifications produced multilayers with a thickness of 3nm per layer, average roughness below 5nm and CdTe packing density of 27%. Electrical measurements showed a stabilisation of the current-voltage (I-V) characteristics. A significant improvement in luminescence occurrence frequency and intensity was also achieved, enabling first spectral analyses. Once a reliable manufacturing procedure was developed, the electrical characterisation commenced with the analyses of samples with a different number of layers, operated in air. A field dependency of the I-V curves was found. Optimal performance was obtained from 30-layer samples, and this number of layers was adopted for subsequent analyses. Best samples showed external quantum efficiencies of 0.51%, with a photometric response of 0.8 lm/W and peak brightness of 1.42 cd/m^2. However, current and electroluminescence (EL) degradation with voltage and operation time were found in the device. Single carrier devices revealed a barrier for electron injection higher than predicted by the band diagram of the structure. The presence of an aluminium oxide layer at the multilayer/cathode interface was postulated, and confirmed through experiments in nitrogen. It was proposed that the growth of this oxide layer is the cause of device degradation during operation in air. However, it was demonstrated that the presence of the oxide favoured radiative recombination prior to degradation, with device efficiencies nearly 10 times higher than in devices without the oxide film. This was justified through three effects: charge accumulation at both sides of the oxide, field concentration across the oxide barrier and a reduction in leakage current. Unequal behaviour of samples with different electrode materials revealed that charge injection was the limiting mechanism for current flow, with a current onset field in the range of 2-3x10^7 V/m. Fowler-Nordheim plots showed that field emission was responsible for hole and electron injection into the device. Also, Fowler-Nordheim plots provided evidence of the dynamic nature of the cathode oxidation process. Dielectric breakdown of the aluminium oxide barrier at a rupture field of 3x10^7 V/m was found as a possible triggering mechanism of oxide layer growth. It was found that a critical field around 4.5x10^7 V/m caused irreversible loss of photoluminescence (PL) of the nanocrystals. This loss was attributed to an avalanche effect within the multilayer. The operational range for the devices is then found to be between 2x10^7 V/m and 4.5x10^7 V/m.
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Hashiba, Hideomi. "Quantum dot detector for passive terahertz imager." Thesis, Royal Holloway, University of London, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.427965.
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Bhat, Jerome C. "Electroluminescent hybrid organic/inorganic quantum dot devices." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298766.
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Ren, Wei, and 任偉. "Electronic transport in the nanotube quantum dot." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B2666530X.
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Böhm, Marcus. "Hybrid ligands in quantum dot solar cells." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708460.
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Zimmer, John P. (John Philip). "Quantum dot-based nanomaterials for biological imaging." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37888.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.Vita.
Includes bibliographical references.
Quantum dot-based fluorescent probes were synthesized and applied to biological imaging in two distinct size regimes: (1) 100-1000 nm and (2) < 10 nm in diameter. The larger diameter range was accessed by doping CdSe/ZnS or CdS/ZnS quantum dots (QDs) into shells grown on the surfaces of pre-formed sub-micron SiO2 microspheres. The smaller diameter range was accessed with two different materials: very small InAs/ZnSe QDs and CdSe/ZnS QDs, each water solubilized with small molecule ligands chosen for their ability not only to stabilize QDs in water but also to minimize the total hydrodynamic size of the QD-ligand conjugates. Indium arsenide QDs were synthesized because nanocrystals of this material can be tuned to fluoresce in the near infrared (NIR) portion of the electromagnetic spectrum, especially in the 700-900 nm window where many tissues in the body absorb and scatter minimally, while maintaining core sizes of 2 nm or less. The QD-containing microspheres were used to image tumor vasculature in living animals, and to generate maps of size-dependent extravasation. With subcutaneously delivered nAs/ZnSe QDs, multiple lymph node mapping was demonstrated in vivo for the first time with nanocrystals. When administered intravenously, < 10 nm QDs escaped from the vasculature, or were efficiently cleared from circulation by the kidney. Both of these behaviors, previously unreported, mark key milestones in the realization of an ideal fluorescent QD probe for imaging specific compartments in vivo. Also presented in this thesis is the growth of single-crystalline cobalt nanorods through the oriented attachment of spherical cobalt nanocrystal monomers.
(cont.) When administered intravenously, < 10 nm QDs escaped from the vasculature, or were efficiently cleared from circulation by the kidney. Both of these behaviors, previously unreported, mark key milestones in the realization of an ideal fluorescent QD probe for imaging specific compartments in vivo. Also presented in this thesis is the growth of single-crystalline cobalt nanorods through the oriented attachment of spherical cobalt nanocrystal monomers.
by John P. Zimmer.
Ph.D.
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Nabanja, Sheila. "Design and fabrication of quantum-dot lasers." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45888.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 87-89).
Semiconductor lasers using quantum-dots in their active regions have been reported to exhibit significant performance advantages over their bulk semiconductor and quantum-well counterparts namely: low threshold current, high differential gain and highly temperature stable light-current characteristics. This thesis investigates the lasing characteristics of a ridge-waveguide laser containing seven layers of quantum dots as the active region. A summary of the electrical and optical performance data of the heterostructure quantum dot lasers, as well as previously fabricated quantum well lasers, is presented. The motivation of using InAs quantum dots in the active region is to produce near infrared emission for telecommunication applications.
by Sheila Nabanja.
S.M.
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Leatherdale, Catherine A. (Catherine Anne) 1972. "Photophysics of cadmium selenide quantum dot solids." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/8828.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2000.Includes bibliographical references.
Semiconductor quantum dots or nanocrystals have size dependent optical and electronic properties that arise from quantum confinement. While the quantum size effect is reasonably well understood, the effect of abrupt interface between the nanocrystal and its dielectric environment is not. In this thesis we study how the dielectric environment affects the quantum dot electronic structure, the optical absorption ~ross-section, charge separation, and transport in cadmium selenide colloidal quantum dots. The electronic states and optical absorption cross-section are found to be less sensitive to changes in the dielectric environment than predicted from theory unless screening from the ligand shell is taken into account. The absolute absorption cross section is measured as a function of quantum dot size; excellent agreement with theory is obtained for absorption far above the band edge. Three-dimensional close packed solids of quantum dots are predicted to act as model artificial solids. Optical absorption measurements indicate that the electronic states of CdSe quantum dots separated by 11 angstroms or more are essentially uncoupled. Photoconductivity measurements suggest that photoexcited quantum confined excitons are ionized by the applied field with a rate that depends on both the size and surface passivation of the quantum dots. The charge generation efficiency decreases with increasing temperature as non-radiative and radiative recombination pathways increasingly compete with charge separation. A simple tunneling model for the initial charge separation step is presented that qualitatively reproduces both the size and surface dependence of the photoconductivity as a function of applied electric field. Finally, we report observations of amplified spontaneous emission from quantum dot solids. The stimulated emission is tunable with quantum dot size and does not sensitively depend upon surface passivation. These measurements demonstrate the feasibility of nanocrystal quantum dot lasers and amplifiers.
by Catherine A. Leatherdale.
Ph.D.
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Giam, Louise R. "Gallium Nitride (GaN) quantum dot layer formation." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/35070.
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Goldberg, Brian 1979. "Magnetic properties of an isolated quantum dot." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/87370.
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Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and, Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.Includes bibliographical references (p. 121-123).
by Brian Goldberg.
M.Eng.
S.B.
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Morgan, Nicole Yen-i. 1971. "Electronic transport in CdSe quantum dot arrays." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/8668.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001.Includes bibliographical references (leaves 148-154).
When an isolated piece of conductor is sufficiently small, the energy to add another electron can be larger than the available thermal energy, and the number of electrons on the island is well-defined. If the island is made still smaller, the quantum confinement energy also becomes large, the electronic states on the dot become discrete, and the island is called a quantum dot, or sometimes an artificial atom. Quantum dots have been a focus of active research for the past decade, both as model systems for exploring physics, and as the ultimate limit in size reduction for conventional transistors. Early quantum dots were made only by lithographic patterning, but more recently the solution-based synthesis of semiconductor nanocrystals, which are much smaller than the lithographic quantum dots, has been developed. The nanocrystals can range in size from 1.5 nm to 8 nm in diameter with a narrow size distribution, and they can form close-packed arrays when deposited from solution, with organic molecules that coat the nanocrystals serving as spacers. These quantum dot arrays have the potential to be model artificial solids, with tunable intersite coupling, site energies, and order. I present results for electronic transport measurements on large arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time and four orders of magnitude in current. Furthermore, we do not observe a steady-state dark current for fields up to 1x106 V/cm and times out to 5 x 104 seconds.
(cont.) Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. We posit the existence of a narrow space charge region near the injecting contact, and provide a consistent interpretation of our results within this model. The observation of extremely long-lived current transients points to the importance of long-range Coulomb interactions between charges on different nanocrystals; in the picture we develop, the interactions within the narrow space charge region determine the current.
by Nicole Yen-i morgan.
Ph.D.
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Hutchings, Matthew D. "Carrier distribution processes in Quantum Dot ensembles." Thesis, Cardiff University, 2012. http://orca.cf.ac.uk/45042/.
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In this thesis the development of new analysis methods that study the carrier distributions in quantum dots (QDs) directly from experimental measurement of spontaneous emission and gain spectra are described. These were applied to three InAs QD structures that are nominally identical except for the doping type in the active region, one p-doped, one n-doped and one left un-doped for comparison. The effect that carrier localisation within individual dots had on this temperature dependence of the carrier distribution under injection was studied and related to key aspects associated with laser device performance. The nature of QD occupation in the three samples was determined through measurement of the carrier temperature (TC) of the electrons populating the QD states. It was found that the un-doped samples QDs were in thermal equilibrium with the bulk lattice down to 200 K. Below this temperature the sample’s QD states become decoupled from the lattice and at 60 K QD occupation was shown to be random. The p-doped sample was shown to be non-thermal between 300 K and 200 K where at 150 K the occupation of QDs became random. The TC was observed to decrease for this sample below 200 K and this was attributed to fewer dopants ionising as the temperature decreased. The n-doped sample was also shown to be non-thermal between 300 K and 200 K with the QD occupation becoming random at 100 K. In all three samples, above 300 K, the measured TC was lower than that predicted by a Fermi-Dirac distribution and this was attributed to the these QDs having a large number of closely spaced hole states leading to a size dependence of the number of these states. This means an individual ΔEf exists for a given set of dot sizes. So emission from an ensemble of dots is “smoothed” across different ΔEf levels leading to a reduction in the apparent TC. These results have a significant effect on the threshold current densities of these samples and suggest that the differences observed due to doping will not be reproduced by calculations assuming a quasi-thermal equilibrium across the QD structure. The temperature dependence of the shift in gain peak energy was determined for the un and p-doped samples. This showed that the blue-shift of the gain peak due to state-filling in un-doped QD structures is independent of temperature, at a given value of peak gain, over the temperature range studied (200 K to 350 K). In the pdoped sample however, the state filling is temperature dependent at any fixed gain with a shift of 8meV observed between 200 K and 350 K. This was attributed to the wide electron state distribution and the lowering of the electronic quasi-Fermi level by the p-doping. This renders p-doped materials unsuitable for any technology application where gain peak wavelength temperature stability is required for efficient operation.
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Lu, Chao-Yang. "Resonance fluorescence and quantum dot spin dynamics." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609382.
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Kuffner, Peter. "Quantum Dot Interdiffusion For Two Colour Quantum Dot Infrared Photodetectors." Thesis, 2006. http://hdl.handle.net/1885/44488.
Full textAbstract:
An investigation into the effects of thermal interdiffusion on the characteristics of quantum dot infrared photodetectors (QDIPs) yields results useful for the creation of a two-colour QDIP. For high temperature rapid thermal annealing, quantum dot interdiffusion is induced, resulting in a large wavelength redshift in the photodetector spectral response, at the cost of a small degradation in device performance. The evaluation of a two-colour QDIP fabricated using selective suppression of interdiffusion during thermal annealing shows uniform performance in the two different detector pixels. This has implications as a useful process for the future fabrication of multi-colour QDIPs.
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Cao, Chuanshun Deppe Dennis G. "Carrier dynamics in quantum dot and GaAs-based quantum dot cascade laser." 2004. http://wwwlib.umi.com/cr/utexas/fullcit?p3143662.
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