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Poppe, Jan. "Spectroelectrochemical Investigations of Semiconductor Nanoparticles." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-162122.
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The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale.The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields.
Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale.
To achieve reliable measurements a new strategy was developed so that dense and homogeneous monolayers of semiconductor particles could be deposited onto transparent electrodes. The films were obtained by exchanging the original bulky ligand shell of the nanocrystals with a reactive alkoxysilane species and subsequent immersion of the substrate into a solution of the modified nanocrystals. SEM and electrochemical investigations have shown a much higher coverage efficiency in comparison with other methods presently established in the literature, which are based on the approach of prefunctionalizing of the substrates prior to coating. Fractional coverages of 80 % were obtained within 24 h while avoiding the time consuming and complicated step of functionalizing the substrates before deposition.
Films of CdSe and CdS nanoparticles deposited on fluorine doped tin oxide (FTO) electrodes were characterized by means of potential modulated absorption spectro-scopy (EMAS). Employing this special spectroelectrochemical technique, bleach signatures in the absorption spectra of the quantum dots induced by electron injection into their respective conduction band states were investigated. The features observed in the spectra and the evaluation of the potential dependence of the signal intensity revealed that only the lowest conduction band state, namely the 1Se state, is populated. The occupancy follows a quasi Fermi-Dirac distribution whose distributional width, in addition to the temperature, also depends on the size distribution of the particle ensemble investigated. On that basis a model was developed to extract the electrochemical potentials of the respective populated lowest conduction band states.
For CdSe quantum dots the four energetically lowest excitonic transitions were found to become bleached as the 1Se state is populated, indicating that these transitions promote electrons from different states in the valence band to the same conduction band state. These findings are in excellent agreement with results obtained from ultra fast optical pump probe experiments, which are methods that usually demand much more experimental efforts than the technique presented in these studies. The determination of the potential of the 1Se state versus a known reference potential allows one to map the top valence band states with respect to an absolute energy scale. This provides the opportunity to compare the energy positions obtained for different samples. Determination of the electrochemical band edge potential clearly features a size dependent shift of the conduction band edge and the valence band edge for both CdSe and CdS quantum dots, which is in excellent agreement with the expected behavior due to the quantum confinement effect.
Investigations in different electrolytes have shown that the immediate environment has a major impact on the electrochemical potentials of the energy levels of the nanoparticles. This observation is particularly important from a technological point of view, as in many applications the semiconductor material is in direct contact with an electrolyte as for example in quantum dot sensitized solar cells, electrochemical sensors and catalysis. In contrast to other “purely physical” methods such as photoelectron spectroscopy or scanning tunneling spectroscopy, potential-modulated absorption spectroscopy provides the ability to probe the materials under their most likely “working” conditions where such environmental influences can be directly taken into account.
Further, it has been shown that potential modulated absorption spectroscopy can be applied to bulk semiconductor electrodes, as long as they are thin enough to allow adequate amounts of light to pass through. The features observed in the EMAS spectra of these samples clearly differ from those obtained for nanoparticle films, as in such materials a continuum of states is progressively filled rather than a single state. Besides band-filling the potential modulation additionally induces changes in the absorption, which can be attributed to the Franz-Keldysh effect resulting from the modulation of the electric field across the space charge layer. The resolution and sensitivity that one can obtain with this comparatively simple and cost-effective setup is quite remarkable. As has been demonstrated it was possible to achieve clearly resolved bleach spectra of submonolayers of quantum dots attached to FTO with optical densities below 0.001.
Recently it has been reported that cyclic voltammetry (CV) can be used to study the size dependent positions of the electronic levels of quantum dots. The intention of the last part of this thesis was to reproduce this work for the nanoparticles investigated within this thesis in order to compare the results with those obtained by EMAS.
However, the experiments undertaken here reveal that the anodic and cathodic peaks observed in the cyclic voltammograms cannot automatically be assigned to the absolute band edge positions of the particles as the size dependent peak positions and their potential differences do not show any evidence for a correlation with respect to the quantum size effect. Rather the voltammetric responses reflect the solid state electrochemical characteristics of CdSe. Theoretical considerations concerning the response expected in a CV due to band filling of semiconductor nanoparticles confined to an electrode surface revealed that the expected currents are quite similar to that of a pseudo-capacitance. However, pronounced signals are only obtained if appropriate amounts of deposited nanoparticles are present which are electronically addressable without hampering the charge transfer. Hence a clear assignment of the peaks obtained in a cyclic voltammogram to the electronic band edges without employing a complementary technique to confirm ones findings therefore seems to be at best questionable.
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Matas, Adams Alba Maria. "Semiconductor Nanoparticles as Platform for Bio-Applications and Energy Related Systems." Doctoral thesis, Universitat Rovira i Virgili, 2015. http://hdl.handle.net/10803/334391.
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Aquesta tesi està dedicada a la síntesi, caracterització i aplicacions de diferents nanomaterials que presenten la propietat de ser semiconductors.
Aquesta dividida en tres blocs, en els quals, en el primer d'ells es parla sobre quantum dots (QDs), que són nanoparticulas fluorescents la longitud d'ona d'emissió varia amb la mida. Aquests materials s'estan utilitzant últimament com a substituts dels colorants orgànics ja que presenten avantatges, la principal és que no perden la seva emissió amb el temps.
Aquests QDs han estat usats per estudiar la seva interacció amb l'or (que augmenta la seva intensitat de fluorescència), han estat encapsulats usant polímers per usar-los com a controls en citometria de flux i per silica per usar-los (un cop units a un peptido i un colorant orgànic adequat) com a detectors de fibrosi quística. Finalment també han estat usats en aquesta tesi per intentar seguir el moviment d'un receptor en plaquetes.
En el segon bloc de la tesi es parla de up conversió nanoparticles, la diferència enfront dels QDs és que s'exciten a major longitud d'ona a la que emeten, pel que són capaços d'absorbir en el infraroig i emetre en el visible, fent-ideals per a aplicacions en biologia. En aquesta tesi es van usar per a reconèixer un receptor en neutrofilos i per introduir-lo dins de hidrotalcites (material que no és reconegut pel cos com estrany) per així poder alliberar-ho en l'organisme.
Finalment, en el tercer bloc s'han sintetitzat materials per catalisis (sulfur de bismut) i per cel·les solars (òxid de titani)Esta tesis esta dedicada a la sintesis, caracterizacion y aplicaciones de diferentes nanomateriales que presentan la propiedad de ser semiconductores. Esta dividida en tres bloques, en los cuales, en el primer de ellos se habla sobre quantum dots (QDs), que son nanoparticulas fluorescentes cuya longitud de onda de emision varia con el tamaño. Dichos materiales se estan usando ultimamente como sustitutos de los colorantes organicos ya que presentan ventajas, la principal es que no pierden su emision con el tiempo. Estos QDs han sido usados para estudiar su interaccion con el oro (que aumenta su intensidad de fluorescencia), han sido encapsulados usando polimeros para usarlos como controles en citometria de flujo y por silica para usarlos (una vez unidos a un peptido y un colorante organico adecuado) como detectores de fibrosis quistica. Finalmente tambien han sido usados en esta tesis para intentar seguir el movimiento de un receptor en plaquetas. En el segundo bloque de la tesis se habla de up conversion nanoparticles, cuya diferencia frente a los QDs es que se excitan a mayor longitud de onda a la que emiten, por lo que son capaces de absorber en el infrarojo y emitir en el visible, haciendolos ideales para aplicaciones en biologia. En esta tesis se usaron para reconocer un receptor en neutrofilos y para introducirlo dentro de hidrotalcitas (material que no es reconocido por el cuerpo como extraño) para asi poder liberarlo en el organismo. Finalmente, en el tercer bloque se han sintetizado materiales para catalisis (sulfuro de bismuto) y para celdas solares (oxido de titanio).
This thesis is dedicated to the synthesis, characterization and application of different nanomaterials that are semiconductors. It is divided in three blocks, in the first one we talk about quantum dots (QDs), that are fluorescent nanoparticles whose wavelength of emission changes with size. Such materials are being used as substitutes of organic dyes, due to the many advantages they present, the main one is that the fluorescence is not lost with time. These QDs have been used to study their interaction with gold ( that increases the fluorescence intensity), they have been encapsulated with polimers to be used as controls in flow cytometry or by silica to use them as sensors for cystic fibrosis (once they have been attatched to the right polymer and dye). Finally, in this thesis, they have been also used to track the movement of a platelet receptor. In the second block we talk about up conversion nanoparticles, which only difference regarding QDs is that they are excited using a longer wavelength than the emission, so they are able to absorb in the infrared and emit in the visible range of light, making them ideal for biological applications. We have use this materials to recognice an specific receptor in neutrophils as well as to be surrounded by hydrotalcite (body friendly material) so it can be released in the organism. Finally, in the third block we have syntesized materials for catalysis (bismuth sulfide) and for solar cells (titanium oxide for perovskite solar cells).
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Dooley, Chad Johnathan. "New Nanomaterials for Photovoltaic Applications: A Study on the Chemistry and Photophysics of II-VI Semiconductor Nanostructures." Thesis, Boston College, 2009. http://hdl.handle.net/2345/705.
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Thesis advisor: Torsten FiebigThis dissertation examines the chemistry and photophysics of semiconductor quantum dots with the intent of studying their capabilities and limitations as they pertain to photovoltaic technologies. Specifically, experiments are presented detailing the first time-resolved measurements of electron transfer in electronically coupled quantum rods. Electron transfer from the conduction band of CdTe was measured to occur on the 400 fs timescale (kET = 2.5 x 1012 s-1), more than 500x faster than previously believed. Additionally, the direct optical promotion of an electron from the valence band of CdTe was observed, occurring on the timescale of the pump pulse (~50 fs). Based on the determined injection rates, a carrier separation efficiency of > 90% has been calculated suggesting these materials are sufficient for use in solar energy capture applications where efficient carrier separation is critical. To this end, model photovoltaic cells were fabricated, and their power conversion efficiency and photon-to-current generation efficiency characterized. In devices based of CdSe and heteromaterial quantum rods we observed fill-factors on the order of 10-20% though with power conversion efficiencies of < 0.02%. It was discovered that using a high temperature annealing step, while critical to get electrochemically stable photoelectrodes, was detrimental to quantum confinement effects and likely removed any hQR specific capabilities. Additionally, a detailed study on the role of nucleotide triphosphate chemistry in stabilizing emissive CdS nanoparticles is presented. Specifically it was observed that in a neutral pH environment, GTP selectively stabilizes CdS quantum dots with diameters of ~4 nm while the other naturally occurring ribonucleotides do not yield emissive product. The selectivity is dependent on the presence of the nucleophilic N-7 electrons near a triphosphate pocket for Cd2+ complexation as well as an exocyclic amine to stabilize the resulting product particles. However, in an elevated pH environment, the nucleobase specificity is relaxed and all NTPs yield photo-emissive quantum dots with PLQEs as high as 10%
Thesis (PhD) — Boston College, 2009
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Chemistry
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Hellström, Staffan. "Exciton-plasmon interactions in metal-semiconductor nanostructures." Doctoral thesis, KTH, Teoretisk kemi och biologi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-93306.
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Semiconductor quantum dots and metal nanoparticles feature very strong light-matter interactions, which has led to their use in many photonic applications such as photodetectors, biosensors, components for telecommunications etc.Under illumination both structures exhibit collective electron-photon resonances, described in the frameworks of quasiparticles as exciton-polaritons for semiconductors and surface plasmon-polaritons for metals.To date these two approaches to controlling light interactions have usually been treated separately, with just a few simple attempts to consider exciton-plasmon interactions in a system consisting of both semiconductor and metal nanostructures.In this work, the exciton-polaritons and surface \\plasmon-polaritons are first considered separately, and then combined using the Finite Difference Time Domain numerical method coupled with a master equation for the exciton-polariton population dynamics.To better understand the properties of excitons and plasmons, each quasiparticle is used to investigate two open questions - the source of the Stokes shift between the absorption and luminescence peaks in quantum dots, and the source of the photocurrent increase in quantum dot infrared photodetectors coated by a thin metal film with holes. The combined numerical method is then used to study a system consisting of multiple metal nanoparticles close to a quantum dot, a system which has been predicted to exhibit quantum dot-induced transparency, but is demonstrated to just have a weak dip in the absorption.QC 20120417
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Jiang, Feng. "Ligand Controlled Growth of Aqueous II-VI Semiconductor Nanoparticles and Their Self-Assembly." Diss., The University of Arizona, 2013. http://hdl.handle.net/10150/311311.
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Colloidal semiconductor nanoparticles (NPs) contain hundreds to thousands of atoms in a roughly spherical shape with diameters in the range of 1-10 nm. The extremely small particle size confines electron transitions and creates size tunable bandgaps, giving rise to the name quantum dots (QDs). The unique optoelectronic properties of QDs enable a broad range of applications in optical and biological sensors, solar cells, and light emitting diodes. The most common compound semiconductor combination is chalcogenide II-VI materials, such as ZnSe, CdSe, and CdTe. But III-V and group IV as well as more complicated ternary materials have been demonstrated. Coordinating organic ligands are used to cap the NP surface during the synthesis, as a mean of protecting, confining, and separating individual particles. This study investigated the impact of the ligand on particle growth and self-assembly into hierarchical structures. ZnSe QDs were synthesized using an aqueous route with four different thiol ligands, including 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), methyl thioglycolate (MTG), and thiolactic acid (TLA). The particle growth was monitored as a function of reaction time by converting the band gaps measured using UV-vis spectroscopy into particle sizes. A kinetic model based on a diffusion-reaction mechanism was developed to simulate the growth process. The growth data were fit to this model, yielding the binding strength in the order TLA < MTG ≈ TGA < MPA. This result showed the relationship between the QD growth rates and the chemical structures of the ligands. Ligands containing electron-withdrawing groups closer to the anchoring S atom and branching promoted growth, whereas longer, possibly bidendate, ligands retarded it. Removing TGA ligands from the surface of CdTe QDs in a controlled manner yielded new superstructures that were composed of either intact or fused particles. Purifying as-synthesized QDs by precipitating them using an anti-solvent removed most of the free ligand in solution. Aging this purified QD suspension for a week caused self-assembly of QDs into nanoribbons. The long time needed for self-assembly was due to the slow equilibrium between the ligands on QD surface and in solution. Accelerating the approach to equilibrium by diluting purifed CdTe QDs with organic solvents triggered rapid self-assembly of superstructures within a day, forming various nanostructures from nanoribbons to nanoflowers. The type of nanostructures that formed was determined by the solvation of TGA in the trigger solvent. Extracting the smallest portion of TGA in methanol promoted vectorial growth into ribbons consistent with dipole-dipole attractive and charge-charge repulsive interactions. Removing more of the TGA layer in IPA caused the dots to fuse into webs containing clustered ribbons and branches, and the directional nature of the superstructure was lost. Completely deprotecting the surface in acetone promoted photochemical etching and dissolved the QDs, yielding
ower-like structures composed of CdS. Nanocrystal (NC) growth mediated by a ligand was also studied in the organic synthesis of FeS₂ nanocubes. Oleylamine was used not only as the ligand but also the solvent and reductant during the reaction. A one hour reaction between iron (II) chloride and elemental sulfur in oleylamine at 200 ℃ and a S to Fe ratio of 6 yielded phase pure pyrite cubes with dimensions of 87.9±14.1 nm. X-ray diffraction (XRD) spectra and Raman peaks for pyrite at 340, 375, and 426 cm⁻¹ confirmed phase purity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that the oleylamine remained on the FeS₂ surface as a ligand. The reaction mechanism includes the production of pyrrhotite Fe₁₋ᵪS (0≤x<0.5) via reduction of S⁰ to S²⁻ by oleylamine and the oxidation of pyrrhotite to pyrite with remaining S⁰.
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Razgoniaeva, Natalia Razgoniaeva. "Photochemical energy conversion in metal-semiconductor hybrid nanocrystals." Bowling Green State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1465822519.
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Fairclough, Simon Michael. "Carrier dynamics within semiconductor nanocrystals." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:857f624d-d93d-498d-910b-73cce12c4e0b.
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This thesis explores how the carrier dynamics within semiconductor nanocrystals can be directly engineered through specific core-shell design. Emphasis is placed on how material characteristics, such as strain or alloying at a core-shell interface, can influence the exciton energies and the recombination dynamics within semiconductor nanocrystals. This study synthesises type-II heterojunction ZnTe/ZnSe core-shell nanocrystals via a diethyl zinc-free synthesis method, producing small size distributions and quantum yields as high as 12%. It was found that the 7% lattice mismatch between the core and shell materials places limitations on the range of structures in which coherent growth is achieved. By developing compositional and strained atomistic core-shell models a variety of physical and optical properties could be simulated and has led to a clear picture of the core-shell architecture to be built. This characterisation provides evidence that the low bulk modulus ZnTe cores are compressed by the higher bulk modulus smaller lattice constant ZnSe shells. Further studies show how strain is manifested in structures with 'sharp' core-shell interfaces and how intentional alloying the interface can influence the growth and exciton energies. A (2-6)-band effective mass model was able to distinguish between the as-grown 'sharp' and 'alloyed' interfaces which indicated that strain accentuates the redshift of the excitonic state whilst reduced strain within an alloyed interface sees a reduced redshift. Single nanocrystal spectroscopy investigations of brightly emitting single graded alloyed nanocrystals and of a size series of commercially available CdSe/ZnS nanocrystals showed almost no fluorescence intermittency (nearly 'non-blinking'). These investigations also identified trion recombination as the main mechanism within the blinking 'off' state. Ultimately this thesis adds to the growing understanding of how specific core-shell architectures manipulate the electronic structure and develops techniques to identify specific material characteristics and how these characteristics influence the physical and optical properties within semiconductor nanocrystals.
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Schill, Alexander Wilhem. "Interesting Electronic and Dynamic Properties of Quantum Dot Quantum Wells and other Semiconductor Nanocrystal Heterostructures." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/11514.
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Some interesting electronic and dynamic properties of semiconductor nanocrystal heterostructures have been investigated using various spectroscopic methods. Semiconductor nanocrystal heterostructures were prepared using colloidal synthesis techniques. Ultrafast transient absorption spectroscopy was used to monitor the relaxation of hot electrons in CdS/HgS/CdS quantum dot quantum wells. Careful analysis of the hot electron relaxation in CdS/HgS/CdS quantum dot quantum wells reveals an energy dependent relaxation mechanism involving electronic states of varying CdS and HgS composition. The composition of the electronic states, combined with the layered structure of the nanocrystal permits the assignment of CdS localized and HgS localized excited states. The dynamic effect of surface passivation is then shown to have the strongest influence on excited states that are localized in the HgS layer.
New quantum dot quantum well heterostructures of different sizes and compositions were also prepared and studied. The dynamic properties of CdS/CdSe/CdS colloidal quantum wells suggest simultaneous relaxation of excited electrons within the CdS core and CdSe shell on the sub-picosecond time scale. Despite the very different electronic structure of CdS/CdSe/CdS compared to CdS/HgS/CdS, the time scales of the relaxation and electron localization were very similar.
Enhancement of trap luminescence was observed when CdS quantum dots were coated with silver. The mechanism of the enhancement was investigated using time-resolved spectroscopic techniques.
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Kairdolf, Brad A. "Development of polymer-coated nanoparticle imaging agents for diagnostic applications." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/31845.
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Thesis (Ph.D)--Biomedical Engineering, Georgia Institute of Technology, 2010.Committee Chair: Nie, Shuming; Committee Member: Bao, Gang; Committee Member: Murthy, Niren; Committee Member: Varma, Vijay; Committee Member: Wang, Zhong Lin. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Zedan, Abdallah. "GRAPHENE-BASED SEMICONDUCTOR AND METALLIC NANOSTRUCTURED MATERIALS." VCU Scholars Compass, 2013. http://scholarscompass.vcu.edu/etd/457.
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Exciting periods of scientific research are often associated with discoveries of novel materials. Such period was brought about by the successful preparation of graphene which is a 2D allotrope of carbon with remarkable electronic, optical and mechanical properties. Functional graphene-based nanocomposites have great promise for applications in various fields such as energy conversion, opteoelectronics, solar cells, sensing, catalysis and biomedicine. Herein, microwave and laser-assisted synthetic approaches were developed for decorating graphene with various semiconductor, metallic or magnetic nanostructures of controlled size and shape. We developed a scalable microwave irradiation method for the synthesis of graphene decorated with CdSe nanocrystals of controlled size, shape and crystalline structure. The efficient quenching of photoluminescence from the CdSe nanocrystals by graphene has been explored. The results provide a new approach for exploring the size-tunable optical properties of CdSe nanocrystals supported on graphene which could have important implications for energy conversion applications. We also extended this approach to the synthesis of Au-ceria-graphene nanocomposites. The synthesis is facilely conducted at mild conditions using ethylenediamine as a solvent. Results reveal significant CO conversion percentages between 60-70% at ambient temperatures. Au nanostructures have received significant attention because of the feasibility to tune their optical properties by changing size or shape. The coupling of the photothermal effects of these Au nanostructures of controlled size and shape with GO nanosheets dispersed in water is demonstrated. Our results indicate that the enhanced photothermal energy conversion of the Au-GO suspensions could to lead to a remarkable increase in the heating efficiency of the laser-induced melting and size reduction of Au nanostructures. The Au-graphene nanocomposites are potential materials for photothermolysis, thermochemical and thermomechanical applications. We developed a facile method for decorating graphene with magnetite nanocrystals of various shapes (namely, spheres, cubes and prisms) by the microwave-assisted-reduction of iron acetylacetonate in benzyl ether. The shape control was achieved by tuning the mole ratio between the oleic acid and the oleyamine. The structural, morphological and physical properties of graphene-based nanocomposites described herein were studied using standard characterization tools such as TEM, SEM, UV-Vis and PL spectroscopy, powder X-ray diffraction, XPS and Raman spectroscopy.
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Dickerson, Bryan Douglas. "Organometallic Synthesis Kinetics of CdSe Quantum Dots." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/27322.
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CdSe quantum dots produced by organometallic synthesis are useful as tunable emitters for photonic devices and as multi-colored protein markers for biomedical imaging, applications requiring bright and narrow emission. A diffusion-limited model helped monitor growth rates via photoluminescence and absorbance spectroscopy, in order to characterize synthesis kinetics in stearic acid, dodecylamine, and in trioctylphosphine oxide. The nucleation rate increased with Se concentration, while the growth rate followed the Cd concentration.
Emission peak widths, emission redshift rates, nanocrystal growth rates, and reactant concentrations all decreased to a minimum when emission reached the critical wavelength, at a reaction completion time, tc. The temperature dependence of 1/tc and of redshift rates followed Arrhenius behavior governed by activation energies, which were tailored by the choice of solvent. Synthesis in solvents, such as stearic acid, with lower activation energies produced faster initial nanocrystal growth and longer critical wavelengths. The highest photoluminescence quantum yield was generally at wavelengths shorter than the critical wavelength, when moderate growth rates enabled surface reconstruction while precursors were still available.Ph. D.
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Lambright, Scott. "Ultrafast Charge Carrier Dynamics in Au/Semiconductor Nanoheterostructures." Bowling Green State University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1404741549.
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Kharin, Alexander. "Group IV nanoparticles for cell imaging and therapy." Thesis, Lyon, 2016. http://www.theses.fr/2016LYSE1032/document.
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La biomédecine et la biophotonique sont des champs de recherches en plein expansion qui grandissent à vive allure, constituant un secteur entier d'activités novatrices. Ce secteur, vraiment interdisciplinaire, comprend le développement de nouveaux nanomatériaux, de sources lumineuses et l'élaboration de nouveaux concepts, de dispositifs/équipements pour quantifier la conversion de photons et leurs interactions. L'importance décisive du diagnostic précoce et du traitement individuel des patients exige des thérapies soigneusement ciblées et la capacité de provoquer sélectivement la mort cellulaire des cellules malades. Malgré les progrès spectaculaires réalisés en utilisant les points quantiques ou des molécules biologiques organiques pour l'imagerie biologique et la libération ciblée de médicaments, plusieurs problèmes restent à résoudre : obtenir une sélectivité accrue pour une accumulation spécifique dans les tumeurs et une amélioration de l'efficacité des traitements. D'autres problèmes incluent la cytotoxicité et la génotoxicité, l'élimination lente et la stabilité chimique imparfaite. Des espérances nouvelles sont portées par de nouvelles classes de matériaux inorganiques comme les nanoparticules à base de silicium ou à base de carbone, qui pourraient faire preuves de caractéristiques de stabilité plus prometteuses tant pour le diagnostic médical que pour la thérapie. Pour cette raison, la découverte de nouveaux agents de marquage et de transport de médicaments représente un champ important de la recherche avec un potentiel de croissance renforcéBiomedicine and biophotonics related businesses are currently growing at a breathtaking pace, thereby comprising one of the fastest growing sectors of innovative economy. This sector is truly interdisciplinary, including, very prominently, the development of novel nanomaterials, light sources, or novel device/equipment concepts to carry out photon conversion or interaction. The great importance of disease diagnosis at a very early stage and of the individual treatment of patients requires a carefully targeted therapy and the ability to induce cell death selectively in diseased cells. Despite the tremendous progress achieved by using quantum dots or organic molecules for bio-imaging and drug delivery, some problems still remain to be solved: increased selectivity for tumor accumulation, and enhancement of treatment efficiency. Other potential problems include cyto- and genotoxicity, slow clearance and low chemical stability. Significant expectations are now related to novel classes of inorganic materials, such as silicon-based or carbon-based nanoparticles, which could exhibit more stable and promising characteristics for both medical diagnostics and therapy. For this reason, new labeling and drug delivery agents for medical application is an important field of research with strongly-growing potential.The 5 types of group IV nanoparticles had been synthesized by various methods. First one is the porous silicon, produced by the electrochemical etching of bulk silicon wafer. That well-known technique gives the material with remarkably bright photoluminescence and the complicated porous structure. The porous silicon particles are the agglomerates of the small silicon crystallites with 3nm size. Second type is 20 nm crystalline silicon particles, produced by the laser ablation of the bulk silicon in water. Those particles have lack of PL under UV excitation, but they can luminesce under 2photon excitation conditions. 3rd type of the particles is the 8 nm nanodiamonds
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Dubavik, Aliaksei. "Hydrophob/hydrophil schaltbare Nanoteilchen für die Biomarkierung." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-82718.
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There is a demand for new straightforward approaches for stabilization and solubilization of various nanoparticulate materials in their colloidal form, that pave way for fabrication of materials possessing compatibility with wide range of dispersing media. Therefore in this thesis a new general method to form stable nanocrystals in water and organics using amphiphilic polymers generated through simple and low cost techniques is presented and discussed. Amphiphilic coating agents are formed using thiolated or carboxylated polyethylene glycol methyl ether (mPEG-SH) as a starting material. These materials are available with a wide variety of chain lengths.
The method of obtaining of amphiphilic NPs is quite general and applicable for semiconductor CdTe nanocrystals as well as nanoscale noble metal (Au) and magnetic (Fe3O4) particles. This approach is based on anchoring PEG segment to the surface of a nanoparticle to form an amphiphilic palisade. Anchoring is realized via interaction of –SH (for CdTe and Au) or –COOH (in the case of magnetite) functional groups with particle’s surface. The resulting amphiphilicity of the nanocrystals is an inherent property of their surface and it is preserved also after careful washing out of solution of any excess of the ligand. The nanocrystals reversibly transfer between different phases spontaneously, i.e. without any adjustment of ionic strength, pH or composition of the phases. Such reversible and spontaneous phase transfer of nanocrystals between solvents of different chemical nature has a great potential for many applications as it constitutes a large degree of control of nanocrystals compatibility with technological processes or with bio-environments such as water, various buffers and cell media as well as their assembly and self-assembly capabilities.
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Gilstrap, Richard Allen Jr. "A colloidal nanoparticle form of indium tin oxide: system development and characterization." Diss., Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/33995.
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A logical progression from the maturing field of colloidal semiconductor quantum dots to the emerging subclass of impurity-doped colloidal semiconductor nanoparticles is underway. To this end, the present work describes the formation and analysis of a new form of Tin-doped Indium Oxide (ITO). The form is that of a colloidal dispersion comprised of pure-phase, 4-6 nanometer ITO particles possessing an essentially single crystalline character. This system forms a non-agglomerated, optically clear solution in a variety of non-polar solvents and can remain in this state, at room temperature, for months and potentially, years. ITO is the most widely used member of the exotic materials family known as Transparent Conductive Oxides (TCOs) and is the primary enabling material behind a wide variety of opto-electronic device technologies.
Material synthesis was achieved by initiating a series of interrelated nucleophilic substitution reactions that provided sufficient intensity to promote doping efficiencies greater than 90% for a wide range of tin concentrations. The optical clarity of this colloidal system allowed the intrinsic properties of single crystalline ITO particles to be evaluated prior to their use in thin-films or composite structures. Monitoring the temporal progression of n-type degeneracy by its effects on the optical properties of colloidal dispersions shed light on the fundamental issues of particle formation, band filling (Burstein-Moss) dynamics, and the very origin of n-type degeneracy in ITO. Central to these studies was the issue of excess electron character. The two limiting cases of entirely free and entirely confined electron motion were evaluated by application of bulk-like band dispersion analysis and the effective mass approximation, respectively. This provided a means to estimate the number of excess conduction band electrons present within an individual particle boundary. The ability to control and optimize the level of n-type degeneracy within the colloidal ITO nanoparticle form by compositional variation was also demonstrated. A key to the widespread adoption of a new material by industry is an ability to produce multi-gram and perhaps, kilogram quantities with no significant sacrifice in quality. Accordingly, a modified synthesis process was developed to allow for the mass production of high-quality colloidal ITO nanocrystals.
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Otto, Tobias. "Herstellung und Charakterisierung von Nanokristall-Lichtemitterdioden." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-78977.
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Die vorliegende Arbeit beschäftigt sich mit dem Aufbau von Nanokristall-LEDs. Dazu werden der Synthese, der Abscheidung und dem Aufbau und der Charakterisierung von Nanopartikeln und LEDs Platz eingeräumt. CdTe-Nanopartikel werden über eine wässrige Synthese, die auf elektrochemisch erzeugten Tellurwasserstoff beruhte, hergestellt. Der Vorteil besteht im geringen Aufwand und der guten Reproduzierbarkeit. Es konnte festgestellt werden, dass sich ein hoher Überschuss an Cadmiumionen in der Lösung positiv auf die Wachstumsgeschwindigkeit der Nanopartikel auswirkt.
Statt des anfänglich benutzten Layer-by-Layer-Tauchverfahrens wurde ein Layer-by-Layer-Sprühverfahren entwickelt, das die schnelle Herstellung homogener Filme, bestehend aus alternierenden Schichten einer Matrix und der Nanopartikel, ermöglichte. Nachteilig ist der hohe Verbrauch an Nanopartikeln gegenüber dem Tauchverfahren. Dem Tauchverfahren lastet der Umstand an, die kolloidalen Lösungen durch häufiges Eintauchen des Substrats zu verunreinigen. Dies wird beim Sprühverfahren vermieden, da alles nicht adsorbierte Material nach unten abfließt.
Es wurde gezeigt, dass sich Polyelektrolyte durch anorganische Gele ersetzen lassen, die über einen Sol-Gel-Prozess darstellbar sind, wobei es möglich wurde „All inorganic“-LEDs aufzubauen mit dem Vorteil der hohen Temperaturstabilität. Wobei sich die Reinheit der dargestellten Aluminiumoxid-Sole stark auf das Bestreben Aluminiumoxid-Kristalle zu bilden, auswirkt, die die Funktionsfähigkeit der LED behindern können. Die Verwendung einer isolierenden Matrix wie Poly-(diallyldimethylammoniumchlorid) oder Aluminiumoxid als Zwischenschicht zum Aufbau von mehrlagigen Nanopartikelschichten stellte sich als unproblematisch heraus, da sich Ladungsträger über einen „hopping“-Mechanismus zwischen den Halbleiternanopartikeln bewegen können. Größere Probleme bereitete die Verwendung von Nanopartikeln größerer Bandlücken wie ZnSe (2,7eV) als Elektrolumineszenz-Emitter. Es konnte nur eine weissbläuliche Emission beobachtet werden. Mit Nanopartikeln kleiner Bandlücke wie CdTe (1,6eV) wurde eine schmalbandige rote Emission festgestellt. Vorteilhaft erwies sich die Verwendung von Kern-Schale-Teilchen wie CdSe/CdS. Mit einer Matrix aus Aluminiumoxid-Gel konnte eine LED mit sehr niedriger Onset-Spannung (2,3V) hergestellt werden, die eine Lebensdauer von 33,5h besaß und noch bei einer Temperatur von 150°C emittierte.
Ein weiterer Weg Nanopartikel zu stabilisieren, stellt der Einbau in makrokristalline Einkristalle durch Mischkristallbildung in Wasser oder organischen Lösungsmitteln dar. Die erhaltenen Kristalle zeichnen sich durch hohe photochemische und thermische Stabilität aus. Sie zeigen die Emissionseigenschaften der Nanopartikel, die nach Auflösung der Matrix wieder kolloidal in Lösung gehen. Allerdings liegen die Nanopartikel in der Kristallmatrix nicht regulär verteilt vor. Die Mischkristalle wurden erfolgreich als Luminophor in einer Gasentladungslampe und als Konversionsschicht einer kommerziellen LED getestet, die die Emission der Nanopartikel aufwies.
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Grimes, Logan. "DEVELOPENT OF A PHOSPHOLIPID ENCAPSULATION PROCESS FOR QUANTUM DOTS TO BE USED IN BIOLOGIC APPLICATIONS." DigitalCommons@CalPoly, 2014. https://digitalcommons.calpoly.edu/theses/1237.
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The American Cancer Society predicts that 1,665,540 people will be diagnosed with cancer, and 585,720 people will die from cancer in 2014. One of the most common types of cancer in the United States is skin cancer. Melanoma alone is predicted to account for 10,000 of the cancer related deaths in 2014. As a highly mobile and aggressive form of cancer, melanoma is difficult to fight once it has metastasized through the body. Early detection in such varieties of cancer is critical in improving survival rates in afflicted patients. Present methods of detection rely on visual examination of suspicious regions of tissue via various forms of biopsies. Accurate assessment of cancerous cells via this method are subjective, and often unreliable in the early stages of cancer formation when only few cancer cells are forming. With fewer cancer cells, it is less likely that a cancer cell will appear in a biopsied tissue. This leads to a lower detection rate, even when cancer is present. This lack of detection when cancer is in fact present is referred to as a false negative. False negatives can have a highly detrimental effect on treating the cancer as soon as possible. More accurate methods of detecting cancer in early stages, in a nonsubjective form would alleviate these problems. A proposed alternative to visual examination of biopsied legions is to utilize fluorescent nanocrystalline biomarker constructs to directly attach to the abnormal markers found on cancerous tissues.
Quantum dots (QDs) are hydrophobic nanoscale crystals composed of semiconducting materials which fluoresce when exposed to specific wavelengths of radiation, most commonly in the form of an ultraviolet light source. The QD constructs generated were composed of cadmium-selenium (CdSe) cores encapsulated with zinc-sulfide (ZnS) shells. These QDs were then encapsulated with phospholipids in an effort to create a hydrophilic particle which could interact with polar fluids as found within the human body. The goal of this thesis is to develop a method for the solubilization, encapsulation, and initial functionalization of CdSe/ZnS QDs. The first stage of this thesis focused on the generation of CdSe/ZnS QDs and the fluorescence differences between unshelled and shelled QDs. The second stage focused on utilizing the shelled QDs to generate hydrophilic constructs by utilizing phospholipids to bind with the QDs. Analysis via spectroscopy was performed in an effort to characterize the difference in QDs both prior to and after the encapsulation process. The method generated provides insight on fluorescence trends and the encapsulation of QDs in polar substances. Future research focusing on the repeatability of the process, introducing the QD constructs to a biological material, and eventual interaction with cancer cells are the next steps in generating a new technique to target and reveal skin cancer cells in the earliest possible stages without using a biopsy.
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Hugall, James T. "On the nature of SERS from plasmonic nanostructures." Thesis, University of Cambridge, 2013. https://www.repository.cam.ac.uk/handle/1810/267496.
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The nature of surface-enhanced Raman scattering (SERS) on nanostructured surfaces is explored using both inorganic and organic-based systems and a variety of environmental perturbation mechanisms. Experimental optical characterisation systems are developed and existing systems extended to facilitate this exploration. SERS of inorganic semiconducting quantum dots (QDs) is observed for the first time, paving the way for their use as spatially well-defined SERS markers. Tuning of the Raman excitation wavelength allows comparison between resonance and nonresonance QD SERS and identifies enhancement due to the plasmonic nanostructure. A gentle mechano-chemical process (carbon dioxide snow jet) is used to rearrange adsorbed organic thiol monolayers on a gold plasmonic nanostructure. The necessity of nanoscale roughness to the large SERS enhancement on pit-like plasmonic nanostructures is shown and demonstrates a new method to boost SERS signals (> 500 %) on plasmonic nanostructures. A multiplexed time-varied exposure technique is developed to track this molecular movement over time and highlights the different origins of the SERS peak and its accompanying background continuum. Using low-temperature cryogenics (down to 10 K) the SERS peak and background continuum intensity are shown to increase as the plasmonic metal damping reduces with temperature. Temperature dependent measurements of QD (resonance) SERS are shown to have strong wavelength dependence due to the excitonic transitions in QDs. Changes to the QD fluorescence at low temperature allows striking comparison between the Raman and fluorescence processes. The role of charge transfer and electromagnetic enhancement in the SERS intensity of p-aminothiophenol (pATP) is investigated on nanostructured plasmonic surfaces coupled to metallic nanoparticles. The results support the importance of charge transfer effects to the SERS of pATP, and highlight the difference between those of electromagnetic origin. Addition of nanoparticles to the nanostructured surface was seen to enhance SERS signals by up to 100×.
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Korkusinski, Marek. "Correlations in semiconductor quantum dots." Thesis, University of Ottawa (Canada), 2004. http://hdl.handle.net/10393/29128.
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In this Thesis, I present a theoretical study of correlation effects in strongly interacting electronic and electron-hole systems confined in semiconductor quantum dots. I focus on three systems: N electrons in a two-dimensional parabolic confinement in the absence and in the presence of a magnetic field, an electron-hole pair confined in a vertically coupled double-quantum-dot molecule, and a charged exciton in a quantum-ring confinement in a magnetic field.
To analyse these systems I use the exact diagonalisation technique in the effective-mass approximation. This approach consists of three steps: construction of a basis set of particle configurations, writing the Hamiltonian in this basis in a matrix form, and numerical diagonalisation of this matrix. Each of these steps is described in detail in the text.
Using the exact diagonalisation technique I identify the properties of the systems due to correlations and formulate predictions of how these properties could be observed experimentally. I confront these predictions with results of recent photoluminescence and transport measurements.
First I treat the system of N electrons in a parabolic confinement in the absence of magnetic field and demonstrate how its properties, such as magnetic moments, can be engineered as a function of the system parameters and the size of the Hilbert space.
Next I analyse the evolution of the ground state of this system as a function of the magnetic field. In the phase diagram of the system I identify the spin-singlet nu = 2 phase and discuss how correlations influence its phase boundaries both as a function of the magnetic field and the number of electrons.
I also demonstrate that in higher magnetic fields electronic correlations lead to the appearance of spin-depolarised phases, whose stability regions separate the weakly correlated phases with higher spin. Further on, I consider electron-hole systems. I show that the Coulomb interaction leads to entanglement of the states of an electron and a hole confined in a pair of vertically coupled quantum dots.
Finally I consider the system of two electrons and one hole (a negatively charged exciton) confined in a quantum ring and in the presence of the magnetic field. I show that the energy of a single electron in the ring geometry exhibits the Aharonov-Bohm oscillations as a function of the magnetic field. In the case of the negatively charged exciton these oscillations are nearly absent due to correlations among particles, and as a result the photoluminescence spectra of the charged complex are dominated by the energy of the final-state electron. The Aharonov-Bohm oscillations of the energy of a single electron are thus observed directly in the optical spectra.
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Larsson, Mats. "Spectroscopy of semiconductor quantum dots /." Linköping : Linköpings universitet, 2005. http://www.bibl.liu.se/liupubl/disp/disp2005/tek976s.pdf.
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Chan, Ka Ho Adrian. "Quantum information processing with semiconductor quantum dots." Thesis, University of Cambridge, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648684.
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Siegert, Jörg. "Carrier dynamics in semiconductor quantum dots." Doctoral thesis, KTH, Mikroelektronik och tillämpad fysik, MAP, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4019.
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This thesis presents results of time-resolved photoluminescence experiments conducted on several different self-assembled InGaAs/GaAs and InAs/GaAs semiconductor quantum dot (QD) structures. Depending on the application in mind, different structural, electronic or optical properties have a different weight of importance. Fast carrier capture and relaxation is critical for QD based lasers, for example. In this thesis, the influence of surplus carriers, introduced through modulation-doping, is studied. It is shown that carrier capture is essentially unaffected whereas the intradot relaxation mechanisms, at least at low carrier concentrations, are fundamentally different. The phonon mediated cascade relaxation found in the undoped reference sample is replaced by efficient scattering with the built-in carriers in the case of the doped structures. Moreover, spin relaxation also depends on presence of extra carriers. During energy relaxation via carrier-carrier scattering, the spin polarization is preserved whereas in the undoped sample the strong interaction of relaxing carriers with LO phonons causes spin relaxation. The decay of the ground state spin polarization proceeds at the same rate for doped and undoped structures and is shown to be caused by acoustic phonons, even up to 300 K. While optimizing QD growth for specific applications, it is imperative to evaluate the influence of nonradiative recombination, which is most often detrimental. While misfit dislocations, deliberately introduced in the substrate, lead to the formation of laterally ordered, uniform dots, these samples are found to suffer from strong nonradiative recombination. Structures with different barrier thicknesses and numerical simulations indicate defects in the vicinity of the QDs as main origin of fast carrier trapping. On the other hand, it is shown that direct dot doping, compared to barrier doping or undoped structures, causes only minor degradation of the optical properties. Directly doped dots even exhibit a significantly weaker photoluminescence quenching with temperature, making them prospective for devices operating at room temperature. Finally, the superior proton radiation hardness of QD structures compared to quantum wells is demonstrated, which is due to the three-dimensional confinement. The increase of photoluminescence intensity at low to moderate doses is interpreted as an enhanced carrier transfer into the dots via the defects introduced into the material by the protons.QC 20100920
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Siegert, Jörg. "Carrier dynamics in semiconductor quantum dots /." Stockholm, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4019.
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Bylsma, Jason Michael. "Multidimensional Spectroscopy of Semiconductor Quantum Dots." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4001.
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The coherent properties of semiconductor nanostructures are inherently difficult to measure and one-dimensional spectroscopies are often unable to separate inhomogeneous and homogeneous linewidths. We have refined and improved a method of performing multidimensional Fourier transform spectroscopy based on four-wave
mixing (FWM) experiments in the box geometry. We have modified our system with broadband beamsplitters in all interferometer arms, high-resolution translation stages and the ability to work in reflection geometry. By improving the phase-stability of our setup and scanning pulse delays with sub-optical cycle precision, we are able to
reproduce 2DFT spectra of GaAs multiple quantum wells. With the FWM signal reflected from the sample surface instead of transmitted through, we show that very low pulse powers can be used to generate coherent 2D signals from colloidal PbS quantum dots. Dephasing times are particularly difficult to measure in small colloidal quantum dots due to environmental broadening effects from the colloidal growth. We show that low-temperature pure excitonic dephasing can be measured via time-integrated measurements as well as from the cross-diagonal linewidths of 2DFT spectra. Ultrafast
sub-picosecond dephasing times are measured at 5 K in 3 nm PbS quantum dots, while excitation-density-dependence is investigated in these dots. By retrieving the global phase with an all-optical method, we are able to retrieve the real-part 2D spectra of PbS quantum dots.
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Chaney, Darren. "Relaxation processes in semiconductor quantum dots." Thesis, University of Leicester, 2004. http://hdl.handle.net/2381/4521.
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In this thesis we investigate the relaxation mechanisms that occur in quantum dots (QDs). First we consider energy relaxation in single particle self-assembled QDs by means of an Auger process. For the first time, relaxation rates are compared for dots of a realistic truncated pyramid shape and for the more elementary dot models considered previously. We find that the fast (pico-second) relaxation necessary for quantum dot based optoelectronics applications is made possible by dot electrons scattering with electrons located in the surrounding bulk material. We show that this relaxation mechanism is dominant by two orders of magnitude over the two-dimensional wetting layer scattering mechanism that has been considered in previous calculations. Exact numerical diagonalisation is used to calculate the two-particle selfassembled QD wave functions. The small size of the QD means that the two electrons in the dot are found to be only weakly interacting. We find the relaxation rate for states of total spin 0 to be larger than the spin I rate by a factor of approximately 2. This is due to the double occupancy of the spin 0 spatial states. We also consider the much slower spin flip relaxation in electrostatic QDs. We include the spin-orbit mixing that results from the bulk inversion asymmetry of the crystal lattice in calculating the exact two-particle states. We find that the spin orbit mixing causes anti-crossings to appear in the energy spectrum and deduce a new conservation rule related to this. We find an oscillation capable of slowing the relaxation time from microseconds to tenths of a second. This oscillation results from the vertical finite well confinement of the QD. It is found to depend on both magnetic field and the QD thickness and is of particular interest for quantum information applications where long-lived excited states are desirable.
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Puebla, Nunez Jorge Luis. "Spin phenomena in semiconductor quantum dots." Thesis, University of Sheffield, 2013. http://etheses.whiterose.ac.uk/4026/.
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This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm.
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Kegel, Ingo. "X-ray diffraction from semiconductor quantum dots." Diss., [S.l.] : [s.n.], 2000. http://edoc.ub.uni-muenchen.de/archive/00000330.
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Plumhof, Johannes David. "Strain-tuning of single semiconductor quantum dots." Doctoral thesis, Universitätsbibliothek Chemnitz, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-83517.
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Polarization entangled photon pairs on demand are considered to be an important building block of quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), which exhibit a certain spatial symmetry, can be used as a triggered, on-chip source of polarization entangled photon pairs. Due to limitations of the growth, the as-grown QDs usually do not exhibit the required symmetry, making the availability of post-growth tuning techniques essential. In this work first the QD-morphology of hundreds of QDs is correlated with the optical emission of neutral excitons confined in GaAs/AlGaAs QDs. It is presented how elastic anisotropic stress can be used to partially restore the symmetry of self-assembled GaAs/AlGaAs and InGaAs/GaAs QDs, making them as candidate sources of entangled photon pairs. As a consequence of the tuning of the QD-anisotropy we observe a rotation of the polarization of the emitted light. The joint modification of polarization orientation and QD anisotropy can be described by an anticrossing of the so-called bright excitonic states. Furthermore, it is demonstrated that anisotropic stress can be used to tune the purity of the hole states of the QDs by modifying the degree of heavy and light hole mixing. This ability might be interesting for applications using the hole spin as a so-called quantum bit.
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Brocke, Thomas. "Electronic Raman spectroscopy on semiconductor quantum dots." Göttingen Cuvillier, 2007.
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Graham, Timothy Carl Maxwell. "Spectroscopy of II-VI semiconductor quantum dots." Thesis, Heriot-Watt University, 2006. http://hdl.handle.net/10399/103.
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Barker, James Alexander. "The electronic properties of semiconductor quantum dots." Thesis, University of Surrey, 2000. http://epubs.surrey.ac.uk/1021/.
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Barbour, Russell James. "A tunable microcavity for semiconductor quantum dots." Thesis, Heriot-Watt University, 2009. http://hdl.handle.net/10399/2298.
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Semiconductor optical microcavities that combine high Q-factors with a small mode volume play a vital role in modifying the interaction between light and matter. Several interesting phenomena arise when an emitter is introduced into such a cavity. These include enhancement or suppression of the spontaneous emission rate (weak coupling) and normal mode splitting (strong coupling). In order to fully exploit the high Q and low mode volume of current microcavities, it is crucial for the emitter to be resonant with the cavity mode. Furthermore the emitter must be located at the exact antinode of the cavity electric eld. Wavelength tuning in semiconductor monolithic microcavities is challenging and is traditionally achieved by altering the temperature or using the Stark shift to alter the emitter's wavelength. Spatially matching the emitter to the electric eld antinode in monolithic cavities is even more challenging. The work in this thesis addresses these challenges. The realisation of a miniaturised, fully tunable Fabry-P erot type microcavity for semiconductor quantum dot experiments is presented. The cavity has a high Q-factor and low mode volume. The cavity wavelength is tuned by altering the air gap between the mirrors to vary the cavity length. This allows a much broader tuning range than is possible using monolithic cavities. In this work the cavity modes are characterised using a xed wavelength laser technique and varying the cavity length. A high nesse is obtained by using a miniaturised concave mirror which laterally con nes the optical mode. Unprecedented in situ control over a single InAs/GaAs quantum dot within the cavity mode is demonstrated at 4 K. The Purcell e ect is demonstrated for a single quantum dot, spatially positioned at the exact antinode of the electric eld. The cavity beam waist at the dot layer is experimentally measured and shown to be in good agreement with the theoretical value. The cavity mode volume is calculated from the measured beam waist. The e ect of weakly coupling a single dot to the cavity transverse modes is also investigated. Photoluminesence data from a high dot density sample within the cavity is presented. An anti-crossing behaviour between an ensemble of dots and the cavity mode is demonstrated. Finally, a dot dependent Fano e ect in the absorption lineshapes in a chargetunable sample is presented. A subtle interaction with an electronic continuum is revealed. The Fano e ect is shown to become more pronounced with increasing excitation power.
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Cheng, Cheng. "Semiconductor colloidal quantum dots for photovoltaic applications." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:07baccd0-2098-4306-8a9a-49160ec6a15a.
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This thesis studies lead suphide (PbS) colloidal quantum dots and their photovoltaic applications. Different sizes of PbS QDs were synthesised and characterised using absorption spectroscopy and transmission electron microscopes. PbS QD Schottky junction devices were fabricated with AM1.5 power conversion efficiency up to 1.8 %. The Schottky junction geometry limits the device performance. A semiconductor heterojunction using ZnO as an electron acceptor was built and the device efficiency increased to 3%. By studying the light absorption and charge extraction profile of the bilayer device, the absorber layer has a charge extraction dead zone which is beyond the reach of the built-in electric field. Therefore, strategies to create a QD bulk heterojunction were considered to address this issue by distributing the junction interface throughout the absorber layer. However, the charge separation mechanism of the QD heterojunction is not clearly understood: whether it operates as an excitonic or a depleted p-n junction, as the junction operating mechanism determines the scale of phase separation in the bulk morphology. This study shows a transitional behaviour of the PbS/ZnO heterojunction from excitonic to depletion by increasing the doping density of ZnO. To utilise the excitonic mechanism, a PbS/ZnO nanocrystal bulk heterojunction was created by blending the two nanocrystals in solution such that a large interface between the two materials could facilitate fast exciton dissociation. However, the devices show poor performance due to a coarse morphology and formation of germinate pairs. To create a bulk heterojunction where a built-in electric field could assist the charge separation, a TiO2 porous structure with the pore size matching with the depletion width was fabricated and successfully in-filled by PbS QDs. The porous device produces 5.7% power conversion efficiency, among one of the highest in literature. The enhancement comes from increased light absorption and suppression of charge recombination.
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Malein, Ralph N. E. "Engineering coherent photons from semiconductor quantum dots." Thesis, Heriot-Watt University, 2015. http://hdl.handle.net/10399/3081.
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Self-assembled semiconductor quantum dots (QDs) have great promise as quantum light sources due to their ability to generate single indistinguishable photons and entanglement. In this thesis, confocal microscopy experiments have been carried using non-resonant photoluminescence (PL) and resonant uorescence (RF) on QDs with the goal of characterising and developing them into high-quality quantum light sources. Through the application of uniaxial strain and an electric eld, single particle energies in a QD and their behaviour with strain are determined using a perturbative Coulomb blockade model. The exciton energy tuning magnitude is found to be a result of the near-cancellation of much larger single electron and hole tuning tuning. In addition, the rate of electron con nement energy tuning with strain is found to be correlated with the nominal unstrained con nement energy. An attempt is made at characterising the composition of the QDs through extracting deformation potentials, but the simple model does not capture the full system. Further, strain tuning of the ne structure splitting (FSS) of the neutral exciton X0 from QDs emitting at telecommunications wavelengths is shown. FSS tuning as large as 46 eV was observed, and using a phenomenological model select QDs were identi ed to achieve FSS < 1 ueV. RF is used to examine noise sources in QDs. Two sources of noise are considered: electric charge noise due to a uctuating charge environment, and nuclear spin noise due to the hyper ne interaction of single electron spins with a large number ( ~105) of nuclear spins. While the charge noise contributes to a loss in overall photon emission rates, but does not negatively impact the photon antibunching or indistinguishability at low Rabi frequencies, spin noise allows inelastic Raman scattering which reduces photon indistinguishability. The application of an external magnetic eld in the Faraday geometry screens the electrons from the nuclear spins, recovering a high degree of photon indistinguishability.
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Konthasinghe, Kumarasiri. "Resonant Light Scattering from Semiconductor Quantum Dots." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6527.
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In this work, resonant laser spectroscopy has been utilized in two major projects --resonance fluorescence measurements in solid-state quantum-confined nanostructures and laser-induced fluorescence measurements in gases. The first project focuses on studying resonant light-matter interactions in semiconductor quantum dots "artificial atoms" with potential applications in quantum information science. Of primary interest is the understanding of fundamental processes and how they are affected by the solid-state matrix. Unlike atoms, quantum dots are susceptible to a variety of environmental influences such as phonon scattering and spectral diffusion. These interactions alter the desired properties of the scattered light and hinder uses in certain single photon source applications. One application of current interest is the use of quantum dots in “quantum repeaters” for which two-photon interference is key. Motivated by such an application we have explored the limits imposed by environmental effects on two quantum dots in the same sample, the scattered light from which is being interfered. We find that both one-photon and two-photon interference, although substantial, are affected in a variety of ways, in particular by spectral diffusion. These observations are discussed and compared with a theoretical model. We further investigated correlations in pulsed resonance fluorescence, and found significant unexpected spectral and temporal deviations from those studied under continuous wave excitation. Under these conditions, the scattered light exhibits Rabi oscillations and photon anti-bunching, while maintaining a rich spectrum containing many spectral features. These observations are discussed and compared with a theoretical model. In the second project, the focus is on the investigation of the possibility of detecting N2+ ions in air using laser induced fluorescence, with potential applications in detection of fissile materials at a distance. A photon-counting analysis reveals that the fluorescence decay rate rapidly increases with increasing N2 pressure and thus limits the detection at elevated pressures, in particular at atmospheric pressure. We show that time-gated detection can be used to isolate N2+ fluorescence from delayed N2 emission. Based on the spontaneous Raman signal from N2 simultaneously observed with N2+ fluorescence, we could estimate a limit of detection in air of order 108-1010 cm3.
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Schmall, Nicholas Edward. "Fabrication of Binary Quantum Solids From Colloidal Semiconductor Quantum Dots." Bowling Green State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1245257669.
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Hsieh, Chang-Yu. "Quantum Circuit Based on Electron Spins in Semiconductor Quantum Dots." Thesis, Université d'Ottawa / University of Ottawa, 2012. http://hdl.handle.net/10393/20738.
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In this thesis, I present a microscopic theory of quantum circuits based on interacting electron spins in quantum dot molecules. We use the Linear Combination of Harmonic Orbitals-Configuration Interaction (LCHO-CI) formalism for microscopic calculations. We then derive effective Hubbard, t-J, and Heisenberg models. These models are used to predict the electronic, spin and transport properties of a triple quantum dot molecule (TQDM) as a function of topology, gate configuration, bias and magnetic field.
With these theoretical tools and fully characterized TQDMs, we propose the following applications:
1. Voltage tunable qubit encoded in the chiral states of a half-filled TQDM. We show how to perform single qubit operations by pulsing voltages. We propose the "chirality-to-charge" conversion as the measurement scheme and demonstrate the robustness of the chirality-encoded qubit due to charge fluctuations. We derive an effective qubit-qubit Hamiltonian and demonstrate the two-qubit gate. This provides all the necessary operations for a quantum computer built with chirality-encoded qubits.
2. Berry's phase. We explore the prospect of geometric quantum computing with chirality-encoded qubit. We construct a Herzberg circuit in the voltage space and show the accumulation of Berry's phase.
3. Macroscopic quantum states on a semiconductor chip. We consider a linear chain of TQDMs, each with 4 electrons, obtained by nanostructuring a metallic gate in a field effect transistor. We theoretically show that the low energy spectrum of the chain maps onto that of a spin-1 chain. Hence, we show that macroscopic quantum states, protected by a Haldane gap from the continuum, emerge.
In order to minimize decoherence of electron spin qubits, we consider using electron spins in the p orbitals of the valence band (valence holes) as qubits. We develop a theory of valence hole qubit within the 4-band k.p model. We show that static magnetic fields can be used to perform single qubit operations. We also show that the qubit-qubit interactions are sensitive to the geometry of a quantum dot network. For vertical qubit arrays, we predict that there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.
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Liu, Tai-Min. "Electronic Interactions in Semiconductor Quantum Dots and Quantum Point Contacts." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1311773375.
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Pang, Hongliang, and 庞鸿亮. "Quantum control of spins in semiconductor nanostructures." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/208042.
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Spins localized in semiconductor nanostructures have been intensively investigated for quantum spintronics. These include the spin of single electron localized by quantum dots or impurities, and spins of the lattice nuclei. These localized spins can be exploited as carriers of quantum information, while in some circumstances they also play the role of deleterious noise sources for other quantum objects through their couplings. Quantum control of the spins in semiconductor nanostructures is therefore of central interest for quantum applications. In this thesis, we address several problems related to the quantum control of electron or hole spin and nuclear spins in semiconductor quantum dots and impurity centers. The first problem studied is the control of nuclear spin bath for a hole spin qubit in III-V semiconductor quantum dot. In quantum dots formed on III-V compounds, the direct band gap of the host material allows ultrafast optical addressability of a single electron or hole spin qubit. However, nonzero nuclear spins of group III and group V elements result in a large statistical fluctuation in the Zeeman splitting of the spin qubit which then dephases in nanosecond time scale. We present a novel feedback scheme to suppress the statistical fluctuation of the nuclear spin field for enhancing the coherence time of the hole spin qubit. We also find positive feedback control which can amplify the magnitude of the nuclear field, so that a bimodal distribution can develop, realizing a quantum environment that can not be described by a single temperature. The second problem addressed here is the control of donor spin qubits in silicon architecture which have ultra-long quantum coherence time. We developed the quantum control scheme to realize the quantum metrology of magnetic field gradient, based on the celebrated Kane’s architecture for quantum computation. The scheme can also be generalized to calibrate the locations of the donors. In the third part of the thesis, we investigate a novel type of quantum dot formed in a new class of two-dimensional semiconductors, monolayer transition metal dichalcogenides (TMDs), which exhibit interesting spin and pseudospin physics. This novel quantum dot system may offer new opportunity for quantum spintronics in the ultimate 2D limit, and we investigate here the valley pseudospin as a possible quantum bit carrier. A main finding is that, contrary to the intuition, the lateral confinement by the quantum dot potential does not lead to noticeable valley hybridization, and therefore the valley pseudospin in monolayer TMDs QD can well inherit the valley physics such as the valley optical selection rules from the 2D bulk which implies a variety of quantum control possibilities.published_or_final_version
Physics
Doctoral
Doctor of Philosophy
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Boyer, de la Giroday Antoine. "Solid-state photonic interfaces using semiconductor quantum dots." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/241720.
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New technologies based on the properties of quantum mechanics promiseto revolutionise the way information is processed by outperforming what ispossible using classical devices. Examples include massively parallel processingusing quantum computers, verifiably secure communication using quantumcryptography, and measurement with sensitivity beyond classical limitationwith quantum metrology. Realising the full potential of these technologiesnecessitates the ability to communicate quantum information over largedistances, a key requirement for future quantum networks. However, developingpractical implementations of long-distance quantum communicationis challenging as it necessitates three major ingredients: light-matter interfaces,elementary quantum operations, and quantum memories. This thesisdescribes work that has been undertaken to address these requirements usingsemiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded insideconventional diode structures constitute high-fidelity controllable interfacesbetween optical qubits and solid-state qubits. Indeed, the polarisationstate of a photon was transferred into the spin state of an electron-hole pairand eventually restored through radiative recombination of the electron andthe hole with a fidelity up to 95%. Moreover, spins were manipulated usingsubnanosecond modulation of a vertical electric field applied to the quantumdots. By controlling this electrical modulation, we demonstrated elementaryphase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiativelifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control thespatial separation of the electron and the hole and therefore prevent theirrecombination. Comprehensive studies of electric field eff ects upon the photoluminescenceof quantum dot molecules lead to a clear understanding anda good control over their physical properties. Single photons were stored inindividual quantum dot molecules up to 1?s and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons wastransferred into the spin state of electron-hole pairs with a fidelity above90%, which does not degrade for storage times up to the 12.5 ns repetitionperiod of the experiment. Our work on single quantum dots could be extended in the near future toallow for two-qubits quantum operations by con fining a second electron-holepair to be electrically manipulated. Storage of a superposition of spin statesin a quantum dot molecule should also be possible if the spin states are madedegenerate, which is feasible using the electric fi eld dependence of the energysplitting between the spin states discussed in this thesis. We believe thatcombining both approaches will lead to the development of a controllablemulti-qubit quantum memory for polarised light, a building block for long distancequantum communication based on semiconductor nanotechnology.
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Kroner, Martin. "Resonant photon-exciton interaction in semiconductor quantum dots." Diss., München Verl. Dr. Hut, 2008. http://d-nb.info/989622762/04.
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Skiba-Szymanska, Joanna. "Electron nuclear spin interaction in semiconductor quantum dots." Thesis, University of Sheffield, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.500176.
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Rueda-Fonseca, Pamela. "Magnetic quantum dots in II-VI semiconductor nanowires." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GRENY015/document.
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Dans ce travail de thèse a été développé et étudié un nouveau type d'objet semiconducteur magnétique : des boîtes quantiques de CdMnTe insérées dans des nanofils de ZnTe/ZnMgTe constituant une structure de type cœur-coquille. L'objectif était d'étudier la croissance par épitaxie par jets moléculaires et les propriétés fondamentales de ces hétéro-structures complexes. Dans ce but deux aspects principaux ont été abordés : i) la qualité et le contrôle des propriétés structurales, électroniques et magnétiques de ces objets, grâce à une maîtrise de leur croissance et ii) l'obtention d'informations quantitatives locales sur la composition chimique de ces nanostructures inhomogènes. Pour atteindre ces objectifs, nous avons divisé notre étude en quatre étapes. La première étape de ce travail a été concentrée sur l'étude quantitative de la formation des particules d'or servant de catalyseurs à la croissance des nanofils. La seconde étape a porté sur l'analyse des mécanismes de croissance et des paramètres gouvernant la croissance des fils de ZnTe. En particulier deux types de fils ont été observés : des fils cylindriques de structure wurtzite et des fils coniques de structures zinc-blende. Un modèle de croissance guidée par la diffusion a été utilisé pour rendre compte de certains des résultats quantitatifs présentés dans cette partie. La troisième étape a concerné l'insertion de boîtes quantiques de CdMnTe dans des nanofils de structure cœur-coquille ZnTe/ZnMgTe. Une étude préalable des paramètres pertinents influençant les propriétés magnéto-optiques de ces objets, tels que le confinement de la boîte quantique, l'incorporation du Mn et l'anisotropie de contrainte créée par la structure, a été menée. La quatrième et dernière étape de ce travail a porté sur l'interprétation quantitative de mesures d'analyse dispersive en énergie effectuées sur des nanofils de structure cœur-multicoquille. Un modèle géométrique a été proposé, permettant de retrouver la forme, les dimensions et la composition chimique des boîtes quantiques et des coquilles. Cette étude a été couplée à des mesures de caractérisation telles que la cathodo-luminescence, la micro-photo-luminescence et la spectroscopie magnéto-optique effectuées sur le même nanofilIn this PhD work a novel type of magnetic semiconductor object has been developed: Cd(Mn)Te quantum dots embedded in ZnTe/ZnMgTe core-shell nanowires. The goal was to investigate the growth, by molecular beam epitaxy, and the fundamental properties of these complex heterostructures. For that purpose, two main issues were addressed: i) gaining control of the structural, electronic and magnetic properties of these quantum objects by mastering their growth; and ii) obtaining quantitative local knowledge on the chemical composition of those non-homogeneous nanostructures. To tackle these topics, our research was divided into four stages. The first stage was devoted to perform a quantitative study of the formation process of the Au particles that catalyze the growth of nanowires. The second stage involved the analysis of the mechanisms and parameters governing the growth of ZnTe nanowires. In particular, two different types of nanowires were found: cone-shaped nanowires with the zinc-blende crystal structure and cylinder-shaped nanowires with the hexagonal wurtzite structure. A diffusion-driven growth model is employed to fit some of the quantitative results presented in this part. The third stage focused on the insertion of pure CdTe quantum dots containing Mn ions in the core-shell nanowires. An initial study of the relevant parameters influencing the magneto-optical properties of these objects, such as the quantum dot confinement, the Mn incorporation, and the strain anisotropy, was performed. The four and last stage of this work concerned the quantitative interpretation of Energy-Dispersive X-ray spectroscopy measurements performed on single core-multishell nanowires. A geometrical model was proposed to retrieve the shape, the size and the local composition of the quantum dot insertions and of the multiple layers of the heterostructures. This study was coupled to other complementary characterization measurements on the same nanowire, such as cathodo-luminescence, micro-photo-luminescence and magneto-optical spectroscopy
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Dakovski, Georgi L. "TIME-RESOLVED TERAHERTZ SPECTROSCOPY OF SEMICONDUCTOR QUANTUM DOTS." Case Western Reserve University School of Graduate Studies / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=case1192470456.
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Chen, Pochung. "Quantum optical control of spins and excitons in semiconductor quantum dots /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC IP addresses, 2002. http://wwwlib.umi.com/cr/ucsd/fullcit?p3061632.
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Frenzel, Johannes. "Structural, electronic and optical properties of cadmium sulfide nanoparticles." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1170678349152-44850.
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In this work, the structural, electronic, and optical properties of CdS nanoparticles with sizes up to 4nm have been calculated using density-functional theory (DFT). Inaccuracies in the description of the unoccupied states of the applied density-functional based tight-binding method (DFTB) are overcome by a new SCF-DFTB method. Density-functional-based calculations employing linear-response theory have been performed on cadmium sulfide nanoparticles considering different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). For saturated particles, the calculated onset excitations are strong excitonic. The quantum-confinement effect in the lowest excitation is visible as the excitation energy decreases towards the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states which lie below the lowest excitations of the completely saturated particles. The molecular orbitals (MOs), that are participating in the excitonic excitations, show the shape of the angular momenta of a hydrogen atom (s, p). Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, a very good agreement of the calculated data to experimentally available spectra and excitation energies is found.
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Frenzel, Johannes. "Structural, electronic and optical properties of cadmium sulfide nanoparticles." Doctoral thesis, Technische Universität Dresden, 2006. https://tud.qucosa.de/id/qucosa%3A23935.
Full textAbstract:
In this work, the structural, electronic, and optical properties of CdS nanoparticles with sizes up to 4nm have been calculated using density-functional theory (DFT). Inaccuracies in the description of the unoccupied states of the applied density-functional based tight-binding method (DFTB) are overcome by a new SCF-DFTB method. Density-functional-based calculations employing linear-response theory have been performed on cadmium sulfide nanoparticles considering different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). For saturated particles, the calculated onset excitations are strong excitonic. The quantum-confinement effect in the lowest excitation is visible as the excitation energy decreases towards the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states which lie below the lowest excitations of the completely saturated particles. The molecular orbitals (MOs), that are participating in the excitonic excitations, show the shape of the angular momenta of a hydrogen atom (s, p). Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, a very good agreement of the calculated data to experimentally available spectra and excitation energies is found.
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Siegert, Jörg. "Semiconductor Quantum Dots Studied by Time-Resolved Luminescence Techniques." Licentiate thesis, KTH, Microelectronics and Information Technology, IMIT, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1789.
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In this thesis time-resolved photoluminescence spectroscopyis presented as a powerful tool to study the carrier dynamicsin various self-assembled quantum dot (QD) structures, whichare potentially attractive for device applications.
The experiments reveal the impact of proton irradiation onInGaAs QDs and comparable quantum wells. Nonradiativerecombination at defectsan important material parameterandmeasureof the structure optical qualityis found to play a much less important role for the QD samples.The superior radiation hardness can be explained as a result ofthe three-dimensional carrier confinement in QDs. Comparisonsbetween the structures show a decrease of photoluminescenceintensity for quantum wells but a slight increase for QDsirradiated at low to intermediate doses. This somewhatunexpected characteristic is described by an enhanced carriertransfer into the dots via the defects introduced in thematerial by the protons.
In a different structure carrier dynamics in spatiallyaligned of InAs QDs are investigated. Alignment along lines isachieved by misfit dislocations deliberately introduced in thesubstrate. Photoluminescence spectra of the dots exhibit muchsmaller inhomogeneous broadening than for the reference sampleas a result of an improved QD uniformity. Samples with varyingbuffer layer thicknesses were grown to study the influence ofdislocation related traps on the observed fastphotoluminescence decay. It is found that the fast carriertrapping is predominantly caused by point defects close to theQDs or at the QD/barrier interfaces.
Additional numerical simulations confirm the roles of thetwo independently acting traps in nonradiativerecombination.
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Issac, Abey. "Photoluminescence Intermittency of Semiconductor Quantum Dots in Dielectric Environments." Doctoral thesis, Universitätsbibliothek Chemnitz, 2006. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200601267.
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The experimental studies presented in this thesis deal with the photoluminescence intermittency of semiconductor quantum dots in different dielectric environments. Detailed analysis of intermittency statistics from single capped CdSe/ZnS, uncapped CdSe and water dispersed CdSe/ZnS QDs in different matrices provide experimental evidence for the model of photoionization with a charge ejected into the surrounding matrix as the source of PL intermittency phenomenon. The distribution of the dark state lifetimes can be described by a power law over a wide range while that of bright state can be described by a power law at shorter times followed by an exponential decay. The lifetimes of the bright and dark states are influenced by the dielectric properties of the surrounding environment. Our experimental results show that the lifetime of the dark state increases with the dielectric constant of the matrix. This is very clear from the linear correlation between αoff and f (ε). We propose a self-trapping model to explain the increase of dark state lifetimes with the dielectric constant of the matrix. A charge will be more stabilized in a medium with high dielectric constant. An energetically more favourable state for an electron in a high dielectric medium decreases the return probability which eventually increases the duration of the off-time. Moreover, the self-trapping model establishes a general model for distribution of states in a matrix. We like to mention, that in the case of bright states, a qualitative observation is the cross over of the on-time power law behavior to an exponential one. The power law part of the decay is nearly matrix independent while the exponential decay, which limits the maximum on-time, strongly depends on dielectric properties of the environment. The exponential part of the on-time probability decays much faster in a high dielectric medium and there exists a linear relation between the time constant of the exponential decay and f (ε). Theoretical background has been provided for the observed results using the recently published DCET model which correlates PL intermittency of QDs with properties of the environment. This supports our previous conjecture of a general model for matrix controlled blinking process. The disagreement between experimentally observed dependence of αoff and f (ε) for different matrices with that of the static tunnelling model proposed by Verberk is due to the fact that the tunneling model considers only an electron transfer between a QD and spatially distributed trap states in vacuum. These states are already stabilized states. It does not assume any medium in between. Therefore, matrix dependent blinking kinetics can not be explained quantitatively by tunneling model even though tunneling between a QD and spatially distributed trap states gives a power law distribution for the blinking kinetics. DCET is a more general (dynamic) model. The bright and dark state parabolas contain QD, charge and the matrix. Therefore, this model could in principle explain matrix dependent blinking kinetics in a better way, for example, the energy difference between the minima of the bright and dark state parabolas (-ΔG0) is defined by the stabilization energy of the system provided by the matrix. However, due to lack of the relevant intrinsic parameters we did not compare this relationship and dependence qualitativelyBetrachtet man die Fluoreszenz einzelner Farbstoffmoleküle oder Halbleiternanokristalle bei kontinuierlicher Anregung, so stellt man fest, dass die im Zeitverlauf beobachtete Intensität einer stochastischen Variation unterliegt, d. h. dass das Chromophor zwischen emittierenden und nicht emittierenden Zuständen, auch Hell- und Dunkelzuständen genannt, hin- und herschaltet. Dieses als Blinken bekannte Phänomen ist physikalisch wie auch technologisch herausfordernd, lässt es doch einerseits die Realisierbarkeit einer Reihe von quantenoptischen Anwendungen, so z. B. auf dem Gebiet der Quantenkryptographie, dem Quantum Computing oder der optischen Schaltungstechnik auf Basis einzelner Quantenobjekte, in naher Zukunft möglich erscheinen. Andererseits setzt es gewissen Anwendungen, die auf die permanente Sichtbarkeit des Chromophors aufbauen, Grenzen, so zum Beispiel der Verwendung als Lumineszenzmarker in der medizinischen Diagnostik. Weiterhin ist festzustellen, dass das Blinken kritisch von den äußeren Bedingungen und von den Umgebungsparametern abhängt. Aus diesen und anderen Gründen ist ein fundamentales Verständnis der physikalischen Ursachen und der Wechselwirkungsprozesse unerlässlich. Die Forschung dazu steckt noch in den Kinderschuhen. Basierend auf umfangreiche Messungen der Fluoreszenzzeitreihen einzelner Nanokristalle aus CdSe und CdSe/ZnS in verschiedenen Umgebungen, zeigt diese Dissertation exemplarisch den Einfluss der Dielektrizitätsparameter auf das Blinken. Zur Erklärung des Sachverhalts wird ein so genanntes Self-Trapping-Modell zu Rate gezogen. Demnach kommt es zu einer Ionisation des Quantenobjekts und anschließender Ladungstrennung, woraufhin die abgetrennte Ladung für eine gewisse Zeit in der Umgebung lokalisiert bleibt. Die Dauer der Lokalisierung und damit der emittierenden und nicht emittierenden Perioden hängt von der dielektrischen Funktion des umgebenden Materials ab. Dies ist als direkter Nachweis für den photoinduzierten Ladungstransfer als Ursache des Fluoreszenzblinkens zu deuten. Die Arbeit demonstriert, dass die experimentellen Zeitreihen die charakteristischen Merkmale eines diffusionsgesteuerten Ladungstransferprozesses besitzen und nimmt dabei den gegenwärtigen wissenschaftlichen Diskurs über geeignete theoretische Modelle des Fluoreszenzblinkens auf
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Shahid, Robina. "Green Chemical Synthesis of II-VI Semiconductor Quantum Dots." Doctoral thesis, KTH, Funktionella material, FNM, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-104980.
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Nanotechnology is the science and technology of manipulating materials at atomic and molecular scale with properties different from bulk. Semiconductor QDs are important class of nanomaterials with unique physical and chemical properties owing to the quantum confinement effect. Size dependent optical properties make research on semiconductor QDs more attractive in the field of nanotechnology. Semiconductor QDs are usually composed of combination of elements from groups II–VI, III–V, or IV–VI of the periodic table. Group II-VI semiconductor QDs (ZnS, ZnSe, ZnO, CdSe, CdS) are most extensively studied systems, having bandgap which can be engineered through the variation of the material composition and size. Most common QDs are made of CdE (E=S, Se, Te) which are toxic. Recent environmental regulations restrict the use of toxic metals and therefore QDs containing nontoxic metals such as Zn are of great importance. The chemical synthesis of QDs involves different methods. Usually high temperature thermal decomposition of organometallic compounds in high boiling point organic solvents is used which needs long reaction time and involves complex synthesis procedures. New simpler and efficient synthetic routes with alternative solvents are required. Recently the synthesis of non-toxic QDs using green chemical routes is a promising approach receiving increasing attention. The aim of this Thesis is to develop novel routes for synthesis of semiconductor QDs employing green nanomaterial synthesis techniques. Therefore, in this work, we developed different green chemical routes mainly for the synthesis Zn-based QDs. Low temperature synthesis routes were developed for the synthesis of ZnS and ZnO QDs. Microwave irradiation was also used as efficient heating source which creates numerous nucleation sites in the solution, leading to the formation of homogeneous nanoparticles with small size and narrow size distribution. Different polar solvents with high MW absorption were used for synthesis of ZnS QDs. We also introduced ionic liquids as solvents in the synthesis of ZnS QDs using microwave heating. ILs are excellent reaction media for absorbing microwaves and are recognized as ‘green’ alternative to volatile and toxic organic solvents. For ZnS systems, the QDs produced by different methods were less than 5 nm in size as characterized by high-resolution transmission electron microscopy (HR-TEM). Selected area electron diffraction (SAED) patterns revealed that ZnS QDs synthesized by low temperature synthesis technique using conventional heating are of cubic crystalline phase while the QDs synthesized by using MW heating are of wurtzite phase. The optical properties were investigated by UV-Vis absorption spectrum and show a blue shift in absorption as compared to bulk due to quantum confinement effect. The photoluminescence (PL) spectra of ZnS QDs show different defect states related emission peaks and depend on different synthesis methods, high bandedge related emission is observed for ZnS QDs synthesized by using ionic liquids. ZnO QDs synthesized by low temperature route were found to be less than 4 nm in size and also show a blue shift in their absorption. The PL spectrum show bandedge related emission which is blue shifted compared with bulk with no emission originating from surface defect levels. The results show that QDs are of high crystalline quality with narrow size distribution. A comparative study of using conventional and MW heating in the synthesis of CdSe QDs was performed. The reactions involving microwave heating showed enhanced rates and higher yields. The developed methods involve all principles for green nanomaterials synthesis i.e. design of safer nanomaterials, reduced environmental impact, waste reduction, process safety, materials and energy efficiency.QC 20121115