Browsing by Subject "Quantum dots"

Now showing 1 - 5 of 5
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    Computational study of confined states in quantum dots by an efficient finite difference method.
    (2010-01) Butt, Salman
    Semiconductor quantum dot systems have gained more attention in quantum computation and optoelectronic applications due to the ease of bandstructure tailoring and three-dimensional quantum confinement. Thus, an accurate solution of energy bandstructure within the quantum dot is important for device design and performance evaluation. In this paper, the solutions of bandstructures of quantum dot systems are presented by implementing finite difference technique. To illustrate our analysis procedure, various configurations of quantum dot systems were taken into account. In order to improve the calculation efficiency of the finite difference solution in terms of time and memory consumption, uneven divisions for the quantum dot confinement region were used. In addition, we identified the optimum combination of divisions for each geometrical configuration. Eventually, the eigenstate wavefunctions and eigenvalues were obtained by directly solving the eigen-value problems. Overall, the generated results agreed consistently with the published results obtained by other solution techniques.
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    Coupling of Surface Plasmons and Semiconductor Nanocrystals for Nanophotonics Applications
    (2015-08) Jayanti, Sriharsha
    The goal of this thesis is to engineer the interaction between surface plasmons and semiconductor nanocrystals for nanophotonic applications. Plasmonic metals support surface plasmon polaritons, hybrid photon and electron waves that propagate along a metal-dielectric interface. Unlike photons, surface plasmons can be confined in sub-diffraction geometries. This has two important consequences: 1) optical devices can be designed at the nanoscale, and 2) the high density of electromagnetic fields allows study of enhanced light-matter interactions. Surface plasmons have been exploited to demonstrate components of optoelectronic circuits, optical antennas, surface enhanced spectroscopy, enhanced fluorescence from fluorophores, and nanolasers. Despite the advances, surface plasmon losses limit their propagation lengths to tens of micrometers in the visible wavelengths, hindering many applications. Recently, the template-stripping approach was shown to fabricate metal films that exhibit larger grains and smoother surface, reducing the grain boundary and roughness scattering. To further improve the plasmonic properties, we investigate the importance of deposition conditions in the template-stripping approach. We provide insight and recipes to enhance the plasmonic performance of the most commonly used metals in the ultraviolet, visible, and near-infrared. We also explore the potential of low temperatures to improve the performance of metal films, where the electron-electron and electron-phonon scattering should be reduced. This sets a limit on the minimum loss metals can exhibit. Using this knowledge, we study the optical properties of quantum-confined semiconductor nanocrystals near metal structures. Semiconductor nanocrystals have many attractive characteristics that make them suitable for solid-state lighting and solar cells among others. Specifically, CdSe nanocrystals have been heavily studied for their large absorption and emission cross-sections, size dependent emission wavelengths, photostability, and high quantum yields. Here, we focus on studying the emission from CdSe nanocrystals near plasmonic structures in the weak and strong coupling regimes. In the weak coupling regime, plasmonic structures can be used to selectively modify the radiative rates at the desired wavelengths. We tailor plasmonic structures to enhance and tune the emission from the surface states of CdSe nanocrystals throughout the visible. Due to their size, a significant fraction of atoms are on the surface; however, electron-hole recombination via surface states is typically dark. We further use electrochemistry to probe the energy levels of the surface states. In the strong coupling regime, the energy levels of the surface plasmons and nanocrystals hybridize to form polariton states. In this regime, we demonstrate polariton emission from CdSe/CdSZnS core/shell/shell nanocrystals on silver hole arrays. Emission from these polariton states should be coherent and has implications for thresholdless lasing. While the above studies focus on the change in nanocrystal behavior near metals, these nanocrystals can also be used to improve plasmonic performance. We study the potential of thin layers of CdSe nanocrystals to amplify surface plasmons and enhance their propagation lengths. When the nanocrystals are excited using an external pump, propagating surface plasmons can stimulate emission from these nanocrystals and amplify. If more surface plasmons are generated than lost, then surface-plasmon signals can propagate over extremely long distances and even amplified. We calculate the gain provided and discuss the importance of key parameters such as the absorption and emission cross section, spacer layer thickness, nanocrystal lifetime, and temperature. Finally, we systematically study the emission properties and exciton decay in Ag-doped CdSe nanocrystals, which were recently shown to exhibit enhanced photoluminescence. Overall, this thesis aims to improve plasmonic performance with and without the presence of a gain medium, and advances the understanding of optical behavior of CdSe nanocrystals near metal structures in the weak and strong coupling regimes.
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    Inelastic scattering in STEM for studying structural and electronic properties of chalcogenide-based semiconductor nanocrystals
    (2013-09) Gunawan, Aloysius Andhika
    Transmission electron microscopy (TEM) relies upon elastic and inelastic scattering signals to perform imaging and analysis of materials. TEM images typically contain contributions from both types of scattering. The ability to separate the contributions from elastic and inelastic processes individually through energy filter or electron energy loss spectroscopy (EELS) allows unique analysis that is otherwise unachievable. Two prominent types of inelastic scattering probed by EELS, namely plasmon and core-loss excitations, are useful for elucidating structural and electronic properties of chalcogenide-based semiconductor nanocrystals. The elastic scattering, however, is still a critical part of the analysis and used in conjunction with the separated inelastic scattering signals. The capability of TEM operated in scanning mode (STEM) to perform localized atomic length scale analysis also permits the understanding of the nanocrystals unattainable by other techniques. Despite the pivotal role of inelastic scatterings, their contributions for STEM imaging, particularly high-angle annular dark field STEM (HAADF-STEM), are not completely understood. This is not surprising since it is currently impossible to experimentally separate the inelastic signals contributing to HAADF-STEM images although images obtained under bright-field TEM mode can be analyzed separately from their scattering contributions using energy-filtering devices. In order to circumvent such problem, analysis based on simulation was done. The existing TEM image simulation algorithm called Multislice method, however, only accounts for elastic scattering. The existing Multislice algorithm was modified to incorporate (bulk or volume) plasmon inelastic scattering. The results were verified based on data from convergent-beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), and HAADF-STEM imaging as well as comparison to experimental data. Dopant atoms are crucial factors which control optical, electronic, and also magnetic properties of semiconductors. Their location inside the materials has become more important with the miniaturization of devices. The precise determination of the position, however, poses a great challenge. Imaging using HAADF-STEM has proven adequate for locating heavy dopant atoms buried in relatively light matrix, particularly using aberration-corrected microscopes. The imaging method has been unsuccessful in detecting dopant atoms with similar atomic number as the matrix. Inelastic core-loss or inner-shell electronic excitations using EELS offer a unique solution when simultaneous imaging and EELS acquisitions are performed. The dopant atoms that are invisible in the images due to the small atomic number differences can be detected via spatial correlation with EELS core-loss data. Three types of samples with varying concentration of Mn dopant atoms in ZnSe nanocrystals were used to confirm such method. Precise locations of the dopant atoms on planes perpendicular to electron beam propagation could be determined although not all of the dopant atoms were detected due to limitations in experimental conditions.Another important type of chalcogenide-based nanocrystals is PbSe which is useful for solar cells. Colloidal method commonly used to synthesize the nanocrystals leave oleic acid capping ligands as surface passivation and size stabilizer. These ligands have critical roles in controlling electrical and optical properties of an individual nanocrystal and their assembly. Deemed insulating due to long chains of carbons, oleic acid is typically treated with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals. Despite its apparent insulating behavior, oleic acid was shown to exhibit surface plasmon coupling under certain circumstances. The geometric arrangement of the ligands was first investigated by HAADF-STEM imaging. Under air exposure, PbSe nanocyrstals easily oxidize to form oxide shells that are responsible for p-type doping by introducing surface acceptor states. At early oxidation stage (partial oxidation), prior to the formation of uniform oxide shells, the nanocrystals appear to form links between neighbors. Localized EELS analysis shows that these links are made of carbon based materials, most likely modified form of oleic acid ligands consisting of conjugated double bonds. Such modification occurred through oxidative dehydrogenation of the oleic acid ligands that is facilitated by the growing oxide shells on the surface of nanocrystals.
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    Interactions of Semiconductor Nanoparticles with Environmentally Relevant Bacteria Model
    (2019-05) Pramanik, Sunipa
    The growth in nanotechnology and the specific interest in the use of semiconductor nanoscale quantum dots have increased recently. The environmental and health concerns over the use of cadmium (Cd) in quantum dots has led to research towards design and synthesis of Cd-free quantum dots. With the growth and synthesis of these alternative quantum dots, research is ongoing to understand their environmental and biological interactions and implications. In this thesis, I have focused on comparison of the toxic effects of conventional CdSe and CdSe/ZnS quantum dots, and various alternative quantum dots such as silicon quantum dots and Zn-based QDs, which are emerging as a potentially benign alternative, using bacteria as a model organism. I have also focused on studying the toxicity and stability of another semiconductor nanoparticle group, copper zinc tin sulfide nanoparticles. This research assesses changes in cell viability, respiration pattern, and cell membrane integrity in the presence of the nanoparticles using colony counting, respirometry and membrane integrity assays, respectively. The association of the QDs with bacterial cell membranes was investigated using transmission electron microscopy (TEM). Mechanism of toxicity is assessed via ion dissolution studies and reactive oxygen species monitoring.
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    Quantum Dot Dispersion in Block Copolymer Matrices
    (2018-08) Wenger, Whitney
    Quantum dots (QDs) have demonstrated viability for a wide set of applications ranging from bioimaging to electronics. Their unique size-tunable band gaps and accessibility for roll-to-roll processing via solution synthesis makes them promising candidates in many of these areas. Several of these applications benefit from carefully manipulated spacing in QD films, often achieved through integration of QDs in a polymer matrix. Although many studies have achieved varying degrees of success in dispersing QDs in polymer matrices, there remains much to be understood about the path dependency of QD integration and how QDs may be integrated into sphere-forming polymer matrices. In this work, CdSe QDs were synthesized via a hot injection technique in an air-free environment. These QDs were fabricated in a range of sizes based on the reaction time and were evaluated for their crystal structure, absorbance, fluorescence, ligand coverage, and dispersion in various solvents. The resulting QDs feature a wurtzite crystal structure and exhibit narrow absorbance and emission peaks. The QDs are well stabilized in nonpolar solvents like hexane and toluene via trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) ligands. In the first approach towards QD integration in polymer matrices, the native TOPO ligands were exchanged for a poly(ethylene glycol) ligand functionalized with a thiol end group. The resulting QDs were qualitatively and quantitatively analyzed to determine the ligand density on the QD surface and the QD dispersion in different solvents. After ligand exchange, the QDs were no longer dispersible in nonpolar solvents like hexane but formed stable dispersions in solvents like tetrahydrofuran, chloroform, and water. Upon ligand exchange, 50-85% of the original ligands were removed and an average of 2-4 poly(ethylene glycol) ligands were installed on each QD surface. The extent of QD dispersion in various homopolymers was evaluated using transmission electron microscopy (TEM). CdSe QDs were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate phase separation of the polymer and the QDs where the spacing between QDs does not change upon addition of the polymer. In a separate study, CdSe QDs were mixed with poly(butadiene) in n-hexane and dropcast into thin films for TEM. These films demonstrated an increase in spacing between the QDs of roughly 2 nm. However, the majority of the polymer does not end up in the space between QDs and is phase separated from the QD crystal phase. Finally, CdSe QDs after ligand exchange with PEG were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate some mixing between the QDs and the polymer. Finally, the extent of QD dispersion in various diblock copolymers was evaluated using small angle x-ray scattering (SAXS), TEM, and fluorescence measurements. CdSe QDs were mixed with a lamellae-forming poly(ethylene-b-cyclohexylethylene) in benzene and dried and heated to T ≈ 200°C and then cooled to 140°C to induce polymer ordering. The resulting solid composites exhibited aggregates of QDs via SAXS and TEM measurements. In a separate study, CdSe QDs were mixed with a body-centered cubic forming poly(styrene-b-butadiene) in benzene. Two types of samples were prepared: one formed from drying the polymer and QDs from the solvent and subsequently heating to 140°C, and one formed from dropcasting the dispersion of polymer and QD from the solvent. The resulting solid composite prepared with temperature processing was microtomed and exhibited QD aggregation in TEM. The dropcast sample exhibited phase separation of the QDs and the polymer. Finally, CdSe QDs were mixed with a body-centered cubic poly(lactide-b-butadiene) in a mixture of n-hexane and chloroform. Micelles with the minority poly(butadiene) block on the outside were formed in similar mixtures of n-hexane and chloroform and observed via dynamic light scattering. QDs were added to the polymer in a mixture of n-hexane and chloroform after micellization (at higher hexane concentrations) and before micellization (at lower hexane concentrations); in the latter case, hexane was added to induce polymer micellization after the addition of QDs. These dispersions were dropcast into films for TEM, which revealed similar film structures for both samples where QDs appeared at the interstices between roughly spherical shapes; these shapes were attributed to the formation of polymer microemulsions during the drying process. The effect of chloroform and n-hexane content of the solvent mixture on the composite formation was tested for various mixtures of n-hexane and chloroform. TEM of QD and polymer samples prepared from these solvent mixtures showed phase separation of QDs and polymer at low n-hexane concentrations and the polymer microemulsion structure for higher n-hexane concentrations. Although this work focused on the integration of CdSe QDs, the insights demonstrated here should prove relevant for other nanoparticle-composite systems.