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

Multijunction solar cells made from a combination of CQDs of differing sizes and thus bandgaps are a promising means by which to increase the energy harvested from the Sun’s broad spectrum. In this dissertation, we first report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device’s collecting electrodes—the heterointerface with electron accepting TiO2 and the deep-work-function hole-collecting MoO3 for ohmic contact—for maximum efficiency. Room-temperature processing enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low thermal-budget larger-bandgap front cell. We report an electrode strategy that enables a depleted heterojunction CQD PV device to be fabricated entirely at room temperature. We develop a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells suitable for use as the back junction in tandem solar cells. We further report in this work the first efficient CQD tandem solar cells. We use a graded recombination layer (GRL) to provide a progression of work functions from the hole-accepting electrode in the bottom cell to the electron-accepting electrode in the top cell. The recombination layers must allow the hole current from one cell to recombine, with high efficiency and low voltage loss, with the electron current from the next cell. We conclude our dissertation by presenting the generalized conditions for design of efficient graded recombination layer solar devices. We demonstrate a family of new GRL designs experimentally and highlight the benefits of the progression of dopings and work functions in the interlayers.

This thesis describes the synthesis and assembly of metal and semiconductor nanoparticles (NPs). The two research topics include i) hetero-assembly of metal and semiconductor NPs, ii) effect of ionic strength on homo-assembly of gold nanorods (GNRs). First, we present hetero-assembly of GNRs and semiconductor quantum dots (QDs) in a chain using biotin-streptavidin interaction. We synthesized alloyed CdTeSe QDs and modified them with mercaptoundecanoic acid to render them water-soluble and to attach streptavidin. We synthesized GNRs by a seed-mediated method and selectively modified the ends with biotin. Hetero-assembly of QDs and GNRs depended on the size, ligands, and ratio of QDs and GNRs. Second, we controlled the rate of homo-assembly of GNRs by varying the ionic strength of the DMF/water solution. The solubility of polystyrene on the ends of GNRs depended on the ionic strength of the solution, which correlated with the rate of assembly of GNRs into chains.

This thesis describes the synthesis and assembly of metal and semiconductor nanoparticles (NPs). The two research topics include i) hetero-assembly of metal and semiconductor NPs, ii) effect of ionic strength on homo-assembly of gold nanorods (GNRs). First, we present hetero-assembly of GNRs and semiconductor quantum dots (QDs) in a chain using biotin-streptavidin interaction. We synthesized alloyed CdTeSe QDs and modified them with mercaptoundecanoic acid to render them water-soluble and to attach streptavidin. We synthesized GNRs by a seed-mediated method and selectively modified the ends with biotin. Hetero-assembly of QDs and GNRs depended on the size, ligands, and ratio of QDs and GNRs. Second, we controlled the rate of homo-assembly of GNRs by varying the ionic strength of the DMF/water solution. The solubility of polystyrene on the ends of GNRs depended on the ionic strength of the solution, which correlated with the rate of assembly of GNRs into chains.

No description available.

This thesis investigates the biodistribution and toxicological effects of amphiphilic polymer coated CdSe/ZnS quantum dots (QDs) in two aquatic species, Daphnia magna (daphnia) and Danio rerio (zebrafish). The use of QDs in the life sciences has become common practice over the past decade. In addition QDs are being incorporated in commercially available light emitting diodes and photovoltaic solar cells. As the widespread commercial use of QDs increases, environmental release is inevitable, and water will contain the highest environmental concentrations based on life cycle assessments. Despite increased attention to the aquatic toxicology of nanomaterials in recent years, little information exists on the biological fate of QDs in aquatic organisms. Quantitative data on the uptake and excretion of QDs from daphnia and zebrafish were collected using fluorescence imaging paired with metal analysis. First, daphnia were examined after aqueous and dietary exposure to amphiphilic polymer coated CdSe/ZnS QDs. Surface coating influenced QD acute toxicity and high particle aggregation correlated with daphnia mortality. QDs were readily ingested by daphnia and accumulated in the intestines. High body burdens of 150-200 μg/g were found in the daphnia, with intestinal QD concentrations significantly elevated above the exposure media concentration. The slow elimination observed in daphnia suggested that trophic transfer of QDs to higher organisms may occur. Using daphnia and zebrafish as a model food chain revealed that QDs can transfer to zebrafish through dietary exposure with body burdens of 8-9.5 μg/g found. However, no biomagnification between daphnia and zebrafish was observed and the biomagnification factor (BMF = 0.04) was significantly less than one. This work demonstrates that aqueous and dietary exposures to QDs can result in high total body concentrations in aquatic organisms with little to no gross toxicity. The low acute toxicity observed for some surface coated QDs encourages further design optimization to improve the biocompatibility and reduce the environmental impact of QDs.

To get an optimal print result with rotogravure on improved newsprint, MF-paper, knowledge aboutwhat adjustment that should be done through the whole production is needed. This degree project investigatesthe reproduction’s effect on the final print result.A test print is done at Quebecor World Nordic Interprint´s regular print unit. The evaluation of theprint shows how important the use of a unit adjusted ICC-profile is for the print quality. A comparisonbetween the two separation methods GCR and UCR shows that Interprint´s present ICC-profile, usingUCR as separation method, gives less missing dots. When the cylinder that prints the black ink isengraving whit a slower speed than normal the result shows that the colour becomes a little darker, thedetails becomes clear but the difference is very small.

In the quantum dot implementation of an intermediate band solar cell presented in this thesis, the offset of the intermediate band with respect to the conduction band is approximated by the ground state energy of a single electron in a single quantum dot heterojunction. The ground state energy is calculated with the radial Schrodinger equation with a Hamiltonian whose potential is composed from the step-like conduction band offset of the quantum dot heterojunction and the 1/r electrostatic potential of the hydrogenic impurity. The position of the intermediate band is tuned by adjusting the radius of the quantum dots. By assuming that the centrally located impurities are ionized, the location of the Fermi energy is guaranteed to be within the intermediate band. An intermediate band solar cell contains three bands: a conduction band, a valence band; and an intermediate band. The addition of an intermediate band augments the photogeneration of carriers. These additional carriers allow for an increased theoretical efficiency as compared to a conventional homojunction solar cell. The challenges in implementing an intermediate band solar cell involve centering the intermediate band at an energy level matched to the solar spectrum and aligning the Fermi energy within the intermediate band. The latter is necessary to ensure both a supply of electrons capable of photon induced transition to the conduction band as well as a large population of holes that allow photon induced electrons to transition from the valence band to the intermediate band. This thesis presents a novel material system, InPAs quantum dots enveloped in AlGaAs barriers grown on GaAs substrates, with which to implement an optimized QD-IBSC. This novel material system is selected based upon a refined set of design rules that include a requirement that the quantum dot/barrier pair offer a negligible valence band offset. With such a design rule the existence of hole levels is avoided, thus reducing bandgap narrowing at the valence band edge and the existence of minibands below the intermediate band.

The objective of this research was to design semiconductor material structures for a number of different devices, including GaN metal-semiconductor-metal (MSM), InGaAs/InAlAs MSM, and InAs/GaAs quantum dot photodetectors, and to study the growth conditions for epitaxial material using molecular beam epitaxy (MBE) augmented with an rf-plasma nitrogen source. GaN was grown on a LiGaO2 substrate, which has multiple advantages over the most commonly used substrates for III-nitride growth. LiGaO2 substrates have a small lattice mismatch of approximately 1% with GaN, which leads to high-quality epitaxy film by optimization of the growth condition. The combination of nitridation, buffer, super lattice, and Ga-rich condition is the key to improving the quality of GaN film grown by MBE on LiGaO2. The first GaN MSM grown on LiGaO2 was reported, which has the dark current in the range of 10-12A. The device was then lift off and bonded on SiO2/Si wafer. The performance of the device did not degrade after integration. However, the orthorhombic crystal structure of LiGaO2 results in the unusual asymmetric strains within GaN, causing changes in the microstructure of GaN and making integration difficult. The strains within GaN grown on LiGaO2 were investigated using high resolution x-ray. It was found that the critical thickness of GaN on LGO was around 10nm, and the strains relieve with film thickness increasing. The dislocation densities were also calculated and confirmed by AFM, which can be as low as 2䥱07/cm2. Through studying of the strains with the insertion of AlGaN/GaN superlattice buffer, it was found that the strain of the epitaxial layer is dependent on the thickness and critical thickness of both epitaxy and buffer layer. For thin GaN films, Al0.12GaN superlattice buffer would relieve the strain most, and for the thicker layers, the Al0.44GaN superlattice buffer relieves the strain most. The dislocation density measurement shows that an insertion of buffer decreases dislocations significantly. Insertion of superlattice buffers does not decrease dislocation density further. The material structure of InGaAs/InAlAs was studied. The device quality was improved by optimizing the material structure. Depends on the application as optical interconnects, the optimized material structure should insure the device with high speed but reasonable responsivity. Finally, devices were fabricated that achieve speeds as high as 50-70 GHz, comparable with the commercial ultra-fast MSM. The research of quantum dots was focused on modification of the size, strains, and structures of quantum dots by annealing the spacer between quantum dot layers using As4 and P2, respectively, at different temperatures. It was found that the annealing under P2 results in surface exchange, and the annealing under As4 mostly changes dots?zes, causing the changes of energy level.

Si QDs embedded in SiOx or SiNx thin films, which could emit light in the entire visible range from 440 nm to 840 nm by controlling their size and/or their matrix, were synthesized by evaporation or plasma enhanced chemical vapor deposition techniques. Various shades of white could be obtained from multi-layered SiNx film structures by controlling the size of Si QDs and layer thickness. It was shown that the combination of these films can produce white emission spectra with superior color rendering properties compared to conventional fluorescent tubes. Such Si-based QDs can be used as down-converting phosphors to coat a blue/UV LED to generate white light, providing a less expensive fabrication process to obtain advanced solid state lighting devices. As a supplement, free CdTe QDs with emission colors spanning 520~700 nm and quantum efficiency up to 54%, were synthesized using a colloidal chemical method for white LED applications. White PL and a range of emission colors were obtained from mixed CdTe QD samples excited by a 420 nm blue LED. Another part of this research was to develop a new x-ray powder phosphor, ZnTe:O, for biological imaging applications used in CCD-based synchrotron x-ray detectors. A unique dry synthesis process, including gaseous dry doping and etching procedures, was developed to synthesize ZnTe:O phosphors. The excellent x-ray luminescence results of oxygen doped ZnTe, including high efficiency, high resolution, fast decay, low afterglow and an improved spectral match to the CCD detector, indicated that ZnTe:O is a promising phosphor candidate for x-ray imaging applications.