Colloidal quantum dots (CQD) are used in the fabrication of efficient, low-cost solar cells synthesized in and deposited from solution. Breakthroughs in CQD materials have led to a record efficiency of 7.0%. Looking forward, any path toward increasing efficiency must address the trade-off between short charge extraction lengths and long absorption lengths in the near-infrared spectral region. Here we exploit the localized surface plasmon resonance of metal nanoparticles to enhance absorption in CQD films. Finite-difference time-domain analysis directs our choice of plasmonic nanoparticles with minimal parasitic absorption and broadband response in the infrared. We find that gold nanoshells (NS) enhance absorption by up to 100% at λ = 820 nm by coupling of the plasmonic near-field to the surrounding CQD film. We engineer this enhancement for PbS CQD solar cells and observe a 13% improvement in short-circuit current and 11% enhancement in power conversion efficiency.

Nanomaterial toxicity is a major concern and could potentially hamper the progress of biomedical nanotechnology development. Dispelling these concerns requires that the consequences of nanomaterial exposure are evaluated, and the findings will determine whether developmental hurdles can be overcome. This thesis evaluates the both in vivo and in vitro impact of quantum dots (QD , zinc sulphide capped cadmium selenide semiconductor nanocrystals) a fluorescent nanoparticle label with potential as an optical in vivo imaging agent. This work reviews nanoparticle characterization techniques and their importance to biological responses, and surveys QD interactions both in vivo and in vitro. We collected pharmacokinetic and toxicity data by a) quantitatively surveying the in vivo absorption, distribution , metabolism and excretion of QDs, and b) measuring the impacts of QDs on relevant organs (in vivo) and cells (in vitro). Neither of these areas had been explored when this thesis was started. In vivo, intravenous QD dosing in Sprague-Dawley rats showed uptake into reticuloendothelial cells with surface coating dependent kinetics, slow degradation, no excretion detected in feces or urine, and no indications of toxicity. The liver took up the majority of dose after 90 minutes and small amounts of QDs appeared in the spleen, kidney, and bone marrow. After 30 days, the cadmium concentration in the kidneys increased to 3µg/g without a proportional amount of zinc, indicating QD breakdown. In vitro we noted phagocytic capacity comparable to in vivo results, QD breakdown, and a retention of normal macrophage function thereby demonstrating that primary rat liver macrophages (Kupffer cells) are an appropriate in vitro system with which to investigate the cellular responses to quantum dots. Such an in vitro model will facilitate faster evaluation of individual nanotechnologies intended for in vivo use. This dissertation addresses a lack of in vivo background information needed to understand the consequences of QD exposure; though QD fail to demonstrate pharmacokinetics desirable for in vivo imaging agents, they are not toxic. Importantly, we provide in vitro data that will lead to the development of accurate and efficient in vitro primary screening methods that will be central to the further development of biomedical nanotechnologies.

No description available.

Nanomaterial toxicity is a major concern and could potentially hamper the progress of biomedical nanotechnology development. Dispelling these concerns requires that the consequences of nanomaterial exposure are evaluated, and the findings will determine whether developmental hurdles can be overcome. This thesis evaluates the both in vivo and in vitro impact of quantum dots (QD , zinc sulphide capped cadmium selenide semiconductor nanocrystals) a fluorescent nanoparticle label with potential as an optical in vivo imaging agent. This work reviews nanoparticle characterization techniques and their importance to biological responses, and surveys QD interactions both in vivo and in vitro. We collected pharmacokinetic and toxicity data by a) quantitatively surveying the in vivo absorption, distribution , metabolism and excretion of QDs, and b) measuring the impacts of QDs on relevant organs (in vivo) and cells (in vitro). Neither of these areas had been explored when this thesis was started. In vivo, intravenous QD dosing in Sprague-Dawley rats showed uptake into reticuloendothelial cells with surface coating dependent kinetics, slow degradation, no excretion detected in feces or urine, and no indications of toxicity. The liver took up the majority of dose after 90 minutes and small amounts of QDs appeared in the spleen, kidney, and bone marrow. After 30 days, the cadmium concentration in the kidneys increased to 3µg/g without a proportional amount of zinc, indicating QD breakdown. In vitro we noted phagocytic capacity comparable to in vivo results, QD breakdown, and a retention of normal macrophage function thereby demonstrating that primary rat liver macrophages (Kupffer cells) are an appropriate in vitro system with which to investigate the cellular responses to quantum dots. Such an in vitro model will facilitate faster evaluation of individual nanotechnologies intended for in vivo use. This dissertation addresses a lack of in vivo background information needed to understand the consequences of QD exposure; though QD fail to demonstrate pharmacokinetics desirable for in vivo imaging agents, they are not toxic. Importantly, we provide in vitro data that will lead to the development of accurate and efficient in vitro primary screening methods that will be central to the further development of biomedical nanotechnologies.

No description available.

No description available.

Quantum-Dot Cellular Automata (QCA) is being investigated as a possible alternative for encoding and processing binary information in an attempt to realize dramatic improvements in device density and processing speed over conventional CMOS design. The binary information is encoded in the locations of two excess electrons in a system of four quantum dots. The dots are arranged with each on a corner of a square, and electrons are able to quantum-mechanically tunnel between dots. Each set of four dots and two excess electrons constitutes a QCA cell. Coulomb repulsion ensures that the electrons will tend to occupy antipodal sites, giving two possible polarizations, or lowest energy ground states for a QCA cell. The electrons would tend to align along one diagonal or the other. Arrangements of QCA cells can be used to pass along input binary information and perform necessary logic operations on the input signal.When electrons tunnel back and forth between dots, it is possible they will occupy excited states in the dots. Two undesirable effects result from this: 1) Energy will be dissipated to the environment and cause thermal heating, and 2) it is possible a cell could become locked in a metastable state, which may be a local energy minimum, but is not one of the ground state polarizations we desire. Through the modulation of the heights of the inter-dot potential barriers, it would be possible to allow electrons to more easily tunnel between dots. This would help prevent the system from reaching excited states. The time variance in the heights of the potential barriers must be greater than the time it takes for the electrons to tunnel between dots, thus, effectively clocking the QCA device.We present theoretical studies of controlling the inter-dot potential barriers in a QCA device using an electric field due to electrostatically charged rods. The amount of charge on the rods is varied in time to increase and decrease the electric field, which will raise and lower the inter-dot potential barriers as desired. Different arrangements of rods provide different time-dependent behavior in the electric field, which may be useful depending on the arrangements of QCA cells required to make a logic device. / Department of Physics and Astronomy

In the framework of the two-band (Dirac) model, we analyze the electronic structure of nanoscale systems, based on narrow-gap semiconductors of Pb,_xSnx (Se, S) type. Themain attention is paid to the influence of properties of the surface, encoded in appropriate boundary conditions, on the size-quantized spectrum. From this point of view we consider two types of systems: spherical (quantum dots) and quasi one-dimensional (films).It is shown that the spectrum of the spherical quantum dot consists not only of usual size-quantized states, located above the gap edge, but also surface modes residing inside the gap. Such states manifest themselves in the far infrared part of the absorption spectrum, the measurement of which allows one to extract information about the dot surface.Next, we consider a film with the energy gap modulated in the <111> (growth) direction. It is shown that the spectrum of the infinite crystal possesses a supersymmetrical structure. The film boundaries, generally speaking, destroy the supersymmetry, i.e. size-quantized subbands turn out to be spin-split. However, there exists a class of boundary conditions that do not lift spin degeneracy. Physically, in this case there is no band mismatch at interfaces. Our central statement, therefore, consists of the following: even when the inversion symmetry is destroyed by the bulk inhomogeneity, the spin-splitting of the spectrum is a purely surface effect. This is illustrated on a simple example, when the energy gap varies linearly over the film width.Finally, we investigate the role of boundary conditions in the problem of scattering of spinor waves by a quantum dot. It is shown that the existence of surface states greatly modifies the scattering data; in particular, outgoing waves may turn out to be fully polarized. / Department of Physics and Astronomy

A nanoscale Aharonov-Bohm (AB) ring with a quantum dot (QD) embedded in each arm is investigated analytically to provide electron transmission characteristics. A parallel magnetic field provides Zeeman splitting of the QD energy levels. Combined Zeeman energy level splitting and AB-effects occur with a perpendicular field. In our device, the AB-ring interferometer, Zeeman splitting of the QD energy levels creates regions of parameter space in which the electron transmission is highly spin-polarized. In addition to Zeeman splitting caused by a parallel magnetic field, combined Zeeman energy level splitting and AB-interference effects occur with a perpendicular field. The weighted spin-polarization function is calculated and presented as a function of magnetic field and electron energy. Due to a unique parameter regime in which the AB-oscillations show extreme sharpening [1], the electron transmission can be tuned to produce spin-polarized currents which can be switched and controlled by small changes of external fields. / Introduction -- AB-oscillations and resonances in a double quantum dot ring -- Results for combined Zeeman and AB effects -- Spin-polarization. / Department of Physics and Astronomy

I present a theoretical study of fault tolerant properties in Quantum-dot Cellular Automata (QCA) devices. The study consists of modeling and simulation of various possible manufacturing, fabrication and operational defects. My focus is to explore the effects of temperature and dot displacement defects at the cell level of various QCA devices. Results of simple devices such as binary wire, logical gates, inverter, cross-over and XOR will be presented. A Hubbard-type Hamiltonian and the inter-cellular Hartree approximation have been used for modeling the QCA devices. Random distribution has been used for defect simulations. In order to show the operational limit of a device, defect parameters have been defined and calculated. Results show fault tolerance of a device is strongly dependent on the temperature as well as on the manufacturing defects. / Cell design -- Basic logic gates -- The exclusive or gate. / Department of Physics and Astronomy