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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

With growing global energy demand there will be an increased need for sources of renewable energy such as solar cells. To make these photovoltaic technologies more competitive with conventional energy sources such as coal and natural gas requires further reduction in manufacturing costs that can be realized by solution processing and roll-to-roll printing. Colloidal quantum dots are a bandgap tunable, solution processible, semiconductor material which may offer a path forward to efficient, inexpensive photovoltaics. Despite impressive progress in performance with these materials, there remain limitations in photocarrier collection that must be overcome. This dissertation focuses on the characterization of charge recombination and transport in colloidal quantum dot photovoltaics, and the application of this knowledge to the development of new and better materials. Core-shell, PbS-CdS, quantum dots were investigated in an attempt to achieve better surface passivation and reduce electronic defects which can limit performance. Optimization of this material led to improved open circuit voltage, exceeding 0.6 V for the first time, and record published performance of 6% efficiency. Using temperature-dependent and transient photovoltage measurements we explored the significance of interface recombination on the operation of these devices. Careful engineering of the electrode using atomic layer deposition of ZnO helped lead to better TiO2 substrate materials and allowed us to realize a nearly two-fold reduction in recombination rate and an enhancement upwards of 50 mV in open circuit voltage. Carrier extraction efficiency was studied in these devices using intensity dependent current-voltage data of an operational solar cell. By developing an analytical model to describe recombination loss within the active layer of the device we were able to accurately determine transport lengths ranging up to 90 nm. Transient absorption and photoconductivity techniques were used to study charge dynamics by identifying states in these quantum dot materials which facilitate carrier transport. Thermal activation energies for transport of 60 meV or lower were measured for different PbS quantum dot bandgaps, representing a relatively small barrier for carrier transport. From these measurements a dark, quantum confined energy level was attributed to the electronic bandedge of these materials which serves to govern their optoelectronic behavior.

Quantum-dot Cellular Automata (QCA) is a new paradigm for computation that utilizes polarization states instead of using current switching. It is being studied because of the realization of the quickly approaching limitation of the current CMOS technology. The location of two excess electrons located within four or five quantum dots on a particular cell can transmit the binary information. These dots are located in the corner of a square cell, and if there is a fifth dot it is located in the center. The electrons are allowed to tunnel freely among the dots, but are restricted from tunneling between neighboring cells. Because of the interaction between the electrons, they will anti-align within the cell giving one of two particular configurations. This configuration can be transmitted to neighboring cells. In other words, data is flowing.We present a numerical study of the fabrication defect's influence on Quantum-dot Cellular Automata (QCA) operation. The statistical model that has been introduced simulates the random distribution of positional defects of the dots within cells and of cells within arrays. Missing dots within a QCA cell structure have also been studied.We have studied specific non-clocked QCA devices using the Inter-cellular Hartree Approximation, for different temperatures. Parameters such as success rate and breakdown displacement factor were defined and calculated numerically. Results show the thermal dependence of the breakdown displacement factor of the QCA devices. It has been shown, that the breakdown displacement factor decreases with increasing temperature. As expected, multiple defects within the same QCA array have shown a reduction in success rate greater than that of a single defect influencing the system. / Department of Physics and Astronomy

To have a useful QCA device it is first necessary to study how to control data flow in a device, then study how temperature and manufacturing defects will affect the proper output of the device. Theoretically a "quantum wire" of perfectly aligned QCA cells at zero Kelvin temperature has been examined. However, QCA processors will not be operating at a temperature of zero Kelvin and inherently the manufacturing process will introduce defects into the system. Many different types of defects could occur at the device level and the individual cell level, both kinds of defects should be examined. Device defects include but are not limited to linear and/or rotational translation, and missing or extra cell(s). The internal cell defects would include an odd sized cell, and one or more miss-sized or dislocated quantum dot(s). These defects may have little effect on the operation of the QCA device, or could cause a complete failure. In addition, the thermal effect on the QCA devices may also cause a failure of the device or system. The defect and thermal operating limit of a QCA device must be determined.In the present investigation, the thermal and defect tolerance of clocked QCA devices will be studied. In order to study tolerance of QCA devices theoretical models will be developed. In particular, some existing computer simulation programs will be studied and expanded. / Department of Physics and Astronomy

Realization of linear quantum computation and establishing secure quantum communication among interacting parties demand for triggered quantum sources delivering genuine single and entangled photons. However, the intrinsic energy level spectrum of nanostructures made by the nature or developed under a random growth process energetically lacks the expected figures of merit to produce such quantized states of photons. Here, I present the semi-empirical modeling and experimental investigation on the spin fine structure of strongly confining quantum dots embedded in III-V nanowires. To this end, the quantum dot is numerically modeled via the Configuration Interaction method at two different levels: 1) single-particle level, where its pure energy level structure is resolved in the presence of strain and spin-orbit interaction. 2) Few-particle level, at which the few-body interactions appear as perturbative energy corrections and orbital correlations. I demonstrate the influence of quantum confinement on the binding energies and spin fine structure of excitons in the absence of hyperfine interaction. Importantly, the high-symmetry character of excitonic orbitals in nanowire quantum dots restore the degeneracy of optically-active ground-state excitons, offering an ideal spectrum for the entangled photon pair generation. To experimentally verify the idea, we design and fabricate defect-free nanowire quantum dots with ultra-clean excitonic spectrum, and construct the time correlation function of emitted photons through performing a series of low-temperature statistical quantum optics measurements. We observe a decent performance in terms of single photon generation under low excitation powers. Moreover, photon pairs emitted from the biexciton-exciton cascade of nanowire quantum dots exhibit color indistinguishability and polarization entanglement owing to the trivial fine structure splitting of the ground-state excitons. We further extend the idea by proposing the hybridized states of a nanowire-based quantum dot molecule as the potential source of higher-order entangled states. Tracing the field-dependent spectrum suggests the appearance of dominant features under the weak localization of electrons and coherent tunneling of holes. In addition to their Coulomb correlation, excitons also remain spatially correlated, opening new transition channels normally forbidden in the ground state of a single dot. The proposed structure can be exploited to create tripartite hybrid, GHZ and W-entangled states.

There is currently interest in developing control over the spin of a single Manganese (Mn) ion, the atomic limit of magnetic memory, in semiconductor quantum dots (QDs). In this work we present theoretical results showing how one can manipulate the spin of Mn ion with light in a QD by engineering Mn-multi-exciton interactions through quantum interference, design of exciton and bi-exciton states and application of the magnetic field. We develop a fully microscopic model of correlated exciton and bi-exciton interacting with the Mn ion. The electrons and heavy holes, confined in the QD, approximated as a two-dimensional harmonic oscillator (HO), interact via direct and short- and long-range exchange Coulomb interactions. The matrix elements of the exchange interaction are computed for the first time in the harmonic oscillator basis and for arbitrary magnetic fields. The exciton and bi-exciton energies and states are computed using the configuration interaction method. The interaction between carriers and the Mn spin is accounted for by the Heisenberg electron-Mn and Ising hole-Mn exchange interactions. For a single exciton confined in a magnetic dot, a novel quantum interference (QI) effect between the electron-hole Coulomb scattering and the scattering by Mn ion is obtained. The QI significantly affects the exciton-Mn coupling, modifying the splitting of the emission/absorption lines from the exciton-Mn complex depending on the degree of electronic correlations in the exciton state. The second signature of the QI are the nonuniform energy gaps between the consecutive emission peaks due to the scattering of carriers by Mn among single-particle orbitals. Magneto-photoluminescence experiments show that the coupling between the exciton and Mn ion does not change in the magnetic field. We report that electron-hole correlations counteract the magnetic squeezing of the single-particle wave functions strengthening the carrier-Mn interactions. As a result, the rate of change of the magneto-photoluminescence spectra with magnetic field is reduced as observed in the experiment. We develop here for the first time a microscopic theory of bi-exciton-Mn complex, and report the presence of the fine structure of bi-exciton-Mn complex, even though as a spin-singlet it is expected to decouple from the localized spin. Theoretical results are compared with experiments in Grenoble and Warsaw.