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.

This thesis is an account of the fabrication of organic field effect transistors using pentacene and two novel oligomers; bis-(4,4'-(biphenyl)ethenyl)phenyl and bis-(4,4'-(octylbiphenyl)ethenyl)phenyl. Organic light emitting diodes were also fabricated using bis-(4,4'-(biphenyl)ethenyl)phenyl and bis-(4,4'-(octylbiphenyl)ethenyl)phenyl in various configurations. A hole field effect mobility of 2.4 cm2-/Vs was observed in pentacene transistor. To our knowledge, this is one of the highest field effect mobilities obtained from a polycrystalline pentacene thin film. bis-(4,4'-(biphenyl)etlhenyl)phenyl displayed extremely poor film forming qualities and did not exhibit any significant field effect. A hole field effect mobility of 0.31 cm2/Vs was achieved using bis-(4,4'-(octylbiphenyl)ethenyl)phenyl. In order to fabricate high quality organic field effect transistors, a self assembled monolayer is usually applied to the gate dielectric prior to the deposition of the organic semiconductor. Three monolayers have been studied: hexamethyldisilazane,n-octyltrichlorosilane, and n-octadecyltrichlorosilane. Through a systematic process, monolayer deposition recipes were developed which resulted in the formation of ultra-smooth surfaces with contact angles of 75-80° for hexamethyldisilazane and 105-110° for the two trichlorosilane derivatives. The formation of ultra-smooth monolayers is critical to fabricating an organic transistor with superior operating characteristics. The oligomers were characterized using cyclic voltammetry to determine the energy levels and differential scanning calorimetry to study their thermal behaviour. Optical absorption and fluorescence spectroscopy was performed to determine the onset of absorption and the main emission wavelengths. The electronic spectra of the oligomers were modeled using semi-empirical quantum mechanical calculations (PM3 and Zindo). Polycrystalline thin films were grown in high vacuum using a Knudsen cell. The substrate temperature was held between room temperature and 225°C. The resulting films were characterized by powder x-ray diffraction and atomic for microscopy. Both of these techniques indicated that the thin films formed a lamellar structure parallel to the substrate surface with the lamella thickness corresponding to the length of the molecule, between 42 and 44 A. The different monolayers did not seem to effect the thickness of the lamella but did increase the size of the grains. Both oligomers were used in single and multi-layer organic light emitting diodes. The single layer organic light emitting diodes displayed faint electroluminescence while the multi-layer devices displayed more intense electroluminescence. The electroluminescence and fluorescence spectra are identical, indicating that recombination occurs solely within the oligomer layer. In the multi-layer organic light emitting diodes, different hole transporting materials such as were used in conjunction with the oligomers, which were employed as the emitter. The devices that generated the most intense electroluminescence were the N,N'-diphenyl-N,N'-bis(3-methylphenyl benzadine)/bis-(4,4'-(octylbiphenyl)ethenyl)phenyl device had a luminance of 174 cd/m2 and the N,N'-diphenyl-N,N'-bis(3-methylphenyl benzadine)/bis(4,4'-(biphenyl)ethenyl)phenyl device was brighter with a luminance of 580 cd/m2.

Microfluidic and nanofluidic technology is revolutionizing experimental practices in analytical chemistry, molecular biology and medicine. Indeed, the development of systems of small dimensions for the processing of fluids heralds the miniaturization of traditional, cumbersome laboratory equipment onto robust, portable and efficient microchip devices (similar to the electronic microchips found in computers). Moreover, the conjunction of scale between the smallest man-made device and the largest macromolecules evolved by Nature is fertile ground for the blooming of our knowledge about the key processes of life. In fact, the conjunction is threefold, because modern computational resources also allow us to contemplate a rather explicit modelling of physical systems between the nanoscale and the microscale. In the five articles comprising this thesis, we present the results of computer simulations that address specific questions concerning the operation of two different model systems relevant to the development of small-scale fluidic devices for the manipulation and analysis of biomolecules. First, we use a Bond-Fluctuation Monte Carlo approach to study the electrophoretic drift of macromolecules across an entropic trap array built for the length separation of long, double-stranded DNA molecules. We show that the motion of the molecules is consistent with a simple balance between electric and entropic forces, in terms of a single characteristic parameter. We also extract detailed information on polymer deformation during migration, predict the separation of topoisomers, and investigate innovative ratchet driving regimes. Secondly, we present theoretical derivations, numerical calculations and Molecular Dynamics simulation results for an electrolyte confined in a capillary of nanoscopic dimensions. In particular, we study the effectiveness of neutral grafted polymer chains in reducing the magnitude of electroosmotic flow (fluid flow induced by an external electric field). Our results constitute the first independent, quantitative verification of theoretical scaling predictions for the coupling between grafted macromolecules and electroosmotic flow. Such simulations will contribute to the rationalization of the existing empirical knowledge about flow control with polymer coatings.

InAs/InGaAsP quantum dots were embedded in laser diode structures grown on (100) InP and tunable lasing has been observed between 1.5 mum and 1.7 mum. We first investigated the dynamics of charge carriers and photons in these laser diodes. It was found that the carriers confined in the quantum dots have discrete energy states like those of a bidimensional harmonic oscillator. The interaction between charge carriers and electromagnetic waves was analyzed through the electrical susceptibility. This has confirmed the possibility of wave amplification if the charge carrier population in the quantum dots is inverted, eventually leading to lasing if enough current is injected. Next, we have studied experimentally three different laser diode structures; their spectral properties and their super linear relation between emitted optical power and injected current demonstrate lasing characteristics. The best optoelectronic performance was obtained for the structure with the deepest charge carrier confinement potential and largest density of quantum dots, but an increase of this second parameter doesn't guarantee an improvement of the performance. At room temperature, a laser threshold current density of 1.1 kA/cm 2 and an external quantum differential efficiency of 9.4% have been measured for this laser diode structure as well as a good internal quantum efficiency of 25% approximately. This efficiency decrease from internal to external was attributed to high internal photon losses with a coefficient of 28 cm-1. Typical increase of the threshold current density with temperature was verified with characteristic temperatures between 52 K and 121 K, but this characteristic temperature abnormally increased above 180 K for two of the three laser diode structures. Spectral tuning potential was assessed by studying the effect of temperature and length of the laser diodes on their lasing wavelength. Promising results incited us to set up an external cavity tunable laser based on our quantum dot laser diodes. A tuning range of 110 nm centered at 1580 nm was reached in a spectral region important for applications of optoelectronic devices in the field of telecommunications.

Ab initio electronic structure calculations are reported which directly relate local chemical and distortion environments of oxygen-coordinated octahedral Fe2+ to corresponding hyperfine parameter distributions in Mossbauer spectroscopy. Changes in the quadrupole splitting (QS) and other properties of the electric field gradient (EFG) with various distortions were investigated on model clusters including the bare octahedra FeO 610- and Fe(OH)64-, and various seven-octahedra sections of an octahedral sheet, through self-consistent charge (SCC) Xalpha ab initio calculations. The percent change in the EFG over the range of distortion parameters found in trioctahedral micas is greatest with flattening for the clusters compared, suggesting that flattening is the most important structural distortion in determining the EFG. The independent parameters of the EFG tensor---particularly the principal value VZZ, the asymmetry parameter eta, and the direction of the principal axis with regards to the octahedral sheet---are examined in detail as functions of octahedral flattening for each of the thirteen possible configurations of Mg2+ and Al3+ cations in the first nearest neighbour octahedra of a Fe2+-centred seven-octahedra cluster. It is demonstrated that the argument that the EFG tensor at a particular site is largely determined by the point group symmetry of the corresponding crystallographic site is not correct. Averages and distributions of eta, principal axis angles and quadrupole splittings are simulated by taking EFG tensor results from all seven-octahedra clusters, and using probabilities for the occurrence of each possible cation configuration in a chemically disordered octahedral sheet of a given bulk composition. Simulations of all hyperfine parameter distributions, including the isomer shift (IS) and the hyperfine magnetic field, were performed using ensembles of Fe(OH)6 4- octahedra generated by applying random distortions to the average structure. Distributions of the QS at 0 and 300 K agree well with experimental results, possessing a low QS tail and a sharp edge at high QS values. This high edge is due to a universal upper bound on the QS, corresponding to the maximum of the QS versus flattening curve. The width-to-mean ratio of the distribution of the nuclear electron spin polarization, the dominant contribution to the hyperfine magnetic field, is large compared to that of the other parameter distributions. The variation of the spin polarization due to local structural variation alone is found to be sufficient to account for the widths and intensities of lines in magnetic hyperfine spectra. The widths of most distributions increase monotonically with the static disorder parameter, and are universal functions of one another, providing a useful constraint in fitting Mossbauer spectra. The correlations between hyperfine parameters are non-linear and considerably more complicated than those supposed in current spectral fitting models. The correlation between QS and isomer shift increases with the amount of disorder in the material. Isomer shift and spin polarization have a strong non-linear correlation. The complexity of the correlations between hyperfine parameters, together with the fact that optimal fits of Mossbauer spectra are not unique, strongly implies that fits to spectra require supporting electronic structure calculations and crystal chemical models of the measured materials for a correct interpretation. It is also shown that published mathematical models of electric field gradient tensor distributions in amorphous materials, derived assuming an isotropic solid, do not describe the distributions in [6]Fe2+ sites in disordered materials, except under special symmetry conditions.

The electronic energy levels of dome-shape InAs self-assembled quantum dots (SAQD) grown by the Stranski-Krastanow mode on GaAs substrates are similar to those obtained from a two-dimensional harmonic-oscillator. A simple selection rule allows transitions only that preserve angular momentum, depicted with atomic-like orbital labels s, p, d, f, etc. This electronic structure was examined with photoluminescence (PL) and photoluminescence excitation (PLE) techniques. As well, in magnetic fields up to 28 Tesla applied parallel to the growth direction, SAQD energy-level degeneracies were lifted. The number of branches observed is correlated to the angular momentum. The ground state (GS) level at zero angular momentum is shifted quadratically under the magnetic field and the behavior could be explained with the Fock-Darwin (F-D) spectral model. The effect of annealing at temperatures from 825°C to 900°C in 25°C steps on the SAQD electronic structure was also examined with the PL technique combined with an applied magnetic field in the Faraday configuration. The PL lines were similar to the F-D spectral lines with their degeneracy lifted by the applied magnetic field. These lines exhibited ten (anti-)crossings: three each at 10 T and 28 T, four at 18 T, while the inter-level spacing and the FWHM were reduced with increasing annealing temperature. Thus an increase in the observed (anti-)crossings resulted for the higher anneal temperatures. The in-plane excitonic reduced-mass was inferred from the systematic splitting of the PL p-branches in a magnetic field. The reduced-mass for all the annealed QD samples was about 0.066 m0 +/- 0.012m0 which decreased slightly with anneal temperature. An 8-band k*p model predicted a similar reduced-mass at low alloying of gallium, but an incorrect trend was observed as the alloying increased with annealing temperature. Unrealistic reduced-masses at 50 percent gallium content were reached. This discrepancy is explained assuming the F-D model is a single (independent) bulk particle picture neglecting many-body effects, and also the k*p model assumes identical disks before and after annealing. The SAQDs were in fact inhomogeneous shallow domes whose height is reduced with annealing temperatures. It is an attempt to reduce the effect of many-body interactions such as exchange, configuration and screened coulomb interactions dominant in the PL technique, the PLE technique was use. In this technique, a single level in a collection or 'ensemble' of dots is excited with tuned laser-light and only the Coulomb interactions are assumed to be important. The PLE peaks were found to be blue-shifted relative to PL peaks. Furthermore, under the influence of a magnetic field, two PLE peaks were observed that corresponded to the p and d energy states. However, three 'd' lines were expected and is hypothesized that one of the d lines remained degenerate. Moreover, the carrier dynamics observed in PLE spectra are much more difficult to interpret than that of the PL spectra. Applying the same method, the analysis of the p-branch peaks suggested an in-plane reduced-mass of ∼0.084m0 +/- 0.002m0, higher than obtained from PL measurement. Since the effective mass is normally associated with the mobility of the carriers, this would imply that the excitons in the PLE measurement are less mobile than in PL. This is despite the reduced many-body effects, suggesting that some extra interactions in the PL excitation may actually enhance the carrier mobility. Given the current interest in devices such as QD infrared photo-detectors and the necessary controls on the number of charge carriers in these devices, a single-layer and 25-layer SAQD samples with doping in the top cap layer were compared to un-doped sample using PLE at various detection energies. No absorption signatures appeared for the doped single layer, whereas they were recovered in the 25-layer doped sample. Evidently either dopants or injected carriers diffused into the QD layers beneath the cap. This diffusion and its influence is expected to be decreasing with depth. Finally, the number of injected charge-carriers in doped GaAs barriers interleaving 50 SAQD layers was studied in order to understand the influence on their electronic structure. From the relation between the dot density and the dopant dose, two to twenty-two charge carriers were estimated to be present in the barriers of each QD. The PLE results indicated that as this number was increased, direct radiative recombination from the higher levels decreased. In addition to Auger scattering and multi-phonon scattering, the enhanced scattering by the dopants impurities appears to add further decay channels toward the lower-energy recombination. This suggests that the PLE technique is sensitive for characterizing the doping effects in SAQD materials. Some fundamental questions regarding the optical and electronic properties of InAs/GaAs SAQD have been answered in this dissertation and the results can be used to support the future development of opto-electronic devices at the nano-scale level.

A source of on-demand single photons for fibre-based quantum information processing applications is highly desirable. To generate single photons at 1.55 mum we use an InAs/InP single quantum dot. Initial measurements to demonstrate the anti-bunched emission from the quantum dot are presented. In order to enhance the emission properties of the quantum dot, it is embedded in a microcavity. A broad variety of micropillar and photonic crystal microcavities are explored in order to find an optimized design to enhance the emission through the Purcell factor. Also, the microcavity output mode has to be suitable to funnel the emitted photons to a communication channel. The microcavity design properties are evaluated by measuring the escaping photons from the embedded high density layer of quantum dots or through complete three-dimensional finite-difference time-domain simulations. A "champion" design using a photonic crystal microcavity is shown to fulfill all the requirements. Record breaking microcavity quality factor (28 000) for an InP-based microcavity is demonstrated. A digital etching technique to tune the photonic crystal microcavity after fabrication is demonstrated. Repetitive removal of an oxide layer formed on the InP with wet chemistry enlarges the photonic crystal holes and reduces the InP layer thickness. Silica nanowire evanescent field coupling is used to probe the mode structure of a microcavity, allowing the extraction of single photons and tuning of the microcavity mode wavelength.

Theoretical studies of the ground state properties of the first-row transition metals (Sc to Zn) are conducted using the Stuttgart TB-LMTO ESC program. The standard deviation of the calculation precision errors (CPE's) and the magnitude of the systematic calculation errors (SCE's) in the output of the TB-LMTO ESC program are estimated. The ground state lattice parameters, local atomic magnetic moment magnitudes and bulk moduli of the first-row transition metals are calculated. Lattice parameters are found to be within 5% to 10% of experimental values, and magnetic moment magnitudes are found to be within 10% to 20% of experimental values. Bulk moduli are found to be within 60% of experimental values. Lattice parameter and magnetic moment magnitude calculations are most accurate when the non-local exchange-correlation functional of Hu and Langreth is used. Bulk modulus calculations are most accurate when conducted using the exchange-correlation functional of Perdew and Yue. The TB-LMTO ESC program is also used to study the volume-controlled low-moment to high-moment (LM-HM) transition of the first-row transition metals (Sc to Ni) when they are constrained to take the FCC crystal structure. A LM-HM transition is predicted to occur in all FCC first-row transition metals if their lattice parameter is sufficiently increased. FCC Fe is found to occupy a unique position in the first-row transition metal series, as its LM-HM transition occurs when its lattice parameter is less than 2.5% larger than its ground state value. The LM-HM transition of the other FCC first-row transition metals occur when their lattice parameters are much larger or much smaller than their ground state values. It is argued that when the lattice parameters of the FCC first-row transition metals are too small, the energy bands of their valence electrons are too wide to allow magnetic moments to form within these metals. A method is proposed for calculating the Helmholtz' free energy of non-magnetic, bulk, crystalline solids consisting of a single chemical species. The method assumes only that an inter-atomic interaction potential can be derived from the minimum total energy versus lattice parameter curve of a solid using results published by Chen, Chen and Wei, and that this potential can accurately reproduce the energy increase that occurs when the atomic nuclei of the solid move away from their equilibrium positions as they undergo small amplitude thermal oscillations. The method is entirely theoretical as the minimum total energy versus lattice parameter curve of a solid can be calculated entirely from first principles using an ESC program. Within the method, the atomic nuclei of a solid are treated in a quasi-harmonic approximation, either as independent harmonic oscillators, or as coupled harmonic oscillators. The thermal expansion of metallic Cu is studied using the proposed method for calculating the Helmholtz' free energy of solids in conjunction with the TB-LMTO ESC program. The lattice parameter versus temperature curve of metallic Cu can be accurately calculated using the method, but its accuracy is sensitive to the functional form of the calculated minimum total energy versus lattice parameter curve of FCC Cu and to the range of the inter-atomic interaction potential. The results of these calculations suggest that the range of the inter-atomic interaction potential extends only to first nearest neighbour atomic nuclei in the metallic Cu solid. These results also suggest that the curvature of the minimum total energy versus lattice parameter curve of FCC Cu is most accurate when the curve is calculated using the exchange-correlation functional of Perdew and Yue even though the ground state lattice parameter of metallic Cu is most accurately predicted using the exchange-correlation functional of Hu and Langreth. However, it may be that imposing a short range to the inter-atomic interaction potential and that using the minimum total energy versus lattice parameter curve obtained with the exchange-correlation functional of Perdew and Yue simply better cancels the errors introduced in the calculation as a result of the quasi-harmonic approximation, as a result of not knowing the correct range of the inter-atomic interaction potential, and as a result of inaccuracies in the calculated minimum total energy versus lattice parameter curve of FCC Cu. Future research efforts should be focussed on the evaluation of the effect of these three factors on the accuracy of the method. We also recommend a direct evaluation of the accuracy of the energy increase that occurs when the atomic nuclei of the solid are displaced slightly from their equilibrium positions, as reproduced using the inter-atomic interaction potential constructed using the results of Chen, Chen and Wei.

Recent advances in nanofabrication technology have made it possible to develop novel quantum opto-electronic devices at the nanometre scale. This thesis demonstrates the ability to precisely position electrostatic gates around the periphery of a deterministically positioned semiconductor quantum dot. This work also demonstrates the first published results of InAs/InP quantum dots subject to electric fields. The InAs/InP material system is particularly attractive for fiber-based quantum cryptography since the ground-state transition can be tuned to the telecommunications wavelength of 1.55 mum. Together, this approach and material system offers a fully scalable route to sources of single photon and entangled photon pairs at telecom wavelengths or arrays of initialized single spins for applications in quantum information science. This thesis presents a detailed study of the electronic and optical properties resulting from electrostatic gating of an individual InAs quantum dot embedded in an InP nanotemplate. The unique device geometry studied here is unprecedented and allows the application of a vertical and an in-plane lateral electric field on the same quantum dot. The results show that application of a vertical electric field along the growth direction precisely controls the charge state or number of electrostatically induced electrons. Important information on the dot morphology is obtained, implying that the InAs dot composition is uniform. In contrast, application of an in-plane lateral electric field across a single quantum dot is shown to strongly modify electron-hole wavefunction overlaps and Coulomb interactions of single excitons and biexcitons while allowing a new optically forbidden transition to appear at finite field involving an s-shell electron and p-shell hole. Of particular significance to the scientific community is the finding that application of the lateral electric field resulted in a crossing of the exciton and biexciton optical transitions, and thus removal of the biexciton binding energy. Theoretical calculations are presented demonstrating that removal of the biexciton binding energy leads to the generation of entangled photon pairs without the need to enforce degeneracy of the two intermediate, single exciton states in the biexciton-exciton radiative cascade. The nanotemplate dimensions are also shown to directly control the biexciton binding energy. A new regime of the biexciton binding energy is studied in larger dots, demonstrating the possibility of entangled photon generation through an 'unbound' biexciton state without the requirement of post-growth tuning.

The steady state and dynamic properties of semiconductor self-assembled island structures made from III-V materials are studied by photoluminescence and time-resolved photoluminescence. The islands layer contains a number of hemispherical caps with a low-gap material inside the cap and a high-gap material outside. The experiments performed are aimed at probing the nature of the bound states inside the islands, looking specifically at the dimensionality of these islands through the mapping of the density of states. First, a system consisting of an $\rm Al\sb{0.45}In\sb{0.55}As$ layer imbedded in an $\rm Al\sb{0.35}Ga\sb{0.65}As$ matrix emitting in the visible (red) is used to directly probe the properties of "individual" islands and compare them with different systems: two-dimensional (quantum well), one-dimensional (quantum wire) and zero-dimensional (quantum dot). The observed temperature independent linewidth and lifetimes are attributed to quantum dot properties. Next, the strong emission obtained under resonant excitation conditions in this system is used to study the influence of phonons on the relaxation processes in zero-dimensional semiconductor heterostructure systems. The other material system studied consists of a single $\rm In\sb{0.5}Ga\sb{0.5}As$ self-assembled layer imbedded in GaAs emitting in the infra-red. A model based on a quantum well with in-plane parabolic confinement is developed and compared with experimental results. Magneto-photoluminescence measurements reveal the symmetry of the electronic shell structure which is found to be consistent with the model developed, thus confirming the zero-dimensional nature of the carrier confinement. The presence of excited states is then exploited by studying the inter-sublevel dynamics of carriers in quantum dots. Despite the discreteness of the density of states, fast inter-sublevel dynamics is observed and all experimental observations are found to be consistent with state-filling dynamics for which the inter-sublevel relaxation is impeded only when lower energy levels are filled.