This thesis contains a theoretical and experimental investigation of semiconductor quantum dots (artificial atoms). The first part presents a numerical study of spin effects in GaAs/AlAs modulation-doped quantum dots containing 0 to 50 electrons. A theoretical model is developed to calculate confinement potentials and ground-state electron density distributions using the Kohn-Sham local spin-density approximation. Spin polarization, defined as the difference between the up- and down-spin electron densities, is predicted to occur spontaneously in symmetric quantum dots and in quantum point contacts in the lowdensity regime as a result of electron exchange interactions. This spontaneous magnetization can be controlled by an applied gate voltage, which opens up applications in spintronics and provides a possible explanation for the non-integer quantization of the quantum point contact conductance. The second part describes experimental techniques to investigate photon-exciton coupling in InAs/GaAs self-assembled quantum dots. Two experiments on resonant excitation of a single quantum dot are proposed, whereby the quantum-dot emission is distinguished from resonant pump light by either photon bunching of collected photons with reference photons, or Michelson interferometry. The feasibility study of the proposed experiments shows that the photon-exciton coupling efficiency must be dramatically increased by putting the quantum dot inside an optical microcavity. Novel types of high-quality, low mode-volume semiconductor microcavities containing quantum dots are designed, fabricated, and studied on a newly built setup. We present the first results of photoluminescence studies of InAs quantum dots inside both GaAs single-defect square-lattice photonic-crystal slabs and GaAs/AlAs micropillars, and InAs artificial molecules formed by vertically coupled strain-assisted quantum dots. The results indicate the potential of these nanostructures for implementing resonant transfer of quantum information, developing quantum repeaters and entangled-photon sources, and studying QED effects in the strong-coupling regime.

The unique optical properties of quantum dots (QDs) have been widely used to develop bioassays based on Fluorescence Resonance Energy Transfer (FRET). The solid-phase assays using QDs as FRET donors have numerous practical advantages, including at least 10-fold enhancement in FRET efficiency, which is not immediately explained by theoretical predictions that model energy transfer processes of QDs in two-dimensional layers. Donor-acceptor separation distance, acceptor and donor concentrations were found to influence FRET efficiency in solid-phase assays. A novel immobilization strategy was implemented which made use of the high affinity of imidazole moieties to QD shells to build solid-phase QD bioassays. A 96-well polystyrene plate is presented as a platform suitable for rapid and convenient multiplexed detection. A typical microtiter plate reader was shown to be capable of discriminating different FRET pairs to picomol detection levels of target oligonucleotides. Furthermore, the QD-FRET bioassays provided for mismatch discrimination, and multiple cycles of regeneration were also demonstrated.

The unique optical properties of quantum dots (QDs) have been widely used to develop bioassays based on Fluorescence Resonance Energy Transfer (FRET). The solid-phase assays using QDs as FRET donors have numerous practical advantages, including at least 10-fold enhancement in FRET efficiency, which is not immediately explained by theoretical predictions that model energy transfer processes of QDs in two-dimensional layers. Donor-acceptor separation distance, acceptor and donor concentrations were found to influence FRET efficiency in solid-phase assays. A novel immobilization strategy was implemented which made use of the high affinity of imidazole moieties to QD shells to build solid-phase QD bioassays. A 96-well polystyrene plate is presented as a platform suitable for rapid and convenient multiplexed detection. A typical microtiter plate reader was shown to be capable of discriminating different FRET pairs to picomol detection levels of target oligonucleotides. Furthermore, the QD-FRET bioassays provided for mismatch discrimination, and multiple cycles of regeneration were also demonstrated.

Silicon quantum dots (QDs) in SiO2 superlattices were fabricated by alternate deposition of silicon oxide (SiO2) and silicon-rich oxide (SRO), i.e. SiOx (x<2), and followed by high temperature annealing. A deposited SRO film is thermodynamically unstable below 1173oC and phase separation and diffusion of Si atoms in the amorphous SiO2 matrix creates nano-scaled Si quantum dots. The quantum-confined energy gap was measured by static photoluminescence (PL) using an Argon ion laser operating at 514.5 nm. The measured energy band gaps of crystalline Si QDs in SiO2 matrix at room temperature (300 K) show that the emission energies from 1.32 eV to 1.65 eV originating Si dot sizes from 6.0 nm to 3.4 nm, respectively. There is a strong blue-shift of the PL energy peak position with decreasing the quantum dot size and this shows the evidence of quantum confinement of our fabricated Si QDs in SiO2 matrix. The PL results indicate that the fabricated Si QDs in SiO2 matrix could be suitable for the device application such as top cell material for all-silicon tandem solar cells. Silicon QD superlattices in nitride matrix were fabricated by alternate deposition of silicon nitride (Si3N4) and silicon-rich nitride (SRN) by PECVD or co-sputtering of Si and Si3N4 targets. High temperature furnace annealing under a nitrogen atmosphere was required to form nano-scaled silicon quantum dots in the nitride matrix. The band gap of silicon QD superlattice in nitride matrix (3.6- 7.0 nm sized dots) is observed in the energy range of 1.35- 1.98 eV. It is about 0.3- 0.4 eV blue-shifted from the band gap of the same sized quantum dots in silicon oxide. It is believed that the increased band gap is caused by a silicon nitride passivation effect. Silicon-rich carbide (SRC, i.e. Si1-xCx) thin films with varying atomic ratio of the Si to C were fabricated by using magnetron co-sputtering from a combined Si and C or SiC targets. Off-stoichiometric Si1-xCx is of interest as a precursor to realize Si QDs in SiC matrix, because it is thermodynamically metastable when the composition fraction is in the range 0 < x < 0.5. Si nanocrystals are therefore able to precipitate during a post-annealing process. SiC quantum dot superlattices in SiC matrix were fabricated by alternate deposition of thin layers of carbon-rich silicon carbide (CRC) and SRC using a layer by layer deposition technique. CRC layers were deposited by reactive co-sputtering of Si and SiC targets with CH4. The PL energy band gap (2.0 eV at 620 nm) from 5.0 nm SRC layers could be from the nanocrystalline ??-SiC with Si-O bonds and the PL energy band gap (1.86 eV at 665 nm) from 6.0 nm SRC layers could be from the nanocrystalline ??-SiC with amorphous SiC clusters, respectively. The dielectric material for an all-silicon tandem cell is preferably silicon oxide, silicon nitride or silicon carbide. It is found that for carrier mobility, dot spacing for a given Bloch mobility is in the order: SiC > Si3N4 > SiO2. By ab-initio simulation and PL results, the band gap for a given dot size is in the order: SiC > Si3N4 > SiO2. However, the PL intensity for a given dot size is in the order: SiC < Si3N4 < SiO2.

Semiconductor nanocrystals exhibit unique optical and physical properties that make them an attractive alternative to organic dyes for fluorescent bioapplications. Although significant advances have been made since their first reported use in biology a decade ago, it still remains a challenge to prepare high quality, biocompatible semiconductor nanocrystals. / In this thesis, studies are described with the aim to prepare robust, biocompatible semiconductor nanocrystals that exhibit each of the properties necessary for their implementation in biological applications. Two different approaches were investigated: ligand exchange and polymer encapsulation, and advances in each are presented. A heterobifunctional ligand suitable for bioconjugation, carboxyl terminated dihydrolipoic acid poly(ethylene glycol) (DHLA-PEG-COOH), was synthesised and characterised to prepare water-soluble, biocompatible semiconductor nanocrystals via ligand exchange. It was found that nanocrystals transferred into water using DHLA-PEG-COOH exhibit the same optical properties and colloidal stability as those prepared using DHLA-PEG. It was demonstrated that the surface charge of the nanocrystals may be controlled by altering the ratio of DHLA-PEG:DHLA-PEG- COOH ligands. In a different approach, colloidally stable, biocompatible nanocrystals were prepared via polymer encapsulation. It was found that by employing a low molecular weight polymer, biocompatible nanocrystals that exhibit a small hydrodynamic diameter could be realised. / Experimental results are presented on the conjugation of biocompatible nanocrystals to protein targets. It was found that while standard coupling chemistries yield protein-dye conjugates, these chemistries did not result in protein-nanocrystal conjugates. In order to overcome the drawbacks of standard coupling chemistries, which are susceptible to hydrolysis, a novel conjugation scheme utilising copper-free click chemistry is proposed. / Finally, the success of nanocrystals in bioapplications depends on the ability to characterise nanocrystal-protein conjugates. By means of analytical ultracentrifugation, data on the sedimentation properties of nanocrystals and nanocrystal-protein conjugates was obtained. Analysis of these data provided information on fundamental physical properties of biocompatible nanocrystals and nanocrystal-protein conjugates, in particular the core crystal size, hydrodynamic size, number of surface ligands and nanocrystal:protein stoichiometry. Such a precise, comprehensive characterisation of nanocrystals in general, and nanocrystal-protein conjugates in particular, will greatly facilitate their use in bioapplications.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.M.S.E.)--University of Delaware, 2007. / Principal faculty advisor: Valeria Gabriela Stoleru, Dept. of Materials Science & Engineering. Includes bibliographical references.

Thesis (Ph.D.). / Written for the Dept. of Chemistry. Title from title page of PDF (viewed 2008/07/23). Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 117-126).

Thesis (doctoral)--Technische Universität München, 2004. / Includes bibliographical references (p. 149-156).