Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

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Next-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments. / text

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Photocarrier relaxation mechanisms in CdTe quantum dots in the strong confinement regime were investigated using femtosecond pump-probe measurements. The quantum dots were formed in films deposited on silica substrates using a sequential RF magnetron sputtering process with heat treatment to grow crystallites of various sizes. Size selection was achieved by tuning the laser to various wavelengths across the first excitation transition. The recombination mechanism showed a biexponential decay, which was fitted to a three-level model. It was shown that recombination occurs increasingly through the intermediate energy level as the size of the dots decreases. The nature of the intermediate level and the role of Auger recombination is discussed.

Optical sensing for imaging applications has traditionally been enabled by single-crystalline photodetectors. This approach has dramatically curtailed monolithic integration of a variety of optically-sensitive materials onto silicon read-out circuits. The advent of solution-processed optoelectronic materials such as colloidal quantum dots offers the potential of a revolution in optoelectronics. Their solution-processibility enables low-cost monolithic integration with an arbitrary substrate. This dissertation presents the first high-sensitivity solution-processed photodetectors. It does so by leveraging the high degree of control offered by nanoscale materials engineering. Material processing routes are developed to achieve sufficient carrier mobility and sensitization that lead to high photoconductive gain up to 10^3 A/W, observed for the first time in soft materials. A method to remove charge-transport-inhibiting moieties from the nanocrystal surface is developed. Surface treatment procedures are then advanced to prolong the carrier lifetime and thus sensitize the material. The sequence of these processing stages is crucial for the noise performance of the device. Processing conditions that lead to high photoconductive gain and low noise current are then reported to achieve highly sensitive photodetectors with reported D* on the order 10^13 Jones. The spectral tunability offered by colloidal quantum dots enables monolithic multispectral photodetectors. The material challenges, imposed by the behaviour of matter in the nanoscale, are addressed to report sensitive photodetectors in the visible and infrared parts of spectrum. Carrier lifetime determines the temporal response of a photoconductor. The abundance of trap states on the nanocrystal surface and their associated carrier lifetimes mandate careful attention in order to preserve the trap states that yield temporal response acceptable for imaging applications. It is shown for the first time that the temporal response of a quantum dot photoconductor can be tailored by careful control over surface chemistry. Materials species were identified as responsible for particular photocurrent temporal components. These findings are then exploited to isolate and remove surface species responsible for undesirably long time constants. A solution-processed photoconductive detector is reported that exhibits high sensitivity (D* ~10^12 Jones) and temporal response of 25 ms, suitable for imaging applications.

Solution-processed optoelectronic materials offer a route to low cost photodetectors, large area solar cells, and integrated optical sources. While significant progress has been reported in organic and polymer spin-cast optoelectronics, colloidal quantum dots offer a distinct further advantage -- the convenient tuning of absorption onset via the quantum size effect. Electronic transport has recently been enhanced in size effect tuned colloidal quantum dot films using ligand exchange, resulting in ultrasensitive photodetectors in both visible and infrared wavelengths. Solid-film ligand exchange, however, generally results in rough film morphologies that are incompatible with high uniformity image sensors. Here, we report a new route to visible-wavelength spin-cast lead sulfide (PbS) nanocrystal photoconductive photodetectors with a sub 1% roughness, compared to the ~10% roughness obtained using previously reported approaches. The new procedure yields devices that exhibit 10 A/W responsivities and reveals an added significant advantage: when illumination conditions change, the photodetectors respond with a single time constant of 20 ms. This compares very favorably to the multi second and multi-time-constant response of previously reported PbS-nanocrystal photoconductive photodetectors.

A strategy to ameliorate non-specific adsorption of oligonucleotides onto Quantum Dots (QDs) is investigated where QDs are being used as a platform for the development of optical bioprobes for nucleic acid detection. Certain zwitterionic structures as coatings on QDs have recently been shown to reduce non-specific binding of proteins. In this thesis, a lysine side chain is attached to a dihydrolipoic acid (DHLA) derivative to create a zwitterionic bidentate ligand that has primary amine and carboxyl termini. Such coatings on CdSe/ZnS QDs were studied to assess adsorption of oligonucleotides across a range of pH to better understand the relationship between surface charge and adsorption. The change of pKa of charged terminal groups on QD surfaces for a variety of different water soluble ligands were evaluated using pseudo-titration curves, and indicated up to 3 orders of magnitude shift of pKa at a QD surface in comparison to pKa in bulk solution.