Fang, Qunling. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 168-170). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Surface passivation of semiconductor quantum dots is essential to preserve their efficient and robust light emitting properties. By using a lattice matched (mismatch = 0.5%) lead halide perovskite matrix, we achieve shell-like passivation of lead sulfide QDs in crystalline films, leading to efficient infrared light emission. These structures are made from a simple one-step spin coating process of an electrostatically stabilized colloidal suspension. Photoluminescence and transient absorption spectroscopy indicate rapid energy transfer between the perovskite matrix and the QDs, suggesting an interface with few trap states. In addition to housing the efficient infrared QD emitters, lead halide perovskites themselves have good carrier mobilities and low trap densities, making these solution-processable heterostructures an attractive option for electrically pumped light emitting devices. The highest performing quantum dots for visible light applications are CdE (E=chalcogenide) core/shell heterostructures. Again, surface passivation plays a huge role in determining the brightness and robustness of visible QD emitters. Multilayer shell passivation is usually used to produce the highest quantum yield particles. Surface trap states are shown to be detrimental to luminescence output, even in thick-shelled particles. Spherical quantum wells allow for thicker shells and with good surface passivation, show promising reduction of biexciton auger recombination, as measured by a time correlated single photon counting (TCSPC) microscope. TCSPC methods were used to diagnose and identify QD architectures for LED applications and explore fundamental recombination dynamics using photon antibunching measurements, and statistical analysis of blinking traces.Introducing new surfaces onto graphitic substrates can be a useful for introducing new electronic properties, patterning device-specific geometries, or appending molecular catalysts. Metal nanoparticles were used to act as a catalyst for the gasification and etching of graphite and graphene. Several methods of controlling the initiation, propagation, and density of these trenches were explored. Patterning defects helped control where initiation occurred, while faceting existing defect sites could also enable more facile initiation and control the direction at the beginning of etching, due to the wetting mechanism of particle movement. Patterning the metal also was shown as a promising avenue to limit unwanted gasification and promote etching in specific, patterned regions. Surface functionalization using reactive gases was performed and characterized with outlook for future experiments.

Some people believe that IC densities are approaching the fundamental limits inherent to semiconductor technologies. One alternative to semiconductors is Quantum-dot Cellular Automata (QCA); QCA is a nanotechnology that offers the potential to build denser IC's that switch at higher frequencies and run on lower power. QCA's most basic building block, the QCA cell, is inherently binary; digital circuits are implemented by arranging these QCA cells in pre-defined configurations on a two dimensional plane. This paper proposes a logic formulation that describes arranging QCA cells on a two dimensional plane; it is presented as a set of rules that can be implemented with basic Boolean variables and operators. This Boolean formulation is general and can be applied to any given specification. In addition, an optimization constraint is defined so that the logic formulation will only validate the most efficient QCA cell arrangements. The correctness of the logic formulation has been empirically verified by testing it with a SAT solver. The effectiveness of the minimization constraint in conjunction with the logic formulation has been tested with a Pseudo-Boolean ILP solver.

Photovoltaic (PV) devices based on PbS quantum dot (QD) solids demonstrate high photon-to-electron conversion yields. However, record power conversion efficiencies remain limited mainly due to bulk and interfacial defects in the light absorbing material (QD solids). Interfacial defects can be formed when a semiconductor, such as QD solid, is contacted by another material and may predetermine the semiconductor/metal or semiconductor/metal-oxide junction properties. The objective of the work described in this dissertation was set to explore whether electrochemical contacting using liquid electrolytes can provide sufficient means of contacting the QD solids to investigate their PV performance without introducing the unwanted interfacial defects. I have initially focused on optimizing processing conditions for efficient QD solids deposition and studied their photovoltaic properties in a standardized solid-state, depleted heterojunction solar cell configuration. Further, a liquid contacting method was developed to study the relationship between photovoltages of QD solids and the energetics (e.g. reduction potentials) of the liquid contacting media. This electrochemical contacting of PbS QD solids was achieved by using anhydrous liquid electrolytes containing fast, non-coordinating, outer-sphere redox couples. Depending on the energetics of a redox couple, both rectifying and non-rectifying (Ohmic) PbS QD solid/electrolyte junctions were successfully formed with both p- and n-type QD solids. Furthermore, application of the liquid solution contacting method in studies of the PbS QD solids has unprecedentedly demonstrated that an ideal behavior of the photovoltage changes with respect to the changes in the energetics of the contacting media can be achieved. This fact supports the initially proposed hypothesis that such liquid contacting method will not introduce surface defects to the studied QD materials, allowing for their intrinsic properties to be better understood. The applicability of this method to both p- and n- type QD solids was demonstrated. Finally, a better understanding of the relationships between the surface and ligand chemistries of both p- and n-type QD solids and their photovoltaic properties was possible via applications of such method in conjunction with XPS and UPS studies.

Quantum Dots (QD) have become a commercial reality for tunable displays and light-emitting diodes. The Department of Energy believes further improvements in efficacy and stability will allow for widespread adoption of solid-state lighting in the United States. QD geometric and compositional architecture, crystal phase and surface chemistry are arguably some of the important aspects governing QD performance in these applications. Chapter I outlines the efforts of QD design, encapsulation and performance for phosphor converted, “on-chip” LEDs. Cadmium chalcogenide QDs with a quantum well geometry and ZnS encapsulation (CdS/CdSxSe1-x/CdS/ZnS) resist photoluminescence bleaching on chip under harsh accelerated ageing tests. Trends in device performance are linked primarily to success of ZnS passivation. Chapter II presents findings regarding crystal structure control (Zinc Blende vs Wurtzite) for CdX (X = S, Se) systems by focusing on crystal phase conversion. The ZB to W transition for CdX is shown to be size, material and surfactant dependent. Chapter III focuses on expanding the precursor compound library for CdSe with aryl substituted cyclic selenones (imidazole and pyrimidine-based compounds). These molecules are shown to react sluggishly at ZB synthetic conditions and that the rate is heavily influenced by compound sterics. Chapter IV presents the findings of a metal carboxylate displacement study on PbS NCs with various L-type ligands. Upon displacement and purification with N,N,N′,N′-tetramethylethylene-1,2-diamine, tri-n-butylamine, and n-octylamine, oriented attachment occurs along the 100 plane and with bis(dimethylphosphino)ethane and tri-n-butylphosphine, attachment is suppressed. This difference allows for the study of ligand density dependent optical properties without the confounding attachment of nanocrystals in solution. A decreasing trend of time resolved photoluminescence lifetime values as a function of ligand density is observed.

<p>Quantum dots are inorganic nanocrystals of semiconductor metals that have unique light emitting properties. Due to their tunable and narrow emission profile, broad absorption spectra, resistance to photobleaching and high level of brightness they have emerged as inorganic fluorophores and numerous applicabilities for in vitro, in situ as well as in vivo studies are present. The chemical nature of the quantum dot surface needs to be altered in order to make the inorganic nanoparticles applicable to biological systems. Water soluble and biocompatible particles that limit unspecific binding to proteins can be obtained through functionalization of the surface coating with appropriate molecules.</p><p> </p><p>In this pilot study, two surface modification strategies were performed upon two commercially available quantum dots in order to attach the zwitterionic molecules L-cysteine and thiolated sulfobetaine methacrylate, both shown to create non-fouling and biocompatible surfaces.</p><p> </p><p>A biphasic exchange method was successfully used to perform ligand exchange of Qdot® ITK™ Organic Quantum Dots (QD-Organic) in order to exchange the structurally unknown, native lipophilic coating to one consisting of the amino acid L-cysteine (QD-Cysteine). The quantum dots transferred from the organic to the aqueous phase after the natively hydrophobic coating was changed to the hydrophilic L-cysteine. A characteristic mass fragment of protonated trioctylphosphine oxide (TOPO) was found for QD-Organic, using TOF-SIMS, suggesting TOPO is a part of the native coating. Further, the mentioned mass fragment was no longer present after the exchange. The C (1s) XPS-spectrum showed a new peak for carboxylic carbon, characteristic for L-cysteine, and expected changes in elemental composition were consistent with measured changes for all relevant elements. Large amounts of buffer remained after purification, suggesting the purification protocol needs further evaluation. Traces of the native coating were found in the C (1s) XPS-spectrum for QD-Cysteine, indicating not all ligands were exchange.    <em></em></p><p> </p><p>Additionally, a strategy for surface functionalization of Qdot® 655 ITK™ amino (PEG) quantum dots (QD-PEG-NH₂) with L-cysteine and thiolated sulfobetaine methacrylate was outlined and performed, using Michael addition and the heterobifunctional linker 3-Maleimidobenzoic acid <em>N</em>-hydroxysuccinimide ester. Unfortunately, no indications of successful attachment of the linker to the quantum dot have been found, neither by TOF-SIMS nor XPS, and thus functionalization with L-cysteine and tSBMA was not achieved. In theory, the proposed coupling chemistry used during the pilot study is promising, but further experiments are needed to obtain a successful and optimized protocol for the functionalization. <strong></strong></p>

Quantum dots (QDs) of II-VI semiconductors (CdS, CdSe, and CdTe) in the size range of 1~12 nm have attracted great interest in both fundamental research and technical applications in recent years. Due to their tunable size-dependent emission with high photoluminescence quantum yields, their broad excitation spectra and narrow emission bandwidths, the semiconductor QDs have been intensively investigated in versatile applications, including thin-film light emitting devices (LEDs), low-threshold lasers, optical amplifier media for telecommunication networks and biological labels. Thus, constructing and fabricating highly ordered QDs are of great importance in the field of nanotechnology. The surface chemistry behavior of the TOPO-CdSe QDs and TOPO-(CdSe)ZnS QDs at the air-water interface was carefully examined by various physical measurements. The surface pressure-area isotherms of the Langmuir monolayers of both types of QDs gave the average diameter which matched the value determined by TEM measurements. Topographic study of the Langmuir monolayers of both QDs revealed the 2D aggregation during the early stage of the compression process. The stability of the Langmuir monolayer of the TOPO-(CdSe)ZnS QDs was measured by the compression/decompression cycle and the kinetic measurements, both of which indicated that TOPO capped (CdSe)ZnS QDs can form stable Langmuir monolayers at the air-water interface. Langmuir-Blodgett (LB) film of the TOPO-(CdSe)ZnS QDs were prepared on quartz slides at different surface pressures and characterized by photoluminescence (PL) spectroscopy. The linear increase of the PL intensity with the increase of the number of layers deposited onto the quartz slide implied a homogeneous deposition of the Langmuir monolayer. The conjugates of 10, 12-pentacosadiynoic acid (PDA) and short chain peptide was used to modify the surface of (CdSe)ZnS core-shell QDs. The PDA-peptide capped QDs formed stable Langmuir monolayer. After the photopolymerization of PDA-peptide-QDs/PDA-peptide system at the air-water interface, a more uniform and robust Langmuir monolayer was constructed. The 3-mercaptopropyltrimethoxysilane (MPS) was linked to (CdSe)ZnS QDs by ligand exchange method. The sol-gel process of the MPS capped QDs Langmuir monolayer was studied under various subphases of pH and reaction time. The fast sol-gel process under a subphase of pH 12.0 led the formation of a more homogeneous Langmuir monolayer. A smooth MPS-QDs LB film deposited under pH 12.0 was also observed by AFM measurements. The imaging of the aggregates of lysozyme using lysozyme/(CdSe)ZnS QDs conjugate as a PL label was investigated. The amyloid fibrils formed by lysozyme/lysozyme-QDs conjugate were observed by epifluorescence microscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements. The emission intensity of the QDs labeled lysozyme was increased about 3 fold after formation of amyloid. This approach, for the first time, provided a convenience method to image the amyloid fibrils by epifluorescence microscopy.

Nanotechnology is a term used to describe nanometer scaled systems. This thesis presents various nanomaterials and systems for the investigation of biologically relevant analytes in general, and in particular for their detection, decontamination, or destruction. The validation of short peptide fragments as models for protein aggregation is initially discussed through applying spectroscopic and microscopic techniques to Langmuir monolayer surface chemistry. Following this validation, the use of nanogold as a photoablative material for the destruction of aggregated protein is investigated. Subsequently, the versatility of nanotechnology is shown by investigating a different form of nanogold; namely, gold quantum dots and the interesting phenomenon that arise when dealing with materials on a nanoscale. Experiments involving a complex between these gold quantum dots and an antibody are performed for the detection of an immunoglobulin in solution. The power of this analytical technique is highlighted by the capability of detecting the analyte at nanomolar concentrations. Finally, a limitation-the multiple synthetic steps necessary for imparting biological activity-- of quantum dots is addressed: a single step reaction is studied that allows for direct stabilization and conjugation of quantum dots with proteins and enzymes. As a representative application of the above mentioned procedure, the detection and decontamination of an organophosphorus compound is explored. In general, methods for overcoming limitations of nanoparticles and nanocrystals are discussed.

Invented in the 1950s, gel electrophoresis has now become a routine analytical method to verify the size of nucleic acids and proteins in molecular biology labs. Conventional gel electrophoresis can successfully separate DNA fragments from several base pairs to a few tens of kilo base pairs, beyond which a point is reached that DNA molecules cannot be resolved due to the size independent mobility. In this case, pulsed field gel electrophoresis (PFGE) was introduced to extend the range of DNA fragment sizes that can be effectively separated. But despite the incredible success of PFGE techniques, some important drawbacks remain. First, separation time is extremely long, ranging from several hours to a few days. Second, detection methods still rely on staining the gel after the run. Real time observation and study of band migration behavior is impossible due to the large size of the PFGE device. Finally, many commercial PFGE instruments are relatively expensive, a factor that can limit their accessibility both for routine analytical and preparative use as well as for performing fundamental studies. In this research, a miniaturized PFGE device was constructed with dimension 2cm x 2.6cm, capable of separating DNA fragments ranging from 2.5kb to 32kb within three hours using low voltage. The separation process can be observed in real time under a fluorescence microscope mounted with a cooled CCD camera. Resolution and mobility of the sample were measured to test the efficiency of the device. We also explored manipulating DNA fragments by end labeling DNA molecules with quantum dot nanocrystals. The quantum dot-DNA conjugates can be further modified through binding interactions with biotinylated single-stranded DNA primers. Single molecule visualization was performed during gel electrophoresis and the extension length, entanglement probability and reorientation time of different conjugates were measured to study their effect on DNA migration through the gel. Finally, electrophoresis of DNA conjugates was performed in the miniaturized PFGE device, and shaper bands were observed compared with the non end-labeled sample. Furthermore, by end-labeling DNA with quantum dots, the migration distance of shorter fragments is reduced, providing the possibility of separating a wider range of DNA fragment sizes on the same gel to achieve further device miniaturization.

Quantum dots are inorganic nanocrystals of semiconductor metals that have unique light emitting properties. Due to their tunable and narrow emission profile, broad absorption spectra, resistance to photobleaching and high level of brightness they have emerged as inorganic fluorophores and numerous applicabilities for in vitro, in situ as well as in vivo studies are present. The chemical nature of the quantum dot surface needs to be altered in order to make the inorganic nanoparticles applicable to biological systems. Water soluble and biocompatible particles that limit unspecific binding to proteins can be obtained through functionalization of the surface coating with appropriate molecules.   In this pilot study, two surface modification strategies were performed upon two commercially available quantum dots in order to attach the zwitterionic molecules L-cysteine and thiolated sulfobetaine methacrylate, both shown to create non-fouling and biocompatible surfaces.   A biphasic exchange method was successfully used to perform ligand exchange of Qdot® ITK™ Organic Quantum Dots (QD-Organic) in order to exchange the structurally unknown, native lipophilic coating to one consisting of the amino acid L-cysteine (QD-Cysteine). The quantum dots transferred from the organic to the aqueous phase after the natively hydrophobic coating was changed to the hydrophilic L-cysteine. A characteristic mass fragment of protonated trioctylphosphine oxide (TOPO) was found for QD-Organic, using TOF-SIMS, suggesting TOPO is a part of the native coating. Further, the mentioned mass fragment was no longer present after the exchange. The C (1s) XPS-spectrum showed a new peak for carboxylic carbon, characteristic for L-cysteine, and expected changes in elemental composition were consistent with measured changes for all relevant elements. Large amounts of buffer remained after purification, suggesting the purification protocol needs further evaluation. Traces of the native coating were found in the C (1s) XPS-spectrum for QD-Cysteine, indicating not all ligands were exchange.      Additionally, a strategy for surface functionalization of Qdot® 655 ITK™ amino (PEG) quantum dots (QD-PEG-NH2) with L-cysteine and thiolated sulfobetaine methacrylate was outlined and performed, using Michael addition and the heterobifunctional linker 3-Maleimidobenzoic acid N-hydroxysuccinimide ester. Unfortunately, no indications of successful attachment of the linker to the quantum dot have been found, neither by TOF-SIMS nor XPS, and thus functionalization with L-cysteine and tSBMA was not achieved. In theory, the proposed coupling chemistry used during the pilot study is promising, but further experiments are needed to obtain a successful and optimized protocol for the functionalization.