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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.

<p>The control of matter and phenomena at the nanoscale is fast becoming one of the most important challenges of the 21st century with wide-ranging applications from energy and health care to computing and material science. Conventional top-down approaches to nanotechnology, having served us well for long, are reaching their inherent limitations. Meanwhile, bottom-up methods such as self-assembly are emerging as viable alternatives for nanoscale fabrication and manipulation.</p><p>A particularly successful bottom up technique is DNA self-assembly where a set of carefully designed DNA strands form a nanoscale object as a consequence of specific, local interactions among the different components, without external direction. The final product of the self-assembly process might be a static nanostructure or a dynamic nanodevice that performs a specific function. Over the past two decades, DNA self-assembly has produced stunning nanoscale objects such as 2D and 3D lattices, polyhedra and addressable arbitrary shaped substrates, and a myriad of nanoscale devices such as molecular tweezers, computational circuits, biosensors and molecular assembly lines. In this dissertation we study multiple problems in the theory, simulations and experiments of DNA self-assembly. </p><p>We extend the Turing-universal mathematical framework of self-assembly known as the Tile Assembly Model by incorporating randomization during the assembly process. This allows us to reduce the tile complexity of linear assemblies. We develop multiple techniques to build linear assemblies of expected length N using far fewer tile types than previously possible.</p><p>We abstract the fundamental properties of DNA and develop a biochemical system, which we call meta-DNA, based entirely on strands of DNA as the only component molecule. We further develop various enzyme-free protocols to manipulate meta-DNA systems and provide strand level details along with abstract notations for these mechanisms. </p><p>We simulate DNA circuits by providing detailed designs for local molecular computations that involve spatially contiguous molecules arranged on addressable substrates via enzyme-free DNA hybridization reaction cascades. We use the Visual DSD simulation software in conjunction with localized reaction rates obtained from biophysical modeling to create chemical reaction networks of localized hybridization circuits that are then model checked using the PRISM model checking software.</p><p>We develop a DNA detection system employing the triggered self-assembly of a novel DNA dendritic nanostructure. Detection begins when a specific, single-stranded target DNA strand triggers a hybridization chain reaction between two distinct DNA hairpins. Each hairpin opens and hybridizes up to two copies of the other, and hence each layer of the growing dendritic nanostructure can in principle accommodate an exponentially increasing number of cognate molecules, generating a nanostructure with high molecular weight. </p><p>We build linear activatable assemblies employing a novel protection/deprotection strategy to strictly enforce the direction of tiling assembly growth to ensure the robustness of the assembly process. Our system consists of two tiles that can form a linear co-polymer. These tiles, which are initially protected such that they do not react with each other, can be activated to form linear co-polymers via the use of a strand displacing enzyme.</p> / Dissertation

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<p>Silicon-based dielectric thin films including amorphous hydrogenated aluminium-doped silicon oxides (<em>a-</em>SiAl_xO_y:H), amorphous hydrogenated silicon nitrides (<em>a-</em>SiN_x:H), and amorphous hydrogenated silicon carbides (<em>a-</em>SiC_x:H) were deposited by remote plasma chemical vapour deposition (RPECVD) techniques including electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD) and inductively-coupled-plasma chemical vapour deposition (ICP-CVD) on silicon (Si) wafers, soda-lime glass microscope slides, and glassy carbon (C) plates. Aluminium (Al) in the SiAlO films was incorporated by way of a metalorganic Al(TMHD)₃ precursor.</p> <p>Thickness, refractive index, and growth rate of the films were measured using variable angle spectroscopic ellipsometry (VASE). Film composition was measured using energy dispersive X-ray spectroscopy (EDX) for the SiAlO films and Rutherford backscattering spectrometry (RBS) for the SiC_x films. Elastic modulus and hardness of the SiAlO and SiC_x films were measured using nanoindentation and their adhesion was characterized via progressive load scratch testing.</p> <p>All films were observed to be optically transparent at near-IR and red wavelengths with many SiN_x and SiC_x films exhibiting significant optical absorption above 2.25eV. Modification of a previously developed deposition recipe produced doubled growth rates in SiN_x and SiC_xfilms. SiAlO films were produced with up to 1.6±0.1at% aluninium (Al) incorporation, while SiC_x films with composition ranging from SiC_0.25:H to SiC₂:H could be produced depending on the growth gas flow ratios. SiAlO films exhibited hardness and reduced modulus (<em>H</em> and <em>E</em>) up to 8.2±0.4 and 75±2GPa, respectively; <em>H </em>and <em>E</em> for the SiC_xfilmsreached 11.9±0.2 and 87±3 GPa. Initially, adhesion to Si wafers was extremely poor with films delaminating at loads of 1.5±0.3N when scratched with a 3/16” alumina (Al₂O₃) sphere; implementation of a rigorous pre-deposition surface cleaning procedure produced films showing only cracking and no delamination up to 30N loads vs. a 200μm radius Rockwell C diamond stylus.</p> / Master of Applied Science (MASc)

<p> Low-k dielectrics have been incorporated into advanced computer chip technologies as a part of the continuous effort to improve computer chip performance. One drawback associated with the implementation of low-k dielectrics is the large leakage current which conducts through the material, relative to silica. Another drawback is that the breakdown voltage of low-k dielectrics is low, relative to silica [1]. This low breakdown voltage makes accurate reliability assessment of the failure mode time dependent dielectric breakdown (TDDB) in low-k dielectrics critical for the successful implementation of these materials. The accuracy with which one can assess this reliability is currently a topic of debate. </p><p> These material drawbacks have motivated the present work which aims both to contribute to the understanding of electronic conduction mechanisms in low-k dielectrics, and to improve the ability to experimentally characterize changes which occur within the material prior to TDDB failure. What follows is a study of the influence of an applied magnetic field on the conductivity of a low-k dielectric, or in other words, a study of the material&rsquo;s magnetoresistance. </p><p> This study shows that low-k dielectrics used as intra-level dielectrics exhibit a relatively large negative magnetoresistance effect (&sim;2%) at room temperature and with modest applied magnetic fields (&sim;100 Oe). The magnetoresistance is attributed to the spin dependence of trapping electrons from the conduction band into localized electronic sites. Mixing of two-electron spin states via interactions between electron spins and the the spins of hydrogen nuclei is suppressed by an applied magnetic field. As a result, the rate of trapping is reduced, and the conductivity of the material increases. </p><p> This study further demonstrates that the magnitude of the magnetoresistance changes as a function of time subjected to electrical bias and temperature stress. The rate that the magnetoresistance changes correlates to the intensity with which the material was stressed. It is postulated that the change in magnetoresistance which occurs as a result of bias temperature stress could be used as an alias for measuring the degradation which contributes to TDDB. </p><p> Finally, it is shown that the magnetoresistance behavior is non-monotonic. That is, for small values of applied magnetic field (&sim;2 Oe) the conductivity initially decreases, while for further increase of the magnetic field the conductivity increases to a saturation. The non-monotonic behavior is consistently described in the context of competing spin mixing mechanisms.</p>

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published_or_final_version / Physics / Doctoral / Doctor of Philosophy