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The field of nanoscience and nanotechnology has achieved significant progress over last thirty years. Complex nanostructures with tunable properties for novel applications have been successfully fabricated and characterized. In this thesis, I will focus on our recent efforts on precise controlled synthesis of zero-dimensional nanostructures as well as fundamental understanding of the physical behavior of as-synthesized nanostructures. Particularly, three topics are presented: (1) Nanoscale crystallinity engineering: we have achieved nanoscale crystallinity control of noble metal nanoparticles with 100% yield by molecular engineering. We have used silver nanoparticles as example to demonstrate synthetic strategy and importance of such control in nanoscale chemical transformation, fundamental electron and phonon couplings and surface pla...
One- dimensional (1D) nanostructures have received growing interest due to their unique physical and chemical properties and promising nanodevice applications, as compared with their bulk counterparts. Complex 1D nanostructures with tunable properties and functionalities have been successfully fabricated and characterized in this thesis. I will show our recent efforts on precise controlled 1D nanostructures by template- assisted electrochemical synthesis as well as fundamental understanding of their physical behavior and growth mechanism of as-synthesized nanostructures. Particularly, three topics are presented: Firstly, a constant current (CC) based anodization technique is newly demonstrated to fabricate and control the structure of an anodic aluminum oxide (AAO) template. This technique has enabled the formation of long- range self-...
The field of nanoscience and nanotechnology has made significant progresses over the last thirty years. Sophisticated nanostructures with tunable properties for novel physics and applications have been successfully fabricated, characterized and underwent practical test. In this thesis, I will focus on our recent efforts to develop new strategies to manipulate the properties of nanostructures. Particularly, three questions have been answered from our perspective, based on the nanomaterials synthesized: (1) How does the composition affect a novel nanostructure? We started from single-molecule precursors to reach nanostructures whose bulk counterparts only exist under extreme conditions. Fe3S and Fe3S2 are used as examples to demonstrate this synthetic strategy. Their potential magnetic properties have bee...
Cleansing effect Scarring effect Creative destruction Learning Demand shocks
Geographic Information Systems (GISs) are attracting more and more interest. For geographically distributed data, value-by-area cartograms provide a highly expressive visualization. Continuously changing CIS spatiotemporal data can be animated by cartogram animation. We propose several value-by-area cartogram animation methods, and a new algorithm for creating single value-by-area cartograms. Our algorithms provide highly expressive animations for GIS spatiotemporal databases. ^ We describe an O(n) time approximate algorithm, which can transform a sequence of n time series data points into a linear constraint database. We also describe how the approximation enables more efficient cartogram animations, as well as more efficient evaluation of database queries. The approximate evaluation has high recall and precision on queries. ^ We ...
This dissertation explores the interactions between resource reallocation, productivity dynamics and business cycles. A theory that combines two driving forces for resource reallocation, learning and creative destruction, is presented to reconcile several empirical findings of gross job flows. The theory suggests a scarring effect, in addition to the conventional Schumpeterian cleansing effect, of recessions on the allocation efficiency of resources. I argue that while recessions kill off some of the least productive businesses, they also impede the development of potentially good businesses -- the ones that might have proven to be efficient in the future are cleared out and lose the opportunity to realize their potential. Calibrations of the model using US manufacturing job flows suggest that the scarring effect is likely to dominate ...
Metallic single-walled carbon nanotubes have been proposed to be good one-dimensional conductors. However, the finite curvature of the graphene sheet that forms the nanotubes and the broken symmetry due to the local environment may modify their electronic properties. We used low-temperature atomically resolved scanning tunneling microscopy to investigate zigzag and armchair nanotubes, both thought to be metallic. “Metallic” zigzag nanotubes were found to have energy gaps with magnitudes that depend inversely on the square of the tube radius, whereas isolated armchair tubes do not have energy gaps. Additionally, armchair nanotubes packed in bundles have pseudogaps, which exhibit an inverse dependence on tube radius. These observed energy gaps suggest that most “metallic” single-walled nanotubes are not true metals, and they have implica...
Intramolecular junctions in single-walled carbon nanotubes are potentially ideal structures for building robust, molecular-scale electronics but have only been studied theoretically at the atomic level. Scanning tunneling microscopy was used to determine the atomic structure and electronic properties of such junctions in single-walled nanotube samples. Metal-semiconductor junctions are found to exhibit an electronically sharp interface without localized junction states, whereas a more diffuse interface and low-energy states are found in metal-metal junctions. Tight-binding calculations for models based on observed atomic structures show good agreement with spectroscopy and provide insight into the topological defects forming intramolecular junctions. These studies have important implications for applications of present materials and pr...
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