Our previous computational studies of two-dimensional quantum waveguide structures formed at the interface of the A1GaAs/GaAs heterostructure have focused on systems with centered attractive potential wells. From those studies we direct our attention to the quantum waveguide structures with an attractive potential well placed asymmetrically in the transverse direction. In particular, we are interested in the conductance spectrum for higher energy regimes where Fano resonances are the dominant resonance form. Of interest is the change and progression of Fano resonance peaks as a function of both the potentials' depth and offset, especially as it relates to the Breit-Wigner resonance forms observed in lower energy regimes. To accomplish this, the hard-wall models and Fortran code in our previous work have been expanded to include the asymmetrical positioning through solving the single-electron Schrodinger and associated equations used in the tight-binding Hamiltonian and recursive Green's functions. The observed Fano resonance structures are fitted to their characteristic equations through the use of zero-pole pairs. / Department of Physics and Astronomy

Silver nanowires (Ag NWs) have a wide range of applications in the electronic industry and are attracting growing world-wide interest because of their unique thermal, chemical, electrical and mechanical properties. Understanding of mechanical properties of Ag NWs and joining processes for them at a nano scale is urgently needed to support exploitation of their applications. Particularly, study of processing-structure-property relationships is of much significance. In the present thesis, the following research works were conducted. Ag NWs were synthesized using the polyol method. Joining of individual Ag NWs in an end-to-end orientation at room-temperature without assistance of external pressure was investigated. Selective surface activation of Ag NWs provided surface free of protective organic layers for metallurgical joining. A similar crystal orientation was maintained between the NWs, and diffusion along the boundary contributed to the nanojunction formation. Monocrystalline V-shaped or zig-zag silver prisms were formed after nanojoining, terminated by twin boundaries and free surfaces. The feasibility of room-temperature pressure-free joining of copper (Cu) substrates using Ag NW paste was conducted and demonstrated for flexible electronic packaging applications. The organic content in water-based Ag NW pastes was largely reduced by a repeated washing process to decrease the joining temperature. The formation of end-to-side or side-to-side joints between Ag NWs was observed concurrently with those joined end-to-end. The mechanical and electrical properties of Ag NW joints were examined. It was found that self-generated local heating within the Ag NW paste and Cu substrate system promoted the joining of Ag-to-Ag and Ag-to-Cu without any external energy input. The localized heat energy could be delivered in-situ to the interfaces and promoted atomic diffusion and metallic bond formation while the bulk component temperature maintaining near room-temperature. The organic layer on the side surfaces of the Ag NWs could be broken down through consumption of the residual PVP by a CuO-PVP reaction and which produced localized heating, increasing activated surface sites dramatically and making three-dimensional networks feasible. Ag NWs were introduced into Ag nanoparticle (NP) matrices joined at low-temperature. Joining was facilitated by solid state sintering of the Ag nanomaterials and metallic bonding at Cu-Ag interfaces. It was found that Ag NWs in a Ag NP matrix acted as a second reinforcement phase. In addition to improving the fracture toughness of joints, the introduction of Ag NWs affected the path of fracture propagation, where necking, breakage and pullout of Ag NWs occurred during loading.

There is a growing interest for relying on cleaner and more sustainable energy sources due to the negative side-effects of the dominant fossil-fuel based energy storage and conversion systems. Cleaner, electrochemical energy storage through lithium-ion batteries has gained considerable interest and market value for applications such as electric vehicles and renewable energy storage. However, capacity and rate (power) limitations of current lithium-ion battery technology hinder its ability to meet the high energy demands in a competitive and reliable fashion. Silicon is an element with very high capacity to Li-ion storage although commercially impractical due to its poor stability and rate capabilities. Nevertheless, it has been heavily researched with more novel electrode nanostructures to improve its stability and rate capability. It was found that silicon nanomaterials such as silicon nanowires have inherently higher stability due to mitigation of cracking and higher rate capability due to the short Li-ion diffusion distance. However, electrode compositions based only on silicon nanowires without additional structural features and a high conductive support do not have enough stability and rate capability for successful commercialization. One structural and conductive support of silicon materials studied in literature is graphene. Graphene-based electrodes have been reported as material capable of rapid electron transport enabling new strides in rate capabilities for Li ion batteries. This thesis presents a novel electrode nanostructure with a simple, inexpensive, scalable method of silicon nanwire synthesis on graphene nanosheets via nickel catalyst. The research herein shows the different electrode compositions and variables studied to yield the highest achievable capacity, stability and rate capability performance. The carbon coating methodology in addition to enhancing the 3D conductivity of the electrode by replacing typical binders with pyrolyzed polyacrylonitrile provided the highest performance results.

The objective of this thesis is to explore the synthesis and characterization of high quality binary ZnTe nanowires with great potential for development of optoelectronic devices including high efficiency photovoltaic cells for energy conversion and high sensitivity photodetectors for green fluorescent protein bioimaging at single molecule level. To systematically explore the fabrication process for high quality nanowires, a chemical vapour deposition system was built for nanowire growth. Computational fluid dynamics simulations were used to optimize the reactor and growth parameters. The simulations were validated by experimental measurements. Room temperature photoluminescence measurements showed that high crystal quality with very low defects by single step growth was achieved. This single step growth technique makes a great improvement compared to the reported growth followed by annealing, which achieved equivalent crystal quality. This simplification could be of use in large scale synthesis of nanowires. The simulation results also showed that reactant species concentration is a key factor influencing the growth. A metal-organic chemical vapour deposition system was thus built to independently control reactant concentrations for ZnTe nanowire growth. Temperature-dependent photoluminescence measurements of as-grown ZnTe nanowires showed a strong near band-edge emission. In addition, a deep level oxygen-related band was observed for the first time. From the detailed analysis of thermal quenching of the photoluminescence, it was shown that the deep level emission was partially from the intermediate band of the material. This is of great importance due to the theoretical absorption efficiency that is as high as 63% for intermediate band materials, which is more than two times of that of current single junction concentrators, and few materials possessing this property. Individual ZnTe nanowires, grown after optimization, were patterned and contacted, and their conductivity and photoconductivity were measured at room temperature. A single ZnTe nanowire serving as a photodetector was shown to have the highest reported visible responsivity of 360 A/W (at 530 nm), and a gain of 8,640 (at 3 V bias). The responsivity is roughly 18 times higher than that of silicon avalanche photodiodes. This demonstrates that ZnTe nanowires are strong candidates for single photon detection.

This dissertation describes experimental studies of the structures and properties, and their correlations in ferromagnetic nanowires and nanotubes fabricated using porous templates. Ferromagnetic Ni and Fe nanowires with diameters 30 ~ 250 nm were electroplated into the pores of anodic aluminum oxide membranes. The effects of nanowire diameter on structural and magnetic properties were investigated. The microstructures of these nanowires were studied using X-ray diffraction and selected-area electron diffraction measurements. The magnetic properties of the nanowires were investigated using magnetic hysteresis measurements and magnetic force microscopy. Additionally, ferromagnetic Ni-P nanotubes were fabricated using an electroless chemical deposition method. Structure and composition analyses were conducted using X-ray diffraction and energy-dispersive spectroscopy. The magnetic properties of the nanotube arrays and the electronic properties of individual nanotubes were studied. Hysteresis measurements revealed that the 250-nm diameter Ni nanowires had a poor squareness in their hysteresis loops, indicating the existence of multi-domain states. In comparison, the squareness in the hysteresis loops of 60-nm and 30-nm Ni nanowires was much improved, suggesting the existence of single domain states in these smaller diameter nanowires. Magnetic force microscopy measurements confirmed the magnetic domain structures suggested by magnetic hysteresis measurements. Similar investigations of Fe nanowires with diameters of 250 nm and 60 nm found that they all have multidomain magnetic structures. This is expected based on their material properties and polycrystalline structures. Furthermore, magnetic structures of Y-branches and multi-wire clusters were also studied using magnetic force microscopy. The as-prepared Ni-P nanotubes had an amorphous structure. Following a heat treatment, however, a structural phase transformation from the amorphous phase to a crystalline phase was observed using X-ray diffraction measurements. The tetragonal crystalline phase of Ni3P and the face-centered-cubic phase of Ni were confirmed via simulations by the GSAS software. The high Ni3P content accounts for the semiconducting behavior and a low magnetic anisotropy observed in the Ni-P nanotubes.

Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 28, 2008) Includes bibliographical references.

Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: Chou, Mei-Yin; Committee Member: First,Phillip; Committee Member: Gao, Jianping; Committee Member: Landman, Uzi; Committee Member: wang, Xiao-Qian.

Thesis (Ph. D.)--Duke University, 2007. / Includes bibliographical references.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references. Also issued in print.