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  Partner: UNT Libraries
 Department: Department of Physics
 Decade: 2010-2019
The Effects of Residual Gases on the Field Emission Properties of Zno, Gan, Zns Nanostructures, and the Effects of Light on the Resistivity of Graphene

The Effects of Residual Gases on the Field Emission Properties of Zno, Gan, Zns Nanostructures, and the Effects of Light on the Resistivity of Graphene

Date: May 2014
Creator: Mo, Yudong
Description: In this dissertation, I present that at a vacuum of 3×10-7 Torr, residual O2, CO2, H2 and Ar exposure do not significantly degrade the field emission (FE) properties of ZnO nanorods, but N2 exposure significantly does. I propose that this could be due to the dissociation of N2 into atomic nitrogen species and the reaction of such species with ZnO. I also present the effects of O2, CO2, H2O, N2, H2, and Ar residual gas exposure on the FE properties of GaN and ZnS nanostructure. A brief review of growth of ZnO, GaN and ZnS is provided. In addition, Cs deposition on GaN nanostructures at ultra-high vacuum results in 30% decrease in turn-on voltage and 60% in work function. The improvement in FE properties could be due to a Cs-induced space-charge layer at the surface that reduces the barrier for FE and lowers the work function. I describe a new phenomenon, in which the resistivity of CVD-grown graphene increases to a higher saturated value under light exposure, and depends on the wavelength of the light—the shorter the wavelength, the higher the resistivity. First-principle calculations and theoretical analysis based on density functional theory show that (1) a water molecule close to ...
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An Electro- Magneto-static Field for Confinement of Charged Particle Beams and Plasmas

An Electro- Magneto-static Field for Confinement of Charged Particle Beams and Plasmas

Date: May 2014
Creator: Pacheco, Josè L.
Description: A system is presented that is capable of confining an ion beam or plasma within a region that is essentially free of applied fields. An Artificially Structured Boundary (ASB) produces a spatially periodic set of magnetic field cusps that provides charged particle confinement. Electrostatic plugging of the magnetic field cusps enhances confinement. An ASB that has a small spatial period, compared to the dimensions of a confined plasma, generates electro- magneto-static fields with a short range. An ASB-lined volume thus constructed creates an effectively field free region near its center. It is assumed that a non-neutral plasma confined within such a volume relaxes to a Maxwell-Boltzmann distribution. Space charge based confinement of a second species of charged particles is envisioned, where the second species is confined by the space charge of the first non-neutral plasma species. An electron plasma confined within an ASB-lined volume can potentially provide confinement of a positive ion beam or positive ion plasma. Experimental as well as computational results are presented in which a plasma or charged particle beam interact with the electro- magneto-static fields generated by an ASB. A theoretical model is analyzed and solved via self-consistent computational methods to determine the behavior and equilibrium ...
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How Cooperative Systems Respond to External Forces

How Cooperative Systems Respond to External Forces

Date: May 2014
Creator: Svenkeson, Adam
Description: Cooperative interactions permeate through nature, bringing about emergent behavior and complexity. Using a simple cooperative model, I illustrate the mean field dynamics that occur at the critical point of a second order phase transition in the framework of Langevin equations. Through this formalism I discuss the response, both linear and nonlinear, to external forces. Emphasis is placed on how information is transferred from one individual to another in order to facilitate the collective response of the cooperative network to a localized perturbation. The results are relevant to a wide variety of systems, ranging from nematic liquid crystals, to flocks and swarms, social groups, and neural networks.
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Synthesis, Characterization, Structural, and Optical Properties of Zinc Oxide Nanostructures Embedded in Silicon Based Substrates

Synthesis, Characterization, Structural, and Optical Properties of Zinc Oxide Nanostructures Embedded in Silicon Based Substrates

Date: May 2014
Creator: Pandey, Bimal
Description: Structural and optical properties of ZnO nanostructures synthesized by low energy ion implantation technique were examined. ZnO molecular ions were implanted into Si/SiO2 substrates at room temperature and then furnace annealed under different temperatures and environments. In all as-implanted samples only Zn nanostructures with varying diameters distributed into the Si/SiO2 matrices were observed. No trace of ZnO was found. The distributions of Zn nanostructures in Si/SiO2 closely matched results from Stopping and Range of Ions in Matter (SRIM) simulations. During annealing at 750 oC, Zn diffused both toward and away from the surface of the substrate and combine with oxygen to form ZnO nanostructures. At higher annealing temperatures ZnO bonding started to break down and transfer to zinc silicate (Zn2SiO4), and at 900 oC the ZnO was completely converted into Zn2SiO4. The average sizes of Zn/ZnO nanostructures depended on the ion fluence. If the fluence increased the average sizes of nanostructures also increased and vice versa. For room temperature photoluminescence (RT-PL), band-edge emission in the ultraviolet (UV) region was observed from all samples annealed at 700 oC/750 oC and were slightly blue shifted as compare to bulk ZnO. Donor-bound exciton (D,X) and acceptor-bound exciton (A,X) transitions were observed in low ...
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Synthesis Strategies and a Study of Properties of Narrow and Wide Band Gap Nanowires

Synthesis Strategies and a Study of Properties of Narrow and Wide Band Gap Nanowires

Date: May 2014
Creator: Sapkota, Gopal
Description: Various techniques to synthesize nanowires and nanotubes as a function of growth temperature and time were investigated. These include growth of nanowires by a chemical vapor deposition (CVD) system using vapor-liquid-solid (VLS) growth mechanism and electro-chemical synthesis of nanowires and nanotubes. Narrow band gap InSb Eg = 0.17 eV at room temp) nanowires were successively synthesized. Using a phase diagram, the transition of the nanowire from metallic- semiconducting- semi-metallic phase was investigated. A thermodynamic model is developed to show that the occurrence of native defects in InSb nanowires influenced by the nanowire growth kinetics and thermodynamics of defect formation. Wide band gap ZnO (Eg = 3.34 eV) and In2O3 (3.7 eV) were also synthesized. ZnO nanowires and nanotubes were successfully doped with a transition metal Fe, making it a Dilute Magnetic Semiconductor of great technological relevance. Structural and electronic characterizations of nanowires were studied for different semiconducting, metallic and semi-metallic nanowires. Electron transport measurements were used to estimate intrinsic material parameters like carrier concentration and mobility. An efficient gas sensing device using a single In2O3 nanowire was studied and which showed sensitivity to reducing gas like NH3 and oxidizing gas like O2 gas at room temperature. The efficiency of the ...
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Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization

Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization

Date: December 2013
Creator: Elmarhoumi, Nader M.
Description: Semi-conductor industry relies heavily on silicon (Si). However, Si is not a direct-band gap semi-conductor. Consequently, Si does not possess great versatility for multi-functional applications in comparison with the direct band-gap III-V semi-conductors such as GaAs. To bridge this gap, what is ideally required is a semi-conductor material system that is based on silicon, but has significantly greater versatility. While sparsely studied, the semi-conducting silicides material systems offer great potential. Thus, I focused on the growth and structural characterization of ruthenium silicide and osmium silicide material systems. I also characterized iron silicon germanide films using extended x-ray absorption fine structure (EXAFS) to reveal phase, semi-conducting behavior, and to calculate nearest neighbor distances. The choice of these silicides material systems was due to their theoretically predicted and/or experimentally reported direct band gaps. However, the challenge was the existence of more than one stable phase/stoichiometric ratio of these materials. In order to possess the greatest control over the growth process, molecular beam epitaxy (MBE) has been employed. Structural and film quality comparisons of as-grown versus annealed films of ruthenium silicide are presented. Structural characterization and film quality of MBE grown ruthenium silicide and osmium silicide films via in situ and ex situ ...
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Cooperation-induced Criticality in Neural Networks

Cooperation-induced Criticality in Neural Networks

Date: August 2013
Creator: Zare, Marzieh
Description: The human brain is considered to be the most complex and powerful information-processing device in the known universe. The fundamental concepts behind the physics of complex systems motivate scientists to investigate the human brain as a collective property emerging from the interaction of thousand agents. In this dissertation, I investigate the emergence of cooperation-induced properties in a system of interacting units. I demonstrate that the neural network of my research generates a series of properties such as avalanche distribution in size and duration coinciding with the experimental results on neural networks both in vivo and in vitro. Focusing attention on temporal complexity and fractal index of the system, I discuss how to define an order parameter and phase transition. Criticality is assumed to correspond to the emergence of temporal complexity, interpreted as a manifestation of non-Poisson renewal dynamics. In addition, I study the transmission of information between two networks to confirm the criticality and discuss how the network topology changes over time in the light of Hebbian learning.
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Effects of Quantum Coherence and Interference

Effects of Quantum Coherence and Interference

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Date: August 2013
Creator: Davuluri, Subrahmanya Bhima Sankar
Description: Quantum coherence and interference (QCI) is a phenomenon that takes place in all multi-level atomic systems interacting with multiple lasers. In this work QCI is used to create several interesting effects like lasing without inversion (LWI), controlling group velocity of light to extreme values, controlling the direction of propagation through non-linear phase matching condition and for controlling the correlations in field fluctuations. Controlling group velocity of light is very interesting because of many novel applications it can offer. One of the unsolved problems in this area is to achieve a slow and fast light which can be tuned continuously as a function of frequency. We describe a method for creation of tunable slow and fast light by controlling intensity of incident laser fields using QCI effects. Lasers are not new to the modern world but an extreme ultra-violet laser or a x-ray laser is definitely one of the most desirable technologies today. Using QCI, we describe a method to realize lasing at high frequencies by creating lasing without inversion. Role of QCI in creating correlations and anti-correlations, which are generated by vacuum fluctuations, in a three level lambda system coupled to two strong fields is discussed.
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A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation

A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation

Date: May 2013
Creator: Kummari, Venkata Chandra Sekhar
Description: SiC has become an attractive wide bandgap semiconductor due to its unique physical and electronic properties and is widely used in high temperature, high frequency, high power and radiation resistant applications. SiC has been used as an alternative to Si in harsh environments such as in the oil industry, nuclear power systems, aeronautical, and space applications. SiC is also known for its polytypism and among them 3C-SiC, 4H-SiC and 6H-SiC are the most common polytypes used for research purposes. Among these polytypes 4H-SiC is gaining importance due to its easy commercial availability with a large bandgap of 3.26 eV at room temperature. Controlled creation of defects in materials is an approach to modify the electronic properties in a way that new functionality may result. SiC is a promising candidate for defect-induced magnetism on which spintronic devices could be developed. The defects considered are of room temperature stable vacancy types, eliminating the need for magnetic impurities, which easily diffuse at room temperature. Impurity free vacancy type defects can be created by implanting the host atoms of silicon or carbon. The implantation fluence determines the defect density, which is a critical parameter for defect induced magnetism. Therefore, we have studied the influence ...
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Zinc Oxide Nanoparticles for Nonlinear Bioimaging, Cell Detection and Selective Cell Destruction

Zinc Oxide Nanoparticles for Nonlinear Bioimaging, Cell Detection and Selective Cell Destruction

Access: Use of this item is restricted to the UNT Community.
Date: May 2013
Creator: Urban, Ben E.
Description: Light matter interactions have led to a great part of our current understanding of the universe. When light interacts with matter it affects the properties of both the light and the matter. Visible light, being in the region that the human eye can "see," was one of the first natural phenomenon we used to learn about our universe. The application of fundamental physics research has spilled over into other fields that were traditionally separated from physics, being considered two different sciences. Current physics research has applications in all scientific fields. By taking a more physical approach to problems in fields such as chemistry and biology, we have furthered our knowledge of both. Nanocrystals have many interesting optical properties. Furthermore, the size and properties of nanocrystals has given them applications in materials ranging from solar cells to sunscreens. By understanding and controlling their interactions with systems we can utilize them to increase our knowledge in other fields of science, such as biology. Nanocrystals exhibit optical properties superior to currently used fluorescent dyes. By replacing molecular dyes with nanoparticles we can reduce toxicity, increase resolution and have better cellular targeting abilities. They have also shown to have toxicity to cancer and antibacterial ...
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