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Device Optimization and Transient Electroluminescence Studies of Organic light Emitting Devices

Description: Organic light emitting devices (OLEDs) are among the most promising for flat panel display technologies. They are light, bright, flexible, and cost effective. And while they are emerging in commercial product, their low power efficiency and long-term degradation are still challenging. The aim of this work was to investigate their device physics and improve their performance. Violet and blue OLEDs were studied. The devices were prepared by thermal vapor deposition in high vacuum. The combinatorial method was employed in device preparation. Both continuous wave and transient electroluminescence (EL) were studied. A new efficient and intense UV-violet light emitting device was developed. At a current density of 10 mA/cm{sup 2}, the optimal radiance R could reach 0.38 mW/cm{sup 2}, and the quantum efficiency was 1.25%. using the delayed EL technique, electron mobilities in DPVBi and CBP were determined to be {approx} 10{sup -5} cm{sup 2}/Vs and {approx} 10{sup -4} cm{sup 2}/Vs, respectively. Overshoot effects in the transient El of blue light emitting devices were also observed and studied. This effect was attributed to the charge accumulation at the organic/organic and organic/cathode interfaces.
Date: August 5, 2003
Creator: Zou, Lijuan
Partner: UNT Libraries Government Documents Department

Investigations of the Electronic Properties and Surface Structures of Aluminium-Rich Quasicrystalline Alloys

Description: The work presented in this dissertation has investigated three distinct areas of interest in the field of quasicrystals: bulk structure, transport properties, and electronic structure. First, they have described the results of a study which explored the fundamental interactions between the atomic species of the icosahedral Al-Pd-Mn quasicrystal. The goal of this work was to determine whether the pseudo-MacKay or Bergman type clusters have a special stability or are merely a geometric coincidence. This was carried out by using laser vaporization to produce gas-phase metal clusters, which were analyzed using time-of-flight mass spectrometry. Both the kinetic and thermodynamic stabilities of the clusters were probed. The data indicated no special stability for either pseudo-MacKay or Bergman type clusters as isolated units. This, however, is not proof that these clusters are simply a geometric coincidence. It is possible that such clusters only have stability in the framework of the bulk matrix and do not exist as isolated units. Next, they have reported their investigations of the bulk thermal transport properties of a decagonal Al-Ni-Co two dimensional quasicrystal in the temperature range 373K-873K. The properties of a sample oriented along the periodic axis and another oriented along the aperiodic axis were measured. A high degree of anisotropy was observed between the aperiodic and periodic directions. Additionally, the properties were measured for a sample miscut to an orientation 45{sup o} off-axis. The properties of the miscut sample were shown to have good agreement with a theoretical model used to describe thermal transport in metallic single crystals. This model only considers thermal transport by a free-electron gas; therefore, agreement with experimental data suggests the validity of the Drude free-electron model for the decagonal Al-Ni-Co at these temperatures. Consequently, the observed anisotropy may be adequately described using classical transport equations. Transport behavior is described in terms ...
Date: August 5, 2003
Creator: Barrow, Jason A.
Partner: UNT Libraries Government Documents Department

Electron-positron production in ultra-peripheral heavy-ion collisions with the STAR experiment

Description: This thesis presents a measurement of the cross-section of the purely electromagnetic production of e{sup +}e{sup -} pairs accompanied by mutual nuclear Coulomb excitation AuAu {yields} Au*Au* + e{sup +}e{sup -}, in ultra-peripheral gold-gold collisions at RHIC at the center-of-mass collision energy of {radical}S{sub NN} = 200 GeV per nucleon. These reactions were selected by detecting neutron emission by the excited gold ions in the Zero Degree Calorimeters. The charged tracks in the e{sup +}e{sup -} events were reconstructed with the STAR Time Projection Chamber. The detector acceptance limits the kinematical range of the observed e{sup +}e{sup -} pairs; therefore the measured cross-section is extrapolated to 4{pi} with the use of Monte Carlo simulations. We have developed a Monte Carlo simulation for ultra-peripheral e{sup +}e{sup -} production at RHIC based on the Equivalent Photon Approximation, the lowest-order QED e{sup +}e{sup -} production cross-section by two real photons and the assumption that the mutual nuclear excitations and the e{sup +}e{sup -} production are independent (EPA model). We compare our experimental results to two models: the EPA model and a model based on full QED calculation of the e{sup +}e{sup -} production, taking the photon virtuality into account. The measured differential cross-section d{sigma}/dM{sub inv} (M{sub inv} - e{sup +}e{sup -} invariant mass) agrees well with both theoretical models. The measured differential cross-section d{sigma}/dp{sub {perpendicular}}{sup tot} (p{sub {perpendicular}}{sup tot} - e{sup +}e{sup -} total transverse momentum) favors the full QED calculation over the EPA model.
Date: August 1, 2003
Creator: Morozov, Vladimir Borisovitch
Partner: UNT Libraries Government Documents Department

Functionalized Materials From Elastomers to High Performance Thermoplastics

Description: Synthesis and incorporation of functionalized materials continues to generate significant research interest in academia and in industry. If chosen correctly, a functional group when incorporated into a polymer can deliver enhanced properties, such as adhesion, water solubility, thermal stability, etc. The utility of these new materials has been demonstrated in drug-delivery systems, coatings, membranes and compatibilizers. Two approaches exist to functionalize a material. The desired moiety can be added to the monomer either before or after polymerization. The polymers used range from low glass transition temperature elastomers to high glass transition temperature, high performance materials. One industrial example of the first approach is the synthesis of Teflon(reg. sign). Poly(tetrafluoroethylene) (PTFE or Teflon(reg. sign)) is synthesized from tetrafluoroethylene, a functionalized monomer. The resulting material has significant property differences from the parent, poly(ethylene). Due to the fluorine in the polymer, PTFE has excellent solvent and heat resistance, a low surface energy and a low coefficient of friction. This allows the material to be used in high temperature applications where the surface needs to be nonabrasive and nonstick. This material has a wide spread use in the cooking industry because it allows for ease of cooking and cleaning as a nonstick coating on cookware. One of the best examples of the second approach, functionalization after polymerization, is the vulcanization process used to make tires. Natural rubber (from the Hevea brasiliensis) has a very low glass transition temperature, is very tacky and would not be useful to make tires without synthetic alteration. Goodyear's invention was the vulcanization of polyisoprene by crosslinking the material with sulfur to create a rubber that was tough enough to withstand the elements of weather and road conditions. Due to the development of polymerization techniques to make cis-polyisoprene, natural rubber is no longer needed for the manufacturing of tires, but ...
Date: May 31, 2003
Creator: Salazar, Laura Ann
Partner: UNT Libraries Government Documents Department

A Model for the Behavior of Magnetic Tunnel Junctions

Description: A magnetic tunnel junction is a device that changes its electrical resistance with a change in an applied magnetic field. A typical junction consists of two magnetic electrodes separated by a nonmagnetic insulating layer. The magnetizations of the two electrodes can have two possible extreme configurations, parallel and antiparallel. The antiparallel configuration is observed to have the higher measured resistance and the parallel configuration has the lower resistance. To switch between these two configurations a magnetic field is applied to the device which is primarily used to change the orientation of the magnetization of one electrode usually called the free layer, although with sufficient high magnetic field the orientation of the magnetizations of both of the electrodes can be changed. The most commonly used models for describing and explaining the electronic behavior of tunnel junctions are the Simmons model and the Brinkman model. However, both of these models were designed for simple, spin independent tunneling. The Simmons model does not address the issue of applied magnetic fields nor does it address the form of the electronic band structure in the metallic electrodes, including the important factor of spin polarization. The Brinkman model is similar, the main difference between the two models being the shape of the tunneling barrier potential between the two electrodes. Therefore, the research conducted in this thesis has developed a new theoretical model that addresses these important issues starting from basic principles. The main features of the new model include: the development of equations for true spin dependent tunneling through the insulating barrier, the differences in the orientations of the electrode magnetizations on either side of the barrier, and the effects of the density of states function on the behavior of the junction. The present work has explored densities of states that are more realistic than the ...
Date: August 5, 2003
Creator: Baker, Bryan John
Partner: UNT Libraries Government Documents Department

Miniaturized Analytical Platforms From Nanoparticle Components: Studies in the Construction, Characterization, and High-Throughput Usage of These Novel Architectures

Description: The scientific community has recently experienced an overall effort to reduce the physical size of many experimental components to the nanometer size range. This size is unique as the characteristics of this regime involve aspects of pure physics, biology, and chemistry. One extensively studied example of a nanometer sized experimental component, which acts as a junction between these three principle scientific theologies, is deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). These biopolymers not only contain the biological genetic guide to code for the production of life-sustaining materials, but are also being probed by physicists as a means to create electrical circuits and furthermore as controllable architectural and sensor motifs in the chemical disciplines. Possibly the most common nano-sized component between these sciences are nanoparticles composed of a variety of materials. The cross discipline employment of nanoparticles is evident from the vast amount of literature that has been produced from each of the individual communities within the last decade. Along these cross-discipline lines, this dissertation examines the use of several different types of nanoparticles with a wide array of surface chemistries to understand their adsorption properties and to construct unique miniaturized analytical and immunoassay platforms. This introduction will act as a literature review to provide key information regarding the synthesis and surface chemistries of several types of nanoparticles. This material will set the stage for a discussion of assembling ordered arrays of nanoparticles into functional platforms, architectures, and sensors. The introduction will also include a short explanation of the atomic force microscope that is used throughout the thesis to characterize the nanoparticle-based structures. Following the Introduction, four research chapters are presented as separate manuscripts. Chapter 1 examines the self-assembly of polymeric nanoparticles exhibiting a variety of surface chemistries and attempts to deconvolute general adsorption rules for their assembly on various substrates. ...
Date: August 5, 2003
Creator: Pris, Andrew David
Partner: UNT Libraries Government Documents Department

Process Optimization for Solid Extraction, Flavor Improvement and Fat Removal in the Production of Soymilk From Full Fat Soy Flakes

Description: Traditionally soymilk has been made with whole soybeans; however, there are other alternative raw ingredients for making soymilk, such as soy flour or full-fat soy flakes. US markets prefer soymilk with little or no beany flavor. modifying the process or using lipoxygenase-free soybeans can be used to achieve this. Unlike the dairy industry, fat reduction in soymilk has been done through formula modification instead of by conventional fat removal (skimming). This project reports the process optimization for solids and protein extraction, flavor improvement and fat removal in the production of 5, 8 and 12 {sup o}Brix soymilk from full fat soy flakes and whole soybeans using the Takai soymilk machine. Proximate analyses, and color measurement were conducted in 5, 8 and 12 {sup o}Brix soymilk. Descriptive analyses with trained panelists (n = 9) were conducted using 8 and 12 {sup o}Brix lipoxygenase-free and high protein blend soy flake soymilks. Rehydration of soy flakes is necessary to prevent agglomeration during processing and increase extractability. As the rehydration temperature increases from 15 to 50 to 85 C, the hexanal concentration was reduced. Enzyme inactivation in soy flakes milk production (measured by hexanal levels) is similar to previous reports with whole soybeans milk production; however, shorter rehydration times can be achieved with soy flakes (5 to 10 minutes) compared to whole beans (8 to 12 hours). Optimum rehydration conditions for a 5, 8 and 12 {sup o}Brix soymilk are 50 C for 5 minutes, 85 C for 5 minutes and 85 C for 10 minutes, respectively. In the flavor improvement study of soymilk, the hexanal date showed differences between undeodorized HPSF in contrast to triple null soymilk and no differences between deodorized HPSF in contrast to deodorized triple null. The panelists could not differentiate between the beany, cereal, and painty flavors. However, the ...
Date: May 31, 2003
Creator: Prawiradjaja, Stanley
Partner: UNT Libraries Government Documents Department

Knowledge and attitudes of preservice teachers towards students who are gay, lesbian, bisexual, or transgendered.

Description: The study used a survey design to ascertain the levels of knowledge and attitudes of special education and non-special education preservice and inservice teachers towards students with different sexual orientations. The results of this study are based on 408 responses from preservice and inservice teachers enrolled at seven institutions of higher education within North Carolina, Virginia, and the District of Columbia offering teacher training programs in regular and/or special education. Two previously developed instruments were used to measure dependent variables in this study. Koch's modified version of The Knowledge about Homosexuality Questionnaire developep by Harris, Nightengale & Owen was used to measure the dependent variable of the preservice and inservice teacher's knowledge about homosexuality. Herek's Attitudes Toward Lesbians and Gay Men (ATLG) measured the dependent variable of attitudes towards homosexuals. The study found no significant differences reported mean scores for knowledge or attitude of homosexuality among the teacher groups surveyed: (a) special education preservice teachers, (b) non-special education preservice teachers, (c) special education inservice teachers, and (d) non-special education inservice teachers. Neither gender nor age were found to be factors in measures of knowledge or attitude of preservice or inservice teachers. Receiving prior instruction in serving the needs of GLBT students, or with a focus GLBT issues, contributed to higher levels of knowledge and more positive attitudes. This research identified current levels of knowledge and attitudes of preservice and inservice teachers towards GLBT youth, and this information may help outline areas of possible changes necessary in teacher preparation programs, research, and policy.
Date: December 2003
Creator: Morgan, Daniel J.
Partner: UNT Libraries

Process environmental philosophy

Description: A process-information approach is examined as a foundation for an environmental philosophy that is dynamic and elastic, with particular emphasis on value, beauty, integrity and stability supporting Aldo Leopold's vision. I challenge one of the basic assumptions of Western philosophy, namely the metaphysical primacy of substance. The classical, medieval and modern metaphysics of substance is presented with particular attention given the paradoxes of substance. Starting from the philosophy of Heraclitus, relatively ignored by the Western tradition of philosophy, a process philosophy is developed as an alternative to standard metaphysical attitudes in philosophy. A possible resolution of Zeno's paradoxes leads to consideration of other paradoxes of substance metaphysics. It is argued that substance metaphysics is incompatible with evidence found in the shifting paradigms of ecology and general science. Process philosophy is explored as a basis for an environmental philosophy, attempting to put the environment back into philosophy.
Date: May 2003
Creator: Corbeil, Marc J.V.
Partner: UNT Libraries

Developmental Patterns of Metabolism and Hematology in the Late Stage Chicken Embryo (Gallus Domesticus) at Two Incubation Temperatures.

Description: How temperature affects physiological development in the chicken embryo is unknown. Embryos incubated at 38°C or 35°C showed no difference in growth or survival. The time to hatching was longer in 35°C than 38°C embryos (23.7 vs. 20.6 days), but unaffected was the relative timing of appearance of developmental landmarks (internal, external pipping). At stage 43-44, 38°C embryos maintained oxygen consumption around 1 mL/g/h despite acute temperature reduction (suggesting thermoregulatory maturation), unlike 35°C embryos. In 35°C embryos the lower oxygen-carrying capacity and temperature insensitive blood O2 affinity (P50 about 30 mmHg) may restrict O2 delivery to tissues, limiting metabolism during decreased ambient temperature. Reduced incubation temperature retards normal hematological and thermoregulatory development.
Date: May 2003
Creator: Black, Juli
Partner: UNT Libraries

An Investigation into the Effects of Long-term Staff Development on Teacher Perceptions and Reading Achievement on Young Children

Description: The effectiveness of long-term staff development (Reading Academy Project-RAP) on students' reading scores on the Texas Assessment of Academic Skills (TAAS) test was examined to determine if teachers transferred newly learned teaching strategies into practice and changed their beliefs about reading instruction. In a four-year cohort longitudinal study in an East Texas rural community, the effects of long-term staff development on third grade students' TAAS test reading scores, teacher practices, and teacher beliefs were explored. Populations included a teacher group (N = 17), an experimental (N = 419), and a control (N = 419) group of students. Children's groups were matched pairs based on five demographic characteristics and membership or non-membership in one or more of six categories. An application survey and four end-of-the-year surveys provided teacher data regarding classroom practices. One interview question provided information about teacher beliefs. Results indicate students who had a RAP teacher for at least one year scored significantly higher on the TAAS reading test in the third grade than those without a RAP teacher. Examination of students having more than one year with an academy teacher failed to produce statistically significant differences in TAAS test reading scores; however, an upward trend was noted. Statistically significant differences were found in 6 of the 20 items on the survey investigating classroom practices. All teachers reported the RAP affected them positively, and 82% confirmed that changes took place in their classroom practices, student behaviors, and teacher responsibilities. Validation of or strengthening existing beliefs accounted for 76% of the teacher responses. Recommendations include adding a population of kindergarten through third graders and following them through high school to determine ultimate reading success, continue surveying teachers to see if effective strategies persist, add a parental involvement component, and replicate this investigation in suburban and metropolitan areas.
Date: May 2003
Creator: Boatman, Vikki
Partner: UNT Libraries

The ends of uncertainty: Air quality science and planning in Central California

Description: Air quality planning in Central California is complicated and controversial despite millions of dollars invested to improve scientific understanding. This research describes and critiques the use of photochemical air quality simulation modeling studies in planning to attain standards for ground-level ozone in the San Francisco Bay Area and the San Joaquin Valley during the 1990's. Data are gathered through documents and interviews with planners, modelers, and policy-makers at public agencies and with representatives from the regulated and environmental communities. Interactions amongst organizations are diagramed to identify significant nodes of interaction. Dominant policy coalitions are described through narratives distinguished by their uses of and responses to uncertainty, their exposures to risks, and their responses to the principles of conservatism, civil duty, and caution. Policy narratives are delineated using aggregated respondent statements to describe and understand advocacy coalitions. I found that models impacted the planning process significantly, but were used not purely for their scientific capabilities. Modeling results provided justification for decisions based on other constraints and political considerations. Uncertainties were utilized opportunistically by stakeholders instead of managed explicitly. Ultimately, the process supported the partisan views of those in control of the modeling. Based on these findings, as well as a review of model uncertainty analysis capabilities, I recommend modifying the planning process to allow for the development and incorporation of uncertainty information, while addressing the need for inclusive and meaningful public participation. By documenting an actual air quality planning process these findings provide insights about the potential for using new scientific information and understanding to achieve environmental goals, most notably the analysis of uncertainties in modeling applications. Concurrently, needed uncertainty information is identified and capabilities to produce it are assessed. Practices to facilitate incorporation of uncertainty information are suggested based on research findings, as well as theory from the literatures of ...
Date: September 2003
Creator: Fine, James
Partner: UNT Libraries Government Documents Department

Quantification of Soil Physical Properties by Using X-Ray Computerized Tomography (CT) and Standard Laboratory (STD) Methods

Description: The implementation of x-ray computerized tomography (CT) on agricultural soils has been used in this research to quantify soil physical properties to be compared with standard laboratory (STD) methods. The overall research objective was to more accurately quantify soil physical properties for long-term management systems. Two field studies were conducted at Iowa State University's Northeast Research and Demonstration Farm near Nashua, IA using two different soil management strategies. The first field study was conducted in 1999 using continuous corn crop rotation for soil under chisel plow with no-till treatments. The second study was conducted in 2001 and on soybean crop rotation for the same soil but under chisel plow and no-till practices with wheel track and no-wheel track compaction treatments induced by a tractor-manure wagon. In addition, saturated hydraulic (K{sub s}) conductivity and the convection-dispersion (CDE) model were also applied using long-term soil management systems only during 2001. The results obtained for the 1999 field study revealed no significant differences between treatments and laboratory methods, but significant differences were found at deeper depths of the soil column for tillage treatments. The results for standard laboratory procedure versus CT method showed significant differences at deeper depths for the chisel plow treatment and at the second lower depth for no-till treatment for both laboratory methods. The macroporosity distribution experiment showed significant differences at the two lower depths between tillage practices. Bulk density and percent porosity had significant differences at the two lower depths of the soil column. The results obtained for the 2001 field study showed no significant differences between tillage practices and compaction practices for both laboratory methods, but significant differences between tillage practices with wheel track and no-wheel compaction treatments were found along the soil profile for both laboratory methods. The K{sub s} measurements and CDE parameters revealed no significant ...
Date: December 12, 2003
Creator: Sanchez, Maria Ambert
Partner: UNT Libraries Government Documents Department

Protein Structure Recognition: From Eigenvector Analysis to Structural Threading Method

Description: In this work, they try to understand the protein folding problem using pair-wise hydrophobic interaction as the dominant interaction for the protein folding process. They found a strong correlation between amino acid sequences and the corresponding native structure of the protein. Some applications of this correlation were discussed in this dissertation include the domain partition and a new structural threading method as well as the performance of this method in the CASP5 competition. In the first part, they give a brief introduction to the protein folding problem. Some essential knowledge and progress from other research groups was discussed. This part includes discussions of interactions among amino acids residues, lattice HP model, and the design ability principle. In the second part, they try to establish the correlation between amino acid sequence and the corresponding native structure of the protein. This correlation was observed in the eigenvector study of protein contact matrix. They believe the correlation is universal, thus it can be used in automatic partition of protein structures into folding domains. In the third part, they discuss a threading method based on the correlation between amino acid sequences and ominant eigenvector of the structure contact-matrix. A mathematically straightforward iteration scheme provides a self-consistent optimum global sequence-structure alignment. The computational efficiency of this method makes it possible to search whole protein structure databases for structural homology without relying on sequence similarity. The sensitivity and specificity of this method is discussed, along with a case of blind test prediction. In the appendix, they list the overall performance of this threading method in CASP5 blind test in comparison with other existing approaches.
Date: December 12, 2003
Creator: Cao, Haibo
Partner: UNT Libraries Government Documents Department

Nitrogen doped zinc oxide thin film

Description: To summarize, polycrystalline ZnO thin films were grown by reactive sputtering. Nitrogen was introduced into the films by reactive sputtering in an NO{sub 2} plasma or by N{sup +} implantation. All ZnO films grown show n-type conductivity. In unintentionally doped ZnO films, the n-type conductivities are attributed to Zn{sub i}, a native shallow donor. In NO{sub 2}-grown ZnO films, the n-type conductivity is attributed to (N{sub 2}){sub O}, a shallow double donor. In NO{sub 2}-grown ZnO films, 0.3 atomic % nitrogen was found to exist in the form of N{sub 2}O and N{sub 2}. Upon annealing, N{sub 2}O decomposes into N{sub 2} and O{sub 2}. In furnace-annealed samples N{sub 2} redistributes diffusively and forms gaseous N{sub 2} bubbles in the films. Unintentionally doped ZnO films were grown at different oxygen partial pressures. Zni was found to form even at oxygen-rich condition and led to n-type conductivity. N{sup +} implantation into unintentionally doped ZnO film deteriorates the crystallinity and optical properties and leads to higher electron concentration. The free electrons in the implanted films are attributed to the defects introduced by implantation and formation of (N{sub 2}){sub O} and Zni. Although today there is still no reliable means to produce good quality, stable p-type ZnO material, ZnO remains an attractive material with potential for high performance short wavelength optoelectronic devices. One may argue that gallium nitride was in a similar situation a decade ago. Although we did not obtain any p-type conductivity, we hope our research will provide a valuable reference to the literature.
Date: December 15, 2003
Creator: Li, Sonny X.
Partner: UNT Libraries Government Documents Department

Anisotropy in CdSe quantum rods

Description: The size-dependent optical and electronic properties of semiconductor nanocrystals have drawn much attention in the past decade, and have been very well understood for spherical ones. The advent of the synthetic methods to make rod-like CdSe nanocrystals with wurtzite structure has offered us a new opportunity to study their properties as functions of their shape. This dissertation includes three main parts: synthesis of CdSe nanorods with tightly controlled widths and lengths, their optical and dielectric properties, and their large-scale assembly, all of which are either directly or indirectly caused by the uniaxial crystallographic structure of wurtzite CdSe. The hexagonal wurtzite structure is believed to be the primary reason for the growth of CdSe nanorods. It represents itself in the kinetic stabilization of the rod-like particles over the spherical ones in the presence of phosphonic acids. By varying the composition of the surfactant mixture used for synthesis we have achieved tight control of the widths and lengths of the nanorods. The synthesis of monodisperse CdSe nanorods enables us to systematically study their size-dependent properties. For example, room temperature single particle fluorescence spectroscopy has shown that nanorods emit linearly polarized photoluminescence. Theoretical calculations have shown that it is due to the crossing between the two highest occupied electronic levels with increasing aspect ratio. We also measured the permanent electric dipole moment of the nanorods with transient electric birefringence technique. Experimental results on nanorods with different sizes show that the dipole moment is linear to the particle volume, indicating that it originates from the non-centrosymmetric hexagonal lattice. The elongation of the nanocrystals also results in the anisotropic inter-particle interaction. One of the consequences is the formation of liquid crystalline phases when the nanorods are dispersed in solvent to a high enough concentration. The preparation of the stable liquid crystalline solution of CdSe nanorods ...
Date: September 1, 2003
Creator: Li, Liang-shi
Partner: UNT Libraries Government Documents Department

SPA-LEED Study of the Morphology and Nucleation of a Novel Growth Mode and the ''devil's staircase'' on Pb/Si(111)

Description: This thesis was developed to address the following questions for the Pb/Si(111) system: (1) Is it possible to control the nano-structure growth by changing the initial substrate; (2) is the nucleation theory applicable to the case of the 7-step growth mode; and (3) what phase or phases could be formed between coverage 6/5 ML and 4/3 ML? The first question was answered in chapter 2, different growth results were observed for different initial substrate, suggesting the possibility of controlling nano-structure growth by selecting the initial substrate. The applicability of nucleation theory was determined to be unclear in chapter 3, from the results that the saturation island density does not depend on deposition rate, in contrary to the prediction of nucleation theory. Chapter 4 revealed a novel ''devil's staircase'' in Pb/Si(111) within the coverage range 6/5 ML and 4/3 ML. Low temperature deposition experiments showed high order of self-organization in such a system. Theoretical studies are needed to understand such a low temperature behavior. In general, this thesis provides possibilities of controlling nano-structure growth, which can be possibly an indication for future application. It also raises interesting questions in fundamental researches: a modified theory of nucleation is needed, and a detailed study of low temperature behavior is required. Details of the conclusions in each of the chapters are collected in the following sections.
Date: December 12, 2003
Creator: Yeh, Wang-Chi Vincent
Partner: UNT Libraries Government Documents Department

Electromagnetic Wave Propagation in Two-Dimensional Photonic Crystals

Description: In this dissertation, they have undertaken the challenge to understand the unusual propagation properties of the photonic crystal (PC). The photonic crystal is a medium where the dielectric function is periodically modulated. These types of structures are characterized by bands and gaps. In other words, they are characterized by frequency regions where propagation is prohibited (gaps) and regions where propagation is allowed (bands). In this study they focus on two-dimensional photonic crystals, i.e., structures with periodic dielectric patterns on a plane and translational symmetry in the perpendicular direction. They start by studying a two-dimensional photonic crystal system for frequencies inside the band gap. The inclusion of a line defect introduces allowed states in the otherwise prohibited frequency spectrum. The dependence of the defect resonance state on different parameters such as size of the structure, profile of incoming source, etc., is investigated in detail. For this study, they used two popular computational methods in photonic crystal research, the Finite Difference Time Domain method (FDTD) and the Transfer Matrix Method (TMM). The results for the one-dimensional defect system are analyzed, and the two methods, FDTD and TMM, are compared. Then, they shift their attention only to periodic two-dimensional crystals, concentrate on their band properties, and study their unusual refractive behavior. Anomalous refractive phenomena in photonic crystals included cases where the beam refracts on the ''wrong'' side of the surface normal. The latter phenomenon, is known as negative refraction and was previously observed in materials where the wave vector, the electric field, and the magnetic field form a left-handed set of vectors. These materials are generally called left-handed materials (LHM) or negative index materials (NIM). They investigated the possibility that the photonic crystal behaves as a LHM, and how this behavior relates with the observed negatively refractive phenomena. They found that in the ...
Date: December 12, 2003
Creator: Foteinopoulou, Stavroula
Partner: UNT Libraries Government Documents Department

Measurement of the beta-neutrino correlation in laser trapped {sup 21}Na

Description: Trapped radioactive atoms are an appealing source for precise measurements of the beta-neutrino correlation coefficient, a, since the momentum of the neutrino can be inferred from the detection of the unperturbed low-energy recoil daughter nucleus. Sodium-21 is produced on-line at the 88'' cyclotron at Lawrence Berkeley National Laboratory, and 8e5 atoms have been maintained in a magneto-optical trap. A static electric field draws daughter Neon-21 ions to a microchannel plate detector and betas are detected in coincidence with a plastic scintillator beta detector. The Neon-21 time-of-flight distribution determines the beta neutrino correlation coefficient, a. The resulting charge-state distribution is compared to a simple model based on the sudden approximation which suggests a small but important contribution from nuclear recoil-induced ionization. A larger than expected fraction of the daughters are detected in positive charge-states, but no dependence on either the beta or recoil nucleus energy was observed. We find a = 0.5243 plus or minus 0.0092, which is in 3.6 sigma disagreement with the Standard Model prediction of a = 0.559 plus or minus 0.003. Aside from a deviation from the Standard Model, a possible explanation for the discrepancy is that the branching ratio to the first excited state is in error.
Date: June 1, 2003
Creator: Scielzo, Nicholas David
Partner: UNT Libraries Government Documents Department

Organogermanium Chemistry: Germacyclobutanes and digermane Additions to Acetylenes

Description: This dissertation comprises two main research projects. The first project, presented in Chapter 1, involves the synthesis and thermochemistry of germacyclobutanes (germetanes). Four new germetanes (spirodigermetane, diallylgermetane, dichlorogermetane, and germacyclobutane) have been synthesized using a modified di-Grignard synthesis. Diallylgermetane is shown to be a useful starting material for obtaining other germetanes, particularly the parent germetane, germacyclobutane. The gas-phase thermochemistries of spirodigermetane, diallylgermetane and germacyclobutane have been explored via pulsed stirred-flow reactor (SFR) studies, showing remarkable differences in decomposition, depending on the substitution at the germanium atom. The second project investigates the thermochemical, photochemical, and catalytic additions of several digermanes to acetylenes. The first examples of thermo- and photochemical additions of Ge-Ge bonds to C{triple_bond}C are demonstrated. Mechanistic investigations are described and comparisons are made to analogous disilane addition reactions, previously studied in their group.
Date: December 12, 2003
Creator: Chubb, Andrew Michael
Partner: UNT Libraries Government Documents Department

Thermal Properties of Starch From New Corn Lines as Impacted by Environment and During Line Development

Description: The objectives of this research were to further characterize exotic by adapted corn inbreds by studying the impact of environment on their starch thermal properties, and investigating the development of starch thermal properties during kernel maturation by using differential scanning calorimetry (DSC). A method to expedite identification of unusual starch thermal traits was investigated by examining five corn kernels at a time, instead of one kernel, which the previous screening methods used. Corn lines with known thermal functions were blended with background starch (control) in ratios of unique starch to control starch, and analyzed by using DSC. Control starch was representative of typical corn starch. The values for each ratio within a mutant type were unique ({alpha} < 0.01) for most DSC measurements. These results supported the five-kernel method for rapidly screening large amounts of corn germplasm to identify unusual starch traits. The effects of 5 growing locations on starch thermal properties from exotic by adapted corn and Corn Belt lines were studied using DSC. The warmest location, Missouri, generally produced starch with greater gelatinization onset temperature (T{sub oG}), narrower range of gelatinization (R{sub G}), and greater enthalpy of gelatinization ({Delta}H{sub G}). The coolest location, Illinois, generally resulted in starch with lower T{sub oG}, wider R{sub G}, and lower {Delta}H{sub G}. Starch from the Ames 1 farm had thermal properties similar to those of Illinois, whereas starch from the Ames 2 farm had thermal properties similar to those of Missouri. The temperature at Ames 2 may have been warmer since it was located near a river; however, soil type and quality also were different. Final corn starch structure and function change during development and maturity. Thus, the changes in starch thermal properties during 5 stages of endosperm development from exotic by adapted corn and Corn Belt lines at two locations ...
Date: December 12, 2003
Creator: Lenihan, Elizabeth M.
Partner: UNT Libraries Government Documents Department

Three-body Forces in Photoreactions on 3He

Description: We have measured the three-body photobreakup of {sup 3}He with the tagged photon beam and the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility, in the photon energy range between 0.35 GeV and 1.55 GeV. This measurement constitutes a wide-ranging survey of two- and three-body processes in the gamma{sup 3}He {yields} ppn reaction channel, thanks to the high statistics and large kinematic coverage obtained with the CLAS. Total and partially integrated differential cross sections for the full ppn data set and for selected kinematics were extracted and are compared to theoretical predictions of Laget (up to 1.0 GeV). At low photon energies, the calculations are generally in fair agreement with the data. The comparison shows evidence of strong contributions of three-body absorption mechanisms, especially in the star kinematics, a symmetric configuration of the three final-state nucleons. Mostly the effects of two-body absorption mechanisms are se en, as expected, in the pp-pair-breakup kinematics, where the neutron does not participate in the reaction. The quasi-two-body breakup shows angular distributions consistent with preliminary gamma{sup 3}He --> pd results, extracted from our experiment. The ratio of cross sections for the star configuration and for the two-body kinematics, shows a maximum for three-body effects at a photon energy of about 0.5 GeV, corresponding to a reduced photon wavelength of 0.4 fm. The 4pi-integrated cross section is in excellent agreement with previous experimental results from DAPHNE up to 800 MeV; no previous results have been obtained above this energy.
Date: February 1, 2003
Creator: Niccolai, Silvia
Partner: UNT Libraries Government Documents Department

Optically Detected Magnetic Resonance and Thermal Activation Spectroscopy Study of Organic Semiconductors

Description: Organic electronic materials are a new class of emerging materials. Organic light emitting devices (OLEDs) are the most promising candidates for future flat panel display technologies. The photophysical characterization is the basic research step one must follow to understand this new class of materials and devices. The light emission properties are closely related to the transport properties of these materials. The objective of this dissertation is to probe the relation between transport and photophysical properties of organic semiconductors. The transport characteristics were evaluated by using thermally stimulated current and thermally stimulated luminescence techniques. The photoluminescence detected magnetic resonance and photoluminescence quantum yield studies provide valuable photophysical information on this class of materials. OLEDs are already in the market. However, detailed studies on the degradation mechanisms are still lacking. Since both optically detected magnetic resonance and thermal activation spectroscopy probe long-lived defect-related states in organic semiconductors, the combined study generates new insight on the OLED operation and degradation mechanisms.
Date: December 12, 2003
Creator: Kim, Chang-Hwan
Partner: UNT Libraries Government Documents Department

Organic Light-Emitting Devices (OLEDS) and Their Optically Detected Magnetic Resonance (ODMR)

Description: Organic Light-Emitting Devices (OLEDs), both small molecular and polymeric have been studied extensively since the first efficient small molecule OLED was reported by Tang and VanSlyke in 1987. Burroughes' report on conjugated polymer-based OLEDs led to another track in OLED development. These developments have resulted in full color, highly efficient (up to {approx} 20% external efficiency 60 lm/W power efficiency for green emitters), and highly bright (> 140,000 Cd/m{sup 2} DC, {approx}2,000,000 Cd/m{sup 2} AC), stable (>40,000 hr at 5 mA/cm{sup 2}) devices. OLEDs are Lambertian emitters, which intrinsically eliminates the view angle problem of liquid crystal displays (LCDs). Thus OLEDs are beginning to compete with the current dominant LCDs in information display. Numerous companies are now active in this field, including large companies such as Pioneer, Toyota, Estman Kodak, Philipps, DuPont, Samsung, Sony, Toshiba, and Osram, and small companies like Cambridge Display Technology (CDT), Universal Display Corporation (UDC), and eMagin. The first small molecular display for vehicular stereos was introduced in 1998, and polymer OLED displays have begun to appear in commercial products. Although displays are the major application for OLEDs at present, they are also candidates for nest generation solid-state lighting. In this case the light source needs to be white in most cases. Organic transistors, organic solar cells, etc. are also being developed vigorously.
Date: December 12, 2003
Creator: Li, Gang
Partner: UNT Libraries Government Documents Department