Browsing by Author "Keith Weninger, Committee Member"

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Application of Diffraction Enhanced Imaging to Bone
(2007-01-24) Connor, Dean Michael Jr.; Zhong Zhong, Committee Member; Etta Pisano, Committee Member; D. Rick Sumner, Committee Member; David S. Lalush, Committee Member; Hans D. Hallen, Committee Chair; Keith Weninger, Committee Member
Diffraction enhanced imaging (DEI) is a new x-ray-based medical imaging modality that is in its early stages of development and testing. In images generated using DEI, contrast is from absorption and refraction of x-rays and from ultra-small angle x-ray scattering (USAXS). Though accepted values for x-ray absorption in biological tissues have been established, only recently have investigators began probing for characteristic refraction and USAXS from biological tissues. For this work, a series of four experiments were performed at the National Synchrotron Light Source (Upton, NY, USA) beamline X15A to help characterize DEI of bone. In the first experiment, the USAXS profile was measured for pre- and post-fatigue loaded cortical bone. Though no clear pattern of change in the USAXS profile was found, the bone samples were shown to have a measurable USAXS signal and it was found that large refracting structures within bone (>100 microns) could be visualized. In the next two experiments, the contrast of DEI?s refraction and apparent absorption images was compared to the contrast in synchrotron radiation (SR) radiographs for planar imaging of gap regions in bone and for imaging of trabecular structure in tomography mode. DEI was shown to have significant contrast-to-noise ratio gains over SR radiographs in both experiments. The planar refraction and apparent absorption signals in the gap imaging experiment were shown to be consistent with their theoretically predicted values. DEI in tomography mode (DECT) was found to have significant resolution gains over comparably obtained SRCT images. In the final experiment, a computer model was developed to predict USAXS from cortical bone and the computer model results were compared to USAXS data obtained using DEI. The scattering widths, as predicted by the computer model, suggest that osteocyte lacunae cause the experimentally measured angular spreading of the x-ray beam. The findings of these experiments provide the impetus for further studies of bone with DEI emphasizing clinical applications.
Coaxial Atmospheric Pressure Plasma Discharge for Treatment of Filaments and Yarns
(2008-01-08) Lee, Kyoung Ook; Keith Weninger, Committee Member; Orlando E. Hankins, Committee Member; Mohamed A. Bourham, Committee Chair
Electronic Transition Imaging of Carbon Based Materials: The Photothreshold of Melanin and Thermionic Field Emission from Diamond
(2006-12-06) Garguilo, Jacob Marshall; Glenn Edwards, Committee Member; Harald Ade, Committee Member; Robert Nemanich, Committee Chair; Keith Weninger, Committee Member; Ron Scattergood, Committee Member
This study explores electronic transitions in carbon based materials through the use of a custom built, non rastering electron emission microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The materials examined fall into two groups, melanosome films isolated from the human body and polycrystalline diamond tip arrays. A novel technique for determining the photothreshold of a heterogeneous material on a microscopic or smaller scale is developed and applied to melanosome films isolated from the hair, eyes, and brain of human donors. The conversion of the measured photothreshold on the vacuum scale to an electrochemical oxidation potential is discussed and the obtained data is considered based on this conversion. Pheomelanosomes isolated from human hair are shown to have significantly lower photoionization energy than eumelanosomes, indicating their likelihood as sources of oxidative stress. The ionization energies of the hair melanosomes are checked with complimentary procedures. Ocular melanosomes from the retinal pigment epithelium are measured as a function of patient age and melanosome shape. Lipofuscin, also found in the eye, is examined with the same microscopy technique and shown to have a significantly lower ionization threshold than RPE melanosomes. Neuromelanin from the substantia nigra is also examined and shown to have an ionization threshold close to that of eumelanin. A neuromelanin formation model is proposed based on these results. Polycrystalline diamond tip arrays are examined for their use as thermionic energy converter emitters. Thermionic energy conversion is accomplished through the combination of a hot electron emitter in conjunction with a somewhat cooler electron collector. The generated electron current can be used to do work in an external load. It is shown that the tipped structures of these samples result in enhanced emission over the surrounding flat areas, which may prove valuable in limiting the negative space charge effect in vacuum energy converting devices. Additionally, the effects of exceeding a threshold temperature for the films are shown, establishing a maximum operating regime for any device which incorporates hydrogen terminated diamond.
Finite-Difference Time-Domain Analysis of Periodic Anisotropic Media
(2007-08-07) Oh, Chulwoo; Michael J. Escuti, Committee Chair; Gianluca Lazzi, Committee Member; Keith Weninger, Committee Member
The Finite-Difference Time-Domain (FDTD) method is a numerical technique for solving electromagnetic propagation and scattering problems. The FDTD method has been one of the most popular numerical tools in the computational electromagnetics since Kane Yee proposed his efficient and stable algorithm, often called the Yee algorithm. Recent rapid development in computer technologies in the last two decades allows us to have more power in computation and memory capacity, which overcomes the computationally intensive nature, the main hindrance of wide use of the FDTD method. Still, emerging new applications need modification to the original FDTD algorithm. For the applications of periodic structures such as gratings and photonic crystals, the FDTD method can be much more efficient and accurate by taking the advantage of the periodicity of structures. The simulation space can be dramatically reduced into only one unit cell by enforcing periodic boundary conditions (PBC). An efficient way of implementing PBCs is the split-field update method. The main advantage of the split-field FDTD method is its capability of wideband simulation at oblique incidence. However, the previous works were limited to materials that are either isotropic or that have diagonal tensors. Here we present a modified FDTD algorithm for periodic structures in more general anisotropic media, which incorporates the nondiagonal permittivity tensor. PBCs are implemented by using the split-field technique. Validation of the new FDTD method is done by applying it for problems of different structures and comparing the results from FDTD simulations with other analytical or numerical solutions. We report a rigorous numerical analysis of the Polarization Grating (PG) at the first time. Diffraction properties such as the diffraction efficiency and the polarization selectivity of each diffraction order are analyzed for all kinds of PGs. We discuss the effect of the grating regimes and the finite grating width on the diffraction properties of PGs. In addition, the minimum number of grating periods within a single pixel to get high diffraction effciency is presented. We apply this FDTD method, as a simulation tool, to analyze three different structures of broadband PGs. The optical performance of each structure is evaluated by the results from FDTD simulations in terms of the bandwidth for the maximum diffraction effciency. The achromatic PG using three gratings shows the best bandwidth and the twist-PG using two gratings with the opposite twist sense is found to be most attractive because of its simple structure and fairly large bandwidth. Preliminary experimental studies are also presented. Here we report a rigorous numerical analysis of polarization gratings for the first time. To this end, we develop the modified FDTD algorithm for periodic anisotropic media. We successfully demonstrate the FDTD method for a number of problems with different structures and the results from FDTD simulations show excellent agreement with other analytical or numerical solutions. Finally, we present the extensive analysis of polarization gratings and novel structures of broadband PG using our FDTD simulation tool. A package of the FDTD code written in the standard C⁄C++ language will be available in public as an open source.
Molecular Dynamics in Self-assembled Monolayers and Polymers studied via sensitive Dielectric Spectroscopy
(2009-08-07) Stevens, Derrick; Laura Clarke, Committee Chair; Keith Weninger, Committee Member; Karen Daniels, Committee Member; David Brown, Committee Member
For many molecular systems, interpreting experimental molecular dynamics, by studying the response of a system to external stimuli, is a difficult task. Often the experimental response cannot be reasonably connected to a specific molecular motion. The aim of this work is to examine molecular systems where this difficulty can be overcome. We use sensitive dielectric spectroscopy to investigate the molecular dynamics of two different systems, chlorosilane self-assymbled monolayers and modified siloxane polymers. The polymers studied responded to changes in their surrounding media by altering their wetting characterstics. Because this macroscopic responsive is present, we are able correlate the microscopic response (as measured by dielectric spectroscopy) to likely molecular motions. The goal of the self-assembled monolayer work is similar albeit by a different approach. In this case, the degrees of freedom were limited by using surface bound molecules. By controlling the molecular density we are able to investigate both local, non-cooperative motions as well as interacting dynamics. Specifically, we will show a connection between the interacting dynamics of the self-assembled monolayers to glass transitions found in more complicated materials.
Optical and Electrical Enhancement of Organic Solar Cells.
(2009-08-10) Meyer, Aric; Harald Ade, Committee Chair; Keith Weninger, Committee Member; Jack Rowe, Committee Member
Organic solar cells have the potential to offer low cost, mass produced, solar energy generation, but further research is required to increase efficiency, improve lifetime, and reduce production costs before the low cost goal can be achieved. Current research toward improving efficiency is focused on developing new materials with better absorption and charge transport properties, and on improving morphology. There are other pursuits that could also significantly improve efficiency yet are relatively neglected by researchers. Two of these are optical light trapping to increase absorption and control of interfaces to reduce energy loss. This thesis makes contributions to both of these goals in three ways. First, some surprising results from the optical modeling of organic solar cells at non-normal incidence are described. Second, a general method for assessing the potential of a promising light trapping geometry is introduced, and discrepancies between modeling and experiment of that structure are explained. Finally, the effect of air exposure on single material and bulk heterojunction solar cells with InGa cathode is investigated. Air exposure is found to increase open circuit voltage, and using this method the open circuit voltage of a common organic solar cell system is increased by 15%. This work exposes the current lack of knowledge of many aspects of band structure in organic solar cells which suggests the need for more research in this area.
Single-molecule Surface Studies of Fibrinogen and DNA on Semiconductors
(2008-11-14) Kong, Xianhua; Tzy-Jiun Mark Luo, Committee Member; Keith Weninger, Committee Member; Robert Riehn, Committee Member; J. E. Rowe, Committee Chair; Robert Nemanich, Committee Co-Chair
Understanding of protein adsorption onto non-biological substrates is of fundamental interest in science, but also has great potential technological applications in medical devices and biosensors. This study explores the non-specific interaction, at the single molecule level, of a blood protein and DNA with semiconductor surfaces through the use of a custom built, non rastering electron emission microscope and a scanning probe microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The protein examined in this study is human plasma fibrinogen, which plays an important role in haemostatis and thrombosis, and deoxyribonucleic acid (DNA) is also studied. A novel technique for determining the photothreshold of biomolecules on single molecule level is developed and applied to fibrinogen molecules adsorbed on oxidized silicon surfaces, using photo-electron emission microscopy (PEEM). Three theoretical models are employed and compared to analyze the experimental photothreshold data. The non-specific adsorption of human plasma fibrinogen on oxidized p- and n- type silicon (100) surfaces is investigated to characterize both hydrophobic interactions and electrostatic forces. The experimental results indicate that hydrophobic interactions are one of the driving forces for protein adsorption and the electrostatic interactions also play a role in the height of the fibrinogen molecules adsorbed on the surface. PEEM images establish a photo threshold of 5.0 Â± 0.2 eV for fibrinogen on both n- type and p- type Si (100) surfaces. We suggest that the photothreshold results from surface state associated Fermi level (EF) pinning and there exists negative charge transfer from the adsorbed fibrinogen onto the p- type silicon substrates, while on n-type silicon substrates negative charge is transferred in the opposite direction. The adsorption of deoxyribonucleic acid (DNA) on mica and silicon is studied in liquid and ambient environments with atomic force microscopy (AFM). Its interactions with fibrinogen proteins co-adsorbed on surfaces exhibit an interesting desorption effect. The photoelectric imaging of DNA adsorbed on silicon is studied in ultra-high vacuum. A contrast reversal is observed on Si (111) depending on different surface pretreatments, which we suggest is due to the surface states induced photoemission. Several semiconductor materials, including Si(100), Si (111), diamond-like carbon (DLC) films, single crystal diamond (SCD) (100), nano-crystalline diamond (NCD) films, silicon carbide (SiC) (0001), and graphene, are examined for biocompatibility in applications such as medical implants and biosensors. In conjunction with other studies in the literature, we suggest that DLC, NCD, and SiC are suitable for biosensor applications.