Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing. A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM.

Analysis of the electronic structures of nitrogen-doped, amorphous carbon samples and of nanodiamond films are carried out in order to determine their sp2 bonding concentration. The amorphous carbon samples under consideration are deposited onto polytetrafluoroethylene (PTFE) polymer substrates by hot wire plasma sputtering of graphite in varying nitrogen concentration atmospheres. The deposition or modification of the substrates surface may lend itself to increasing hardness and wear resistance. Eventually these polymer substrates may be used for applications in the field of biomaterials, focusing on cardiovascular surgery, where a low blood/surface interaction is important. The primary technique used in this study is x-ray absorption spectroscopy, measured at the Advanced Light Source synchrotron at the Lawrence Berkeley National Laboratory, Berkeley, USA. A method of analyzing these spectra was then developed to determine the sp2 bonding concentrations in carbon films. Through this newly developed analysis method, the sp2 bonding concentrations in these samples increases from 74 to 93% with growing nitrogen doping. The diamond films presented here are deposited on silicon wafer substrates in a methane atmosphere by microwave plasma deposition. Various deposition conditions, such as bias voltage and methane atmosphere concentration, affect the purity of the diamond film. This analysis reveals sp2 bonding concentrations in these samples from, typically, a few percent to 25%.

Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing. A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM.

We focus on determining the underlying physical cause of a Seyfert galaxy's appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical Seyfert 1 nuclei reddened by central dusty tori or by nuclear dust lanes/spirals in the narrow-line region? Or, are they similar to NGC 2992, objects that have intrinsically weak continua and weak broad emission lines? Our study compares measurements of the reddenings of the narrow and broad-line regions with each other and with the X-ray column derived from XMM Newton 0.5-10 keV spectra to determine the presence and location of dust in the line of sight for a sample of 35 Seyfert 1.8s and 1.9s. From this, we find that Seyfert 1.9s are an almost equal mix of low-flux objects with unreddened broad line regions, and objects with broad line regions reddened by an internal dust source, either the torus or dust structures on the same size scale as the narrow line region. The 1.9s that recieved this designation due to a low continuum flux state showed variable type classifications. All three of the Seyfert 1.8s in our study are probably in low continuum states. Many objects have been misclassified as Seyfert 1.8/1.9s in the past, probably due to improper [N II]/H-alpha deconvolution leading to a false detection of weak broad H-alpha.

Analysis of the electronic structures of nitrogen-doped, amorphous carbon samples and of nanodiamond films are carried out in order to determine their sp2 bonding concentration. The amorphous carbon samples under consideration are deposited onto polytetrafluoroethylene (PTFE) polymer substrates by hot wire plasma sputtering of graphite in varying nitrogen concentration atmospheres. The deposition or modification of the substrates surface may lend itself to increasing hardness and wear resistance. Eventually these polymer substrates may be used for applications in the field of biomaterials, focusing on cardiovascular surgery, where a low blood/surface interaction is important. The primary technique used in this study is x-ray absorption spectroscopy, measured at the Advanced Light Source synchrotron at the Lawrence Berkeley National Laboratory, Berkeley, USA. A method of analyzing these spectra was then developed to determine the sp2 bonding concentrations in carbon films. Through this newly developed analysis method, the sp2 bonding concentrations in these samples increases from 74 to 93% with growing nitrogen doping. The diamond films presented here are deposited on silicon wafer substrates in a methane atmosphere by microwave plasma deposition. Various deposition conditions, such as bias voltage and methane atmosphere concentration, affect the purity of the diamond film. This analysis reveals sp2 bonding concentrations in these samples from, typically, a few percent to 25%.

<p>Silicon ceramics are popular in both commercial applications and material research. The purpose of this thesis is to present measurements and analysis of four different silicon ceramics: á, â and ã phases of silicon nitride and silicon oxynitride using soft x-ray spectroscopy, which analyses the electronic structure of materials by measuring the absorption and emission of x-ray radiation. Absorption and emission spectra of these materials are presented, many of which have not be previously documented. The results are compared to model spectra and together they provide information about the electronic structure of the material.</p><p>Assignments of emission features to element, orbital, and site symmetry are performed for each material. Combinations of silicon and nitrogen emission spectra provide insight into the strained bonding structure of nitrogen. It is concluded that p-dð interaction plays a role in the bonding arrangement of nitrogen and oxygen sites within these structures. The emission features of non-equivalent silicon sites within ã-Si3N4 are identified, which represents some of the first analysis of same element, non-equivalent sites in a material.</p><p>Silicon absorption and emission spectra were plotted on the same energy scale to facilitate measurement of the band gap. Since previously measured band gaps are not well represented in literature, the measured band gaps were compared to values predicted using DFT calculations. The band gap values are in reasonable agreement to calculated values, but do not vary as widely as predicted.</p>

An x-ray fluorescence detection system has been designed by our research group for quantifying the amount of gold nanoparticles presented within the phantom and animals during gold nanoparticle-aided cancer detection and therapy procedures. The primary components of the system consist of a microfocus x-ray source, a Pb beam collimator, and a CdTe photodiode detector. In order to optimize and facilitate future experimental tasks, a Monte Carlo model of the detection system has been created by using the MCNP5 code. Specifically, the model included an x-ray source, a Pb collimator, a CdTe detector, and an acrylic plastic phantom with four cylindrical columns where various materials such as gold nanoparticles, aluminum, etc. can be inserted during the experiments. In this model, 110 kVp x-rays emitted into a 60o cone from the focal spot of the x-ray source were collimated to a circular beam with a diameter of 5 mm. The collimated beam was then delivered to the plastic phantom with and without a gold nanoparticle-containing column. The fluence of scattered and gold fluorescence x-rays from the phantom was scored within the detector's sensitive volume resulting in various photon spectra and compared with the spectra acquired experimentally under the same geometry. The results show that the current Monte Carlo model can produce the results comparable to those from actual experiments and therefore it would serve as a useful tool to optimize and troubleshoot experimental tasks necessary for the development of gold nanoparticle-aided cancer detection and therapy procedures.

This research developed an on-line measurement systemfor determining the amount of fluorochemicals on carpet fibers using energy-dispersive X-ray fluorescence (EDXRF).This system is designed as a complementary tool to an existingchemical burn test certified by the American Association ofTextile Chemists and Colorists (AATCC), which is performed off-line on randomly selected carpet samples and time consuming.This research reviewed XRF principles and determined parameters that affect XRF spectra such as measurement time, measurement number, X-ray tube voltage, X-ray tube current, primary beam filter, and carpet characteristics. For this application, XRF calibrations must be performed for carpets of all styles and types. However, preparing actual carpet calibration samples is expensive. This research introduced a methodology to synthetically generate reference spectra using XRF spectra from standard fluorochemical stock solution samples and from base carpet samples for each carpet type to be tested. Thus, actual, physical standards are not required for each carpet type or style. This study showed that the synthetically generated XRF spectra alone were not always sufficient to guarantee the confidence interval required by the certified AATCC test. Thus, it is recommended that for on-line implementation, burn test results should be used to create a historical data base for each carpet type to reduce margin of error for calibrations generated from the synthetic spectra.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2002. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

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