Microelectromechanical systems (MEMS) devices have been increasing in popularity for radio frequency (RF) and microwave communication systems due to the ability of MEMS devices to improve the performance of these circuits and systems. This interdisciplinary field combines the aspects of lithographic fabrication, mechanics, materials science, and RF/microwave circuit technology to produce moving structures with feature dimensions on the micron scale (micro structures). MEMS technology has been used to improve switches, varactors, and inductors to name a few specific examples. Most MEMS devices have been fabricated using planar micro fabrication techniques that are similar to current IC fabrication techniques. These techniques limit the thickness of individual layers to a few microns, and restrict the structures to have planar and not vertical features. <p> One micro fabrication technology that has not seen much application to microwave MEMS devices is LIGA, a German acronym for X-ray lithography, electroforming, and moulding. LIGA uses X-ray lithography to produce very tall structures (hundreds of microns) with excellent structural quality, and with lateral feature sizes smaller than a micron. These unique properties have led to an increased interest in LIGA for the development of high performance microwave devices, particularily as operating frequencies increase and physical device size decreases. Existing work using LIGA for microwave devices has concentrated on statically operating structures such as transmission lines, filters, and couplers. This research uses these unique fabrication capabilities to develop dynamically operating microwave devices with high frequency performance. <p>This thesis documents the design, simulation, fabrication, and testing of MEMS variable capacitors (varactors), that are suitable for fabrication using the LIGA process. Variable capacitors can be found in systems such as voltage-controlled oscillators, filters, impedance matching networks and phase shifters. Important figures-of-merit for these devices include quality factor (Q), tuning range, and self-resonant frequency. The simulation results suggest that LIGA-MEMS variable capacitors are capable of high Q performance at upper microwave frequencies. Q-factors as large as 356 with a nickel device layer and 635 with a copper device layer, at operational frequency, have been simulated. The results indicate that self-resonant frequencies as large as 45 GHz are possible, with the ability to select the tuning range depending on the requirements of the application. Selected capacitors were fabricated with a shorter metal height for an initial fabrication attempt. Test results show a Q-factor of 175 and a nominal capacitance of 0.94 pF at 1 GHz. The devices could not be actuated as some seed layer metal remained beneath the cantilevers and further etching is required. As such, LIGA fabrication is shown to be a very promising technology for various dynamically operating microwave MEMS devices.

The sensitivity reduction or ghosting mechanism of x-ray photoconductor is studied based on Monte Carlo simulation techniques. We have calculated the sensitivity reduction for different detector operating conditions (applied electric field, x-ray spectrum and photoconductor thickness) and for different levels of carrier trapping. We have analyzed the effect of photoconductor biasing (positive or negative) on ghosting. The following effects are taken into account in modeling the ghosting phenomena: (i) recombination between trapped and oppositely charged drifting carriers, (ii) trap filling, (iii) nonuniform electric field, (iv) detrapping of trapped holes, and (v) x-ray induced trap generation. Our calculation shows that not only the recombination between trapped and oppositely charged drifting carriers but the x-ray induced trap generation is also responsible for ghosting in photoconductor based x-ray image detectors. Moreover not all the trapped carriers take part in recombination; rather only a fraction of the trapped carriers are involved in recombination. Electric field also plays an important role in ghosting calculations via the electron hole pair generation mechanism. Trap filling has also non trivial effects on ghosting. The simulation results show that the amount of ghosting strongly depends on the applied electric field. Ghosting increases with decreasing applied electric field and vice versa. It is observed that ghosting is higher at high carrier trapping level than at low trapping level. Again ghosting is more pronounced in chest radiographic detector than mammographic detector. In chest radiographic detector, carrier trapping is high due to greater thickness hence recombination and electric field effects are prominent in chest radiographic detector. Biasing dependent ghosting depends on the carrier mobility lifetime product. For positively biased detectors, ghosting is less if the mobility lifetime product of hole is higher than that of electron and vice versa for negatively biased detectors. It also appears that the use of only recombination to calculate ghosting, as believed the primary source of ghosting in some literatures, will lead to significant error in the calculation of ghosting.

The desire for low cost electronics has led to a huge increase in research focused on organic materials. These materials are appealing due to their unique electrical and material-processing properties and are rapidly being adopted in old and new electronic applications. To create practical devices requires a further understanding of the charge transport properties of the unique anisotropic molecular crystal structures. This work looks at how doping with the transition-metal element manganese can alter the electronic structure of the organic material pentacene. It has been found that using manganese as a dopant provides novel physical characteristics previously not encountered in organic field effect transistors based on pentacene. These organic thin films were characterized using X-ray absorption spectroscopy and the results compared to computational density functional theory analysis.

The thesis presented here describes the steps that were taken in an attempt to solve the protein structure of MenD via molecular replacement and multiple wavelength anomalous dispersion. The introduction provides background on menaquinone biosynthesis and the role of MenD in this metabolic pathway. Also, a detailed discussion of the DC Family of enzymes, a subgroup of ThDP dependent enzymes, which MenD is a part of, is included.<p> Utilizing various software packages a 1.9 Å data set was processed and analyzed in an attempt to provide a molecular replacement result. When molecular replacement was deemed incapable of solving the phase problem of the data set, the production of SeMet protein was attempted to allow for MAD phasing.<p> A homology model of MenD was produced using the program Modeller with benzaldehyde lyase as a template. A structure based sequence alignment was done with all DC Family enzymes with structures published. Then a second structure based sequence alignment was done to compare the same set to the Modeller model. This was done to gain a deeper understanding of MenD and how it interacts with its cofactors ThDP and Mg2+. Furthermore, these results were used to implicate potential active site residues.

The "n-like layer" is important in multilayer layer amorphous selenium (a-Se) based Xray detector structures because it blocks the injection of holes from the positive electrode. The dark current in these devices is controlled primarily by hole injection,and the introduction of the n-like layer to block hole injection was a key development in the commercialization of a-Se X-ray detectors. An n-like a-Se layer is defined as a layer in which the electron range is much greater than the hole range, ¦Ìe¦Óe >> ¦Ìh¦Óh, where ¦Ó and ¦Ì are the lifetime and drift mobility of the charge carriers and the subscript e and h represent electrons and holes.<p> This thesis examines the effect of doping a-Se with Group II elements (in particular Mg) towards finding a better n-like layer ¨C that with relatively long electron range (drift mobility ¡Á lifetime) , trap limited hole transport and which is stable against crystallization. Conventional Time of Flight (TOF) and Interrupted Field Time of Flight (IFTOF) transient photoconductivity measurements were used to characterize the electron and hole transport in various Group II doped a-Se layers. The dependence of the electron and hole lifetimes and drift mobilities on the composition of the n-like layer was examined. The addition of Group II materials converts the a-Se starting material from p-like into n-like. It was found that increasing the concentration of Mg increases the electron range while limiting the hole range by modifying the population of deep traps. The addition of As further limits the hole transport but does not alter the electron range. The clear reproducibility of the thermal properties obtained from the Differential Scanning Calorimetry (DSC) implies that small amounts of Mg can be used as a suitable n-type dopant.

X-ray medical imaging provides invaluable medical information, while subjecting patients to hazardous ionizing radiation. The dosage that the patient is exposed to may be reduced, at the cost of image resolution. A technology that promises lower dosage for a given resolution is direct conversion digital imaging, typically based on amorphous Selenium semiconductor. Sufficient exposure should be used for the first exposure to avoid subsequent exposures; a challenge is then to reduce the necessary exposure for a suitable image. To quantify how little radiation the detector can reliably discriminate, one needs an analysis of the variance that 1/f and white noise contribute to the signal of such detectors. An important consideration is that the dark current, which varies with time, is subtracted from the photo-current, to reduce the spurious spatial variance in the image. In this thesis, the variance that 1/f noise contributes to integrating detectors is analysed, for a very general integrating detector. Experiments were performed to verify the theoretical results obtained for the 1/f noise variance contribution.

An overview of synchrotrons and synchrotron radiation is presented, along with the theory and practical considerations behind several types of X-ray spectroscopy. The theory and practical considerations of density functional theory are also given, with direct reference to some specific software packages.<p> Some synchrotron-excited X-ray spectroscopy measurements and density functional theory calculations of selenium and arsenic-doped selenium films are then outlined. The physical structure of crystalline and amorphous selenium and the electronic structure of amorphous selenium are discussed and comparison is made to the experimental results.<p> A weak feature in the conduction band is identified as a "fingerprint" of the degree of crystallization in amorphous selenium from X-ray absorption measurements. Similarly, a weak feature corresponding to lone-pairs in the valence band is identified as a "fingerprint" of the arsenic concentration from X-ray emission measurements.<p> Finally a detailed model of the structure of amorphous selenium is explained, and compared to experiment. This model is tested both by direct calculations and by a reverse Monte Carlo approach. The implications of this model with respect to the structure of amorphous and arsenic-doped amorphous selenium are discussed. Calculations suggest that simply randomizing the arrangement of "perfect" trigonal selenium is unable to reproduce the measurements of amorphous selenium; a moderate variation in the bond angle of "perfect" trigonal selenium is also necessary.

Chronic arsenic poisoning due to arsenic contamination of groundwater is a serious public health problem in Bangladesh and neighboring countries. Severe health effects associated with chronic exposure to arsenic include melanosis and several kinds of cancer. It is now generally agreed that the arsenic contamination of groundwater in Bangladesh is of geological origin. Arsenic naturally present in aquifers may be mobilized into drinking water by microbial action.<p> The formation of a novel arsenic-selenium compound: seleno-bis (S-glutathionyl) arsinium ion, [(GS)2AsSe]-, and its subsequent excretion in rabbit bile has been demonstrated previously. This molecular basis for the in vivo antagonism between arsenic and selenium was discovered using X-ray absorption spectroscopy. There is growing evidence that, in Bangladeshi people who are suffering long term chronic lowlevel arsenic poisoning, this antagonism is causing a selenium deficiency. Administering selenium supplements might provide a simple but highly effective treatment of the Bangladeshi arsenic poisoning.<p> In order to examine the disposition of [(GS)2AsSe]-, a set of rabbits were intravenously injected with selenite, arsenite or both. Whole blood, red blood cell and plasma samples were collected at different time intervals within 2hrs after injection and cecotrope samples 24hr after injection. Samples were examined using X-ray absorption spectroscopy and both arsenic and selenium K-near edge spectra were recorded.<p> iii Speciation of arsenic and selenium will be discussed in this thesis. Results indicate that [(GS)2AsSe]- is formed in blood very rapidly after injection of both arsenite and selenite, and then is removed from blood stream within 2hrs post injection. Results also show that [(GS)2AsSe]- is assembled in red blood cells, with no [(GS)2AsSe]- detected in plasma samples. [(GS)2AsSe]- is also found in cecotrope samples after injection of both arsenite and selenite.<p> The results of this study in rabbits will contribute to the understanding of chronic arsenic poisoning in humans.

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This thesis presents a lithic attribute and geochemical analysis of the lithic material recovered from coastal site of Fatumafuti, on Tutuila Island, in the Samoan archipelago during 1050-520 BP. The goal of this thesis is to clarify the nature of stone tool production and to add to our current understanding of the cultural transformations from Lapita to a Polynesian identity. To complete this goal four research questions are addressed. What is the stage of reduction (cha ne operatoire) at Fatumafuti? Does the assemblage vary over space and time? Where did the source material come from? And, what was the organization of lithic craft production? Specifically, is there evidence for specialization? The lithics at Fatumafuti contain multiple segments in the technical sequence of tool manufacture (cha ne operatoire). The two major segments are middle stage and late stage reduction, and two minor segments are early stage reduction and tool rejuvenation. Expedient tools found on site indicate that prehistoric groups did not rely on a completely curated technology. Tool manufacture was geared toward producing a variety of tools, as opposed to a specific product. Production was most intense towards the coastal portion of the site during the earlier cultural component and then shifted towards the talus base during the later cultural component. Using non-destructive Energy Dispersive X-Ray Fluorescence (EDXRF), elemental concentrations were analyzed and compared to those of Tataga-matau, Lau?agae, Asiapa and Alega. One, possibly two, sources were utilized at this site; however, they are not chemically similar to Tatagamatau, Lau'agae, Asiapa and Alega. I conclude that people of Fatumafuti practiced independent household production at the end of the Aceramic and beginning of the Recent period. Either the intensification of lithic craft production that is seen during the height of complex chiefdoms is not seen at Fatumafuti, or these social transformations had not yet taken hold. With more cases that date to this time, we may find that Samoan chiefdoms had not attained full complexity at this point.