A series of accident simulations were performed using INL's thermal hydraulics code RELAP5-3D to analyze steady-state and transient behavior of a sodium cooled fast reactor. The reactor chosen for this study was General Electric's S-PRISM, which is a 1,000 MWt pool-type sodium-cooled fast reactor, designed for either an Oxide or Metal fueled core. Once key core characteristics including power profiles, reactivity feedback coefficients and delayed neutron parameters were calculated, S-PRISM was redesigned for a Nitride fueled core to take advantage of the Nitride fuel's high thermal conductivity and melting temperature. Loss of flow, loss of heat sink, loss of power and inadvertent control rod withdrawal accidents were simulated for each core at beginning, middle and end of cycle to determine if one fuel type provides significant safety advantages over the others.

This work targeted the growth of gadolinium (Gd)-doped gallium nitride (GaN) thin films (Ga₁₋ₓGdₓN) by metal organic chemical vapor deposition (MOCVD). Characterization and evaluation of these Ga₁₋ₓGdₓN thin films for application in spintronics/optoelectronics devices also formed part of this work. This work presents: (1) the first report of stable, reproducible n- and p-type Ga₁₋ₓGdₓN thin films by MOCVD; (2) the first Ga₁₋ₓGdₓN p-n diode structure; and (3) the first report of a room temperature spin-polarized LED using a Ga₁₋ₓGdₓN spin injection layer. The Ga₁₋ₓGdₓN thin films grown in this work were electrically conductive, and co-doping them with Silicon (Si) or Magnesium (Mg) resulted in n-type and p-type materials, respectively. All the materials and structures grown in this work, including the Ga₁₋ₓGdₓN-based p-n diode and spin polarized LED, were characterized for their structural, optical, electrical and magnetic properties. The spin-polarized LED gave spin polarization ratio of 22% and systematic variation of this ratio at room temperature with external magnetic field was observed.

Shock compaction experiments were performed on soft magnetic phases Fe₄N and Fe₁₆N₂, and hard magnetic phases Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ in order to determine their thermo-mechanical stability during shock loading and explore the possibility of fabricating a textured nanocomposite magnet. Gas gun experiments performed on powders pressed in a three capsule fixture showed phase transformations occurring in Fe₄N, Fe₁₆N₂, and Nd₂Fe₁₄B, while Sm₂Fe₁₇N₃ was observed to be relatively stable. Shock compaction of FCC Fe₄N resulted in a partial transformation to HCP Fe₃N, consistent with previous reports of the transition occurring at a static pressure of ~3 GPa. Shock compaction of Fe₁₆N₂ produced decomposition products alpha-Fe, Fe₄N, and FeN due to a combination of thermal effects associated with dynamic void collapse and plastic deformation. Decomposition of Nd-Fe-B, producing alpha-Fe and amorphous Nd-Fe-B, was observed in several shock consolidated samples and is attributed to deformation associated with shock compaction, similar to decomposition reported in ball milled Nd-Fe-B. No decomposition was observed in shock compacted samples of Sm-Fe-N, which is consistent with literature reports showing decomposition occurring only in samples compacted at a pressure above ~15 GPa. Nd-Fe-B and Sm-Fe-N were shown to accommodate deformation primarily by grain size reduction, especially in large grained materials. Hard/Soft composite magnetic materials were formed by mixing single crystal particles of Nd-Fe-B with iron nanoparticles, and the alignment-by-magnetic-field technique was able to introduce significant texture into green compacts of this mixture. While problems with decomposition of the Nd₂Fe₁₄B phase prevented fabricating bulk magnets from the aligned green compacts, retention of the nanoscale morphology of the alpha-Fe particles and the high alignment of the green compacts shows promise for future development of textured nanocomposite magnets through shock compaction.

As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interests worldwide. In recent years, the graphitic carbon nitride (g-C3N4) has shown new possible applications in the photocatalytic field due to its unique properties. However, the photocatalytic efficiency of the pristine g-C3N4 is greatly limited by the high recombination rate of the photo-induced electron-hole pairs. In this thesis, the aim is to design and fabricate efficient g-C3N4 based photocatalysts with enhanced photocatalytic activities under a visible light irradiation. In order to achieve this goal, two strategies have been employed in the present thesis. First, the as-obtained g-C3N4 was used as the host material to construct staggered-aligned composite photocatalysts by selecting semiconductors with suitable band positions. By this method, three kinds of g-C3N4-based composite photocatalysts such as g-C3N4/ZnS nanocage, g-C3N4/m-Ag2Mo2O7 and g-C3N4/MIL-88A were successfully fabricated. Second, the microstructure of the g-C3N4 was modified by the H2O2-treatment at an elevated temperature and ambient pressure. In this study, the g-C3N4 was prepared by a simple pyrolysis of urea. As for all the as-synthesized phtocatalysts, the structures, morphologies and the optical properties were carefully characterized by the following techniques: XRD, SEM, TEM, FT-IR and DRS. Also, the band edge positions of m-Ag2Mo2O7 and MIL-88A were studied by the Mott-Schottky methods. Thereafter, the photocatalytic activities were evaluated by using a solution of rhodamine B (RhB) as a target pollutant for the photodegradation experiments performed under a visible light irradiation. The results showed that all the aforementioned g-C3N4-based photocatalysts exhibited enhanced photocatalytic activities in comparison with the pristine g-C3N4. For the case of the g-C3N4-based composite photocatalysts, the enhancement factor over the pristine g-C3N4 can achieve values ranging from 2.6 to 3.4. As for the H2O2-treated g-C3N4, the degradation rate constant can be 4.6 times higher than that of the pristine g-C3N4. To understand the key factors in new materials design, we also devote a lot of efforts to elucidate the basic mechanisms during the photocatalytic degradation of organic pollutant. Based on the results of the active species trapping (AST) experiments, the main active species in each photocatalytic system were determined. In the g-C3N4/m-Ag2Mo2O7 and the g-C3N4/MIL-88A system, three kinds of active species of ·O2-, h+ and ·OH were found to be involved in the photocatalytic reaction. Among them, the ·O2- and h+ were the main active species. In the g-C3N4/ZnS and H2O2-treated g-C3N4 photocatalytic systems, the main active species was determined as the ·O2-. The reaction pathways of these active species were also demonstrated by comparing the band edge positions with the potentials of the redox couple. In addition, the relationship between the active species and the photocatalytic behaviors of N-de-ethylation and conjugated structure cleavage were studied. Finally, possible mechanisms to explain the enhanced photocatalytic activities were proposed for each photocatalytic system. The results in this thesis clearly confirm that the photocatalytic activity of the g-C3N4 based photocatalyst can efficiently be enhanced by constructions of staggered-aligned composites and by modification of the microstructure of the g-C3N4. The enhanced photocatalytic performance can mainly be ascribed to the efficient separation of the photo-induced electron-hole pairs and the increase of the active sites for the photocatalytic reaction. / <p>QC 20150909</p>

The thesis is dedicated to investigation of carrier dynamics in InN, InGaN, and GaAsBi heterostructures by using light-induced transient gratings and differential transmission techniques. The experimental studies in a wide range of excess carrier densities and temperatures revealed that trap-assisted Auger recombination is the dominant recombination mechanism in MBE-grown InN layers at room temperature. Investigation of carrier dynamics in In-rich InGaN alloys revealed that density of fast nonradiative recombination centers increases with Ga content. The correlation between excess carrier lifetime and diffusion coefficient in MOCVD-grown single InGaN layer with 13% In is governed by diffusive flow to the extended defects. Investigations of carrier lifetime and diffusivity dependence on excitation fluence indicated that both nonradiative and radiative recombination contribute to an increase of excess carrier recombination rate at high photo-excitation levels in MOCVD-grown InGaN multiple quantum wells. Transient grating measurements in MBE-grown GaAsBi layers with different Bi content revealed that Bi induced potential fluctuations determine the tenfold decrease in nonequilibrium hole mobility, if compare to GaAs. / Disertacija skirta nepusiausvirųjų krūvininkų dinamikos tyrimams InN, InGaN ir GaAsBi heterosandarose naudojant šviesa indukuotų dinaminių gardelių ir skirtuminio pralaidumo metodikas. Atlikti tyrimai plačiame nepusiausvirųjų krūvininkų tankių ir bandinio temperatūrų intervale parodė, kad krūvininkų rekombinacijos sparta MBE būdu užaugintuose InN sluoksniuose dominuojantis rekombinacijos mechanizmas kambario temperatūroje yra gaudyklių įtakota Ožė rekombinacija. Nustatyta koreliacija tarp krūvininkų gyvavimo trukmės ir difuzijos koeficiento MOCVD būdu užaugintame InGaN sluoksnyje su 13% In parodė, kad krūvininkų gyvavimo trukmę lemia difuzinė jų perneša link rekobinacijos centrų. Parodoma, kad MBE metodu užaugintuose InGaN sluoksniuose su dideliu In kiekiu (x>0,7) didėjant Ga kiekiui didėja nespindulinės rekombinacijos centrų tankis, o krūvininkų rekombinacijos sparta yra termiškai aktyvuojama. MOCVD metodu užaugintose InGaN kvantinėse sandarose dinaminių gardelių tyrimais parodoma, kad spartėjančią krūvininkų rekombinaciją didėjant sužadinimo intensyvumui lemia ne tik spindulinė rekombinacija, tačiau reikia atsižvelgti ir į nespindulinės rekombinacijos spartėjimą. Nustatyta, kad į GaAs įterpiant Bi atomus daugiau nei dešimt kartų sumažėja skylių judris dėl Bi atomų kuriamų valentinės juostos fliuktuacijų.

Disertacija skirta nepusiausvirųjų krūvininkų dinamikos tyrimams InN, InGaN ir GaAsBi heterosandarose naudojant šviesa indukuotų dinaminių gardelių ir skirtuminio pralaidumo metodikas. Atlikti tyrimai plačiame nepusiausvirųjų krūvininkų tankių ir bandinio temperatūrų intervale parodė, kad krūvininkų rekombinacijos sparta MBE būdu užaugintuose InN sluoksniuose dominuojantis rekombinacijos mechanizmas kambario temperatūroje yra gaudyklių įtakota Ožė rekombinacija. Nustatyta koreliacija tarp krūvininkų gyvavimo trukmės ir difuzijos koeficiento MOCVD būdu užaugintame InGaN sluoksnyje su 13% In parodė, kad krūvininkų gyvavimo trukmę lemia difuzinė jų perneša link rekobinacijos centrų. Parodoma, kad MBE metodu užaugintuose InGaN sluoksniuose su dideliu In kiekiu (x>0,7) didėjant Ga kiekiui didėja nespindulinės rekombinacijos centrų tankis, o krūvininkų rekombinacijos sparta yra termiškai aktyvuojama. MOCVD metodu užaugintose InGaN kvantinėse sandarose dinaminių gardelių tyrimais parodoma, kad spartėjančią krūvininkų rekombinaciją didėjant sužadinimo intensyvumui lemia ne tik spindulinė rekombinacija, tačiau reikia atsižvelgti ir į nespindulinės rekombinacijos spartėjimą. Nustatyta, kad į GaAs įterpiant Bi atomus daugiau nei dešimt kartų sumažėja skylių judris dėl Bi atomų kuriamų valentinės juostos fliuktuacijų. / The thesis is dedicated to investigation of carrier dynamics in InN, InGaN, and GaAsBi heterostructures by using light-induced transient gratings and differential transmission techniques. The experimental studies in a wide range of excess carrier densities and temperatures revealed that trap-assisted Auger recombination is the dominant recombination mechanism in MBE-grown InN layers at room temperature. Investigation of carrier dynamics in In-rich InGaN alloys revealed that density of fast nonradiative recombination centers increases with Ga content. The correlation between excess carrier lifetime and diffusion coefficient in MOCVD-grown single InGaN layer with 13% In is governed by diffusive flow to the extended defects. Investigations of carrier lifetime and diffusivity dependence on excitation fluence indicated that both nonradiative and radiative recombination contribute to an increase of excess carrier recombination rate at high photo-excitation levels in MOCVD-grown InGaN multiple quantum wells. Transient grating measurements in MBE-grown GaAsBi layers with different Bi content revealed that Bi induced potential fluctuations determine the tenfold decrease in nonequilibrium hole mobility, if compare to GaAs.

The current trend in pushing photo lithography to smaller and smaller resolutions is becoming increasingly difficult and expensive. Extreme ultra-violet lithography is an alternate method that has the potential to provide feature sizes down to 30 nm, however, it will come at an even greater cost. Nanoimprint lithography (NIL) is another lithographic technique which is promising to provide very high resolutions at a relatively low cost. Imprinting works by using a mold with a surface patterned with the required nano structures and pressing it into a substrate coated with a deformable polymer. Due to its direct pattern replication technique, it is very capable of reproducing three-dimensional structures, however limited research has been performed on this to date. In this study, investigations have been performed into developing a reliable process for creating SiN molds with sub-100 nm structures with variable height control. The process relies on a negative tone electron beam resist which can be patterned to various thicknesses by varying the exposure dosage. This allows for the creation of complex multi-layer structures in a single electron beam lithography step. These patterns then have been transferred into the SiN substrate by a single reactive ion etch. From here the mold is ready for use in imprinting. Study has also been performed into imprinting process as well. This includes the development of an imprint press, the manner in which NIL works. Investigations have been performed into the imprinting performance of 3D molds. Thermal expansion issues have been found and addressed, as have adhesion problems. Some other aspects of 3D NIL which have not been addressed in this study have been outlined in future work for further investigation.

Rare-earth ions holmium (Ho+3), Thulium (Tm+3), and Samarium (Sm+3) were investigated for infrared emission and their possible biomedical applications by a photoluminescence (PL) system. Holmium’s (Ho+3) emission peaks were the result of transitions 5 S2 → 5 I7, and 5 S2 → 5 I5 respectively. Samarium’s (Sm+3) emission peaks were 936 nm and 1863 nm. Thulium’s (Tm+3) emission peaks were the a result of transitions 3 H4 → 3 H6, 3 H5 → 3 H6 , and 3 F4 → 3 H6 respectively. Erbium Oxide nanoparticles (Er2O3) mixed with water by a photoluminescence (PL) system. Erbium Oxide’ (Er2O3) nanoparticle’s emission peaks were the a result of transitions 4 I15/2 → 4 S3/2 , 4 I15/2 → 4 I13/2 respectively. The process was also repeated in vacuum and it was found that the green emission enhances tremendously when the nanoparticles are excited in vacuum. This enhanced luminescence from the Erbium Oxide nanoparticles shows their potential importance in the optical devices and Biomedical applications. / Department of Physics and Astronomy

The prospect of producing nanometer channel-length thin film transistors (TFTs) for active matrix addressed pixelated arrays opens up new high-performance applications in which the most amenable device topology is the vertical thin film transistor (VTFT) in view of its small area. The previous attempts at fabricating VTFTs have yielded devices with a high drain leakage current, a low ON/OFF current ratio, and no saturation behaviour in the output current at high drain voltages, all induced by short channel effects. To overcome these adversities, particularly dominant as the channel length approaches the nano-scale regime, the reduction of the gate dielectric thickness is essential. However, the problems with scaling the gate dielectric thickness are the high gate leakage current and early dielectric breakdown of the insulator, deteriorating the device performance and reliability. A novel ultra-thin SiNx film suitable for the application as the gate dielectric of short channel TFTs and VTFTs is developed. The deposition is performed in a standard 13.56MHz PECVD system with silane and ammonia precursor gasses diluted in nitrogen. The deposited 50nm SiNx films demonstrate excellent electrical characteristics in terms of a leakage current of 0.1 nA/cm?? and a breakdown electric field of 5.6MV/cm. Subsequently, the state of the art performances of 0.5??m channel length VTFTs with 50 and 30nm thick SiNx gate dielectrics are presented in this thesis. The transistors exhibit ON/OFF current ratios over 10^9, the subthreshold slopes as sharp as 0.23 V/dec, and leakage currents in the fA range. More significantly, a high associated yield is obtained for the fabrication of these devices on 3-inch rigid substrates. Finally, to illustrate the tremendous potential of the VTFT for the large area electronics, a 2.2-inch QVGA AMOLD display with in-pixel VTFT-based driver circuits is designed and fabricated. An outstanding value of 56% compared to the 30% produced by conventional technology is achieved as the aperture ratio of the display. Moreover, the initial measurement results reveal an excellent uniformity of the circuit elements.

Possessing a wide band gap and large break down field, gallium nitride (GaN) is of interest for a host of high power, high frequency applications including next generation cellular base stations, advanced military radar, and WiMAX networks. Much of this interest stems from the continued development of the AlGaN/GaN high electron mobility transistor (HEMT) that is capable of operating at sizable power densities and switching speeds. The same fields responsible for this performance, however, also elicit acute device heating and elastic loads. These induced thermomechanical loads limit both performance and reliability thus necessitating continued improvement in the management and characterization of the coupled environments. In response, this study establishes a new implementation of Raman spectroscopy capable of simultaneously measuring the operational temperature and stress in a HEMT using only the Stokes response. First, the linewidth (FWHM) of the Stokes signal is utilized to quantify the operating temperature of a HEMT independent to the influences of stress. Second, a new method, incorporating the use of the linewidth and peak position in tandem, is developed to estimate the biaxial thermoelastic stress that arises during device operation. With this capability, the HEMT's resultant load is assessed, highlighting the large role of the residual stress on the total mechanical state of the device. Subsequently, this same linewidth is leveraged to identify the distinct effect that electrical carriers have on the thermally relevant decay of longitudinal optical phonon modes. Further investigation of the lattice transport then concludes the study by way of an analytical treatment describing the significant influence of interfacial disorder on the energy transport at GaN/substrate boundaries.