Master of Science / Department of Chemical Engineering / James H. Edgar / 4H-silicon carbide (4H-SiC) is a wide band gap semiconductor with outstanding capabilities for high temperature, high power, and high frequency electronic device applications. Advances in its processing technology have resulted in large micropipe-free single crystals and high speed epitaxial growth on off-axis silicon face substrates. Extraordinarily high growth rates of high quality epitaxial films (>100 [Mu]m per hour) have been achieved, but only on off-axis substrates (misoriented 4° to 8° from the (0001) crystallographic plane). There is a strong incentive to procure an on-axis growth procedure, due to the excessive waste of high quality single crystal associated with wafering off-axis substrates. The purpose of this research was to develop a reliable process for homoepitaxial growth of 4H-SiC on on-axis 4H-SiC. Typically the use of on-axis SiC for epitaxial growth is undesired due to the increased probability of 3C-SiC inclusions and polycrystalline growth. However, it is believed that the presence of chlorine during reaction may reduce the presence of 3C-SiC and improve the quality of the epitaxial film. Therefore homoepitaxial SiC was deposited using methyltrichlorosilane (MTS) and ethane sources with carrier gases consisting of argon-hydrogen mixtures. Ethane was used to increase the C/Si ratio, to aid in the prevention of 3C-SiC, and to help eliminate silicon droplets deposited during epitaxial growth. Deposition occurred in a homemade, quartz, cold wall chemical vapor deposition reactor. Epitaxial films on on-axis 4H-SiC were deposited without the presence of 3C-SiC inclusions or polycrystalline SiC, as observed by defect selective etching, scanning electron microscopy and optical microscopy. Large defect free areas, [similar to]5 mm[superscript]2, with epitaxial film thicknesses of [similar to]6 [Mu]m were grown on on-axis 4H-SiC. Epitaxial films had approximately an 80%, [similar to]20 cm[superscript]-2, decrease in defect density as compared to the substrates. The growth rate was independent of face polarity and orientation of the substrate. The optimal temperature for hydrogen etching, to promote the smoothest epitaxial films for on-axis substrates (both C- and Si-polarities), is [similar to]1550 °C for 10 minutes in the presence of 2 slm hydrogen. The optimum C/Si ratio for epitaxial growth on on-axis 4H-SiC is 1; excess carbon resulted in the codeposition of graphite and cone-shaped silicon carbide defects.

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / Laser-assisted machining (LAM) is a promising non-conventional machining technique for advanced ceramics. However, the fundamental machining mechanism which governs the LAM process is not well understood so far. Hence, the main objective of this study is to explore the machining mechanism and provide guidance for future LAM operations. In this study, laser-assisted milling (LAMill) of silicon nitride ceramics is focused. Experimental experience reveals that workpiece temperature in LAM of silicon nitride ceramics determines the surface quality of the machined workpiece. Thus, in order to know the thermal features of the workpiece in LAM, the laser-silicon nitride interaction mechanism is investigated via heating experiments. The trends of temperature affected by the key parameters (laser power, laser beam diameter, feed rate, and preheat time) are obtained through a parametric study. Experimental results show that high operating temperature leads to low cutting force, good surface finish, small edge chipping, and low residual stress. The temperature range for brittle-to-ductile transition should be avoided due to the rapid increase of fracture toughness. In order to know the temperature distribution at the cutting zone in the workpiece, a transient three-dimensional thermal model is developed using finite element analysis (FEA) and validated through experiments. Heat generation associated with machining is considered and demonstrated to have little impact on LAM. The model indicates that laser power is one critical parameter for successful operation of LAM. Feed and cutting speed can indirectly affect the operating temperatures. Furthermore, a machining model is established with the distinct element method (or discrete element method, DEM) to simulate the dynamic process of LAM. In the microstructural modeling of a β-type silicon nitride ceramic, clusters are used to simulate the rod-like grains of the silicon nitride ceramic and parallel bonds act as the intergranular glass phase between grains. The resulting temperature-dependent synthetic materials for LAM are calibrated through the numerical compression, bending and fracture toughness tests. The machining model is also validated through experiments in terms of cutting forces, chip size and depth of subsurface damage.

Doctor of Philosophy / Department of Physics / Jingyu Lin / Time-resolved deep ultraviolet (DUV) Photoluminescence (PL) spectroscopy system has been employed to systematically monitor crystalline quality, identify the defects and impurities, and investigate the light emission mechanism in III-nitride semiconducting materials and photonic structures. A time correlated single photon counting system and streak camera with corresponding time resolutions of 20 and 2 ps, respectively, were utilized to study the carrier excitation and recombination dynamics. A closed cycle He-flow cryogenic system was employed for temperature dependent measurements. This system is able to handle sample temperatures in a wide range (from 10 to 900 K). Structural, electrical, and morphological properties of the material were monitored by x-ray diffraction (XRD), Hall-effect measurement, and atomic force microscopy (AFM), respectively. Most of the samples studied here were synthesized in our laboratory by metal organic chemical vapor deposition (MOCVD). Some samples were bulk AlN synthesized by our collaborators, which were also employed as substrates for homoepilayer growth. High quality AlN epilayers with (0002) XRD linewidth as narrow as 50 arcsec and screw type dislocation density as low as 5x10[superscript]6 cm[superscript]-2 were grown on sapphire substrates. Free exciton transitions related to all valence bands (A, B, and C) were observed in AlN directly by PL, which allowed the evaluation of crystal field (Δ[subscript]CF) and spin-orbit (Δ[subscript]SO) splitting parameters exerimentally. Large negative Δ[subscript]CF and, consequently, the difficulties of light extraction from AlN and Al-rich AlGaN based emitters due to their unique optical polarization properties have been further confirmed with these new experimental data. Due to the ionic nature of III-nitrides, exciton-LO phonon Frohlich interaction is strong in these materials, which is manifested by the appearance of phonon replicas accompanying the excitonic emission lines in their PL spectra. The strength of the exciton-phonon interactions in AlN has been investigated by measuring the Huang-Rhys factor. It compares the intensity of the zero phonon (exciton emission) line relative to its phonon replica. AlN bulk single crystals, being promising native substrate for growing nitride based high quality device structures with much lower dislocation densities (<10[superscript]4 cm[superscript]-2), are also expected to be transparent in visible to UV region. However, available bulk AlN crystals always appear with an undesirable yellow or dark color. The mechanism of such undesired coloration has been investigated. MOCVD was utilized to deposit ~0.5 μm thick AlN layer on top of bulk crystal. The band gap of strain free AlN homoepilayers was 6.100 eV, which is ~30 meV lower compared to hetero-epitaxial layers on sapphire possessing compressive strain. Impurity incorporation was much lower in non-polar m-plane growth mode and the detected PL signal at 10 K was about an order of magnitude higher from a-plane homo-epilayers compared to that from polar c-plane epilayers. The feasibility of using Be as an alternate p-type dopant in AlN has been studied. Preliminary studies indicate that the Be acceptor level in AlN is ~330 meV, which is about 200 meV shallower than the Mg level in AlN. Understanding the optical and electronic properties of native point defects is the key to achieving good quality material and improving overall device performance. A more complete picture of optical transitions in AlN and GaN has been reported, which supplements the understanding of impurity transitions in AlGaN alloys described in previous reports.

Doctor of Philosophy / Department of Chemical Engineering / Vikas Berry / Graphene - a single atom thick two dimensional sheet of sp[superscript]2 bonded carbon atoms arranged in a honeycomb lattice - has shown great promise for both fundamental research & applications because of its unique electrical, optical, thermal, mechanical and chemical properties. Derivatization of graphene unlocks a plethora of novel properties unavailable to their pristine parent “graphene”. In this dissertation we have synthesized various structural and chemical derivatives of graphene; characterized them in detail; and leveraged their exotic properties for diverse applications. We have synthesized protein/DNA/ethylenediamine functionalized derivatives of graphene via a HATU catalyzed amide reaction of primary-amine-containing moieties with graphene oxide (GO) – an oxyfunctional graphene derivative. In contrast to non-specificity of graphene, this functionalization of GO has enabled highly specific interactions with analytes. Devices fabricated from the protein (concanavalin – A) and DNA functionalized graphene derivatives were demonstrated to enable label-free, specific detection of bacteria and DNA molecules, respectively, with single quanta sensitivity. Room temperature electrical characterization of the sensors showed a generation of ~ 1400 charge carriers for single bacterium attachment and an increase of 5.6 X 10[superscript]12 charge carriers / cm[superscript]2 for attachment of a single complementary strand of DNA. This work has shown for the first time the viability of graphene for bio-electronics and sensing at single quanta level. Taking the bio-interfacing of graphene to the next level, we demonstrate the instantaneous swaddling of a single live bacterium (Bacillus subtilis) with several hundred sq. micron (~ 600 µm[superscript]2) areal protein-functionalized graphene sheets. The atomic impermeability and high yield strength of graphene resulted in hermetic compartmentalization of bacteria. This enabled preservation of the dimensional and topological characteristics of the bacterium against the degrading effects of harsh environments such as the ultrahigh vacuum (~ 10[superscript]-5 Torr) and high intensity electron beam (~ 150 A/cm[superscript]2) in a transmission electron microscope (TEM) column. While an unwrapped bacterium shrank by ~ 76 % and displayed significant charge buildup in the TEM column; a wrapped bacterium remained uncontracted and undamaged owing to the graphenic wraps. This work has shown for the first time an impermeable graphenic encasement of bacteria and its application in high vacuum TEM imaging without using any lengthy traditional biological TEM sample preparation techniques. In an inch-scale, we fabricated robust free-standing paper composed of TWEEN/Graphene composite which exhibited excellent chemical stability and mechanical strength. This paper displayed excellent biocompatibility towards three mammalian cell lines while inhibiting the non-specific binding of bacteria (Bacillus cereus). We predict this composite and its derivatives to have excellent applications in biomedical engineering for transplant devices, invasive instrument coatings and implants. We also demonstrate a novel, ultra-fast and high yield process for reducing GO to reduced graphene oxide (RGO) using a facile hydride-based chemistry. The RGO sheets thus-produced exhibited high carrier mobilities (~ 100-600 cm[superscript]2/V•s) and reinstatement of the ambipolar characteristic of graphene. Raman spectra and UV-Vis spectroscopy on the RGO sheets displayed a high degree of restoration of the crystalline sp2 lattice with relatively low defects. We fabricated graphene nanoribbons (GNRs) – 1D structural derivatives of graphene – using a nano-scale cutting process from highly oriented pyrolytic graphite (HOPG) blocks, with widths pre-determinable between 5 nm to 600 nm. The as-produced GNRs had very high aspect ratio in the longitudinal direction (~ 0.01); exhibited predominantly mono-layered structure (< 10 % bilayer); and smooth edges (Raman I[subscript]D/G ~ 0.25 -0.28). Low temperature electrical transport measurements on back-gated thin film GNR devices were performed and a carrier mobility of ~ 20 ± 4 cm[superscript]2/V•s with sheet resistances of 2.2-5.1 MΩ / □ was extracted. Despite the ~ 50 nm thicknesses of the films, a clear bandgap scaling was observed with transport via variable range hopping (VRH) in 2 and 3 dimensions. This work demonstrates the first fully functional narrow pristine GNR thin-film field effect transistors (FETs). In addition we fabricated graphene quantum dots (GQDs) – 0D derivatives of graphene with dimensions < 100 nm – using a slight variation of our nano-scale cutting strategy, where the cleavage process is carried out in two dimensions. A high degree of control on the dimensions (Std. Dev. of ~ 5 nm for 50 X 50 nm square GQDs) and shape (pre-determinable between square, rectangle, triangle and trapezoid) of the as-synthesized GQDs is demonstrated. The optical properties of the GQDs such as the UV-Vis absorbance and photoluminescence were studied and their facile tunability was demonstrated depending on their dimensions. This work demonstrates for the first time the high throughput fabrication of GQDs with tunable dimensions and shape.

Master of Science / Department of Grain Science and Industry / Xiuzhi Susan Sun / Soybean oil-based resin for transparent flexible coating applications were formulated by dihydroxyl soybean oil (DSO) with commercial epoxy monomers (i.e., epoxidized soybean oil (ESO) and 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (ECHM)). The resin was formed to thermoset polymers using cationic ring-opening photopolymerization. The ether crosslinking and post-polymerization of the polymeric network were observed using Fourier transform infrared spectroscopy. Thermal properties of the bio-based coating materials and their copolymerization behaviors were examined using a differential scanning calorimetry and a thermogravimetric analyzer. Crosslink density and molecular weight between crosslink were obtained from dynamic mechanical analysis. ECHM/DSO (1: 1.43 weight ratio) films showed the highest elongation at break (49.2 %) with a tensile strength of 13.7 MPa. After 2 months storage, the elongation at break and tensile strength of films were 32 % and 15.1 MPa, respectively. ESO/DSO films (w/w ratios of 1:0.1, 1:0.15, and 1:0.2) exhibited stable flexibility around 11-13 % of elongations at break without significant reductions of tensile strengths (2.5 to 4.4 MPa) during 2-months shelf life. Optical transparencies of the films were comparable to commercial glass and polymers, and water uptake properties (0.72 and 2.83%) were significantly low.

Master of Science / Department of Civil Engineering / Robert J. Peterman / With limited research being conducted solely on lightweight concrete prestressed bond and current development-length equations based on tests performed on normal-weight members, more investigation on lightweight concrete prestress bond is necessary. Additionally, the effects of water-reducing agents on normal-weight and lightweight concrete need further exploration. The aim of this study was to examine these areas using two locally available lightweight aggregates from Kansas and one from North Carolina to determine if lightweight prestressed concrete bridge girders are a useful alternative for the Kansas Department of Transportation. The lightweight concrete mixes developed were capable of attaining 5000 psi compressive strength in 16 hours and 7000 psi in 28 days. During the large block pull-out test, the average maximum force at pull-out and first observable slip was higher for the block cast with a three inch slump then the companion specimen poured at a nine-inch slump. During flexural testing, the two beams not reaching nominal moment capacity, KC-9 and STA-9, failed in compression without strand end slip. The moment capacity was considerably greater for three-inch slump members than the companion specimen placed with nine-inch slump concrete.

Master of Science / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / Purification of cadmium, zinc and tellurium was attempted to improve the quality of cadmium-zinc-telluride (CdZnTe) crystal growth. Specifically, vacuum distillation, zone refining and H[subscript]2 gas flow assisted zone refining were all investigated as methods to purify the constituent elements of CdZnTe. A unique multi-chamber ampoule was used to enable a purification sequence starting with double vacuum distillation followed by zone refining all without sample handling after the initial step. Modifications due to unique material properties of Cd and Zn were developed. Glow discharge mass spectroscopy (GDMS) analysis was used to measure impurity concentrations of 74 elements. Cd purification using vacuum distillation proved to be an effective method to reduce the impurity level of 5N starting material to a purity between the range of 6N5 and 7N5, as measured using GDMS and laser ablation mass spectroscopy. Combined Zn double vacuum distillation and zone refining in an enclosed Ar atmosphere using 5N starting material yielded material with a purity between the range of 5N8 to 6N8. Tellurium purification using combined double vacuum distillation followed by zone refining under continuous H[subscript]2 flow of 4N specified raw material resulted in high purity tellurium between the range of 6N3 and 7N4.

Doctor of Philosophy / Department of Chemistry / Viktor Chikan / This dissertation reports the colloidal synthesis of iron silicide, hafnium oxide core-gold shell and water soluble iron-gold alloy for the first time. As the first part of the experimentation, plasmonic and superparamagnetic nanoparticles of gold and iron are synthesized in the form of core-shell and alloy. The purpose of making these nanoparticles is that the core-shell and alloy nanoparticles exhibit enhanced properties and new functionality due to close proximity of two functionally different components. The synthesis of core-shell and alloy nanoparticles is of special interest for possible application towards magnetic hyperthermia, catalysis and drug delivery. The iron-gold core-shell nanoparticles prepared in the reverse micelles reflux in high boiling point solvent (diphenyl ether) in presence of oleic acid and oleyl amine results in the formation of monodisperse core-shell nanoparticles. The second part of the experimentation includes the preparation of water soluble iron-gold alloy nanoparticles. The alloy nanoparticles are prepared for the first time at relatively low temperature (110 oC). The use of hydrophilic ligand 3-mercapto-1-propane sulphonic acid ensures the aqueous solubility of the alloy nanoparticles. Next, hafnium oxide core-gold shell nanoparticles are prepared for the first time using high temperature reduction method. These nanoparticles are potentially important as a high κ material in semiconductor industry. Fourth, a new type of material called iron silicide is prepared in solution phase. The material has been prepared before but not in a colloidal solution. The Fe3Si obtained is superparamagnetic. Another phase β-FeSi2 is a low band gap (0.85 eV) semiconductor and is sustainable and environmentally friendly. At last, the iron monosilicide (FeSi) and β-FeSi2 are also prepared by heating iron-gold core-shell and alloy nanoparticles on silicon (111) substrate. The nucleation of gaseous silicon precursor on the melted nanoparticles results the formation of nanodomains of FeSi and β-FeSi2. A practical application of these nanoparticles is an important next step of this research. Further improvement in the synthesis of β-FeSi2 nanoparticles by colloidal synthetic approach and its application in solar cell is a future goal.

Three-dimensional macroporous structures (‘opals’ and ‘inverse opals’) can be produced by colloidal crystal templating, one of the most intensively studied areas in materials science today. There are several potential advantages of lithium-ion battery electrodes based on inverse opal structures. High electrode surface, easier electrolyte access to the bulk of electrode and reduced lithium diffusion lengths allow higher discharge rates. Highly open structures provide for better mechanical stability to volume swings during cycling. Silicon is one of the most promising anode materials for lithium-ion batteries. Its theoretical capacity exceeds capacities of all other materials besides metallic lithium. Silicon is abundant, cheap, and its use would allow for incorporation of microbattery production into the semiconductor manufacturing. Performance of silicon is restricted mainly by large volume changes during cycling. The objective of this work was to investigate how the inverse opal structures influence the performance of silicon electrodes. Several types of silicon-based inverse opal films were synthesised, and their electrochemical performance was studied. Amorphous silicon inverse opals were fabricated via chemical vapour deposition and characterised by various techniques. Galvanostatic cycling of these materials confirmed the feasibility of the approach taken, since the electrodes demonstrated high capacities and decent capacity retentions. The rate performance of amorphous silicon inverse opals was unsatisfactory due to low conductivity of silicon. The conductivity of silicon inverse opals was improved by crystallisation. Nanocrystalline silicon inverse opals demonstrated much better rate capabilities, but the capacities faded to zero after several cycles. Silicon-carbon composite inverse opal materials were synthesised by depositing a thin layer of carbon via pyrolysis of a sucrose-based precursor onto the silicon inverse opals in an attempt to further increase conductivity and achieve mechanical stabilisation of the structures. The amount of carbon deposited proved to be insufficient to stabilise the structures, and silicon-carbon composites demonstrated unsatisfactory electrochemical behaviour. Carbon inverse opals were coated with amorphous silicon producing another type of macroporous composites. These electrodes demonstrated significant improvement both in capacity retentions and in rate capabilities. The inner carbon matrix not only increased the material conductivity, but also resulted in lower silicon pulverisation during cycling.

Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / Solar energy driven catalytic systems have gained popularity in environmental remediation recently. Various photocatalytic systems have been reported in this regard and most of the photocatalysts are based on well-known semiconducting material, Titanium Dioxide, while some are based on other materials such as Silicon Dioxide and various Zeolites. However, in titania based photocatalysts, titania is actively involved in the catalytic mechanism by absorbing light and generating exitons. Because of this vast popularity of titania in the field of photocatalysis it is believed that photocatalysis mainly occurs via non-localized mechanisms and semiconductors are extremely important. Even though it is still rare, photocatalysis could be localized and possible without use of a semiconductor as well. Thus, to support localized photocatalytic systems, and to compare the activity to titania based systems, degradation of organic air pollutants by nanostructured silica, titania and mixed silica titania systems were studied. New materials were prepared using two different approaches, precipitation technique (xerogel) and aerogel preparation technique. The prepared xerogel samples were doped with both metal (silver) and non-metals (carbon and sulfur) and aerogel samples were loaded with Chromium, Cobalt and Vanadium separately, in order to achieve visible light photocatalytic activity. Characterization studies of the materials were carried out using Nova BET analysis, DR UV-vis spectrometry, powder X-ray diffraction, X-ray photoelectron Spectroscopy, FT-IR spectroscopy, Transmission Electron Microscopy, etc. Kinetics of the catalytic activities was studied using a Shimadzu GCMS-QP 5000 instrument using a closed glass reactor. All the experiments were carried out in gaseous phase using acetaldehyde as the model pollutant. Kinetic results suggest that chromium doped silica systems are good UV and visible light active photocatalysts. This is a good example for a localized photocatalytic activity. In contrast, our xerogel system shows comparatively high visible light photocatalytic activity for the titania based system, showing the importance of non-localized nature of photocatalysis. The Cobalt doped silica system shows interesting dark catalytic activity towards acetaldehyde and several other pollutants. Thus, in summary, based on the different activities we observed during our studies these materials could be successfully used to improve the quality of both indoor and outdoor air.