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<p> The next generation of electronic devices faces the challenge of adequately containing and controlling extremely high charge densities within structures of nanometer dimensions. Atomic-scale transistors must be thin and be able to control extremely high charge densities (>10e13/cm</p><p>2). Silicon devicestypically have two-dimensional electron gas (2DEG) densities around 10e12/cm</p><p>2.Nitride-based devices can sustain densities an order of magnitude higher. The "complex oxides" have recently emerged as an attractive materials system to support these developments. The demonstration of a 2DEG at the SrTiO₃/LaAlO3 interface has triggered an avalanche of research, including the unprecedentedly high density of 3x10e14/cm</p><p>2 at SrTiO₃/GdTiO3and SrTiO₃/SmTiO₃ interfaces. Metal-insulator (Mott) transitions that are inherent to some of these complex oxides could offer even greater prospects for enhanced functionality or novel device concepts. </p><p> The materials and heterostructures that have been explored to date are clearly only a small subset of the vast number of materials combinations that could lead to interesting phenomena. In this work we use first-principles methods to build greater understanding of the interface phenomena, so that searches can be better informed and more focused. We also develop a set of criteria that the materials and their heterostructures should satisfy to develop a high-performance 2DEG-based device. We focus in particular on the band alignment, calculating it for a variety of different potential materials. </p><p> Next, we study GdTiO₃/SrTiO₃/GdTiO₃ heterostructures in depth, where each interface contributes excess electrons into the SrTiO₃. We calculate the 2DEG formation for a superlattice containing six layers of SrTiO₃, and compare with angle-resolved photoemission spectroscopy results. Together, the experimental and theoretical results conclusively show that the 2DEG results from the interface itself, and does not originate from a secondary source such as oxygen vacancies. These heterostructures also exhibit a metal-to-insulator transition as the SrTiO₃ layer thickness decreases, which could possibly be used as a "Mott field effect transistor" &mdash; the system is very close to a metal-to-insulator transition, and modulating a small fraction of the electron density would lead to switching between the metallic and insulating phases. The mechanism behind this transition is unraveled, and we construct a bulk model of the transition based on the surprising observation that SrTiO₃ itself can become a Mott insulator when doped with an extremely high density of electrons. </p><p> Building on our study of the SrTiO₃/GdTiO3 interfaces, we investigate the electronic structure of GdTiO3 in detail - our calculated band gap differs markedly from past experimental values, but is consistent with recent photoluminescence measurements. We find that the presence of small hole polarons leads to a feature in the optical absorption spectrum which was previously interpreted to be the band gap. Since small hole polarons are present in all the rare-earth titanates, not only GdTiO3, the values of the band gaps (also based on optical absorption measurements) across the series will likely have to be revised. Lastly, to understand the formation of small hole polarons in the rare-earth titanates, we study point defects and impurities in GdTiO3. We also investigate how defects may impact the behavior of GdTiO3 in electronic devices.</p>

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

<p>Light-emitting diode (LED) curing lights were introduced to the dental market promising a higher curing efficiency than halogen-based lights. The earlier generation curing lights, however, proved not to be as effective as halogen lights. As a result 3M ESPE introduced a new high-powered LED curing light, the Elipar FreeLight 2, that delivers a greater irradiance. and threfore greater energy density than its precursor. Due to these changes, the light's manufacturer claims that the FreeLight 2 can cure resin composites at half of their recommended curing time. The aim of this study was to compare the effectiveness of cure when a FreeLight 2 was used to sure composite samples at 100% and at 50% of the recommended curing time.</p>

<p>Engineering thin, organic materials with tailored properties requires both the understanding of the conformation of thin organic films and their conformational response to changes in the environment, and the accurate characterization the mechanical properties of the materials as a thin layer on surfaces. These issues have not yet been sufficiently addressed due to the paucity of appropriate tools and data interpretation approaches to reveal the nanometer scale conformation and mechanics of surface-grafted, thin, organic films. In this dissertation, I report on the characterization of conformational and mechanical properties of thin organic films, and the development of techniques that allow more detailed and reliable measurement of these material properties. First, I co-developed a novel approach to evaluate neutron reflectivity data and to simulate the conformational structure for thin stimulus-responsive polymer brushes. In this approach, we used a molecular-based lattice mean-field theory, augmented with experimentally obtained parameters to describe the polymer chains. The approach and fitting results required fewer fitting parameters, and captured the thermal response of the sample self-consistently. </p><p>Second, I demonstrated the capability of force-modulation microscopy in imaging surface-grafted, organic thin films in aqueous environments, with high spatial resolution and sensitivity to conformational details that affect the contact mechanics. To this end, I developed a new parameter-selection approach. This approach allowed both highly sensitive mapping of subtle differences in the molecular packing of thiol molecules on the substrate surface, and generation of high-contrast contact-stiffness images of end-grafted protein patterns on a surface. Finally, I discussed model selection and error estimation in calculating the reduced Young's modulus of soft materials on surfaces. I found that the detailed characterization of probe apex profiles, using probe-reconstruction techniques, provide only marginal improvements in calculating the reduced Young's modulus of thin films, compared with analytical models of equivalent probe radii; however, I found that a hybrid worn-cone model is appropriate for large indentations on soft materials, and benefits from the characterization of the probe apex profile. Additionally, we rendered error maps of several common scenarios, referenced to indentation and probe radius values, in the determination of the reduced Young's modulus.</p> / Dissertation

<p> We consider a size-structured population model where individuals may be recruited into the population at different sizes. First and second order finite difference schemes are developed to approximate the solution of the model. The convergence of the approximations to a unique weak solution is proved. We then show that as the distribution of the new recruits become concentrated at the smallest size, the weak solution of the distributed states-at-birth model converges to the weak solution of the classical Sinko-Streifer type size-structured model in the weak* topology. Numerical simulations are provided to demonstrate the achievement of the desired accuracy of the two methods for smooth solutions as well as the superior performance of the second-order method in resolving solution-discontinuities. A least-squares method is developed for estimating parameters in a size-structured population model with distributed states-at-birth from field data. The first and second order finite difference schemes for approximating solution of the model are utilized in the least-squares problem. Convergence results for the computed parameters are established. Numerical results demonstrating the efficiency of the technique are provided. </p>

<p> The conversion efficiency of conventional silicon (Si) photovoltaic cells has not been improved significantly during last two decades but their cost decreased dramatically during this time. However, the higher price-per-watt of solar cells is still the main bottleneck in their widespread use for power generation. Therefore, new materials need to be explored for the fabrication of solar cells potentially with lower cost and higher efficiency. The n-type zinc oxide (n-ZnO) and p-type Si (p-Si) based single heterojunction solar cell (SHJSC) is one of the several attempts to replace conventional Si single homojunction solar cell technology. There are three inadequacies in the literature related to n-ZnO/p-Si SHJSC: (1) a detailed theoretical analysis to evaluate potential of the solar cell structure, (2) inconsistencies in the reported value of open circuit voltage (VOC) of the solar cell, and (3) lower value of experimentally achieved VOC as compared to theoretical prediction based on band-bending between n-ZnO and p-Si. Furthermore, the scientific community lacks consensus on the optimum growth parameters of ZnO. </p><p> In this dissertation, I present simulation and experimental results related to n-ZnO/p-Si SHJSC to fill the gaps mentioned above. Modeling and simulation of the solar cell structure are performed using PC1D and AFORS-HET software taking practical constraints into account to explore the potential of the structure. Also, unnoticed benefits of ZnO in solar cells such as an additional antireflection (AR) effect and low temperature deposition are highlighted. The growth parameters of ZnO using metal organic chemical vapor deposition and sputtering are optimized. The structural, optical, and electrical characterization of ZnO thin films grown on sapphire and Si substrates is performed. Several n-ZnO/p-Si SHJSC devices are fabricated to confirm the repeatability of the VOC. Moreover, the AR effect of ZnO while working as an n-type layer is experimentally verified. The spatial analysis for thickness uniformity and optical quality of ZnO films is carried out. These properties turn out to play a fundamental role in device performance and so far have been overlooked by the research community. Three different materials are used as a quantum buffer layer at the interface of ZnO and Si to suppress the interface states and improve the VOC. The best measured value of VOC of 359 mV is achieved using amorphous-ZnO (a-ZnO) as the buffer layer at the interface. Finally, supplementary simulations are performed to optimize the valence-band and conduction-band offsets by engineering the bandgap and electron affinity of ZnO. </p><p> After we published our initial results related to the feasibility of n-ZnO/p-Si SHJSC [Sol. Energ. Mat. Sol. Cells 139 (2015) 95&ndash;100], different research groups have fabricated and reported the solar cell performance with the best efficiency of 7.1% demonstrated very recently by Pietruszka et al. [Sol. Energ. Mat. Sol. Cells 147 (2016) 164&ndash;170]. We conclude that major challenge in n-ZnO/p-Si SHJSC is to overcome Fermi-level pinning at the hetero-interface. A potential solution is to use the appropriate material as buffer layer which is confirmed by observing an improvement in VOC using a-ZnO at the interface as buffer layer. Once the interface quality is improved and the experimental value of VOC matched the theoretical prediction, the n-ZnO/p-Si SHJSC can potentially have significant contribution in solar cells industry.</p>

<p> The responses of structure and properties to composition and temperature have been investigated for glasses in TiO₂-SiO₂ and Na₂O-TiO₂-SiO₂ systems. Additionally, the response of Na₂O-TiO₂-SiO₂ glasses to plastic deformation has been studied. (x)TiO₂-(1-x)SiO₂ glasses were prepared through the sol-gel process with compositions 0 &le; x &le; 10 mol% and compared to commercial glasses prepared through flame hydrolysis deposition with x = 0, 5.4, and 8.3 mol%. (x) Na₂O - (y) TiO₂ - (1-x-y) SiO₂ glasses were prepared with x = 10, 15, 20, and 25 mol% and y = 4, 7, and 10 mol% through a melt-quench process. Density and index of refraction of glasses was measured through the Archimedes's method and using a prism coupler, respectively. The glass transition temperature of Na₂O-TiO₂-SiO₂ glasses was measured through differential thermal analysis. </p><p> The structure and elastic moduli have been studied through Raman spectroscopy and Brillouin light scattering, respectively, at room temperature and in-situ up to 1200 &deg;C for TiO₂-SiO₂ glasses and up to 800 &deg;C for Na₂O-TiO₂-SiO₂ glasses. Young's modulus was observed to decrease from 72 GPa to 66 GPa with the addition of 8.3 mol% TiO₂ in TiO₂-SiO₂ glasses and to increase from 65 GPa to 73 GPa with the addition of 10 mol% TiO₂ in 10 Na₂O - (0-10) TiO₂-SiO₂ glasses. The addition of TiO₂ was observed to shift the 460, 490, and 600 cm-1 Raman peaks to lower frequencies in TiO₂-SiO₂ glasses, suggesting a more open and flexible network, and the 720, 800, and 840 cm^-1 Raman peaks to higher frequencies in Na₂O-TiO₂-SiO₂ glasses, suggesting a lower free volume and stiffer network. The addition of TiO₂ has little effect on the temperature response of the elastic moduli in either system, but decreases the thermal expansion and increases the frequency shifts in the 950 and 1100 cm^-1 Raman peaks in the TiO₂-SiO₂ system while the thermal expansion increases with initial additions of TiO₂ and then remains constant in the Na₂O-TiO₂-SiO₂ system. </p><p> Changes in structure and property with composition have been discussed, and structural models were proposed. The reduction of thermal expansion and elastic moduli in TiO₂-SiO₂ glasses occurs through the promotion of cooperative, inter-tetrahedral rotations facilitated by the longer and weaker Ti-O bonds. The increase in elastic moduli in the Na₂O-TiO₂-SiO₂ glasses occurs through the formation of small clusters with local, relatively high Ti and Na concentrations, promoted by Ti adopting a five-fold coordination in a square-pyramidal geometry. These clusters work to shield the silica network from non-bridging oxygens from the presence of Na while simultaneously increasing the volume bond density of the glass. </p><p> For Na₂O-TiO₂-SiO₂ glasses, the response to mechanical damage and plastic deformation has been examined through Vickers indentation experiments at loads from 10 mN to 49 N. Fracture toughness was measured through the single-edge precracked beam method. The permanent deformation volumes around Vickers indents were investigated through atomic force microscopy. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. Vickers indents were observed to change from a mixture of radial/median and cone cracks to radial/median and lateral cracks as Poisson's ratio increases. As Poisson's ratio increases hardness decreases from 5.5 GPa to 4.5 GPa, the average radial/median crack length roughly doubles, and fracture toughness remains constant. A minimum in the critical crack initiation load was observed at &nu; = 0.21&ndash;0.22. The volume of glass deformed through shear flow during indentation increases gradually with increasing Poisson's ratio, becomes larger than the densified volume at &nu; = 0.237. The densified volume increases between &nu; = 0.18 and &nu; = 0.21 and decreases rapidly from 16.5 &micro;m³ to 8.7 &micro;m³ at &nu; = 0.235&ndash;0.237. A correlation between the minimum in crack initiation load and the change in deformation mechanisms over the same Poisson&rsquo;s ratio range was observed.</p>

Solid oxide fuel cells (SOFCs) are one of the most efficient and environment-friendly devices for electricity generation. One critical challenge of SOFC commercialization is high cell operating temperatures (800°C-1000°C), which lead to high material costs, high performance degradation rates, long start-up and shutdown times, and limited portable applications. Intermediate temperature (600°C-800°C) operation of SOFCs is limited by sluggish electrode reaction kinetics. The objective of this research is to improve intermediate temperature performance of commercially available Ni-YSZ cermet anode supported SOFCs by liquid infiltration of the anode. One effective method to improve kinetics of electrochemical reactions at the anode is to increase the density of reaction sites, which are known as the triple phase boundaries (TPBs). The porous Ni-YSZ cermet anodes were liquid infiltrated with Ni nanoparticles, leading to a four-fold increase in TPB density in the anode. The improved electrochemical performance of the infiltrated cells compared to the uninfiltrated cells highlights the effectiveness of anode infiltration in facilitating improved anode electrochemical reaction kinetics. However, the post-electrochemical testing characterization revealed that Ni nanoparticles were not stable due to Ni coarsening and were mostly isolated indicating that not all of the additional TPBs were fully utilized in electrochemical reactions due to the lack of an electronic pathway between the Ni nanoparticles. In order to improve microstructural stability of the infiltrated Ni nanoparticles, and to fully utilize the added TPBs, co-infiltration of Ni with a mixed ionic and electronic conductor (MIEC) was carried out. Two MIEC materials are chosen based on their chemical stability and conductivity in the anode operating environments; Gd0.1Ce0.9O2-δ (GDC), a predominantly an ionic conductor, and La0.6Sr0.3Ni0.15Cr0.85¬O3-δ (LSNC), a predominantly electronic conductor, and cells were successfully co-infiltrated to form Ni-GDC and Ni-LSNC nanostructures with the MIEC phases connecting the Ni nanoparticles. Stability tests demonstrated that both MIECs inhibited Ni nanoparticle coarsening. Electrochemical studies showed that Ni-GDC is the most effective for improved anode kinetics. A long-term (120 hours) electrochemical test indicated that infiltration of Ni-GDC into Ni-YSZ cermet anode effectively improves overall cell performance at intermediate temperatures and maintains the performance gain for a long period of time.

Photonic metamaterials are materials whose optical properties are derived from artificially-structured sub-wavelength unit cells, rather than from the bulk properties of the constituent materials. Examples of metamaterials include plasmonic materials, negative index materials, and electromagnetic cloaks. While advances in simulation tools and nanofabrication methods have allowed this field to grow over the past several decades, many challenges still exist. This thesis addresses two of these challenges: fabrication of photonic metamaterials with tunable responses and high-throughput nanofabrication methods for these materials. The design, fabrication, and optical characterization of a microelectromechanical systems (MEMS) tunable plasmonic spectrometer are presented. An array of holes in a gold film, with plasmon resonance in the mid-infrared, is suspended above a gold reflector, forming a Fabry-Perot interferometer of tunable length. The spectra exhibit the convolution of extraordinary optical transmission through the holes and Fabry-Perot resonances. Using MEMS, the interferometer length is modulated from 1.7 μm to 21.67 μm , thereby tuning the free spectral range from about 2900 wavenumbers to 230.7 wavenumbers and shifting the reflection minima and maxima across the infrared. Due to its broad spectral tunability in the fingerprint region of the mid-infrared, this device shows promise as a tunable biological sensing device. To address the issue of high-throughput, high-resolution fabrication of optical metamaterials, atomic calligraphy, a MEMS-based dynamic stencil lithography technique for resist-free fabrication of photonic metamaterials on unconventional substrates, has been developed. The MEMS consists of a moveable stencil, which can be actuated with nanometer precision using electrostatic comb drive actuators. A fabrication method and flip chip method have been developed, enabling evaporation of metals through the device handle for fabrication on an external substrate. While the MEMS can be used to fabricate over areas of approximately 100 square μm, a piezoelectric step-and repeat system enables fabrication over cm length scales. Thus, this technique leverages the precision inherent to MEMS actuation, while enhancing nanofabrication thoughput. Fabricating metamaterials on new substrates will enable novel and tunable metamaterials. For example, by fabricating unit cells on a periodic auxetic mechanical scaffold, the optical properties can be tuned by straining the mechanical scaffold.