Browsing by Subject "Thin film"
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Item The application of light trapping structures and of InGaAs/GaAs quantum wells and quantum dots to improving the performance of single-junction GaAs solar cells(2012-05) McPheeters, Claiborne Ott; Yu, Edward T.; Alu, Andrea; Bank, Seth R.; Chen, Ray T.; Zhang, John X.High efficiency photovoltaic solar cells are expected to continue to be important for a variety of terrestrial and space power applications. Solar cells made of optically thick materials often cannot meet the cost, efficiency, or physical requirements for specialized applications and, increasingly, for traditional applications. This dissertation investigates improving the performance of single-junction GaAs solar cells by incorporating InGaAs/GaAs quantum wells and quantum dots to increase their spectral response bandwidth, and by incorporating structures that confine light in the devices to improve their absorption of it. InGaAs/GaAs quantum dots-in-wells extend the response of GaAs homojunction devices to wavelengths >1200 nm. Nanoparticles that are randomly deposited on the top of optically thick devices scatter light into waveguide modes of the device structures, increasing their absorption of electromagnetic energy and improving their short-circuit current by up to 16%. Multiply periodic diffractive structures have been optimized using rigorous software algorithms and fabricated on the back sides of thin film quantum dot-in-well solar cells, improving their spectral response at wavelengths 850 nm to 1200 nm, where only the quantum dot-in-well structures absorb light, by factors of up to 10. The improvement results from coupling of diffracted light to waveguide modes of the thin film device structure, and from Fabry-Perot interference effects. Simulations of absorption in these device structures corroborate the measured results and indicate that quantum well solar cells of ~2 µm in thickness, and which are equipped with optimized backside gratings, can achieve 1 Sun Airmass 0 short-circuit current densities of up to ~5 mA/cm2 (15%) greater than GaAs homojunction devices, and of up to >2 mA/cm2 (7%) greater than quantum well devices, with planar back reflectors. A combination of Fabry-Perot interference and diffraction into waveguide modes of the thin devices is shown to dominate the simulated device response spectra. Simulations also demonstrate the importance of low-loss metals for realizing optimal light trapping structures. Such device geometries are promising for reducing the cost of high efficiency solar cells that may be suitable for a variety of traditional and emerging applications.Item Catalytic activity of 2-D bimetallic surfaces and 3-D metal organic frameworks(2020-04-10) Han, Sungmin (Ph. D. in chemistry); Mullins, C. B.; Keitz, Benjamin K; Hwang, Gyeong S; Sitz, Greg O; Henkelman, Graeme AThe first part of this dissertation involves understanding the fundamentals of 2-D palladium (Pd) - gold (Au) bimetallic surfaces. The special catalytic capabilities of Pd-Au bimetallic surfaces are mainly attributed to ensemble effects related to the compositions of inert Au atoms and active Pd atoms. In particular, we focus on the activation of O₂ and various oxidative reactions on the Pd–Au surfaces. Using molecular beam techniques under ultrahigh vacuum (UHV), we prove that H₂O and O₂ admolecules form hydrogen bonded clusters, which enhance O₂ activation. The direct dissociative adsorption of O₂ is also possible on the Pd–Au surfaces. We experimentally estimate the activation barriers for O₂ dissociation and the reactivity of oxygen adatoms as a function of the Pd coverage on the Au(111) surface, where a relatively low O₂ dissociation barrier (high Pd coverages) corresponds to a higher reaction barrier for oxygen adatoms (due to the higher Pd coverages). Based on these results, we also discover that acetaldehyde molecules can be selectively oxidized to acetic acid on less Pd deposited surfaces, since the small Pd ensembles on this surface can prevent the decomposition of acetate, which is an intermediate state in the formation of acetic acid. The second part of this dissertation involves growing and analyzing HKUST-1 metal-organic framework (MOF) thin films. MOFs are a new class of ultra-porous material based on inorganic-organic hybrid structures. We explain a new growth method for HKUST-1 thin films by sequential deposition of Cu and H₃BTC under vacuum. This procedure resulted in first MOF thin film to be controllably grown under vacuum, and the strategy can be applied in various applications. Since this growth method allows delicate quantity control of Cu, we successfully grow 4-6 nm or 8-12 nm Cu nanoparticles (NP) incorporated in HKUST-1 thin films. Applying temperature programmed desorption under vacuum, Cu NP incorporated HKUST-1 thin films show different catalytic activity for the methanol oxidation depending on the Cu NP sizes. The film with smaller Cu NP’s has improved selectivity for formaldehyde, and the film with larger Cu NP’s generates formaldehyde along with other products, CO₂ and H₂.Item Coordination complex design for applications in thin films and electrochemical energy storage(2020-12-11) Hall, Justin Walker; Jones, Richard A., 1954-; Que, Emily; Rose, Michael; Ekerdt, John G; Campion, AlanSeveral applications of d-block organometallic chemistry will be presented, spanning chemical vapor deposition precursors, lithium ion battery electrode materials, and redox flow battery electrolytes. In the first chapter, the coordination chemistry of phosphine (PH3) with Ruthenium will be explored in the context of attempts to design a carbon-free Ru(P) thin film chemical vapor deposition precursor. In the second chapter, a novel application of organometallic proton exchange to the synthesis of amorphous Li-ion battery electrodes with the goal of increased capacity retention will be demonstrated. In the third chapter, the identity and electrochemistry of the redox active species in a novel redox flow battery system will be elucidated.Item Effects of elevated temperature on the physical aging and gas transport of sub-micron polybenzimidazole gas separation membranes(2020-07-16) Merrick, Melanie Mae; Freeman, B. D. (Benny D.); Paul, Donald R; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel AThe promising potential of polybenzimidazole (PBI) membranes for high temperature (~200 °C), hydrogen-selective gas separations has been reported for many membrane geometries (e.g., bulk, composite, and hollow fiber), but never for sub-micron, spin-cast membranes. Numerous studies have shown that the performance of spin-cast membranes, which simulate commercially relevant thicknesses, declines more quickly with time than that of thick membranes due to accelerated physical aging. However, because most existing membranes are used near ambient temperature, the physical aging of sub-micron, spin-cast membranes has never been studied at temperatures above 55 °C. Because physical aging is dependent on both thickness and temperature, emerging high temperature membrane applications make it both intellectually and practically imperative to characterize spin-cast membranes in this new temperature regime. For the first time, physical aging studies of spin-cast, sub-micron membranes have been extended to elevated temperatures. PBI membranes, cast from commercial-grade Celazole®, were aged in a high-temperature permeation system while the gas permeabilities were periodically measured over more than 1500 hours. When aging at 190 °C, membrane gas permeabilities decreased rapidly then plateaued after 300 hours of aging. The observation of a plateau (i.e., equilibration) had never before been seen for a membrane, nor, to our knowledge, for any polymer ~250 °C below its glass transition temperature. Decreases in membrane permeability were accompanied by increases in selectivity for H₂, which are traditionally represented by Robeson upper bound plots. These shifts were consistent with previous membrane physical aging studies and indicate membrane size-sieving ability improves with aging. Celazole®’s permeability reductions at lower aging temperatures (e.g., 175 °C) were qualitatively similar to those at 190 °C, but occurred over a longer time period. When graphed vs. the logarithm of aging time, the permeability reductions at various temperatures could be superimposed via time-temperature superposition, which is a hallmark of physical aging. A thorough review of physical aging studies in the polymer physics literature is presented to give context for this unexpectedly short equilibration time far below the glass transition. Comparisons are then made between the current study and previous aging studies in the polymer physics field. Overall, the observation of a plateau at short aging times for a polymer deep in the glassy state casts doubt on our understanding of physical aging’s temperature-dependence and our ability to predict membranes’ long-term stability in elevated temperature applications.Item Fatigue characterization of asphalt binders using a thin film poker chip test(2016-05) Hajj, Ramez Muhammad; Bhasin, Amit; Motamed, ArashAsphalt binder is the adhesive that holds together aggregate particles of different sizes of an asphalt mixture. The tensile properties of an asphalt binder can greatly affect the performance of the asphalt mixture under repeated traffic loading. While the current performance grade specification has been in use for a long time to characterize the asphalt binders with regards to fatigue, it has been shown to be largely ineffective. This study was performed with the goal of investigating a strength-based measure to evaluate the fatigue cracking resistance of the asphalt binder. The poker chip geometry was used for this purpose. The test involved tensile loading of a thin film of asphalt binder between two rigid substrates. The first part of this study focused on determining failure criteria for the test. The second part was a study of the binders that have a similar grade based on the current performance grade specification but are expected to perform differently due to difference in their chemical makeup. Finally, the third part involved a study of the effects of nanomaterials as additives on the strength of the binder based on poker chip test results. The results demonstrated that failure strain criteria is promising as a material property, but still needs further study for validation. It was also observed that binders with similar performance grade had significantly different tensile strength. Finally, it was observed that nanomaterials had a significant impact on the test results of unaged binder, but had less effect on aged asphalt binders.Item Fluid dynamics of thin films in semiconductor manufacturing processes(2021-08-03) Ban, Yang Hun; Bonnecaze, R. T. (Roger T.); Ekerdt, John G; Ganesan, Venkat; Sepehrnoori, KamyNanostructures and nanomaterials are means to dramatically improve the performance of LEDs, solar cells, hard disk drives, laser diodes, and displays. This improvement comes from fabricating nanostructures inside or on the surface of a substrate. Fabrication of these nanostructures often involves the coating and imprinting of thin films onto a substrate. However, defects produced during the manufacturing process destroy the functionality of nanostructures. Thus, the fluid dynamics of thin fluid films involved in the manufacturing process is an interesting scientific and engineering problem. The first system studied in this dissertation is the UVNIL. It is a promising high-resolution nanopatterning process and a key technique for the commercialization of nanostructure applications. However, the UVNIL suffers from low throughput. The bottleneck is the resist filling step, so the fluid flow in UVNIL has been studied extensively to find the minimum time required for completing filling of features can be found by studying the flow behavior of resist. The second system is a thin film resist used in the lithography industry. Thin liquid coatings, which have thicknesses of tens of nanometers, are frequently used in lithography. However, generating ultra-thin films is challenging because thinner films are more susceptible to defects and disturbances. Efforts to model the evolution of film profile have been made and flow and leveling dynamics of thin-film were modelled successfully.Item Growth and characterization of the Zintl-phase SrAl₄ on LaAlO₃(2012-08) Schlipf, Lukas Philipp; Demkov, Alexander A.; Ekerdt, John G.We present an experimental study of thin films of SrAl₄ on a LaAlO₃ substrate, with special emphasis on the Zintl-Klemm-type properties of the thin films that we grow using molecular beam epitaxy. We quantify the orientation and stoichiometry of the films and the surface morphology using reflection high energy electron diffraction (RHEED), x-ray diffraction (XRD) and atomic force microscopy (AFM). Furthermore, we present measurements of electronic properties using x-ray photoelectron spectroscopy (XPS) and ultraviolet spectroscopy (UPS). We determine the core level shifts due to the chemical environment in SrAl₄-films, which will underline the Zintl-Klemm character of the material. We measure the work function of (001)-oriented SrAl₄. Additionally we analyze the electronic transport properties of the grown thin films including the resistivity, carrier density and mobility.Item Mechanistic study of plasma damage to porous low-k : process development and dielectric recovery(2010-05) Shi, Hualiang; Ho, Paul S.; Niu, Qian; Shi, Li; Swift, Jack B.; Yao, ZhenLow-k dielectrics with porosity are being introduced to reduce the RC delay of Cu/low-k interconnect. However, during the O2 plasma ashing process, the porous low-k dielectrics tend to degrade due to methyl depletion, moisture uptake, and densification, increasing the dielectric constant and leakage current. This dissertation presents a study of the mechanisms of plasma damage and dielectric recovery. The kinetics of plasma interaction with low-k dielectrics was investigated both experimentally and theoretically. By using a gap structure, the roles of ion, photon, and radical in producing damage on low-k dielectrics were differentiated. Oxidative plasma induced damage was proportional to the oxygen radical density, enhanced by VUV photon, and increased with substrate temperature. Ion bombardment induced surface densification, blocking radical diffusion. Two analytical models were derived to quantify the plasma damage. Based on the radical diffusion, reaction, and recombination inside porous low-k dielectrics, a plasma altered layer model was derived to interpret the chemical effect in the low ion energy region. It predicted that oxidative plasma induced damage can be reduced by decreasing pore radius, substrate temperature, and oxygen radical density and increasing carbon concentration and surface recombination rate inside low-k dielectrics. The model validity was verified by experiments and Monte-Carlo simulations. This model was also extended to the patterned low-k structure. Based on the ion collision cascade process, a sputtering yield model was introduced to interpret the physical effect in the high ion energy region. The model validity was verified by checking the ion angular and energy dependences of sputtering yield using O2/He/Ar plasma, low-k dielectrics with different k values, and a Faraday cage. Low-k dielectrics and plasma process were optimized to reduce plasma damage, including increasing carbon concentration in low-k dielectrics, switching plasma generator from ICP to RIE, increasing hard mask thickness, replacing O2 by CO2 plasma, increasing CO addition in CO/O2 plasma, and increasing N2 addition in CO2/N2 plasma. By combining analytical techniques with the Kramers-Kronig dispersion relation and quantum chemistry calculation, the origin of dielectric loss was ascribed to the physisorbed water molecules. Post-ash CH4 plasma treatment, vapor silylation process, and UV radiation were developed to repair plasma damage.Item Novel insights into macromolecularly imprinted polymers for the specific recognition of protein biomarkers(2011-08) Kryscio, David Richard; Peppas, Nicholas A., 1948-; Hilt, James Z.; Roy, Krishnendu; Maynard, Jennifer A.; Ellison, Christopher J.Bulk imprinted polymers were synthesized using traditional small molecular weight imprinting techniques for the recognition of bovine serum albumin (BSA). Reproducibility and capacity concerns prompted the use of circular dichroism to investigate the potential effects that conditions commonly employed have on the structure of the protein prior to polymerization. These studies clearly showed a substantial change in the secondary structure of three common model protein templates when in the presence of various monomers and crosslinkers. Molecular docking was used to further examine the interactions taking place at the molecular level. Docking simulations revealed that significant amounts of non-covalent interactions are occurring between the amino acid side chains and ligands; although, the interactions taking place amongst the analyte and polypeptide backbone are responsible for the experimentally observed conformational change. The computational studies also showed that several of the ligands preferentially ‘docked’ to the same amino acids in the protein, indicating that if multiple monomers are employed, this competition for similar binding sites will potentially result in non-specific recognition. These findings are important as they offer insight into the fundamental reasons why recognition of macromolecular templates has proven difficult as well as provide guidance for future success in the field. Using this information, novel surface imprinted polymers were synthesized via a facile technique for the specific recognition of BSA. Thin films based on 2-(dimethylamino)ethyl methacrylate (DMAEMA) as the functional monomer and varying amounts of either N,N’ methylenebisacrylamide (MBA) or poly(ethylene glycol) (400) dimethacrylate (PEG400DMA) as crosslinker were synthesized via UV free-radical polymerization. A clear and reproducible increase in recognition of the template was demonstrated for these systems as 1.6-2.5 times more BSA was recognized by the MIP sample relative to the control polymers. Additionally, these polymers exhibited specific recognition of the template relative to similar competitor proteins with up to 2.9 times more BSA adsorbed than either glucose oxidase or bovine hemoglobin. These synthetic antibody mimics hold significant promise as the next generation of robust recognition elements in a wide range of bioassay and biosensor applications.Item Optical and electro-optical phenomena in transition metal oxide thin film heterostructures(2019-05-13) Ortmann, John Elliott, Jr.; Demkov, Alexander A.; Lai, Keji; Tsoi, Maxim; de Lozanne, Alejandro; Ekerdt, John GBeginning in the mid-20th -Century and continuing to the present day, integrated circuit technology has advanced at a remarkable pace. Dedicated materials research has been at the heart of this advancement, with materials development preceding technological advancement at nearly every stage. As humanity barrels onward into the 21st -Century and our data and computational demands grow ever larger, new computing hardware designed to handle increasingly difficult computational challenges is quickly becoming necessary in order to continue the historical breakneck pace of advancement. Just as in the early days of the integrated circuit, materials advancement will likely be the key to developing the next generation of computing hardware. In this thesis, I investigate two materials systems well-suited for implementation in next-generation optical computing technologies: transition metal oxide quantum wells and Pockels-active BaTiO₃ thin film heterostructures. Both materials systems are promising for use in a wide variety of optical and electro-optical devices central to integrated photonic technologies, including quantum cascade lasers, photodetectors, electro-optic modulators and switches. For the case of transition metal oxide quantum wells, I focus on the famous SrTiO₃/LaAlO₃ materials system. I first investigate the structural and optical properties of arbitrarily thick, high-quality SrTiO₃/LaAlO₃ heterostructures grown on oxide substrates. Then, I demonstrate the monolithic integration of these heterostructures on silicon, bringing them one step closer to technological relevance. Finally, I present detailed simulations of the optical and electro-optical performance of integrated photonic devices based on SrTiO₃/LaAlO₃ heterostructures. In bulk form, the transition metal oxide BaTiO₃ has some of the largest known Pockels coefficients. However, early work suggests the coefficients are reduced by roughly a factor of ten when fabricated as a thin film. Here, I demonstrate the first BaTiO₃-based integrated devices showing bulk-like Pockels coefficients. Then, I iterate on the initial design of the devices in order to optimize them for ultra-low-power refractive index tuning. The resulting devices achieve refractive index tuning with power consumption many orders of magnitude less than previous reports. Taken together, the investigations in this thesis will hopefully open the door for the development of new kinds of optical and electrooptical devices for use in integrated photonics technologiesItem Photoemission study of stepped surface, thin film and nanowire growth(2008-12) Zhou, Xubing; Erskine, James L.Steps on a high index metal or semiconductor surface may play a fundamental role for electronic structure, adsorption, film growth, chemical reaction and catalysis. The surface atomic and electronic structures of stepped W(110) surfaces have been investigated by a few research groups during the past 20 years. But there is still a lot of controversy. We use high resolution core level photoemission to study several different stepped tungsten surfaces. Curve fittings of the spectra permit tests of core-level binding- energy shift models that relate local atomic coordination to binding -energy differences associated with terrace and step-edge atoms. For the first time we find a well resolved W4f₂/₇ peak associated with step edge atoms. We attribute previous failure to directly detect the step-edge effects in core level photoemission to contamination by hydrogen. The well resolved peaks for surface atoms with different coordinations can serve as a “finger print” for specific atoms. Experiments in which stepped surfaces are systematically dosed by H₂ clarify the role played by H contamination. We also grow Ag nanowires on the stepped W(110) surface and use angle resolved photoemission to study the band structure. We find distinct dispersion for the nanowires along the step edge direction while there is only little dispersion perpendicular to the wires. The second part of the research is core level photoemission study on Cesium film growth on Cu(100) surface. We study the phonon broadening effect for Cs at different temperatures. We compare our data with previous theoretical models and get good results on surface and bulk Debye temperatures and zero temperature phonon broadening. The binding energy shifts for the Cs 5p₂/₇ at different temperatures have also been investigated. The results fit the lattice expansion model very well except at temperature higher than 200 K. The higher temperature deviation is caused by thermal evaporation of Cs films. This conclusion is checked by the following coverage dependent core level peaks study on the Cs/Cu(100) system.Item Spectroscopic studies of boron carbo-nitride(2010-12) Ahearn, Wesley James; Ekerdt, John G.; Hwang, Gyeong S.BCxNy films were characterized as a potential pore sealing layer for low κ dielectrics. The changes in chemical bonding were studied as a function of growth temperature to aid in understanding the variation in electrical performance of these films. Thermal chemical vapor deposition of BCxNy using dimethylamine borane and ethylene were deposited on porous methylsilsesquioxane substrates at 335 °C and 1 Torr. BCxNy was able to seal the porous interface with a thickness of 3.9 nm for both blanket and patterned substrates. BCxNy films deposited over a temperature range of 300-400 °C with dimethylamine borane and either ethylene or ammonia coreactant gas were characterized. Films deposited with ethylene became more concentrated in B at the expense of C with increasing temperature. These films favored C-B intermixing over C-C and B-B bonding at higher temperature. H was detected in the form of B-H and C-H bonds. Films deposited with ammonia became concentrated in N at the expense of B, and favored B-N viii bonding at higher temperatures. H was found in the films as B-H, C-H, and N-H bonds. The amount of H in the films decreased with increasing growth temperature for both ethylene and ammonia coreacted films. The valence band offset of C-rich films increased from 0.17 ± 0.22 eV to 0.32 ± 0.22 eV when deposited at 300°C and 400 °C. For the Nrich films, the valence band offset shifted from 0.26 ± 0.28 at 300 °C to -0.15 ± 0.24 eV at the same deposition conditions. High temperature annealing from 400-800 °C in forming gas caused all BCxNy films to decrease in thickness up to 30%. At the same time, the index of refraction increased, and likely, the dielectric constant. X-ray photoelectron spectroscopy revealed little change in the constituent bonding, suggesting that volatile –H containing species were removed during the annealing process.Item Studies of singlet exciton fission in perylenediimide films and triplet exciton transfer at organic:inorganic interfaces(2021-01-27) Bender, Jon Alexander; Roberts, Sean T.; Rose, Michael J; Biaz, Carlos R; Milliron, Delia J; Vanden Bout, David AHerein, I showcase three studies that have formed the backbone of my work at UT Austin. First, we studied singlet exciton fission (SF) in a common perylenediimide (PDI) derivative often used as a molecular organic semiconductor, C8-PDI, using pump-probe spectroscopy to model SF and singlet-singlet annihilation (SSA). Therein, we were surprised to report a SF rate orders of magnitude slower than predicted by computational studies of PDI dimers. In Chapter 3, we study a suite of six PDI derivatives that adopt different crystal structures to further assess the nature of SF in polycrystalline films grown on sapphire and fused-quartz. We developed a kinetic model across the PDI series based on the analysis of time-resolved photoluminescence, free from SSA contributions to kinetics that must otherwise be carefully modelled for TA data of these films. We confirm that Redfield theoretic approximations of the SF rate in these materials better captures the trend in kinetics implying the important role the charge-transfer character in these excitons play in mediating the process. The variation in intermolecular organization and associated changes in the Coulombic and exchange coupling between nearest-neighbor molecules correctly captures a qualitative trend in the observed SF rate, though the observed rates are an order of magnitude smaller than expected. We propose this discrepancy arises because the PDI dimer model we use for our predictive model for polycrystalline PDI thin films neglects changes in the excited state character/energetics that become important in strongly interacting molecular solids. The contents of Chapter 4 are then a study of colloidal suspensions of PbS nanocrystals (NC) decorated with a TIPS-pentacene ligand, 2-CP. We set out to search for evidence of the formation of spin-triplet excitons on the 2-CP ligands after photoexcitation of the PbS NC. Triplet exciton formation is observed with no clear observation of an intermediate charge separated species. However, an intermediary state is observed and carefully assigned to a surface associated state on the PbS NC. This hypothesis is further supported by the presence of a multitude of triplet excited states found in constrained DFT computations and fluence dependent pump-probe data providing evidence for the photopassivation the intermediate surface state. In totality, my studies have elucidated excited state dynamics in singlet fission capable polycrystalline films of perylenediimide molecules and contributed to our growing understanding of triplet exciton transfer between PbS NCs and molecular ligands.Item Theory for Hydration Forces in Thin Films of Aqueous Electrolytes(1993-08) Basu, Subhayu; Sharma, Mukul ManiA theory is presented to quantify the electrostatic forces in thin aqueous electrolyte films between two charge regulating surfaces. The Poisson-Boltzmann equation (PBE) is modified to include the effects of dielectric saturation of the double layer and the corresponding change in hydration free energy of ions. The results obtained agree remarkably well with the experimental data of Pashley (1981) and Israelachvili et. al. (1978) at low electrolyte concentration and pH. At higher concentration and pH, the model provides qualitative agreement with experimental observations. Finite ion sizes needs to be incorporated into the PBE to obtain quantitative agreement with experimental data in the high ion concentration range. The reduction in dielectric constant with increasing electric field increases the hydration energy of the ions in the double layer, giving rise to a repulsive hydration force. It is concluded that dielectric saturation of the double layer near a charged interface plays an important role in the electrostatic interactions in thin electrolyte films.Item Thin film growth and deposition of functional perovskite oxides(2019-09-13) Edmondson, Bryce Isaiah; Ekerdt, John G.; Demkov, Alexander A; Milliron, Delia J; Korgel, Brian A; Willson, Carlton GThe research documented in this work focuses on the growth and deposition of thin film perovskite oxides for their electronic and photonic functionalities. The ever increasing demand for faster electronic devices, particularly in the realm of micro and nanoelectronics, is requiring diversification of the materials used in typical semiconductor-based integrated circuits. Metal oxides, particularly those in the perovskite family of materials, offers a wide range of functionalities that can both increase device performance and add new capabilities such as optical interconnects. The process in which perovskite oxide thin films are deposited and integrated with one another and conventional semiconductors can have many effects on the properties of the resulting device. This work explores these effects in the context of BaTiO₃ (BTO) and Ba [subscript x] Sr₁- [subscript x] TiO₃ (BST) second order nonlinear optical effects, which includes second harmonic generation (SHG) and the linear electro-optic effect (Pockels effect). The work also studies the growth effects on the apparent interfacial conductivity of LaTiO₃/SrTiO₃ (LTO/STO) heterostructures. BTO thin films grown epitaxially on STO(001) by molecular beam epitaxy (MBE) experience strain relaxation due to misfit dislocations for increasing thickness while epitaxial BST thin films remain strained at equivalent thicknesses due to the smaller lattice mismatch. These strained BST films exhibit larger second harmonic generation than relaxed BTO films, for particular compositions, suggesting that the epitaxial strain enhances their polarization and second order nonlinear susceptibility beyond what is capable in equivalently thick, relaxed BTO films. We also explore the deposition of BTO and BST thin films by a chemical solution method under atmospheric conditions. The films are epitaxial with STO(001) template layers prepared by MBE on Si(001) wafers. Presence of the Pockels effect is confirmed in the films, although optical hysteresis and remanent polarization is not observed owing to the films’ small (10-100 nm) grain size, structural defects, and residual tensile stress. Effective Pockels coefficients decrease monotonically with decreasing Ba composition. Non-zero Pockels response is observed in even Sr-rich films, which is explained by the tensile stress that deforms the normally cubic crystal structure into a non-centrosymmetric structure. Post-deposition annealing can increase the Pockels response by approximately four times and is attributed to an increase in grain size and the elimination of structural defects. Finally, we study the effects of oxygen vacancies caused by varying MBE growth conditions of LTO thin films on STO(001) single crystals and thin STO layers on Ge(001). Electrical conductivity of the heterostructures is LTO-thickness dependent, and LTO growth on STO/Ge structures requires orders of magnitude higher partial pressures of molecular oxygen in order to achieve crystalline LTO with proper oxidation states. This is explained by the propensity of LTO to scavenge oxygen from STO during growth, which is limited in LTO/STO/Ge heterostructures, and generates many free carriers and leads to the observed conductivity of the LTO/STO systems BTO thin films grown epitaxially on STO(001) by molecular beam epitaxy (MBE) experience strain relaxation due to misfit dislocations for increasing thickness while epitaxial BST thin films remain strained at equivalent thicknesses due to the smaller lattice mismatch. These strained BST films exhibit larger second harmonic generation than relaxed BTO films, for particular compositions, suggesting that the epitaxial strain enhances their polarization and second order nonlinear susceptibility beyond what is capable in equivalently thick, relaxed BTO films. We also explore the deposition of BTO and BST thin films by a chemical solution method under atmospheric conditions. The films are epitaxial with STO(001) template layers prepared by MBE on Si(001) wafers. Presence of the Pockels effect is confirmed in the films, although optical hysteresis and remanent polarization is not observed owing to the films’ small (10-100 nm) grain size, structural defects, and residual tensile stress. Effective Pockels coefficients decrease monotonically with decreasing Ba composition. Non-zero Pockels response is observed in even Sr-rich films, which is explained by the tensile stress that deforms the normally cubic crystal structure into a non-centrosymmetric structure. Post-deposition annealing can increase the Pockels response by approximately four times and is attributed to an increase in grain size and the elimination of structural defects. Finally, we study the effects of oxygen vacancies caused by varying MBE growth conditions of LTO thin films on STO(001) single crysals and thin STO layers on Ge(001). Electrical conductivity of the heterostructures is LTO-thickness dependent, and LTO growth on STO/Ge structures requires orders of magnitude higher partial pressures of molecular oxygen in order to achieve crystalline LTO with proper oxidation states. This is explained by the propensity of LTO to scavenge oxygen from STO during growth, which is limited in LTO/STO/Ge heterostructures, and generates many free carriers and leads to the observed conductivity of the LTO/STO systems.Item Towards reproducible graphene synthesis on optimized copper substrates(2016-05) Lee, Alvin Lynghi; Akinwande, Deji; Register, Leonard F; Banerjee, Sanjay; Tutuc, Emmanuel; Cullinan, MichaelAs more knowledge is accumulated in the synthesis of 2D materials, such as graphene, graphene analogues, and transition metal dichalcogenides (TMDs), we are confronted with widely varying qualities of materials synthesized. In order to integrate into current VLSI technology and beyond to wearable technologies, the synthesis of the materials needs to be facile and provide reproducible quality. In order for reproducible graphene to be made, we must have a better understanding of the mechanism that causes the differences in quality of graphene synthesized. Different groups report graphene mobilities with larges variance. Even within the same research group there can be differences in graphene quality from piece to piece. To further graphene research, the quality should be close to the level that we would expect less than a 10% difference from batch to batch or group to group. We examine graphene grown on copper of varying purities. Commercially available copper was chosen to be able to have this work be easily replicated. Additionally, we produced copper film using E-beam evaporation, which is a commonly used method. Graphene was synthesized using methods that have already been outlined in literature to examine the role of the substrate. To enable a transition to flexible substrates for wearable devices, we investigate the use of polymer electrets to provide electrostatic doping to graphene. The benefit of using polymer electrets is that they are solution processable and have well understood properties. These properties hold promise for developing wearable graphene devices. The properties of electrets can be further extended to other 2D materials providing the same benefits that they afforded graphene.Item Understanding the electrochemistry and reaction mechanisms of solid-state sulfides with application to the lithium-sulfur battery system(2017-05) Klein, Michael James; Manthiram, Arumugam; Goodenough, John B; Ferreira, Paulo J; Yu, Guihua; Hwang, Gyeong SThe lithium-sulfur (Li-S) battery is a highly promising technology for next-generation high energy density storage. This high energy density has its roots in the conversion chemistry of the Li-S system, which also imparts numerous challenges to the realization of practically viable cells. This dissertation focuses on improving the performance and understanding of insulating solid-state lithium sulfides, which are the source of many of the challenges inherent to Li-S batteries. First, a facile strategy is presented to generate a manganese sulfide surface layer on Li₂S particles, which dramatically improves cycling performance. Analysis of this reaction mechanism demonstrates how surface layers with limited conductivity but high electrochemical stability and facile charge transfer can profoundly improve the solid Li₂S charge mechanism. The role of solid sulfur-sulfur bonding in the cycling mechanism was then analyzed by direct chemical synthesis and isolation of insoluble sulfur-sulfur bonded species (i.e., Li₂S₂-type species). While these syntheses are shown not to generate Li₂S₂ separate from Li₂S, the insoluble polysulfide species were isolated from the soluble polysulfides. These isolated insoluble sulfides are used to demonstrate that solid-state sulfur-sulfur bonds can be reduced in the absence of soluble polysulfides, and the formation of Li₂S₂ is thus not inherently limiting to the capacity of Li-S batteries. To further clarify the fundamental limitation of Li₂S thickness on Li-S battery rate performance, a system was built to sputter-deposit air-sensitive lithium sulfide films of arbitrary thickness. It is shown that while the deposition initially generates a novel sulfide structure containing polymer-like Li₂S units, highly pure crystalline films of Li₂S can be generated with annealing. These Li₂S films are used to systematically determine the maximum thickness of Li₂S that can be charged at a practical rate is approximately 40 nm at a local charge density of 1 μA cm-2. This systematic approach additionally identified the appearance of the activation overpotential when charging Li₂S to be associated with the generation of soluble polysulfide species. Finally, these results are used to develop a model for the rational design of Li-S cathodes by tailoring the conductive pore structure around the local charge density and total sulfur content.