Browsing by Subject "Perovskite"
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Item Carbonatites(2009-07) Barker, Daniel S.Item Characterization of manganese-oxide perovskites, exhibiting a colossal magnetoresistance(2003) Liu, Guoliang; Goodenough, John B.The complex phase diagrams of the La1-xSrxMnO3 ( 0 ≤ x ≤ 0.35), La 1- xCaxMnO3 ( 0 ≤ x ≤ 0.40 ), and R0.7A0.3MnO3 ( 0.949 ≤ t ≤ 0.997 ) systems, in which R is one or more the trivalent rare earth ions, A is one or two divalent alkaline earth, and t is tolerance factor, are presented; they were mapped out with measurements of resistivity ρ(T), thermoelectric power α(T), specific-heat Cp(T), and magnetization M(T) on a series of melt-grown single-crystal samples. A transition from localized to itinerant behavior of electrons of e-orbital parentage in the presence of a localized t3 configuration with spin S = 3/2 was shown to be characterized by an electronic phase segregation into orbitally ordered, electron-rich antiferromagnetic regions and orbitally disordered, conductive ferromagnetic regions that grow in an applied magnetic field. This two-phase electronic character in the crossover compositions gives rise to spin-glass behavior and a colossal magnetoresistance. Particular emphasis was placed on the evolution of the specific-heat data Cp(T) on crossing the transition from localized to itinerant electrons and the fact that Cp(T) is suppressed at a spin-glass freezing temperature.Item Computational investigation of functional perovskites(2018-06-12) Li, Xinyu, Ph. D.; Henkelman, Graeme; Zhou, Jianshi; Goodenough, John; Hwang, GyeongFunctional perovskites have been investigated extensively for many years. Thousands of new perovskites are synthesized and studied every year. Many functional perovskites have been widely employed in industry. Density functional theory (DFT) calculations have been used to obtain a better understanding of functional perovskites, especially their electronic and structural properties. During my graduate study, I investigated perovskite’s properties on ionic transport, magnetic ordering, ferroelectricity, physical property and phase transition using DFT calculations. In the first case, I simulated the ionic transport process in several Ruddlesden- Popper (RP) phases. Climbing image nudged elastic band (CI-NEB) calculation was used to get accurate oxygen interstitial migration barrier. I established a linkage between interstitial migration barrier and perovskite’s octahedral rotation with symmetry mode approach. Two factors, including A-site atom radius and epitaxial strain, were used to reduce interstitial migration barrier in my simulation. My study on ionic transport in RP phases provides guidance on the design of fast ionic transport in perovskite oxides. In the second case, DFT calculation was employed to investigate a double perovskite’s magnetic and electronic properties. A new ferroelectric mechanism in perovskite, associated with the displacement of coplanar Mn²⁺, was discovered experimentally. My DFT calculation explained the origin of coplanar displacement from an orbital point of view. In addition, DFT simulations were used in the design of ferroelectricity enhancement perovskite. In the last case, I simulated structural behaviors under pressure of several double perovskites. The results show that these double perovskites can be divided into two groups based on their octahedral rotations under pressure. The origin of their distinct volume reduction mechanisms was studied through DFT simulations. The difference between the two mechanisms and their influence on bulk modulus were discussed based on my computational results.Item Development of electrode materials with matched thermal expansion for solid oxide fuel cells(2018-07-09) Lai, Ke-Yu; Manthiram, Arumugam; Goodenough, John B.; Kovar, Desiderio; Hwang, Gyeong S.Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with a conversion efficiency of over 50 % from fuel to electricity. Their high operation temperature (600 - 1000 °C) enables SOFCs to directly utilize hydrocarbon fuels without an external fuel reforming system or precious-metal catalyst. However, several critical electrode challenges impede the mass commercialization, such as high thermal stress, electrode material decomposition, unwanted reactions between neighboring components, and impurity poisoning. A rapid SOFC failure during operation is mainly caused by the mismatch of thermal expansion coefficients (TECs) among device components. Unfortunately, few electrode materials with suitable TECs and adequate electrochemical activities have been reported. With the aim of achieving high phase stability and enhancing catalytic activity, new anode and cathode materials with compatible TECs are developed. YBaCo₄O₇-based swedenborgite oxides with Y-site dopants (In³⁺ and Ca²⁺) and Co-site dopants (Ga³⁺, Al³⁺, and Fe³⁺) are investigated as cathode materials in intermediate-temperature SOFCs (600 - 800 °C). The high-spin state of the Co cation in a tetrahedral coordination prevents spin transition at elevated temperatures and makes the TECs of YBaCo₄O₇-based materials much lower than those of Co-containing perovskite oxides. However, YBaCo₄O₇-based materials may decompose at > 600 °C. Hence, the cation doping effect on the long-term phase stability is examined with 50 compositions. The electrical conductivity, TECs, thermal behavior, catalytic activity toward the oxygen reduction reaction, and SOFC performance and stability are comprehensively evaluated. A Co-doped chromite perovskite oxide with self-regenerating Co-Fe nanoparticles is utilized as a catalytically-active anode. The moderate TEC of the chromite perovskite oxide is slightly higher than the TECs of common electrolyte materials. Unlike the conventional Ni - electrolyte cermet anode, the oxide anode exhibits high redox phase stability without irreversible performance degradation during a reduction and oxidation (redox) cycle. The performance is significantly enhanced with exsolved Co-Fe nanocatalysts. The sulfur impurity tolerance and coking resistance are evaluated with an electrolyte-supported single cell by various fuels. Meanwhile, the self-regeneration behavior of exsolved nanoparticles on the oxide surface is described by carefully observing the surface evolution during a redox cycle at 700 and 800 °C.Item The effect of epitaxial strain and R³+ magnetism on the interfaces between polar perovskites and SrTiO₃(2011-05) Monti, Mark Charles; Markert, John T.; Markert, John T.; de Lozanne, Alex; Tsoi, Maxim; Yao, Zhen; Campion, AlanWe have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO₂ terminated SrTiO₃ (STO) substrates and have grown epitaxial thin films of LaAlO₃ (LAO), LaGaO₃ (LGO), and RAlO₃ on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R³+ magnetism on the interface between RAlO₃ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10¹⁴ cm[superscript -2] and 4.5 x 10¹⁴ cm[superscript −2], respectively. Rare-earth aluminate films, RAlO₃, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R³+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO₃ films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R³+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering.Item Electrochemical materials for the production and storage of renewable energy(2020-06-23) Wygant, Bryan Russell; Mullins, C. B.; Crooks, Richard M; Roberts, Sean T; Eberlin, Livia S; Yu, Edward TThe production of electricity from renewable sources, including solar power, is increasingly important as our society seeks to move to cleaner energy sources. Organolead halide perovskites, a class of thin film photovoltaic (PV) materials, are an exciting competitor to traditional Si devices, but suffer from poor material stability. Further, PV power is intermittent, creating a need for efficient energy storage during times when solar power is unavailable. H₂ gas, produced via electrochemical water electrolysis, is a promising way to store this energy in chemical bonds but efficient electrocatalysts are required to drive the reaction. This need for electrocatalysts is particularly acute for the complementary oxygen evolution reaction (OER), the rate-limiting half reaction of electrolysis. Here, we address both halves of the renewable energy problem above, production and storage, and study how the chemistry of PV and OER electrocatalyst materials impacts electrochemical performance and material stability. In regard to production, we studied the performance and stability of quasi-2D Ruddlesden-Popper phase (RPP) perovskites under humid conditions. We found that RPP perovskites are more stable than typical 3D perovskites due to a unique moisture-driven disproportionation mechanism that passivates and protects the surface of the RPP perovskite. This process can also result in the formation of discrete RPP crystallites within the bulk of a perovskite film or device. We also found that changing the composition of the RPP perovskite enables control of the halide diffusion barrier, further impacting material stability. We next investigated energy storage, and studied how elemental composition affected the performance of two transition metal-based OER electrocatalyts. We found that for a Co-containing oxide perovskite, changes in the crystal structure of the catalyst from hexagonal to orthorhombic had little effect on OER performance, while adding small amounts of Fe improved catalytic behavior. Likewise, we found that the addition of Se to a nickel sulfoselenide material improved OER performance, even though the sulfoselenide material itself oxidizes during electrocatalysis to produce a catalytically-active nickel (oxy)hydroxide surface. Altogether, our work highlights the importance chemical composition when studying the material stability and electrochemical performance of both PV and electrocatalytic materials for renewable energy applications.Item The high pressure synthesis, crystal growth and physical properties of transition metal perovskites(2013-12) Marshall, Luke Gordon; Goodenough, John B.; Zhou, JianshiThe perovskite structure has an incredible versatility that results in myriad compounds with varied and eccentric behaviors. Perovskite oxides have been extensively studied and used for over 60 years. In order to expand on our already thorough knowledge of these compounds, it is necessary to use modern and creative experimental techniques. High-pressure synthesis and high oxygen-gas pressure annealing techniques are used to synthesize oxygen stoichiometric RNiO₃ (R = lanthanide). The particularly rich phase diagram of this compound allows for the study of the crossover from localized to itinerant electronic behavior and from an enhanced Pauli to a Curie-Weiss law paramagnetism. Single crystals of RFeO₃ are grown in order to analyze the spin canting in these antiferromagnetic samples. The size of the rare earth-cation is used to tune the magnitude of octahedral-tilt distortions. This tuning allows distinguishing between the two possible drivers for spin canting and weak ferromagnetism in these compounds, the octahedral-tilt-dependent single-ion anisotropy and the octahedral-tilt-independent Dzyaloshinskii-Moriya interaction. Although it is a fluoride compound, KCuF₃ has been used as an analogue to transition-metal oxide perovskites such as LaMnO₃ because of the similarity of their orbital ordering. Through the use of high-temperature neutron diffraction, it is shown that the orbital ordering and Jahn-Teller distortion in this compound are not lifted at the predicted temperature. Another mechanism for orbital ordering is identified. La₂[subscript-x] Sr [subscript x] CuO₄ has long been of interest as the progenitor system of the highTc superconductors. Despite having an exceedingly well-studied phase diagram in the over-doped region of its superconducting dome, little is known about this system in the region x > 0.3 because of the difficulty of synthesizing fully oxygen-stoichiometric samples. With high-oxygen-gas-pressure annealing and high-pressure synthesis, the completion of the phase diagram up to x = 1.0 is attempted. Finally, like many iridates, post-Perovskite CaIrO₃ exhibits a very strong spinorbit coupling of its 5d electrons. Because its magnetism is very weak, traditional methods to measure the magnitude of its orbital moment and spin-orbit coupling, such as neutron powder diffraction, are not viable. In order to address this issue, direct measurement of the orbital moments was conducted by using x-ray absorption spectroscopy and x-ray magnetic circular dichroism techniques.Item Hybrid organic-inorganic perovskites : humidity stability and CdTe tandem photovoltaics(2019-09-17) Siegler, Timothy Daniel; Korgel, Brian Allan, 1969-; Keitz, Benjamin K; Milliron, Delia J; Vanden Bout, David ASolution-processed hybrid organic-inorganic perovskites (HOIPs) have garnered significant interest for tandem photovoltaics (PVs)-solar cell architectures that employ two absorber layers to overcome the PV thermodynamic efficiency limit. Perovskites have been coupled in tandem with silicon, CIGS, and other perovskites to boost PV efficiency. However, they have not been employed in tandem with popular PV material CdTe. Additionally, HOIPs suffer from rapid humidity-induced degradation, which limits their commercial application in general. Here, compositional engineering of the B-site is used to tune humidity stability of model HOIP semiconductors, and HOIP materials for tandem CdTe-HOIP PV are developed. Substituting Pb²⁺ 5% with Bi³⁺ in model HOIP CH₃NH₃PbI₃ (MAPI) is seen to stabilize MAPI at 90% humidity, but destabilize MAPI at 60% humidity, making bismuth the first HOIP additive observed to have a stabilizing and destabilizing effect at different humidity conditions. From mechanistic insight and kinetic modeling, this stabilizing and destabilizing effect is shown to be due to Bi³⁺ impacting the kinetics of different steps of the degradation reaction mechanism distinctly. The related humidity-induced degradation/deliquescence of transition metal halide thin films is then made use of in thin films of NiI₂. Water vapor uptake is seen to rapidly modulate light transmittance uniformly across the visible spectra, making NiI₂ films candidates for color-neutral smart windows. HOIP-CdTe tandem cells are then studied. Traditional iodide-based HOIPs have band gaps that are too similar to CdTe for efficient tandem PV; therefore, CdTe is coupled with wide band gap CH₃NH₃PbBr₃ (MAPBr). MAPBr films exhibit a significant amount of haze due to optical loss in the MAPBr layer, which correlates with poor tandem PV performance. The Semiconductor Capacitance Simulator (SCAPS) software package is then utilized to determine the degree of haze that can be tolerated in a MAPBr-CdTe tandem before MAPBr begins to worsen CdTe PV efficiency. Inclusion of Ag⁺ is seen to produce MAPBr films with uniform crystallographic orientation due to surface segregation of Ag and respective lowering of the surface energy of (100) MAPBr facets. Finally, initial steps towards Tl-based low band gap HOIP nanostructures for CdTe-HOIP tandem PV with ideally matched band gaps are undertakenItem Imaging the diffusion of photoexcited free charge carriers in organic-inorganic trihalide perovskites(2019-05) Wannlund, David Grant; Lai, Keji, 1978-Organic-inorganic perovskites have recently attracted significant attention because of their large - and increasing - power conversion efficiencies (PCEs) reaching upwards of 20%. With the promise of application in photovoltaics and optoelectronics, Methylammonium Lead Triiodide (MAPbI₃), one such organic-inorganic trihalide perovskite, warrants further investigation of the carrier dynamics at the nanoscale level. After introducing organic-inorganic trihalide perovskites in Chapter 1, I discuss the construction and design challenges of the new Cryochamber Light Assisted Microwave Impedance Microscope (L-MIM) in Chapter 2. In Chapter 3, using the recently constructed Cryochamber L-MIM, I spatially map the intrinsic photoconductivity of MAPbI₃ with nanoscale resolution to reveal diffusion lengths below 10μm independent of the incident laser power density. I demonstrate the utility of L-MIM in revealing nanoscale electronic properties of emerging materials and discuss future applications of this powerful tool in Chapter 4.Item Manufacturing of intermediate-temperature solid oxide fuel cells using novel cathode compositions(2007-05) Torres Garibay, Claudia Isela, 1972-; Kovar, DesiderioThe development of intermediate temperatures solid oxide fuel cells (IT-SOFC) with YSZ electrolytes imposes a double requirement in their manufacturing. First, the electrolyte has to be kept as thin as possible to minimize ohmic polarization losses. Second, the cathode compositions used must exhibit an adequate catalytic activity at the operating temperature (600 – 800 ºC). Current methods to manufacture thin YSZ electrolytes require complex processes, and sometimes costly equipment. Cathode compositions traditionally used for high temperature solid oxide fuel cells, such as (La,Sr)MnO3 do not exhibit good catalytic properties at intermediate temperatures. These challenges present areas of opportunity in the development of original manufacturing techniques and new cathode compositions. This study presents a low-cost fabrication procedure for IT-SOFC using tape casting, co-firing and screen printing. The electrochemical performance of the cells is evaluated using a known cathode composition for IT-SOFC, such as La0.6Sr0.4CoO3-δ (LSC), novel perovskite oxides, such as Nd0.6Sr0.4CoO3-δ (NSC), and perovskite-related intergrowth oxides compositions, like Sr0.7La0.3Fe1.4Co0.6O7-δ (SLFCO7) and LaSr3Fe1.5Co1.5O10-δ (LSFCO10). The impact of conductivity is studied by substituting Fe for Co in the case of the perovskite oxides, with compositions such as La0.6Sr0.4Co0.5Fe0.5O3-δ (LSCF), and Nd0.6Sr0.4Co0.5Fe0.5O3-δ (NSCF) and by infiltration of NSCF with silver. The effect of the cathode sintering temperature is studied using LSC and LSCF cathodes. It is found that there is generally a correlation between cell performance and conductivity. However, the microstructure of the cathode is also important in determining cell performance by tailoring the cathode sintering temperature. IT-SOFC with SLFCO7 cathodes show a performance comparable to cells with LSFC cathode. In the case of LSFCO10, the performance loss associated with its lower conductivity compared to LSC can be more than offset by tailoring the microstructure.Item Monolithic integration of crystalline oxides on silicon and germanium using atomic layer deposition(2015-05) McDaniel, Martin Douglas; Ekerdt, J. G. (John G.); Demkov, Alexander A; Yu, Edward T; Mullins, Charles B; Manthiram, ArumugamInside your microelectronic devices there are up to a billion transistors working in flawless operation. Silicon has been the workhorse semiconductor used for the transistor; however, there must be a transition to materials other than silicon, such as germanium, with future device sizes. In addition, new dielectric oxide materials are needed. My research has examined a type of crystalline oxide, known as a perovskite, which is selected for its ability to bond chemically to Si and Ge, and eliminate the electrical defects that affect performance. Many perovskite oxides are lattice-matched to the Si (001) and Ge (001) surface spacing, enabling heteroepitaxy. To date, the majority of research on crystalline oxides integrated with semiconductors has been based on strontium titanate, SrTiO3, epitaxially grown on Si (001) by molecular beam epitaxy. Alternative low-temperature growth methods, such as atomic layer deposition (ALD), offer both practical and economic benefits for the integration of crystalline oxides on semiconductors. My initial research informed the broader community that four unit cells (~1.5 nm) of SrTiO3 are required to enable heteroepitaxy on Si. The research has also shown that heteroepitaxial layers can be monolithically integrated with Si (001) without the formation of a SiOx interlayer between the Si (001) surface and the SrTiO3 layer because ALD is performed at lower temperatures than are typical for MBE. Thus, a combined MBE-ALD growth technique creates possible advantages in device designs that require the crystalline oxide to be in contact with the Si (001) surface. In recent work, I have demonstrated a method for integrating crystalline oxides directly on Ge by ALD. Germanium is being explored as an alternative channel material due to its higher hole and electron mobilities than Si, potentially enabling device operation at higher speed. This all-chemical growth process is expected to be scalable, is inherently less costly from a manufacturing cost of ownership, and is based on current manufacturing tool infrastructure. The impact of my research will be in continued scaling of device dimensions with novel materials that will provide faster speed and lower power consumption for microelectronic devices.Item Monolithic integration of functional perovskite structures on Si(2014-08) Choi, Miri; Demkov, Alexander A.; Shih, Chih-KangFunctional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods.Item Oxide materials at the two-dimensional limit(2017-05-05) Kormondy, Kristy Joy; Demkov, Alexander A.; Lai, Keji; De Lozanne, Alejandro L; Tsoi, Maxim; Ekerdt, John GEmergent phenomena in transition metal oxide films are receiving considerable attention with the development of techniques for the preparation of well-controlled oxide surfaces. On the macroscopic scale, such display novel physics phenomena including superconductivity, magnetism, ferroelectricity, and more. On the nanometer scale, the properties of epitaxial interfaces are further impacted by strain, band alignment, and crystal imperfections that may affect the long-range as well as the short-range order. Furthermore, symmetry lowering at the interface creates entirely new environments that are not accessible in the bulk environment. Thus, thin-film oxide materials are increasingly important in many applications. My work focuses on epitaxial oxides of the perovskite, spinel, and rocksalt structure and covers two main phenomena: (1) the two-dimensional electron gas at epitaxial oxide interfaces, and (2) thin epitaxial electro-optic oxides. Because polar oxides are of prominent interest as a mechanism for the formation of the two-dimensional electron gas, I start with a study of polar semiconductor Co₃O₄. Ellipsometry reveals a direct band gap of 0.75 eV, and magnetic measurements show the signature of antiferromagnetic ordering at 49 K, higher than the typical bulk value. Next, I look closer at the role of defects by studying the highly conducting layer at the crystalline [gamma]-alumina/SrTiO₃ (STO) interface which is attributed to oxygen vacancies. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator. Building upon these results, I show that even at room temperature, out-diffusion of oxygen from SrTiO₃ during epitaxy of highly spin-split semiconductor EuO epitaxy creates a highly conductive layer of oxygen vacancies on the SrTiO₃ side of the interface. The films are ferromagnetic with a Curie temperature of 70 K and display giant magnetoresistance below the transition temperature. Leveraging this approach offers an as-yet unexplored route to seamlessly integrate ferromagnetism and the oxide two-dimensional electron gas for the development of novel nano-oxide spintronic devices. The large effective Pockels coefficient for high-quality epitaxial BaTiO₃ (BTO) films on Si distinguishes BaTiO₃ as a highly promising material for integrated silicon nanophotonics. However, the linear electro-optic effect in BaTiO₃ thin films determined in previous experiments clearly shows deteriorated properties compared to bulk BTO crystals. First, I study BaTiO₃ films of varied thickness in order to quantify the Pockels coefficient with respect to crystalline orientation. As a next step, I report on the strong dependence of the Pockels effect in BaTiO₃ thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. The 25× enhancement of the Pockels coefficient indicates a promising route to increase the performance of nonlinear oxides in the two-dimensional limit for the development of novel hybrid silicon photonic platform.Item Oxide-ion conduction in anion-deficient perovskites(1995-05) Feng, Man; Not availableItem Perovskite and Ruddlesden-Popper oxides for electrochemical energy conversion and storage(2018-12-07) Forslund, Robin Paul; Vanden Bout, David A.; Johnston, Keith P., 1955-; Stevenson, Keith J; Mullins, Charles B; Henkelman, Graeme AWith the growing threat of climate change and impending scarcity of fossil fuels it has become necessary to develop more efficient means of converting and storing energy from intermittent, renewable sources such as wind and solar. In an effort to help solve these broad issues, perovskite oxides with the nominal formula ABO₃ and derivatives of the perovskite crystal structure have been selected to investigate their possible application as materials to convert and store energy, either to catalyze the anodic reaction in the generation of hydrogen or to store charge themselves as pseudocapacitor materials. One commonly studied anodic reaction used in tandem with the evolution of hydrogen gas is the oxygen evolution reaction, and a series of Ruddlesden-Popper La [subscript 0.5] Sr [subscript 1.5] Ni [subscript 0.7] Fe [subscript 0.3] O [subscript 4+δ] materials, a derivative of the perovskite structure, were used to demonstrate how previously proposed descriptors such as increased M - O bond covalency, Ni - Fe interactions, and utilization of a reaction mechanism that involves lattice oxygen in the evolution of oxygen gas could all be incorporated to yield a highly active catalyst and demonstrate how hybridization of electronic bands near the Fermi level may serve as a guiding design principle for future catalysts. In an effort to further reduce the cell voltage needed to generated hydrogen, these types of materials were applied to the electrooxidations of urea, methanol and ethanol. LaNiO₃ perovskite was shown to display greater activity than materials containing Ni in lower oxidation states, and further raising the oxidation state of Ni in a series of Ruddlesden-Popper La [subscript 2-x] Sr [subscript x] NiO [subscript 4+δ] materials past Ni³⁺ continued to raise their activities toward the oxidations of these small molecules. Finally, CaMnO [subscript 3-δ] perovksite and Ca₂MnO [subscript 4-δ] Ruddlesden-Popper oxides were used to demonstrate the roles of surface redox reactions and bulk oxygen diffusion at varius scan rates in psedocapacitive charge storage through anion intercalation.Item Perovskite oxide electrode materials for energy conversion and storage(2016-09-16) Mefford, John Tyler; Rose, Michael J., Ph. D.; Stevenson, Keith J.; Johnston, Keith P; Crooks, Richard M; Henkelman, GraemeThe development of renewable energy conversion and storage technologies has become a leading focus of the fields of Materials Chemistry and Electrochemistry, with the ultimate goal of converting energy generated by renewable sources such as wind and solar into chemical fuels that can be stored and then utilized with clean energy conversion technologies. With energy densities primarily being a factor of fuel weight, metal-air batteries and fuel cell technologies are attractive candidates, having energy densities near gasoline. In addition, storage solutions that are both fast charging/discharging and have extended lifetimes are needed to implement renewable generation technology on a wide scale. In this vain, perovskite oxide catalysts were chosen as a model system to investigate both their applications as pseudocapacitor electrodes for storing energy and as air electrodes for both the reduction of oxygen and water electrolysis during the generation of energy. For energy storage, LaMnO [subscript 3±δ] was shown to be an active pseudocapacitor electrode, taking advantage of oxygen vacancies are charge storage sites that could intercalate oxygen anions from the electrolyte. For energy generation, a number of systems were studied. Firstly, the connection between the element in the active site and the chemical functionalities of the carbon support were identified using a number of perovskite systems, LaBO3 (B = Mn, Co, Ni, Ni₀.₇₅5Fe₀.₂₅), and high surface area carbons that were either unfunctionalized (reduced graphene oxide) or nitrogen-doped. Out of this work came the hypothesis of lattice-oxygen being an intermediate reactant during the OER and that the chemical functionalities of the carbon support were crucial to the ORR. This work was extended further by examining the system La₁₋ [subscript x] Sr [subscript x] CoO [subscript 3-δ] (0 ≤ x ≤ 1). Through a combination of rigorous characterization of the materials coupled with computation modeling, the connection was made between the covalency of the Co-O bond and access to an OER pathway utilizing lattice oxygen. On the ORR side, the connection between support interactions with the catalyst was extended to demonstrate that the ORR followed a 2-site series pathway with O₂ being reduced to HO₂⁻ on the carbon support and HO₂⁻ being further reduced to OH⁻ on the perovskite surface.Item Perovskites oxides for metal-air batteries and pseudocapacitor applications(2019-06-12) Alexander, Caleb Tyler; Stevenson, Keith J.; Johnston, Keith P., 1955-; Hwang, Gyeong; Milliron, DeliaWind and solar energy’s rapid development has created a significant need for low-cost energy storage to enable renewables at grid level. To meet these challenges, metal-air batteries and fuel cells are being considered for base-load energy storage while high power applications like frequency regulation and uninterruptable power supplies (UPS) can be addressed using high energy pseudocapacitors. The major bottleneck to metal-air battery and fuel cell commercialization is the sluggish oxygen reactions at the positive electrode that are industrially catalyzed using expensive precious metal catalysts like Pt and IrO₂. Here, the aim is to replace precious metal-catalysts with low-cost LaNiO₃ perovskites and N-doped CNTs in alkaline conditions and study their synergistic interactions and composite stability. The work is continued by studying the anion-intercalation pseudocapacitance in a perovskite oxide library with composition La [subscript 1-x] Sr [subscript x] BO [subscript 3-δ] (B = Mn, Fe, Co; 0 ≤ x ≤ 1) and found that increasing oxygen vacancy content universally increases the pseudocapacitance while the B-site element controlled the redox potential. The most pseudocapacitive materials were then used to make the first all perovskite asymmetric pseudocapacitors with a maximum energy density of 31 Wh kg⁻¹. This work was followed by using the principles learned to further extent the redox voltage potential difference using LaNi [subscript 1-x] Fe [subscript x] O [subscript 3-δ] and brownmillerite-SrFeO [subscript 2.5] to make an asymmetric pseudocapacitor. Doing this, the redox discharge potential was pushed all the way to 1.1 V which is the highest asymmetric pseudocapacitor discharge peak potential reported to date.Item Photoluminescence and stability of perovskite-phase CsPbI₃ nanocrystals and development of nickel metal-organic decomposition inks(2022-12-02) Abney, Michael Keith; Korgel, Brian Allan, 1969-; Ekerdt, John G; Milliron, Delia J; Vanden Bout, David ALead halide perovskite nanocrystals (LHP NCs) exhibit interesting and exceptional optical properties such as near-unity photoluminescence (PL) quantum yield and narrow PL emission line widths. As a result, LHP NCs have demonstrated promising potential in an array of optoelectronic devices such as photovoltaics (PVs), light-emitting diodes (LEDs), optical detectors, and lasers. For CsPbI₃ NCs in particular, one of the major obstacles to their commercial success is structural instability of the perovskite phase, which must be managed. Here, a closer look is taken at the PL emission of CsPbI₃ NC films and how it is affected by light and environmental conditions. The thermal phase stability of CsPbI₃ and the impact of surface area and composition are also investigated. Light-induced changes in photophysical and electronic properties in LHPs can affect their performance in device applications. It is revealed that light excitation induces a slow, reversible enhancement in PL lifetime and intensity in films of perovskite-phase CsPbI₃ NCs. Placing the films under vacuum or nitrogen for several minutes was also found to increase the PL lifetime and intensity. A model of slow, humidity and light-sensitive surface states in CsPbI₃ NCs is proposed. Perovskite-phase CsPbI₃ nanocrystals convert to the optically inactive δ-phase at elevated temperature. Alloying with Br was found to improve the phase stability when the films were relatively thin. Films of mixed-halide CsPbI₂.₅Br₀.₅ nanocrystals less than 30 nm thick showed no conversion to the δ-phase even after 1 hour of heating at 250°C, while thicker films still reverted to the δ-phase after heating. These results show that compositional changes and film thickness can have a significant, cumulative effect on the stability of perovskite nanocrystal films. The role of surface area/energy in CsPbI₃ nanocrystal stability is discussed. CsPbI₃ NC-based solar cells were successfully fabricated. This inspired collaborative work on the development of printable, conductive metal-organic decomposition (MOD) inks, which could play a role in the contacts of perovskite PV for low-cost, flexible, and low-temperature applications. Early development and characterization of a screen-printable, air-curable Ni-based MOD ink is detailed. Current performance of this ink is limited by residual carbon contamination, which is a focus of further development.Item Physical properties of transition metal oxides synthesized by floating zone method and spark plasma sintering(2018-05-04) Li, Zongyao; Goodenough, John B.; Zhou, JianshiTransition metal oxides have attracted growing attention over the last few decades because of rich physical properties they exhibit. Perovskite structure transition metal oxides AMO₃ are of particular interest to the design of functional materials in modern techniques, since a variety of ways can be used to tune the physical properties of AMO₃. Single crystals of Y₁₋ₓLaₓTiO₃ are grown by floating zone method to study the magnetic transition from ferromagnetic in YTiO₃ to G-type antiferromagnetic in LaTiO₃. Y₁₋ₓ LaₓTiO₃ shows similar magnetic phase diagram with RTiO₃ family, and the magnetism and the transition temperature can be finely tuned by varying the La doping x. By measuring the change of magnetic transition temperatures on single crystal samples under uniaxial stress, the correlation between the lattice distortions and the cooperative orbital ordering can be distinguished. Double perovskite CaMnTi₂O₆ is the first columnar A-site ordered perovskite exhibiting ferroelectric property. Spark plasma sintering (SPS) is used to successfully synthesize gram-level Ca₂₋ₓMnₓTi₂O₆, which has the same crystal structure and similar high-T [subscript c] ferroelectric property. Through neutron diffraction, the detailed information of the structure is obtained, and the driving force for ferroelectricity is identified. Inspired by the successful synthesis of double perovskite Ca₂₋ₓMnₓTi₂O₆, perovskites La₁₋ₓPrₓRuO₃ are obtained by SPS as well. The substitution of La by smaller rare earth ion Pr gives rise to the crossover from itinerant to localized electronic behavior. A systematical study of physical properties is made and an unusual second-order metal insulator transition is found in La₁₋ₓPrₓRuO₃. The A²⁺V₂O₄ spinels have the smallest gap caused by electron-electron correlations in the single-valent spinels, and the V-V bond length in these spinels decreases as the A-site cation is replaced by cations in the order of A = Cd, Mn, Fe, Mg, Zn, Co. The density functional theory (DFT) calculation and transport properties of CoV₂O₄ under pressure indicate that CoV₂O₄ might be at the crossover between localized electron and itinerant electronic behavior. In order to clarify this, the series of AV₂O₄ spinels (A = Cd, Mn, Fe, Mg, Zn, Co) are studied with in situ high-pressure x-ray and neutron diffraction at different temperatures.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.