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Browsing by Subject "Perovskites"

Now showing 1 - 4 of 4
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    Electronic Structure Theories for Periodic Systems
    (2021-05) Pham, Hung
    Electronic structure theory for periodic system is a crucially important branch of quantum chemistry and condensed matter physics. For many solid-state phenomena, for instance, high temperature superconductivity, the effect of the strong correlation between electrons is profound and cannot be accurately described by means of a mean-field theory like density functional theory (DFT). The inherent single-reference nature of DFT together with the absence of a systematic approach to approximate the exchange-correlation functional result in its poor performances in predicting several solid-state properties, such as cohesive energy, heat of formation, and band gap. Quantum chemical treatment beyond DFT for strongly correlated solids is often impractical due to their poor scaling with the system size. Alternatively, quantum embedding theories is a powerful ansatz to overcome the computational cost challenge. This is achieved by fragmentating the system into smaller pieces and only treating the chemically relevant fragments using an expensive yet accurate method. In this thesis, I discuss our effort in combining density matrix embedding theory (DMET) with wave function theories to treat electron correlation in solids. We introduce an algorithm to perform DMET on periodic system where the unit cell can be seen as a chemically relevant fragment. Mostly importantly, we demonstrate how multireference methods such as complete active space self-consistent field and n-electron valence state perturbation theory can be extended to solid-state systems within the DMET framework. I hope that our exploration of multireference wave function theories discussed in this dissertation can inspire further developments of electronic structure theories and numerical algorithms for periodic solids.
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    Functional Oxide Depositions on Dielectric Substrates for Optical Applications
    (2015-11) Block, Andrew
    Complex oxides of the transition metals are essential materials in the fabrication of photonic devices due to their high transparency in the infrared and the exotic properties they exhibit. Unfortunately, integrating them onto semiconductor platforms has proven to be a challenge due to significant differences in lattice constant (12-15Å compared to 5Å) and thermal expansion coefficient (~10-5 compared to 10-6) leading to cracking in films during thermal processing. This work focuses on methods to integrate two complex oxides onto semiconductor platforms while minimizing crack formation and maximizing the optical properties used for photonic devices. The two oxides to be discussed are yttrium iron garnet (YIG), an oxide that exhibits the magneto-optical effect, and barium strontium titanate (BSTO), which exhibits the electro-optical effect. The discussion will begin with the fabrication procedures to make each oxide, focusing on the novel reactive sputter and rapid thermal anneal method used to achieve film crystallization. Film properties will then be discussed, including transparency, dielectric constant, and crystallization. Each of the two complex oxides exhibited extremely high photonic effects, magneto-optical for YIG and electro-optical for BSTO, respectively. From the work optimizing oxide thin films, methods to fabricate complex oxide waveguides were developed that resulted in high transparency and transmission of a guided mode. These materials are highly effective for use in semiconductor integrated photonics.
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    Magneto-electronic phase separation in doped cobaltites.
    (2009-09) He, Chunyong
    This thesis work mainly focuses on magneto-electronic phase separation (MEPS), an effect where chemically homogeneous materials display inhomogeneous magnetic and electronic properties. A model system La1-xSrxCoO3 (LSCO) is chosen for the study of MEPS. The doping evolution of MEPS in LSCO single crystals is extensively studied through complementary experimental techniques including heat capacity, small angle neutron scattering, magnetometry, and transport. It is found that there exists a finite doping range over which MEPS occurs. The doping range determined from different experimental techniques is found to be in good agreement. Also, this same doping range is reproduced by statistical simulations incorporating local compositional fluctuations. The excellent agreement between experimental data and statistical simulations leads to the conclusion that the MEPS in LSCO is driven solely by inevitable local compositional fluctuations at nanoscopic length scales. Such a conclusion indicates that nanoscopic MEPS is doping fluctuation-driven rather than electronically-driven in LSCO. The effect of microscopic magneto-electronic phase separation on electrical transport in LSCO is also examined. It is demonstrated (i) that the T = 0 metal-insulator transition can be understood within double exchange-modified percolation framework, and, (ii) that the onset of a phase-pure low T ferromagnetic state at high x has a profound effect on the high T transport. In addition, a new origin for finite spin Co ions in LaCoO3 is revealed via a Schottky Anomaly in the heat capacity, which was not previously known. Such a discovery casts a new understanding of the spin state at low temperature. Via small-angle neutron scattering and d.c. susceptibility, it is revealed that short-range ordered FM clusters exist below a well-defined temperature (T*) in highly doped LSCO. It is demonstrated that the characteristics of this clustered state appear quite unlike those of a Griffiths phase. Finally, through magenetometry and SANS, the magneto-crystalline anisotropy of highly doped LSCO is studied and the easy and hard magnetization axes are determined.
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    Thin-Film Synthesis of Metal Halide Perovskites for Optoelectronics
    (2020-08) Clark, Catherine
    Metal halide perovskites (MHPs), like the archetypal methylammonium lead iodide (MAPbI3), have emerged in the last decade as promising materials for efficient, low-cost optoelectronics. MHP solar cells have already reached efficiencies >25%, rivaling established technologies like single-crystal Si. Yet several challenges prevent the widespread commercialization of MHPs, including their instability in ambient conditions, their toxicity, and the need for scaleable fabrication techniques. Fundamentally, the origins of important material properties relating to carrier transport and recombination are still not well understood. Thin film deposition techniques that enable detailed study of process-structure-property relationships and are commercially relevant are consequently becoming increasingly essential. This thesis seeks to address these challenges through the design, implementation, and utilization of a carrier-gas assisted vapor deposition (CGAVD) method that can grow MHP films with highly tunable stoichiometries and morphologies. Alongside the design of a CGAVD system with six independently controllable experimental parameters, an analytical model is developed and experimentally validated that allows the determination of robust and repeatable growth regimes and the prediction of material deposition rates. Harnessing this technique, we demonstrate the ability to deposit MASnI3 and MASnBr3 films and to systematically vary their compositions across a wide range, and realize corresponding changes in film microstructures (grain size, coverage) and electronic properties (resistivity, carrier concentration, mobility). Control of grain size and film texturing is also achieved independent of stoichiometry via modulation of chamber pressure and substrate temperature. The benefits of CGAVD are further highlighted by the successful growth of novel all-MHP heterojunctions. Two stable pairings are identified: MAPbBr3/MASnBr3 and CsPbBr3/MASnBr3. Design rules to control the mixing of heterojunctions are developed by exploring the dependence of mixing rate on MHP layer composition and grain size. Finally, through a collaboration with Physical Electronics, we optimize the use of XPS depth-profiling for MHPs and investigate which ions are diffusing in a layered structure that exhibits mixing. Moving forward, the incorporation of CGAVD-grown heterojunctions and Pb MHPs into optoelectronic devices will harness the tunability of this system towards a deeper understanding of process-structure-property relationships in MHP thin films and novel layered structures.