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NORTHWESTERN UNIVERSITY Understanding Atomic Structure NORTHWESTERN UNIVERSITY Understanding Atomic Structure and Structural Evolution of Perovskite Oxides at the 2-D Limit: From Surface to Thin Film A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUREMENTS for the degree DOCTOR OF PHILOSOPHY Field of Materials Science and Engineering By Tassie K. Andersen EVANSTON, ILLINOIS September 2018 2 © Copyright by Tassie K. Andersen 2018 All Rights Reserved 3 ABSTRACT Understanding Atomic Structure and Structural Evolution of Perovskite Oxides at the 2-D Limit: From Surface to Thin Film Tassie K. Andersen Perovskite oxide materials for the wide array of properties that make them candidate materials for application ranging from catalysis, to electronics, to beyond-Moore computation. As many of these oxides share similar structures they can be combined in a seemingly-endless number of ways to produce the desired properties for a given application. Understanding of their surface structures and interface formation during growth however, is an area not given enough attention, especially in a class of materials that relies heavily on interfaces to produce its unique properties. The objective of this dissertation is to present theories and tools for probing and understanding the atomic structure of perovskite materials at the 2-D limit (i.e. surfaces and thin films). In this dissertation the applicability of Pauling’s rules to prediction and determination of oxide surface structures is presented. Examples of solved surface reconstructions on SrTiO3 (1 0 0), (1 1 0), and (1 1 1) are considered as well as nanostructures on these surfaces and a few other ABO3 oxide materials. These rules are found to explain atomic arrangements for reconstructions and thin films just as they apply to bulk oxide materials. Using this data and Pauling’s rules, the fundamental structural units of reconstructions and their arrangement are discussed. Pauling’s rules are applied to the SrTiO3 (1 1 1) surface to solve the atomic structures of two reconstructions, (√7 × √7)R19.1° and (√13 × √13)R13.9°. These structures were determined using a combination of density functional theory and scanning tunneling microscopy data and APW+lo density functional theory minimizations and simulations. These reconstructions belong 4 to the same structural family made up of an interconnected, single layer of edge-sharing TiO6 and TiO5[] octahedra. This family is found to include the previously-solved (2 × 2)a reconstruction. This reconstruction family and the calculations of surface energies for different hypothesis structures also shed light on the structure of Schottky defects observed on these reconstructed SrTO3 (1 1 1) surfaces. Moving from surfaces to thin films, growth of strontium cobalt oxide thin films by molecular beam epitaxy, and conditions necessary to stabilize different defect concentration phases are reported. In situ X-ray scattering is used to monitor structural evolution during growth, while in situ X-ray absorption near-edge spectroscopy is used to probe oxidation state and measure changes to oxygen vacancy concentration as a function of film thickness. Experimental results are compared to kinetically-limited thermodynamic predictions, in particular, solute trapping, with semi-quantitative agreement. Agreement between observations of dependence of cobaltite phase on oxidation activity and deposition rate, and predictions indicates that a combined experimental/theoretical approach is key to understanding phase behavior in the strontium cobalt oxide system. To facilitate studies of thin film structure, a portable metalorganic gas delivery system was designed and constructed to interface with an existing molecular beam epitaxy chamber at beamline 33-IDE of the Advanced Photon Source. This system offers the ability to perform in situ X-ray measurements of complex oxide growth via hybrid molecular beam epitaxy. Performance of the hybrid molecular beam epitaxy system while delivering metalorganic source materials is described. The high-energy X-ray scattering capabilities of the hybrid molecular beam epitaxy system are demonstrated both on oxide films grown solely from the metalorganic 5 source, and ABO3 oxide perovskites containing elements from both the metalorganic source and a traditional effusion cell. 6 Approved by Professor Laurence D. Marks Department of Materials Science and Engineering Northwestern University, Evanston, IL 60208, U.S.A 7 Acknowledgement Firstly, I would like to thank my advisors, Laurence Marks and Dillon Fong. Without their support and encouragement over the years this dissertation would not be possible. I want to thank Laurie for his wholistic approach to research: his insistence on the importance of following the science and acquiring whatever skills and knowledge are necessary to do so have shaped my approach to experiments and broadened my views. I will always appreciate how he urged me to see connections between the different areas of my work and consider the forest as well as the trees. His curiosity and critical eye have been both invaluable and inspiring. I want to thank Dillon for his depth of knowledge in guiding me and the freedom in projects he trusted me with. He has also tolerated an astonishing amount of stupid questions, and I think him for many interesting discussions. His creative approaches to complicated problems have shown me different ways of tackling hard questions and he has been a wonderful resource in navigating the professional side of my career. I also would like to thank my thesis defense committee, Professor Michael Bedzyk, Dr. John Freeland, and Professor James Rondinelli for their valuable time and discussions. I am exceptionally grateful to all my collaborators for their help in my Ph.D studies. First and foremost, I would like to thank my colleague Dr. Seyoung Cook for the countless hours of assistance during beamtimes and many fruitful discussions. Without his help, the many beamtimes this work relied upon would not have been pleasant or productive. I want to thank Dr. Yang Liu for his guidance in learning to operate the equipment at the beamlines and his expertise in oxide growth. I would also like to thank Dr. I-Cheng Tung for his advice and assistance during beamtimes as well as for sharing his wealth of knowledge in many discussion on X-ray methods and oxide growth. I would like to thank Dr. Hawoong Hong for his unfailing patience and 8 contributions to beamtimes, as well as his expertise. I would like to thank Erika Benda for her mechanical knowledge and for creating beautiful CAD renderings of the hybrid molecular beam epitaxy instrumentation. I would also like to thank Dr. Bharat Jalan, Dr. Roman Engel-Herbert, Dr. Matthew Brahlek, and Dr. Craig Eaton for their valuable insight into construction and troubleshooting of the hybrid system. Furthermore, I would like to thank my collaborator at Oxford University, Professor Martin Castell, for the experimental expertise and many years of back and forth with the thorny problems of various oxide reconstructions. I would also like to thank Shuqiu Wang at Oxford for her assistance with STM images and unfailing willingness to accommodate my most specific questions. Without the assistance of many staff members at the Advanced Photon Source beamlines this work would have been far more difficult and much less enjoyable. I would like to thank Dr. Zhan Zhang, Evguenia Karapetrova, and Roger Ranay. They have all assisted with logistics, scheduling, questions, and at times have lent a helping wrench or screwdriver. I would also like to acknowledge the staff of the Department of Materials Science and Engineering who have always been attentive and swift in their responses. I would also like to thank all the previous and current members of the Marks group for their friendship, conversation, and engaging company. Special thanks are due to Emily Hoffman for her enthusiasm in convincing me to join the group, and Pratik Koirala for his humor and patience with all things DFT-related. For their companionship, many all-you-can-eat sushi dinners, and general commiseration I thank Betty, Lawrence, Chris, Tiffany, Alex, and Zach. 9 The many friends I have made during my time at Northwestern in the graduate school community, department, and the Chicago area also deserve mention- without you the time spent outside of work would be bland and I appreciate all the color and fun you have brought me. A special thanks goes to my partner Dale, who has been a source of encouragement and consistency in my life throughout these years. He has tolerated complaints, strange working hours, and sat through all manner of presentation practices with patience and encouragement. I am very grateful that he has been there to celebrate achievements and prop me up when I need it most. I look forward to everything yet to come. Finally, I would like to thank my family. My brother Tom has provided me with the type of perspective that only an older brother can- leading by example while knowing exactly what to say to make me smile. Thank you to my mother and father who have always cared and listened, while supporting me in all academic and creative endeavors. Their drive has always inspired me to pursue my goals with my entire focus, and their perspective has always kept me grounded. I am forever grateful for all they have done. I would also like to officially thank my funding. The work in Chapters Four and Five was used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562. Specifically, it used the Bridges system, which is supported by NSF award number ACI-1445606, at the Pittsburgh Supercomputing Center (PSC) through allocation DMR160023P. The work in Chapters Six, Seven, and Eight was performed at Argonne National Laboratory, including the Advanced Photon Source, was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences, under Contract 10 No. DE-AC02-06CH11357.
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