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Surface Structures of the Metal-Oxide Materials Strontium Titanate and Lanthanum

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Surface Structures of the Metal-Oxide Materials Strontium Titanate and Lanthanum NORTHWESTERN UNIVERSITY Surface Structures of the Metal-Oxide Materials Strontium Titanate and Lanthanum Aluminate A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS for the degree DOCTOR OF PHILOSOPHY Field of Materials Science & Engineering By Danielle M. Kienzle Evanston, Illinois July 2013 2 © Copyright by Danielle M. Kienzle 2013 All Rights Reserved 3 Abstract A wide array of techniques were applied in this research to investigate the perovskite materials SrTiO3 and LaAlO3 with the goal of furthering the understanding of oxide surfaces. Specifically, a combination of transmission electron diffraction, direct methods and density functional theory was used to determine the structure of the SrTiO3 (001) (√13×√13)R33.7º surface reconstruction. It has a TiO2-rich surface with a 2D tiling of edge or corner-sharing TiO5 octahedra. By tiling these units and forming network surface structures ranging from ordered, like the 2x1 and c(4x2), to pseudo-ordered, like the c(6x2), to a disordered glass-like surface layer made up of TiOx . The LaAlO3 (110) 3x1 surface reconstruction, here reported for the first time, was found to have a hydroxylated Al-rich surface with X-ray photoelectron spectroscopy. Transmission electron diffraction data and direct methods revealed a high resemblance to the previously solved SrTiO3 (110) 3x1 reconstruction leading to a hydrated version that fits the for LaAlO3 3x1 structure. The hydroxyl groups are necessary to balance the surface polarity, an issue arising from the difference in cationic valences between La/Sr and Al/Ti. Also reported and investigated here for the first time is a LaAlO3 (100) 5x2 reconstruction. A direct methods analysis was done for several sets of recorded diffraction pattern; however the results have yet to lead to an atomic surface structure solution. X-ray photoelectron spectra were collected over a range of detector-to-surface-normal angles elucidating an Al-rich surface layer. X-ray photoelectron intensities were calculated for a model of alternating Al and La layers over a 4 range of grazing angles and varying amounts of Al in the top surface layer. An Al concentration of 0.5 was found to give the best fit to experimental results. Approved by Professor Laurence D. Marks Department of Materials Science and Engineering Northwestern University, Evanston, IL 60208, U.S.A. 5 Acknowledgments First and foremost, I would like to thank my adviser, Laurie Marks, without which this dissertation would not be possible. I am truly grateful for the opportunity and the experiences I have had at Northwestern in the L. D. Marks Group. For as much as I have learned about surfaces, I have also learned a lot about myself and what I can accomplish. I would also like to thank my Qualifier committee: Professors Scott Barnett, Chris Wolverton, and Peter Stair, as well as my defense committee: Professors Scott Barnett, Thomas Mason, and Ken Poeppelmeier, for their time and energy. I would like to thank the staff at the Electron Probe Instrumentation Center (EPIC) at Northwestern University for their expertise, interest, and readiness to help me with any sample preparation or instrumentation. Dr. Shuyou Li and Dr. Jinsong Wu, in particular, were indispensable to me and could always be counted on for getting me what I needed, be it more rr f r h p gr r r h r I g “ ” fr h TEM r … g . I w k h k K E r fr H h f r h h p w h maintenance and repairs for our groups UHV-TEM. Thanks to the Materials Science Department ff p P gg A f r f g f “ gr ” q ; h Chemistry Department staff and Pat Fulton at the Catalysis for Catalysis for helping me schedule group meetings over the years. Thank you to all past and present L. D. Marks members: Courtney, Yingmin, Yougui, Jim C., Brian, for teaching me how to make a TEM sample, use SPEAR, and tend goal on the MatSci floor hockey team; Jim E. and Andres for your collaboration on all things STO; Ariel, 6 Emilie, Yifeng, Chuandao, Yuyuan for providing a sounding board for me to work through research problems; Thanks to Richard Zou who worked with me as an undergraduate and did much of the sample preparation for the LAO 3x1 in Chapter 5. A special thanks to all of the friends I have made at Northwestern, Steph, Minna, Marie, Marc, Jason, Matt, Megna, Ted, David, Mitch, Alok, Jon, Andres, Ariel, Brian, Prad, for all the good times and shared experiences that will stay with me forever. Thank you to my parents for their unwavering support and love no matter the circumstances. And to Justin, who has been encouraging and supporting me through the last few years of my graduate career, just meeting you may have been my greatest accomplishment. 7 Table of Contents Abstract .......................................................................................................................................... 3 Acknowledgments ......................................................................................................................... 5 Table of Contents .......................................................................................................................... 7 List of Abbreviations .................................................................................................................. 10 List of Figures .............................................................................................................................. 12 List of Tables ............................................................................................................................... 15 1. Introduction ........................................................................................................................ 16 1.1 Motivation ...................................................................................................................... 16 1.2 Organization ................................................................................................................... 22 2. Techniques .......................................................................................................................... 23 2.1. Sample Preparation ............................................................................................................ 23 2.2. Transmission Electron Microscopy and Diffraction .......................................................... 24 2.3. Direct Methods ................................................................................................................... 28 2.4. Density Functional Theory ................................................................................................. 30 2.5. Bond Valence Sums ........................................................................................................... 34 2.6. X-ray Photoelectron Spectroscopy ..................................................................................... 35 3. SrTiO3 Reconstructions ...................................................................................................... 37 3.1. Strontium Adatom .............................................................................................................. 37 3.2. Oxygen Vacancy ................................................................................................................ 39 3.3. Double layers Reconstruction ............................................................................................ 39 3.3.1. Surface: 2x1, c(4x2), c(6x2) ....................................................................................... 39 3.3.2. 2x2 ............................................................................................................................... 44 3.3.3. (√2x√2)R45° ............................................................................................................... 44 3.3.4. SrTiO3 (110) ................................................................................................................ 46 3.4. Conclusions ........................................................................................................................ 47 8 4. SrTiO3 (001) (√13×√13)R33.7º Surface Reconstruction .................................................. 49 4.1. Introduction ........................................................................................................................ 49 4.2. Background ........................................................................................................................ 49 4.3. Experimental ...................................................................................................................... 50 4.4. Results ................................................................................................................................ 52 4.5. Discussion .......................................................................................................................... 61 4.6. Conclusions ........................................................................................................................ 65 5. LaAlO3 (110) 3x1 Surface Reconstruction ........................................................................ 66 5.1. Introduction ........................................................................................................................ 66 5.2. Experimental ...................................................................................................................... 67 5.2.1. Transmission Electron Microscopy and Direct Methods ...........................................
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