Multiscale Study of BaTiO3 Nanostructures and Nanocomposites

Advancements in integrated nanoelectronics will continue to require the use of unique materials or systems of materials with diverse functionalities in increasingly confined spaces. Hence, research on finite-dimensional systems strive to unearth and expand the knowledge of fundamental physical properties in certain key materials which exhibit numerous concurrent and exploitable functions. Correspondingly, ferroelectric nanostructures, which particularly display a plethora of complex phenomena, prevalent in countless fields of research, are noteworthy candidates. Presently, however, the assimilation of zero-(0D) and one-dimensional (1D) ferroelectric into microor nano-electronics has been lagging, in part due to a lack of applied and fundamental studies but also due to the paucity of synthetic strategies yielding high quality monocrystalline structures. In this work, the problematics of size reduction, which affects many aspects of electronic devices, was addressed. Furthermore, the depolarizing effects associated with finite thickness in ferroelectric nanostructures was investigated in connection with other crucial boundary conditions. The work reported in this dissertation concerned isolated 0D and 1D BaTiO3 nanocrystals and nanocomposites composed of periodic arrays of BaTiO3 nanowires embedded in a matrix formed by another ferroelectric material. A systematic investigation was conducted for those three types of nanostructures from a quantum mechanical and atomistic perspective using both direct-firstprinciples and first-principles-derived methods. Using first-principles-based calculations, the structural phase sequences in 0D (cubic-to-tetragonalto-monoclinic-to-rhombohedral) and 1D (cubic-to-tetragonal-to-orthorhombic-to-monoclinic) BaTiO3 nanoparticles revealed differences from that of the bulk and thin film systems. The monoclinic symmetry found in the 0D compounds, and as for the ground-state of 1D systems, were also affected by size effects and tuned by varying parameters related to the depolarizing effect. Strong electromechanical responses characteristic to the monoclinic symmetry, were also found. In addition, by partially screening the uncompensated charges at the surface of the nanodots, a small range existed (∼ 87% to ∼ 95% screening) where both the polarization and toroidal moment coexisted within the nanoparticles. Ferroelectric nanocomposites are novel systems that were also examined and were found to exhibit completely original properties not yet observed in either constituents alone. The temperaturedependent properties such as the structural phases and behavior of the polarization within these nanocomposites were obtained. Interesting new features related to flux-closure configurations were discovered. Transitions associated with the cores of electric dipole vortices were correlated to the direction of in-plane polarization. In addition, vortex-antivortex pairs in a peculiar phase-locked configuration were ascertained in these structures. Complementary density-functional theory calculations were also performed for BaTiO3 nanowires with dissociated-water adsorbates as a function of the out-of-plane lattice constant. Topological defects with winding numbers ranging from 1 to -3 were found in the water-covered nanowires. The ground-state was found to be of triclinic symmetry. Ab-initio calculations were also performed for nanocomposites to investigate the electronic properties of the phase-locked configuration. Similarly to the Monte-Carlo simulations, a configuration containing both vortices (not localized in the nanowires though) and antivortices was found to be the ground state. Mastery of nanomaterials requires merging theoretical research with experimental observation, hence a synthesis project was developed to obtain BaTiO3 nano-tubes and wires using direct pore filling of nanoporous templates. The preliminary results suggested the synthesis of polycrystalline nanostructures depend on the template pore surface polarity and size. The results presented in this dissertation suggested that ferroelectric nanostructures continue to be of great fundamental value and may substantially impact advancement in certain technologies. Furthermore, the work on nanocomposites offered a glimpse to the novel functionalities in ferroelectrics. The following signatories attest that all software used in this dissertation was legally licensed for use by Lydie Louis for research purposes and publication. Ms. Lydie Louis, Student Dr. Laurent Bellaiche, Dissertation Director This dissertation was submitted to http://www.turnitin. com for plagiarism review by the TurnItIn companys software. The signatories have examined the report on this dissertation that was returned by TurnItIn and attest that, in their opinion, the items highlighted by the software are incidental to common usage and are not plagiarized material. Mr. Ken Vickers, Program Director Dr. Laurent Bellaiche, Dissertation Director c ©2013 by Lydie Louis All Rights Reserved ACKNOWLEDGEMENTS I would like to first and foremost thank God for carrying me through this period of my life. I would like to express my deepest gratitude to all those who helped me see this journey through its completion. I would like to thank Dr. Marty Gregg, and prof. Ken Vickers for accepting to be the referees to my dissertation. I would also like to thank Dean Collis Geren for agreeing to be a member of my committee. I am very grateful and honored to count you three among the members of my committee. I would like to express my deepest and sincerest gratitude to my advisor, Dr. Laurent Bellaiche, for giving me the opportunity to pursue this doctorate degree. I am grateful to you for your guidance, patience and support during this dissertation. You have opened the door to a world of opportunities and have allowed me to accomplish one of my dearest dreams, and for that I am very grateful. I would also like to express my sincerest gratitude to my mentors and advisors at Ecole Centrale Paris (ECP), Dr. Brahim Dkhil, Dr. Grégory Geneste, and Dr. Igor Kornev. I would like to thank particularly my mentor Dr. Brahim Dkhil for facilitating the “dreadful” task of navigating the nightmarish “paperasserie administrative” and for your always entertaining coffee breaks. I am thankful to my mentor, Dr. Grégory Geneste for introducing and teaching me the many intricacies of the density functional theory, and to Dr. Igor Kornev for accepting to be my advisor at ECP. All of you have made my stay at ECP an enriching and very enjoyable experience that I will cherish always. I would like to thank my colleagues both past and present: Dr. Inna Ponomareva for introducing me to the effective Hamiltonian and for many useful discussions. Thank you to the members of the CCMP group: Dr. Sergey Prosandeev, Dr. Sergey Lisenkov, Zhigang Gui, Dr. Wei Ren, Dr. Yurong Yang, for you help and for teaching me a little bit about the Russian and Chinese culture. I would like to extend a special thank to Dovran Rahmedov for all the metaphysical conversations. At ECP, I would like to first thank Dr. Jean-Michel Kiat for accepting me in the SPMS Laboratory. I would like to thank particularly Bertrand Dupé, Julienne Chaigneau and David Albrecht, and Preethi Meher for the many insightful conversations. I would like to thank the laboratory engineers and technicians: Christine Bogicevic and Fabienne Karolak for their help in teaching me to understand and develop the experimental part of this dissertation. Thank you to Françoise Garnier for the SEM images and Agnès Bénard for your technical support. I am thankful to M. Jean-Hubert Schmitt, the director of the research department at ECP for providing the financial support for the part of my dissertation in France. I would like to thank Catherine Lhopitale at the the secretariat of the “Ecole doctorale” for her invaluable administrative help in completing this dual degree project. I am also grateful for the financial support provided by the National Science Foundation grants DMR-1066158 and DMR-0701558. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Finally, I thank my sisters in Christ: Renee Hearon, Jaime Baker, Enma Yahoska Sirias Saldaa, and Ujjwala Darvemula for your support, strength, many uplifting conversations and delicious food. I would like to thank my dearest friends, Yanna Bambouvert, Manola Mischer, Rochel Bent and Dr. Kim Sablon for your friendship, encouraging words, trips (Cancale, Senegal, Danmark, · · · ), hébergement, etc · · · Last but not least, I am grateful to my family for their patience, love and support throughout this period. Thank you mom for your unwavering support and love. David and Damas, I love you guys dearly and I thank you for always minding me. To my father and Nickmann, I think of you always and I miss you terribly.