Date of Graduation

8-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Laurent Bellaiche

Committee Member

Igor Kornev

Second Committee Member

Brahim Dkhil

Third Committee Member

Gregory Geneste

Fourth Committee Member

Collis R. Geren

Fifth Committee Member

Ken Vickers

Sixth Committee Member

Marty Gregg

Abstract

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 micro- or 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-first-principles and first-principles-derived methods.

Using first-principles-based calculations, the structural phase sequences in 0D (cubic-to-tetragonal-to-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 temperature-dependent 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.

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