THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Wednesday, August 29, 2018

3:00 PM
in MoSE 4100F

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Yeu-Wei Harn

"Nonlinear Block Copolymer Nanoreactor-Enabled Synthesis of Metal Oxide Nanoparticles with Controlled Dimensions, Compositions and Architectures and Investigation into Their Ferroelectric, Ferromagnetic and Magnetoelectric Properties"

Committee Members:

Dr. Zhiqun Lin, Advisor, MSE

Dr. Seung Soon Jang, MSE

Dr. Vladimir Tsukruk, MSE

Dr. Younan Xia, BME

Dr. Lei Zhu, School of Macromolecular Science and Engineering, CWRU

Abstract:

Multiferroic materials represent a class of materials that possess at least two or more ferroic properties. Among them, the interplay between electricity and magnetism has garnered much attention due to their intriguing properties for fundamental studies and potential applications in advanced spintronics devices, capacitors, actuators, transducers, electromagnetic sensors, communications devices, etc. In these materials, due to the coupling between the ferroelectric and ferromagnetic orders, the electric polarization can be induced by a magnetic field, and conversely the magnetization can be induced by an electric field. Despite such tunable degree of freedom these materials offer, few single-phase materials have been discovered to possess both ferroelectric and ferromagnetic properties due to their intrinsic contradiction among ferroelectric and ferromagnetic materials. To this end, recent researches have been centered on synthesis of nanocomposites consisting of the ferroelectric and ferromagnetic components, thereby leading to larger coupling effect at room temperature. Nanostructured ferroelectric materials show great potential in electronic applications. The grand challenge in nanostructured ferroelectrics is the decreased dielectric property and the destabilized phase and domains as their dimensions decrease. Thus, it is highly desirable to develop a new approach to produce ferroelectric nanocrystals while retaining high dielectric constant.

In this thesis, we aim to explore the size- and shape-dependent ferroelectric, ferromagnetic and magnetoelectric coupling properties of ferroelectric and ferromagnetic plain nanoparticles and ferroelectric/ferromagnetic core/shell nanoparticles, respectively. These nanoparticles possess uniform size and shape, compositions and architectures, rendered by capitalizing on judiciously designed block copolymers as nanoreactors. We first synthesized plain ferromagnetic iron oxide (Fe3O4) nanoparticles, ferroelectric barium titanate (BaTiO3) and lead titanate (PbTiO3) nanoparticles with controlled dimensions, demonstrating the capability of synthesizing a rich variety of ferromagnetic and ferroelectric nanomaterials of different size using star-like diblock copolymers. Their size- and shape-dependent ferroelectric and ferromagnetic properties will then be investigated.

Subsequently, by utilizing star-like triblock copolymers as nanoreactor, multiferroic materials composed of ferromagnetic Fe3O4 as the core and ferroelectric BaTiO3 as the shell with precisely controlled core diameter and shell thickness are expected to be successfully synthesized. As the coordination bonding between functional groups of hydrophilic blocks in star-like triblock copolymers and metal moieties of precursors, the core and shell will only be formed within the specific region occupied by polymer blocks within star-like triblock copolymers, thus enabling the dimension tuning of the core and shell components, independently, which can hardly be achieved by other synthetic approaches. Furthermore, core/shell structures of Fe3O4/BaTiO3 nanoparticles may demonstrate a larger coupling effect due to the large interfaces between these two dissimilar materials. Surface capping can also be altered by changing the third polymer block within star-like triblock copolymers, thus offering a diversity of applications these nanomaterials may have. Their size- and shape-dependent magnetoelectric coupling properties will then be studied. We will also use star-like diblock copolymers with different number of arms as nanoreactors for crafting high-dielectric-constant materials. By changing the number of arms within the nanoreactor (from 8, 16, 21, to 42 arms), tunable ferroelectric domains are expected. The interaction between organic polymer chains and inorganic ferroelectric nanoparticles will be elucidated, and the possible role of polymer chains to stabilize ferroelectric domains will be corroborated. By changing the ability to coordinate of the inner polymer block with precursors and the polymer capped on the surface of as-synthesized ferroelectric nanoparticles, an optimized synthetic condition and the underlying mechanism will be explored.