(Advisor: Professor Nazanin Bassiri-Gharb)

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Committee

• Prof. Lauren Garten – School of Materials Science and Engineering
• Prof. Joshua Kacher – School of Materials Science and Engineering
• Prof. Asif Khan – School of Electrical and Computer Engineering
• Prof. Eric Vogel – School of Materials Science and Engineering

Abstract

Antiferroelectrics are materials that transition from a macroscopic nonpolar state to a polar state upon application of a sufficiently high electric field. The unique functional properties that accompany the field-induced transition make antiferroelectrics attractive for applications including high strain-high blocking force actuators, energy storage devices, and more. Lead zirconate (PbZrO3), often considered the prototypical antiferroelectric, is classically modeled with a double antiparallel configuration of dipoles between adjacent unit cells, resulting in a null macroscopic polarization in the absence of an applied electric field. However, there is a growing body of work reporting structures and ferroic properties of PbZrO3 that are inconsistent with the classical framework of antiferroelectricity. The existence of a ferrielectric ground state, resulting in ferroelectric responses along some directions and antiferroelectric responses along others, has been hypothesized, and work is ongoing to re-evaluate the understanding of antiferroelectricity in PbZrO3. However, the challenges of processing undoped, phase-pure (perovskite) PbZrO3 hinder experimental efforts to probe the stability of antiferroelectric, ferroelectric, and ferrielectric states, which are susceptible to the effects of stoichiometry, crystal anisotropy, and mechanical boundary conditions. While the former two have been investigated extensively, studies on the effects of thickness reduction and size confinement in PbZrO3 are especially lacking. This work proposes to leverage chemical solution processed PbZrO3 ultra-thin (<200 nm thick) films and nanostructures to probe the effects of mechanical boundary conditions on the stability of antiferroelectric, ferroelectric, and ferrielectric phases.

In the initial studies of this work, PbZrO3 thin films were prepared by chemical solution deposition to study the effects of stoichiometry, presence of secondary phases, and crystal anisotropy on the processing-structure-properties relationships. To compensate for eventual Pb loss during processing, over-stoichiometric Pb content was introduced in bulk through the precursor solution, and locally by deposition of PbO layers at interfaces. Pb over-compensation resulted in porous microstructures and lossy polarization switching, while Pb-deficiency resulted in formation of nanocrystals at the film surface and crystallization interfaces. Crystallization interfaces were Pb-deficient and contained both Pb-rich and Pb-deficient secondary phases. Both bulk and local Pb compensation were necessary for processing highly 001-oriented PbZrO3 films and for mitigating the presence of secondary phases. Work is ongoing to investigate the possibility of finer stoichiometric control through variation of the concentration of PbO capping layers (deposited after crystallization of the final PbZrO3 layer). Insights into the processing-structure-properties relationships from these initial studies will inform the processing of ultra-thin PbZrO3 films and nanostructures in subsequent studies.

The influence of mechanical boundary conditions, which are expected to affect the stability of phases in antiferroelectrics, will be investigated. A critical thickness below which the antiferroelectric phase transforms to a ferroelectric or ferrielectric phase has been reported in PbZrO3 and other antiferroelectrics. Thickness reduction, resulting in residual stresses, is expected to stabilize the size-induced phase. Through processing of ultra-thin PbZrO3 films, the effects of thickness reduction on the macroscopic structural and functional properties as well as the nanoscale polarization switching response will be evaluated. The size-induced phase transition in antiferroelectrics has also been attributed to a surface effect. Such an intrinsic effect should then be present in size-confined PbZrO3 nanostructures, which have higher surface-to-volume ratio than ultra-thin films. Through processing of PbZrO3 nano-islands, the effects of size confinement on the nanoscale topography and polarization switching response will be evaluated. Finally, the superposition of temperature on the aforementioned boundary conditions will be investigated to evaluate the effects on the stability of high- and low-temperature phases in PbZrO3.