Graduation Year

2017

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Inna Ponomareva, Ph.D.

Committee Member

Lilia Woods, Ph.D.

Committee Member

Pritish Mukherjee, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Keywords

ferroelectric, antiferroelectric, nanowire, effective, Hamiltonian, electrocaloric

Abstract

Perovskite ferroelectrics and antiferroelectrics have attracted a lot of attention owing to their potential for device applications including THz sensors, solid state cooling, ultra high density computer memory, and electromechanical actuators to name a few. The discovery of ferroelectricity at the nanoscale provides not only new and exciting possibilities for device miniaturization, but also a way to study the fundamental physics of nanoscale phenomena in these materials. Ferroelectric nanowires show a rich variety of physical characteristics which are advantageous to the design of nanoscale ferroelectric devices such as exotic dipole patterns, a strong dependence of the polarization and phonon frequencies on the electrical and mechanical boundary conditions, as well as a dependence of the transition temperatures on the diameter of the nanowire. Antiferroelectricity also exists at the nanoscale and, due to the proximity in energy of the ferroelectric and antiferroelectric phases, a phase transition from the ferroelectric to the antiferroelectric phase can be facilitated through the application of the appropriate mechanical and electrical boundary conditions. While much progress has been made over the past several decades to understand the nature of ferroelectricity/antiferroelectricity in nanowires, many questions remain unanswered. In particular, little is known about how the truncated dimensions affect the soft mode frequency dynamics or how various electrical and mechanical boundary conditions might change the nature of the phase transitions in these ferroelectric nanowires. Could nanowires offer a distinct advantage for solid state cooling applications? Few studies have been done to elucidate the fundamental physics of antiferroelectric nanowires. How the polarization in ferroelectric nanowires responds to a THz electric field remains relatively underexplored as well. In this work, the aim is to to develop and use computational tools that allow first-principles-based modeling of electric-field-induced phenomena in ferroelectric/antiferroelectric nanowires in order to address the aforementioned questions.

The effective Hamiltonian approach is a well validated model which reliably reproduces many static and dynamic properties of perovskite ferroelectric and antiferroelectrics. We begin by developing an effective Hamiltonian for the prototypical ferroelectric potassium niobate, a lead-free material which undergoes multiple structural phase transitions. Density functional theory calculations within the LDA and GGA are used to determine the effective Hamiltonian parameters for KNbO3 . By simulating an annealing within an NPT ensemble, we find that the KNbO3 parameters found from first principles underestimate the experimental transition temperatures. We apply a universal scaling technique to all of the first-principles derived parameters and are thus able to more accurately reproduce the transition temperatures predicted by experiment as well as a number of other static and dynamic properties of potassium niobate.

Having determined the parameters of the effective Hamiltonian for KNbO3 , we use this as well as previously determined effective Hamiltonian parameters for PbTiO3 and BaTiO3 to study the electrocaloric effect in nanowires made of these materials. We determined that, in general, the electrocaloric effect in ferroelectric nanowires is diminished due to the reduced correlation length resulting from the finite lateral dimensions. However, certain temperature ranges were identified near ambient temperature where the electrocaloric response is enhanced with respect to bulk. The effective Hamiltonian model was also employed to study the response of the spontaneous polarization and temperature to tailored electric fields. We identified a novel means of reversing the polarization in ferroelectric nanowires which could potentially be used in the design of nanoscale THz sensors of ultra high density ferroelectric memory devices.

While the soft mode frequency dynamics of bulk ferroelectrics under various mechanical boundary conditions have been studied extensively, the effects of different mechanical boundary conditions on the soft mode dynamics in ferroelectric nanowires remains relatively under-explored. We conduct a comprehensive study on PbTiO3 nanowires which explores the effects of hydrostatic pressure, applied uniaxial stress, and biaxial strain on the structural properties, transition temperatures, and soft mode dynamics. We found that depending on the particular type of mechanical boundary condition, the nanowire can exhibit either monodomain or polydomain vortex phases, drastically different from what is found for PbTiO3 bulk and originates from the critical role of the depolarizing field. We found a rich variety of dipole patterns, particularly for the polydomain states with the dipoles arranged in single and double polarization vortices depending on the type and strength of the mechanical boundary conditions. The soft mode frequency dynamics are also strongly affected by the mechanical boundary conditions. In particular we find that the frequency of the E mode in the P4mm phase is significantly larger than the A 1 mode which is in contrast with bulk PbTiO3 . This striking finding is attributed to the presence of the depolarizing field along the truncated directions which leads to mode hardening.

In the last chapter, we identify the emergence of a ferroelectric state in antiferroelectric PbZrO3 nanowires and describe possible ways to stabilize the ferroelectric phase. Finally, we explore how our findings could potentially be used to improve existing technologies such as energy storage devices and electromechanical actuators as well as future technologies like solid state cooling devices.

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