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Title:Novel phase and phase transition behavior of a superionic conductor observed on the nanoscale
Author(s):Heo, Jaeyoung
Director of Research:Jain, Prashant K
Doctoral Committee Chair(s):Zuo, Jian-Min
Doctoral Committee Member(s):Shim, Moonsub; Chen, Qian
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemical structure
Defects in solids
Transmission electron microscopy
fast-ion transport
Abstract:This dissertation documents the work performed in the pursuit of a PhD degree in the research group of Prof. Prashant K. Jain at the University of Illinois at Urbana-Champaign. I employed in-situ transmission electron microscopy (TEM) for understanding the ionic structure, vacancy ordering, phase transitions, and ion transport in a prototypical fast-ion conductor, copper selenide. Major objectives of my work were to elucidate i) the nature of the structural phase transition by which copper selenide becomes a superionic phase and ii) the microscopic mechanisms of ionic motion in this material. My studies were performed on nanostructures of copper selenide. Chapter 1 introduces the concept of a superionic conductor and describes one particular example, copper selenide, which is the model system studied in my work. Also, I briefly discuss in-situ high-resolution TEM the primary technique I used, and the advantages and capabilities this method offered for my studies of copper selenide. Chapter 2 presents an interesting finding made in the course of my TEM studies: the observation of a novel phase of cuprous selenide in nanocrystals. This novel phase exhibits an unusually longer-range of Cu-vacancy ordering and is found from density functional theory (DFT) calculations to be a metastable phase. After thorough characterization of the different phases of copper selenide in nanocrystals under TEM, I investigated, using in-situ TEM, the phase transition behavior of copper selenide nanocrystals from their ordered state to their superionic phase, the results of which are described in Chapter 3. From the spatiotemporal kinetics interrogated by in-situ TEM, we determined the precise nucleation sites of the phase transition and establish a correlation between the dynamics of the cations and anions in the transition. Next, we studied using in-situ TEM the ionic structure and dynamics in copper selenide nanowires, which are one-dimensional structures with anisotropy unlike zero-dimensional nanocrystals. As described in Chapter 4, I observed anti-phase boundaries, a type of planar defect, in the nanowires. Investigation of the phase transition behavior near anti-phase boundaries led us to conclude that this planar defect impedes the growth of the superionic phase. Also, I observed the motion of this defect under electron beam irradiation. From the characteristics of the motion, we infer how copper ions migrate in the copper selenide lattice. Taken together, my work leveraged well-defined nanostructures and in-situ TEM to shed broadly applicable insights into the atomistic dynamics involved in a phase transition, vacancies and defects in ionic compounds, and the nature of cation transport in superionic materials.
Issue Date:2020-12-03
Rights Information:Copyright 2020 Jaeyoung Heo
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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