Exploring nanoscale properties in metal halide perovskites and multiferroics via scanning probe microscopy

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Embargoed until 2019-12-01
Copyright: Kim, Dohyung
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Abstract
Visualizing polar and magnetic domains in recent years has become an important topic in the physical research field due to their potential applications in microelectronics such as data storage and photonic devices. Scanning probe microscopy (SPM) as a large group of the microscopy techniques has been a very useful instrument in studying micro to nano sized structures whereby resolution ranges from atomic level to the nanoscale. In this thesis, various techniques based on SPM were used to study piezoelectric, electrical, and magnetic properties of different materials. The materials were chosen based on their potential applications in science and technology. Each chapter in this dissertation is allocated to specific materials. Organic-inorganic metal halide perovskites have gained considerable attention for next-generation photovoltaic cells due to rapid improvement in power conversion efficiencies. In this dissertation, fundamental understanding of underlying mechanisms related to light and bias induced effects at the nanoscale is systematically studied in mixed halide perovskites and single crystal MAPbI3. Firstly, mixed halide perovskites show that periodically striped ferroelastic domains can be modulated significantly under illumination as well as by electric bias. This leads to strain disorder with changed external conditions such as light, bias, and temperature. Secondly, the role of domain walls is explicitly explored in mixed halide perovskites. The results show the observation of enhanced ion migration at the domain walls and its effect in local charge separation and collection. Thirdly, single crystal MAPbI3 is chosen to investigate types of ionic defects for specific (100) and (112) lattice facets. The results reveal significant anisotropic properties. These findings provide fresh microscopic insight for designing more efficient perovskite-based devices. Multiferroics have recently attracted considerable attention due to their unusual properties and for exploiting them in potential applications such as memories, sensors, field-effect transistors, transducers and actuators. First of all, among various multiferroics, iron vanadate (FeV2O4) spinel oxide is considered as an interestingly synthesized material as it undergoes various structural and magnetic phase transitions at low temperature (~below 140 K) with complex spin structures. Here, a cryogenic MFM is used to visualize temperature-dependent magnetic domains of (001)-oriented single crystalline epitaxial iron vanadate thin films. The results show that the magnetic domains are varied as a function of temperature accompanied by domain coarsening and shrinking. Secondly, conductivity of domains and domain walls in polycrystalline BTO ceramics is studied to observe conduction behaviour as a function of temperature. The results reveal distinct temperature-dependent electric conduction at certain grain boundaries and domains. These findings on magnetic and polar domain structures provide the materials to understand underlying mechanism regarding interesting phenomena. In summary, these studies suggest that SPM work provides highly detailed physical properties of the materials and an in-depth insight into novel functional properties in (FAPbI3)0.85(MAPbBr3)0.15, MAPbI3 single crystal and FeV2O4, BaTiO3.
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Author(s)
Kim, Dohyung
Supervisor(s)
Seidel, Jan
Yun, Jae Sung
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Publication Year
2019
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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