https://scholar.gist.ac.kr/handle/local/7936 Fri, 05 Dec 2025 17:39:13 GMT 2025-12-05T17:39:13Z https://scholar.gist.ac.kr/handle/local/31978 Title: X-ray Study of Polymorphism in SnO₂ Thin Films Synthesized via RF Powder Sputtering Author(s): Sukjune Choi Abstract: This dissertation investigates the polymorphic behavior of tin dioxide (SnO₂) thin films grown on single-crystal substrates via radio-frequency (RF) powder sputtering. Particular attention is paid to the crystallization mechanisms, phase competition, and epitaxial stabilization of the rutile-type (R–SnO₂) and columbite-type (C–SnO₂) polymorphs under varying growth and annealing conditions. In the first part of the study, amorphous SnO₂ films deposited at room temperature on Al₂O3(0001) were subjected to in situ annealing, revealing the coexistence of R and C phases during solid-phase epitaxy. High-resolution X-ray diffraction (HRXRD) and off-specular scans demonstrated that both polymorphs nucleate competitively under strain and thermal activation, with their relative volume fractions modulated by film thickness and interfacial strain. Building on these findings, direct epitaxial growth was achieved by heating the substrate during deposition. While similar R/C phase coexistence was observed, detailed off-specular diffraction and reciprocal space mapping enabled the unambiguous identification of phase-specific Bragg peaks and domain orientations. Unexpectedly, room-temperature-deposited films also exhibited partial crystallinity, and thickness- dependent domain evolution was tracked using rocking curve analysis and atomic force microscopy. These results highlighted a critical role of surface roughness, domain alignment, and strain relaxation in determining crystalline quality and phase distribution. In contrast, SnO₂ films grown on YSZ(001) under identical sputtering conditions exhibited stabilization of the C–SnO₂ phase across all thickness examined. Off-specular diffraction and azimuthal ϕ-scans confirmed multi-domain epitaxy involving C[100] and C[010] domains. The absence of R–SnO₂ was attributed to unfavorable lattice matching and anisotropic bonding incompatibility with the cubic YSZ substrate. Comparative analysis suggests that the choice of substrate governs the polymorph selection via strain accommodation and interface energetics, rather than bulk thermodynamic stability. Taken together, this study provides a comprehensive framework for understanding and controlling SnO₂ polymorphism in epitaxial thin films. The results offer new insights into how strain, symmetry, and thermal conditions interplay to govern phase stability and domain orientation in complex oxide systems. These findings contribute to the rational design of polymorphic thin films for advanced electronic and optoelectronic applications. Tue, 31 Dec 2024 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/31978 2024-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19865 Title: Ultrafast and nonlinear optical investigations on van der Waals materials Author(s): Hwiin Ju Abstract: Van der Waals (vdW) materials, due to their highly anisotropic nature, have become a novel platform for creating flat 2-dimensional layers and investigating the 2-dimensional properties of the material. Research on these materials has further expanded through the realization of novel emergent phenomena in heterostructures and superlattices made by artificial stacking, where weakly bonded interlayer structure plays a key role. Therefore, understanding the properties of materials based on their structure is pivotal for this field of research. Within this context, we investigated various properties of vdW materials and interpreted them from a structural perspective using ultrafast and nonlinear optical methods. We characterize the thermal transport properties of TMPS3 (TM = Mn, Ni, Fe) using time-domain thermoreflectance method and interpreted them with their structural disarrangement. The multiferroicity of NiI2 was traced using optical second harmonic generation, and we demonstrated the possible existence of multiferroicity down to the bilayer limit, where interlayer interaction is largely suppressed. We also applied hydrostatic pressure using a diamond anvil cell. Under the hydrostatic pressure application, we discovered that transition metal ditellurides undergo interlayer structural phase transitions at low pressure and intralayer structural phase transitions at high pressure. We discuss various physical properties emerging in the high-pressure phase based on their structural changes. Furthermore, we discovered that the stacking angle of bi layer transition metal ditelluride, WSe2, can be continuously controlled with the application of hydrostatic pressure. These characterizations of vdW materials from a structural perspective will contribute to the advancement of research based on vdW materials. Sun, 31 Dec 2023 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19865 2023-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/31972 Title: Time-dependent Analysis of Electron–Ion Temperature Evolution in Solid-density Samples Heated by Laser-accelerated Ion Beams Chiwan Song Gwangju Institute of Science and Technology Author(s): CHIWAN SONG Abstract: This study uses computational modeling to investigate the time-dependent evolution of electron and ion temperatures in solid-density materials heated by laser-accelerated ion beams. A two-temperature model simulates the thermal response of the target, incorporating cold stopping power data from SRIM, electron-ion coupling factors from the literature, and SESAME equation-of-state tables. The analysis examines the effects of ion species, energy spectra, and time-of-flight on energy deposition and the resulting temperature distributions. Simulation results provide time-resolved spatial profiles of electron and ion temperatures, emphasizing heating uniformity. The study offers a theoretical basis for interpreting ion-beam driven heating experiments and supports the design of future warm dense matter generation experiments using high-energy ion beams. Tue, 31 Dec 2024 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/31972 2024-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19837 Title: The study of defect-related surface reaction on metal oxide model system utilizing ambient pressure X-ray photoelectron spectroscopy Author(s): Hojoon Lim Abstract: The development of surface characterization techniques can contribute to significant progress in cutting-edge science regarding sustainable energy conversion and environment-friendly technology at the molecular level. In the last decade, the advanced operando observation tools have been widely utilized in structural, compositional, and thermodynamic behavior observations at the gas-solid interface in surface science and heterogeneous catalysis. In particular, the synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) plays an important role in fundamental investigations of complicated reactions to reveal physicochemical properties on the material surface under the more realistic condition. In this thesis, I will introduce technical details on the hemispherical electron analyzer equipped with the differential pumping system to build high-flux photon delivery into the analysis chamber at elevated pressure conditions. By using AP-XPS, surface chemical states and interfacial structures have been studied on representative metal oxide model systems such as Ga2O3-x, SrTiO3 (001), and SrRuO3/SrTiO3 (001). The core-level spectra analysis results highlight that the surface redox process and the variation of bandgap on the Ga2O3-x during post-annealing process, the nature of electron depletion and band bending with vacancy formation on the SrTiO3 (001) substrate, and the influence of surface migration of lattice oxygen process at the SrRuO3-SrTiO3 interface on the structural phase transition. Those observed phenomena at the surface show that the bunch of reactive molecules’ interaction at the gas-solid interface could affect the surface electronic structure near the Fermiedge on the condensed matter, which provides more reliable spectroscopic evidence for understanding energy conversion issues on the surface in the real world. Fri, 31 Dec 2021 15:00:00 GMT https://scholar.gist.ac.kr/handle/local/19837 2021-12-31T15:00:00Z