https://scholar.gist.ac.kr/handle/local/7936 2026-09-08T20:42:59Z 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. 2024-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/33417 Title: X-ray Diffraction Studies on the Metal-Insulator Transition in Vanadium Oxide Nanocrystals Author(s): Mohd Faiyaz Abstract: In this dissertation, we study the comprehensive structural changes accompanying metal insulator transition (MIT) in vandium dioxide (VO2) nanocrystals using in-situ synchrotron x-ray diffraction (XRD) measurements, which is an ideal and robust technique to precisely investigate changes in the lattice structure at the nanoscale level. Recently, transition-metal oxides (TMOs) such as VO, VO2, V2O3, Fe3O4 etc have attracted immense discussion due to their MIT transition property. Among them, VO2 is especially attractive since its MIT, where electrical conductivity and optical refctance change drastically, occurs near 68 °C, close to room temperature (RT). Due to the existence of this MIT, VO2 thin films and nanostructures have been investigated for practical applications such as sensor, energy harvesting systems, thermoelectric room temperature devices, ultrafast nonvolatile memories, and optical switches. This MIT transition is accompanied by a structural phase transition (SPT) where a low temperature (T) monoclinic (M) structure changes to a high temperature rutile (R) structure. The origin of this MIT transition including the role of the structural modification has been extensively studied but still remains disputable. Recent research has been directed towards the possibility of controlling the MIT properties by modifying the morphology of nano structures, structural defects, stoichiometry, and strain. Since the electronic band structure of VO2 depends sensitively on the small lattice distortion leading to pairing of vanadium atoms, the MIT property is altered critically by the nature and density of structural defects. It is reported that grain boundaries affect the MIT characteristics as they might act as nucleation centers of the metallic phase, and the propagation of phase boundaries are controlled by them. In this dissertation we have also demonstrated the tuning of the MIT temperature using the morphology of the twinned VO2 nanocrystals. The details of studies are as follows. In chapter 1, we write motivation for research related to vanadium oxide and its significance in the cutting-edge research. Principles, the theories and features of x-ray diffraction study are introduced in chapter 2. Chapter 3 contains literature survey and scientific research related to vanadium oxide such as synthesis study, MIT transitions and their control etc. Chapter 4 deals with the contents of experimental setups and methods used for the synthesis of VO2 nanocrystals and their characterizations. In chapter 5 we discuss about the synthesis and structure of ’V’ shape twinned VO2 nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)M twin plane played a key role in the determination of the morphology of VO2 nanocrystals. The growth of VO2 nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (aM -axis) direction resulting in ‘V’ shape twin crystals with the angle between the sides of around 115.4°. At relatively low growth temperature, 900 °C, the growth of the sides of ’V’ was limited producing ’coffee-bean’ shape crystals in which flat crystal facet regions are connected with rounded edges. The twin crystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (bM -axis) normal to the substrate. In the in-plane direction, the (001)M planes of the VO2 twin crystals was aligned to the direction ±2.3° away from the sapphire (112 ̅0) plane. The sides of ’V’ exhibit a rectangular cross-section truncated by the substrate. A synchrotron x-ray diffraction measurement across the metal-insulator transition of the twin crystals implies that the phase transition of the coffee-bean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown V-shape nanocrystals. In chapter 6 we investigated the structural transformation from the insulating monoclinic to the metallic rutile phase in VO2 using synchrotron x-ray diffraction in a grain orientation specific manner using a two-dimensional x-ray pixel detector. The XRD data averaged in all grain orientation corresponding to typical powder diffraction profile, the transition occurred over a broad temperature range of about 8.9K. However, the transition temperature and range vary greatly among the grains composing a bulk VO2 specific. We attribute the broadness of the transition in the averaged profile to this variance of the transition temperature among grains. Consistently, the transition was much sharper in the nanoparticle specimen, where were able to isolate signals from even fewer grains than the bulk power specimen. To understand the intrinsic physical characteristics, one has to carefully analyze data from a bulk system, and it is preferable to study VO2 single crystals. And finally in the last, chapter 7 contains the conclusions of this dissertation. 2020-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/32800 Title: Ultrafast x-ray absorption and emission spectroscopy using broad band x-ray pulse sources Author(s): Anwar Mahar Muhammad Ijaz Abstract: This thesis describes the ultrafast x-ray absorption and emission spectroscopy using broad band x-ray pulse sources. X-rays played very vital role in all field of science specially physics since after Wilhelm Conrad Roentgen's discovers of "Roentgenstrahlen" or X-rays in 1895. The biggest contribution of x-ray towards research is the information about the atomic structure of matter. The electronic configurations changes during chemical reactions, molecular or lattice vibrations, and structural phase transitions occur on a time scale of femtoseconds therefore in order to study and understand these process we required a radiation source which have pulse duration of femtosecond. During recent years x-ray science enters a new era of scientific research due to the emerging of novel type of pulsed x-ray sources which are equipped with very short pulse duration up-to few tens of femtoseconds. These ultra-short pulses are generated by the electron accelerators. In synchrotron femtosecond laser are used to cut out a femtosecond slice from the picosecond electron bunches and these sources are known as slicing source. Ultrashort x-ray pulses are also obtained from the laboratory based laser plasma x ray sources. These ultra-short x-ray pulse sources are very effective for the time resolved studies in pump probe scheme. We used laser plasma x ray source to perform these studies in which x rays from the laser plasma x ray source are used as the probe to observe the changes in the electronic system made by the pump laser pulse. Ultrafast time-resolved x-ray absorption near edge spectroscopy (XANES) experiment was performed on a magnetite Fe3O4 film using a femtosecond laser plasma x-ray source delivering Bremsstrahlung radiation. Temporal evolution of the XANES of Fe3O4 following an excitation by an infra-red (IR) laser pulse was observed in a pump-probe scheme. The charge transfer also causes an ultrafast increase in the IR transmission. We developed a hard x-ray spectrometer for the purpose of measuring x-ray emission spectrum excited by a single x-ray free electron laser (XFEL) pulse. A highly oriented pyrolytic graphite (HOPG) crystal was placed in von Hamos geometry to focus x-rays of the equal energy on a specific point in the detector plane. The spectrometer was tested at PAL-XFEL using Ni and NiO films. This spectrometer can be utilized to conduct various XFEL x-ray emission and absorption measurements using XFELs. 2018-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/33107 Title: Ultrafast X-ray & EUV Absorption Spectroscopy of Non-Equilibrium Warm Dense Matter Author(s): Jong-won Lee Abstract: Experimental setups for time-resolved extreme ultraviolet (EUV) and X-ray absorption spectroscopy were developed and a series of femtosecond and picosecond measurements have been performed to study the ultrafast electron dynamics in non-equilibrium warm dense matter (WDM). First, using high-harmonic generation (HHG) of a Ti:sapphire laser, femtosecond EUV absorption measurement setup for warm dense aluminum was developed. An aluminum nano foil was highly excited by intense femtosecond laser pulses, and the ultrafast evolution of EUV absorption around the L-edge was measured. The results indicate the existence of non-equilibrium electron distribution right after optical excitation, and the Fermi–Dirac distribution with a finite electron temperature is established in about 300 fs. This measurement can visualize the equilibration process of highly excited electronic system in aluminum, and the thermalization time-scale is an order of magnitude longer than the time scales of known electron-electron scattering processes. Second, using femtosecond X-ray pulses from an X-ray free electron laser (XFEL), an experimental setup for X-ray absorption spectroscopy was developed at the soft X-ray beamline of PAL-XFEL. As a representative of noble metal, a copper nano foil is excited by a femtosecond laser pulse. The femtosecond changes in the X-ray absorption around the L3-edge of warm dense copper show the highly excited d-band which thermalizes with sp-electrons over the duration of a picosecond. This result showed the widely used two-temperature model is non-applicable and questioned the fast thermalization concept to describe the nascent stage of intensively photoinduced material responses. Finally, by using picosecond X-ray Absorption Near Edge Structure (XANES) measurement setup at the Advanced Light Source, the electron mean-free-path in warm dense gold was investigated. A gold/copper nano foil is used as a target. The front Au layer is heated with a femtosecond laser pulse. The real Cu layer is heated via hot electrons transferred from Au at delayed time. The evolution of temperature at the real side of Au is determined by tr-XANES measurement of Cu L-edge. We investigated the electron motion is either ballistic or diffusive. In Au with a few eV temperatures, the electron mean-free-path seems to be less than 50 nm, which is a half of the known value (100 nm) in the ambient gold. This result infers the thermal conductivity of warm dense matter should be re-evaluated, and even for WDM experiments using nano-structured targets, the effects by thermal gradient should be considered more carefully. 2019-12-31T15:00:00Z