https://scholarworks.unist.ac.kr/handle/201301/77 2026-04-08T00:30:10Z 2026-04-08T00:30:10Z PANDURO, MIGUEL LORENZO ANTONIO ESPINOZA https://scholarworks.unist.ac.kr/handle/201301/91045 2026-03-26T13:15:06Z 2026-01-31T15:00:00Z

Title: Diffuse Nonthermal Radio Structures in Galaxy Clusters Author(s): PANDURO, MIGUEL LORENZO ANTONIO ESPINOZA Abstract: This thesis presents a systematic search and multi-frequency radio analysis of the galaxy cluster Abell 2811, with the primary objective of identifying and characterizing a cluster that could host a radio megahalo—a recently discovered class of extended, ultra-steep spectrum synchrotron sources in galaxy clusters. Employing deep observations with the Murchison Widefield Array (MWA) in the 88-216 MHz range, I investigate the spatial and spectral properties of diffuse radio emission in Abell 2811, combining advanced imaging, compact source subtraction, and spectral mapping techniques. The cluster reveals a remarkable ultra-steep spectrum radio halo, extending over more than 1 Mpc at the lowest frequencies and exhibiting spectral index gradients indicative of advanced synchrotron ageing and turbulent reacceleration processes. Morphological and spectral index maps show broad, low- brightness extensions typical of megahalos, with a central region of ongoing particle acceleration and peripheral zones shaped by radiative losses. The association between radio and X-ray emission supports a scenario where cluster mergers drive turbulence and magnetic field amplification on megaparsec scales. These results strongly position Abell 2811 as a promising candidate for hosting a megahalo structure, contributing key observational evidence to the population of diffuse nonthermal cluster sources. The findings have important implications for our understanding of particle acceleration, magnetic fields, and the prevalence of large-scale nonthermal phenomena in galaxy clusters, motivating further systematic searches with next-generation radio telescopes. Major: Department of Physics

2026-01-31T15:00:00Z Jeon, Hye-Ran https://scholarworks.unist.ac.kr/handle/201301/91044 2026-03-26T13:15:05Z 2026-01-31T15:00:00Z

Title: Generation and Characterization of Diamond Color Centers Using Ultra-Low-Fluence Ion Implantation at KAERI Author(s): Jeon, Hye-Ran Abstract: Diamond color centers provide a versatile solid-state platform capable of operating across multiple defect-density regimes—from ensemble color centers to isolated single emitters—owing to their stable optical emission and controllable spin properties at room temperature. However, the absence of fabrication techniques capable of deterministically controlling this transition has hindered domestic progress toward scalable single-photon–based quantum devices. This study aims to establish an ion- implantation–based fabrication method that enables precise control of defect density and spatial distribution, thereby realizing both ensemble and single diamond color centers within a single integrated platform. Nitrogen and silicon ions were implanted into single-crystal diamond substrates with systematically varied implantation energies and fluences. SRIM simulations were used to estimate the projected ion ranges, which guided the selection of implantation conditions. Through systematic optical and spin characterization of ensemble color centers, optimal annealing temperatures and surface treatment conditions were identified to maximize formation yield and spectral quality. These optimized parameters were subsequently applied to the fabrication of single centers, where we aimed to secure stable single-photon emission despite the inherently low implantation fluences. The optical and spin properties of the fabricated centers were characterized using confocal microscopy, photoluminescence (PL) spectroscopy, and Optically Detected Magnetic Resonance (ODMR). NV centers exhibited resonance splitting at 2.87 GHz, confirming their spin-triplet ground-state characteristics, while SiV centers showed narrow zero-phonon lines suitable for coherent photon emission. Single-photon emission was investigated using a Hanbury–Brown–Twiss interferometer, where the observation of an antibunching dip indicated the potential realization of single-color centers. This work not only demonstrates a reproducible ion-implantation–based process capable of generating both ensemble and single diamond color centers, but also provides a comprehensive optical characterization of the fabricated defects, thereby offering essential insights for future developments in quantum sensing, quantum communication, and photonic devices in Korea. Major: Department of Physics

2026-01-31T15:00:00Z RAHMATULLOH, IMASDA https://scholarworks.unist.ac.kr/handle/201301/91043 2026-03-26T13:15:04Z 2026-01-31T15:00:00Z

Title: Chemical Vapor Deposition of Molybdenum Disulfide for Optoelectronic Device Applications Author(s): RAHMATULLOH, IMASDA Abstract: The growth of two-dimensional (2D) transition metal dichalcogenides, especially molybdenum disulfide (MoS2), has become a focus of research owing to their potential in next-generation electronic and optoelectronic applications. Although numerous studies have demonstrated the excellent electronic and optoelectronic properties of mechanically exfoliated MoS2, the reliance on exfoliation and transfer processes severely limits scalability and practical device integration. These limitations show the need for scalable and epitaxial growth techniques to enable broader technological applications. In this thesis, we focus on chemical vapor deposition (CVD) as a scalable synthesis method for MoS2 and explore its application in optoelectronic devices. In Chapter 1, we detail the fundamental properties of MoS2 and discuss its potential in electronic and optoelectronic applications. We show the limitations of commonly used mechanical exfoliation and transfer-based approaches and emphasize the necessity of epitaxial growth methods. Based on these limitations, we define the objectives of this thesis to investigate CVD growth parameters for MoS2 and to demonstrate MoS2 LEDs. The motivation for studying CVD-grown MoS2 and its application in light-emitting devices is also presented. In Chapter 2, a detailed literature review is provided. The electronic and optical characteristics of MoS2, including bandgap and spin-orbit coupling, are discussed, followed by a review of MoS2 synthesis techniques from mechanical exfoliation to CVD. The status and challenges of scalable and epitaxial MoS2 growth systems are also addressed. In Chapter 3, we describe the experimental methods employed in this study. We first introduce our CVD growth system and the experimental growth profiles used to synthesize MoS2. We then explain the characterization techniques used to evaluate the grown MoS2, including scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, and X-ray diffraction. Device fabrication and characterization methods are also described. In Chapter 4, we focus on the optimization of CVD growth conditions for MoS2, which represents the main experimental contribution of this research. We investigated the growth of MoS2 on different substrates and studied the effects of growth temperature. Subsequently, we examined the influence of growth time on MoS2 layers. Through these studies, we identified optimized growth conditions that enabled the formation of continuous and uniform MoS2 layers. In Chapter 5, we present our research on an epitaxial MoS2 LED. We fabricated a ZnO/MoS2/GaN heterostructure and investigated its electrical and optical properties. The electrical and optical performance of the fabricated LED was analyzed, demonstrating the feasibility of CVD-grown MoS2 for light-emitting applications. Chapter 6 summarizes the overall conclusions of this thesis and discusses future research directions. Major: Department of Physics

2026-01-31T15:00:00Z Kim, Seokhyun https://scholarworks.unist.ac.kr/handle/201301/88159 2025-11-06T00:57:24Z 2025-07-31T15:00:00Z

Title: Temperature dependent dynamics of nanoconfined water at terahertz frequencies Author(s): Kim, Seokhyun Abstract: The dielectric properties of water confined at the nanoscale differ markedly from those of bulk water due to limited molecular mobility and altered hydrogen-bonding networks. In this study, tera- hertz time-domain spectroscopy (THz-TDS) was employed to investigate how temperature modulates the dielectric response of water under confinement within gold nanogap structures. As temperature in- creases, pronounced changes in transmitted amplitude and resonance frequency were observed within narrower gaps, exceeding the relative variation seen in wider nanogaps that more closely resemble bulk water behavior. These results suggest that temperature can partially alleviate the effects of nanoscale confinement, promoting increased dipolar mobility. While the system does not fully recover bulk-like dynamics, the enhanced temperature sensitivity in narrower gaps points to a thermally induced loosen- ing of confinement-driven constraints. These findings highlight that the structural ordering and dipolar suppression imposed by nanoconfinement are not entirely static, but can be modulated to some extent by thermal energy. Major: Department of Physics

2025-07-31T15:00:00Z