https://scholarworks.unist.ac.kr/handle/201301/54 2026-04-08T00:32:33Z 2026-04-08T00:32:33Z Chae, Yujin https://scholarworks.unist.ac.kr/handle/201301/90993 2026-03-26T13:14:20Z 2026-01-31T15:00:00Z

Title: Synthesis of Titanium-based Carbides and Nitrides for Electrochemical Energy Applications Author(s): Chae, Yujin Abstract: The global transition toward electrification, renewable energy integration, and data-intensive technologies has elevated the demand for advanced electrochemical systems capable of delivering high energy density, long-term stability, and efficient catalytic conversion. MXenes, a versatile family of two dimensional (2D) transition-metal carbides and nitrides, have emerged as promising candidates for such applications due to their exceptional electrical conductivity, tunable surface chemistry, and structural versatility. Specially, the performance and applicability of MXenes are strongly dictated by their synthesis pathways, precursor quality, and interfacial engineering, highlighting the need for systematic research on both carbide and nitride MXene systems. Chapter 2 focuses on titanium carbide MXenes, highlighting how controlled combinations of etchants and intercalants enable precise modulation of Ti3C2Tx flake dimensions throughout the synthesis process. This further demonstrates their integration with silicon-based anodes (Si, SiG) for lithium-ion batteries (LIB), showing how tailored MXene flake size, improved conductivity pathways, and optimized composite architectures significantly enhance capacity retention, and mechanical resilience under operating conditions. The dissertation further investigates titanium nitride MAXene structures for hydrogen production (Chapter 3). By optimizing nitride MAX synthesis, partially extracting the A-layer, and carbon nanoplating, the resulting carbon-coated MAXene provides a conductive and chemically durable platform for MoS2, enhancing active-site exposure and interfacial charge transport. The engineered heterostructure demonstrates improved HER activity and operational stability, revealing the strong catalytic promise of nitride-derived two-dimensional materials. An improved saturated salt solution etching (S3) route for titanium nitride MAX phase is presented, offering a substantial advancement over conventional molten-salt methods (Chapter 4). The resulting layered nitride MXenes form reliably under milder conditions and exhibit effective electromagnetic interference (EMI) shielding, demonstrating both their functional versatility and scalable potential. In summary, this dissertation establishes a framework for synthesizing and engineering titanium-based carbides and nitrides for electrochemical energy applications (Chapter 5). The integrated advances in battery anode design, HER catalysis, and nitride MXene synthesis provide both fundamental insights and practical methodologies that broaden the technological potential of MXenes in next-generation energy storage and conversion applications. Major: Department of Materials Science and Engineering

2026-01-31T15:00:00Z SALEH, IBRAHIM RABI https://scholarworks.unist.ac.kr/handle/201301/90994 2026-03-26T13:14:21Z 2026-01-31T15:00:00Z

Title: Engineering Multifunctional Nanoparticle-Polymer Composites for Targeted Biomedical Systems Author(s): SALEH, IBRAHIM RABI Abstract: Advances in nanotechnology and biomaterials have opened new frontiers in the design of intelligent systems capable of sensing, responding, and adapting within biological environments. This thesis focuses on the engineering of multifunctional nanoparticle-polymer composites that integrate chemical specificity with physical responsiveness to enable next-generation targeted biomedical systems. Two complementary material platforms were developed to demonstrate this concept. The first, a polysuccinimide-silica nanocomposite, was engineered to enhance molecular recognition and surface interaction in microneedle-based devices. By modulating surface charge and hydrophilic-lipophilic balance, the platform achieved significant improvement in protein conjugation and biofluid capture efficiency, validated through in vitro and in vivo analyses. The second platform, a polyethylene glycol hydrogel integrated with room-temperature multiferroic nanoparticles, leveraged magnetoelectric coupling to achieve dual-mode, label-free biosignal transduction via ferroelectric and ferromagnetic mechanisms. This hybrid hydrogel demonstrated exceptional sensitivity and stability, detecting protein analytes at femtomolar concentrations while maintaining mechanical compliance and biocompatibility. Together, these studies establish a unified framework for adaptive nanoparticle–polymer composites that bridge biochemical recognition and physical actuation. The insights gained extend beyond biosensing toward applications in regenerative engineering, therapeutic feedback systems, and real-time physiological monitoring. This work highlights the potential of multifunctional nanocomposites to serve as foundational materials for future bio-integrated platforms capable of dynamic interaction, precise control, and targeted response within complex biological systems. Major: Department of Materials Science and Engineering

2026-01-31T15:00:00Z Jeon, Changbeom https://scholarworks.unist.ac.kr/handle/201301/90992 2026-03-26T13:14:19Z 2026-01-31T15:00:00Z

Title: Rational Design of Carbon Fibers via Mechanistic and Structural Studies Author(s): Jeon, Changbeom Abstract: Carbon fibers are promising materials for a sustainable society and have been widely used across various application fields. In the case of high mechanical performance group, carbon fibers are utilized as structural components in automotive, aerospace, and sporting goods industries. While high-mechanical-performance carbon fibers dominate over 90% of the market, highly porous activated carbon fibers also exist and are applied in separation, purification, and electrochemistry. In the carbon fiber market, polyacrylonitrile (PAN)-based carbon fibers represent almost 90% of the market size due to their high mechanical properties, scalability, and low processing costs. Nevertheless, there remains room for improving the performance of carbon fibers. To address this challenge, understanding the structure-process- property relationships is essential. In this dissertation, the fundamental microstructural evolution/development during heat treatments of PAN-based fibers was unveiled in Chapters 2–4. Through various heat treatments, different types of carbon fibers (carbonized fibers, graphitized carbon fibers, and activated carbon fibers) were fabricated. In Chapter 2, PAN fibers were carbonized in a continuous carbonization oven, and microstructural changes were observed using advanced X-ray analysis (voids and crystallites) and radial heterogeneity analysis. Microstructural analysis revealed correlations carbon crystallites, voids, and tensile modulus. Moreover, radial structure variations showed trends closely correlating with the tensile strength of carbon fibers. As a result, optimal continuous carbonization conditions were identified. In Chapter 3, carbon fibers were graphitized in a continuous graphitization oven. Structural changes during graphitization were observed through void analysis, crystallite analysis, and mechanical property analysis. At a certain graphitization temperature, tensile strength decreased dramatically while tensile modulus increased linearly with temperature. This trade- off phenomenon was elucidated through microstructural analysis. During graphitization, the high thermal conductivity along the in-plane direction, characteristics of carbon fibers, promoted structural reorganization and merging of crystallites near the fiber surface. This resulted in an accumulation of grain boundaries at the surface layer, which contributed to the observed trade-off reaction characteristics. In Chapter 4, PAN fibers were stabilized and activated to manufacture activated carbon fibers that resolve the bottlenecks of current sorbents. The engineered ACFs possess an extraordinary micro/mesoporous structure with a surface area exceeding 2,900 m2 g-1 while maintaining mechanical and thermal stability. The resulting fibers demonstrate a superior iodine capture capacity of 2.89 g g-1 and a capture rate of 2.56 g g-1 h-1. Furthermore, a novel oxygen-doping strategy was developed to enhance iodine capture performance beyond conventional methods. Strategic oxygen doping dramatically improves performance, achieving 66% higher capacity (4.41 g g-1) and 91% faster rate (4.89 g g-1 h-1). The key objectives of this dissertation are to provide a fundamental understanding of structural changes in PAN fibers during various heat treatments and to demonstrate the relationships between structure and properties of PAN-based carbon fibers. Major: Department of Materials Science and Engineering

2026-01-31T15:00:00Z Lee, Dongryeol https://scholarworks.unist.ac.kr/handle/201301/90990 2026-03-26T13:14:18Z 2026-01-31T15:00:00Z

Title: Ligand Mediated Surface Chemistry in Perovskite Nanocrystals for Light-Emitting Application Author(s): Lee, Dongryeol Abstract: Metal halide perovskite nanocrystals (PNCs) have emerged as exceptional light-emitting materials, distinguished by their high photoluminescence quantum yields (PLQYs), narrow emission linewidths, and precisely tunable bandgaps spanning the entire visible spectrum. These remarkable optical properties position PNCs as promising candidates for next-generation display technologies and advanced photonic applications. However, realizing their full potential in practical optoelectronic devices requires overcoming several fundamental challenges inherent to their ionic nature and surface dominated properties. The soft ionic lattice of perovskites renders them susceptible to ion migration under electrical bias and thermal stress, leading to operational instability and performance degradation. Furthermore, the insulating organic ligands necessary for colloidal stability and defect passivation simultaneously impede efficient charge transport in solid-state films. Beyond these issues, the anisotropic optical properties arising from quantum confinement effects and the accurate determination of electronic structures at buried interfaces remain poorly understood, limiting rational device optimization strategies. This thesis presents a comprehensive investigation into ligand mediated surface chemistry of colloidal PNCs, establishing systematic strategies to address these challenges through molecular-level engineering. By elucidating the fundamental relationships between ligand structure, surface chemistry, and device performance, this work provides rational design principles for achieving high-efficiency, stable, and functionally advanced perovskite-based light-emitting devices. In Chapter 2, a multisite coordination strategy is developed using tetrafluoroborate (BF4⁻) as a pseudo-halide ligand to achieve robust defect passivation in FAPbBr3 nanocrystals. Unlike conventional monodentate ligands that bind exclusively to undercoordinated Pb2+ sites, BF4⁻ enables simultaneous coordination with both inorganic lead and organic formamidinium cations through hydrogen bonding interactions. This multisite binding significantly strengthens ligand attachment, effectively suppressing ion migration and enhancing PLQY to 98.8% by mitigating nonradiative recombination pathways. Computational analysis confirms that the multiple fluorine atoms in BF4⁻ establish stronger binding energies compared to single halide ligands. Building upon this enhanced surface stability, a post- synthetic ligand exchange using short chain FABr is introduced to replace insulating oleate ligands without compromising optical properties. This synergistic combination of strong BF4⁻ passivation and FABr mediated conductivity enhancement yields high performance green emissive LEDs with a maximum external quantum efficiency (EQE) of 25.2% at 4,474 cd/m², maintaining over 20% EQE up to approximately 8,000 cd/m². Device characterization under applied electric fields demonstrates significantly suppressed ion migration in BF4⁻-passivated films, validating the effectiveness of multisite coordination in achieving both defect suppression and improved charge transport. Chapter 3 establishes a ligand engineering strategy to systematically control the orientation of self- assembled CsPbBr3 nanoplatelets (NPLs) and their transition dipole moment (TDM) alignment. By partially substituting native ligands with ammonium bromide-based ligands of varying chain lengths and steric properties—specifically oleylammonium bromide (OAMBr), phenethylammonium bromide (PEABr), and butylammonium bromide (BABr)—both crystallization kinetics and inter-ligand interactions are precisely modulated. Long-chain OAMBr restricts lateral growth during synthesis, producing smaller NPLs that preferentially assemble in edge-up orientation through enhanced ligand interdigitation. This vertical alignment of NPLs results in out-of-plane TDMs, generating highly linearly polarized emission with a degree of polarization (DOP) of 14.6%. Conversely, short-chain BABr permits faster growth, yielding larger NPLs that adopt face-down orientation due to reduced inter-ligand interactions. Face-down NPLs exhibit in-plane TDMs with minimal polarization (DOP = 3.0%) but superior optical outcoupling efficiency in the surface-normal direction, achieving LED performance with EQE of 3.14%. The aromatic PEABr produces intermediate-sized NPLs with mixed orientations due to π–π interactions. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and angle-dependent photoluminescence measurements confirm the correlation between ligand structure, NPL orientation, and optical anisotropy. These findings establish clear design principles linking molecular engineering to TDM alignment, enabling tailored emission characteristics for polarized light sources and enhanced outcoupling efficiency. In Chapter 4, a refined methodology for constructing accurate electronic structures and interfacial energy diagrams of CsPbBr3 nanocrystals is established. Conventional linear scale ultraviolet photoemission spectroscopy (UPS) analysis is shown to significantly overestimate ionization energies due to the unusually low density of states near the valence band maximum. By introducing logarithmic scale UPS analysis, the intrinsic valence band edge positions are more accurately determined, revealing energy alignments consistent with experimental device characteristics. Device studies demonstrate that turn-on voltage depends primarily on electron transport layer mobility rather than hole injection barriers, validating the corrected energy diagram. Furthermore, ligand induced surface dipoles are identified as critical modulators of vacuum level alignment and interfacial energetics. Didodecyldimethylammonium bromide (DDAB)-capped PNCs, possessing weaker surface dipoles, enable efficient and loss-free electron injection with minimal thermal dissipation. In contrast, native primary ammonium-capped PNCs induce larger vacuum level shifts that, despite permitting charge injection, cause excessive carrier thermalization and heat generation during device operation. Infrared thermal imaging confirms that DDAB-based devices operate at significantly lower temperatures, directly correlating with extended operational stability. This comprehensive framework integrating photoemission spectroscopy with device-level analysis provides essential insights into accurate electronic structure determination and interfacial dipole engineering for optimized PNC-based optoelectronics. Collectively, this thesis demonstrates that ligand chemistry serves as a powerful and versatile tool for controlling the structural, optical, electronic, and interfacial properties of perovskite nanocrystals. Through systematic investigation of ligand binding mechanisms, crystallization dynamics, and interfacial dipole effects, this work establishes fundamental design principles that bridge molecular level surface engineering with macroscopic device performance. The findings not only deepen the fundamental understanding of perovskite surface chemistry but also provide practical guidelines for achieving effective defect passivation, orientation control, charge transport optimization, and energy level alignment. These advances pave the way toward realizing next-generation perovskite optoelectronic devices, including high performance LEDs, linearly polarized light sources for advanced display and photonic applications. Major: Department of Materials Science and Engineering

2026-01-31T15:00:00Z