https://scholar.gist.ac.kr/handle/local/7955 2025-12-08T05:04:34Z https://scholar.gist.ac.kr/handle/local/19900 Title: X-ray scattering and spectroscopy study on structural properties of VO2 and perovskite oxides Author(s): Sung Soo Ha Abstract: In this thesis, using the various synchrotron x-ray techniques, the behaviors of transition metal oxides were discussed in thermal process. Chapter 1 describes the behavior of the defect in vanadium oxide across the metal insulator transition using the 3-dimentional (3D) x-ray diffraction technique. Chapter 2 describes the same phenomenon by the resonant hard x-ray emission spectroscopy technique, in other words, electron behavior. In Chapter 3, the difference in the diffraction pattern between the M1 phase and the M2 phase in VO2 nanocrystals and the difference in the structure phase transition were investigated. Chapter 4 describes the segregation and strain change in thickness effect and annealing effect for SrTi0.5Fe0.5O3−δ thin films grown on LaAlO3 substrate. Experimental techniques in 3D x-ray diffraction, fitting process for analysis, conversion of reciprocal space in real, and correlation fuction obtained using FFT for diffraction signals are introduced. Also, x-ray reflectivity was used in conjunction with the grazing incidence x-ray flourescence (GIXRF) technique to describe the thin film behavior by adding anomalous x-ray reflectivity fitting obtained from the energy near the absorption edge of the target atom. Additionally, AP-XPS results are added there, and strain is discussed through diffraction. Finally, it describes the simultaneous measurement of atomic and electron behavior across the phase transition section through resonant hard x-ray emission spectroscopy. 2021-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19893 Title: Wearable energy storage electrode based on liquid crystalline carbon nanotube fiber Author(s): Hayoung Yu Abstract: With the development of wearable devices and the miniaturization of various home appliances, fiber-type solid-state supercapacitors(FSSCs) are attracting a lot of attention. FSSCs made of carbon-based nanomaterials such as graphene and carbon nanotube has excellent flexibility, lightweight and robust. In particular, carbon nanotube fiber (CNTF) based on liquid crystal (LC) spinning has shown remarkable properties with electrical conductivity (~ MS/m), excellent mechanical strength (~ GPa), lightweight, and good flexibility. These outstanding properties of LC-spun CNTF originates from 1) the structural characteristics of the fiber with high-packing density and -alignment and 2) the fiber composed of highly crystalline CNTs with low defects prerequisite for the development of LC phase. Ironically, the nature of LC-spun CNTF also acts as disadvantages for directly applying to FSSCs because of the low specific area and low functional groups for incorporating active materials. Thus, it is essential to modify CNTF for FSSCs while preserving its outstanding properties. This thesis is about effective strategies for the architecture structure of LC-spun CNTF for use as a flexible electrode for wearable FSSCs. Chapter 1 in this thesis overviews the type of CNTF manufacturing method and the latest studies and trends on CNTF as energy devices. Especially, among the various methods for manufacturing CNTF, the liquid crystalline spinning method of CNT and liquid crystalline of CNTs are closely introduced. Chapter 2~4 in this thesis describes the strategies to overcome the disadvantages of LC-spun CNTF electrodes when applied directly to FSSCs. In Chapter 2, we propose an effective strategy to increase the surface area of electrodes with 2D materials to fabricate high-performance and flexible FSSCs. We employ graphene oxide (GO) as 3D architectural material of surface of CNTF. Chapter 3 is a continuous work of Chapter 2, which overcomes the process drawback of chapter 2. The issue of chapter 2 is that due to the weight of GO sheet, the 3D structures of GO easily collapse without a solidification process. Therefore, as a follow-up study, we employ tunicate cellulose nanofibrils (TCNF) as a filler to support a stereoscopic 3D structure of GO and fabricate a stable hybrid 3D structure on the CNTF without additional solidification process. Chapter 4 depicts a study to develop LC-spun CNTF using surface-functionalized CNTs, followed by an investigation of its electrochemical activity without any additional materials for directly use as a freestanding electrode for energy storage devices. In chapter 2 and 3, other nanomaterials such as GO and TCNF are employed, but CNTFs is solely used for FSSCs in this chapter. Oxygen-containing functional groups (–COOH and –OH) are introduced onto the surface of CNTs by well-known acid treatment and the lyotropic LC phase thereof is developed for use as a dope for liquid crystalline spinning (i.e., wet-spinning). The study on the electrochemical activity of CNTF as a function of functionalization provides new insight into the development of FSSCs. In this study, we utilize the outstanding properties of LC-CNTF and present effective surface modification strategies to improve the electrochemical properties of LC-CNTF for use as an electrode for FSSCs. 2022-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19881 Title: Utilization of glycosaminoglycans to improve biological activities of biomaterials Author(s): Meei Chyn Goh Abstract: Glycosaminoglycans (GAGs) are present abundantly in the extracellular matrix (ECM) of higher organisms. GAGs play an important role on many cell surfaces in connective tissues and ECM. They displayed a variety of biological functions and modulated many biological processes through their interactions with a wide range of proteins in ECM. Therefore, in an attempt to capture these biological functions, GAGs have been incorporated into a range of biomaterials either in 2D or in 3D systems for tissue engineering, protein/drug delivery, and regenerative medicine applications. Herein, in this thesis, GAGs (heparin and hyaluronic acid) are utilized for improving the bioactivities of the biomaterials through surface modification and physical incorporation. A general introduction of GAGs in terms of their structure, their role in the biomedical application as well as the way of incorporation into biomaterials for bioactivities improvement were first reviewed in chapter 1. In chapter 2, heparin possesses specific binding affinities and controlled release capability to bone morphogenetic protein-2 (BMP-2) was coated on the polyetheretherketone (PEEK) to improve its performance in terms of osteogenesis. PEEK is a biocompatible synthetic thermoplastic polymer that has gained increased interest as an alternative material in orthopedic and dental applications recently mainly due to its closely matched elastic modulus with natural bone. However, the bio-inertness of PEEK which results in poor osseointegration has limited its potential applications. Delivery of bone morphogenetic protein-2 (BMP-2) in a controlled manner has emerged as a potential approach for osseointegration improvement through osteogenic differentiation of osteoblast progenitors or stem cells. Therefore, in this study, surface modification of PEEK with heparin was carried out via a combination of ozone and UV treatment. Heparin was successfully grafted on PEEK through the thiol-ol reaction. BMP-2 loading efficiency on PEEK was enhanced and controlled release of it was achieved in the presence of heparin on it compared to pristine PEEK. Surprisingly, bioactivity enhancement was observed on hydrophilic heparin-grafted PEEK itself in terms of proliferation rate as well as osteogenic differentiation of MG 63 compared to pristine PEEK. However, the synergistic effect of BMP-2 on PEEK especially in osteogenic differentiation of MG 63 became significant only on heparin-grafted PEEK. Overall, we demonstrated a relatively safe method where no harsh chemical reagent or organic solvent is involved in the process of heparin grafting onto PEEK. The BMP-2 loaded heparin-grafted PEEK could be served as a potential platform for osseointegration improvement. In chapter 3, hyaluronic acid (HA), the most abundant glycosaminoglycans in native ECM of stem cells was physically incorporated into cellulose nanofiber (CNF) microbeads to improve the bioactivities of human adipose-derived stem cells (hADSCs). Cell microencapsulation is a process to entrap viable and functional cells within a biocompatible and semi-permeable matrix that aims to provide a favorable or closely mimic microenvironment to the cells. Cellulose nanofiber (CNF), a low-cost and resources sustainable cellulose-derived natural polymer has been widely studied as the matrix for 3D stem cell culture in the form of a bulk hydrogel. Therefore, in chapter 3, the formation of CNF microbeads by ion crosslinking for the 3D culture of human adipose-derived stem cells (hADSCs) was first demonstrated followed by HA incorporation. Since HA can affect most of the stem cell activities, such as cell proliferation, signaling, adhesion, and differentiation through the interaction with stem cell surface receptors, hence, it is expected that the bioactivities of encapsulated cells in CNF-HA microbeads could be enhanced in the presence of HA. All the CNF-HA microbeads showed no significant differences from CNF microbeads in terms of rheological property. Moreover, hADSCs encapsulated in HA incorporated CNF microbeads (CNF-HA) showed the enhancement in cell proliferation, endogenous growth factor secretion as well as stemness and differentiation ability preservation compared to CNF microbeads. The results were noticeable at the highest molecular weight (MW) and concentration of HA (700kDa and 0.2%) incorporated CNF microbeads. All of our results demonstrate that the CNF microbeads which are tunable with HA could serve as a promising matrix for hADSCs 3D culture. In chapter 4, heparin possesses anti-inflammatory potential was physically absorbed on tempo-oxidized CNF (CNF) film together with chitosan by layer-by-layer technique to reduce the pro-inflammatory property of bare CNF film. The demand for biomaterials and medical devices has rapidly increased. However, foreign body reaction is still one of the big challenges for successful implantation. The foreign body response is an immune-mediated reaction to implanted materials that leads to the rejection of implanted devices. Therefore, the implanted materials with anti-inflammatory and anti-fibrotic properties are needed to achieve successful implantation. Cellulose nanofiber (CNF) is considered a biocompatible nanomaterial that has been widely studied in various biomedical applications. However, there is still no certainty that CNF-based biomaterials possess either pro-inflammatory or anti-inflammatory. CNF is a relatively low risk for human health, as well as the environment and it is represented as a low-cost and sustainable biomaterial to be widely used in biomedical applications compared to the other types of biopolymer materials. Hence, it is worth reducing the inflammatory properties of CNF-based biomaterials as this could provide more beneficial effects for their various biomedical applications. Herein, a simple and cost-effective layer-by-layer method was utilized to deposit chitosan-heparin (Chi/Hep) multilayers film on CNF film to reduce its pro-inflammatory property. This process could be carried out without any modification of CNF film due to its intrinsic negatively charged property. By heparin deposition, the macrophage's attachment, as well as the pro-inflammatory cytokine secretion were significantly decreased compared to the only CNF and chitosan deposited CNF films. The positive effects of heparin deposition were significant compared to CNF films as the (Chi/Hep) bilayers increased from 1 to 3 layers. Overall, the present study demonstrated the potential of Chi/Hep multilayer films to improve the anti-inflammatory property of CNF film. In summary, this thesis described the use of heparin and hyaluronic acid in improving the bioactivities of the biomaterials where they are incorporated on or in. 2021-12-31T15:00:00Z https://scholar.gist.ac.kr/handle/local/19880 Title: Utilization of Eco-Friendly Materials for Printed Large-Area Organic Photovoltaics with High Efficiency and Stability Author(s): Nara Han Abstract: Solution processed organic solar cells (OSCs) provide several advantages, such as low-cost, flexibility, and suitability for mass production through roll-to-roll processing based on various printing methods. The state-of-the-art OSCs have achieved commercially applicable power conversion efficiency (PCE) exceeding 18%, which exhibits similar level with other solar cells. However, in order to broaden and develop into the next generation OSCs more, further researches that can optimize each layer in OSCs are necessary to ensure the device performance and their practical application in the future. In this study, the extensive research findings include three following strategies using eco-friendly materials, which will detailedly discussed in chapter 2~4. (Chapter 1) It provides a general introduction and background to OSCs, printed electronics, and layer engineering in OSCs. (Chapter 2) Various studies in OSCs have been conducted on scalable coating methods that are compatible with large-area production of organic photovoltaic modules. However, it is still difficult to control the bulk heterojunction (BHJ) morphology of the active layer during large-scale fabrication of OSCs. This study reports a morphology-controllable strategy in OSCs using water treatment (WT) in the stirring process of the active solution, thus resulting in vortex agitation. The effects of WT and water injection volume are investigated based on three reference cells for the optimization of small- and large-area devices, and the physicochemical and optical properties of the films are compared with those without WT. The thin films with WT exhibit a smoother morphology than those without WT, indicating well-dispersed donor (D)–acceptor (A) phases. Therefore, enhanced efficiencies of the films are achieved via WT. Furthermore, large-area solar cell modules with a total effective area of 10 cm2 are fabricated, and they exhibit superior PCEs as high as 11.92% (H-NF-DIW10), indicating that the WT method is a simple and effective strategy to fabricate large-area organic photovoltaic modules. (Chapter 3) The introduction of molecular doping process is necessary to enhance the optic and electronic properties of organic semiconductors for facilitating charge transport. In particular, since the doping process has a positive influence on the charge transfer interaction between the host semiconductor and dopant, improved mobility has been efficiently achieved via these doping methods using p- or n-type dopants. Despite its advantages, doping technologies in OSCs, which are generally known to have better hole mobility, are emphasized to the development of n-type dopants used for balancing the electron and hole as increasing the electron mobility. In addition, since the BHJ microstructure in OSCs has randomly blended phases of the D and A, it is important to optimize charge extraction without loss by controlling the morphology. In this study, I report OSCs by p-type doping with formic acid (FA) into a BHJ photoactive layer comprised of PM6 and Y6. The resulting champion device yields a significantly improved power conversion efficiency from 14.3% to 15.3% with a high fill factor of 71.7%. It is found that the p-doped photoactive layer exhibits enhanced conductivity, improved carrier mobilities, suppressed charge recombination, and lowered leakage current. The p-type dopant, FA, simultaneously acts as a film morphology controller of the photoactive layer with enhanced phase separation to transport the charge efficiently. Thus, the doping process with FA can maintain the device performance in long-term stability tests (95.6% remaining of its initial PCE). This work demonstrates that controlling the charge transport and trap formation via directly introducing a small amount of FA dopant into photoactive layers is a promising strategy for further improvement of device efficiency and stability in OSCs. (Chapter 4) In OSCs, the role of the interlayer is important for efficient charge transport from the photoactive layer to electrodes. However, as a hole-transporting layer (HTL), molybdenum oxide (MoOx) and poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) are still used for commercialization despite their drawbacks. The development of novel materials with suitable energy levels and solvent orthogonality is required to enhance hole extraction and optimize the film morphology. Herein, oxidized carbon soot (OCS) and OCS-functionalized octadecylamine (OCS-ODA) nanoparticles as HTL materials are synthesized. As a large number of oxygen functional groups are produced via Hummer’s method, the devices with the OCS-ODA exhibit high hole extraction ability. The ODA long alkyl chains functionalized by facile process also improve film morphology to minimize contact resistance and charge recombination. The small-area (SA), large-area, and flexible (F) OSCs with OCS-ODA show power conversion efficiencies of 15.04%, 14.57%, and 12.73%, respectively. In particular, OSCs with OCS-ODA are further demonstrated to possess storage stability in SA-OSCs (71% retention after 450 h) and mechanical stability in F-OSCs (78% retention after 1000 bendings). 2022-12-31T15:00:00Z