Angle-independent structural colors from hollow silica nanospheres

Kim, Seunghyun Sungkyunkwan university 2020 국내박사

The colors present in the surroundings derive from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. And the color represented by light is generally represented by dyes, but there are structural colors represented by the interference of light from aligned structures in nature. In a perfectly aligned structure (photonic crystal structure), light is reflected by Bragg's law to give a color, but accordingly, different colors vary depending on the angle. On the other hand, an amorphous structure (photon glasses structure) is disorderly as a whole, but has a narrow region of aligned structures, and constructive interference between such narrow regions of aligned structures appears. Therefore, it has uniform scattering property in all directions and shows color independent of angle. In order to artificially prepare such a structure, multiple scattering of non-resonant wavelengths should be minimized. In addition, the form factor represented by the particles and the structure factor represented by the structure must be controlled. However, in general, when the red color is represented by the structure factor, it is difficult to all realize RGB colors because the color by the form factor is blue. For this reason, there is a part to be solved in order to all realize RGB color using the photon glass structure. Hollow particles have a longer scattering cross section than regular particles, so the transport length is longer, resulting in less multi-scattering, resulting in a distinct color. And, the organic part can be easily doped with carbon, which serves to reduce multiple scattering. In addition, since the shell thickness can be easily adjusted, not only the refractive index can be adjusted, but also the distance between the particles can be easily adjusted when manufacturing the photon glass structure. Also, due to the advantages of excellent color and harmless to the human body it is possible to apply to color pigments. Finally, the light mass has the advantage that does not settling well compared to other particles even if left for a long time during the electric drive. In this dissertation, to realize RGB, photonic glasses structures are prepared using hollow silica particles. And I describe them. In particular, the focus is on red color, which is difficult to implement in general. The first chapter introduces the basic description of photonic crystal structure and photon glass structure, how to make it, and the application field. In the second chapter, I present the fabrication of a photon glass structure using two hollow silica particles with the same core size and different shell sizes. In particular, I describe a method of fabricating an inverse structure by refractive index matching a shell of a particle and a matrix and a method of implementing RGB color. And, I also describe a theoretical calculation method for the structure factor. The third chapter describes how to introduce carbon into the shell part of the hollow silica particles, and the color implementation by the form factor of the particles suppressed by multi-scattering. I also demonstrate the potential of infrared reflective color pigments through the reflection of the infrared region of structural factors and the color implementation of particles in solvents and polymers. The fourth chapter demonstrates to demonstrate the applicability of hollow silica particles to electrophoresis. Finally, I summarize for implementing RGB color using hollow silica particles and outline the challenges and outlook on photonic glasses for controlling light scattering using hollow particles.

A Study on Morphological, Electrochemical Properties and the Reaction Mechanism of Hollow NiCo2O4 Anode Materials with Porous Shell for Lithium Ion Batteries

유재승 성균관대학교 일반대학원 2017 국내석사

이원 금속 산화물은 리튬 이온 배터리에서 각광받는 음극 소재이다. 특히, NiCo2O4는 낮은 율 특성 및 수명 특성과 같이 현재 사용되는 음극 소재의 단점을 극복하기 위한 물질 중 하나이다. Co3O4에서 Co 성분에 의한 비싼 가격과 독성 때문에 Co를 다른 전이금속으로 치환하는 것은 리튬 이온 배터리에서 중요한 요소이다. 또한, Hollow structure는 높은 비표면적을 가지고 있기 때문에 각광받고 있는 Morphology이다. 이에 본 연구는 3차원 Hollow structure의 NiCo2O4를 합성하기 위하여 Co-precipitation의 손쉬운 합성 방법을 사용하였다. 이러한 방법은 그램 단위의 많은 양을 균일하게 합성할 수 있다. N2 adsorption-desorption isotherms, Scanning electron microscopy (SEM) 및 Transmission electron microscopy (TEM) 등을 통하여 Hollow NiCo2O4의 Morphology를 분석하였고, 50 nm의 Nanoparticles/flakes shell에 의하여 3~4 nm의 Pore size를 가지고 134 m2 g-1의 높은 비표면적을 가진 Porous hollow structure를 확인할 수 있었다. 전기화학적 특성에서는 높은 용량, 우수한 율 특성 및 수명 특성을 나타내었다. 또한, 방사광 X선을 사용한 ex situ X-ray diffraction, ex situ X-ray absorption spectroscopy 및 TEM 분석을 통하여 이 물질의 초기 두 사이클에서의 반응 메커니즘을 깊이 있게 알아보았다. Cyclic voltammetry 또한 수 사이클 동안의 반응 메커니즘을 연구하기 위하여 사용하였다. Binary metal oxides are regarded as one of promising anode materials for lithium ion batteries. Especially, NiCo2O4 is a candidate to overcome the currently existing disadvantages of anode materials such as rate capability and cycle life. The substitution of Co content to other transition metal due to high cost and toxicity compared with pure Co3O4 makes it more significant for lithium ion batteries. Also, hollow structures have been attracting due to the high surface area. In this thesis, a facile synthesis method is reported following by co-precipitation and calcination in the air to achieve three-dimensional hollow structured NiCo2O4. This method helps to produce uniformly synthesized structure with gram quantities. N2 sorption analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that the surface area of the hollow NiCo2O4 is determined to be 134 m2 g-1 with a unique porous morphology of narrow pore size distribution of 3 to 4 nm including nanoparticles/flakes in a size range of about 5 to 50 on the surface of each particles. It is evidently exhibited by hollow structures NiCo2O4 electrode which has the increased capacity, excellent rate capability and cycling stability. Also, we have investigated the advanced reaction mechanism of this electrode material by ex situ X-ray diffraction, ex situ X-ray absorption spectroscopy using synchrotron X-ray beam, and TEM observations during initial two cycles tested at low scan rate of 100 mA g−1. Cyclic voltammetry is also used for the study of reaction mechanism after subsequent cycles.

Comprehensive studies on the feasibility of dual catalyst electrode comprising Pt-hollow-structured metal oxides/Pt-C, and 3-D structured Pt-rGO/MWCNT as electrocatalysts in PEMFC

고영돈 건국대학교 대학원 2019 국내석사

In this study, two different electrochemical catalysts were comprehensively studied for their feasibility as self-humidifying catalyst in proton exchange membrane fuel cell: that is, dual catalyst electrode comprising Pt-deposited hollow-structured metal oxides and Pt-C, and Pt-deposited 3-D structured reduced graphene oxide (rGO)/multiwall carbon nanotube (MWCNT). In the first part of this study, Pt-HZrO2 with the shell thickness of 27 nm was synthesized. Pt-HZrO2 exhibited the significant enhancement in water retention ability and BET surface area. Dual catalyst electrode (DCE) comprising Pt-HZrO2 and Pt-C was applied to both anode and cathode or anode only. The visual cell test suggests that Pt-HZrO2 dual layer absorbs the water molecules produced at cathode. Higher flow rate of O2 to the cathode seems to help the prevention of water flooding expelling the water molecules, finally leading to the enhancement of the cell performance. In case of the second type of dual catalyst electrode fabricated with Pt-HTiO2/Pt-C at anode and Pt-C at cathode, the effect of pH on the agglomeration of SiO2 template nanoparticles and the size of TiO2 deposited on the external surface of SiO2 nanoparticles was investigated. The pH adopted in this study was acidic, neutral and alkali conditions such as pH 5.0, 7.0 and 9.0. At acidic and neutral condition, the repulsive forces between negatively charged silica particles led to the significant agglomeration due to the offset by protons. On the other hand, the hydrolysis and condensation during the sol-gel process, it was accelerated under alkali condition, enhancing the nucleation and thus resulting in the formation of a number of smaller particles. As a consequence, the hollow structured HTiO2 comprising small particles exhibits higher BET surface area, water uptake and ECSA. These higher physical and electrochemical properties enhanced water retention ability and thus cell performance under zero humidity in PEMFC. In the second part of this study, the 3-D structured Pt-rGO/MWCNT was prepared by inserting MWCNT as a spacer into Pt-rGO to prevent the restacking of Pt-rGO due to van der Waals force. In order to improve the deposit of Pt nanoparticles on MWCNT, cationic polyethyleneimine (PEI) was functionalized on the surface of MWCNT (PMWCNT). Then, different mass ratio of rGO and MWCNT was mixed to form 3-D structured supporting material followed by the deposit of Pt nanoparticles onto it. The insertion of PMWCNT between rGO sheets increased the surface area where 1 to 1 mass ratio of rGO and PMWCNT exhibited the highest surface area and best cell performance. 본 연구에서는 두가지 종류의 전기화학 촉매에 대한 연구가 진행되었다. 첫째로 무가습 작동용 연료전지의 구동을 위한 백금을 담지한 중공구조의 금속산화물과 백금을 담지한 탄소의 이중 촉매층에 대한 연구이다. 두번째는 그래핀과 탄소나노튜브를 이용하여 3D 구조를 형성하여 그래핀의 재결합을 막아 연료전지에 사용되는 촉매 활성을 높이는 연구이다. 첫번째 주제는 27nm의 두께를 가지는 중공구조의 산화 지르코늄에 백금을 담지하여 공기극과 연료극 모두 이중 촉매층의 연료전지를 구성하였다. 중공구조의 산화 지르코늄은 큰 비표면적과 높은 수분 유지력을 가지고 있다. 연료전지의 기체 유로를 볼 수 있는 비주얼 셀을 이용하여 중공구조의 산화 지르코늄 촉매층이 공기극에서 생성되는 물을 흡수하는 것을 확인하였다. 공기극의 유속의 증가에 따라 물 넘침 현상을 예방하고 연료전지의 성능증가도 확인할 수 있었다. 또한 다른 금속산화물인 중공구조의 산화 티타늄을 이용하여 이중 촉매층을 구성하였고 pH에 따라 실리카의 뭉침 현상과 산화 티타늄의 사이즈에 대한 연구가 진행되었다. 산성과 중성의 용액에서 실리카는 반발력의 감소로 인해 뭉쳤고 산화 티타늄의 크기도 빠른 속도의 수화, 탈수 반응 속도로 인해 증가하였다. 염기성의 용액에서 합성한 실리카와 중공구조의 산화 티타늄이 가장 큰 비표면적, 높은 수분 유지력, 및 많은 전기화학적 활성부분을 가지고 있었다. 이 특성을 활용하여 무가습 작동용 연료전지의 성능을 향상시켰다. 두번째 주제는 3D 구조의 백금을 담지한 그래핀과 탄소나노튜브를 이용한 촉매에 대한 연구이다. 그래핀의 층간에 탄소나노튜브를 삽입하여 재결합에 대한 문제점을 해결하여 비표면적의 증가를 이루었고 탄소나노튜브에는 PEI를 코팅하여 양전하의 제타포텐셜을 가지게 하여 나노 사이즈의 백금의 담지량을 증가시켰다. 그래핀과 탄소나노튜브의 중량비를 1:1로 하였을 때 가장 뛰어난 성능을 보여주었고, 전극의 내구성도 가장 좋은 것으로 나타났다.

Investigation of functional micro-structure in optical fiber waveguides

이세진 Graduate School, Yonsei University 2013 국내박사

For the last two decades, Optical fiber has developed from step index structures to micro structures of sub-wavelength scale. Various structures of optical fibers was related to the requirement for flexible control of light in waveguide and it provided wider range of functions and applications of optical fiber in light guiding. For example, a periodic structure of refractive index which is called photonic crystal was embedded to optical fiber, and they have been applied for engineering and scientific researches. Also, micro structures of ring core in hollow optical fiber (HOF) and micro tapered optical fiber provided novel applications in engineerings and sciences. In this thesis, we discuss for fundamental properties and functional applications of optical fibers with the novel structures, micro hollow structure or micro tapered structure. Fundamental properties and functional applications of three types optical fibers which have micro hollow structure are dealt with in the 1st part. Then, a novel system with micro optical tapered and thermoplastic composite structure for semiconductor nanowire manipulation will be introduced with a practical demonstration in the 2nd part.

Hollow porous CoO@reduced graphene oxide self-supporting flexible membrane for high performance lithium-ion battery

Junxuan Zhang 동국대학교 일반대학원 2023 국내석사

The rapid development of today's society has put forward higher requirements for energy storage devices with higher energy density, higher power density, and higher cycle life. As a mature high-efficiency energy storage device, the structural characteristics of the anode material of lithium-ion batteries have a crucial impact on the overall performance of the battery. Metal compounds with relatively high theoretical specific capacity have become a very promising anode material for the new generation of Li-ion batteries. However, their large volume expansion and poor electrical conductivity have severely restricted their rapid development. In this paper, we mitigate the problems of large volume expansion and poor electrical conductivity of metal compounds by compounding them with carbon-based conductive materials. We use a rapid preparation method of rGO-based flexible self-supporting film electrode by compounding Co-MOF with graphene oxide and use the etching effect of ammonium sulfide on Co-MOF combined with the heat treatment process to rapidly prepare CoO@rGO flexible self-supporting film composite with hollow porous structure. This unique hollow porous structure can effectively shorten the ion transport path and provide more active sites for lithium ions. The high conductivity of reduced graphene oxide further facilitates the rapid charge transfer and provides sufficient buffer space for the hollow Co-MOF nanocubes. Thanks to the synergistic effect of hollow porous structure and 3D reduced graphene oxide network, this would be a promising new strategy for synthesizing hollow porous structured rGO-based self-supported flexible electrodes. 오늘날의 사회의 급속한 발전은 에너지 밀도가 높고, 전력 밀도가 높으며, 사이클 수명이 높은 에너지 저장 장치에 대한 요구 사항을 더욱 높게 제시하고 있다. 성숙한 고효율 에너지 저장 장치로서 리튬이온전지 음극재의 구조적 특성은 전지의 전체적인 성능에 결정적인 영향을 미친다. 이론 비용량이 상대적으로 높은 금속 화합물은 차세대 Li-ion 전지에 매우 유망한 음극 재료가 되었다. 그러나, 그들의 큰 부피 팽창과 낮은 전기 전도도는 그들의 빠른 발전을 심각하게 제한했다. 본 논문에서는 금속 화합물을 탄소계 도전재와 혼합하여 부피 팽창이 크고 전기 전도도가 떨어지는 문제를 완화한다. 산화그래핀과 Co-MOF 를 복합화하여 rGO 기반의 플렉시블 자기지지막 전극을 신속하게 제조하는 방법을 사용하고 열처리 공정과 결합된 Co-MOF 에 황화암모늄의 식각 효과를 이용하여 중공 다공성 구조의 CoO@rGO 플렉시블 자기지지막 복합체를 신속하게 제조한다. 이러한 독특한 중공 다공성 구조는 효과적으로 이온 전달 경로를 단축시키고 리튬 이온의 보다 활성 사이트를 제공할 수 있다. 환원된 산화그래핀의 높은 전도도는 빠른 전하 이동을 더욱 용이하게 하며, 중공의 Co-MOF 나노큐브를 위한 충분한 완충 공간을 제공한다. 중공 다공성 구조 및 3D 환원된 그래핀 옥사이드 네트워크의 시너지 효과 덕분에, 이는 중공 다공성 구조 rGO 기반의 자체 지지 유연 전극을 합성하기 위한 유망한 새로운 전략이 될 것이다.

자기유도 약물전달용 중공형 하이드로겔 입자의 제조와 중공구조 형성 메커니즘에 관한 연구

박성진 한양대학교 일반대학원 2015 국내석사

본 연구에서는 poly(MAA/EGDMA) 하이드로겔 마이크로 입자를 템플레이트로 사용해서 코어를 제거 하기 위한 별도의 열처리나 에칭 과정 없이, 자성을 띄며 중공구조를 가진 유/무기 복합체인 poly(MAA/EGDMA)/Fe3O4를 제조 하였다. Fe2+ 금속산화물 전구체는 Poly(MAA/EGDMA) 고분자 사슬의 정전기적 인력과 농도 차에 의한 확산을 통해 입자 내부로 도입되었다. 중공구조를 가진 유/무기 복합체는 하이드로겔 작용기의 음전하와 전구체의 양전하간의 정전기적 인력을 통해 형성 되는데, 하이드로겔의 표면에서부터 전구체-사슬간의 복합체가 증가하게 되고 그렇게 생성된 소수성인 부분과 하이드로겔 내부에 존재하던 물과의 상분리로 인해 내부에 공간이 형성되게 된다. 이것은 겔 내부의 고분자 사슬들이 고분자-전구체로 구성된 복합체와의 소수성 상호작용으로 인해 중심부로부터 복합체 방향으로의 인력 때문에 발생하는 것이다. 도입된 전구체는 환원과정을 통해서 Fe3O4로 전환되며, 유/무기 복합체의 구조적, 기계적으로 안정화 시키게 된다. 내부에 도입시키는 전구체의 양에 따라서 자성특성을 조절할 수 있으며, pH의 조절에 따라서도 자성특성을 조절 할 수 있음을 조사했다. Poly(MAA/EGDMA)/Fe3O4 의 약물전달시스템으로써 실험은 액상에서 양전하를 띄는 Doxorubicin을 모델 약물로 사용하여 각각의 다른 pH(산성, 중성, 염기성)에서 진행하였다. 염기조건에서는 입자와 약물간의 정전기적 인력을 통해서 DOX가 방출되는 것이 저지 되었고 산성조건에서는 정전기적 인력의 감소로 인하여 DOX가 대부분 방출되는 경향을 보였다. 제조된 입자의 모폴로지는 SEM, FIB-SEM, TEM, HR-TEM을 통하여 조사하였으며, 중공구조가 형성되는 것은 Zeta potential과 CLSM을 통해서 측정 하였다. 자성특성과 약물방출상태는 VSM과 UV를 통해 측정하였다. This study presents a facile fabrication method for monodisperse poly(methacrylic acid/ethylene glycol dimethacrylate)/Fe3O4 [poly(MAA/EGDMA)/Fe3O4] composite microcapsules with magnetic properties and hollow structures for use as a targeted drug delivery system. In aqueous solution, the iron ions diffuse into the negatively-charged spherical polymer particles by electrostatic attraction. Since the obtained polymer-metal complexes decrease the repulsive force between the negatively-charged carboxyl groups of the polymer chains, the chelating parts of the polymer particles lose their hydrophilic properties and become relatively hydrophobic. As the number of iron ions continues to increase in the polymer particles, an internal cavity begins to be formed by interaction between the hydrophobic domains and the inner polymer chains. After the reducing agent is added into the synthetic system, Fe3O4 nanoparticles formed in the shells of the polymer particles retain the hollow structure. Confocal laser scanning microscopy and zeta potential are employed to confirm the formation of the hollow structure during the diffusion of the various metal ions into the polymer gel. The magnetic properties of the poly(MAA/EGDMA)/Fe3O4 composite microcapsules synthesized under diverse experimental conditions are characterized by a vibrating sample magnetometer. The controlled release behavior of the model drug doxorubicin chloride from the polymer-metal hollow composite microcapsules is investigated under different pH conditions.

Hollow Zeolitic Imidazolate Frameworks (ZIF) for Enhanced Low Dielectric Performance

이나현 숙명여자대학교 대학원 2025 국내석사

차세대 전자 및 통신 기술의 고속화 및 고집적화에 따라, 신호 간섭을 최소화하고 에너지 소비를 줄이기 위한 저유전율 소재 개발의 중요성이 더욱 부각되고 있다. 기존의 저유전율 소재로는 대표적으로 실리카가 널리 활용되어 왔으며, 상대적으로 낮은 유전율과 공정 호환성으로 주목받아왔다. 그러나 실리카는 구조적 조절의 한계, 기계적 취약성, 높은 습도나 고온 환경에서의 안정성 저하와 같은 문제를 동반한다. 이러한 한계를 극복하기 위해, 구조적으로 정밀하게 설계된 다공성 소재 기반의 복합화와 같은 새로운 전략이 요구되고 있다. 금속-유기 구조체(MOF)는 복합화 전략을 위한 유망한 소재로 주목받고 있다. MOF는 금속 클러스터와 유기 리간드로 이루어진 다공성 물질로, 높은 비표면적과 조정 가능한 기공을 특징으로 한다. 이러한 특성은 MOF가 구조적 특성 및 기공 조정을 통해 내부 공극률을 조절함으로써, 초저유전율 특성을 구현할 수 있는 다공성 필러임을 보여준다. 본 연구에서는 MOF 중 소수성 특성과 자체로 고유한 저유전율 특성을 갖는 ZIF-8을 기반으로, 중공 구조를 설계하여 내부 공기 함량을 증가시키는 전략을 채택하였다. 중공 ZIF-8 입자는 선택적 에칭법과 템플릿 기반 합성을 통해 제작되었으며, 열적 및 기계적 안정성을 유지하면서 내부 다공성을 극대화하였다. 합성된 ZIF-8 및 중공 ZIF-8 입자를 고내열성 고분자인 폴리이미드 전구체에 분산시켜 복합 필름을 제작하였고, 상온 조건에서 유전율 특성을 확인하였다. 연구 결과, ZIFs가 혼합된 복합 필름은 기존 폴리이미드 필름 대비 우수한 저유전율 특성을 보였으며, 특히 중공 구형 ZIF-8의 복합 필름에서 가장 뛰어난 성능을 확인하였다. 본 연구는 MOF의 중공 구조 설계를 활용하여 저유전율 특성 향상의 가능성을 입증하였으며, 고분자 필름 기반 차세대 절연소재 설계에 있어 MOF의 응용 범위를 확장하는 기반을 마련하였다. 또한, MOF의 우수한 기공 조절 능력과 구조적 안정성을 바탕으로 고주파 환경 및 고집적 소자에 적합한 기능성 절연 소재로의 실용화 가능성을 제시한다. With the advancement of next-generation electronic and communication technologies toward higher speed and integration, the development of low-dielectric materials has become increasingly important to minimize signal interference and reduce energy consumption. Among conventional low-dielectric materials, silica has been widely utilized due to its relatively low dielectric constant and process compatibility. However, silica suffers from limitations such as restricted structural tunability, mechanical fragility, and degradation under high humidity or elevated temperatures. To overcome these challenges, new strategies such as the incorporation of structurally engineered porous materials into composites are being explored. Metal-organic frameworks (MOFs) have emerged as promising candidates for such hybridization strategies. Composed of metal clusters and organic ligands, MOFs are porous materials characterized by high surface areas and tunable pore structures. These properties enable control over internal void fractions through structural and pore size engineering, making MOFs as effective porous fillers for achieving ultra-low dielectric performance. In this study, zeolitic imidazolate framework-8, ZIF-8, a hydrophobic MOF known for its intrinsically low dielectric constant, was employed. A hollow structure was designed to increase the internal air content, thereby enhancing the low-k performance. Hollow ZIF-8 particles were fabricated via selective etching and template-based synthesis methods, maximizing internal porosity while maintaining thermal and mechanical stability. The synthesized ZIF-8 and hollow ZIF-8 particles were dispersed in heat-resistant polyimide to fabricate composite films, and their dielectric properties were evaluated under ambient conditions. The results showed that the composite films containing ZIFs exhibited superior low dielectric performance compared to pristine polyimide film, with the composite incorporating hollow spherical ZIF-8 showing the most outstanding performance. Our findings demonstrate the potential of hollow MOF structures for enhancing low dielectric performance, establishing a foundation for expanding their applications in the design of next-generation polymer-based insulating materials. Furthermore, the excellent pore tunability and structural stability of MOFs suggest their practical applicability as functional insulating materials suitable for high-frequency environments and highly integrated devices. This study underscores the potential of MOF engineering to achieve enhanced low dielectric performance and provide energy-efficient solutions in microelectronics.

Photocatalytic and thermoelectric properties of hollow titanium oxynitride nanoparticle

서슬기 Graduate School, Yonsei University 2013 국내석사

Environmental problems due to the improvement of science and technologies have become major concern. Photocatalysis is one of the solutions for these problems. Photocatalytic reaction removes harmful substances, like organic compounds or nearby bacteria, by oxidation and reduction. Among the photocatalytic materials, TiO2 is a promising material but photocatalytic reactions rarely occur in the visible range due to a wide band gap. In this thesis, hollow rock-salt titanium oxynitride nanoparticles were synthesized by a simple reduction process for the first time. To confirm the structural properties, XRD, XPS, TEM and neutron scattering were conducted. The mechanism for formation of hollow structures is presented. Compared to P25 (TiO2), titanium oxynitride nanoparticles exhibit improvement of photocatalytic performance in visible light. Hollow rock-salt titanium oxynitride nanoparticles were fabricated by reduction under NH3/N2 flow at 800 oC. A hollow structure was generated by direct reduction. During the reduction, oxygen vacancies were generated and then diffused and merged inside particles. The titanium oxynitride nanoparticles had a rock-salt structure which is observed above a transition temperature of 1250 oC. Incorporation of N was related to the structural properties and stability. The composition of particles was metal deficient Ti0.7O0.7N0.3. Photocatalytic reaction of titanium oynitride and P25 were characterized by photodegradation of MB and phenol. Hollow TiO1-xNx particles were composed 90% of MB but P25 was composed of 45% of MB in the UV-visible range. That is because the band gap of the hollow TiO1-xNx particles was decreased by N incorporation. Furthermore, after the photodegradation experiment, TiO1-xNx particles preserved their structure. Therefore, hollow rock-salt TiO1-xNx particles are a new promising photocatalyst.

Development of Sulfur Electrode Materials for High-Performance Lithium-Sulfur Battery

Jeong-Hoon Yu DGIST 2026 국내박사

Surface structure and refractive index engineered sapphire substrates for high brightness LEDs

성영훈 Graduate School, Korea University 2020 국내박사

The GaN based light-emitting diodes (LEDs) have attracted considerable interest because they can be a solid-state lighting source. GaN based LEDs are used as illumination sources in mobile electronics, the back light unit for display panel, automobiles and traffic signal light because of their low power consumption and long lifetime. Regardless of the high energy conversion efficiency of GaN based LEDs, their efficiency still needs to be improved. The large differences in the lattice constant and thermal expansion coefficient between GaN and sapphire substrate create high density threading dislocations and wafer bow. Also, total internal reflection of the emitted light due to the large difference in the refractive index between the GaN epitaxial layer and outside (air) is one of the factors that reduce light extraction efficiency. To achieve the high efficiency and productivity for cost reduction of the GaN-based LEDs, these technical issues need to be resolved. In this research, in order to overcome these problems, nanoimprint lithography (NIL) was proposed as a cost effective and mass productive patterning tool for the nano patterned sapphire substrates, SiO2 patterned sapphire substrate, and hollow patterned sapphire substrates. In chapter2, Conical-shape nano-patterns were fabricated on the sapphire wafer using a nanoimprint technique and dry etching method. The conical-shape nano-patterns were fabricated for two reasons. First, in order to increase the light extraction efficiency in the UV region, which has a shorter wavelength than visible light. Secondly, the nano scaled pattern was carried out to reduce the wafer bowing phenomenon by reducing the thickness of the GaN epitaxial layer. In chapter 3, full wafer scale SiO2 nano patterned sapphire substrate was employed to GaN based LEDs to improve the light extraction efficiency. The conical-shaped SiO2nano-patterns were fabricated on a 2-inch sapphire wafer using direct imprinting of hydrogen silsesquioxane (HSQ) material. The SiO2 nano-patterns were fabricated for two reasons. First, in order to increase the light extraction efficiency by increasing refractive index differences in GaN and pattern. Secondly, SiO2pattern was carried out to selective growth of the AlGaN epitaxial layer. In chapter 4, Hollow patterns were fabricated by polymer nanoimprint lithography, amorphous alumina deposition process and annealing process. The hollow patterns were fabricated for two reasons. First, in order to maximize the light extraction efficiency by increasing refractive index differences in GaN and hollow pattern. Secondly, hollow pattern was carried out to absorb residual stress after GaN growth.