Synthesis and Characterization of Low-Dimensional Transition Metal Oxide with Energy Functionalities

제 1장에서는 a-MnO2의 망간 자리에 루테늄 4가 이온을 부분적으로 치환하여 전기화학 촉매 성능을 향상시킴으로써 망간 산화물의 이작용성 전기화학 촉매 성능 및 망간 산화 상태를 결합 경쟁을 이용해 효과적으로 조절할 수 있음을 보였다. 망간 이온의 자리에 전기음성도가 큰 루테늄 이온을 치환하는 것은 결합 경쟁을 통해 마주보는 (Mn-O) 결합을 약화시키는 데 효과적이며, 이는 Jahn-Teller 활성이 있는 망간 3가 이온종을 안정화시킴과 동시에 전기화학적으로 활성이 있는 루테늄 반응 자리를 제공할 수 있다. 결과적으로, 합성된 a-Mn1-xRuO2 나노와이어는 산소 발생 반응 및 산소 환원 반응 모두에서 a-MnO2와 RuO2의 물리적 혼합물보다 우수한 전기화학 촉매 성능을 보였으며, 이는 망간 산화물의 이작용성 전기화학 촉매 활성에 (Mn-O) 결합의 공유 결합성이 주요한 역할을 한다는 것을 시사한다. 구조의 무질서성을 증가시키는 방법 등을 이용한 이전 연구들처럼, 전기음성도가 높은 양이온을 치환하는 본 연구 방법 또한 망간 산화물의 전기화학 촉매 성능을 최적화하는 새로운 효과적인 방법이 될 수 있다. 제 2장에서는 층상 금속 산화물의 층간의 강한 전기적인 결합을 조절하는 방법을 통해 유기 양이온을 이용하지 않고 2차원 나노시트를 합성하는 효과적인 액체 상 박리법을 개발하였다. 층상 금속 산화물의 층간 전기적인 결합은 층간에 삽입되어 있는 알칼리 금속 이온을 큰 사이즈를 가지는 하이드로늄 이온으로 치환함으로써 약화될 수 있다. 결과적으로, 하이드로늄 이온이 삽입되어 층간 상호작용이 약화된 층상 금속 산화물은 유기 양이온을 첨가하지 않고도 액체 상에서 초음파 에너지를 이용하여 박리되었다. 액체 상 박리에서 사용되는 유기 용매의 성질은 금속 산화물 나노시트의 박리율에 큰 영향을 미치게 된다. 밀도와 점도가 높은 포름아마이드와 같은 용매는 두껍고 무거운 Ti1-xO2 및 RuO2 나노시트를 박리하는 데 효과적이며, 반대로 얇고 가벼운 MnO2 나노시트는 알콜 류 용매의 -OH기와 수소 결합을 형성하며 박리될 수 있다. 수득된 나노시트들은 높은 등방성의 2차원 나노시트 형태를 보였으며, -30~-50 mV 사이의 표면 음전하를 띄었다. 본 방법을 통해 합성된 나노시트는 전기적인 상호작용을 통해 다양한 무기 고체소재와 혼성화하여 향상된 기능성을 가진 나노혼성체를 합성하는 데 유용한 빌딩블록으로 응용될 수 있다. 일례로, 본 연구에서는 CdS 및 그래핀-층상이중층수산화물 등의 나노소재에 나노시트를 혼성화하여 광촉매 성능 및 전극 성능을 향상시켰다. 본 연구는 하이드로늄이 삽입된 층상 금속 산화물이 유기 양이온을 사용하지 않는 층상 금속 산화물 나노시트 콜로이드 및 기능성 나노혼성체의 효율적인 전구체로 응용될 수 있음을 시사한다. ;In chapter I, a critical role of bond competition in tailoring Mn valence state and bifunctional electrocatalyst activity of manganese oxide is evidenced by the remarkable improvement of the electrocatalyst activity of a-MnO2 upon the partial substitution of electronegative Ru4+ ion. The replacement of Mn4+ ion with more electronegative Ru4+ one is quite effective in weakening adjacent (Mn-O) bonds in terms of bond competition, leading to the stabilization of Jahn-Teller active Mn3+ species, as well as in providing electrocatalytically active Ru sites. The resulting Ru-substituted a-Mn1-xRuxO2 nanowires show much higher electrocatalyst activities for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) than does the physical mixture of a-MnO2 and RuO2, indicating the main role of (Mn-O) bond covalency in the optimization of the bifunctional electrocatalyst activity of manganese oxide. The present study underscores that, like the previous strategy of structural disorder enhancement, the substitution of highly electronegative cation can provide a novel efficient way of improving the electrocatalyst performance of manganese oxide via the bond competition between adjacent (Ru-O) and (Mn-O) bonds. In chapter II, a scalable organic-free liquid exfoliation route to 2D nanosheets of layered metal oxides is developed with the control of strong electrostatic interlayer interaction in these materials. The electrostatic interlayer bonding of layered metal oxides can be significantly weakened by the replacement of interlayer alkali metal ions with larger hydronium ions, leading to the marked expansion of basal spacings. The resulting weakening of interlayer interaction in the hydronium-intercalated derivatives makes possible the liquid exfoliation of layered metal oxide nanosheet without any assistance of organic cations. The nature of solvent employed for liquid exfoliation has profound effect on the exfoliation yield of these metal oxide nanosheets; the highly dense and viscous solvents like formamide are effective for the exfoliation of thicker and heavier Ti1-xO2 and RuO2 nanosheets, whereas the exfoliation of thinner and lighter MnO2 nanosheet can occur efficiently in alcoholic solvents via significant hydrogen bonding with OH group of solvent molecules. All the obtained Ti1-xO2, MnO2, and RuO2 nanosheets show highly anisotropic 2D morphologies and distinct negative surface charges with a zeta potential of -30~-50 mV. These nanosheets can be employed as useful hybridization matrices for diverse inorganic solids in terms of electrostatic interaction, yielding novel hybrid materials with enhanced functionalities. The hybridization with liquid-exfoliated metal oxide nanosheet is quite effective in improving the photocatalytic activity of CdS quantum dot and the electrode functionality of graphene-layered double hydroxide nanohybrid. The present study underscores that the hydronium-intercalated derivatives of layered metal oxides can be used as efficient scalable precursor for the organic-free colloidal suspensions of layered metal oxide nanosheets and their functional nanohybrids.Chapter I. An Effective Way to Improve Bifunctional Electrocatalyst Activity of Manganese Oxide via Control of Bond Competition 1 1. Introduction 2 2. Experimental Section 5 2.1. Sample Preparation 5 2.2. Characterization 5 2.3. Electrochemical Measurement 6 3. Results and Discussion 8 3.1. Powder XRD Analysis 8 3.2. EXAFS and Micro-Raman Spectroscopic Analyses 8 3.3. XPS and XANES Analyses 10 3.4. FE-SEM, TEM, and EDS-Elemental Mapping Analyses 12 3.5. N2 Adsorption-Desorption Isotherm Analysis 12 3.6. Test of Electrocatalyst Activity for OER and ORR 13 4. Conclusion 21 5. References 23 6. Appendixes 27 Chapter II. Organic-Free Exfoliative Synthesis of Layered Metal Oxide Nanosheets via the Weakening of Interlayer Electrostatic Interaction 44 1. Introduction 45 2. Experimental Section 48 2.1. Sample Preparation 48 2.2. Characterization 49 2.3. Photocatalytic and Electrochemical Measurement 50 3. Results and Discussion 52 3.1. Effect of solvent on the liquid exfoliation yield of Ti1xO2, MnO2, and RuO2 nanosheets 52 3.2. Hybridization effect of liquid-exfoliated TMO nanosheet on the photocatalytic activity of CdS 57 3.3. Hybridization effect of liquid-exfoliated TMO nanosheet on the electrode activity of LDHrGO nanohybrid 59 4. Conclusion 62 5. References 64 6. Appendixes 67 Abstract (in Korean) 8

초고유전율 물질의 게이트 적층 기반 Al/TiO2/Si 커패시터의 특성 분석

MasterCMOS technology has continued to improve performance of device by scale-down. The device scaling scheme made the oxide thickness shrink as well as other parameters of the device. So, as a result of continued scaling, extremely thin oxide layer was developed, which causes direct tunneling. To prevent the leakage current caused by direct tunneling, high-k gate dielectric that is physically thick but maintains the same gate capacitance as SiO2 was required. In response to this requirement, since 45nm technology node, hafnium based oxide (HfO2) that has large permittivity and is compatible to conventional CMOS technology have been used. Due to further scaling, however, HfO2 also has reached scaling limitation and so much higher-k dielectric have been needed again. TiO2 is a good candidate for replacement material for HfO2 owing to its large dielectric constant (~ 80). However, TiO2 has a small energy band gap, which can make it show large leakage current. Because of this problem, before TiO2 can be used as a gate dielectric, its leakage current must be reduced. Therefore, the cause of its leakage current must be understood. In this study, Al/TiO2/Si, Metal/Insulator/Semiconductor (MIS), capacitors were fabricated so that their electrical properties could be quantified. TiO2 was deposited by sputtering on prepared n-doped silicon wafer, then rapid thermal annealing (RTA) process was conducted under 3-different temperatures to analyse the characteristics depending on RTA temperature; 750 ℃, 1000 ℃ and 550 ℃ that was used in additional experiment. A k value of over 30 was obtained in the case of RTA at 750 ℃ and 1000 ℃ by capacitance measurement, which is sufficiently large value. Current-Voltage characteristic showed large leakage current as concerned. However, the current density of the film processed RTA at 750 ℃ was smaller than as-deposited film and that of the film processed RTA at 1000 ℃ increased again. This reversal implies that TiO2 has an optimum RTA temperature at which leakage current is lowest. The RTA process caused development of an interfacial layer that attributes to prevent the leakage current and phase transition of TiO2. TiO2 can have various crystal phase depending on process temperature; amorphous state at the point of deposition, anatase phase below 600 ℃ and rutile phase above 800 ℃. In addition, when TiO2 has anatase phase, it presents larger resistivity than TiO2 with rutile phase. So if the ratio of anatase phase in TiO2 increase, total resistivity of TiO2 also increases. Further study on MIS capacitor using TiO2 processed RTA at 550 ℃ that would have more anatase phase than RTA at 750 ℃ was conducted to confirm that the optimal RTA temperature is in a range lower than 750 ℃. The film annealed at 550 ℃ has lower leakage current than film annealed at 750 ℃. In addition, the crystalline structure of TiO2 annealed at 550 ℃ showed quite similar result to RTA 750 ℃ case, which means the phase of TiO2 changed to anatase. These results provide evidence that the optimal RTA temperature exists under 750 ℃ to minimize leakage current flowing through TiO2

H2 Plasma and PMA Effects on PEALD-Al2O3 Films with Different O2 Plasma Exposure Times for CIS Passivation Layers

In this study, the electrical properties of Al2O3 film were analyzed and optimized to improve the properties of the passivation layer of CMOS image sensors (CISs). During Al2O3 deposition processing, the O2 plasma exposure time was adjusted, and H2 plasma treatment as well as post-metallization annealing (PMA) were performed as posttreatments. The flat-band voltage (Vfb) was significantly shifted (ΔVfb = 2.54 V) in the case of the Al2O3 film with a shorter O2 plasma exposure time; however, with a longer O2 plasma exposure time, Vfb was slightly shifted (ΔVfb = 0.61 V) owing to the reduction in the carbon impurity content. Additionally, the as-deposited Al2O3 sample with a shorter O2 plasma exposure time had a larger number of interface traps (interface trap density, Dit = 8.98 × 1013 eV−1·cm−2). However, Dit was reduced to 1.12 × 1012 eV−1·cm−2 by increasing the O2 plasma exposure time and further reduced after PMA. Consequently, we fabricated an Al2O3 film suitable for application as a CIS passivation layer with a reduced number of interface traps. However, the Al2O3 film with increased O2 plasma exposure time deteriorated owing to plasma damage after H2 plasma treatment, which is a method of reducing carbon impurity content. This deterioration was validated using the C–V hump and breakdown characteristics.11Ysciescopu