京都大学 / 0048 / 新制・論文博士 / 博士(工学) / 乙第13240号 / 論工博第4178号 / 新制||工||1720(附属図書館) / (主査)教授 木本 恒暢, 教授 藤田 静雄, 教授 山田 啓文 / 学位規則第4条第2項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18285号 / 工博第3877号 / 新制||工||1595(附属図書館) / 31143 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 藤田 静雄, 准教授 掛谷 一弘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

In this research we investigated the generation of methanol and the utilization of CO2 using heterogeneous catalysts. Heterogeneous catalysts are frequently used in industry due to their multiple benefits, which include long-term thermal and mechanical stability, as well as reusability. Our research has demonstrated a variety of heterogeneous catalysts for sustainable methanol production and CO2 utilization, including the novel dendritic mesoporous metal oxides support. We have also designed and screened multiple active metals on the dendritic mesoporous metal oxide catalysts, modified active metal dispersion, and further reduced metal oxides to utilize silica-based catalysts, among other things. Comprehensive characterization of the final products was performed using N2 adsorption and desorption, XRD, HR-TEM, SEM, ICP-OES, XPS, H2-TPD, CO2-TPD, Raman spectroscopy, pulse-chemosorption and DRIFT, in order to determine the chemical and physical properties of the catalysts. The catalysts were found to have the following characteristics. We obtained a CO2 conversion of 25.5 % and a MeOH yield of 6.4 % after at least three cycles of usage in an avantium fixed bed reactor system with a PdCu/CZ-3 catalyst. Additionally, continuous methanol production with a higher yield (6.9 %) has been achieved using our PdZn/CZ-3 catalysts, and the best ultra-dispersed Pd nanoparticles over CZZ catalyst produces more than 12 % methanol yield with constant selectivity to methanol even after a lengthy catalytic test (more than 100 h), demonstrating their industrial viability. Additionally, our PdZn/CeTi-DMSN exhibits a high methanol production of up to 10% and better long-term stability with lower metal oxides content. The adsorption and activation of CO2 to react with the spilled over hydrogen to generate methanol has been researched for the CO2 hydrogenation and utilization reaction. Catalysts' redox, acidic, and basic characteristics all play a crucial part in this reaction and in the formation of the various products. With 2.0 percent Pd, the supported dendritic CeZrZn catalyst exhibits the highest catalytic performance (29.1% conversion and 40.6% MeOH selectivity). Comprehensive analysis revealed in this research not only identified effective catalysts with high activity for a variety of applications, but also established a link between catalytic performance and the material's nature. These discoveries may also aid the researcher in the near future in resolving global environmental problems.

Resistive random access memory (RRAM), based on a two-terminal resistive switching device with a switching element sandwiched between two electrodes, has been an attractive candidate to replace flash memory owing to its simple structure, excellent scaling potential, low power consumption, high switching speed, and good retention and endurance properties. However, due to the current limited understanding of the device mechanism, RRAMs research are still facing several issues and challenges including instability of operation parameters, the relatively high reset current, the limited retention and the unsatisfactory endurance. In this study, we investigated the switching mechanisms, conditions and applications of oxygen vacancy (Vo) filament formation in resistive memories. By studying the behavior of conductive Vo nanofilaments in several metal/oxide/metal resistive devices of various thicknesses of oxides, a resulting model supported by the data postulates that there are two distinct modes of creating oxygen vacancies: i) a conventional bulk mode creation, and ii) surface mode of creating oxygen vacancies at the active metal-dielectric interface. A further investigation of conduction mechanism for the Vo CF only based memories is conducted through insertion of a thin layer of titanium into a Pt/ Ta2O5/Pt structure to form a Pt/Ti/ Ta2O5/Pt device. A space charge limited (SCL) conduction model is used to explain the experimental data regarding SET process at low voltage ranges. The evidence for existence of composite copper/oxygen vacancy nanofilaments is presented. The innovative use of hybrid Vo/Cu nanofilament will potentially overcome high forming voltage and gas accumulation issues. A resistive floating electrode device (RFED) is designed to allow the generation of current/voltage pulses that can be controlled by three independent technology parameters. Our recent research has demonstrated that in a Cu/TaOx/Pt resistive device multiple Cu conductive nanofilaments can be formed and ruptured successively. Near the end of the study, quantized and partial quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10. / Ph. D.

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(工学) / 甲第24622号 / 工博第5128号 / 新制||工||1980(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 白石 誠司, 准教授 小林 圭 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Excessive CO2 emissions can negatively impact society and our planet. Reduction of CO2 is one potential avenue for its abatement. One of the most significant challenges to reducing CO2 is its extremely stable linear form. Experimentally, Cu/TiO2 has shown promise for CO2 photocatalytic reduction. Dispersed atomic catalysts can achieve high catalytic efficiency on a per atom basis. Active sites also typically having lower coordination number, and therefore may be more reactive. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom catalysts and CO2 reduction. Cu atoms with Ov have shown to aid in the process of bent, anionic CO2 formation. In the first step involving CO2 dissociation (CO2* --> CO* + O*), a single Cu atom in Ov lowered the activation barrier to 0.10 - 0.19 eV, which could enable fast reduction of CO2 even at room temperature, in agreement with experimental findings. A photoexcited electron model was shown to readily promote Cu binding to the surface vacancy, and CO2 adsorption and direct dissociation. Finally, we briefly compare our results to calculations of supported single Pt atoms to determine how metals besides Cu may behave as photocatalysts for CO2 reduction, and we found a single Pt with Ov can promote CO2 dissociation. Our results show that tailoring TiO2 surfaces with defects in conjunction with atomic catalysts may lead to useful catalysts in the photoreduction of CO2.

Oxidkeramische Materialien sind zunehmend von praktischem Interesse für neue Technologien, die in Brennstoffzellen, Sensoren und Ionentransport-Membranen Anwendung finden. Einige dieser Oxide mit Perowskitstruktur ABO3 zeigen hohe Ionen- und Elektronenleitung, ausreichende chemische Stabilität sowie thermisch-mechanische Eigenschaften, wie sie für die Anwendung als Sauerstofftransportmembran benötigt werden. Oxidionentransport erfolgt über einen Oxidionen-Leerstellenmechanismus. Die charakteristische Schwierigkeit für die Anwendung solcher Materialien besteht aber darin, dass die gestellten Forderungen wie hoher Ionentransport und hohe Stabilität sich diametral gegenüberstehen. In dieser Arbeit wurde eine systematische Untersuchung der Beziehungen zwischen Zusammensetzung, Struktur und Stöchiometrie der ferritischer und cobaltitischer Mischoxide und den Transporteigenschaften sowie der thermischen Ausdehnung durchgeführt. Erstmalig wurden thermodynamische und kinetische Parameter von Reihen von Oxiden in einem weiten Bereich von Temperatur und Sauerstoffpartialdruck systematisch bestimmt. Aus den Ergebnissen konnten Empfehlungen gegeben werden für die Zusammensetzung von Perowskitoxiden, die zum Aufbau von Sauerstofftransportmembranen unter bestimmten pO2/T-Bedingungen geeignet sind.

Oxidkeramische Materialien sind zunehmend von praktischem Interesse für neue Technologien, die in Brennstoffzellen, Sensoren und Ionentransport-Membranen Anwendung finden. Einige dieser Oxide mit Perowskitstruktur ABO3 zeigen hohe Ionen- und Elektronenleitung, ausreichende chemische Stabilität sowie thermisch-mechanische Eigenschaften, wie sie für die Anwendung als Sauerstofftransportmembran benötigt werden. Oxidionentransport erfolgt über einen Oxidionen-Leerstellenmechanismus. Die charakteristische Schwierigkeit für die Anwendung solcher Materialien besteht aber darin, dass die gestellten Forderungen wie hoher Ionentransport und hohe Stabilität sich diametral gegenüberstehen. In dieser Arbeit wurde eine systematische Untersuchung der Beziehungen zwischen Zusammensetzung, Struktur und Stöchiometrie der ferritischer und cobaltitischer Mischoxide und den Transporteigenschaften sowie der thermischen Ausdehnung durchgeführt. Erstmalig wurden thermodynamische und kinetische Parameter von Reihen von Oxiden in einem weiten Bereich von Temperatur und Sauerstoffpartialdruck systematisch bestimmt. Aus den Ergebnissen konnten Empfehlungen gegeben werden für die Zusammensetzung von Perowskitoxiden, die zum Aufbau von Sauerstofftransportmembranen unter bestimmten pO2/T-Bedingungen geeignet sind.

L'équipe micro-capteurs de l'IM2NP développe des capteursde gaz dont le principe de détection est basé sur la mesure de la variationde la conductance en présence de gaz. Le matériau utilisé comme élémentsensible est l'oxyde de tungstène (WO3) en couches minces. L'objet de cettethèse est donc d'étudier la surface de WO3 dans sa reconstruction c(2x2),obtenue par clivage selon la direction [001]. Cette étude a été également suivied'une étude des lacunes par des calculs ab initio basés sur la DFT, dans lesdeux approximations LDA et GGA. Ensuite, l'dsorption de molécules de gazsimples (O3, COx, NOx) sur des surfaces plus ou moins riches en oxygènea été effectuée. Pour simuler ces systèmes, nous avons fait le choix du codeSIESTA basé sur la DFT et qui présente l'avantage de pouvoir travailler. / The team of micro sensors at IM2NP mainly focuses onthe development of gas sensors based on measurement in conductancevariation in presence of gas. The material used as sensitive element istungsten oxide (WO3) thin film. The objective of present thesis is to studythe surface properties of WO3 in its reconstruction c(2x2), obtained bycleavage along the [001] direction. This study is also followed by a gapanalysis using ab initio calculations based on DFT in both LDA andGGA approximations. Then, the adsorption of molecules of simple gases((O3, COx NOx) for these surfaces (more or less rich in oxygen), is performed.To simulate these systems, we have chosen the SIESTA code based onDFT which is used for the larger number of atoms as compared to other codes.

A study of defects in semiconductors is vital as defects tend to influence device operation by modifying their electrical and optoelectronic properties. This influence can at times be desirable in the case of fast switching devices and sometimes undesirable as they may reduce the efficiency of optoelectronic devices. ZnO is a wide bandgap material with a potential for fabricating UV light emitting diodes, lasers and white lighting devices only after the realization of reproducible p-type material. The realization of p-type material is greatly affected by doping asymmetry. The self-compensation behaviour by its native defects has hindered the success in obtaining the p-type material. Hence there is need to understand the electronic properties, formation and annealing-out of these defects for controlled material doping. Space charge spectroscopic techniques are powerful tools for studying the electronic properties of electrically active defects in semiconductors since they can reveal information about the defect “signatures”. In this study, novel Schottky contacts with low leakage currents of the order of 10-11 A at 2.0 V, barrier heights of 0.60 – 0.80 eV and low series resistance, fabricated on hydrogen peroxide treated melt-grown single crystal ZnO samples, were demonstrated. Investigations on the dependence of the Schottky contact parameters on fabrication techniques and different metals were performed. Resistive evaporation proved to produce contacts with lower series resistance, higher barrier heights and low reverse currents compared to the electron-beam deposition technique. Deep level transient spectroscopy (DLTS) and Laplace-DLTS have been employed to study the electronic properties of electrically active deep level defects in ZnO. Results revealed the presence of three prominent deep level defects (E1, E2 and E3) in the as-received ZnO samples. Electron-beam deposited contacts indicated the presence of the E1, E2 and E3 and the introduction of new deep level defects. These induced deep levels have been attributed to stray electrons and ionized particles, present in the deposition system during contact fabrication. Exposure of ZnO to high temperatures induces deep level defects. Annealing samples in the 300°C – 600°C temperature range in Ar + O2 induces the E4 deep level with a very high capture cross-section. This deep level transforms at every annealing temperature. Its instability at room temperature has been demonstrated by a change in the peak temperature position with time. This deep level was broad, indicating that it consists of two or more closely spaced energy levels. Laplace-DLTS was successfully employed to resolve the closely spaced energy levels. Annealing samples at 700°C in Ar and O2 anneals-out E4 and induces the Ex deep level defect with an activation enthalpy of approximately 160 – 180 meV. Vacuum annealing performed in the 400°C – 700°C temperature range did not induce any deep level defects. Since the radiation hardness of ZnO is crucial in space applications, 1.6 MeV proton irradiation was performed. DLTS revealed the introduction of the E4 deep level with an activation enthalpy of approximately 530 meV, which proved to be stable at room temperature and atmospheric pressure since its properties didn’t change over a period of 12 months. / Thesis (PhD)--University of Pretoria, 2013. / Physics / unrestricted