Les films magnétiques ultra-minces font partie intégrante des technologies d'aujourd'hui, comme l'illustre leur omniprésence dans de nombreuses applications courantes telles que les disques durs. A cause de leurs dimensions réduites, les propriétés magnétiques spécifiques à ces échelles conduisent à la formation de structures de spin exotiques et de taille nanométrique. Pour explorer ces matériaux en détail, nous utilisons un magnétomètre à balayage développé dans notre laboratoire et qui est basé sur un défaut de spin unique dans le diamant. Ce capteur non-invasif peut mesurer à l'échelle nanométrique à la fois le champ magnétique et la topographie, et fonctionne aux conditions ambiantes. En développant une méthode d'évaluation originale du champ magnétique, nous déterminons la structure interne de parois de domaines ferromagnétiques et quantifions la force de l'interaction Dzyaloshinskii-Moriya dans des hétérostructures à couches minces. Ensuite, nous mettons en évidence le rôle clé du désordre et de l'histoire magnétique sur la stabilisation des skyrmions dans un échantillon de bicouche magnétique. Enfin, nous visualisons dans l'espace réel une spirale de spin de 70 nm de période dans le matériau multiferroïque BiFeO3 et nous manipulons sa direction de propagation avec des champs électriques. Les connaissances tirées de ces études aideront à exploiter au maximum les capacités des matériaux magnétiques à couche ultra-mince et à les mettre en œuvre dans de nouveaux dispositifs de spintronique. / Thin film magnetic materials are an integral part of today’s technology and widespread applications like the magnetic hard drive disk mirror their potential. Due to their reduced dimensions, size-specific magnetic properties induce the formation of nanoscale, exotic spin structures. To explore such materials in detail, we utilize a home-built nitrogen vacancy scanning magnetometer, based on a single defect in diamond. This non-perturbative probe combines nanoscale magnetic field- and spatial resolution and works under ambient conditions. We develop a new way to determine the inner structure of magnetic domain walls and quantify the strength of the Dzyaloshinskii-Moriya interaction in thin film heterostructures. We reveal the key role of disorder and magnetic history on the stabilization of skyrmions in a magnetic bilayer sample. Finally, we reveal the 70nm-pitch spin spiral in the multiferroic bismuth ferrite in real space and manipulate its propagation direction with electric fields. The insight gained from these studies will help to exploit the full capacity of thin film magnetic materials for spintronic application.

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

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19002号 / 工博第4044号 / 新制||工||1622(附属図書館) / 31953 / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 白川 昌宏, 教授 田中 庸裕, 教授 濵地 格 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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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.

O mundo passa pelo processo de urbanização de sua população. As áreas urbanas são pólos atratores devido aos muitos benefícios proporcionados pela aglomeração de pessoas. Ao mesmo tempo, também são palco do agravamento de problemas relacionados à pobreza. Muitos desses novos problemas estão relacionados à necessidade de proporcionar abrigo a todos os moradores das crescentes metrópoles. No Brasil, e mais especificamente na Região Metropolitana de São Paulo, a combinação da pobreza e rápido crescimento urbano culminou na dualidade do mercado habitacional: a existência do mercado formal e informal. Além disso, o mercado informal muitas vezes oferece condições de vida precárias, por não seguir a legislação apropriada. Na Região Metropolitana de São Paulo mesmo a alternativa no mercado formal de ocupar as periferias disponibiliza uma infra-estrutura urbana insuficiente. Apesar destes serem problemas de proporções significativas, não está claro como afetam a estrutura urbana das metrópoles, ou o funcionamento do mercado de habitação. Portanto, esta tese tem como objetivos esclarecer o funcionamento do mercado habitacional de São Paulo e estudar o seu processo de ocupação do solo, levando em conta a existência destes problemas urbanos. O primeiro problema analisado é o espraiamento urbano, pois a urbanização acontece de forma cada vez menos compacta. O segundo, das taxas de vacância residenciais, expõe a necessidade de gerenciar melhor os recursos da cidade: se o déficit habitacional é expressivo, porque temos tantos domicílios vagos? Por fim tratamos da interação entre o mercado de habitação formal e informal, estimando o impacto da proximidade a favelas no preço dos lançamentos imobiliários. Os resultados evidenciam o papel dos problemas gerados pela pobreza urbana no funcionamento de toda a metrópole. No estudo do espraiamento urbano identificamos como o fenômeno é incentivado pela periferização das camadas de renda baixa. Este problema também explica o padrão espacial das taxas de vacância residencial, alto no centro e baixo nas periferias, detectado no segundo ensaio. Este estudo ressaltou a importância da dinâmica do mercado imobiliário na diferenciação destas taxas. No último ensaio estimamos empiricamente uma externalidade negativa do mercado de habitação formal: a proximidade a favelas, evidenciando as relações entre os mercados formal e informal no município de São Paulo. / The worlds population is becoming urban in its majority. Urban areas attract people because of the benefits they provide. Still, they are a place where urban poverty problems are worsened. Among these new problems, many are related to the need to shelter the new inhabitants. In Brazil, and more specifically in São Paulo Metropolitan Area, the combination of poverty and fast urban growth has led to a duality in the housing market: the formal and informal markets. Moreover, because the informal market doesnt follow the urban laws, it often offers an unhealthy and unsafe environment. Even the formal market solution of suburbanization doesnt warrant an urban infra-structure good enough. Although these are significantly big problems, it is not clear how they affect the urban structure or the functioning of the housing market. Therefore this thesis has as target the study of São Paulo Metropolitan Areass housing market and its land use evolution, taking into account the urban problems above mentioned. First of all, we examine the problem of urban sprawl, since urbanization has been less compact. In the second essay we deal with residential vacancy rates. It shows the need to better manage the citys resources. If we have such a big housing deficit, why are there so many vacant dwellings? In the last essay, we present the interaction between the formal and informal housing market. We estimate how proximity to slums impact the price of real estate. Results highlight the role of urban poverty problems in the organization of the whole metropolis. Suburbanization of low income families is identified as one of the incentives for the urban sprawl. This problem also explains the spatial pattern of vacancy rates: high downtown, low in suburbs. The second essay also points out the importance of real estate market dynamics to the differentiation of these rates. In the third essay we estimate empirically one of the negative externalities of the formal housing market: the proximity to slums.

Cette thèse aborde la problématique du rapport entre ville et commerce.Depuis un siècle, la recherche urbaine est venue rappeler avec constance l’existence d’une relation d’évidence entre ces deux termes. Cette relation est pourtant interrogée par un phénomène de découplage croissant entre l’évolution des surfaces de commerce et l’évolution de la consommation des territoires.Cette recherche avance l’hypothèse que ce découplage témoigne d’un processus de «déterritorialisation » du commerce, au sens où l’activité de ce secteur relève désormais en majorité de grandes entreprises intégrées ou de conglomérats d’activités qui, dans le sillage des firmes globales, développent de nouvelles stratégies de développement au sein de chaînes de valeur globale, dont l’un des effets majeurs est de les émanciper des contraintes d’implantation sur un territoire donné, qu’il soit national, régional, voire local. En des termes régulationnistes, les mutations du mode d’accumulation du commerce définissent un nouvel espace économique tendant à se dissocier de l’espace politique de sa régulation par les territoires.La démarche s’engage sur une réflexion épistémologique visant à révéler les hypothèses fondatrices du cadre d’analyse de cette relation réputée « d’évidence », entre ville et commerce. Elle se poursuit par un questionnement de ces hypothèses, mené à partir de la mesure empirique de deux phénomènes de disjonction entre l’évolution de la fonction commerciale et l’évolution de la structure commerciale des villes. La question de la vacance commerciale est particulièrement traitée. Elle s’achève par la proposition d’un cadre d’analyse du rapport entre ville et commerce revisité à travers le concept de déterritorialisation. / For over a century, town planning constantly insisted on the link between these two notions.Nevertheless, this relation is coming under scrutiny because of a decoupling phenomenon between the evolution of the trade surface versus the evolution of the territories consumption.This research works from the hypothesis that the decoupling is the result of a process of "deterritorialization" of trade, meaning that the trade activities are part of large integrated companies or conglomerates following the same example than global firms developing new strategies based on global values. As a result, the trade installation in a given territory is emancipated from the national, regional or local constraints.In legal terms, the changes of the accumulation process for trade defines a new economic space tending to separate itself from the political sphere and its regulations by the territories.This analysis is based on a systematic reflection about the founding hypotheses linking city and trade as an obvious "fait accompli". The hypotheses will be challenged by a study based on observations of two disjunction phenomenoms between the evolution of trade levels and the evolution of trade infrastructure within cities. The question of commercial vacancy will be examined in details. In the end, a new proposal will be presented to revisit the connection between city and trade through the concept of "deterritorialization".