Dissertations / Theses on the topic 'Oxygen vacancy'
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1
Kullgren, Jolla. "Oxygen Vacancy Chemistry in Ceria." Doctoral thesis, Uppsala universitet, Strukturkemi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-167521.
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Cerium(IV) oxide (CeO2), ceria, is an active metal oxide used in solid oxide fuel cells and for the purification of exhaust gases in vehicle emissions control. Behind these technically important applications of ceria lies one overriding feature, namely ceria's exceptional reduction-oxidation properties. These are enabled by the duality of the cerium ion which easily toggles between Ce4+ and Ce3+. Here the cerium 4f electrons and oxygen vacancies (missing oxygen ions in the structure) are key players. In this thesis, the nature of ceria's f electrons and oxygen vacancies are in focus, and examined with theoretical calculations. It is shown that for single oxygen vacancies at ceria surfaces, the intimate coupling between geometrical structure and electron localisation gives a multitude of almost degenerate local energy mimima. With many vacancies, the situation becomes even more complex, and not even state-of-the-art quantum-mechanical calculations manage to predict the experimentally observed phenomenon of vacancy clustering. Instead, an alternative set of computer experiments managed to produce stable vacancy chains and trimers consistent with experimental findings from the literature and revealed a new general principle for surface vacancy clustering. The rich surface chemistry of ceria involves not only oxygen vacancies but also other active oxygen species such as superoxide ions (O2−). Experiments have shown that nanocrystalline ceria demonstrates an unusually large oxygen storage capacity (OSC) and an appreciable low-temperature redox activity, which have been ascribed to superoxide species. A mechanism explaining these phenomena is presented. The ceria surface is also known to interact with SOx molecules, which is relevant both in the context of sulfur poisoning of ceria-based catalysts and sulfur recovery from them. In this thesis, the sulfur species and key mechanisms involved are identified.
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Chen, Haiyan. "Probing Defects and Electronic Processes on Gadolinia-doped Ceria Surfaces Using Electron Stimulated Desorption." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/10427.
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Probing Defects and Electronic Processes on Gadolinia-doped Ceria Surfaces Using Electron Stimulated Desorption
Haiyan Chen
133 Pages
Directed by Professor Thomas M. Orlando
Polycrystalline gadolinia-doped ceria (GDC) has been widely investigated as a promising low temperature solid oxide fuel cell (SOFC) electrolyte and as part of composite electrodes. In this thesis, electron stimulated desorption (ESD) has been used to probe the defect related electronic properties of GDC surfaces and the interactions of water and molecular oxygen with these surfaces.
In particular, the electron irradiation induced surface charging of GDC has been found to be dependent on the incident electron energy: negative at lower energy and positive at higher energy. Trapping of electrons and holes by the gadolinium aggregated, oxygen vacancy rich grain boundaries has been considered as the origin of surface charging. Depending on the sample treatment, there can be various defects, hydroxyl groups, chemically adsorbed water molecules, or water dimers on GDC surfaces. Water and molecular oxygen interact primarily with defect sites.
Systematic investigations of electron stimulated O+ desorption have yielded activation energies relevant to oxygen vacancy production on ceria surfaces, and to surface positive charge dissipation related to ionic conduction of GDC. Highly efficient electron stimulated O+ desorption from GDC surfaces has been attributed to the lowered charge density on oxygen ions coordinated with oxygen vacancy clusters and thus may be used as a probe for surface defect types.
Electron stimulated desorption of O2+ from GDC surfaces during molecular oxygen adsorption has shown the ability of ESD to detect chemically adsorbed O2. The velocity distributions of O2+ can be used to probe intermediate adsorption species such as O2, as well as the positive charge of the surface.
Overall, this thesis has demonstrated that ESD can provide important information on the kinetics and dynamics of surface charging, charge transport, adsorption and reactions occurring at defective insulating metal oxides materials. The abilities to probe the defects and their roles in surface processes make ESD a valuable technique for surface chemistry and catalysis studies.
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Wang, Xiaoshuang. "Multiscale modeling of oxygen and vacancy diffusion in dilute ferritic iron alloys." Technische Universität Dresden, 2020. https://tud.qucosa.de/id/qucosa%3A72722.
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Iron-based ferritic alloys are used for a plethora of industrial applications. These alloys contain foreign atoms purposely employed to improve certain properties as well as some unwanted impurities introduced during fabrication. Materials properties are decisively influenced by diffusion processes. Very often diffusion cannot be avoided during fabrication and application. Therefore, many efforts are made to understand the underlying atomic-level mechanisms by both experimental and theoretical investigations. In this thesis work a multiscale modelling approach is used to study oxygen and vacancy diffusion in dilute ferritic iron alloys. Due to the extremely low solubility of oxygen the measurement of oxygen diffusion in iron is difficult. Only few experimental data are available. Experimental investigation of vacancy migration is still more complicated. The lack of reliable experimental data is therefore an important motivation for theoretical investigations. Gaining fundamental data on oxygen and vacancy diffusion in dilute iron alloys is essential for many applications. Oxygen plays a crucial role in the corrosion of iron-based alloys. Oxygen and the vacancy are also important in the formation and evolution of Y-Ti-O nanoclusters in oxide dispersion strengthened ferritic Fe-Cr alloys, which are considered as promising candidates for structural materials of future fusion and fission reactors. Furthermore, vacancies are formed during neutron and ion irradiation and their diffusion affects radiation-induced nanostructure formation in ferritic alloys.
In the first part of this thesis work, the diffusion of interstitial oxygen under the influence of substitutional atoms or solutes (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo and W) in bcc Fe is investigated by the combination of Density Functional Theory (DFT) and Atomistic Kinetic Monte Carlo (AKMC) simulations. The substitutional atoms are assumed to be immobile because oxygen diffusion is much faster than that of the solutes. DFT is applied to gain data on binding energies between interstitial oxygen and the substitutional foreign atoms, and to calculate the migration barriers for oxygen in the environment of the solutes. Using the migration barriers obtained by DFT, the diffusion coefficient of oxygen is determined by AKMC simulation. It is found that Si, P, Ni, Mo, and W have negligible influence on the oxygen diffusion coefficient. Al, Cr, Mn, S, Ti, and Y cause a considerable reduction of oxygen mobility. In these cases, the temperature dependence of oxygen diffusivity shows deviations from Arrhenius behavior. This is explained in detail by the significant temperature dependence of the ratio between residence times in the respective states.
In the second part of the work a method is presented which allows for an efficient calculation of the diffusion coefficient of oxygen and other interstitial atoms in dilute alloys. The method is applied to examples considered in the first part of the work. The calculation procedure is based on the separation of the diffusion path into a contribution related to migration in the interaction region between the mobile interstitial and the substitutional solute and another part related to diffusion in perfect bcc Fe. In this manner AKMC simulation must be performed only for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations using analytical expressions containing binding energies between the interstitial and the substitutional solute.
The focus of third part of the work is on the mutual dependence of oxygen and vacancy diffusion in bcc Fe and dilute iron alloys. Here both O and v must be considered as mobile while the substitutional atoms are assumed to be immobile. DFT is applied to determine the binding energy between O and v for different distances, the migration barriers for O in the environment of v, and the corresponding barriers of v in the vicinity of O. In agreement with previous work O and v have a very strong binding at the 1st neighbor distance. On the other hand, the calculations show that the Ov pair at the 6th neighbor distance is instable. The newly found simultaneous or coupled jumps of both O and v compensate the lack of jump paths that would occur due to this instability. The DFT results are employed to determine the diffusion coefficient of O and v using the scheme of the AKMC-based calculation method presented in the second part of the thesis work. At first a model system with fixed O and v concentrations is studied. It is found that a small v content of some ppm can already lead to a strong reduction of the O diffusivity. A similar effect is obtained for v diffusion under the influence of O. Furthermore, investigations on the interdependence of O and v diffusion during thermal processing of oxide dispersion strengthened iron alloys are performed, and the influence of the substitutional atoms Y and Ti is studied. A simple thermodynamic model is employed to determine the concentration of O, Y, and Ti monomers as well as the total v concentration, for a typical total content of O, Y, and Ti. These results are used in calculations of the diffusion coefficients of O and v. Not only a strong mutual dependence but also a significant influence of Y on O diffusion is found. Finally, O and v diffusivities in a system with a total O content close to the thermal solubility are calculated. The monomer O concentration as well as the total v concentration was determined using two different models considering equilibrium of O and v with Ov, or equilibrium of O and v with Ov and O2v or Ov2. Despite the very small value of thermal solubility of O in bcc Fe, both the O and v diffusion coefficient are very different from that in pure iron. Even for such a low amount of O in the alloy the diffusion coefficients differ strongly from those in perfect bcc Fe.
The results of the present work have important consequences for planning and performing new experiments on O and v diffusion in dilute iron alloys. In particular, a very precise knowledge of the concentrations of O and v, as well as of other foreign atoms and traps such as dislocations is required.
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Wang, Xiaoshuang [Verfasser], Jürgen [Gutachter] Faßbender, and Karsten [Gutachter] Albe. "Multiscale modeling of oxygen and vacancy diffusion in dilute ferritic iron alloys / Xiaoshuang Wang ; Gutachter: Jürgen Faßbender, Karsten Albe." Dresden : Technische Universität Dresden, 2020. http://d-nb.info/1227311273/34.
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Lin, Hong-Ying. "Improving the optoelectronic property and photoactivity of nano-structured titanuim dioxide effect of particle size, oxygen vacancy, and nitrogen doping /." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 1581 p, 2008. http://proquest.umi.com/pqdweb?did=1609302401&sid=2&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Thesis (Ph.D.)--University of Delaware, 2008.Principal faculty advisors: Chin-Pao Huang, Dept. of Civil and Environmental Engineering, and S. Ismat Shah, Dept. of Materials Science Includes bibliographical references.
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Girdauskaite, Egle. "Thermodynamische und kinetische Untersuchungen zum Sauerstoffaustausch in perowskitischen Mischoxiden auf Basis von Ferriten und Cobaltiten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1195658113234-25483.
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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.
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Iwata, Tatsuya. "Study on Resistive Switching Phenomenon in Metal Oxides for Nonvolatile Memory." 京都大学 (Kyoto University), 2014. http://hdl.handle.net/2433/188598.
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Peng, Yung-Kang. "Surface mapping of faceted metal oxides by chemical probe-assisted NMR for catalytic applications." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:7b56021f-71fb-437b-8c6b-0569705ef68e.
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Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate energy. The electronic structure, light absorption and charge transport properties have made the transition metal oxides an attractive material as photocatalyst. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used in the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR and XPS) used for materials characterization (oxygen vacancy, hydroxyl group, cation, etc.) are not truly surface specific but analyzing a range from surface few layers to bulk. Accordingly, they can only provide very limited information on chemical states of the surface active features and their distribution among facets, causing difficulties to unambiguously correlate facet-dependent results with activity. As a result, this often leads to different interpretations amongst researchers during the past decades. As the publications of titanium and zinc ranked top two among studies of first row of transition oxides in the past decades, this thesis will firstly review on the disagreements generated among researchers when they correlated the performance of ZnO and TiO2 with their facet activities based on traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. To address these issues, we have developed a new technique "probe-molecule-assisted NMR" which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features among facets. In light of the new technique, this thesis will revisit the facet-dependent photocatalytic properties and shed light on these issues.
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Nishi, Yusuke. "Nonpolar Resistive Switching Based on Quantized Conductance in Transition Metal Oxides." Kyoto University, 2019. http://hdl.handle.net/2433/242544.
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Umeda, Yuji. "Rational design of dielectric oxide materials through first-principles calculations and machine-learning technique." Doctoral thesis, Kyoto University, 2020. http://hdl.handle.net/2433/245844.
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京都大学0048
新制・課程博士
博士(工学)
甲第22159号
工博第4663号
新制||工||1727(附属図書館)
京都大学大学院工学研究科材料工学専攻
(主査)教授 田中 功, 教授 中村 裕之, 教授 邑瀬 邦明
学位規則第4条第1項該当
Doctor of Philosophy (Engineering)
Kyoto University
DFAM
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Shojaee, Kambiz. "Fundamental aspects of ammonia oxidation on cobalt oxide catalysts." Thesis, The University of Sydney, 2014. http://hdl.handle.net/2123/13657.
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The current thesis deals with the ammonia oxidation on cobalt oxide catalyst at the molecular level. The catalytic oxidation of ammonia to NO is crucial in the industrial process of nitric acid production. Cobalt oxide catalysts are being used together with platinum gauzes to reduce the production cost and emission of greenhouse gas N2O. However, the fundamentals of ammonia oxidation on cobalt oxides are not known. This thesis aims to provide insights into our fundamental understanding of ammonia oxidation on Co3O4 surfaces. The performance of cobalt oxide catalysts in the oxidation of NH3 strongly depends upon the exposed surface terminations. Results indicate that different surfaces of Co3O4 behave markedly differently in oxidative reactions due to the difference in binding energy and O recombination energies and oxygen vacancy formation. Overall, NH3 oxidation follows stepwise dehydrogenative route (NH3* → NH2* → NH* → N*) on Co3O4 surfaces. Desorption of lattice products results in the formation of O vacancy sites opening the way for a Mars-van Krevelen mechanism. The successive dehydrogenation of ammonia preferably occurs on the surfaces exposing active lattice O sites. Removal of active lattice O sites from the Co3O4 surfaces in the form of products results in the surface reduction. If the rate of reduction is faster than that of re-oxidation, a CoO-like phase might form. The formation of CoO in Co3O4 catalysts during NH3 oxidation not only reduces the NH3 conversion but also alters the selectivity towards N2 rather than NO due to weak ability of lattice O at the CoO surface to assist the hydrogen abstraction process. A surface with a lower oxygen vacancy formation energy and a higher binding energy of hydrogen exhibits a higher activity towards ammonia oxidation to NO.
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Lortie, Maxime. "Reverse Water Gas Shift Reaction over Supported Cu-Ni Nanoparticle Catalysts." Thesis, Université d'Ottawa / University of Ottawa, 2014. http://hdl.handle.net/10393/31733.
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CuNi nanoparticles were synthesized using a new polyol synthesis method. Three
different CuxNi1-x catalysts were synthesized where x = 20, 50 and 80. The nanoparticles were deposited on carbon, C, gamma-alumina, γ-Al2O3, yttria-stabilized zirconia, YSZ, and samariumdoped ceria, SDC. Each set of catalysts was tested using the Reverse Water Gas Shift, RWGS, reaction under atmospheric pressure and at temperatures ranging from 400°C-700°C. The experiments were repeated 3 times to ensure stability and reproducibility. Platinum nanoparticles
were also deposited on the same supports and tested for the RWGS reaction at the same conditions. The CuNi nanoparticles were characterized using a variety of different techniques. Xray diffraction, XRD, measurements demonstrate the resence of two CuNi solid solutions: one Cu rich solid solution, and the other a Ni rich solid solution. X-ray photo electron spectroscopy, XPS, measurements show Cu enrichment on all catalytic surfaces. Scanning electron microscopy, SEM, measurements show CuNi nanoparticles ranging in size from 4 nm to 100 nm.
Some agglomeration was observed. SDC showed the best yield with all catalysts. Furthermore, high oxygen vacancy content was shown to increase yield of CO for the RWGS reaction. Cu50Ni50/SDC shows the combination of highest yield of CO and the best stability among CuNi catalysts. It also has similar yields (39.8%) as Pt/SDC at 700°C, which achieved the equilibrium yield at that temperature (43.9%). The catalyst was stable for 48 hours when exposed to high temperatures (600-700°C). There was no CH4 observed during any of the experiments when the
partial pressure of the reactant gases was fed stoichiometrically. Partial pressure variation experiments demonstrated the presence of CH4 when the partial pressure of hydrogen was increased to twice the value of the partial pressure of CO2.
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Saines, Paul James. "Structural Studies of Lanthanide Double Perovskites." Thesis, The University of Sydney, 2008. http://hdl.handle.net/2123/3939.
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This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.
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Saines, Paul James. "Structural Studies of Lanthanide Double Perovskites." Faculty of Science. School of Chemistry, 2008. http://hdl.handle.net/2123/3939.
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Doctor of Philosophy(PhD)This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.
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Kang, Yuhong. "Mechanisms, Conditions and Applications of Filament Formation and Rupture in Resistive Memories." Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/77593.
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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.
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Chen, Junbo. "Effect of Defects and Photoexcited Electrons on CO2 Reduction using Supported Single Atom Catalysts." Digital WPI, 2018. https://digitalcommons.wpi.edu/etd-theses/1266.
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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.
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Ali, Fawad. "Investigation of metal oxides thin films developed by PVD system for perovskite solar cells." Thesis, Queensland University of Technology, 2019. https://eprints.qut.edu.au/127139/1/Fawad_Ali_Thesis.pdf.
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This research presents thin film deposition and characterization of metal oxides using industrially viable Physical Vapour Deposition (PVD) techniques. The research examines low temperature processed electron and hole transport metal oxides for high performance and stable perovskite solar cells. The physical, chemical, optical and electronic properties of the films were investigated and their device performance has been evaluated. The performance of the device improved and the materials cost reduced by replacing the expansive organic materials with more stable inorganic metal oxides.
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Asplet, William. "Etude des interactions entre les défauts lacunaires et les solutés Y,O, Ti pour mieux comprendre leur rôle dans la formation des nanoparticules d'oxydes dans les aciers ODS." Thesis, Orléans, 2018. http://www.theses.fr/2018ORLE2056/document.
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Cette thèse est dédiée à l’étude des interactions entre les défauts lacunaires et les solutés (Y, Ti, O) pour mieux comprendre la formation des nanoparticules d’oxyde dans les aciers ODS (Oxide Dispersion Strengthened). Ces aciers sont envisagés comme matériau de structure dans la prochaine génération de réacteurs nucléaires fission et fusion. Leurs bonnes propriétés reposent essentiellement sur une répartition homogène de nanoparticules (YxTiyOz) de très faible taille. Cependant, l’obtention de cette répartition n’est pas encore maitrisée et le mécanisme de formation de ces nanoparticules n’est pas encore bien défini. Des modèles théoriques montrent que la présence de lacunes pendant l’étape de broyage pourrait impacter la formation de ces nanoparticules. Cette étude fait suite à la thèse de C. He et apporte de nouveaux résultats et de nouvelles interprétations et conclusions. Des implantations avec des ions Y, Ti, O ont été réalisées afin de simuler l’étape de broyage de ces aciers ODS. Elles ont permis d’introduire des défauts et les solutés désirés dans la matrice de fer α. Nous avons ensuite étudié les interactions entre les solutés et les défauts générés dans la matrice. La nature des défauts induits a été étudiée en fonction de l’ion implanté et de traitements thermiques après implantation par spectroscopie d’annihilation de positons (PAS) et corrélée avec les profils en profondeur des solutés obtenus par spectrométrie de masse des ions secondaires (SIMS). Les caractéristiques d’annihilation de certains défauts encore inconnues ont pu être déterminées. Les mesures SIMS ont montré que le titane ne migre pas entre 100 et 450°C et que l’oxygène présente un comportement complexe de migration et de piégeage dépendant de la température et de la microstructure des matériaux. Les résultats PAS montrent que les implantations ioniques conduisent à la formation de clusters lacunaires, de complexes lacunes-solutés et de dislocations dont la proportion change en fonction de la profondeur et de la nature de l’ion implanté. Les clusters de lacunes Vn et les dislocations sont détectés au-delà du pic d’implantation avec une fraction plus importante pour les dislocations indiquant que les défauts ont pu migrer pendant l’implantation. La proportion des complexes lacunes-solutés est maximale dans la zone d’arrêt des ions. Elle est en accord avec les énergies de liaison théoriques des complexes lacunes-soluté. La nature et la distribution des défauts formés évoluent en fonction de la température de recuit. Les clusters lacunaires Vn disparaissent entre RT et 300°C alors que les dislocations sont éliminées à partir de 400°C. Des phases oxydes sont détectées pour des recuits à 500 et 550°C en lien avec la contamination en oxygène. Des défauts dont la nature est non identifiée ont été mis en évidence pour des recuits réalisés entre 300 et 400°C dans les implantations O, Y et Y+OThis PhD thesis is dedicated to the study of interaction between vacancies and Y, Ti,O solutes for a better understanding of formation of oxide nanoparticles in ODS steel (Oxide Dispersion Strengthened). These ODS steels are considered as structural material for the next generation of fission and fusion nuclear reactors. Their good properties are induced by the fine dispersion of low size oxide nanoparticles. However, obtaining this distribution is not mastered and atomic scale clustering is not yet defined. Furthermore, it was shown by theoretical models that the presence of vacancy during mechanical alloying could affect the formation of these nanoparticles. This study follows upon on a previous study made by C.He, and bring new results, new interpretation and conclusions. Some implantations with Y, Ti, O ions with several energy have been made in order to simulate the mechanical alloying step used for ODS steel fabrication. Theses irradiations have induced defects and solutes into the iron matrix. Then we characterized samples using positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry (SIMS). The nature of defects was studied according to nature of the implanted ion and the annealing temperature by PAS and correlated to depth profiles of solutes obtained by SIMS. Annihilation characteristics of some defects still unknown were able to be determined thanks to positron lifetime measurements. SIMS analysis showed that titanium doesn’t migrate for annealing experiments between 100°C and 450°C and that oxygen show a complex behavior of migration and trapping dependent on the microstructure of the material. PAS results show that ionic implantations produce vacancy clusters, dislocations and solutes-vacancies complex. Their proportion changes as a function of depth and nature of these irradiations. Vacancy clusters and dislocations are detected deeper than the implantation peak with a higher fraction for the dislocations indicating that the defects were able to migrate during implantations. The fraction of vacancy-solutes complexes is the highest in the ion stopping zone and is in a good agreement with the theoretical binding energy of vacancies-solutes complex. The nature and the distribution of the defects evolve according to the annealing temperature. Vacancy clusters disappear between RT and 300°C while the dislocations are eliminated from 400°C. Oxide phases are detected for annealing at 500 and 550°C in relation with the oxygen contamination during these annealings. Some defects which the nature is not yet identified were highlighted for annealing between 300 and 400°C for Y, O and Y+O irradiations
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Saadi, Lama. "Etude de l'adsorption des molécules simples sur WO3 : application à la détection des gaz." Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4346/document.
Full textAbstract:
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 travaillerThe 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
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Er-Rakho, Lahcen. "Oxydes de cuivre a valence mixte : perovskites deficitaires en oxygene." Caen, 1987. http://www.theses.fr/1987CAEN2036.
Full textAbstract:
Mise en evidence et etude de deux grandes familles de composes dans le systeme la::(2)o::(3)-ao-cuo(a=ca,ba,sr) : la::(2-x)ba::(1-x)cu::(1-x/2)o::(5-x)(ln=la,nd) ou cu est essentiellement au degre d'oxydation 2 et une seconde famille caracterisee par la valence mixte du cuivre, les quantites de cuivre 3 pouvant atteindre dans certains cas 40%. Tous ces oxydes ont en commum leur appartenance a la structure perovskite. Proprietes electriques et magnetiques
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Guttman, Jeremy. "Polymer-based Tunnel Diodes Fabricated using Ultra-thin, ALD Deposited, Interfacial Films." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469125487.
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Vitoux, Laura. "Etude électrochimique et structurale du système NaxMoO2." Thesis, Bordeaux, 2016. http://www.theses.fr/2016BORD0362/document.
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Ce travail de thèse présente l’étude du diagramme de phase des oxydes lamellaires NaxMoO2dans le cadre de la recherche de nouveaux matériaux. L’identification des transitionsstructurales au cours de l’intercalation et désintercalation électrochimique du sodium dans lesdomaines de composition ½ ≤ x ≤ 1 et ¼ < x ≤ ½ a été faite par électrochimie combinée à ladiffraction des rayons X in situ. Il a été montré que le profil très accidenté de la courbegalvanostatique résulte de multiples réarrangements structuraux au cours du cyclage.Notamment l’existence de nombreuses phases NaxMoO2 particulières a été mise en évidence,pour lesquelles des mises en ordre des ions sodium et des atomes de molybdène sont attendues.Des composés Na~1/2MoO2, Na~2/3MoO2 et NaMoO2 ont été synthétisés ex situ par voieélectrochimique ou chimique et leur caractérisation révèle des arrangements structurauxcomplexes, tel que des chaînes de clusters de molybdène dans les feuillets [MoO2] de NaMoO2This work concerns the investigation of the phase diagram of sodium layered oxides NaxMoO2in the search of new materials. Structural transitions upon sodium electrochemical(de)intercalation were studied by electrochemistry combined with in situ X-ray diffraction forcompositions ½ ≤ x ≤ 1 et ¼ < x ≤ ½. It was shown that the very undulating aspect of theelectrochemical curve results from multiple structural rerarrangements upon cycling. Especiallynumerous NaxMoO2 specific phases have been evidenced, for which sodium/vacancy orderingsas well as the formation of Mo-Mo bonds are expected. Na~1/2MoO2, Na~2/3MoO2 et NaMoO2compounds have been (electro)chemically synthesized and their structural characterizationreveals complex structures, such as chains of diamond-like molybdenum clusters in NaMoO2
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Huang, Mei-tzu, and 黃眉滋. "Oxygen vacancy dependent magnetism in CeO2 nanoparticles." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/gtgfy7.
Full textAbstract:
碩士國立臺灣科技大學
工程技術研究所
99
This study investigated the magnetism of CeO2 nanoparticles prepared by thermal decomposition method. At first, it is found that the properties of CeO2 NPs were strongly related to the synthesis parameters. CeO2 NPs prepared by using Octyl ether as solvent have higher degree of oxygen deficiency than that of NPs using Benzyl ether a solvent. The concentration of Ce3+ is 42% of the former and 18% of the latter. Importantly, the former one is paramagnetic while the later one is ferromagnetic at room temperature. Then the CeO2 NPs were undergo different redox treatments. It includes the annealing in different oxygen partial pressure at 500℃ for 2hrs, and the annealing in air at 500℃ for different time. Combined the XAS and TEM analysis, it was found that the oxygen vacancy tends to locate at the surface of NPs. In addition, it was found that the magnetism of CeO2 NPs is related to the concentration of Ce3+ at surface. According to our results, it is proposed that the highest Ms will be obtained in CeO2 NPs with 35%~40% Ce3+ at surface.
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Lin, Yu-shiang, and 林裕翔. "The oxygen vacancy and spin polarization of cuprous oxide nanoparticle." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/50981798243992667228.
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碩士國立中央大學
物理研究所
95
In studying the magnetic properties of cuprous oxide nanoparticle. We got a group of M-H curve measurements of cuprous oxide nanoparticle and discovered hysteresis at 1.8 K and 5 K. Then we pressed the cuprous oxide sample to control the interparticle separation, and dicussed the effect of interparticle interaction. The range of compacting density (CD) of particles is from 3 % to 77 % . There are two magnetic components in cuprous oxide nanoparticle system. We infer that one component is surface spin polarization and the other is magnetic characters of Cu2O0.9nanoparticle. The saturated magnetization (Ms) of spin polarization increases as the interparticle separation is decreased. The behavior can be taken the form of magnetic dipole-dipole interaction. After pressing the sample,we never discovered hysteresis again and diamagnetic properties was revealed.
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Yu, Szu Chun, and 尤思淳. "Threshold Voltage Tuning and Oxygen Vacancy Passivation for Advanced FinFET." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/35ms4m.
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碩士國立清華大學
工程與系統科學系
103
FinFET devices have higher on current and lower leakage current due to better gate control ability compared to the conventional single gate planar FETs. With the scaling of gate length, threshold voltage tuning by channel doping will be an issue. With the scaling of EOT, flat band voltage roll-off caused by oxygen vacancy needs to be solved. There are three parts in this thesis to dicuss the impacts of high-k/metal gate process on device characteristics, which are plasma nitridation, ALD pulse time and ALD growth temperature modulation. In the first part, different time of plasma assisted nitridation is implemented on metal gate to adjust work function and achieve multi-Vt FinFET. It has been reported that threshold voltage in PFinFET decreases with a higher nitrogen concentration in metal gate. However, results in this work are contrary to expectation. It supposes that nitrogen concentration of metal gate is already saturated, and effective work function may be decreased with further nitridation treatment. The on current is increased over 5-6% for PFinFET and slightly decreased for NFinFET, which are consistent with the results of EOT. Two possible mechanisms are proposed to explain electrical characteristics of FINFET, which are nitrogen diffusion into high-k layer and interfacial layer regrowth. In the second part, two different H2O pulse time in ALD are implemented to study the oxygen vacancy effect. The crystallization of high-k is transferred into monoclinic phase with higher oxygen content by a longer H2O pulse time. Less oxygen vacancy can be also obtained, which is verified by the increased and decreased threshold voltage for NFinFET and PFinFET, respectively. Sub-threshold swing, Dit, and leakage current are decreased, and good reliability is obtained by longer H2O pulse time in ALD although EOT is slightly increased. In the third part, two different deposition temperatures in ALD are implemented to study the oxygen vacancy effect. The crystallization phase of high-k is transferred to more stable monoclinic by a higher deposition temperature in ALD. Smaller standard deviation of threshold voltage can be achieved by a higher deposition temperature in ALD because of the reduced oxygen vacancy. EOT is slightly increased due to the higher oxygen content in high-k, and it doesn’t decrease the on current. Sub-threshold swing, Dit, and leakage current are decreased, and good reliability is obtained as well.
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Karapetrova, Euguenia. "Factors influencing the crystallization, phase and oxygen vacancy concentration in zirconia." Thesis, 1997. http://hdl.handle.net/1957/34055.
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In order to achieve a better understanding of the processes that occur during
formation and sintering of zirconia, various chemical and physical techniques were
used. Along with Perturbation Angular Correlation spectroscopy, that allowed us to
investigate microscopic properties inside the nanometer-size zirconia grains, such
techniques as Scanning Electron Microscopy and X-ray diffraction were used for
determining the size of particles before and after sintering, and Neutron Activation
Analysis was employed for measuring the impurity levels in zirconia powders.
By controlling the initial conditions and heat treatment of the powders, we
investigated the dependence of formation of the charged defects on the existing
molecular structure and morphology of zirconia particles.
During the study, it was discovered that at low temperature the PAC frequencies of
tetragonal zirconia behave very similarly for all materials that were used in this study.
If stabilization is achieved by heavy doping, there are shifts and line-broadening due to
the presence of dopants but no obvious differences in the essential physics. One
material included in this group is Nb-doped zirconia that has no oxygen vacancies. It
was concluded that there are no detectable oxygen vacancies in our pure or lightly
doped tetragonal zirconia powders before they are heated into the temperature region
where sintering occurs.
Vacancies are incorporated as the samples are heated above 1050��C, the
temperature at which sintering becomes important. The oxygen vacancies in samples
that have been heated to 1200��C remain when cooled. We see no vacancy
concentration dependence on the atmosphere for samples not doped with +5 valent
elements in order to reduce the vacancy density at 1200��C. In several instances,
samples that had been heated to a maximum temperature of 1050��C or 1100��C
contained a vacancy density that was small (<100 ppm) but measurable. A reduced
oxygen pressure increased the oxygen vacancy density by a measurable amount in
these samples. Samples that are tetragonal at 800��C are well-sintered after being
heated to 1200��C. Samples that are monoclinic below 1170��C are very poorly sintered
at 1200��C and contain few vacancies. Flowing Cl in the system as the samples are
sintering retards the densification of the grains. These samples had the smallest density
of oxygen vacancies.Graduation date: 1998
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Lin, Ching-Hsien, and 林京憲. "First-Principle Investigation on Oxygen Vacancy in High-K Dielectric Materials." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/18940294968448298313.
Full textAbstract:
碩士國立高雄應用科技大學
電子工程系
97
Recently, the thickness of gate oxide made from SiO2 in nano-scale MOSFET has reached a physical limit according to the current leakage. The High-K dielectric materials have been found to be a good solution to the problem. However, there is a tradeoff between the dielectric constant and the energy gap in High-K materials, a larger dielectric constant usually associated with a smaller energy gap. To solve this problem a multilayer structure is adopted, for example a TiO2 grown on HfO2. In this work, an alloy oxide of Hf(1-X)TiXO2 is studied because the properties of TiO2 are complementary to HfO2. The density function theory (DFT) with the DMol3 and CASTEP codes is applied to investigate High-K alloy dielectric materials of Hf(1-X)TiXO2 and Hf(1-X)TiXOY. The mix-atom mode is used to model the alloy oxide. In out study, we find that (i) the position of minority atoms strongly affects the physical properties of bulk material, (ii) the high Ti concentration is easier to produce oxygen vacancies than the low concentration, and (iii) the increase of the Ti concentration decreases the energy gap but increases the dielectric constant. According to these properties, the Hf(1-X)TiXO2 may have possibility to be used as a High-K dielectric layer in nano-scale MOSFET.
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Roh, Bumwook Macdonald Digby D. "Defect properties of anodic oxide films on titanium and impact of oxygen vacancy on oxygen electrode reactions." 2007. http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-1878/index.html.
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HSIEH, TSU-HSIU, and 謝祖修. "High Sensitivity NO gas sensor using SnO2 nanofiber by oxygen vacancy engineering." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/43bha3.
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Huang, Cheng-Yen, and 黃丞言. "Studies of Oxygen Vacancy Defects and Magnetism in Co-Doped Cerium Oxide Nanoparticles." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/45604922345335992055.
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Wang, Wen-Chieh, and 王玟捷. "Gigantic Enhancement in Sensing Ability of Nanowire Sensor by Increasing the Oxygen Vacancy Amounts." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/64488472717223223958.
Full textAbstract:
碩士淡江大學
物理學系碩士班
101
Recently, there are many metal-oxide-based nano materials have been studied, most article indicated that the conductivity of the metal-oxide nano materials due to oxygen vacancies. In this research work, the SnO2-X and SnO2 nanostructures have been used to study the detection ability. The sensing abilities of metal-oxide-based nanosensors will be affected by the oxygen vacancy amounts of nanowire surface. In UV sensing, compared to the sensing abilities of SnO2 schottky contacted devices, the SnO2-X schottky contacted devices improved the sensitivity in different O2 concentration environment. The sensitivity and schottky barrier height variation with the increasement of O2 concentration of SnO2-X are much larger than SnO2, and the reset time is also faster. In gas sensing, the sensitivity of SnO2-X schottky contacted devices is better than SnO2 in different measurement temperature and different CO concentration. The response and reset time are faster, either. This result indicated that the sensing abilities can be gigantically enhanced by increasing the oxygen vacancy amounts. This research work provides a potential method for enhancing the detection ability of nanosensors.
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Lin, Ching Hsuan, and 林敬軒. "Threshold Voltage Tuning and Oxygen Vacancy Passivation in FinFET by High-k/Metal Gate Processes." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/4tg64p.
Full textAbstract:
碩士國立清華大學
工程與系統科學系
103
As device scaling, the leading edge technology is moving to FinFET process. In order to improve the FinFET performance, the thesis focuses on the study of High-k/metal gate process. There are three parts in this work to discuss the effects of High-k/metal gate process on device characteristics. In the first study, Vth tuning by the PMA process after MG-1 gate stack is investigated. The EWF, Vth, and other electrical characteristics are discussed. It is found that there are two mechanisms affecting the work function value during the PMA process. First, Work function (WF) could be lower by M1 diffusion. Second, work function could be higher by dipole formation. Therefore, the Vth can be modified by the PMA process. PMA X+100℃ is the best annealing condition for MG-1 gate stack, which shows the best electrical performance. In the second study, Vth tuning by different metal alloys (MG-2 vs. MG-1) is studied. （MG-1：Metal A and Metal C，MG-2：Metal B and Metal C） The thermal stability and electrical characteristics of two different metal alloys are discussed with the same PMA process. The results indicate that the thermal stability of Metal B is worse than that of Metal A. For MG-2 gate stack, PMA X℃ is the best annealing condition which shows good performance. For FinFET process, MG-1 as metal gate stack is better than MG-2. In the last study, O2 PDA at low temperature and low partial pressure are implemented; in order to passivate oxygen vacancy in High-k. The variation of Vth and Ion, CET, other electrical characteristics and reliability are discussed. The results show that CET of O2 PDA is a little higher than that of STD. For PFinFET, most device performance can be improved. Lower Vth variation is obtained, which indicates the oxygen vacancy can be passivated. However, the improvement of device performance (Vth, σVth, Ion) for NFinFET is not achieved. For NFinFET and PFinFET, the reliability can be improved by O2 PDA.
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Yu, Shen-Wei, and 俞勝為. "Evaluation for Oxygen Vacancy on Co-Ce-Y Composite Oxides and Steam Reforming of Ethanol." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/78827177806809738326.
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碩士國防大學理工學院
化學工程碩士班
102
The effect of accessible oxygen on the steam reforming of ethanol (SRE) over Co3O4-CeO2 and Co3O4-CeO2-Y2O3 catalysts was investigated. Both equal molar ratio of Co3O4-CeO2 catalysts were prepared by hydrothermal co-precipitation (H) and hydrothermal ultrasonic-assisted co-precipitation (UH) methods, and finally the UH method was applied to prepare 2.5, 5, and 10 wt % of yttrium-doped Co3O4-CeO2 catalysts, respectively. All catalysts were characterized through X-ray diffraction (XRD), temperature programmed reduction/temperature programmed oxidation (TPR/TPO), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), elemental analysis (EA) and CO2 - temperature programmed desorption (CO2-TPD) techniques at various stages. The results indicated that the incorporation of cobalt ion into the ceria lattice could increase the dispersion of ceria, oxygen vacancies and promote the oxygen-storing and releasing capability of ceria, especially over the catalyst prepared by ultrasonic-assisted method. The accessible oxygen played an important role on the SRE reaction and resistant to carbon deposition. The Co-Ce(UH) catalyst was more active and selective, i.e., ethanol conversion achieved complete and hydrogen selectivity (SH2) approached 90% at 400 C. The high oxygen storage capacity (OSC) and high accessible oxygen for the Co-Ce(UH) catalyst allowed oxidation/gasification of deposited carbon as soon as it formed, and less coke was detected. However, cobalt ions were not easily incorporated into CeO2 lattice with less formation of oxygen vacancies when the yttrium was added, which led to decrease of activity and more formation of carbonaceous deposits.
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Jiu-XingHuang and 黃久倖. "Influence of oxygen vacancy reservoirs on resistive switching characteristics of oxide based resistance switching memories." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/33367771155989874222.
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Yu-LungChung and 鍾裕隆. "Influences of oxygen vacancy drifting on resistive switching behaviors in TiOx and TaOx based nonvolatile memories." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/xkcv88.
Full textAbstract:
博士國立成功大學
材料科學及工程學系
103
This dissertation is devoted to study the influence of oxygen vacancy on resistive switching behaviors in the valence change memories (VCM) and electrochemical metallization (ECM) memories. The VCM and ECM memory devices were prepared with the device structures of Pt/TiOx/Pt/TiOx/Pt and Ag/TaOx/Pt, respectively. The current-voltage (I-V) characteristics including the electroforming, unipolar resistive switching (URS), bipolar resistive switching (BRS) and reset current characters were investigated and understood in terms of the migration of oxygen vacancies in VCM and ECM system. In the first part this dissertation, the TiOx-based memory devices were fabricated for electrical and material characterization. A distinct unipolar but single-polarity resistive switching behavior is observed in a TiOx/Pt/TiOx active layer, formed by thermal oxidation of a Ti/Pt/Ti stack. Introduction of the Pt mid-layer creates two additional Schottky barriers, which mediate the band bending potential at each metal-oxide interface and attains a rectifying current conduction at the high resistance state (HRS). Experimental evidences proving the single-polarity switching behavior is a combination of bias-induced Schottky barrier modification and conduction filament construction, both associated with the bias-driven migration of oxygen vacancies. In addition, the rectifying conduction behavior is also observed with an AFM-tip as the top electrode (TE), which implies the rectifying property is still valid when miniaturizing the device to nano-meter scale. In the second part of this dissertation, we explored the electroforming and resistive switching behaviors in the Ag/TaOx/Pt trilayer structure under a continual change of temperatures between 300 K and 100 K to distinguish the contributions of Ag ions and oxygen vacancies in developing of conducting filaments. We found that either electroforming or resistive switching, a significantly higher forming/set voltages is needed as the device is operated at 100 K, as compared to that observed when operating at 300 K. The temperature dependence of the pulsed switching time (tsw) measurement results indicates that larger activation energy (Ea) of Ag diffusion in TaOx (EaeV) will lead to a faster decay of Ag ions mobility in TaOx with decreasing temperature. Thus, the disparity in electroforming/set voltages of Ag/TaOx/Pt operating at 300 K and 100 K can attribute to the mobility difference of Ag ions at 300 K and 100 K. This mobility difference of Ag ions also leads a presence of metallic filament at 300 K while a filament composed of oxygen vacancy and Ag is formed at 100 K. The presence of oxygen vacancy segment in the conducting filament also modifies the reset current from a gradually descending behavior (at 300 K) to a sharp drop (at 100 K). Furthermore, the characteristic set voltage (Vset) and reset current are irreversible as the operation temperature is brought from 100 K back to 300 K, indicating the critical role of filament constituents on the switching behaviors of Ag/oxide/Pt system.
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Chu, Fu Chuan, and 褚福荃. "Tuning oxygen vacancy distributions in High-K dielectric and scaling interfacial layer thickness for 16nm FinFET." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/q8s2eu.
Full textAbstract:
碩士國立清華大學
工程與系統科學系
105
There are several challenges about gate dielectric thickness scaling in MOSFET, such as increased equivalent oxide thickness(EOT) value due to the growth of interfacial layer(IL)、reliability, and oxygen vacancies induced threshold voltage(Vt) variation. This thesis consists of three parts to discuss the impacts on characteristics of gate stack process of 16 nm FinFET, which are interfacial layer desorption by high temperature annealing to reduce IL thickness and defects、reducing Vt variation by passivaMetal nitrideg oxygen vacancy at interface and reducing EOT by incorporating few oxygen vacancy into high-K with modulating oxygen content in atomic layer deposition(ALD), and scavenging IL by increasing oxygen diffusion with a post metal annealing(PMA) and effects of PMA temperature and nitrogen gas flow ratio on metal cap. In the first parts, desorption of IL was carried out at high temperature for different annealing periods in N2 ambient. The desorption of IL in I/O FinFET can be obtained, which in confirmed by the reduced EOT. With increasing annealing time the on current and transconductance of devices increase. Reduced off current、enhanced mobility and reliability for PFinFET are achieved. The improvement may be attributed to the densification of IL after annealing. However, off current、subthreshold swing(S.S.), and mobility in NFinFET degrade after annealing. Which may be due to more diffusion in source draun junctions and defect generation at interface caused by a high temperature annealing. In the second part, high-K deposition is formed with different Oxygen precursor and Metal precursor cycle time in ALD to tune the oxygen vacancy distribution. Results of Core FinFET show that the EOT decreases, but S.S.、mobility and reliability degrade for sample with oxygen vacancy rich at both interface and bulk. Sample with oxygen vacancy deficiency at interface shows improved Vt variance 、off current and reliability, but increased EOT induced degraded S.S. and on current. Sample oxygen vacancy rich at interface shows the reduced Vt roll off、reduced Vt variation and S.S. , and improved reliability. The on current also increases due to reduced EOT. In the third part, PMA on metal nitride cap were performed at varies temperatures and gas ambients. EOT slightly increases with increasing anneal temperature and decreasing flow ratio of nitrogen gas, which induced S.S. and DIBL degradation. It may be attributed to interfacial layer regrowth with increasing the annealing temperature. IL regroeth caused by oxygen diffusion from metal nitride cap is more by decreasing nitrogen gas flow ratio in ambient.
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Huang, Chien-Ming, and 黃千鳴. "First Principles Study of the Effect of Oxygen Vacancy and Mechanical Strain on the Photocatalytic Activity of Anatase TiO2." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/95510636079016948999.
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碩士國立臺灣大學
材料科學與工程學研究所
98
This study tried to use First Principles to study the effects of induced oxygen vacancies and mechanical strain on formation energy of oxygen vacancies, electronic density of states, band structure and optic absorption coefficient. Our results showed that in oxygen defective TiO2, the relative sites of vacancies have strong influence on the interaction between vacancies, which leads to totally different electronic and optical properties. For different oxygen vacancy concentration, the tendency of oxygen vacancy configuration differs, and this also makes difference on the properties. For TiO2-x system, when x, the concentration of oxygen vacancy, increases, the optical absorption coefficient also increases. However, when x is larger than 0.125, the configuration tendency of oxygen vacancies switches from B-type to A-type, and this leads to higher probability of defect level. Defect level may become the recombination center of electron-hole pairs and the life time of carriers is largely shortened. This has negative effects on photovoltaic conversion. On the other hand, we also used First-Principles Calculations to study the effects of mechanical strain on thin film anatase TiO2. We discussed the stress parallel to (001), (010) and (101) planes. The mechanical strain we induced on the system is no larger than 8%. We use this system to discuss how the deformation that induced by lattice mismatch affects the electronic and optical properties. Our result shows that when the stress is parallel to (001) plane, the biaxial tensile stress on [010] and [100] direction and uniaxial tensile stress on [100] direction decreases the band gap, and the absorption spectrum shift toward longer wavelength side. We also found out that if we induce uniaxial compressive stress on [001] direction parallel to (010) plane or uniaxial tensile stress on [010] direction and uniaxial compressive stress on [1 ̅01] direction parallel to (101) plane, there will be similar results. Otherwise, there is no significant improvement on photocatalysis ability. Our results may give an aid to the process of TiO2 photocatalyst.
38
McCrate, Joseph Michael. "Characterization of low density oxide surface sites using fluorescent probes." 2013. http://hdl.handle.net/2152/23078.
Full textAbstract:
Low density surface sites are believed to play an important role in processes occurring on oxide surfaces, including catalysis and particle and film nucleation. However, our understanding of the role and chemical nature of such sites play in these processes is limited by the inability to experimentally detect minority surface sites in many oxide systems. The research performed for this dissertation is focused on developing a surface science technique utilizing fluorescent molecules to titrate specific surface sites on planar fused silica surfaces in an ultra-high vacuum (UHV) environment. High sensitivity (low detection limit) is achieved by using derivatives of perylene, a high quantum yield fluorophore. High specificity is attained by employing perylene derivatives with functional groups designed to react chemically with and titrate various sites. In addition to titrating the well-studied hydroxyl sites with perylene-3-methanol (density ~ 10¹⁴ cm⁻²), which is used to establish the technique, the detection of strained siloxane sites (~ 10¹² cm⁻²), ) with perylene-3-methanamine and oxygen vacancy sites (~ 10¹¹ cm⁻²), ) with 3-vinyl perylene is demonstrated. Particle nucleation on oxides is suspected to involve defects that trap adatoms and form critical nuclei. Using this technique, the possible role strained siloxane and oxygen vacancy sites play in trapping adatoms during the nucleation of Ge nanoparticles on silica surfaces is examined.text
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Lin, YungHsun, and 林詠勛. "Construction of the Carrier Concentration(Oxygen-Vacancy and Cation-Substitution) Separation Model & Metal Doping Effect on the Carrier Concentration in Conductive Oxides." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/02238829690685739233.
Full textAbstract:
博士國立中央大學
化學工程與材料工程研究所
99
In this thesis, we would propose a method which can decouple and estimate the oxygen-vacancy concentration and the cation-substitution concentration in an extrinsic metal oxide thin film separately. Also, we further propose how the properties of the dopant affect the cation-substitution reaction and the oxygen-vacancy concentration by observing the annealing ambient effects. In chapter 3, we propose the method which could estimate the oxygen-vacancy concentration and the cation-substitution concentration by measuring the carrier concentration of the thin film, which is annealed in different oxygen partial pressure. In chapter 4, we would discuss that how the difference of the electronegativity and the electron configuration would govern the cation-substitution reaction and the oxygen-vacancy formation reaction n. And how do oxygen-partial pressure and annealing temperature affect on the oxygen-vacancy concentration and the cation-substitution concentration. We found that the electronegativity would affect the cation-substitution concentration when the thin films were annealed at different oxygen pressure. And the electron configuration would affect the band diagram and affect the oxygen-vacancy formation energy. Besides the basic electrical properties, we also study the optical properties of ITO based thin films. In chapter 5, we found that the as-deposited ITO/Al structure would absorb particular light (400 nm- 600 nm) after thermal treatment. It is because the Al diffusion in ITO would locate at the interstitial site and serve as a defect level. Yet, we found that the pre-annealed ITO thin film is a good diffusion barrier layer which can retard the Al diffusion. On the other hand, by measuring the transmittance of the M:ITO thin films, we found that the transmittance of the extrinsic metal oxide in the near-UV region could be enhanced by doping the ternary metal. The transmittance of ITO thin film at near-UV region (380 nm) could be enhanced 22 % after doping Ti. Thus, after applying the Ti:ITO thin film on near-UV LED as the current spreading layer, the light output power of the near-UV LED is enhanced 52.1 %.
40
Mtangi, Wilbert. "Electrical characterization of process, annealing and irradiation induced defects in ZnO." Thesis, 2012. http://hdl.handle.net/2263/30356.
Full textAbstract:
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
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