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.

ZnO thin film is a suitable material for the phosphor layer of green emission of the electroluminescence (EL) device. Therefore, the luminescence mechanism of green emission of ZnO thin film is a key issue to be investigated. In this thesis, ZnO thin films are deposited on SiO2/Si substrates using sol-gel method and sputtering technology, and then post-annealed by a rapid thermal annealing (RTA) process under various annealing temperatures (200¢J~900¢J) and atmospheres (vacuum, ambient atmosphere and oxygen). The physical and photoluminescence (PL) characteristics were first discussed. Secondly, the relationship between the chemical composition and the PL properties were also investigated to figure out the dominant luminescent center of ZnO thin film. Finally, ZnO thin film was applied as the phosphor layer of AC thin film EL device and the characteristics were discussed. According to the experimental results of ZnO thin film prepared using sol-gel method and RTA process, the XRD patterns show a preferred (002) orientation after annealing. The grain size became larger with the increasing annealing temperature. From PL measurement, two ultraviolet (UV) luminescence bands were obtained, and the intensity became stronger when the annealing temperature was increased. The strongest UV light emission appeared at annealing temperature of 900¢J in oxygen. The X-ray photoelectron spectrum (XPS) demonstrated that a more stoichiometric ZnO thin film was obtained upon annealing in oxygen and more excitons were generated from the radiative recombination carriers consistently, and resulted in the strong UV emission. However, no green emission was obtained from ZnO thin film prepared by sol-gel method. The XRD patterns also exit an excellent preferred (002) orientation of ZnO thin film deposited using sputtering and RTA process. The grain size of ZnO thin film annealed at 200¢J~500¢J increased with the increasing annealing temperature, and then exhibited a melting state with the temperature of 600¢J~700¢J. A large and complete grain was observed at the temperature of 900¢J. The PL spectrum illustrated that a stronger UV emission intensity appeared at annealing temperature of 500¢J. On the other hand, the green light emission could be obtained as ZnO films were annealed above 500¢J and reached a maximum intensity at 900¢J. Based on the XPS analysis, the O1s peak of ZnO film revealed that the concentration of oxygen vacancy increased with the annealing temperature from 600¢J to 900¢J under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900¢J. According to the experimental results manifested above, room temperature was used to deposit films to increase the ratio of Zn atoms inside the thin film when using sputtering technique to deposit ZnO thin film. With the modulation of the annealing parameters, stronger green light emission could be obtained. The luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies. In addition, only UV light emission of ZnO thin film prepared using sol-gel method was obtained because of the better stoichiometry.

No description available.

In recent years, due to the rapid development of electronic products, non-volatile memory has become more and more important. However, flash memory has faced some physical limits bottleneck with size scaling-down. In order to overcome this problem, alternative memory technologies have been extensively investigated, including ferroelectric random access memory (FeRAM), magneto resistive RAM (MRAM), phase-change RAM (PRAM), and resistive RAM (RRAM). All of this potential next generation non-volatile memory, the resistive random access memory has most advantages such as simple structure, lower consumption of energy, lower operating voltage, high operating speed, high storage time and non-destructive access, which make it be the most potential candidate of the next generation non-volatile memory. Many studies have proposed to explain the resistance switching phenomenon, which is due to the metallic filament or the oxygen vacancies. Therefore, in order to investigate the influence of resistance switching characteristic by metal or oxygen, we choose the non-metal contained boron oxy-nitride film as the insulator layer and successfully make the resistance has the switchable characteristic of this device. Furthermore, we improved the iv stability by using the Gadolinium-doped method in the boron oxy-nitride based film. In addition, we observed the negative current differential phenomenon during the set process, which can further controlled by lower operating voltage to achieve the interfacial resistance switching. We think that is due to the formation of nitrogen titanium oxide at the interface between insulator layer and titanium nitride electrode, which caused the Schottky barrier formation and reduced the current flow. In addition, current conduction fitting can also confirm this hypothesis. Besides, titanium nitride easily bond with oxygen ions; moreover, the oxygen ions can be easily disturbed at higher temperature ambient. We believed there may easily form the nitrogen titanium oxide layer in higher temperature environment; which also improve by a series of varied temperature experiments. However, this nitrogen titanium oxide layer formed naturally very easily, resulting in an inevitable problem of data retention time, which wish to be resolved in the future.

Non-stoichiometric barium titanate (BaTiO3) powder of TiO2-excess compositions has been investigated using both reducing sintering and acceptor-doping. Crystalline phases were analysed by XRD. Attention has been paid to the analysis of the corresponding sintered microstructure by adopting scanning and transmission electron microcopy. Reducing sintering was in the low oxygen partial pressure, so as to dominate the oxygen-deficient. According to the defect chemistry, the purpose of acceptor-doping was the same as reducing sintering. We look out for phenomena which may be indicative that oxygen vacancies generated by acceptor-doping and reducing sintering have resulted in the metastable retention of high temperature hexagonal-BaTiO3 to an ambient temperature. In the Mg-doped study investigated the possibility that Mg2+ substitutes on Ti4+ site rather than the Ba2+ site, as expected from the radii. According to the unknown phase was indexed a supercell of MgTiO3, that showed evidence of Mg2+ dissolves in BaTiO3 and occupies the Ba2+ site. To reduce in a hydrogen atmosphere was a high dark conductivity. The Ti3+ content was determined via colorimetry. Because of the defect chemistry led to oxygen-deficient h-BaTiO3, i.e.BaTi1-xTixO3-x/2. The observed volume expansion behavior under Ar-H2 atmosphere demonstrates the possibility of having various microstructures via control of oxygen partial pressure. The transformation matrix described the relation between the two reciprocal lattices of the twinning. Investigation of reciprocal lattices was shown that ordering oxygen deficient on the BaO3 layer in the twin boundary. There was evidence of XRD patterns and surface energy that explained more and more twins in the microstructure via control of the low oxygen partial pressure. According to this theory, lamellae twins were generated by oxygen-deficient. The hexagonal phase might be also expressed as the cubic BaTiO3 containing twin boundary at BaO3 planes every three layers. That demonstrates the possibility of hexagonal phase retention in BaTiO3 was oxygen vacancies.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23459号 / 理博第4753号 / 新制||理||1681(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 倉田 博基, 教授 島川 祐一, 教授 寺西 利治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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.

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

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.

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.