Diluted magnetic semiconductor oxides (DMSOs) have received great attention recently due to their outstanding applications in optoelectronic and spintronic devices. Ever since the initial observation of ferromagnetism at room temperature in cobalt-doped titania, extensive effort is concentrated on preparation of transition metal doped wide band gap semiconductors, especially Mn- doped ZnO. Compared to Mn-doped ZnO, magnetic interactions in SnO! and In!O! semiconductors have been underexplored. SnO! and In!O! semiconductors have many applications, owing to their high charge carrier density and mobility as well as high optical transparency. Investigation on electronic structure changes induced by dopants during the synthesis procedure can effectively influence magnetic interactions between charge carriers. In this work, a combination of structural and spectroscopic methods was used to probe as-synthesized SnO! and In!O! nanocrystals doped with Mn!! and Mn!! as precursors. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy are powerful techniques to explore formal oxidation state of manganese dopant, electronic environment, number of nearest neighbors around the absorbent, and bond lengths to the neighboring atoms. Analysis reveals the presence of multiple oxidation states in the doped nanocrystals, and establishes a relation between !"!! ratio and expansion or contraction of lattice parameters. !"!! Although doping semiconductors are crucial for manipulating the functional properties, the influence of dopants on nanocrystals structure is not well understood. Nanocrystalline films prepared from colloidal Mn-doped SnO! and In!O! nanocrystals through spin coating process exhibit ferromagnetic behavior in temperatures ranging from 5 K to 300 K. Magnetic transformation from paramagnetic in free-standing Mn-doped nanocrystals to strong ferromagnetic ordering in nanocrystalline films is attributed to the formation of extended structural defects, e.g., oxygen vacancies at the nanocrystals interface. Magnetic circular dichroism (MCD) studies clearly show that Mn!! occupies different symmetry sites in indium oxide, when bixbyite and rhombohedral In!O! nanocrystals (NCs) are compared.

No description available.

The purpose of the research work, presented in this thesis is to offer a detailed atomic level study of interfaces created by adsorption of organic molecules on metals and metal oxides to point out significant impact of substrate, dye structure as well as different mediators on the charge transfer at these interfaces, which is proven to influence the device performance to a great extent. Adsorption of organic photosensitive molecules on metals and metal-oxides is the main focus of this thesis. Phthalocyanines which are organic semiconductors offer a broad range of properties, such as thermal and chemical stability, high charge mobility and strong absorption coefficient in the visible and near-IR regions, which make them very attractive to be applied in various systems and devices. Fuel cells, organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) and solar cells are examples of phthalocyanine’s applications. The main focus of this work is to characterize the interfaces of Dye Sensitized Solar Cells (DSSCs). DSSC was invented by Michael Grätzel and Brian O’Regan in 1988. At the heart of this cell there is an oxide which is coated by a photosensitive dye. Under illumination, an electron is excited from HOMO to LUMO of the molecule, which can be further transferred to the conduction band of the oxide by a proper energy level alignment. The original state of the dye is regenerated by electron donation via the electrolyte, which usually is an organic solvent containing a redox couple e.g., iodide/triiodide. The iodide is regenerated by reduction of triiodide at the counter electrode. To improve the functionality of the cell, different additives can be added to the electrolyte. To mimic the interfaces of this cell, molecular layers of MPc (M: Fe, Zn, Mg) are adsorbed on both metallic surfaces, Au(111) and Pt(111), and rutile TiO2(110). Layers of iodine were inserted between metallic substrates and dyes to investigate the electronic properties and charge transfer at these multi-interface systems. 4-tert-butyl pyridine is a significant additive to the electrolyte and has proven to enhance the cell’s performance. This molecule was also adsorbed on Pt(111) and TiO2(110). Phthalocyanines were deposited by organic molecular beam deposition and 4TBP was evaporated at room temperature. Surface structures and reconstructions were confirmed by LEED measurements. Surface sensitive synchrotron radiation based spectroscopy methods, XPS and NEXAFS were applied to characterize these surfaces and interfaces. STM images directly give a topographical and electronic map over the surface. All measurements were carried out in UHV condition. When MPc was adsorbed on Au(111) and TiO2(110), charge transfer from molecule to substrate is suggested, while the opposite holds for MPc adsorbed on Pt(111). Moreover, stronger interaction between MPc and Pt(111) and TiO2(110) compared to Au(111) also demonstrates the effect of substrate on the charge transfer at the interface. The stronger interaction observed for these two substrates disturbed the smooth growth of a monolayer; it also resulted in bending of the molecular plane. Interaction of MPc with metallic surfaces was modified by inserting iodine at the interface. Another substrate-related effect was observed when MgPc was adsorbed on TiO2(110);  and -cross linked surfaces, where the surface reconstruction directly affect the molecular configuration as well as electronic structure at the interface. Besides, it is shown that the d-orbital filling of the central metal atom in MPc plays an important role for the properties of the molecular layer as well as charge transfer at the interface. Upon adsorption of 4TBP on Pt(111), C-H bond is dissociatively broken and molecules is adsorbed with N atoms down. Modification of surface by iodine, prevent this dissociation. In the low coverage of iodine, there is a competition between 4TBP and iodine to directly bind to Pt(111). Investigation on the adsorption of 4TBP on TiO2(110) illustrated that these molecules in low coverage regime, prefer the oxygen vacancy sites and their adsorption on these sites, results in a downward band bending at the substrate’s surface. / <p>QC 20131203</p>

In the present thesis the electronic properties of two organic molecules were studied by means of density functional theory (DFT) in connection to their possible applications in organic photovoltaics and molecular spintronics respectively. The first analysed system is the C60 derivative PCBM extensively used in polymer solar cells for the charge separation process. Since fullerenes have been shown to undergo modifications as a result of light exposure, investigating their electronic structure is the first step in elucidating the photodegradation process. The electronic excitations from core levels to unoccupied molecular orbitals reveal not only the empty level structure of the molecule, but provide additional information related to the chemical bonds involving a specific atom type. In this way, they represent a means of determining the chemical changes that the molecule might withstand. The electronic transitions from carbon 1s core levels to unoccupied states are explained for the unmodified PCBM by a joint theoretical (DFT) and experimental study using the near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The second investigated system is the transition metal phthalocyanine with a manganese atom as the metal center. Manganese phthalocyanine (MnPc) is a single molecular magnet in which the spin switch process can be triggered by various methods. It has been shown, for instance, that the adsorption of hydrogen to the Mn center changes the spin state of the molecule from 3/2 to 1. More interestingly, the process is reversible and can be controlled, opening up the possibility of using MnPc as a quantum bit in magnetic memory devices. Up to this date, the d orbital occupation in MnPc has been under a long debate, both theoretical and experimental studies revealing different configurations. In this thesis the electronic structure of the phthalocyanine is thoroughly analysed by means of DFT and the calculated results are compared to photoelectron spectroscopy measurements. The combination of theoretical and experimental tools reveals that in gas phase at high temepratures the molecule exhibits a mixed electronic configuration. In this light, the possible control of the specific electronic state of the central metal represents an interesting prospect for molecular spintronics.

The photosensitive molecule adsorption on titanium dioxide (TiO2) forms the so-called “dye sensitized TiO2” system, a typical organic/oxide heterojunction, which is of great interest in catalysis and energy applications, e.g. dye-sensitized solar cell (DSSC). Traditionally, the transition metal complex dyes are the focus of the study. However, as the fast development of the organic semiconductors and invention of new pure organic dyes, it is necessary to expand the research horizon to cover these molecules and concrete the fundamental understanding of their basic properties, especially during sensitization.In this work, we focus on two different photosensitive molecules: phthalocyanines and triphenylamine-based dyes. Phthalocyanines are organic semiconductors with symmetric macro aromatic molecular structures. They possess good photoelectrical properties and good thermal and chemical stability, which make them widely used in the organic electronic industries. Triphenylamine-based dyes are new types of pure organic dyes which deliver high efficiency and reduce the cost of DSSC. They can be nominated as one of the strong candidates to substitute the ruthenium complex dyes in DSSC. The researches were carried out using classic surface science techniques on single crystal substrates and under ultrahigh vacuum condition. The photosensitive molecules were deposited by organic molecular beam deposition. The substrate reconstruction and ordering were checked by low energy electron diffraction. The molecular electronic, geometric structures and charge transfer properties were characterized by photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and resonant photoelectron spectroscopy (RPES). Scanning tunneling microscopy is used to directly image the molecular adsorption.For phthalocyanines, we select MgPc, ZnPc, FePc and TiOPc, which showed a general charge transfer from molecule to the substrate when adsorbed on rutile TiO2(110) surface with 1×1 and 1×2 reconstructions. This charge transfer can be prevented by modifying the TiO2 surface with pyridine derivatives (4-tert-butyl pyridine (4TBP), 2,2’-bipyridine and 4,4’-bipyridine), and furthermore the energy level alignment at the interface is modified by the surface dipole established by the pyridine molecules. Annealing also plays an important role to control the molecular structure and change the electronic structure together with the charge transfer properties, shown by TiOPc film. Special discussions were done for 4TBP for its ability to shift the substrate band bending by healing the oxygen vacancies, which makes it an important additive in the DSSC electrolyte. For the triphenylamine-based dye (TPAC), the systematic deposition enables the characterization of the coverage dependent changes of molecular electronic and geometric structures. The light polarization dependent charge transfer was revealed by RPES. Furthermore, the iodine doped TPAC on TiO2 were investigated to mimic the electrolyte/dye/TiO2 interface in the real DSSC.The whole work of this thesis aims to provide fundamental understanding of the interaction between photosensitive molecules on TiO2 surfaces at molecular level in the monolayer region, e.g. the formation of interfacial states and the coverage dependent atomic and electronic structures, etc. We explored the potential of the application of new dyes and modified of the existing system by identifying their advantage and disadvantage. The results may benefit the fields of dye syntheses, catalysis researches and designs of organic photovoltaic devices. / QC 20111114

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (p. [151]-156).

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 156-168).

The As- and Mo- bearing secondary mineral phases formed during the neutralization of uranium mill wastes were studied for a variety of ore blends including current and future ore sources at the Key Lake milling operation, northern Saskatchewan, Canada. A lab-scale plant model was employed to characterize secondary precipitates obtained during the mill waste neutralization process. Three scenarios of ore blends were processed through the lab-scale plant to produce mill waste solutions for neutralization before combination into final tailings. Slurry samples (n = 12) were collected from the secondary precipitates formed during the neutralization of mill wastes (raffinate) by precipitation with Ca(OH)2 (slaked lime) from pH 1.5 to 10.5. Synchrotron based X-ray absorption spectroscopy of mill and lab-scale plant precipitates showed arsenate adsorbed to ferrihydrite was the dominant As mineral phase regardless of pH or sample blend (53-77%), with fractional contributions from ferric arsenates, and adsorption to aluminum phases (AlOHSO4, As(OH)3 and hydrotalcite). Molybdate adsorbed to ferrihydrite was the dominant Mo mineral phase, regardless of pH or sample blend, with fractional contribution decreasing with increasing pH, and minor contributions from calcium molybdate, ferric molybdate and nickel molybdate. These results were used in geochemical modelling to predict the source terms for these mineral phases in tailings facilities. Sequestration of As and Mo in the model showed solubility was controlled by adsorption to both Fe and Al oxide surfaces as well as by direct precipitation with other dissolved constituents (Ni, Ca and SO4).The models developed pH profiles of mineral phase precipitation to explain the solubility of As, Mo, Fe, Al, Mg and Ni during sequestration from pH 1.5 to 10.5 that were consistent regardless of ore blend used in simulations. Since adsorption of anions to the surface of ferrihydrite has been shown to slow conversion to crystalline forms of Fe oxides (goethite and hematite) and sequestration of arsenate effectively controls As solubility at high pH (pH >10), As-bearing mineral phases are expected to be stable for thousands of years. With adsorption as well as direct precipitation considered, Mo phases though effectively sequestering below pH 8, became unstable and released Mo back into the tailings porewater (pH >10), as predicted by the thermodynamic model. Historical data obtained from as-discharged tailings as well as previously published U mill tailings studies agree with these findings.

The safety case for a proposed geological disposal facility (GDF) for radioactive wastes relies upon a series of engineered and natural barrier systems to limit the migration of harmful radionuclides into the geosphere over geological timescales. Natural minerals, dominantly phyllosilicates, are expected to be the most reactive components of both the host rock and the clay-based backfill surrounding the highly radioactive waste canisters for as long as 100,000 years. Upon eventual canister degradation, alpha-emitting radionuclides will leach into the backfill material (and eventually beyond) and the constituent mineral systems will accumulate radiation damage upon radionuclide uptake and/or surface precipitation. The following study is an assessment of the structural and chemical effects caused by alpha-particle bombardment of silicate minerals, as proxies for the radiation stability of natural materials present in the near and far field of a GDF.Microscopy and spectroscopy studies from naturally occurring radiation damage accumulated in silicates over geological timescales (forming distinct 'radiohaloes') have shown that both alpha-particle and alpha-recoil bombardment results in altered unit cell dimensions caused by the accumulation of point (Frenkel) defects. In the example of highly damaged biotite, structural breakdown through the reorientation of discrete lattice crystallites was observed; the variability of the interlayer spacing within these regions reveal the potential for damaged mica to adopt the structure of phyllosilicate breakdown products over geological time. Controlled alpha-particle irradiation using the Dalton Cumbrian Facility's 5 MV tandem pelletron ion accelerator, combined with microfocus spectroscopy analysis has revealed the mechanisms of high fluence alpha-radiation damage across 2:1 phyllosilicate minerals (biotite and chlorite); reducing the layered structures into a series of loosely connected domains of alternating lattice expansion and collapse. Radiation induced Fe redox changes have been revealed, with Fe reduction apparent at relatively low alpha-particle doses, giving way to Fe oxidation at high doses. A 'redox gradient', based on alpha-particle energy deposition through a silicate structure has therefore been proposed. In addition, the increase in 'edge' sites generated by structural deformation has been shown to be favourable for the adsorption of the Se(IV) oxyanion to the mica surface. Comprising a body of additional work, a core sample has been extracted from a spent nuclear fuel pond wall at the decommissioned Hunterston A nuclear power station and the radioactive contamination on the painted core surface has been analysed by microfocus spectroscopy. The contaminant radiostrontium has been shown to be associated with the Ti rich pigment in the surface paint, resulting in a 'patchy' accumulation of radioactivity at the core surface. In addition, inert Cs reactivity experiments using the underlying concrete have shown that Cs is preferentially uptaken by phyllosilicates within the altered mafic clasts used in the concrete aggregate.

The current proposed method for the long-term management of intermediate and high level radioactive waste in the UK is via geological disposal. It is known that redox sensitive elements such as uranium and neptunium will significantly contribute to the total waste inventory. Recently, studies have indicated that both U and Np can be stabilised by interaction with minerals. Over long periods of time (1000’s -10,000’s years) steel canisters that encase radioactive waste in geodisposal systems will undergo anaerobic corrosion, potentially leading to the release of radionuclides, including U and Np. Anaerobic corrosion will also result in the formation of a number of oxide phases, including iron (oxyhydr)oxides e.g. magnetite and green rust. The interaction of U and Np with such forming iron (oxyhydr)oxides may lead to the sequestering of radionuclides in the environment through a range of processes such as adsorption to a mineral surface and incorporation into a mineral structure. Therefore the interactions between iron (oxyhydr)oxides and radionuclides are important to determine their fate if potentially released within the wider environment. In this study, the fate of U(VI) and Np(V) when in contact with a range of iron (oxyhydr)oxides was considered. These systems were selected to help understand the detailed mechanisms that may occur between radionuclides and iron (oxyhydr)oxides. XRD and TEM were used to characterise mineralogy, whilst acid digestions determined the distribution of U within the mineral phase. Synchrotron based XAS was used to determine oxidation state, site geometry and local bonding environment of the radionuclides associated with the mineral phases. The data suggests that: U(V) is stabilised and incorporated in octahedral coordination into both the magnetite and green rust structure in a uranate-like coordination; with increasing U concentration mineral formation favours uraninite and Fe(III) (oxyhydr)oxides; the limit of U incorporation into magnetite is 0.45 mol % U ± 0.23; Np(V) is reduced to Np(IV) on the iron (oxyhydr)oxide surface forming a bidentate binuclear complex; and that upon reoxidation, Np(IV) is partially reoxidised back to Np(V) but not released back into solution. These results highlight the significance in understanding the mechanisms when both Np and U are in contact with iron (oxyhydr)oxides which can contribute towards site environmental clean-up and waste management in the nuclear industry.