Magnetite (Fe3O4) is of both scientific and technological interest because of its fascinating magnetic properties. It has a high Curie temperature of 860 K and a theoretical 100% spin polarization at the Fermi level. There are a variety of deposition techniques to form thin films of magnetite, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), iron oxidation, sputtering and so on. In comparison with other deposition methods mentioned above, electrodeposition has a key advantage of relatively low processing temperature. The intention of this work was to investigate magnetite (Fe3O4) thin films grown via an electrochemical route by using various kinds of characterization techniques, especially on morphology, chemical composition, structure and magnetic properties. Fe3O4 thin films were obtained by using a galvanostatic or potentiostatic deposition from simple aqueous solutions of ferrous salts. Iron oxide thin films have been grown at different current densities and temperatures onto polycrystalline copper substrates. XRD results indicate that Fe3O4 is formed at 90 oC at an applied current density of 0.05 mA·cm-2. Lower growth temperatures can cause the formation of another phase, a-FeOOH at a certain concentration of Fe2+ and pH buffer. Time-dependent growth of the iron oxides exhibits nucleation and coalescence. In order to obtain uniform Fe3O4 film surface, longer deposition times are needed. The influence of applied potential on the characteristics of the deposited iron oxide was examined. The formation of Fe3O4 in a low potential regime (< 100 mV) vs. gold reference electrode while iron oxyhydroxides such as goethite (a-FeOOH) and lepidocrocite (?-FeOOH) are favoured for E > 100 mV. The magnetic properties of the films were found to be strongly dependent on the deposition potential. The multi-layer structure of Fe3O4/a-FeOOH/Fe3O4 onto NiO/Ni substrates has been demonstrated via successive deposition. A TEM cross-section image shows a-FeOOH is coherently formed between two ferromagnetic layers. ADF-STEM micrographs show that Fe3O4 has a columnar structure and has less composition variation compared to that grown onto a polycrystalline copper substrate. Synchrotron techniques, i.e. x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD), were performed to examine the iron oxide film. Fe K-edge x-ray absorption spectra demonstrate that the films grown at low potential regime (< 100 mV) have a comparable valency state with the standard Fe3O4 sample. The identification of the iron oxide was further confirmed by using XMCD technique. The calculation of the asymmetry ratio suggests that the total magnetic moment increased with decreasing applied potential. In addition, vibrating sample magnetometer (VSM) data show that the magnetic response is somewhat slower for the iron oxide grown at higher potential regime. A change of pH in the electrolyte does not change the lattice constant and film morphology or texture but does affect particle sizes in Fe3O4 thin films. This decrease with the pH is due to the reaction of FeOH+ ions with molecular oxygen in electrolyte.

Electron energy­‐loss spectroscopy (EELS) is a powerful method for providing detailed information on the bonding, chemical structure and electronic structure of materials. In this work, EELS has been used to correlate variations in magnetic properties of cobalt ferrite films with film thickness and post‐processing conditions. Magnetometry performed on as‐deposited and oxygen post­‐annealed films has shown saturation magnetization (Ms) to be strongly affected by post processing. This has been attributed to an enhancement in superexchange by reoxidation and cation ordering processes during post­‐anneal. To date this has not been confirmed using nanoanalytical techniques. This work addresses this issue. In particular, it is of interest to determine local changes in the degree of inversion of the ferrite spinel in order to link local chemical changes to bulk magnetic properties. Two sample preparation techniques were used to produce electron transparent sections – conventional ion beam milling and focussed ion beam (FIB) milling using a dual beam system. The suitability of each technique is discussed in terms of, sample damage, thickness, reproducibility and reliability. Aberration corrected HRTEM was used to investigate the microstructure of the thin films. Lattice strain and defect strain were quantified at increasing distance from the substrate/interface in as‐deposited and oxygen post­‐annealed cobalt ferrite films and structural defects responsible for misfit accommodation were characterised. Local variation in cation valence and coordination cobalt in an oxygen post­‐annealed film was investigated by monochromated EELS of the iron and cobalt L2,3­‐edges in the electron energy­‐loss spectrum. A method to determine the spinel degree of inversion (λ) by multiple linear least squares fitting was developed using data acquired from reference materials. A commercially available full multiple scattering code (FEFF 8.2) was used to aid interpretation of reference spectra and the fitting technique used to determine λ was applied to the cobalt ferrite thin film in order to identify variations in λ.

The increasing demand for high performance electronics that can be fabricated onto large area substrates employing low manufacturing cost techniques in recent years has fuelled the development of novel semiconductor materials such as organics and metal oxides, with tailored physical characteristics that are absent in their traditional inorganic counterparts such as silicon. Metal oxide semiconductors, in particular, are highly attractive for implementation into thin-film transistors because of their high charge carrier mobility, optical transparency, excellent chemical stability, mechanical stress tolerance and processing versatility. This thesis focuses on the development of high performance transistors based on zinc oxide (ZnO) semiconducting films grown by spray pyrolysis (SP), a low cost and highly scalable method that has never been used before for the manufacturing of oxide-based thin-film transistors. The physical properties of as-grown ZnO films have been studied using a range of techniques. Despite the simplicity of SP, as-fabricated transistors exhibit electrical characteristics comparable to those obtained from ZnO devices produced using highly sophisticated deposition processes. In particular, electron mobility up to 25 cm2/Vs has been achieved in transistors based on pristine ZnO films grown at 400 °C onto Si/SiO2 substrates utilising aluminium source-drain (S-D) electrodes. A strong dependence of the saturation mobility on the work function of S-D electrodes and the transistor channel length (L) has been established. Short channel transistors are found to exhibit improved performance as compared to long channel ones. This was attributed to grain boundary effects that tend to dominate charge transport in devices with L < 40 μm. High mobility, low operating voltage (<1.5 V) ZnO transistors have also been developed and characterised. This was achieved through the combination of SP, for the deposition of ZnO, and thermally stable solution-processed self-assembling monolayer gate dielectrics. Detailed study of the temperature dependence of the operating characteristics of ZnO transistors revealed a thermally activated electron transport process that was described by invoking the multiple trapping and release model. Importantly, ZnO transistors fabricated by SP are found to exhibit highly stable operating characteristics with a shelf lifetime of several months. The simple SPbased fabrication paradigm demonstrated in this thesis expands the possibilities for the development of advanced simple as well as multi-component oxide semiconductors far beyond those accessible by traditional deposition methods such as sputtering. Furthermore, it offers unprecedented processing scalability hence making it attractive for the manufacturing of future ubiquitous oxide electronics.

Phthalocyanines (Pcs) form crystals whose structure and morphology depend on the growth conditions, leading to changes in the physical properties which are still little understood. Pc thin films and nanostructures have already been exploited in optoelectronic applications and could form the basis of spintronic devices but little or contradictory structural information is available because they are challenging systems to study. Hence the precise determination of the molecular order in these systems is of considerable interest both from a fundamental and technological point of view but requires a combination of complementary techniques. Crystalline powders of α-copper phthalocyanine (CuPc), α-metal-free phthalocyanine (H2Pc) and their mixtures are studied using powder X-ray diffraction (XRD) and found to be isomorphous and adopt a triclinic structure first proposed for α-CuPc (Hoshino et al., 2003). This information is used to study highly textured crystalline α-Pc thin films. The texture reduces the available crystallographic information but allows for the manipulation of the anisotropic physical properties. The Pc molecular plane lies 82±11° to the substrate when deposited on a weakly interacting substrate but at 7 or 9±5° when templated by a layer of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). Such an interpretation is different to all those previously given. The change in the texture is confirmed by high resolution transmission electron microscopy (HRTEM) of ultramicrotomed cross-sections of the films. The optimum TEM operating conditions were first determined on sections of CuPc single crystals which demonstrated an information limit of ~5Å with HRTEM. The technique was then applied to the films and the morphology, crystallinity and texturing of the layers is largely retained by the sectioning process. With further refinements it is hoped that this technique could be used to study the properties of interfaces and individual domains in multilayers and blends of organic thin films. Lastly the crystal structure of a new CuPc phase designated as η which forms nanowires as thin as 10nm and shows enhanced absorption in the infra-red (IR) is proposed. XRD, transmission electron diffraction (TED) and lattice potential energy (LPE) minimisation were used to determine the crystal structure: monoclinic P21/a, Z = 2, a = 24.8±0.2Å, b = 3.77±0.02Å, c = 13.2±0.1Å and β = 106±1°. The LPE minimisation was validated by correctly predicting the atomic coordinates of β-CuPc to within 0.05Å.

Recently there has been a great level of interest in the effect of interfaces in oxide ionic conductors with a view to eventual application in solid oxide fuel cells (SOFCs). Enhancements in electrical conductivity of one half to eight orders of magnitude have been reported in simplified thin film and multi-layered heterostructure systems. Often this is reported to be enhanced oxygen ion conduction with little supporting evidence. The aim of this work is to investigate these reports and develop an understanding of the underlying mechanisms. Several series of samples were investigated to achieve this goal. The first, designed to emulate an anomalous result from literature with alternating samarium doped ceria (SDC) and undoped ceria layers, featured an increasing total number of layers of equal individual thickness, and thus a constant interfacial density. This system featured no enhancement in conductivity and exhibited a level of tracer diffusion comparable with that in bulk SDC. Electron energy loss spectroscopy (EELS) studies, however, revealed a significant level of Ce III in the undoped layers. The second and third series used similar materials but tested the hypothesis proposed in many works, that conductivity enhancement was related to tensile strain in the conducting material at the heterointerfaces. The second, manipulating the strain at the interface by varying the dopant (Nd, Sm, Y) in films with alternating doped and undoped ceria layers and a range of interfacial densities. This series exhibited minimal change in conductivity with strain or interfacial density. The third series replaced the doped ceria with yttria-stabilised zirconia (YSZ) in order to achieve a higher level of tensile strain. This again featured minimal change in conductivity. Tracer diffusion and secondary ion mass spectrometry (SIMS) studies suggested that the undoped ceria layers featured vacancy-rich regions, close to the interfaces, possibly with compensating Ce III. The final series of multilayers comprised alternating praseodymium nickel copper gallate (Pr1.91 Ni0.71 Cu0.24 Ga0.05 O4) and SDC layers which exhibited a high level of conductivity and evidence of reduced levels of p-type conduction with decreasing SDC layer thickness, suggesting enhanced ionic conductivity. Oxygen tracer studies revealed, however, that the dominant charge carrier was not oxygen. Finally a study of the effect of dislocations in ionic conductors was performed on deformed single crystal YSZ. Impedance measurements revealed a small enhancement in conductivity in the orientation parallel to the dislocation cores however diffusion measurements showed a change that could be negated by the consideration of the inherent errors.

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