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Two sets of nanostructured Zinc Oxide (ZnO) thin films doped with varying nominal concentrations of rare earth (RE) ions were prepared by pulsed laser deposition (PLD). One set was doped with europium ions (ZnO:Eu³⁺) while the other was doped with erbium ions (ZnO:Er³⁺). The nominal concentration of RE ions ranged from 0.025 to 5 atomic %. The produced films were structurally, morphologically and optically characterised using different techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), combined excitation and emission spectroscopy (CEES) and X-ray photoelectron spectroscopy (XPS). All films were found to possess a single-crystal hexagonal structure and were strongly oriented along the c-axis. However, the crystallinity of the investigated films seemed to deteriorate as the concentration of the rare earth ions increased. This deterioration is assumed to be due to the local distortion of the ZnO structure (host material) caused by the insertion of the relatively large RE ions, hence inducing structural stresses. Importantly, XRD measurements showed that no other crystalline phases related to europium or erbium, such as Eu₂O₃ or Er₂O₃, were observed. Surprisingly, the ZnO lattice constant (c) tended to become smaller as more RE³⁺ ions were added to the films. An explanation is offered whereby this observation can be taken as further evidence that Zn²⁺ ions were successfully substituted by RE³⁺ ions. Interestingly, doping ZnO films with RE³⁺ ions of a nominal concentration of ≥ 0.5 at.% or higher exhibited a drastic effect on the optical properties of the host matrix (ZnO) in which the near band edge luminescence characteristic of pure ZnO completely disappeared. According to SEM images, morphological changes also occur as dopant concentrations increase. Well-defined grains (crystallites) were clearly seen in films doped with ˂ 0.5 at.% of RE ions. However, these grains became hardly distinguishable at higher RE ion concentrations. Typical intra-4f shell transitions of RE³⁺ ions were observed when these ions were non-resonantly excited with UV radiation, indicating that energy had been efficiently transferred from ZnO to the rare earth ions. A plausible physical mechanism for this energy transfer is proposed. The radiative optical centres of rare earth ions were studied by CEES. In these experiments, both sets of films exhibited multiple optical sites. ZnO:Eu³⁺ thin films were found to have two distinct optical sites with differing site symmetries, whereas up to four optical sites were detected in the ZnO:Er³⁺ films.

This thesis concerns experimental studies of nanocrystalline tungsten trioxide thin films. Functional properties of WO3 have interesting applications in research areas connected to energy efficiency and green nanotechnology. The studies in this thesis are focused on characterization of fundamental electronic and optical properties in the semiconducting transition metal oxide WO3. The thesis includes also applied studies of photocatalytic and photoelectrochemical properties of the material.     All nanocrystalline WO3 thin films were prepared using DC magnetron sputtering. It was found that structures like hexagonal and triclinic phase with different properties can be produced with sputtering technique. Thin film deposition has been performed using different process parameters with emphasis on sputter pressure and films that mainly consist of monoclinic γ-phase, with small contributions of ε-phase. Changes in the pressure are shown to affect the number of oxygen vacancies in the WO3 thin film, with close to stoichiometric WO3 formed at high pressures (30 mTorr), and slightly sub-stochiometric WO3-x, x = 0.005 at lower pressures (10 mTorr). Both stoichiometric and sub-stoichiometric thin films have been characterized by several structural, optical and electronic techniques.    The electronic structure and especially band gap states have been explored and optical properties of WO3 and WO3-x have been studied in detail. The band gap has been determined to be in the range 2.7-2.9 eV. Absorption due to polaron absorption (W5+  -W6+), oxygen vacancy sites (Vo -W6+), and due to differently charged oxygen vacancy states in the band gap have been determined by spectrophotometry and photoluminescence spectroscopy, in good agreement with resonant inelastic x-ray spectroscopy and theoretical calculations. The density of electronic states in the band gap was determined from cyclic voltammetry measurements, which correlate with O vacancy concentration as compared with near infrared absorption.      By combining different experimental methods a thorough characterization of the band gap states have been possible and this opens up the opportunity to tailor the WO3 functionalities. WO3 has been shown to be visible active photocatalyst, and a promising electrode material as inferred from photo-oxidation and water splitting measurements, respectively. Links between device performance in photoelectrochemical experiments, charge transport and the electronic structure have been elucidated.

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Vertically aligned nanocomposite (VAN) oxide thin films are unique nanostructures with two-phase self-assembled, heteroepitaxially grown on single-crystal substrates. Both phases tend to grow vertically and simultaneously on a given substrate with lattice matching in the system. The nanostructured thin film system could form different in-plane morphologies including nano-checkerboard, nanopillar in matrix and nanomaze structures. The VAN thin films with tunable vertical lattice strain and novel microstructures provide fascinating approaches to achieve enhanced functionalities. In this dissertation, the microstructure and vertical strain effect on low-field magnetoresistance (LFMR) have been investigated in heteroepitaxial La0.7Sr0.3MnO3 (LSMO):CeO2 and LSMO:ZnO VAN thin films with a vertical strain of 0.13 % and 0.5 %, respectively. We demonstrate that LFMR can be tuned by column width and vertical strain in these VAN systems, i.e., smaller column width and larger vertical strain could result in a larger LFMR in the vertical nanocomposite heteroepitaxial thin films. The physical mechanism of enhanced LFMR in LSMO-based VAN has been explored. Single-phase LSMO and LSMO-based VANs have been grown on different substrates with different secondary phase compositions. Substrate effect in single-phase LSMO films shows that LFMR tends to increase with grain misorientation factor because the cross-section of electron conduction paths reduces as grain misorientation factor increases. (LSMO)1-x:(ZnO)x VAN heteroepitaxial films without large angle grain boundary (GB) have been used to study the pure phase boundary (PB) effect on the LFMR. It shows that increased PBs tends to reduce the cross-section of the conducting path and thus favor the spin-dependent tunneling in nanomaze structures with ferromagnetic/insulating/ferromagnetic vertical sandwiches. Tilted aligned LSMO nanostructured films with artificial GBs have been designed to investigate pure GBs influence on LFMR. The results indicate that decoupling of neighboring ferromagnetic (FM) domains by artificial GBs is necessary to achieve enhanced LFMR properties; and the strength of the GBs can be controlled by post-annealing to tune the LFMR effect. The VAN heteroepitaxial films display excellent microstructure compatibility and strain tuning. Perovskite oxides can be combined with many other oxide materials to form VAN architectures. The microstructure and lattice strain in the unique heteroepitaxial VANs can be used to engineer and tune the existing/new functionalities.

Electrochromic films undergo a colour change when small ions and electrons are inserted into them, under the influence of an applied electric field. These films are also known as ion-intercalation electrodes, and may be incorporated into glazing structures more commonly known as 'smart windows'. Smart windows are that which may be used to control the amount of heat and light entering a building and may therefore be used to minimise the energy consumption associated with heating, cooling and lighting. The commercial success of smart windows, requires that they operate reproducibly at temperatures up to approximately 70ºC, for many tens of thousands of colouring and bleaching cycles. An understanding of the underlying kinetic processes over a wide temperature range is therefore needed, in order to determine suitable control strategies and switching conditions capable of fulfilling these requirements. The research detailed in this thesis has involved an investigation into the kinetic behaviour of ion-intercalation electrodes, and simulation of the electrical response as a means of developing a tool for predicting and then optimising electrochromic switching. More specifically, the electrical and optical properties of electrochromic thin films of WO3/TiO2 have been studied over a wide temperature range, appropriate for the operation of electrochromic windows. The magnitude of the voltages required for coloration and bleaching significantly reduces as temperature increases. Some irreversibility was observed at high temperature, as well as a reduction in coloration efficiency. Further investigation revealed that self-bleaching and irreversibility effects were caused by the presence of water, and this problem was exacerbated at high temperature. Post-experiment chemical analysis of a film sample revealed that some trapping of the inserted ions had occurred, however the amount of ions remaining in the film was much smaller than expected. The results suggested that a large quantity of the lithium ions injected into the film were lost to the electrolyte after many cycles, possibly accompanied by some film dissolution. Experimental work carried out in a dry-box showed that films may be cycled reversibly in a very dry environment, and the optical properties were independent of temperature under these conditions. Unfortunately, the conditions which led to reversible cycling and good electrochromic memory, also resulted in very long response times for film bleaching. This result implies that a good electrochromic memory and a fast response are mutually competitive aims. Data from high temperature experiments was simulated with a mathematical model and the mobility of lithium ions inside the electrochromic films was estimated in the process. The estimated diffusion coefficients agreed well with published values, and exhibited an Arrhenius dependence on temperature. Activation energies for diffusion were calculated and the results were very reasonable. Some deviation from ideal Arrhenius behaviour was observed for the estimated diffusion coefficients at high temperature. It is likely that the rate limiting mechanism changes from diffusive motion of ions at low temperature, to charge transfer at high temperature.

The surface force apparatus (SFA) has been an important technique for making direct force measurements and has contributed enormously to our understanding of colloidal interactions. The conventional SFA has been limited to measuring forces between solid surfaces, until recently when a modified SFA was developed at the Ian Wark Research Institute [1]. A fluid drop (mercury) is introduced into the apparatus which allows a range of deformable surfaces to be studied in the SFA. This project is an extension of this technique. Interactions between a mica sheet and a mercury drop are studied, including the modification of mercury with self-assembled monolayers (SAMs) of thiol surfactants, and the drop deformation due to non-equilibrium adsorption effects and hydrodynamic forces.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references (ℓ. 70-73)