When supported, thin films demonstrate remarkable structural transformations, with important implications for catalysis, sensors, electrochemistry, semiconductors or superconductors. At present, the tools available to characterize solid-solid systems cannot provide atomic level resolution of, for example mixed screw-edge dislocations. Therefore atomistic simulation can provide an invaluable complement to experiment. In this work atomistic simulation was employed to generate models of oxide thin films. First an atom deposition methodology was used to create an SrO thin film on a BaO(001) support. The evolution of the thin film from small clusters (submonolayer coverage), to five atomic layers, which includes cracks in its structure, was studied. Specifically, information related to growth and nucleation processes can be explored using this methodology. Secondly an amorphisation and recrystallisation methodology was developed to explore the more complex system, that of ceria deposited on zirconia and yttrium stabilized zirconia. Simulated amorphisation and recrystallisation involves forcing the thin film to undergo a transformation into an amorphous state prior to recrystallising and therefore the recrystallisation process rather than the (perhaps artificial) initial structure will dictate the final structure. The recrystallisation process enables the evolution of all the important structural modifications as the thin film evolves structurally in response to the support. These include dislocations (pure edge and mixed screw-edge), dislocation networks, grain-boundaries and defects (interstitials, vacancies and substitutionals, including complex defect association) all within a single simulation cell.

The focus of this research project was the investigation of the functional properties of thin films that incorporate a secondary nanoparticulate phase. In particular to assess if the secondary nanoparticulate material enhanced a functional property of the coating on glass. In order to achieve this, new thin film deposition methods were developed, namely use of nanopowder precursors, an aerosol assisted transport technique and an aerosol into atmospheric pressure chemical vapour deposition system. Aerosol assisted chemical vapour deposition (AACVD) was used to deposit 8 series of thin films on glass. Five different nanoparticles silver, gold, ceria, tungsten oxide and zinc oxide were tested and shown to successfully deposit thin films incorporating nanoparticles within a host matrix. Silver nanoparticles were synthesised and doped within a titania film by AACVD. This improved solar control properties. A unique aerosol assisted chemical vapour deposition (AACVD) into atmospheric pressure chemical vapour deposition (APCVD) system was used to deposit films of Au nanoparticles and thin films of gold nanoparticles incorporated within a host titania matrix. Incorporation of high refractive index contrast metal oxide particles within a host film altered the film colour. The key goal was to test the potential of nanopowder forms and transfer the suspended nanopowder via an aerosol to a substrate in order to deposit a thin film. Discrete tungsten oxide nanoparticles or ceria nanoparticles within a titanium dioxide thin film enhanced the self-cleaning and photo-induced super-hydrophilicity. The nanopowder precursor study was extended by deposition of zinc oxide thin films incorporating Au nanoparticles and also ZnO films deposited from a ZnO nanopowder precursor. Incorporation of Au nanoparticles within a VO: host matrix improved the thermochromic response, optical and colour properties. Composite VC/TiC and Au nanoparticle/V02/Ti02 thin films displayed three useful properties photocatalysis, photo-induced super-hydrophilicity and thermochromism.

The slow deformation of a conducting drop surrounded by a viscous insulating fluid subject to a uniform electric field is considered. Two analytic models are presented for inviscid drops. The first makes use of a time-evolving spheroidal shape along with an energy balance to determine the drop behaviour. For fields below a critical value there exist equilibrium shapes. In this case, the evolution of the drop to the equilibrium shape is obtained. Above the critical value no equilibrium shapes exist and the drop has a period of slow elongation before undergoing rapid expansion. The spheroidal model is shown to be accurate up to aspect ratios of about 5. The second model uses slenderbody theory to model the drop behaviour. A similarity solution, exhibiting a finite-time singularity is obtained. Finally, detailed numerical computations, based on a boundary integral formulation, are presented for inviscid and viscous drops. The deformation of the drop right up to breakup is obtained. The type of breakup seen depends on the viscosity ratio of the drop to the surrounding fluid, and on the electric field strength. The different types of breakup seen are small droplets being emitted from the ends of the drop with a charge greater than the Rayleigh limit, the formation of what appear to be conical ends with the subsequent ejection of thin jet-like structures, or the formation of thin jet-like structures without the conical ends. Also, local analytic solutions that allow for a conical end are derived. However, none of the analytic solutions seem to correspond well with the numerical results. Finally, the behaviour of the drop near the critical electric field strength is examined in detail and time scales for the drop evolution are determined.

No description available.

The magnetron sputtering process has become established as the process of choice for the deposition of a wide range of industrially important coatings. However, despite its successes, there are inherent limitations in the current process. The ion-to-atom ratio incident at the substrate, which has a profound effect on coating properties, cannot readily be varied using present technology. Consequently, film properties may not be optimal. The relation between the incident charge and the energy delivered to the surface is another parameter that cannot be controlled in classical magnetron sputtering systems. This in turn can lead to difficulties in controlling the reaction between reactive gases and deposited materials, controlling the surface status prior, during and after deposition, monitoring the natural growth of native oxides and their combined effect on the adhesion of the thin film and the optical and electrical properties of the coated surface.

Multi-nuclear solid state nuclear magnetic resonance has been used to investigate the local atomic structure of the relaxor ferroelectric materials, lead magnesium niobate titanate (PMN-PT) and sodium potassium bismuth titanate (NKBT). In addition to these two series of materials, numerous precursor and model niobate compounds have also been analysed in order to gain a insight into the structures and phases present in these materials. The PMN-PT series was investigated using 93Nb, 207Pb and 170 NMR techniques. A total of 14 PMN-PT samples, from pure PMN to PMN-90PT, were investigated in order to fully understand the transitions taking place over the entire compositional range. 9~b proved to be the most informative nucleus, owing to its high sensitivity to the changes occurring at the B-site of the perovskite structure. We discovered three distinct niobium environments. We then proposed a new randomsite random-layer model explaining the distribution of the cations among two different layers ß' and ß". The high level of correlation between the theoretical predictions and the experimental results suggests that there are actually two different ways that PMN-PT behaves, one for titanium concentrations less than 25% and the other for concentrations over 25%. This was also clearly visible in our PMN-PT spectra, as a sharp line present in titanium concentrations below 25%, that disappears in the concentrations above 25%. We have also tied in our results with the existing literature on PMN-PT to identify possible links to the dielectric response and phase transitions in the material. NKBT was investigated using both 23Na and 39K MAS NMR techniques. The 23Na data proved most informative and results were obtained at different fields and different spinning speeds. We were then able to extract calculated isotropic chemical shift values and quadrupolar parameters to understand the subtle changes taking place. The preliminary results hint that there are some interesting changes taking place around the morphotropic phase boundary in the material.

Calcium aluminate compound, 12CaO.7Al2O3, was prepared via an improved sol-gel technique in the form of thin film on magnesium oxide (MgO) single crystal substrate as well as powder. The microstructures of the films were observed before and after crystallization, and the effect of solution processing parameters, including the molar fractions of the ingredients, on the continuity of the films and the formation of surface defects was studied. An optimized sol-gel process using a new solution recipe was developed based on the microstructural observations. Homogeneous thin films of 12CaO.7Al2O3 with high critical thickness (~ 5 − 6 μm)were produced using this optimized technique. The chemical composition of the films was determined using energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Raman and Fourier transform infrared (FTIR) spectral analyses were employed in order to investigate the effect of heat treatment temperature on the crystallization of 12CaO.7Al2O3 film on magnesium oxide substrate. The results of the phase analysis show that a single-phase film of 12CaO.7Al2O3 is formed at a temperature of 1300 oC. A crystallized structure with well-defined grain boundaries is obtained after 2 hr of heat treatment at this temperature under normal air atmosphere. The phase formation of 12CaO.7Al2O3 in powder form was investigated via room-temperature and high-temperature X-ray diffraction (XRD) and crystallization of 12CaO.7Al2O3 and CaO.Al2O3 powders started taking place simultaneously at a temperature of ~ 900 oC. A comparison between the FTIR results of the films with XRD results of the powder proved the crystallization of 12CaO.7Al2O3 thin film to start at a higher temperature compared to the powder. Furthermore, a single-phase 12CaO.7Al2O3 tends to form in thin film on MgO substrate, whereas the formation of 12CaO.7Al2O3 is accompanied by the formation of secondary phases of CaO.Al2O3 and 3CaO.Al2O3. The optical absorption properties of the 12CaO.7Al2O3 films were investigated at different temperatures from room temperature to 300 oC and the experimental data were analysed in Tauc and Urbach regions. The optical band gap decreased from 4.088 eV at 25 oC to 4.051 eV at 300 oC, while Urbach energy increased from 0.178 eV at 25 oC to 0.257 eV at 300 oC. The relationship between the optical band gap and the Urbach energy at different temperatures showed an almost linear relationship from which the theoretical values of 4.156 and 0.065 eV were evaluated for the band gap energy and Urbach energy of a 12CaO.7Al2O3 crystal with zero structural disorder at 0 K.

No description available.

New thin-film solar cell materials and a greater understanding of their properties are needed to meet the urgent demand for sustainable, lower-cost and scalable photovoltaics. Computational techniques have been used to investigate Cu2ZnSnS4, CuSbS2 and CuBiS2 , which are potential absorber layer materials in thin-film photovoltaics. Their low cost, low toxicity and their constituent’s relative abundance make them suitable replacements for current thin-film absorbers, which are CdTe or Cu(In, Ga)(S, Se)2 based systems. Firstly, we have used hybrid Density Functional Theory (DFT) calculations to study CuSbS2 and CuBiS2. We calculate band gaps of 1.69 eV and 1.55 eV respectively, placing CuBiS2 within the optimal range for a viable absorber material. The density of states for both these materials indicate that formation of electron hole charge carriers will occur in the Cu d10 band. Consequently, photoexcitation leads to the oxidation of Cu(I). Secondly, we have derived interatomic potentials which describe the complex structure of Cu2ZnSnS4 accurately. We find that the Cu/Zn antisite defect represents the lowest energy form of intrinsic defect disorder. For these antisite defects, we find a preference for small neutral defect clusters, which suggests a degree of self-passivation exists. Investigations of Cu-ion transport find VCu migration is possible via a vacancy hopping mechanism. There are pathways which can be connected to give 3D long-range diffusion. Investigations of the Cu/Zn site disorder in Cu2ZnSnS4 find that configurations which are kesterite-like will dominate synthetic samples. However, perfectly ordered kesterite will not be formed due to entropic effects. The simulations indicate the stannite and stannite-like polymorphs are less favourable, and can only account for ≈2.5% of a sample. Investigations of the surfaces of Cu2ZnSnS4, suggest that the vast majority of the low index surfaces are dipolar and that only the (1 1 2), (0 1 0) and (1 0 1) surfaces have low surface energies.

A novel dislocation locking technique is used to study the behaviour of nitrogen and oxygen in silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen or oxygen diffuses to the dislocations. The stress required to move the dislocations away from the impurities is then measured. Measurement of this unlocking stress as a function of annealing time and temperature allows information on the transport of nitrogen and oxygen to be deduced. Despite being present in a concentration of just 3E14cm-3 in some specimens, nitrogen is found to provide substantial benefits to the mechanical properties of float-zone silicon (FZ-Si). The segregation of nitrogen at dislocations is stable to at least 1200 degrees centigrade and the unlocking stress measured at 550 degrees centigrade is of similar magnitude to that found previously for oxygen in Czochralski silicon (Cz-Si). The unlocking stress initially rises linearly with annealing time, before it takes a constant value. The rate of the initial rise is dependent on temperature and the 1.5eV activation energy found agrees with that found previously. The rate of the initial rise also depends on nitrogen concentration. In the 500 to 700 degrees centigrade temperature range, the unlocking stress is found to decrease linearly as the temperature at which the unlocking process takes place increases. The results of a pre-annealing experiment confirm that oxygen monomers and dimers in Cz-Si exist in thermodynamic equilibrium at 550 degrees centigrade. Numerical simulation of oxygen diffusion to dislocations allows values of the effective diffusivity of oxygen in Cz-Si with four different oxygen concentrations to be deduced. At 500 degrees centigrade, the effective diffusivity depends upon oxygen concentration in a way which is consistent with oxygen dimers being responsible for transport. The transport of oxygen in Cz-Si at 550 to 600 degrees centigrade is found to be unaffected by nitrogen doping at a level of 2.1E15cm-3. The dislocation locking technique has also been used to study the effect of high concentrations of shallow dopants on oxygen transport in Cz-Si in the 350 to 550 degrees centigrade temperature range. Oxygen transport has been found to be unaffected by a high antimony concentration ~3E18cm-3, but is found to be enhanced by, on average, a factor of approximately 44 in Cz-Si with a high boron concentration ~5E18cm-3. Furthermore, deep-level transient spectroscopy (DLTS) and high-resolution DLTS (HR-DLTS) are used to study the electrical activity of defects in silicon. A deep-level with an enthalpy of 0.50eV and a concentration of order 10E11cm-3 is found in n-type nitrogen-doped FZ-Si and n-type nitrogen-doped neutron transmutation doped FZ-Si. No additional deep-levels are found in either material, for which the detection limit is 6E10cm-3. No deep-levels are found in p-type nitrogen-doped Cz-Si, for which the detection limit is approximately 10E12cm-3. DLTS and HR-DLTS are also used to investigate the electrical activity of oxygen-decorated dislocations in Cz-Si and states associated with oxygen at dislocation cores have been identified.