YBCO thin films were grown on c-axis and vicinal (001) STO substrates by pulsed laser deposition at the Cambridge Interdisciplinary Research Centre. The superconducting and structural properties of these films were examined by a variety of different analytical techniques. It was found that YBCO thin films grown on vicinal substrates required less energy (a lower laser fluence, or a lower deposition temperature) for optimal growth than the films grown on c-axis oriented substrates. A report on the effect the deposition rate had on the properties of the films is presented. It was found that to balance obtaining films with high T_cs and a smooth surface morphology, a deposition rate of 5Hz was required. Finally, experiments were conducted to observe how the film grows from nucleation to a chosen thickness of 200nm. YBCO thin films were grown to various thicknesses on 0°, 2° and 8° vicinal substrates to observe how the growth of films evolves and is influenced by the vicinal off-cut. The surface and properties of the films at these various thicknesses and on substrates of varying vicinality were analysed. It has been suggested in the literature that there is a critical thickness to consider, and this will be explained further in Chapter 6. The growth of the films on all three types of substrates was found to be very similar until this critical thickness is exceeded. A thickness range is determined where the formation of holes is believed to occur.

The technique of infrared-visible sum-frequency spectroscopy (SFS) has been used to investigate surfactants and self-assembled monolayers (SAMs) adsorbed on solid surfaces. In Chapter 1, the technique of SFS is introduced in terms of its general application and theory with comparisons drawn to other interface sensitive techniques. The experimental methodology and analytical approach are described in Chapter 2. While references are made to surfactants in Chapters 1 and 2, discussion of surfactant behaviour <I>per-se</I> is postponed until Chapter 3. In this chapter, <I>in-situ</I> investigations of surfactant adsorption from aqueous solution onto model hydrophobic substrates of octadecanethiol (ODT) are detailed. The focus is on the behaviour of novel sugar-based surfactants as a function of chain length, solution concentration and temperature. Monolayers of decanol and DDAB are studied at the same interface by way of comparison. Chapter 4 is the central chapter in terms of technique development, describing the extension of SES to monolayers on hydrophilic surfaces, principally mica. A novel experimental configuration is employed where the mica is back-coated with gold. This variant of SFS has been called the displaced metal surface or DIMS method to highlight the fact that interpretation requires careful consideration of the mica thickness. SAMs formed from octadecyltrichloro-silane (OTS) are used to aid the development of the DIMS method. The self-assembly process is then investigated for alkylsilanes with focus on reproducibility in monolayer preparation and water stability. OTS was also adsorbed directly on bare gold and studied by conventional SFS, highlighting the insensitivity of monolayer structure to the type of hydrophilic substrate and serving as a reference for the DIMS spectra. In Chapter 5, feasibility studies are described using SFS in non-aqueous environments. The solid/toluene interface is probed by DIMS SFS using an OTS monolayer, and by conventional SFS using a partially fluorinated substrate. Deuterated toluene was used in this work to allow transmission of the IR beam.

This thesis describes a study into the plasmonic properties of nanostructured metallic films. Structures are produced by electrochemically depositing metal through a self-assembled template of polymer micro-spheres. This versatile technique allows nano-structures made from metals which can be electrodeposited to be produced quickly and cheaply. Geometries ranging from arrays of shallow dishes, to sharp metallic spikes and encapsulated spherical cavities can all be produced on the same sample. This thesis presents an in-depth study into the properties of delocalised and localised surface plasmons on these structures. These plasmons can be tuned in energy by controlling the sample geometry and local dielectric environment. Techniques are explored for modifying the energy, absorption strength and field distribution of plasmon modes for specific applications. With an understanding into the plasmonic properties of the metallic nanostructures, research is undertaken to explore how the associated local electric-field couples to molecules adsorbed onto a sample surface. The role of specific plasmon modes in enhanced Raman scattering is identified, and then optimised using multilayer nanostructures with tailored plasmon modes. Finally, the use of flexible elastomeric substrates for mechanically tuneable plasmonic substrates is explored.

Due to the unavailability of bulk defect-free GaN substrates, GaN is grown hetero-epitaxially on substrates which have a substantial lattice mismatch. As a consequence, a large population of threading dislocations (TDs), typically 10⁹ – 10¹⁰ cm^-2, and other defects are generated in the GaN thin film. TDs act as non-radiative recombination centres. The main aim of this thesis is to investigate the effect of <i>in situ </i>silane dosing (simultaneous flow of SiH₄ and NH₃) and varying the growth parameters on reducing the density of threading dislocations. Optimising the growth parameters led to a TD density ~ 10⁸ cm^-2. Binding of threading dislocations through 90° has been found to be important to reduce their density and a new mechanism is described to explain the bending of all types of TDs through 90°. Moreover, a new modified growth technique for the reduction of TDs is proposed which might lead to a very low TD density in GaN during metal organic vapour phase epitaxy (MOVPE) growth. In recent developments, the combination of MOVPE and halide vapour phase epitaxy (HVPE) growth techniques show good promise for reducing the TD density below 10⁶ cm^-2: some of these results are also presented. Additionally, two new techniques based on transmission electron microscopy (TEM) and atomic force microscopy (AFM) are described for counting the density of all types of threading dislocations reliably. The origin of additional threading dislocations in AlGaN growth on GaN using a relaxed AIN interlayer is presented. The quality of the AIN templates grown on sapphire as a function of the initial growth conditions {i.e nitridation of substrate, tri-methyl ammonia (TMA) pre-dosing, etc} is described. The growth of GaN and AlGaN on this AIN template is briefly described. Finally, strain relaxation and the origin of misfit dislocations in ‘high’ indium content (20%) green single quantum well (SQW) and multi quantum well (MQW) hetero-structures are described and analysed.

No description available.

A detailed experimental study using magnetooptics has been performed into magnetic switching and domain structure predominantly in high quality ultrathin epitaxial Ag/Fe/Ag(001) films, for both in-plane and out of plane magnetisation. When the magnetisation is in-plane, magnetic switching has been found to proceed by a series of irreversible jumps of the magnetisation direction, each of which is mediated by the sweeping of domain walls. A simple phenomenological model has been developed which explains the switching and which highlights the role of magnetic anistropy and domain wall pinning by defects. Further micromagnetic modelling, combined with experiments, showed that the defects which determine coercivity are atomic steps on the Fe surface (named 'micropins'). A series of time resolved studies revealed that in addition to the micropins there also exists a second domain wall pinning mechanism, named 'macropinning' which is due to extrinsic defects such as scratches and other surface damage and which is not primarily responsible for coercivity. It has been found possible to engineer artificially the magnetic properties of a Permalloy film by introducing controlled macropins through lithographic structuring. The interplay between dipolar effects and intrinsic anisotropy leads to a novel domain structure during switching which could have important technological applications. In contrast to the in-plane magnetised case, in which the domain structure is an interim state which mediates the magnetic switching, the out of plane magnetised system was found to adopt a domain structure as its preferred ground state for certain temperatures and film thicknesses. These have been described by a magnetic phase diagram.

The work in this dissertation is focused on the insulating side of the La_1-xCa_xMnO₃ and Pr_1-xCa_xMnO₃ phase diagram, in particular on the low temperature ‘charge ordered’ phase. This phase exhibits a superstructure whose periodicity depends on the doping of the sample, among other factors. Firstly the microscopic nature of the charge ordered phase is probed using a variety of techniques including TEM, heat capacity measurements and neutron diffraction measurements. Secondly, the charge ordered phase is controlled via the manipulation of strain. In the first set of experiments, polycrystalline samples are subject to extrinsic effects, and so although it is possible to observe spatial variations in the superstructure, it is not possible to systematically control possible causes of the variation. By growing thin films it was possible to control the level of strain in the film, which is one of the extrinsic effects. This creates a spatial variation in the superstructure which could be observed. The results of this experiment can then be used to provide insight into the small scale nature of the charge ordering superstructure.

Metal clusters can be regarded as nanometre-scale building blocks for the construction of novel materials, but before this technology can be utilised to its full extent, the processes involved in the creation of nanometre size structures on a surface need to be more fully understood. This thesis describes investigations of the diffusion of metallic atoms and clusters on a graphite surface, on the nanometre scale. In chapter one, I review recent advances in the field of cluster physics, concentrating on the formation of clusters, both from beam sources and by epitaxial growth on a surface. Chapter two details a study of the nucleation and growth of Ag clusters on a highly oriented pyrolytic graphite surface using high resolution scanning electron microscopy (SEM). Atomic clusters of diameters between 4-20nm have been grown by the diffusion and aggregation of Ag atoms across the surface terraces. The growth of three dimensional clusters at the surface steps has also been investigated as a function of the substrate temperature, between the temperatures of 20<SUP>o</SUP>C and 200<SUP>o</SUP>C, and an atomistic theory has been used to calculate the activation energy for diffusion of the atoms along the steps on graphite. At higher temperatures the cluster growth has been confined to the steps, resulting in the formation of quasi one-dimensional cluster wires. The mobilities of atoms and clusters on the surface have also been investigated. Chapter three contains a study of epitaxially grown Ag and Au clusters on graphite surfaces, using a scanning tunnelling microscope (STM) in air. The average size of the clusters observed with the STM has been found to be smaller than that observed with the SEM. This discrepancy has been attributed to an interaction between the STM tip and the larger size clusters, resulting in their movement from the scanning area. Evidence for this process is presented, and a size dependent cluster-surface binding energy has been postulated to explain the results.

Quantitative low energy electron diffraction has been used to determine the surface structure of a range of adsorbates, from simple atomic species to more complex molecular adsorbates, on Ir{100}. Only the clean surface structures of Ir{100} have been studied by LEED, so firstly simple atomic adsorbates, oxygen and carbon, were studied to build up the knowledge base for this surface, before progressing onto more complex adsorbates. After determining the surface structures for oxygen and carbon, a simple molecular adsorbate, carbon monoxide, was studied, followed by a more complex molecular adsorbate, benzene. Adsorption behaviour on both the (1x5) and (1x1) surfaces of Ir{100} was considered. All of the adsorbates lift the (1x5) reconstruction, although only oxygen lifts it completely. On the (1x1) surface all the adsorbates form ordered structures, except benzene, which forms an ordered overlayer after thermal decomposition to benzyne. Oxygen adatoms are found to adsorb on bridge sites in a p(1x2) arrangement where rows of first layer iridium atoms pair up. In contrast, carbon is adsorbed on the four-fold hollow site with <I>c</I>(2x2) periodicity. Carbon monoxide also forms a <I>c</I>(2x2) overlayer but is adsorbed on the atop site: the molecule bonds through carbon with the C-O bond perpendicular to the surface. Formation of benzyne from benzene results in a <I>c</I>(2x4) LEED pattern. LEED analysis of this structure shows that the benzyne molecule is adsorbed on the bridge site and tilted 48° from the surface normal. This is the first full structure determination of adsorbed benzyne.

The quantitative results from a recent molecular beam study have been used to formulate an improved model which accurately reproduces the observed kinetic oscillations during CO oxidation on Pr {100}. The model includes an experimentally determined power-law which manifests as a flux-dependent sticking probability describing the CO-induced lifting of the hex reconstruction. The calculations reveal that the inclusion of the non-linear power-law in the modelling is critical in order to accurately reproduce the experimentally observed oscillations. The supersonic molecular beam scattering apparatus was used to extend the recent studies in adsorbate-induced restructuring processes on Pr{100} to the structurally similar Ir{100} surface. To this effect, the adsorption kinetics of CO, oxygen and hydrogen chemisorption on both the (1 x 5) and (1 x 1) surfaces of Ir{100} have been studied. Due to competition between desorption from the (1 x 5) phase and growth of (1 x 1) islands, the sticking probability of CO on the initial Ir{100}-(1 x 5) surface is strongly flux dependent at surface temperatures in the range 480 - 510 K. It is shown that, as with Pt{100}, this is due to a strongly non-linear dependence of the growth rate on the local CO coverage on the (1 x 5) substrate, with an apparent reaction order of around 5. The heat of adsorption of CO on both surface phases of Ir{100} has been determined. The zero coverage adsorption energy on the (1 x 1) substrate is 196 kJ/mol and on the (1 x 5) substrate it is around 150 kJ/mol. The formation of (1 x 1) islands during oxygen adsorption on Ir{100} occurs between two limiting surface coverages. Between 350 and 600 K the local oxygen coverage on the (1 x 1) is about 0.28 ML during the prevailing phase transformation, whereas it is 0.20 ML in the temperature range 700 to 900 K. This "biphasic" behaviour is explained by the enhancement of surface diffusion at temperatures above 650 K. The adsorption kinetics of oxygen on Ir{100} at 1080 K also how a pronounced flux dependence of the sticking probability due to a non-linear growth law for the formation of (1 x 1) islands. The average apparent reaction order for this process is around 4.5.