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The research work presented in this thesis comprises an electron microscopy and spectroscopy study of crystal defects that relate to the occurrence of different colours in natural and synthetic diamonds. Two principal lines of investigation have been covered, each with its own objective. The first aims to identify the source of brown colour in natural and synthetic diamond and the second to ascertain the distribution of colour inducing point defects in synthetic diamond. An outline of both areas of research is given below.1) Brown colour in natural and synthetic diamondsColour is a physical property that can be very difficult to characterise in diamond and consequently it receives regular attention from scientists working in the gem industry. In this work, the crystal structures of brown and colourless natural type IIa diamonds are compared along with brown coloured synthetic diamonds manufactured using the chemical vapour deposition (CVD) process. Numerous attempts have been made to trace the origin of brown tints in natural diamond, with the most likely sources, dislocations and nitrogen impurities, ruled out through the application of various analytical techniques. Recently more emphasis has been placed on the study of vacancy related defects in natural diamond and their influence on colour. Differences between the annealing characteristics of brown coloured natural and CVD diamonds suggest that the defect or defects responsible for the brown colour might be different for each type of diamond. The focus of this research work is the analysis of vacancy defects of the order of 1nm in size using aberration corrected scanning transmission electron microscopy (AC-STEM). The sub-nanometre size probe afforded by this technique allows such defect structures to be resolved much more readily than with conventional high resolution transmission electron microscopy (HR-TEM). Small-scale contrast variations are apparent in the lattice images of brown diamonds but not of the colourless variety. These features have been compared to simulated phase contrast images of vacancy clusters in diamond.2) Yellow / Green coloured synthetic diamonds grown using metal solvent catalystSynthetic diamonds for jewellery and industrial applications are routinely manufactured under high pressure-temperature (HPHT) conditions that closely resemble those found during the creation of natural diamonds. Although the manufacturing equipment can vary in design, the HPHT process that occurs inside the reaction vessel remains essentially the same. During processing, the carbon source material is dissolved into a molten metal and then precipitated onto tiny seed diamonds that are added to the reaction chamber. Much time and effort has been expended in refining this process to reduce impurities and defects in the finished diamonds. The presence of remnant transition metal atoms (e.g. nickel) in the crystal structure influences the electronic properties and in particular the colour of the diamonds. The position and configuration of these metallic defects has previously been studied by a variety of analytical techniques, including optical absorption-luminescence spectroscopy and electron paramagnetic resonance spectroscopy (EPR). These studies have proposed a number of optically active nickel centres at both substitutional and interstitial sites. Their association with vacancies and nitrogen atoms has also been highlighted. This work uses electron microscopy and spectroscopy to characterise the nickel defects in synthetic type 1b diamonds.

Since its discovery, the one atom thick material graphene has been at the centre of growing interest in two-dimensional materials. Due to its exceptional properties, graphene is a rich topic to explore by physicists, chemists, engineers and materials scientists. In addition to its use in the fundamental research, graphene is also a promising candidate for future electronics, photonics and energy storage devices.The project presented in this thesis was carried out to explore the structure of suspended graphene in particular in order to probe the metal-graphene interaction via Transmission Electron Microscopy, as most graphene applications require interfacing with metals. As the work was based on free standing graphene, graphene layers obtained by mechanical cleavage or growth on a substrate were transferred onto TEM-grids. Therefore, fabrication, suspended sample preparation and identification of graphene layers were first discussed for a better understanding of how to obtain high quality graphene, as this was essential for the rest of the project.Structural, topographic and chemical analysis of pristine suspended graphene layers were investigated in detail via Transmission Electron Microscopy and Scanning Tunnelling Microscopy. The latter technique was also employed for graphene on a substrate along with establishing annealing conditions for residue free graphene.Metal deposited suspended graphene layers were then investigated in the electron microscopes. Different metal behaviours were observed on the graphene surfaces for the same amount of metal evaporation. Generally, metals interact only weakly with graphene as they are not observed on clean (residue free) parts and are mainly clustered. On the other hand, graphene etching has been observed in the presence of metals. The etching was initiated with graphene vacancy formation as a result of the interaction between metal and carbon atoms on clean graphene. Once a vacancy was created, a hole quickly formed and eventually the graphene layers were destroyed. However, those holes created by metals were healed spontaneously either by non-hexagonal or perfect hexagonal rings. The possible etching and healing mechanisms of the suspended graphene were also discussed.

The application of gold and silver nanoparticles to areas such as medical research is based on unique optical properties exhibited by some metals. These properties are a direct consequence of localised excitations occurring at visible frequencies known as Surface Plasmon Resonances (SPRs). The exact frequency of an SPR induced in a nanoparticle can be 'tuned' in the optical range by, for example, changing the size of gold and silver nanoparticles, or by varying the relative concentrations of gold and silver within an alloy nanoparticle. Whatever the desired frequency, it is critical that the majority of nanoparticles exhibit the frequency within the resolution limit of the imaging system. The research presented here utilises the high resolution imaging and spectroscopy techniques of (Scanning) Transmission Electron Microscopy ((S)TEM) and Electron Energy Loss Spectroscopy (EELS). It is common practice to analyse the optical properties of alloy nanoparticles using techniques that acquire a single spectrum averaged over multiple particles such as Ultraviolet-Visible (UV-Vis) spectroscopy. However, this technique cannot detect any optical variation between the nanoparticles resulting from compositional change. In this research the author demonstrates through the use of EELS that the SPR can be determined for individual gold/silver alloy nanoparticles, for the purpose of determining the extent of their homogeneity. Importantly, the data presented here suggest dramatic variation in SPR frequency between particles and even within the same particle, indicative of large variations in alloy composition. This puts the assumption that alloying can be scaled down to the nanometre-scale to the test. In order to resolve and extract the SPR in both the pure gold and gold and silver alloy nanoparticles, the author has successfully applied multiple post acquisition techniques such as Richardson-Lucy deconvolution and Principle Component Analysis (PCA) to the EELS Spectrum Imaging (SI) acquisition method. Additionally, the valence band EELS data are supported by complementary electron microscopy techniques; Core loss EELS, Energy Dispersive X-Ray Spectroscopy (EDX) and High Angle Annular Dark Field (HAADF) imaging.

The research carried out here employed analytical and imaging transmission electron microscopy and scanning transmission electron microscopy to gain a good understanding of local structure and composition of semiconductor nanocrystals and quantum dots for electronics and optoelectronics applications. One of the world's most advanced analytical scanning transmission electron microscopes in the field, the Daresbury SuperSTEM, was used to scrutinise the structure and composition of the samples. Three nanostructure systems are investigated in this thesis: 1. Structures consisting of Ge-nanocrystals (NCs) in alumina. Here HRTEM suggests relaxed and twinned smaller NCs grown annealed at lower temperature compared to elongated non-faulty bigger NCs annealed at higher temperature. HRTEM also suggests a polycrystalline structure of the matrix. 2. With regards to the InAs/GaAs quantum dots (QD) the study aims in particular at elucidating QD formation by investigating samples grown with and without growth interrupt (GI). Diffraction contrast TEM shows formation of buried dots in the sample prepared with GI whereas for the sample without GI the immediate growth of GaAs after InAs inhibits diffusion and segregation of In adotoms, and no footprint of buried dots has been observed. HRTEM and HAADF show coherent QDs in the sample with GI and abrupt InAs/GaAs interfaces in the sample without GI. In executing energy electron loss spectroscopy (EELS) and geometric phase analysis (GPA) the distribution of In in InGaAs/GaAs QDs has been obtained in samples grown in the critical thickness regime for quantum dot formation. The highest In percentage achieved in the dots grown with a nominal fraction of 100% was ~70%. EELS shows variations in the In concentration within the QD structure and wetting layer 3. In the case of Er-doped Si-NCs in silica this research tries to provide an understanding of structure, composition and position of excess Si and Er in the silica matrix of materials prepared under different growth conditions and to correlate this information with the PL emission, all with the aim to find preparation routes for optimum optical efficiency for applications of this materials system in silicon photonics. High spatial correlation between Si-NCs, Er and O in the Er and Si co-implanted sample with strong indication of an Er-oxide/Si core-shell structure had been found. The lack of an Er-oxide plasmon indicates, however, that the shell structure and its interface with the SiNCs is highly defective and a likely cause for non-radiative recombination. The sample with similar excess Er and Si concentrations but prepared in a two-stage implantation and annealing process shows a 10 times improvement in the optical emission. Here no spatial correlation between Er and Si-NCs was found in core loss EELS. EELS and HAADF evidenced more highly, near-atomically dispersed Er in the matrix with no formation of a core-shell structure as compared to the co-implanted sample. No footprint of Er-silicide plasmon was observed by low loss valence band EELS investigation in the co-implanted sample.