Studies of distant clusters of galaxies

A technique of constructing crude, low-resolution Spectral Energy Distributions (SEDs) for galaxies in distant clusters, using a set of intermediate bandwidth filters and a CCD detector, is developed which is capable of redressing many of the problems which have previously beset work in this field. The technique has been used to study galaxies in the distant clusters 0016+16 (z = 0.54) and Abell 370 (z = 0.37).These SEDs are then used to individually classify each object in the CCD field, ascribing both an estimated redshift and a galaxy type. The SEDs have been extended into the rest-frame ultraviolet (~ 270 nm) by imaging high redshift galaxies in blue passbands. Monitoring the behaviour of the Colour-Magnitude effect in the optical and -ultraviolet (uv) regions, indicates the presence of a new class of object which exhibits excess emission in the uv whilst having optical colours similar to nearby E/SO galaxies. The significance of this uv-excess is addressed by examining the available uv spectroscopy of nearby early-type galaxies obtained from observations carried out on the International Ul traviolet Explorer satellite. This study, in conjunction with a series of crude evolutionary models, leads to the conclusion that the uv-excess is most likely a manifestation of evolutionary differences in the spectral properties of galaxies at high redshifts, resulting from increased levels of star formation. Having developed such methods for using distant clusters of galaxies as evolutionary probes, a catalogue of candidate distant clusters is constructed from high contrast copies of deep 4m photographic plates. Finally, a series of possible future observations bcised on such a resource, combining a wide range of techniques, is outlined

Utilising DualEELS to probe the nanoscale mechanisms of the corrosion of Zircaloy-4 in 350 °C pressurised water

Characterisation of materials utilised for fuel cladding in nuclear reactors prior to service is integral in order to understand corrosion mechanisms which would take place in reactor. Zircaloy-4 is one such material of choice for nuclear fuel containment in Pressurised Water Reactors (PWRs). In particular, the metal-oxide interface has been a predominant focus of previous research, however, due to the complex oxidation process of zirconium cladding, there is still no clear understanding of what is present at the interface. Using Scanning Transmission Electron Microscopy (STEM) and Dual Electron Energy Loss Spectroscopy (DualEELS), we have studied the corrosion of this material under conditions similar to those that could be encountered in service. It is shown that under all conditions, whether during faster oxidation in the early stages, slow growth just prior to the transition to a new growth regime, or in the faster growth that happens after this transition, the surface of the metal below the scale is loaded with oxygen up to around 33 at%. Approaching transition, in conditions of slow growth and slow oxygen supply, an additional metastable suboxide is apparent with a thickness of tens of nm. By studying changes in both chemical composition and dielectric function of the material at the oxide scale – metal interface with nanometre resolution, quantitative mapping could be achieved, clearly showing that this is a suboxide composition of ZrO and a Zr oxidation state close to +2

Spectrum imaging of complex nanostructures using DualEELS: I. digital extraction replicas

This paper shows how it is possible to use Dual Electron Energy Loss Spectroscopy (DualEELS) to digitally extract spectrum images for one phase of interest in a complex nanostructured specimen. The specific cases studied here concern Nb or V precipitates, a few nanometres in size, in high manganese steels. The procedures outlined allow the extraction of the precipitate signal from the Fe–Mn matrix, as well as correction for surface oxide and any surface carbon contamination. The resulting precipitate-only spectrum images are then suitable for quantitative analysis of the precipitate chemistry. This procedure results in much improved background shapes under all edges of interest, mainly as a result of the removal of the extended electron loss fine structure (EXELFS) from the elements in the matrix. This allows the reliable extraction of even tiny quantities of elements, such as low levels of nitrogen in some carbide precipitates. As well as being relevant to precipitation in steels, these techniques will be widely applicable to the separation of chemically-distinct phases in complex nanostructured samples, and can be viewed as a digital version of the extraction replica technique

Accurate measurement of absolute experimental inelastic mean free paths and EELS differential cross-sections

Methods are described for measuring accurate absolute experimental inelastic mean free paths and differential cross-sections using DualEELS. The methods remove the effects of surface layers and give the results for the bulk materials. The materials used are VC0.83,TiC0.98,VN0.97and TiN0.88but the method should be applicable to a wide range of materials. The data were taken at 200 keVusing a probe half angle of 29mradand a collection angle of 36mrad. The background can be subtracted from under the ionisation edges, which can then be separated from each other. This is achieved by scaling Hartree-Slater calculated cross-sections to the edges in the atomic regions well above the threshold. The average scaling factors required are 1.00 for the non-metal K-edges and 1.01 for the metal L-edges (with uncertainties of a few per cent). If preliminary measurements of the chromatic effects in the post-specimen lenses are correct, both drop to 0.99. The inelastic mean free path for TiC0.98 was measured as 103.6±0.5 nm compared to the prediction of 126.9 nm based on the widely used Iakoubovskii parameterisation

Getting the most out of a post-column EELS spectrometer on a TEM/STEM by optimising the optical coupling

Ray tracing is used to find improved set-ups of the projector system of a JEOL ARM 200CF TEM/STEM for use in coupling it to a Gatan 965 Quantum ER EELS system and to explain their performance. The system has a probe aberration corrector but no image corrector. With the latter, the problem would be more challenging. The agreement between the calculated performance and that found experimentally is excellent. At 200kV and using the 2.5mm Quantum entrance aperture, the energy range over which the collection angle changes by a maximum of 5% from that at zero loss has been increased from 1.2keV to 4.7keV. At lower accelerating voltages, these energy ranges are lower e.g. at 80kV they are 0.5keV and 2.0keV respectively. The key factors giving the improvement are an increase in the energy-loss at which the projector cross-over goes to infinity and a reduction of the combination aberrations that occur in a lens stack. As well as improving the energy-loss range, the new set-ups reduce spectrum artefacts and minimise the motion of the diffraction pattern at low STEM magnification for electrons that have lost energy. Even if making the pivot points conjugate with the film plane gives no motion for zero-loss electrons, there will be motion for those electrons that have lost energy, leading to a false sense of security when performing spectrum imaging at low magnifications. De-scanning of the probe after the objective lens is a better way of dealing with this problem

EELS at very high energy losses

Electron energy-loss spectroscopy (EELS) has been investigated in the range from 2 to >10 keV using an optimized optical coupling of the microscope to the spectrometer to improve the high loss performance in EELS. It is found that excellent quality data can now be acquired up until about 5 keV, suitable for both energy loss near edge structure (ELNES) studies of oxidation and local chemistry, and potentially useful for extended energy loss fine structure (EXELFS) studies of local atomic ordering. Examples studied included oxidation in Zr, Mo and Sn, and the ELNES and EXELFS of the Ti-K edge. It is also shown that good quality electron energy-loss spectroscopy can even be performed for losses above 9.2 keV, the energy loss at which the collection angle becomes ‘infinite’, and this is demonstrated using the tungsten L3 edge at about 10.2 keV

Aberration-corrected scanning transmission electron microscopy for atomic-resolution studies of functional oxides

Electron microscopy has undergone a major revolution in the past few years because of the practical implementation of correctors for the parasitic lens aberrations that otherwise limit resolution. This has been particularly significant for scanning transmission electron microscopy (STEM) and now allows electron beams to be produced with a spot size of well below 1 Å, sufficient to resolve inter-atomic spacings in most crystal structures. This means that the advantages of STEM, relatively straightforward interpretation of images and highly localised analysis through electron energy-loss spectroscopy, can now be applied with atomic resolution to all kinds of materials and nanostructures. As this review shows, this is revolutionising our understanding of functional oxide ceramics, thin films, heterostructures and nanoparticles. This includes quantitative analysis of structures with picometre precision, mapping of electric polarisation at the unit cell scale, and mapping of chemistry and bonding on an atom-by-atom basis. This is also now providing the kind of high quality data that are very complementary to density functional theory (DFT) modelling, and combined DFT/microscopy studies are now providing deep insights into the structure and electronic structure of oxide nanostructures. Finally, some suggestions are made as to the prospects for further advances in our atomistic understanding of such materials as a consequence of recent technical advances in spectroscopy and imaging