Towards robust and reliable short-range order measurement using electron spectroscopy

The atomic structure of amorphous materials is a key to many applications, whether optical coatings for interferometers, filters for narrow-band photodetectors, metallic glasses, or bioglasses. But reliable determination of the structure of glasses is still a major area for development, especially when they are in thin film form. For bulk glasses, beamline techniques like X-ray PDF (XPDF) measurement using short wavelength X-rays gives a good picture of the total scattering from all atoms. Combining that with Extended X-ray Absorption Fine Structure (EXAFS) gives a more atom-specific probe of local atomic environments. For thin films, transmission electron microscopy is more appropriate because of the ease with which sub-nanometre electron beams can be formed. Diffraction-based techniques like radial distribution function measurement from high angle electron diffraction (analogous to XPDF) or fluctuation microscopy for examining medium-range order (to about 1-3 nm) are useful methods for investigating the total scattering from all atoms. In principle, an analogy to EXAFS is possible with electrons, EXtended Energy Loss Fine Structure. However, whilst the effects of EXELFS are well known as a disturbance on the background in Electron Energy Loss Spectroscopy, it has not been extensively used for the determination of short range order around specific atoms. There are several reasons for this: various features of electron gun and microscope design can lead to perturbations of the continuum background in the spectrum that prevent accurate background fitting, shortcomings in the optical design of microscopes with regard to chromatic effects can also perturb the background at higher energy losses, and detectors can be noisy and give non-uniform readings across their range. Major advances in recent years mean that these problems can now be largely overcome and it may be possible to make significant progress in converting EXELFS from an intriguing possibility into a widely useable technique. This will include the use of new developments in the compensation of chromatic effects in electron microscope optics at Glasgow, a better understanding of how to work with gun stray scattering, and new developments in detection technologies for EELS (in collaboration with researchers at Drexel University, Philadelphia, PA). We will use this to understand the structure of glasses containing Si, Ti and possibly other cations in materials like SiO2, and mixed oxide glasses containing Ti, as well as investigating as to whether O atom environments can also be investigated by this technique. The resulting publications will both aid the understanding of structures in thin films of such materials, as well as driving the methodological development of the technique for application to a wide variety of glassy materials.
This work relates to the following EPSRC research areas:
Functional Ceramics and Inorganics - Especially glasses for optical coatings
Chemical Structure - Determination of structure in solids
Biomaterials - Especially bioglasses
Optoelectronic Devices and Circuits, Sensors and Instrumentation - both in understanding glasses for filters for wavelength selectivity
Superconductivity - amorphous superconducting thin film structure