Digital Precession Electron Diffraction

Structure solution - determining the arrangement of atoms in a material - using diffraction is one of the outstanding achievements of 20th century science. It has been very successfully used on a vast range of inorganic and organic materials and compounds, from complex ionic crystals such as Mg2Sn to the structure of DNA. One of the main limitations of the principal method, X-ray diffraction, is its failure when applied to materials with sizes smaller than a few times the X-ray wavelength - i.e. tens of nm or less. This is a real limitation for any researcher working on nanometre-scale thin films or particles, or materials which have an inherent nm-scale microstructure.

Electron diffraction is not limited in this way, since the wavelengths are more than 50 times smaller than those of commonly used X-rays. Here, the main limitation is the problem of multiple scattering, which changes the intensity of diffracted beams in a complicated way. Although the physics of electron diffraction are well-understood, its sensitivity to very small changes in the beam-specimen geometry (0.1 degrees or smaller) has made accurate analysis of large numbers of diffracted beams practically impossible.

Recent advances in electron microscope techniques, computer control, and data acquisition mean that new approaches can be used which would previously have been too time-consuming. This project aims to overcome the 'multiple scattering problem' through the use of automated acquisition of a large number of diffraction patterns while the electron beam is precessed around a hollow cone, combined with digital image analysis. This Digital Precession Electron Diffraction (D-PED) produces a data set that contains all the information needed for full structure solution and overcomes the problems with existing precession electron diffraction, in which the intensities are averaged over a precession cycle and this information is lost. The means to extract accurate structure factors from electron diffraction data, using dynamical electron diffraction theory, already exist when applied to conventional convergent beam electron diffraction and we will adapt these routines to analyse our precession data, with the aim of fully automating both data acquisition and analysis. This will revolutionise the field of structure solution, allowing a vast range of nanometre-scale materials to be analysed which cannot be tackled at present. We will apply D-PED to key problems in materials science and make both the acquisition and analysis software widely available to other researchers as a routine analysis tool.

Our aim is to make electron diffraction the equal that of X-ray and neutron diffraction in general, and to out-perform these techniques at the nanoscale. Its role will be complementary to these and other techniques. Accurate determination of crystal structure is fundamental to many disciplines, including materials science, solid-state physics, nanotechnology, pharmaceuticals, biology, geology and chemistry. There are obvious applications in thin-film and nanoscale research, which is now the main focus of many of these disciplines, as well as the private sector (e.g. catalysis, pharmaceuticals, nanotechnology) and third sector institutions such as museums identifying compounds in historical artefacts. The contribution in these areas will have scientific, technological and ultimately social impacts.

In the short term, the impact will be felt within the community of scientists working on structure solution and crystallography as well as our chosen materials applications. In the long term, our aim is to develop an integrated system which can be used on any machine with the appropriate capabilities; world-wide, the potential is many thousands of systems. Manufacturers of electron microscopes and/or secondary systems are likely to be in the first tier of private sector companies to benefit from this research. Nevertheless, our guiding principle is to advance scientific understanding by maximising uptake of the technique rather than develop a direct commercial interest. This approach matches that taken by the X-ray crystallography community, who have developed a wide range of open source software, and are one of the main target audiences of this research. The UK is already one of the leading countries in identification of new crystal structures and exploitation of novel properties (e.g. one of the major repositories for new structures is the Cambridge Crystallographic Data Centre), and the addition of D-PED to the armoury of available techniques will give the UK significant competitive advantage in this area.

Results will be communicated through high-impact papers where the unique advantages of D-PED leads to genuine advances, in specialist journals in electron microscopy and crystallography, and presentations at national and international conferences. Preprints will be made available via arXiv.org and the Centre for Scientific Computing's internationally-indexed Eprint server. We aim to hold an international workshop on PED in 2013, providing a route for dissemination within the scientific community. It is highly desirable that the capabilities and limitations of the technique are evaluated on a wide range of materials, and this is best done by engaging a wide range of users who will be reached through this and similar meetings.
As shown by the letters of support, there is already interest in the oxide research community and we will use samples provided by these and other researchers where appropriate for development of the technique. This will provide a strong basis for future collaborations and development.

We will protect IP and copyright code, licensing our approach to manufacturers who wish to use it while retaining free access for academic purposes. We have entered into collaboration with JEOL as part of our drive to become a leading centre in microscopy and have support from Gatan Inc, giving two immediate exploitation routes. Where appropriate, microscope control scripts will be uploaded to the dm3 script database (www.felmi-zfe.tugraz.at/dm_scripts/) for use by other researchers.

The external impact activities will be jointly undertaken by the investigators and PDRA. All will be involved in progress reports and publications, as well as presenting results to conferences, workshops, and meetings. Dissemination across the internet will take place through the University website, the Science City Research Alliance as well as press releases and showcase i-casts through the Warwick Communications Office.