Modelling and quantitative interpretation of electron energy-loss spectra using novel density functional theory methods

The research proposed here aims to further our ability to use electron energy-loss spectra to solve real problems in Materials Science by developing new computer modelling methods and by using these methods to study real-world materials problems. We have identified in 2 carefully selected materials types key problems proving extremely difficult to study with other techniques; the doping of carbon nanotubes and what determines the oxidation resistance of nuclear fuel cladding alloys. Much of our understanding of how macroscopic materials properties relate to atomic structure and bonding, and how we can control properties by manipulating these, is a result of the development of techniques to characterise materials on very short length scales. A particularly powerful characterisation method is to measure the energy lost by the electrons as they pass through a thin sample, so called electron energy-loss spectroscopy (EELS). The energy lost by the electrons is highly dependent on the elements present, allowing the composition of the material to be determined. Furthermore, the spectra also contain information on how the atoms are chemically bonded to each other. The nature of the bonding strongly affects the fine-scale detail in the spectra, but interpreting these details in a quantitative way is not straightforward. This proposal aims to develop and test methods of using computer modelling to predict spectra for trial materials systems to allow the features observed in spectra from real materials to be quantitatively explained and interpreted.To calculate the features in the spectra, it is necessary first to calculate how the material's own electrons are involved in bonding - an inherently quantum mechanical problem. The most common methods for doing this are currently based on density function theory (DFT), which provides an ideal balance between accuracy and computational efficiency. Even so, the number of atoms that can be included in a model for a reasonable computation time is still limited. The situation can be improved using efficient implementations of DFT, in particular using so-called pseudopotentials. Relatively little use has been made of pseudopotential methods to model EELS spectra because other (so called full potential or all-electron) methods provide a simpler, albeit slower approach. We propose to enhance the pseudopotential approach by implementing new ways of computing the EELS spectrum so that the only the initial calculation of the bonding, and not the subsequent computation of the resulting EELS spectrum, is the significant time consuming step.A key aim of the project is to increase the size of the system that can be modelled to allow real materials problems to be solved. The newly developed methods will then be used in proof-of-principle analysis of two materials where characterisation of key features has proved to be extremely problematical. The first involves developing an understanding of how the addition of nitrogen and boron impurity atoms to carbon nanotubes controls their properties. These materials have potential applications in a wide range of novel sensing and computing applications. The second application aims to improve the lifetime of nuclear fuel rods by studying the critical mechanisms of oxidation in zirconium alloy cladding. Finally, we wish to test the hypothesis that placing a lens after the sample to refocus the electrons that have lost energy may allow the symmetry of the bonding to be directly imaged. The optical configuration to do this has been called the energy-filtered scanning confocal electron microscope (EFSCEM). To do this, we need to calculate how the electrons are scattered in the material much like the calculations we need to compute the spectra. The methods developed as described above will be very valuable in calculations to test this hypothesis to decide whether this is a viable experiment to which to allocate future experimental resources.

We have identified a number of pathways by which the outputs of the proposed research will impact more widely than the more obvious academic research implications. These pathways are identified in the diagram presented in the attached Impact Plan, and are summarised here. The pathways may be identified as being direct (i.e. having an immediate, direct causal potential benefit) or indirect (where a link occurs indirectly by supporting other areas of science research). An obvious direct impact of the proposed research is the further training and development of a promising young scientist in this field (Nicholls). By the end of this project, we are confident that Nicholls will be a strong candidate for an independent research fellowship in a UK university. The economic direct impact of this research is the potential for commercialisation of its outputs as follows: (i) Development of modelling methods for EELS interpretation: A basic EELS modelling module is now available as part of the Materials Studio package from Accelrys (www.accelrys.com), and there is excellent potential for the commercial licensing of the much more advanced methods we propose to develop. A letter of support for this research from Accelrys is attached. (ii) Energy-filtered scanning confocal electron microscopy (EFSCEM): The commercial success of the confocal optical scanning microscope naturally leads to a great interest in confocal electron microscopy. Work strand 6 of this proposal is to indentify whether bonding effects can be observed in the EFSCEM. Success in this area would greatly enhance the prospects for commercially exploiting this IP through ISIS Innovation. The outputs of this research can also be identified as impacting indirectly in the areas of environment, economy and education. (i) Environment: Probably the biggest current challenge for human civilisation is the development of low-carbon energy. Nuclear energy has been identified (Meeting the Energy Challenge: Government White Paper January 2008) as being a vital contribution to the UK's low carbon energy portfolio, at least while renewables are further developed. Work strands 4 and 5 of this proposed research directly contributes to efforts to improve the burn-up (energy extracted per uranium mass) and to develop fuel assemblies that can operate under more severe fuel duty cycles, including higher coolant temperatures, higher discharge burn-ups and longer in-core residence times to produce significantly more energy. Carbon nanotube-based technologies have also been identified as having potential uses for photovoltaic applications, nanoelectronics, bio-engineering, fuel cells, catalyst supports, structural materials, fillers in composites, multi-functional nanomaterials, etc.(see Royal Society report Nanoscience and nanotechnologies, opportunities and uncertainties, July 2004, for which Grobert (Co-I) was one of the Working Group) (ii) Economy: The re-development of nuclear power in the UK as a significant contribution to a low carbon energy will necessarily generate significant economic activity. The UK is currently playing catch-up in this area having allowed its expertise base to decay to dangerous levels. In order to gain maximum advantage from the economic activity surrounding new nuclear build, it is crucial that we start to rebuild a foundation of people and capabilities in the area. Economic activity in nanotechnology already stands at many billions of $ annually worldwide and in 2008/9 the EPSRC committed over 75m of research grants to the areas in the UK. (iii) Education: Oxford Materials already engages significantly in outreach activities, described further at http://outreach.materials.ox.ac.uk/index.php. The details of these activities, and how they will be strengthened by the research proposed here, are described further in the Impact Plan.