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

Completed

Nuclear Fission and Fusion(Nuclear Fission, Nuclear supporting technologies)
Not Energy Related

Basic and strategic applied research

PHYSICAL SCIENCES AND MATHEMATICS (Metallurgy and Materials)

Not Cross-cutting

Dr PD Nellist
Materials
University of Oxford

Standard

EPSRC

01 April 2010

31 March 2012

24 months

£272,055

Materials sciences

South East

NC : Physical Sciences

Dr PD Nellist, Materials, University of Oxford

Dr N Grobert, Materials, University of Oxford
Professor C Grovenor, Materials, University of Oxford
Dr S Lozano-Perez, Materials, University of Oxford
Dr JR Yates, Materials, University of Oxford

Dr R Nicholls, University of Oxford

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 ofnitrogen 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

22/10/10