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

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

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.

Planned Impact

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.

Publications


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Koós A (2012) N-SWCNTs production by aerosol-assisted CVD method in Chemical Physics Letters
McCulloch DG (2012) The near edge structure of cubic boron nitride. in Micron (Oxford, England : 1993)
Nicholls R (2012) Low-loss EELS of 2D boron nitride in Journal of Physics: Conference Series
Nicholls RJ (2012) Boron-mediated nanotube morphologies. in ACS nano
Nicholls RJ (2015) Crystal Structure of the ZrO Phase at Zirconium/Zirconium Oxide Interfaces. in Advanced engineering materials
 
Description The findings from this project will be discussed under the headings of the initial objective for the project:



Development of pseudopotential-based DFT EELS modelling:



We have developed OptaDOS, a software package which allows a detailed analysis of density of states and theoretical spectra based on the output of a bandstructure code. It calculates projected and joint densities of states, core and low-loss electron energy loss spectroscopy (EELS) spectra and other optical properties. OptaDOS employs linear extrapolation and adaptive smearing (methods not available within any other code) to construct the (joint) density of states, which is then used as the basis of spectra property calculations. It is currently compatible with the output of CASTEP, which is freely available to UK academics. It will soon also be compatible with ONETEP and Abinit.



Application of modelling methods to interpret EELS spectra from B, N-doped carbon nanotubes.



EELS has been applied to understand how doping effects the structure and properties of boron-doped and phosphorus, nitrogen co-doped carbon nanotubes. The EELS signals have been used to measure the distribution of the dopants and the variations in their bonding state. The state of the dopants has then been related to observed variations in the morphology of the tubes.



In addition, we have combined the approaches described above with in-situ current measurements. EELS has been used to monitor the chemical composition of both nitrogen and boron-doped nanotubes whilst current was passed. In both cases, current-induced restructuring as a result of Joule heating is found to remove the dopant atom and cause changes in the electronic properties.



We also extended our study to several different layered materials: hexagonal boron nitride, tungsten disulphide and molybdenum disulphide. It has been possible to understand the experimentally observed change in the EELS spectrum as the number of layers in reduced by modelling the changes in the dielectric function and show that the changes in the EELS spectrum provide a probe of the number of layers.



Application of modelling methods to interpret EELS spectra from Zr oxides formed in nuclear fuel-rod cladding.



Since the discovery of a sub-oxide phase and its importance in explaining the break-away behaviour in zirconium nuclear fuel cladding materials, and the impact on fuel-rod lifetime, it has been a priority to identify the crystallographic structure of this phase. The sub-oxide layer is known to appear under high stress conditions which are also a prerequisite for the loss of the protective nature of the oxide. An understanding of how this phase forms could mean that we can design better alloys or optimise the operating conditions of the current materials. The plane-wave pseudopotential-based methods described above have provided sufficient speed for many potential structure candidates to be considered. By combining experimental EELS and electron diffraction with data simulated from this range of models, we have been able to identify the structure of this sub-oxide phase. A publication on this work is currently being prepared, and it is expected to have considerable importance in understanding fuel-rod degradation.
Exploitation Route The ongoing work in Oxford involving both doped carbon nanotubes and nuclear fuel-rod cladding already includes several industrial collaborators, and these collaborators will undoubtedly make use of this research. The OptaDOS code developed as part of this project is freely available to UK academics.



The findings of this work relating to the structure and properties of doped carbon nanotubes and the structure of zirconum oxides for nuclear fuel rods provide a basis for further development of these materials.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy
URL http://www-stemgroup.materials.ox.ac.uk/Projects/EELSModelling
 
Description The post-doc employed on this grant has used this outcomes from the grant in a number of schools outreach settings. The work is an excellent way of demonstrating the importance of mathematics to understanding materials.
First Year Of Impact 2014
Sector Education
Impact Types Societal
 
Title OptaDOS: For high quality theoretical DOS, Projected-DOS, Joint-DOS, Optics and core-loss spectroscopy 
Description This is the main output from the theoretical development part of funded project. It is a piece of software that allows spectra to be simulated from first-principles calculations of materials. 
Type Of Technology Software 
Year Produced 2012 
Impact Subsequently used in several publications relating to research following on from this grant. 
URL http://www.optados.org