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

这里提出的研究旨在通过开发新的计算机模拟方法并通过使用这些方法来研究现实世界的材料问题，来进一步提高我们使用电子能量损失谱来解决材料科学中的实际问题的能力。我们已经在两种精心挑选的材料类型中确定了用其他技术证明极难研究的关键问题：碳纳米管的掺杂以及决定核燃料覆层合金抗氧化性的因素。我们对宏观材料性质与原子结构和成键的关系，以及我们如何通过操纵这些性质来控制性质的大部分理解，都是在很短的尺度上表征材料的技术发展的结果。一种特别强大的表征方法是测量电子通过薄样品时的能量损失，即所谓的电子能量损失谱(EELS)。电子损失的能量高度依赖于存在的元素，从而可以确定材料的组成。此外，光谱还包含有关原子如何相互化学键的信息。成键的性质强烈影响光谱中的精细细节，但用定量的方式解释这些细节并非易事。这项提议旨在开发和测试使用计算机模拟来预测实验材料系统光谱的方法，以便对从真实材料中观察到的光谱中的特征进行定量解释和解释。为了计算光谱中的特征，有必要首先计算材料本身的电子如何参与成键--这是一个固有的量子力学问题。目前最常见的方法是基于密度泛函理论(DFT)，它在精度和计算效率之间提供了理想的平衡。即便如此，在合理的计算时间内可以包含在模型中的原子数量仍然是有限的。使用DFT的有效实现，特别是使用所谓的伪势，可以改善这种情况。相对而言，较少使用赝势方法来模拟鳗鱼光谱，因为其他(所谓的全势或全电子)方法提供了一种更简单的方法，尽管速度较慢。我们建议通过实现计算EELS谱的新方法来增强赝势方法，以便只有成键的初始计算，而不是随后产生的EELS谱的计算是非常耗时的一步。该项目的一个关键目标是增加可以建模的系统的规模，以便能够解决真实的材料问题。然后，新开发的方法将用于对两种材料进行原则证明分析，在这些材料中，关键特征的表征被证明是非常有问题的。第一个涉及发展对碳纳米管中氮和硼杂质原子的添加如何控制其性质的理解。这些材料在一系列新颖的传感和计算应用中具有潜在的应用。第二个应用旨在通过研究锆合金包覆层氧化的关键机理来提高核燃料棒的寿命。最后，我们希望测试一个假设，即在样品后面放置一个透镜，以重新聚焦失去能量的电子，可能会允许直接成像成键的对称性。这样做的光学配置被称为能量过滤扫描共聚焦电子显微镜(EFSCEM)。要做到这一点，我们需要计算电子在材料中的散射方式，就像我们计算光谱所需的计算一样。如上所述开发的方法将在计算中非常有价值，以检验这一假设，以确定这是否是一个可行的实验，并为其分配未来的实验资源。

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.