Modelling high-precision TEM phase imaging with Density Functional Theory

由于最近的技术进步，如像差校正，现代电子显微镜现在能够常规地实现亚埃分辨率。扫描透射电子显微镜（STEM）内的电子重叠关联成像提供了测量透射通过材料的电子波的相移的手段。照射光束的高度会聚性质导致以不同角度散射的波的重叠和干涉，并且干涉可以用于解决允许图像反转的相位问题。因此，重叠关联技术是衍射和成像技术的桥梁。重叠关联成像的一个应用是克服由物镜透镜的数值孔径所施加的衍射分辨率限制。我们最近表明，测量是准确的，足够精确，以提供一个本地的，真实的空间测量的电荷重新分配由于bonding.The定量解释这样的实验在很大程度上依赖于与模拟比较。该项目将开发一种新的第一性原理量子力学方法，用于预测和解释相敏图像，例如由重叠关联提供的图像。该方法将使用CASTEP材料建模代码中的密度泛函理论（DFT）。该项目的最初目标是评估密度泛函，纳入热效应，以及发展在厚样品中模拟图像的能力，即一个以上原子层的材料，其中弱相近似是无效的。一旦完成，该项目的第二个目标将是应用这些技术来解释实验数据，包括模拟六方氮化硼中的缺陷，钛酸锶中的界面和过渡金属氧化物。实验数据也将由约翰逊Matthey提供，他还将为学生提供3个月的实习。该项目的成果将为电子显微镜界提供一个有价值的工具，用于解释相敏图像-因此是一种在原子尺度上表征材料的强大方法。然而，它将有显着的好处，为材料建模社区的方法将提供一个独特的方法来评估性能的交换相关功能，最重要的近似在现代电子结构建模。该项目福尔斯EPSRC物理科学、能源和制造未来研究领域。

Modern electron microscopes are now capable of routinely achieving sub-Angstrom resolution, due to recent technological advancements such as aberration correction. Electron ptychography within the scanning transmission electron microscopy (STEM) provides means to measure the phase shift of an electron wave transmitted through a material. The highly convergent nature of the illuminating beam leads to overlap and interference of waves scattered at different angles and the interference can be used to solve the phase problem allowing inversion to an image. Ptychography therefore bridges diffraction and imaging techniques. One application of ptychography is overcoming the diffraction resolution limit imposed by the numerical aperture of the objective lens. We have recently shown that the measurement is accurate and sufficiently precise to provide a local, real-space measurement of charge redistribution due to bonding.The quantitative interpretation of such experiment relies heavily on comparison with simulation. This project will develop a novel first-principles quantum mechanical approach for the prediction and interpretation of phase sensitive images, such as those provided by ptychography. The approach will use density functional theory (DFT) within the CASTEP materials modelling code. The initial project aims are the assessment of density functionals, the inclusion of thermal effects, as well as developing the capability to model images in thick samples i.e. materials of more than one atomic layer, in which the weak phase approximation is not valid. Once this is complete the second aim of the project will be to apply the techniques to interpret experimental data including simulations of defects in hexagonal boron nitride, interfaces in strontium titanite, and transition metal oxides. Experimental data will also be provided by Johnson Matthey, who will also provide the student with a 3 month internship. The outcome of this project will provide the electron microscopy community with a valuable tool for the interpretation of phase sensitive images - and hence a powerful approach for the characterization of materials at the atomic scale. However, it will have significant benefits for the materials modelling community as the approach will provide a unique method for evaluating the performance of exchange-correlations functionals, the most significant approximation made in modern electronic structure modelling. In turn this may lead to the development of more accurate modelling approaches.This project falls within the EPSRC Physical sciences, Energy and Manufacturing the Future research areas.