Digital Precession Electron Diffraction

利用衍射测定材料中原子排列的结构解是20世纪科学的杰出成就之一。它已经非常成功地应用于广泛的无机和有机材料和化合物，从复杂的离子晶体，如Mg2Sn到DNA的结构。x射线衍射法的主要局限性之一是，当它应用于尺寸小于x射线波长几倍的材料时，即几十纳米或更小的材料，它就失效了。这对于任何研究纳米级薄膜或颗粒或具有固有纳米级微观结构的材料的研究人员来说都是一个真正的限制。电子衍射不受这种方式的限制，因为它的波长比常用的x射线的波长小50倍以上。在这里，主要的限制是多重散射问题，它以复杂的方式改变了衍射光束的强度。虽然电子衍射的物理原理已经被很好地理解，但它对光束试样几何形状的微小变化（0.1度或更小）的敏感性使得对大量衍射光束的精确分析实际上是不可能的。电子显微镜技术、计算机控制和数据采集的最新进展意味着可以使用以前过于耗时的新方法。该项目旨在克服“多重散射问题”，通过使用自动采集大量的衍射图案，同时电子束在空心锥体周围进动，并结合数字图像分析。这种数字进动电子衍射（D-PED）产生的数据集包含了完整结构解所需的所有信息，克服了现有进动电子衍射的问题，即在一个进动周期内平均强度而丢失这些信息。利用动态电子衍射理论从电子衍射数据中提取准确的结构因子的方法，在应用于常规会聚束电子衍射时已经存在，我们将采用这些例程来分析我们的进动数据，目的是实现数据采集和分析的完全自动化。这将彻底改变结构解决方案领域，允许对目前无法解决的大量纳米尺度材料进行分析。我们将把D-PED应用于材料科学中的关键问题，并使采集和分析软件作为常规分析工具广泛地提供给其他研究人员。

Structure solution - determining the arrangement of atoms in a material - using diffraction is one of the outstanding achievements of 20th century science. It has been very successfully used on a vast range of inorganic and organic materials and compounds, from complex ionic crystals such as Mg2Sn to the structure of DNA. One of the main limitations of the principal method, X-ray diffraction, is its failure when applied to materials with sizes smaller than a few times the X-ray wavelength - i.e. tens of nm or less. This is a real limitation for any researcher working on nanometre-scale thin films or particles, or materials which have an inherent nm-scale microstructure.Electron diffraction is not limited in this way, since the wavelengths are more than 50 times smaller than those of commonly used X-rays. Here, the main limitation is the problem of multiple scattering, which changes the intensity of diffracted beams in a complicated way. Although the physics of electron diffraction are well-understood, its sensitivity to very small changes in the beam-specimen geometry (0.1 degrees or smaller) has made accurate analysis of large numbers of diffracted beams practically impossible.Recent advances in electron microscope techniques, computer control, and data acquisition mean that new approaches can be used which would previously have been too time-consuming. This project aims to overcome the 'multiple scattering problem' through the use of automated acquisition of a large number of diffraction patterns while the electron beam is precessed around a hollow cone, combined with digital image analysis. This Digital Precession Electron Diffraction (D-PED) produces a data set that contains all the information needed for full structure solution and overcomes the problems with existing precession electron diffraction, in which the intensities are averaged over a precession cycle and this information is lost. The means to extract accurate structure factors from electron diffraction data, using dynamical electron diffraction theory, already exist when applied to conventional convergent beam electron diffraction and we will adapt these routines to analyse our precession data, with the aim of fully automating both data acquisition and analysis. This will revolutionise the field of structure solution, allowing a vast range of nanometre-scale materials to be analysed which cannot be tackled at present. We will apply D-PED to key problems in materials science and make both the acquisition and analysis software widely available to other researchers as a routine analysis tool.