Electron Nano-Crystallography: Precession Electron Diffraction in an Aberration-Free Environment

电子纳米晶体学：无像差环境中的进动电子衍射

• 批准号: EP/H017712/1
• 负责人: Paul Midgley
• 金额: $ 44.89万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FH017712%2F1
• 关键词: Electron; Nano; Crystallography; Precession; Diffraction

The vast majority of important materials are crystalline in nature. As such it is important that techniques are available which enable the crystal structure to be determined for new materials. X-ray and neutron methods are now very sophisticated and accurate but often fail to solve structures because the material is multi-phased, the crystal of interest has too small a volume or there is too much disorder at the lengthscale of the x-ray/neutron beam. Electron microscopy offers a way to record diffraction data from nm-sized regions overcoming the limitations of x-ray and neutron techniques. However electrons interact strongly with the crystal and the resultant diffraction intensities cannot in general be used directly to determine the crystal structure. Precession electron diffraction is a method by which the electron beam in scanned in a hollow cone above the specimen and then de-scanned below to give rise to diffracted intensities which are integrated through the Bragg condition; this geometry is equivalent to precessing the crystal about a fixed beam. The diffracted precession intensities are less prone to dynamical effects and may be used to solve crystal structures. In broad terms, the larger is the scan, or precession, angle, the smaller the effects of dynamical scattering on the diffracted intensities. In this proposal we plan to implement precession on an aberration-corrected electron microscope so that large precession angles can be used whilst retaining ultra-small nanoscale beams. On conventional instruments the spherical aberration of the probe-forming lens gives rise to very large probes for all but the smallest of precession angles.Precession will be optimised to generate the smallest probe for the largest precession angle. We will evaluate two ways of achieving this. The first requires dynamical control of the aberrations, changing the correction optics as the beam precesses. In theory this gives a very small beam but will be difficult to implement in practice. The second, probably preferred, option is to choose a lens defocus that enables a pass-band to match the precession angle and limit the distortion in the overall beam as it scans around the precession cone. Here the correction is fixed and far easier to implement in practice. We then plan to determine universal curves for the optimal precession angle for particular material types using a large number of simulations from many different structure types to establish trends and similarities. Running concurrently will be a development of precession theory using electron channelling and the concept of scattering strength from atomic strings. Normalizing data is crucial to successful structure solution and this will be investigated thoroughly. How best to incorporate high order (HOLZ) data, which contain high spatial frequency information will also be studied. The PI recently published a paper in Acta Cryst A detailing a new method to solve crystal structures using electron data combining 'charge-flipping' and phase symmetry. We plan to extend this to combine with elements of more conventional 'direct methods', a more statistical approach to structure analysis. By combining diffraction patterns recorded about a tilt axis it should be possible to solve crystal structures directly in 3D. This combination of tomographic acquisition and precession diffraction should prove to be a powerful tool. These methods can be applied to many crystal systems. In the proposal, we focus on three: The first in bismuth manganite, a multiferroic (ferroelectric and ferromagnetic) material, whose behaviour depends upon the crystal structure and oxygen sub-stoichiometry. The second is rhenium oxide with a negative thermal expansion dependent on the crystallography. Lastly, we plan to study metal-organic frameworks and how precession can be optimised to study this fascinating class of new materials, which have promise for catalysis, fuel cell technology and gas storage.

绝大多数重要的物质本质上都是晶体。因此，重要的是，技术是可用的，使晶体结构，以确定新的材料。X射线和中子方法现在非常复杂和准确，但通常无法解决结构，因为材料是多相的，感兴趣的晶体体积太小，或者在X射线/中子束的长度尺度上有太多的无序。电子显微镜提供了一种记录纳米级区域衍射数据的方法，克服了x射线和中子技术的局限性。然而，电子与晶体强烈相互作用，并且所产生的衍射强度通常不能直接用于确定晶体结构。旋进电子衍射是一种方法，通过这种方法，电子束在样品上方的空心圆锥中扫描，然后在下面去扫描，以产生通过布拉格条件积分的衍射强度;这种几何结构相当于围绕固定光束旋进晶体。衍射旋进强度不太容易受到动力学效应的影响，可以用来求解晶体结构。在广义上，扫描或进动角度越大，动态散射对衍射强度的影响越小。在这个提议中，我们计划在像差校正电子显微镜上实现进动，以便可以使用大的进动角，同时保留超小的纳米级光束。在传统仪器上，形成探针的透镜的球面像差导致对于除了最小旋进角之外的所有旋进角都产生非常大的探针。我们将评估实现这一目标的两种方法。第一种需要动态控制像差，随着光束旋进改变校正光学器件。理论上，这给出了非常小的波束，但在实践中将难以实现。第二种可能是优选的选择是选择透镜散焦，其使得通带能够匹配旋进角，并且在整个光束围绕旋进锥扫描时限制整个光束中的失真。在这里，校正是固定的，并且在实践中更容易实现。然后，我们计划使用来自许多不同结构类型的大量模拟来确定特定材料类型的最佳进动角的通用曲线，以建立趋势和相似性。同时进行的将是利用电子通道和原子弦散射强度概念的进动理论的发展。规范化的数据是至关重要的成功的结构解决方案，这将进行彻底的调查。如何最好地纳入高阶（HOLZ）数据，其中包含高空间频率信息也将进行研究。PI最近在Acta Cryst A上发表了一篇论文，详细介绍了一种使用电子数据结合“电荷翻转”和相位对称来求解晶体结构的新方法。我们计划将其扩展到联合收割机与更传统的“直接方法”的元素相结合，这是一种更统计的结构分析方法。通过结合关于倾斜轴记录的衍射图案，应该可以直接在3D中求解晶体结构。这种层析成像采集和进动衍射的组合应该被证明是一种强大的工具。这些方法可以应用于许多晶体系统。在该提案中，我们专注于三个：第一个是铋锰氧化物，一种多铁性（铁电和铁磁）材料，其行为取决于晶体结构和氧的亚化学计量。第二种是具有负热膨胀的氧化物，其热膨胀取决于晶体学。最后，我们计划研究金属有机框架，以及如何优化岁差，以研究这类迷人的新材料，这些材料有望用于催化，燃料电池技术和气体储存。

项目成果

Aberration-corrected and energy-filtered precession electron diffraction

像差校正和能量过滤进动电子衍射

• DOI: 10.1524/zkri.2013.1565
• 发表时间: 2013
• 期刊: Zeitschrift für Kristallographie - Crystalline Materials
• 作者: Eggeman A

Ultrafast electron diffraction pattern simulations using GPU technology. Applications to lattice vibrations.

• DOI: 10.1016/j.ultramic.2013.05.013
• 发表时间: 2013-11
• 期刊: Ultramicroscopy
• 影响因子: 2.2
• 作者: A. Eggeman;Andrew J. London;P. A. Midgley

Analytical electron tomography

分析电子断层扫描

• DOI: 10.17863/cam.477
• 发表时间: 2016
• 作者: Leary R