Robert H L, Lobato I, Lyu F J, Chen Q, Van Aert S, Van Dyck D, Müller-Caspary K
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C), Forschungszentrum Jülich, Wilhelm-Johnen-Strasse, 52428 Jülich, Germany; 2nd Institute of Physics, RWTH Aachen University, Templergraben 55, 52062 Aachen, Germany.
Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.
Ultramicroscopy. 2022 Mar;233:113425. doi: 10.1016/j.ultramic.2021.113425. Epub 2021 Nov 13.
We report a study of scattering dynamics in crystals employing momentum-resolved scanning transmission electron microscopy under varying illumination conditions. As we perform successive changes of the probe focus, multiple real-space signals are obtained in dependence of the shape of the incident electron wave. With support from extensive simulations, each signal is shown to be characterised by an optimum focus for which the contrast is maximum and which differs among different signals. For instance, a systematic focus mismatch is found between images formed by high-angle scattering, being sensitive to thickness and chemical composition, and the first moment in diffraction space, being sensitive to electric fields. It follows that a single recording at one specific probe focus is usually insufficient to characterise materials comprehensively. Most importantly, we demonstrate in experiment and simulation that the second moment μ+μ=〈p〉 of the diffracted intensity exhibits a contrast maximum when the electron probe is focused at the top and bottom faces of the specimen, making the presented concept attractive for measuring local topography. Given the versatility of 〈p〉, we furthermore present a detailed study of its large-angle convergence both analytically using the Mott scattering approach, and by dynamical simulations using the multislice algorithm including thermal diffuse scattering. Both approaches are in very good agreement and yield logarithmic divergence with increasing scattering angle.
我们报告了一项在不同照明条件下，采用动量分辨扫描透射电子显微镜对晶体中的散射动力学进行的研究。当我们连续改变探针焦点时，会根据入射电子波的形状获得多个实空间信号。在大量模拟的支持下，每个信号都显示出具有一个最佳焦点，在该焦点处对比度最大，并且不同信号的最佳焦点有所不同。例如，在由对厚度和化学成分敏感的高角度散射形成的图像与对电场敏感的衍射空间中的一阶矩之间，发现了系统的焦点失配。由此可见，在一个特定的探针焦点处进行单次记录通常不足以全面表征材料。最重要的是，我们在实验和模拟中证明，当电子探针聚焦在样品的顶面和底面时，衍射强度的二阶矩μ+μ=〈p〉表现出最大对比度，这使得所提出的概念对于测量局部形貌具有吸引力。鉴于〈p〉的通用性，我们还分别使用莫特散射方法进行解析研究，以及使用包括热漫散射的多层算法进行动态模拟，对其大角度收敛进行了详细研究。两种方法非常吻合，并且随着散射角的增加产生对数发散。
