Phase-contrast transmission electron microscopy at high resolution by combination of physical phase plates with correction of aberrations and its application on nano-materials

由于像差校正器的发展，高分辨率透射电子显微镜（HRTEM）是空间高分辨率材料分析的首选方法，可以分析到原子水平。由于由轻元素制成的材料通常对电子波的振幅和相位只有微弱的影响，并且对于HRTEM来说必须非常薄，因此这种图像通常只显示弱对比度，并且由于在图像形成过程中相位和振幅混合在一起而难以解释。物理相位板（PPs）提供了一种可能，以一种可控的方式放大图像中相衬的贡献，因为它已经在各种应用中得到了证明。在后焦平面上，电子波的散射部分和非散射部分在空间上是分开的，在这两部分之间会产生额外的相移。该项目致力于首次将像差校正和PP结合应用于HRTEM图像的相位对比放大和控制，并提高对它们的理解。结合像差校正器，一个PP允许在理想条件下的图像采集，无论是纯幅度或相衬贡献。为了实现这些目标，生产了两种不同的PPs并将其合并到像差校正显微镜中。静电扎克PP是一种通过隔离、屏蔽和开放电极产生非均匀静电电位的微结构器件。通过定位在后焦平面上并施加相应的电压，可以根据需要调节感应相移。所谓的无孔PP由一层非晶态碳薄膜组成，其上必要的相移是由后焦平面上的强零光束产生的。实现后，分析了组合成像特性，并将其应用于合适的样品。由于碳基及相关纳米材料的尺寸小，它们特别适合于HRTEM，但通常只表现出微弱的对比度。像差校正和PP的结合是为了改进它们的分析，特别是为了调查故意的修改或缺陷。对于提议的项目，n掺杂石墨烯的分析，功能化碳纳米管或无机过渡金属二硫族化合物的缺陷测定计划。项目范围不仅包括对分辨率最高的点缺陷的分析，而且还包括对由这种缺陷引起的主体材料的改性的研究，这些缺陷通常具有较大的尺寸，因此特别适合用PPs进行分析。

High-resolution transmission electron microscopy (HRTEM) is, thanks to the development of aberration correctors, the method of choice for spatially high-resolution material analyses down to the atomic level. Since materials made of light elements usually only have a weak influence on the amplitude and phase of the electron wave and have to be very thin for HRTEM, such images often show only weak contrast and are difficult to interpret because phase and amplitude are mixed during the image formation process. Physical phase plates (PPs) offer a possibility to amplify the contribution of phase contrast in the image in a controlled way, as it has already been demonstrated in various applications. In the back focal plane, where scattered and unscattered parts of the electron wave are spatially separated, an additional phase shift is induced between the two parts. The proposed project is dedicated to the first combined application of aberration correction and PP for the amplification and control of phase contrast in HRTEM images and for their improved understanding. In combination with an aberration corrector, a PP allows the acquisition of images under ideal conditions to which either pure amplitude or phase contrast contributes. To achieve these goals, two different PPs are produced and incorporated into an aberration-corrected microscope. The electrostatic Zach PP is a microstructured device which generates an inhomogeneous electrostatic potential by means of an isolated, shielded and open electrode. By positioning in the back focal plane and applying a corresponding voltage, the induced phase shift can be adjusted as required. The so-called hole-free PP consists of a thin film of amorphous carbon, on which the necessary phase shift is generated by the intensive zero beam in the back focal plane. After implementation, the combined imaging properties are analyzed and applied to suitable samples. Due to their small dimensions, carbon-based and related nanomaterials are particularly suitable for HRTEM, but often exhibit only weak contrast. The combination of aberration correction and PP is intended to improve their analysis and, in particular, to investigate deliberate modifications or defects. For the proposed project the analysis of N-doped graphene, functionalized carbon nanotubes or defect determination in inorganic transition metal dichalcogenides is planned. Not only the analysis of point defects with highest resolution is in the scope of the project, but especially the investigation of the modification of the host material induced by such defects, which usually has larger dimensions and is therefore particularly suitable for the analysis with PPs.