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

点击翻译按钮获取中文摘要

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.