Retrieval of material’s 3D structure using new phase-contrast STEM methods

相衬成像技术应用于扫描电子显微镜(STEM)不仅提高了电子剂量的效率，而且为纳米材料的研究提供了更多的可能性。相衬成像技术是在20世纪70年代提出的，近年来得到了发展。这些相衬成像技术主要分为三类：(A)用于研究位错、磁结构或电子结构的差分相衬(DPC)；(B)用于测量电荷密度分布的差分相衬(DDPC)；(C)用于生成扫描样品表面电势图的积分DPC(IDPC)方法。大多数基于DPC的方法不能产生各向同性的对比度传递函数，通常用于研究薄样品。我们认为IDPC方法很有前途，并希望在本项目中改进目前用于厚度相关研究的IDPC模式。此外，在理论研究的基础上，我们计划通过在STEM中应用像素化探测器来从最少的曝光次数中恢复厚样品的内部结构。结合理论工作和我们对(S)电子显微镜成像的剂量依赖关系的研究，我们想要发展一种基于离散脉冲编码的方法，以最小的电子剂量输入获得样品的三维结构，并在实验上实现。

Phase-contrast imaging techniques applied for scanning transmission electron microscope (STEM) not only improve the efficiency of electron dose compared with conventional imaging modes, but also provide more possibilities to investigate nanomaterials. The techniques for phase-contrast imaging were proposed in the 1970s and have been developed in recent years. These phase-contrast imaging techniques are mainly classified in three categories: (a) differential phase contrast (DPC) preferred for investigating dislocations, the magnetic structure, or the electronic structure; (b) differentiated DPC (DDPC) preferred for measuring charge density distribution; (c) integrated DPC (IDPC) method for generating the potential map of the scanned sample surface. Most DPC-based methods cannot produce an isotropic contrast transfer function and are usually applied for investigating thin samples. We find the IDPC method promising and would like to improve the current IDPC mode for thickness-dependent study in this project. Furthermore, based on theoretical studies we plan to retrieve the inner structure of a thick sample from a minimum number of exposures by applying a pixelated detector in STEM. Combining the theoretical work with our dose-dependent study on (S)TEM imaging, we would like to develop an IDPC-based method to acquire the three-dimensional structure of a sample with the minimal input of electron dose and realize it experimentally.