Inversion of dynamical electron scattering for atomically-resolved structural analysis

4D扫描透射式电子显微镜(4D-STEM)的方法包括扫描穿过样品的电子探针，并在每个光栅处记录衍射图。通过迭代算法，可以重建原子级结构和化学结构。在这个项目中，当代纳米结构被用来证明结构表征的精确度提高到皮米范围，并最大化化学敏感性，从而探索目前隐藏的结构-性质关系。通过将冻结声子近似下的散射问题描述为神经网络，消除了与热扩散散射(TDS)有关的持久模型破坏。这样的网络是在实验的基础上通过有效地计算关于任意单一物理参数的梯度来训练的，从而产生作为直接测量结果的原子位置和类型。该项目致力于四个主要目标：1.量化BaTiO_3中的铁电区，而O-和Ti-离子与它们的对称位置只有几个皮米的差异。由于具有1-2 nm厚的隧道结有望成为未来的数据存储器件，PM能级映射应变感生磁畴被认为可以揭示纳米层系统中磁畴形成的物理机制和关于观察到的铁电畴的不可切换性的器件失效。2.在原子尺度上定量测量了轴向InAs/AlAsSb纳米线异质结中的偏析，以了解高电子迁移率晶体管(HEMT)的电子性能。通过显式开发TDS，我们实现了比已建立的光刻技术更高的化学灵敏度，取代了Z-对比度STEM中的固定前向散射模型，并消除了界面上的复杂动态散射。3.从化学和结构上探索了合金化的二卤化物单分子膜Mo{S(X)Se(1-x)]。合金化的2D晶体允许一个新的自由度x来调节异质双层中的电子能带排列。关于第VI族位置的化学组成的梯度被用来以聚合的方式设计实验条件。低剂量的4D-STEM实验用于绘制VI族单体、同聚体和异二聚体的图谱，并使人们能够理解用可见光获得的光谱。4.对重建算法进行了扩展，使化学键在实验上变得可用。使用模拟和实验来开发额外的网络层并设置它们的参数化。通过实现ResNet模块，研究了网络对试件厚度的可微性。总之，开发了一种通用的重建方法，并在社区中发布以供科学使用。

The method of 4D scanning transmission electron microscopy (4D-STEM) consists of scanning an electron probe across a specimen, and recording a diffraction pattern at each raster position. By iterative algorithms, atomic-scale structure and chemistry can be reconstructed. In this project, contemporary nanostructures are used to demonstrate precision enhancement of the structural characterization to the picometre-range, and to maximise chemical sensitivity so as to explore currently hidden structure-property relationships. By formulation of the scattering problem in frozen phonon approximation as a neural network, persistent model violations as to thermal diffuse scattering (TDS) are eliminated. Such networks are trained based on experiments via efficient calculation of gradients with respect to arbitrary solely-physical parameters, yielding atom positions and types as the direct measurement result. The project addresses four main objectives: 1. Ferroelectric domains in BaTiO3 are quantified, whereas O- and Ti-ions differ from their symmetry positions by a few picometres only. Since tunnel junctions with 1-2nm thick layers are promising for future data storage devices, pm-level mapping the strain-induced domains is supposed to shed light on the physics of domain formation in nanolayer systems and device failure regarding the observed non-switchability of ferroelectric domains. 2. Segregation in axial InAs/AlAsSb nanowire heterostructures is measured quantitatively at the atomic scale, so as to understand the electronic performance of high electron mobility transistors (HEMT). By exploiting TDS explicitly, we achieve enhanced chemical sensitivity in comparison to established ptychography, the replacement of fixed forward scattering models in Z-contrast STEM, and the elimination of the complex dynamical scattering at interfaces. 3. Alloyed dichalcogenide monolayers Mo{S(x)Se(1-x)] are explored chemically and structurally. Alloyed 2D crystals enable a new degree of freedom x to tune the electronic band alignment in heterobilayers. Gradients with respect to the chemical composition of group-VI sites are used to design experimental conditions in paradigmatic manner. Low-dose 4D-STEM experiments serve to map group-VI monomers, homomers and heterodimers and enable the understanding of spectra obtained with visible light. 4. The reconstruction algorithm is expanded such that chemical bonding becomes experimentally accessible. Simulations and Experiments are used to develop additional network layers and setting up their parametrization. By implementing ResNet modules, differentiability of the network as to the specimen thickness is studied. In summary, a generic reconstruction method is developed and released for scientific use in the community.