Two-dimensional (2D) materials have been the focus of research, driven by the diversity of physical properties discovered in these materials that could potentially be harnessed for developing high-performance electronic devices. However, the controlled fabrication of high-quality crystalline 2D materials still remains a challenge. Namely, numerous structural defects created during the growth process cause deviations from the properties theoretically predicted in defect-free models. To address this issue, a robust approach for evaluating the quality of grown 2D materials and understanding growth mechanisms and defect-induced properties is essential. Aberration-corrected scanning transmission electron microscopy (STEM) stands out as one of the most powerful techniques for structural and chemical analysis of 2D materials at the atomic scale. Recently, a new imaging mode called 4-dimensional STEM (4D-STEM) has emerged, allowing for the recording of a 2D diffraction pattern image at each electron beam scanning position. In nanobeam conditions, this imaging mode provides information about crystalline polymorphs, orientation, and polarity, which can be extracted from the recorded diffraction disk positions and intensities. On the other hand, in convergent beam conditions, the technique is sensitive to atomic-scale projected electric fields, potentially providing information on the electronic structure of the material. Recent advancements in direct electron detector technologies have enabled the precise measurement of the transmitted beam, and its deviation from the optical axis can be analyzed using the center-of-mass (CoM) technique, giving direct access to the local electric field. However, the lack of quantitative understanding and interpretation of CoM images is the main reason this imaging technique is not yet routinely used for the study of 2D materials. This PhD thesis aims to explore the use of the 4D-STEM technique for multiscale quantitative analysis of structural and electronic features in synthesized 2D materials, spanning analysis from micron to Ångström scales.
First, 4D-STEM is employed for large-scale structural mapping of epitaxially grown materials. In this context, anomalous contrast in Friedel pair diffraction spots is explored theoretically, with a focus on establishing optimal sample tilt conditions for enhanced detection of polarity, followed by experimental demonstration for several monolayer 2D TMDs (MoS2, WS2, WSe2), providing critical information about the large-scale distribution of 1D defects, namely inversion domain boundaries. In the second part of this thesis, 4D-STEM is used in convergent beam condition for atomic-scale mapping of projected electric field and potential in vanadium-doped WSe2. A quantitative agreement is achieved by comparing the experimentally measured fields to density functional theory based STEM image simulations, including the effect of residual probe aberrations. Finally, the negative charge induced by single vanadium dopant atoms is detected as a potential drop in the reconstructed potential maps, demonstrating that 4D-STEM can be used for imaging single dopant charge states in semiconducting TMDs.