Abstract text (incl. figure legends and references)

In recent years, two-dimensional transition metal dichalcogenides (2D TMDs) have been intensively studied to explore their electronic and optical properties. While basic research on these materials can be conducted with exfoliated monolayers and manually assembled heterostructures, the focus is more and more placed on the application of materials prepared with scalable bottom-up techniques such as chemical vapor deposition (CVD). However, CVD-grown materials can have very different quality in terms of defect densities, grain sizes, and the number of layers.

In this study, we discuss how transmission electron microscopy techniques help to elucidate the relation between the growth parameters, atomic structure, number of layers, and electronic and optical properties of TMDs and their heterostructures. We investigate mono- and few-layer MoS₂ and MoTe₂, as well as lateral MoSe₂-WSe₂ heterostructures. The TEM techniques we apply include high-resolution TEM and STEM imaging at low voltages (20–80 kV), electron diffraction, energy-dispersive X-ray spectroscopy (EDX), and electron energy-loss spectroscopy (EELS). The results from all techniques are compared with simulations.

We first address the question of the exact number of layers in few-layer TMDs. We demonstrate that 3D electron diffraction and momentum-resolved EELS allow for unambiguous identification of 1–4 layers of MoS₂ or MoTe₂ [1]. Secondly, we show that the density of intrinsic defects in TMD monolayers can be obtained from atomically resolved Cc/Cs-corrected TEM images. The evaluation of the defect density can be performed very efficiently using convolutional neural networks, which can be trained using simulated TEM images and are able to identify atomic positions and chalcogen vacancies [2]. Thirdly and lastly, we evaluate that lateral heterostructures of monolayer TMDs can be best investigated with STEM, using the high-angle annular dark-field signal as well as EDX elemental mapping. These signals can be used to analyse the widths of interfaces between TMDs with different transition metals like Mo and W.

In summary, a variety of TEM techniques have been used to study 2D TMDs and their heterostructures. Specific applications have been found for each technique, be it the determination of the number of layers, the analysis of intrinsic defects, or the width of interfaces in lateral heterostructures.

We acknowledge funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (GrapheneCore3).

[1] J. Köster, A. Storm, et al., Micron 160, 103303 (2022).
[2] J. Schulz, M.Sc. thesis, Ulm University (2022).