Abstract text (incl. figure legends and references)

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS₂, MoSe₂, WS₂ and WSe₂, exhibit exciting electrical and optical properties due to their low dimensionality and are therefore promising candidates for next generation optoelectronic devices [1]. Comparable to graphene, TMDs consist of molecular layers which are bound to each other only by weak van der Waals interlayer interactions and are covalently bound within each layer. TMDs show varying physical properties depending on the number of stacked layers, such as a direct band gap for a monolayer of WSe₂ compared to an indirect band gap of its bulk form [2,3]. Many of these layer thickness-dependent properties are determined by the electric field and charge density distributions around individual atoms and defects within the crystal structure. However, only a few measurement methods allow characterization of electric fields with such a high spatial resolution. One of these is differential phase contrast (DPC) imaging in STEM, which, in combination with state-of-the-art Cs correction, allows the investigation of electric fields with subatomic resolution even at the sufficiently low acceleration voltage of 80 kV.

It is, thus, the objective of this study to image and quantify atomic electric fields and the field distribution of defects in mono- and multilayer WSe₂ using STEM-DPC imaging.

In order to investigate 2D WSe₂ by STEM-DPC, mechanically exfoliated WSe₂ flakes with different thicknesses are transferred to TEM grids. STEM-DPC imaging is performed using an eight-fold segmented STEM detector at an acceleration voltage of 80 kV. The measurement of electric fields by DPC in general relies on the measurement of the center of mass (CoM) of the bright-field intensity distribution. Due to Coulomb interaction of the incident electron beam with the electrostatic potential inside the specimen, a shift of the CoM in the detection plane can be measured in presence of electric fields. These electric fields can be quantified by the difference of intensity on opposing detector segments and an adequate calibration. Measurements are compared with corresponding multislice image simulations.

We show the atomic structure as well as the corresponding electric field and charge density distribution of 2D WSe₂, which are in good qualitative agreement with multislice image simulations. As 2D materials are prone to various defects which can influence physical properties, we also show the electric field distribution of defects and disclose characteristic changes in the electric field distribution in the vicinity of defects.

In conclusion, DPC measurements on atomically thin WSe₂ offer a great possibility to reveal the electronic structure achieve a deeper understanding of the optoelectronic properties of this material system.

[1] J. An, et al., Perspectives of 2D Materials for Optoelectronic Integration, Adv. Funct. Mater. 32 (2022)
2110119. https://doi.org/10.1002/adfm.202110119.

[2] J. Gusakova, et al., Electronic Properties of Bulk and Monolayer TMDs: Theoretical Study Within DFT
Framework (GVJ-2e Method), Phys. Status Solidi A. 214 (2017) 1700218.
https://doi.org/10.1002/pssa.201700218.

[3] T. Yan, et al., Photoluminescence properties and exciton dynamics in monolayer WSe₂,
Appl. Phys. Lett. 105 (2014) 101901. https://doi.org/10.1063/1.4895471.