Imaging the electron charge density in monolayer MoS2 at the {\AA}ngstrom scale

TL;DR

This study uses advanced 4D-STEM imaging to map the electron charge density in monolayer MoS2, revealing the contributions of core and valence electrons and emphasizing the influence of probe shape on data interpretation.

Contribution

The paper demonstrates a method to distinguish core and valence electron contributions in charge density maps using 4D-STEM, highlighting the impact of probe shape on results.

Findings

Core electrons significantly contribute to charge density.

Valence electrons form a nearly featureless background.

Probe shape affects interpretation of charge density maps.

Abstract

Four-dimensional scanning transmission electron microscopy (4D-STEM) has recently gained widespread attention for its ability to image atomic electric fields with sub-{\AA}ngstrom spatial resolution. These electric field maps represent the integrated effect of the nucleus, core electrons and valence electrons, and separating their contributions is non-trivial. In this paper, we utilized simultaneously acquired 4D-STEM center of mass (CoM) images and annular dark field (ADF) images to determine the electron charge density in monolayer MoS2. We find that both the core electrons and the valence electrons contribute to the derived electron charge density. However, due to blurring by the probe shape, the valence electron contribution forms a nearly featureless background while most of the spatial modulation comes from the core electrons. Our findings highlight the importance of probe shape in interpreting charge densities derived from 4D STEM.