  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.

Materials Science;Applied Physics