Imaging the electron charge density in monolayer MoS2 at the Ångstrom scale

Abstract

Four-dimensional scanning transmission electron microscopy (4D-STEM) has recently gained widespread attention for its ability to image atomic electric fields with sub-Ångstrom 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 projected electron charge density in monolayer MoS2. We evaluate the contributions of both the core electrons and the valence electrons 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 and the need for smaller electron probes.

Direct Quantifying Charge Transfer by 4D-STEM: A Study on Perfect and Defective Hexagonal Boron Nitride

“…In principle, the stability of the integrated charges under probe effects may also be affected by the probe shape which is defined by high-order aberrations. Indeed, recent investigations have demonstrated that in TMD monolayers the 4D-STEM estimated charge density at atomic sites can be influenced by second-order 3-fold astigmatism C 2,3 , and this effect depends upon the azimuthal angle of the aberration. , Notably, an apparent charge inversion between different atomic species can be observed under specific orientations of the probe aberrations. To investigate this effect, we convolved the DFT charge density considering a probe function with a size of 1.15 Å, consistent with our experimental estimations, and a 3-fold astigmatism of 30 nm, corresponding to the tolerance for the spherical aberration corrector of the STEM microscope employed (see Section V in the Supporting Information).…”

“…32,34,35 Attaining a direct quantitative analysis of atomic-scale charge density variations would require probe sizes well below one angstrom, but this requirement currently exceeds the limits of current technology. 35 An alternative approach involves integrating experimental results with multislice image simulations based on ab initio calculations. Nevertheless, to prevent misinterpretations, it has been recommended that the probe size and shape should be known with very high accuracy.…”

“…Nevertheless, even in the simple case of weak phase objects, probe size effects might complicate the 4D-STEM image interpretation . These limitations have been highlighted on various studies on 2D materials. ,, Attaining a direct quantitative analysis of atomic-scale charge density variations would require probe sizes well below one angstrom, but this requirement currently exceeds the limits of current technology . An alternative approach involves integrating experimental results with multislice image simulations based on ab initio calculations.…”

Metrology for 2D materials: a perspective review from the international roadmap for devices and systems

“…Importantly, recent work based on 4D-STEM with center of mass (CoM) images and annular dark field (ADF) have been used to extract the projected electron charge density in monolayer MoS 2 . 28 Similarly, Wen et al , reported on the atomic scale structure and fluctuations of electric fields around edge atoms with different bonding states than bulk atoms in MoS 2 . 29 …”

The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure–property relationships of Pt-based fuel cells electrocatalysts

“…COM analysis, for example, can be used to obtain electric field and charge distribution maps. 123 This is particularly useful for investigating catalyst–support interactions, in cases where charge transfer between the two is significant. The structure and chemical composition of the catalyst/support system are frequently shown to govern the electronic structure and the resulting activity and stability of the material.…”