Detecting charge transfer in defective 2D WS2 with electron ptychography

Abstract text (incl. figure legends and references)

Detecting the electron redistribution in a material is a challenging, but important task as it can reveal fundamental information about chemical bondings. Conventional electron microscopy techniques such as annular dark field scanning transmission electron microscopy (ADF-STEM) are only sensitive to the nuclei rather than the electrons. We demonstrate the use of electron ptychography to detect charge transfer in two-dimensional WS2 at atomic resolution. Compared to other phase contrast imaging methods, it has the advantage of higher dose efficiency, correcting residual aberration after data collection as well as distinguishing heavy and light elements with the additional ADF signal. Importantly for this analysis, we introduce a new method to analyse phases which is robust to different experimental parameters. This is crucial as the changes due to charge transfer are very small.

We investigate the charge transfer of single layer WS2 by comparing structural potentials obtained using the independent atom model (IAM) and density functional theory (DFT). Based on them, we extract the atomic phases using single side band ptychography (SSB) and compare them with the experimentally obtained phases. The phase extraction is performed via an optimization approach where a simplified simulation of the structure is matched to the target image. The simulation is obtained via convolution of a kernel corresponding to the contrast transfer function of SSB with a 2D point phase model. The method is demonstrated to be robust to noise and sample tilt.

We show that charge redistribution can be reliably detected and occurs in the pristine WS2 structure as well as in their defects. While in the pristine structure, Sulphur obtains electrons from Tungsten, S vacancies result in a back transfer of electrons from S to W. Interestingly, the charge transfer is directly related to the density of defects. Our studies open up a new route to analyse charge transfers towards a better understanding of the effects of chemical bonds.