Revealing the stacking configuration of ideal Moiré structures in CVD grown MoSe₂-WSe₂ heterobilayers

Abstract text (incl. figure legends and references)

A MoSe₂-WSe₂ heterobilayer (HBL) consists of stacked monolayers (MLs) of each respective material. Because of slightly different lattice parameters a Moiré superstructure is formed by periodic self-reconstruction, with regions showing ideal Moiré patterns and according transitional regions in between [1]. When using chemical vapour deposition (CVD) to fabricate a HBL the actual stacking is hardly under control. In theory several R- and H-type ideal stacking configurations are possible. Depending on the Moiré interference pattern varying electrical and optical properties arise, as has been indicated by spectroscopic analysis [2]. In order to reliably link these properties to the respective stacking, knowledge is required about which Moiré structures are present. Due to the periodicity of the Moiré superstructure with small Moiré cells, and the limited spatial resolution of the applied spectroscopic methods, it could not yet be fully determined how the different properties relate to each stacking.

A probe-corrected STEM operated at 120kV was used for high-angle annular dark field (HAADF) imaging and recording of four-dimensional (4D) scanning transmission electron microscopy (STEM) data with a Merlin Medipix3 camera. The MoSe₂-WSe₂ HBL was prepared by CVD and transferred on a Cu Quantifoil TEM grid. Multislice simulations of 4D-STEM data of ideal Moiré patterns were conducted.

In this study the local Moiré stacking in MoSe₂-WSe₂ HBLs was identified using HAADF and 4D-STEM, supported by multislice simulations. We use both signals to determine the stacking order, vacancies and additional atoms. By evaluating the gradients of the pixel wise 4D-STEM and HAADF signals with respect to stacking, atom positions and vacancies we evaluate the validity of the results from the respective signals.

The atom structure in the experimental HAADF-STEM image shown in Fig.1(a) represent three groups of differently stacked atom columns because of their different Z-contrast. Simulated ADF-STEM signals of ideal Moiré patterns are shown in Fig.1(b-e). Ideal Moirés that, in contrast to the experiment, do not show an additional centred atom in the hexagonal ring were excluded. By comparing the relative intensities in the experimental HAADF image with the relative intensities expected from the respective atom columns and which constituting atom species are known, a R_X^M stacking depicted in Fig.1(e) was in best agreement with the experiment. The bright columns represent 1Mo+2Se, the darker ones W or 2Se atoms. The line profiles in Fig.1(f) highlight the differences in the ADF signal of each ideal Moiré. The according atomic structures are shown in Fig.2.

[1] S. Zhao et al. Excitons in mesoscopically reconstructed Moiré heterostructures, pre-print, (2022)
[2] M. Förg et al. Moiré excitons in MoSe₂-WSe₂ heterobilayers and heterotrilayers. Nat Commun 12, 1656 (2021)
[3] We acknowledge financial support from the DFG under grant number EXC 2089/1-390776260 (e-conversion) and the EQAP project Munich.

Fig.1 Comparison of (a) an experimental HAADF-STEM image (noise filtered) with (b,c,d,e) simulated images of the ideal Moiré structures H_M^M, H_X^X, R_M^X and R_X^M of a MoSe₂-WSe₂ HBL. Profiles along red arrows are shown in (f); field of view in b,c,d,e is 1.5nm

Fig.2 Atomic structures of ideal Moiré configurations of the HBL; super- and subscript indices in the notation represent the top and bottom layer atoms stacked on top of another; R and H indicate the stacking type