Multiscale Structural Analysis of Epitaxially Grown 2D Layers using Four-Dimensional Scanning Transmission Electron Microscopy

Epitaxial growth is a route to achieve highly crystalline continuous 2D layers. In recent years, large scale epitaxial growth of graphene and various transition metal dichalcogenides (TMDs) have been demonstrated by many research groups. However, high quality layer production with expected electrical properties is still challenging due to the imperfection of nucleation control. For instance, slight misalignement and symmetrical crystal inversion in neighbouring nucleation sites result in domain junctions, at the stage of coalescence, containing a large number of atomic defects and which drastically degrades material quality. The presence of polymorphe, such as 2H and 1T‘ in the TMDs, also modifies their electronic properties.<br/>In order to control growth process and to predict intrinsic properties of grown material up to wafer scale as a patchwork of defect structures, multiscale structural analysis should be accessible in routine way. X-ray diffraction (XRD) gives a precise information on material quality regarding the crystallinity and the angular distribution at mm scale, while scanning transmission electron microscopy (STEM) imaging offers today the capability to study the detailed local atomic structures such as vacancies, grain boundaries as well as the associated chemical composition at atomic scale. However, these techniques do not reveal complete material characteristics because of the gap in scale between the information obtained by different techniques. Four dimensional (4D)-STEM is a new acquisition technique allowing to simultaneously record 2D images in real and recoprocal spaces. Multiple information, from structure to electric field, at different scales can be reconstructed from signals appearing in diffraction pattern acquired at each pixel of the beam scan.<br/>In this work, we demonstrate an analytical process using 4D-STEM to study epitaxial 2D materials grown in both laboratory and industrial scales. Orientation, polarity and phase maps in various TMD monolayers, such as WSe₂ and WS₂, are reconstructed at micron scale from 4D-diffraction datasets and directly correlated with both large scale XRD analysis and related atomic defect structures revealed by STEM imaging. This structural mapping technique allowed the localization of various domain junctions as well as quantitative information on their density. In addition, the technique was also applied in epitaxial vdW heterostructures for mapping local twist angles. The results show the capacity to construct structural overview of synthesized materials from large scale down to atomic scale, which provides reliable information necessary to develop high quality 2D materials and further to enable theoretical prediction of material properties basis on large-scale realistic structure models.