Mapping Domains and Domain Junctions in Mono- and Multi-Layer TMDs Using 4D-STEM—Toward Property Controlled 2D Device Materials

Despite the increasing interest in 2D material-based devices, synthesized 2D layers generally contain inevitable intrinsic growth-related atomic defects, which are the cause of a discrepancy between the properties measured in synthesized materials and those theoretically predicted from a perfect model system. Among many atomic defects, a domain boundary is one of the dominant structural defects determining the intrinsic properties of 2D materials [1]. Identification of domains and quantitative localization of domain junctions on a large scale have become critical requirements for the development of high quality 2D materials and theoretically modeling 2D material-based device performance.<br/>In this work, we demonstrate multi-scale analytical process using four-dimensional scanning transmission electron microscopy (4D-STEM); a new acquisition technique allowing to record a diffraction pattern at each pixel of the electron beam scanning position [2]. The generated 4D-datacubes provide local structural information with high precision and enable numerically reconstructing selected structural information as real space images. We present here the application of 4D-STEM techniques for the domain investigations in mono- and multi-layer 2D films.<br/>First, orientation and polarity mapping is performed on wafer-scale highly oriented WS₂ epitaxial layers. Typical domain junctions are identified and characterized using atomic resolution STEM imaging with the help of DFT calculations. Both rotational and polar inverted domains are then independently visualized at the micron scale by 4D-STEM mapping, using the position of 6-fold and 3-fold symmetry in diffraction patterns to extract quantitative local information. The distribution and density of different types of domain boundaries are quantitatively analyzed [3].<br/>Secondly, the lateral and vertical distribution of domains in 2D multilayer PtSe₂ films is analyzed by combining 3-dimensionally resolved in-plane orientation maps. It has been previously reported that the electronic properties of PtSe₂ films vary with increasing number of layers, from semiconducting to semi-metallic in a single crystal [4]. The position and the in-plane and out-of-plane sizes of individual domains are analyzed by deconvoluting the multiple diffraction patterns resulting from the twisted domains superimposed within the 15ML films. The effects of the domain arrangement and the out-of-plane domain size on the conductivity and electronic properties of the PtSe₂films are discussed by comparing samples with identical film thickness.<br/>Finally, the results demonstrate the capability of the 4D-STEM multi-structural mapping method to characterize 2D films from the atomic to the micro-structures. This opens up possibilities for the construction of device scale structural models and further understanding of structure-property relationships in synthesized 2D device materials.<br/>[1] Hue Ly et al., Nature Communications, 7 (2016) 10426.<br/>[2] Ophus et al., Microsc. Microanal. 25 (2019) 563.<br/>[3] D. Dosenovic et al., 2D Materials, 10 (2023) 045024.<br/>[4] Villaos et al., npj 2D Materials and Applications 3 (2019) 2.