Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS₂ Monolayer Films

Abstract

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and ReaxFF reactive force field-based molecular dynamics simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resolution imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical analysis of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire

“…The line defects can be described as translational boundaries, and detailed analysis of their structure and geometry indicates that they arise from coalescence of WS 2 domains with well-defined edge orientations. Because these edges do not follow the zigzag edge as expected from previous studies on isolated domains obtained using powder vaporization, they give rise to the irregular shapes within the coalesced monolayer in the DF-TEM images . Translational boundaries are the dominant type of line defect present in all of the WS 2 monolayers examined, in contrast to prior reports for MoS 2 and WSe 2 monolayers grown on sapphire where IDBs were predominant.…”

“…To determine the structure of the two regions and their boundaries, the DF-TEM maps were correlated with atomic-resolution ADF-STEM imaging. The details of this correlation are discussed in the Supporting Information (Figure S9), the Materials and Methods section, and a related publication . DF-TEM images of the WS 2 monolayer from the center and edge of the wafer are shown in Figure S10 to highlight the microstructural uniformity of the film across the entire 2 in.…”

Controllable growth of two-dimensional materials on noble metal substrates

“…Therefore, the epitaxial 2D domains can only be imperfectly stitched. Even coalesced 2D grains with the same in-plane crystalline orientation are still separated by translational grain boundaries as they are anchored on the solid substrates with poor rheology ( Chubarov et al., 2021 ; Hickey et al., 2021 ).…”

A ReaxFF Force Field for 2D-WS2 and Its Interaction with Sapphire

“…To date, many experimental studies have focused on the synthesis and characterization of 2D TMD materials by applying various growth and visualization techniques, representing great progress. − Annular dark-field scanning transmission electron microscopy (ADF-STEM) has been deployed to investigate characteristics of materials on the atomic scale in real time and has provided valuable insights into the atomic structure and dynamics of structural impurities, such as point and line defects. , These experimental achievements have increased the need for a fundamental understanding of the scalable synthesis and characterization of TMDs. Quantum mechanical (QM) methods have been intensively utilized for the past two decades for the theoretical investigation of TMDs and have provided highly accurate insights into their structural characteristics; however, they are limited to relatively small systems of up to 1000 atoms due to their heavy computational burden.…”

“…This potential has a simple many-body form including terms that enable the simulation of the nonlinear characteristics of a material, such as thermal conductivity and nonlinear elastic effects at a low computational cost. However, it has difficulty describing nonequilibrium states . The REBO bond-order potential was developed by Han et al to model WS 2 –WSe 2 and MoS 2 –WSe 2 heterostructures.…”