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

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-Angstrom to multimicrometer length scales.

Automated summary

The study investigates the crystal growth mechanisms and defect formation of WS2, revealing its microstructure and orientation characteristics through electron microscopy imaging and molecular dynamics simulations. It reveals the relationship between defects and grain boundaries, as well as the tilting of the film when released from the substrate. Statistical analysis reveals that microstructural features are constructed from nanometer-scale building blocks, describing the system across length scales from sub-Angstrom to multimicrometer.