Intrinsically patterned two-dimensional transition metal halides

Patterning and defect engineering are key methods to tune 2D materials' properties. However, generating 2D periodic patterns of point defects in 2D materials has been elusive until now, despite the well-established methods for creating isolated point defects and defect lines. Herein, we report on intrinsically patterned 2D transition metal dihalides on metal surfaces featuring periodic halogen vacancies that result in alternating coordination of the transition metal atoms throughout the film. Using low-temperature scanning probe microscopy and low-energy electron diffraction, we identified the structural properties of patterned FeBr$_2$ and CoBr$_2$ monolayers grown epitaxially on Au(111). Density-functional theory reveals that the Br-vacancies are facilitated by low formation energies and accompanied by a lateral softening of the layers leading to a significant reduction of the lattice mismatch to the underlying Au(111). We demonstrate that interfacial epitaxial strain engineering presents a versatile strategy for controlled patterning in 2D. In particular, patterning 2D magnets provides new pathways to create unconventional spin textures with non-collinear spin.