  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.
