Theoretical Investigation of Intrinsically Patterned 2D Transition Metal Halides: Defects, Structure, and Magnetic Phenomena

In the quest for complex structured functional materials, defect engineering and patterning in two-dimensional (2D) systems are critical for tuning material properties and enabling new functionalities. However, generating 2D periodic patterns of point defects in 2D materials, such as vacancy lattices that can serve as antidot lattices, has been elusive until now. Herein, we report on intrinsically patterned 2D transition metal dihalides (TMDs) on a gold surface, featuring periodic halogen vacancies in the upper and bottom halide layers that result in alternating coordination of the transition metal atoms throughout the film.

Density-Functional Theory (DFT), a widely used method for investigating material properties, is employed to explore the formation pathways leading to periodic halogen vacancies and their role in modifying the electronic and magnetic structure of TMDs. The defect-engineered vacancy lattice not only stabilizes the structure but also provides atomic-level control over the material's properties. The calculations show that Br vacancies are energetically favorable, and the resulting vacancy lattice significantly reduces the lattice mismatch with the underlying Au(111) surface. The structure formation of the 2D FeBr$_2$ on the surface and the presence of defects are further analyzed using STM simulations. The excellent match between the experimental findings and the STM calculations confirms the intrinsic vacancy lattice. Moreover, our spin-polarized, non-collinear DFT calculations predict the emergence of unconventional magnetic textures, driven by the interplay of defect-induced strain and transition metal coordination. The calculated magnetic anisotropy energy for pristine FeBr$_2$ favors an out-of-plane orientation, while the introduction of vacancies shifts the preferred magnetization to an in-plane direction, demonstrating that vacancies can tune the magnetic properties of the films. By coupling our theoretical results with experimental observations, we provide a comprehensive framework for understanding the structure formation, electronic and magnetic properties of 2D materials, advancing the design of complex structured materials with tunable properties.