Coverage-Dependent Structural Evolution of CoBr₂ at the Au(111) Interface

Unraveling the growth mechanism of van der Waals materials is crucial for their device implementation, as this improves the overall film quality, allowing precise control of their electronic and magnetic properties in nanoscale applications. The initial structure formed on the substrate during growth is often assumed to be bulk-like, thereby neglecting the role of the surface in the assembly. Here, the coverage-dependent growth of CoBr₂ on Au(111) from a stoichiometric molecular powder is studied using a combination of experimental techniques, machine-learning-driven molecular dynamics simulations and density functional theory calculations. It is found that CoBr₂ molecules initially form a molecular precursor phase characterized by three-molecule clusters arranged in a surface-stabilized structure with long-range order and a periodic coincidence with Au(111). As the surface coverage is increased, this phase subsequently undergoes a transition to form the equilibrium van der Waals crystal layered structure observed for the bulk material. These findings challenge conventional views of direct van der Waals layer formation and provide new insight into the role of the substrate during the growth process.