Data-driven quest for two-dimensional non-van der Waals materials

Two-dimensional (2D) materials are frequently associated with the sheets which form bulk layered compounds bonded by van der Waals (vdW) forces. The anisotropy and weak interaction between the sheets have also been the main criteria in the computational search for new 2D systems, which predicted about 2000 exfoliable compounds. However, several representatives of a new type of non-vdW 2D systems, such as hematene or ilmenene, which have no layered 3D analogues, and which, unlike, e.g. silicene, do not need to strongly interact with the substrate to be stable, were recently manufactured. The family of non-vdW 2D materials is an attractive playground for data-driven high-throughput approaches as computational design principles are still missing. Here, we outline a new set of 8 binary and 20 ternary candidates by filtering the AFLOW-ICSD database according to the structural prototype of the first two template systems realized in experiment. All materials show a strong structural relaxation upon confinement to 2D which is essential to correctly estimate the bonding strength between facets. The oxidation state of the cations at the surface of the sheets is demonstrated to regulate the inter-facet binding energy with low oxidation states leading to weak bonding. We anticipate this descriptor to be highly useful to obtain novel 2D materials, providing clear guidelines for experiments. Our calculations further indicate that the materials exhibit a vast range of appealing electronic, optical and magnetic properties, which is expected to also make them attractive for potential applications particularly in spintronics.
Comment: 26 pages, 6 figures