Topology-Scaling Identification of Layered Solids and Stable Exfoliated 2D Materials

Abstract

The Materials Project crystal structure database has been searched for materials possessing layered motifs in their crystal structures using a topology-scaling algorithm. The algorithm identifies and measures the sizes of bonded atomic clusters in a structure's unit cell, and determines their scaling with cell size. The search yielded 826 stable layered materials that are considered as candidates for the formation of two-dimensional monolayers via exfoliation. Density-functional theory was used to calculate the exfoliation energy of each material and 680 monolayers emerge with exfoliation energies below those of already-existent two-dimensional materials. The crystal structures of these two-dimensional materials provide templates for future theoretical searches of stable twodimensional materials. The optimized structures and other calculated data for all 826 monolayers are provided at https://materialsweb.org.The combination of modern computational tools and the growing number of available crystal structure databases with high-throughput interfaces have accelerated recent efforts to map the materials genome. One of the most recently discovered branches of the materials genome is the class of two-dimensional (2D) materials, which generally have properties that are markedly different from their three-dimensional counterparts. The canonical example is the graphite/graphene system, but monolayers have been exfoliated from many other layered compounds as well [1][2][3][4]. Stable 2D materials can also be obtained by deposition [5][6][7][8] or chemical exfoliation [9,10]. Because the contribution of interlayer interactions to these materials free energies is typically quite small, the existence of a mechanically exfoliable bulk precursor generally indicates the relative stability of a freestanding single layer, regardless of how it is synthesized.In an effort to discover novel 2D materials, two recent studies searched the inorganic crystal structure database (ICSD) for compounds with large interlayer spacings, which are characteristic of weak interlayer bonding that could be overcome by mechanical exfoliation [11,12]. They used the intuitive criteria of a low packing fraction based on the covalent radii of the atoms and an interlayer gap larger than the sum of the covalent radii of atoms at the layers' surfaces along the c-axis to identify layered compounds in the ICSD. They discovered almost 100 layered phases, nearly half of which had monolayers that had not been the subject of any prior publications.Here, we extend their method to identify a large number of layered compounds that were missed using the packing factor and c-axis interlayer gap criteria. We further add the constraint that a bulk material must be thermodynamically stable to be of interest during our search. Therefore, we use the Materials Project (MP) database [13], an online repository of crystallographic and thermodynamic data for over 65,000 compounds calculated with density-functional theory (DFT). Our algorithm is designed to correctly identify ad...

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