Framework for identifying non–van der Waals two-dimensional materials

Abstract

Two-dimensional (2D) materials are categorized into van der Waals (vdW) and non-vdW types. However, no relevant descriptors have been proposed for identifying the latter. Here, we identify the non-vdW 2D materials by calculating the thickness-dependence of total energy of thin films trun- cated from surfaces. The non-vdW 2D materials exhibit a deviation from the law of finite-thickness excess energy (FTEE) inverse to the number of layers in the monolayer limit. This framework is applied to single-component systems, which successfully identifies non-vdW 2D materials including silicene and goldene that are overlooked in the dimensional analysis of the parent crystals. The anomalies of FTEE at specific thickness are found in Pb and Ga thin films, supporting the creation of plumbene and gallenene. The framework is also applied to multi-component systems with cubic and hexagonal symmetry (zincblende- and fluorite-types and tungsten carbide- and PtSbBa-types, respectively). Among them, several non-vdW 2D materials are identified to be dynamically stable. Structural transformation from hexagonal to almost tetragonal symmetry is observed in metallic TiS and ZrTe thin films. Our framework is useful for identifying non-vdW 2D materials.journal articl