Thickness-dependent surface reconstructions in non-van der Waals two-dimensional materials

Bismuth oxychalcogenides (Bi₂O₂X, X = S, Se, Te), a family of non-van der Waals (non-vdW) two-dimensional (2D) semiconductors, are attracting significant attention due to their outstanding semiconducting properties and huge potential in various applications of electronic and optoelectronic devices. Surface imperfections (e.g., surface vacancies) and surface reconstructions are more likely to appear and may cause intriguing physical properties and novel phenomena in the non-vdW 2D materials than the vdW cases. Here, we explore the impacts of surface vacancies and surface reconstructions on the properties of the surfaces and 2D structures of Bi₂O₂X by using the first-principles method. We find that the dimerization of surface X-vacancies occurs in Bi₂O₂S and Bi₂O₂Te (001) surfaces, like that happening in Bi₂O₂Se. Unexpectedly, the electronic structures of Bi₂O₂X (001) surfaces show strong tolerance to the order of surface X-vacancies. Furthermore, we find a phenomenon of thickness-dependent surface reconstructions for non-vdW Bi₂O₂X ultrathin films. For a monolayer, the zipper-surface is more stable, while the dimer-surface is generally more stable for thicker films. Calculated exfoliation energies of the Bi₂O₂X monolayer and multi-layers are close to those of common vdW 2D materials, indicating that 2D Bi₂O₂X belong to easily fabricated 2D materials, even though the inter-layer binding interaction is of the non-vdW type. Our results suggest that non-vdW 2D materials can possess intriguing properties because of surface imperfections and reconstructions in comparison with vdW 2D materials.