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 (<i>e.g.</i>, 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.