Unified classification of metal-2D interactions for tailored functional materials

The modification of two-dimensional (2D) materials with metal atoms can lead to additional structures and properties; however, a systematic classification of metal-2D material interactions is still lacking. We propose a general framework for classifying these interactions based on two primary modes: metal atom adsorption on the surface and intercalation between layers. This framework, validated by first-principles calculations on representative van der Waals layered and nonlayered 2D materials interacting with transition metals (TMs) and alkali metals, provides a comprehensive explanation for experimental observations across various systems. Furthermore, the classification serves as a guide for designing stable and functional 2D materials. Our investigation of nonlayered borophene shows that interlayer-intercalated TM borides (TMBs) exhibit significantly higher stability than intralayer-doped TMBs. Physical property calculations further reveal that these sandwich-type TMBs possess superior mechanical stability and additional electronic properties, underscoring their potential for future applications.