How can the unstable two-dimensional Sn₂Bi be experimentally realized on Si(111)?

A versatile two-dimensional (2D) material, Sn₂Bi, has been grown on a semiconducting Si(111) substrate. However, the freestanding form of 2D Sn₂Bi was found to be dynamically unstable due to the out-of-plane deformation. Theoretical calculations indicate that the freestanding 2D Sn₂Bi corresponds to a saddle point in the potential energy surface, while in the supported cases, newly formed covalent bonds between the Sn atoms and the Si atoms underneath result in giant adsorption energy of Sn₂Bi and Si(111), which significantly alters the potential energy landscape of 2D Sn₂Bi, and inhibits the out-of-plane deformation of Sn₂Bi. Thus, the huge interfacial interaction eventually stabilized the 2D Sn₂Bi, which is the underlying mechanism responsible for the unexceptional synthesis of 2D Sn₂Bi on Si(111). We also propose a simplified phenomenological model to rationally understand the enhanced stability of 2D materials on the substrates, which can well capture the lattice dynamics of 2D materials in the presence of substrate. In addition, the direct estimation of Rashba spin–orbit coupling (SOC) strength gives rise to a giant value of 1.22 eVÅ at the Γ point for the lowest conduction band. Such a giant Rashba SOC implies the 2D Sn₂Bi would be a promising candidate in spintronic applications.