Interlayer Bonding in Two-Dimensional Materials: The Special Case of SnP₃ and GeP₃

Stacked two-dimensional (2D) heterostructures are evolving as the "next-generation" optoelectronic materials because of the possibility of designing atomically thin devices with outstanding characteristics. However, most of the existing 2D heterostructures are governed by weak van der Waals interlayer interactions that, as often is the case, exert limited impact on the resulting properties of heterostructures relative to their constituting components. In this work, we investigate the optoelectronic properties of a novel class of 2D MP₃ (M = Ge and Sn) materials featuring strong interlayer interactions, applying a robust theoretical framework combining density functional theory and many-body perturbation theory. We demonstrate that the remarkable intrinsic vertical strain (of ∼40% relative to the monolayers) promotes the exfoliation of these materials into bilayers and profoundly impacts their electronic structure, charge transport, and optical properties. Most strikingly, we observe that the strong interlayer hybridization indicates continuous optical absorption across the entire visible range that, together with high charge carrier mobility, makes these 2D MP₃ heterostructures attractive for photoconversion applications.