Intrinsic instabilities and mechanical anisotropy in halide perovskite monolayers

Abstract Atomically thin halide perovskites are promising materials, with a growing number of experimental studies exploring their properties and potential applications. They combine the remarkable optoelectronic properties of halide perovskites with the versatility of 2D materials, yet a comprehensive understanding of the properties across different stoichiometries remains lacking. Here, we investigate the structural, elastic, and electronic properties of lead halide perovskite monolayers using first-principles simulations. We examined different monolayer stoichiometries CsPbX 3 , CsPbX 4 , and Cs 2 PbX 4 (X = I, Br, Cl), and showed that CsPb X 4 monolayers are intrinsically unstable, whereas CsPbX 3 , and Cs 2 PbX 4 can be stabilized depending on the chemical potential. We further demonstrate that the difference in stiffness between Pb-halogen bonds and other bonds accounts for the mechanical anisotropy of the monolayers. Finally, first-principles electronic simulations reproduced the experimentally observed differences in work function and Scanning Tunneling Microscopy images between Pb- and Cs-terminated surfaces, while revealing pronounced Rashba splitting in CsPbX 3 monolayers.