How Stable are Perovskite Monolayers? A First-Principles Study of Mechanical and Electronic Properties of Perovskenes

Halide perovskites have been highly studied owing to their excellent optoelectronic properties. Recent studies indicate the importance of understanding the properties of low dimensional halide perovskites. To investigate the structural and electronic properties of halide perovskite monolayers (i.e., perovskenes), this work explores their mechanical, thermodynamic, and electronic structure through first-principles simulations. We have studied three different stoichiometries (ABX3, ABX4, and A2BX4) and structural phases for iodides, bromides, and chloride perovskite monolayers. Their thermodynamic behavior was evaluated through the construction of phase diagrams, highlighting the instability of the ABX4 stoichiometry, which was reinforced by the mechanical instability. Structurally, the covalent characteristics of the Pb–X bond, in contrast to the Cs–X bonds, induce a strong variation of the Young's modulus and Poisson's ratio along different crystallographic directions, and a lower stiffness observed in the phases where the octahedra are not aligned. The electronic properties are somewhat similar to the 3D counterparts, but with a slightly larger bandgap; in the monolayers, the band gap increases with the halogen electronegativity (I, Br, Cl) and octahedral tiltings. Our work lays the groundwork for a deeper understanding of low-dimensional structures in these materials.