Computational Material Database of Free-Standing 2D Perovskites

Inspired by the recent developments in the free-standing 2D complex oxide perovskite membranes and bulk 2D halide perovskite photoabsorbers, here, we put forward a new computational material database that has screened the stabilities and physical properties of 5,112 free-standing 2D nanostructures for 213 ABX3 cubic perovskites in 3 crystallographic orientations and 4 thicknesses, each with 2 different types of terminating surface chemistries. Our screening results show that many ABX3 compounds may form free-standing 2D structures that are both chemically and dynamically stable. Generally, the optimized 2D structures exhibit a shrinkage of the in-plane B-X bond lengths that are compensated by the elongation of the bonds normal to the 2D plane. As a result, the electron density distributions along the out-of-plane directions are strongly perturbed leading to a complex structural-dependent variation in the space-charge polarizations in these free-standing 2D perovskites. However, the out-of-plane ionic contributions to the dielectric permittivity are completely suppressed because of the structural confinement effect. The dominant contribution to the ionic dielectric permittivity thus comes from the in-plane optical phonons, the change in the frequencies of which from their bulk counterparts exhibits a complex dependency with the structures and chemistries of the 2D structures. Furthermore, without considering surface passivations and structural defects, the majority of the free-standing 2D perovskites exhibit metallic behavior. In particular, transition metal halides show exclusive spin-polarization at the Fermi level, which may be harnessed for applications such as nano spin-polarizer. Finally, unsupervised machine learning is further applied to discover the intriguing chemical and structural patterns among the computationally screened free-standing 2D perovskite structures, allowing us to reveal in a fully `data-driven' manner, the dependence of the energetic stabilities upon the surface termination, ionic radii and the layer thicknesses of these nanostructures. With fully open-sourced data and analysis code, we anticipate the current work can inspire the future theoretical and experimental works in the developments of free-standing 2D perovskites to harness their ubiquitous properties for new applications in nanoelectronics, sensing, energy harvesting and many others.