nanocrystals using first-principles calculations*

Owning to its soft lattices, halide perovskites exhibit unique structural properties including high anharmonicity and dynamical instabilities.
While their bulk have been extensively investigated, the structural dynamics of 2D and nanocrystal perovskites are yet to be explored.
We demonstrate, for the first time, that localized surface phonons exists in the cubic phase of CsPbI3 slabs using density functional theory.
The surface localized modes include tilting and breathing of the octahedra, as well as cation displacements, both of which vanish into the bulk region.In comparison, the optically inactive “yellow” phase shows no obvious surface phonons, indicating higher structural dynamics for the cubic surfaces. We also predicted projected density of states for these surface phonons, which can be compared to experimental measurements such as Raman/IR spectrum and electron energy loss spectrum.
Our understanding of the lattice dynamics of the perovskite surfaces are key for understanding the stability of the materials in reduced dimensions.