(Invited) Crystal Phase Engineering of Cesium Lead Iodide By Nanoconfinement

Abstract

Nanoscale dimensional reduction leads to perturbed electronic structure, a foundational concept in the study of quantum dots. In this work, we extend this concept to consider how nanoscale dimensionality perturbs the crystal phase diagram of a solid. We apply this to the phase behavior of cesium lead iodide, a material that possesses favorable optoelectronic properties in its perovskite crystal polymorph at high temperature, and exhibits an undesirable crystal phase at room temperature. We develop and test the hypothesis that for solids such as cesium lead iodide that simultaneously exhibit both predominantly covalent bonding (Pb—I) and predominantly ionic bonding (Cs—I), low-density, high-symmetry phases can be favored by restricting crystal dimensions below 100 nm. This is based on a model in which short-range, orbital-orbital repulsive forces, which favor low-density and high-symmetry phases, are in competition with long-range electrostatic forces, which favor high-density. The electrostatic forces, being long-range in nature, are more diminished by truncating the crystal. Experimentally, we show that the phase sequence can be inverted and the equilibrium shifted by as much as 250 ºC through various nanostructuring approaches, including colloidal nanocrystal co-assembly and crystallization within nanoporous scaffolds. Ultimately, we seek to develop a phase diagram in which temperature and size are the independent variables.