Size-Dependent Lattice Structure and Confinement Properties in CsPbl3 Perovskite Nanocrystals: Negative Surface Energy for Stabilization

CsPbI3 nanocrystals with narrow size distributions were prepared to study the size-dependent properties. The nanocrystals adopt the perovskite (over the nonperovskite orthorhombic) structure with improved stability over thin-film materials. Among the perovskite phases (cubic alpha, tetragonal beta, and orthorhombic gamma), the samples are characterized by the gamma phase, rather than a, but may have a size-dependent average tilting between adjacent octahedra. Size-dependent lattice constants systematically vary 3% across the size range, with unit cell volume increasing linearly with the inverse of size to 2.1% for the smallest size. We estimate the surface energy to be from -3.0 to -5.1 eV nm(-2) for ligated CsPbI3 nanocrystals. Moreover, the size-dependent bandgap is best described using a nonparabolic intermediate confinement model. We experimentally determine the bulk bandgap, effective mass, and exciton binding energy, concluding with variations from the bulk alpha-phase values. This provides a robust route to understanding gamma-phase properties of CsPbI3.