Size-Dependent Lattice Structure and Confinement Properties in CsPbI₃ Perovskite Nanocrystals: Negative Surface Energy for Stabilization

Abstract

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 α, tetragonal β, and orthorhombic γ), the samples are characterized by the γ phase, rather than α, 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 α-phase values. This provides a robust route to understanding γ-phase properties of CsPbI₃.

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