Long-Lived Polarization Memory from Piezoelectric Electron-Acoustic Phonon Coupling in Symmetric Spherical-like Cesium Lead Bromide Nanocrystals

Abstract

The anisotropic nature of halide perovskite nanostructures gives rise to intriguing polarized optical transitions. While shape-induced anisotropy in halide perovskites is well-documented, the role of intrinsic structural symmetry remains elusive. Using symmetric spherical-like cesium lead bromide nanocrystals to isolate shape effects, we correlate intrinsic symmetry with polarized optical transitions via polarization-selective ultrafast transient absorption spectroscopy and 8-band k·p modeling. We demonstrate that polarization anisotropy in an ensemble of photoexcited perovskite nanocrystals arises from transition dipole moment asymmetry inherent to the orthorhombic structure. This anisotropy persists for ∼2 ps─exceeding conventional semiconductors─owing to piezoelectric electron-transverse acoustic phonon coupling, which results in an unusual temperature dependence following τ ∝ T–0.6. Furthermore, the anisotropy amplitude and lifetime show a strong size dependence driven by internal shear strain. Reducing the nanocrystal size enhances the shear strain, leading to a concomitant enhancement in both the anisotropy amplitude and lifetime. Collectively, our findings provide deep insights into the role of intrinsic structural symmetry in polarized transitions, which is crucial for engineering polarization-sensitive perovskite light-emitting devices.