Ligand-Controlled Phonon Dynamics in CsPbBr₃ Nanocrystals Revealed by Machine-Learned Interatomic Potentials

Halide perovskite nanocrystals are leading candidates for next-generation optoelectronics, yet the role of surface ligands in controlling their phonon dynamics remains poorly understood. These lattice dynamics critically govern energy up-conversion, phonon-assisted anti-Stokes emission, and nonradiative relaxation. Conventional ab initio methods, while accurate, are computationally infeasible for experimentally relevant nanocrystal sizes that require thousands of atoms to capture realistic ligand shells and dynamic disorder. Here, we introduce a machine-learned interatomic potential fine-tuned on small CsPbBr₃ nanocrystals with diverse ligands, enabling accurate prediction of ligand-induced phonon properties far beyond the spatial and temporal scales of ab initio methods. We find that both cationic and anionic ligands systematically redshift Pb-Br-Pb stretching modes while blueshifting the PbBr₆ ^4- octahedral rotation mode, with stronger overall effects for anionic passivation. Notably, anionic ligands stiffen the rotation mode nonmonotonically with respect to the ligand binding energy. The nanocrystal models also reveal a strong site dependence of these phonon-mode shifts, with corner and edge sites showing the largest response. Our findings reveal important roles of cationic and anionic ligands in modulating key dynamic modes of halide perovskite nanocrystals associated with detrimental nonradiative losses, offering mechanistic insights and design principles for high-performance perovskite nanocrystal optoelectronics.