Interfacial Ligand Density Regulates Solvent-Induced Phase Transformation in CsPbBr ₃ /Cs ₄ PbBr ₆ Nanocrystals

Solvent-driven phase transformation in halide perovskite nanocrystals is often attributed to solvent polarity and ion extraction, but the role of the organic ligand shell in regulating solvent access remains less defined. The kinetics of Cs 4 PbBr 6 -to-CsPbBr 3 phase transformation in CsPbBr 3 /Cs 4 PbBr 6 heterophase nanocomposites were investigated as a function of ligand-associated molecular density. By varying the synthesis temperature and applying postsynthetic ligand titration, the surface organic environment was tuned from a sparse ligand regime of 1.1 molecules nm –2 to an organic-rich matrix of approximately 26.3 molecules nm –2 . Real-time photoluminescence measurements during alcohol exposure showed that dense organic loading delays the emission evolution, whereas sparse ligand coverage allows rapid solvent response. Size-selective solvent probing indicated that the transformation involves two contributions: solvent polarity provides the driving force for lattice reorganization, while solvent molecular size and ligand-associated molecular density influence kinetic access to the reactive surface. Density functional theory calculations support this picture by showing that ligand passivation weakens direct alcohol interaction with the Cs 4 PbBr 6 surface and limits solvent-induced lattice destabilization. The ligand-gated transformation was further evaluated for dynamic optical authentication through coupled excitation, solvent, and lifetime readouts, with ligand-associated molecular density influencing metastability and programmable optical responses in perovskite nanocrystals.