Combined NMR and Isothermal Titration Calorimetry Investigation Resolves Conditions for Ligand Exchange and Phase Transformation in CsPbBr₃ Nanocrystals

Inorganic halide perovskite nanocrystals (NCs), such as CsPbBr₃ quantum dots, have emerged as an intriguing alternative to traditional semiconductors in optoelectronic devices, but their rational development is hindered by limited stability, including reactions that lead to other solid phases at ambient conditions. Dimethyldidodecyl ammonium bromide (DDAB) is one of the most widely studied ligands in efforts to stabilize CsPbBr₃ NCs through surface modification. While some researchers have reported improved quantum yield, optoelectronic performance, and stability through such ligand exchange, others have reported it to cause a phase transformation to poorly fluorescent two-dimensional (2D) CsPb₂Br₅ nanoplatelets. Here, we investigated the thermodynamics of this ligand-mediated phase transformation of CsPbBr₃ NCs using absorption spectroscopy and matched NMR and isothermal titration calorimetry measurements. We were able to resolve two different processes that occur upon the introduction of DDAB. Ligand exchange, displacing native oleate and oleylammonium ligands, proceeds readily at low DDAB concentrations with an exchange equilibrium constant of ∼10² and is endothermic with ΔH° ∼30 kJ mol^–1. Larger equivalencies bring about a second process that is exothermic and corresponds to a displacement of PbBr_x complexes from NC surfaces; these complexes ultimately lead to the formation of the 2D phase. Resolving these processes through a direct thermal measurement helps to reconcile contradictory conclusions in prior studies of surface passivation with quaternary ammonium bromides. In addition to revealing conditions that lead to instability of CsPbBr₃ NCs, the present findings could also guide the intentional formation of 2D CsPb₂Br₅ and 2D/3D CsPbBr₃/CsPb₂Br₅ composite structures of interest for selected applications.