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 similar to 102 and is endothermic with Delta H degrees similar to 30 kJ mol(-1). Larger equivalencies bring about a second process that is exothermic and corresponds to a displacement of PbBrx 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.

Automated summary

The thermodynamics of ligand-mediated phase transformation of CsPbBr3 NCs using DDAB were investigated, revealing two processes: facile ligand exchange displacing native ligands, and exothermic displacement of PbBrx complexes from NC surfaces leading to 2D phase formation. These findings help reconcile conflicting conclusions in prior studies and could aid in intentional formation of composite structures for selected applications.