Control of Facet Preference and Stability with Halogen Passivation of CsPbBr₃ Perovskite

Cesium lead bromide perovskite nanocrystals (CsPbX₃, X = Cl, Br, I) have garnered significant attention due to their facile controllable synthesis and remarkable optoelectronic properties. Herein, we reveal the energetics of surfaces to account for the formation preference of surface orientations and terminations through first-principles calculations. Surface energetics, which are difficult to quantitatively measure in experiments, are the underlying thermodynamic indicators that govern the sample morphology, growth direction, and surface stability. For the first time, we establish a hierarchy of surface energy for various surfaces (named as Miller indices-terminations), ordered under the CsBr-rich condition as follows: (001)-CsBr < (100)-Br < (110)-PbBr₂ ≈ (001)-PbBr₂ < (100)-PbBr < (010)-CsPbBr < (100)-CsPbBr < (100)-Br₂. Hence, the lowest CsBr-terminated (001) surface tends to be the most popular and surviving one in CsPbBr₃ nanocrystals. Interestingly, adoption of appropriate halogen atoms (F, Cl, and I) as adsorbents can lead to a reversal in the trend of surface energies. This allows intentional control of the popularity of certain surface indexes with enhanced performance. Our work presents an atomic-scale mechanism and proposes an effective strategy for improving the surface stability of CsPbBr₃, which is crucial for guiding experimentalists on designing more efficient and stable perovskite nanocrystals.