Controllable Black-to-Yellow Phase Transition by Tuning the Lattice Symmetry in Perovskite Quantum Dots

The black-to-yellow phase transition in perovskite quantum dots (QDs) is more complex than in bulk perovskites, regarding the role of surface energy. Here, with the assistance of in situ grazing-incidence wide-angle and small-angle X-ray scattering (GIWAXS/GISAXS), distinct phase behaviors of cesium lead iodide (CsPbI3) QD films under two different temperature profiles-instant heating-up (IHU) and slow heating-up (SHU) is investigated. The IHU process can cause the phase transition from black phase to yellow phase, while under the SHU process, the majority remains in black phase. Detailed studies and structural refinement analysis reveal that the phase transition is triggered by the removal of surface ligands, which switches the energy landscape. The lattice symmetry determines the transition rate and the coexistence black-to-yellow phase ratio. The SHU process allows longer relaxation time for a more ordered QD packing, which helps sustain the lattice symmetry and stabilizes the black phase. Therefore, one can use the lattice symmetry as a general index to monitor the CsPbI3 QD phase transition and finetune the coexistence black-to-yellow phase ratio for niche applications.

Automated summary

In this research, the phase transition behavior of cesium lead iodide (CsPbI3) quantum dot (QD) films under different temperature profiles was investigated using in situ scattering techniques. It was found that the phase transition from black phase to yellow phase can be induced by the instant heating-up (IHU) process, while the majority of QDs remained in the black phase under the slow heating-up (SHU) process. Further analysis revealed that the phase transition is triggered by the removal of surface ligands, which alters the energy landscape. The lattice symmetry plays a crucial role in determining the transition rate and the coexistence black-to-yellow phase ratio. Therefore, the lattice symmetry can be used as an indicator to monitor the phase transition of CsPbI3 QDs and adjust the coexistence ratio for specific applications.