Defect-Induced Inhomogeneous Phase Transition in 2D Perovskite Single Crystals at Low Temperatures.

The temperature-induced phase transition in two-dimensional (2D) layered perovskites was recently found to be incomplete even if the temperature dropped to tens of kelvin. However, its intrinsic cause still remains unclear, and the information on the phase transition in individual single crystals (SCs) is also limited. Herein, we study the phase transition process in individual ( n -C 4 H 9 NH 3 ) 2 PbI 4 SCs using a home-built photoluminescence (PL)-scanned image microscope. At 83 K, the phase transition is indeed incomplete, leading to the coexistence and inhomogeneous distributions of room-temperature and low-temperature phases. We map the distribution of phase transition degree on individual SCs at 83 K, which exhibits a strong positive (negative) correlation with the distribution of local defects (PL lifetimes) at 293 K, indicating that the phase transition is enhanced by initial defects. Our findings might provide new insight into the phase transition of ( n -C 4 H 9 NH 3 ) 2 PbI 4 crystals, which is of potential value for applications based on 2D layered perovskites.