Phase transition and rapid temperature response of lead-free perovskite Cs-Cu-I nanocrystals enabled by their size

Cu-based halide perovskites have attracted extensive attention in the optoelectronic field owing to their non-toxic nature and excellent stability. Nevertheless, the growth mechanism of Cu-based halide perovskite nanocrystals (NCs) remains unclear, and the size and uniformity of NCs are difficult to control, which seriously hinders the development of these materials. Herein, a facile hot-injection method was applied to control the size and uniformity of blue-emitting (450 nm) Cs3Cu2I5 NCs. The dimensions of Cu-based perovskites were tuned from zero-dimensional NCs to one-dimensional nanorods. Additionally, the sizes of Cs3Cu2I5 NCs could be tuned from 22.1 +/- 1.5 to 8.9 +/- 1.1 nm by varying the reaction temperature and the dosage of cesium oleate, which is also extended to tune the size of Cs3Cu2X5 (X = Br and Cl) NCs. Interestingly, the photoluminescence (PL) maximum at 450 nm was almost unchanged with the size tuning, indicating that PL is from the radiation recombination of self-trap exciton emission. Moreover, the phase transition temperature of Cs3Cu2X5 NCs was regulated by varying the size, where a small size is beneficial to the decrease of the phase transition temperature. As a demonstration, the different-sized Cs3Cu2X5 NCs exhibited a size-dependent temperature response, and the response time became shorter as the size decreased. The temperature response of the small-sized Cs3Cu2X5 NCs could be decreased to only 15 s, which is better than that of their counterparts reported previously. This work may deepen the understanding of the size-dependent phase transition process in Cu-based perovskite NCs. Reducing the amount of Cs-oleate can reduce the size of Cs3Cu2I5 lead-free perovskite nanocrystals from 22 to 10 nm. The as-obtained nanocrystals show a more uniform size distribution and a more rapid response to temperature change of only 15 s.