Tuning Pressure-Induced Phase Transitions, Amorphization, and Excitonic Emissions of 2D Hybrid Perovskites via Varying Organic Amine Cations

The existence of organic components in 2D hybrid organic–inorganic perovskites gives them significant structural flexibility in response to pressure. Here we study the high-pressure behavior and associated physical properties of a family of 2D hybrid perovskites, [C n H 2 n +1 NH 3 ] 2 PbI 4 ( n = 4, 8, 12). Our high-pressure synchrotron X-ray diffraction experiments and ab initio molecular dynamics simulations show that the critical pressures for driving structural transitions and amorphization are proportional to the length of alkylamine cations, which arise from their scaling counterbalance to compression energy via conformational rearrangements. In addition, we reveal that their photoluminescence alters from free excitons to bound excitons, then to self-trapped excitons under successive pressure manipulation. Furthermore, we discovered that the high-pressure treatment could transform the samples from bulk micrometer powders to thin nanosheets. Our findings suggest that these 2D hybrid perovskites structurally evolve and amorphize in a remarkably different way compared with conventional 2D materials because of their high degrees of structural freedom.