Insight into the structure-property relationship of two-dimensional lead-free halide perovskite Cs3Bi2Br9 nanocrystals under pressure

Lead halide two-dimensional (2D) nanomaterials (NCs) have attracted continuous attention owing to their unique optoelectronic properties. However, the toxicity of lead largely prevents their commercialization. Therefore, environmentally friendly lead-free 2D perovskite NCs are sorely needed in the aspects of application. Recently, 2D lead-free halide perovskite Cs3Bi2Br9 NCs have attracted intense attention because of their environmentally friendly and potentially useful photovoltaic behavior. In this work, we explored the structure-property relationship of Cs3Bi2Br9 NCs by using high pressure techniques. The band gap of Cs3Bi2Br9 NCs was narrowed by about 0.69 eV, originating from Bi-Br bond contraction and Br-Bi-Br bond angle changes in the [BiBr6](3-) octahedra. Angle dispersive synchrotron X-ray diffraction patterns and Raman spectra show that the Cs3Bi2Br9 NCs experienced a phase transition from the trigonal phase to monoclinic phase, and then amorphization with increasing pressure. The results also indicate that the changes are reversible after pressure relief. First-principles calculations further demonstrate that the band gap is largely influenced by the orbital interactions, which is related to the distortion of the Bi-Br octahedral network under pressure. Our research not only strengthens the basic understanding of Cs3Bi2Br9 NCs, but also enables pressure processing as an efficient and environmentally friendly strategy to improve the application of design-by-design materials.