Two-Step Chemical Vapor Deposition for Fabrication of FAPbI₃ Single-Crystal Microsheets with High Exciton Binding Energy

Hybrid perovskites exhibit highly efficient optoelectronic properties and find widespread applications in areas such as solar cells, light-emitting diodes, photodetectors, and lasers. Here, we report the innovative synthesis of formamidinium lead iodide (FAPbI 3 ) single-crystal microsheets via a two-step chemical vapor deposition (CVD) method. The microsheets exhibit hexagonal and trapezoidal shapes, with hexagonal FAPbI 3 growing parallel to the substrate and trapezoidal FAPbI 3 growing perpendicular to the substrate. The dominant role of single-exciton recombination in the photoluminescence (PL) of these microsheets is observed, especially pronounced at low temperatures, attributed to the relatively large exciton binding energies of the samples. Calculations reveal exciton binding energies as high as 110.8 meV for hexagonal and 133.3 meV for trapezoidal FAPbI 3 single-crystal microsheets, attributed to reduced rotational freedom of the formamidinium (FA) ions. Further investigation into low-temperature phase transitions indicates lower transition temperatures (around 100 K) for these microsheets, suggesting reduced FA ion rotational freedom and consequently higher exciton binding energies.