Figure 7.

a) 2D crystal structure of Cs₃Bi₂X₉ was displayed viewed down the b‐axis and c‐axis. Reproduced with permission.^{[ 154 ]} Copyright 2019, American Chemical Society. b) 0D Bi‐based halide perovskite structure consists of dioctahedral face‐sharing (Bi₂X₉)³⁻ clusters with hexagonal phase. Reproduced with permission.^{[ 147 ]} Copyright 2015, Wiley‐VCH. c) Absorption and PL spectra of MA₃Bi₂Br₉ nanocrystals. Reproduced with permission.^{[ 143 ]} Copyright 2016, Wiley‐VCH. d) Photographs of MA₃Bi₂Br₉ nanocrystal solutions passivated with different amount of Cl⁻ under a 325 nm UV lamp excitation. Reproduced with permission.^{[ 59 ]} Copyright 2018, American Chemical Society. e) Photographs (top) and steady‐state absorption and PL spectra (bottom) of colloidal Cs₃Bi₂X₉ (X = Cl, Cl_0.5Br_0.5, Br, Br_0.5I_0.5, I) nanocrystals. f) Excited dynamics model of Cs₃Bi₂Br₉ nanocrystals via combining time‐resolved PL and transient absorption. Reproduced with permission.^{[ 144 ]} Copyright 2017, Wiley‐VCH. g) PL spectra for the Cs₃Bi₂I₉, MA₃Bi₂I₉ and MA₃Bi₂I₉Cl_x thin films. Reproduced with permission.^{[ 147 ]} Copyright 2015, Wiley‐VCH. h) Normalized absorption and PL spectra of Cs₃Bi₂I₉ colloidal nanocrystals. i) Proposed recombination process in Cs₃Bi₂I₉ nanocrystals. Reproduced with permission.^{[ 152 ]} Copyright 2017, American Chemical Society.