Crystal Structure and Optical Properties Characterization in Quasi-0D Lead-Free Bromide Crystals (C₆H₁₄N)₃Bi₂Br₉·H₂O and (C₆H₁₄N)₃Sb₃Br₁₂

Abstract

Low dimensional organic inorganic metal halide materials have shown broadband emission and large Stokes shift, making them widely used in various fields and a promising candidate material. Here, the zero-dimensional lead-free bromide single crystals (C 6 H 14 N) 3 Bi 2 Br 9 •H 2 O (1) and (C 6 H 14 N) 3 Sb 3 Br 12 (2) were synthesized. They crystallized in the monoclinic crystal system with the space group of P2 1 and P2 1 /n, respectively. Through ultraviolet−visible−near-infrared (UV−vis−NIR) absorption analysis, the band gaps of (C 6 H 14 N) 3 Bi 2 Br 9 •H 2 O and (C 6 H 14 N) 3 Sb 3 Br 12 are found to be 2.75 and 2.83 eV, respectively. Upon photoexcitation, (C 6 H 14 N) 3 Bi 2 Br 9 •H 2 O exhibit broad-band red emission peaking at 640 nm with a large Stokes shift of 180 nm and a lifetime of 2.94 ns, and the emission spectrum of (C 6 H 14 N) 3 Sb 3 Br 12 are similar to those of (C 6 H 14 N) 3 Bi 2 Br 9 •H 2 O. This exclusive red emission is ascribed to the self-trapping exciton transition caused by lattice distortion, which is confirmed through both experiments and first-principles calculations. In addition, due to the polar space group structure and the large spin−orbit coupling (SOC) associated with the heavy elements of Bi and Br of crystal 1, an obvious Rashba effect was observed. The discovery of organic inorganic metal bromide material provides a critical foundation for uncovering the connection between 0D metal halide materials' structures and properties.