First-principles study of two-dimensional all-inorganic halide perovskites with different crystal facets

Two-dimensional (2D) inorganic halide perovskites have demonstrated potential in optoelectronic applications, with selective exfoliation along different crystallographic planes serving as a key strategy for performance optimization. Using first-principles density functional theory calculations, we systematically investigated the structural stability, exfoliation feasibility, and electronic properties of two representative series of 2D inorganic halide perovskites, i.e., A2BX4 and A3B'2X9, derived from the cubic perovskite along the (100) and (111) crystallographic planes. A3B'2X9-type perovskites derived from the (111) plane show exfoliation energies in the range of 23.1-62.1 meV/Å2, substantially outperforming their (100) plane-derived counterparts, A2BX4-type materials (59.7-174.0 meV/Å2). Rb3Bi2I9 exhibits an exceptionally low exfoliation energy of 23.1 meV/Å2, comparable to or even superior to that of graphene, indicating significant potential for the A3B'2X9 perovskite in producing high-quality single-layer nanosheets via mechanical exfoliation. The A2BX4 monolayers exhibit direct band gaps, while A3B'2X9 monolayers display indirect band gap behavior. Moreover, Rb2SnBr4 is predicted to possess an ideal direct band gap of 1.34 eV (calculated based on the HSE06 hybrid functional), in excellent agreement with the Shockley-Queisser limit for single-junction solar cells. Our first-principles investigation demonstrates that the (111) plane represents the optimal orientation for synthesizing readily exfoliable, while the (100) plane is more conducive to obtaining a direct band gap monolayer with superior optoelectronic properties. These findings provide critical theoretical guidance for experimental synthesis and precise tuning of material performance through crystallographic plane engineering.