Inorganic-Cation Pseudohalide 2D Cs₂ Pb(SCN)₂ Br₂ Perovskite Single Crystal

Most of the reported 2D Ruddlesden-Popper (RP) lead halide perovskites with the general formula of A_{n +1} B_n X_{3 n +1} (n = 1, 2, …) comprise layered perovskites separated by A-site-substituted organic spacers. To date, only a small number of X-site-substituted RP perovskites have been reported. Herein, the first inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs₂ Pb(SCN)₂ Br₂ , is reported. It is synthesized by the antisolvent vapor-assisted crystallization (AVC) method at room temperature. It exhibits a standard single-layer (n = 1) Ruddlesden-Popper structure described in space group of Pmmn (#59) and has a small separation (d = 1.69 Å) between the perovskite layers. The SCN^- anions are found to bend the 2D Pb(SCN)₂ Br₂ framework slightly into a kite-shaped octahedron, limiting the formation of a quasi-2D perovskite structure (n > 1). This 2D single crystal exhibits a reversible first-order phase transformation to 3D CsPbBr₃ (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV-one of the lowest for 2D perovskites (n = 1). A Cs₂ Pb(SCN)₂ Br₂ -single-crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W^-1 and detectivity of ≈1.2 × 10¹⁰ Jones at a low bias voltage of 0.5 V.