Inorganic-Cation Pseudo-Halide Two-dimensional Cs2 Pb(SCN)2 Br2 Perovskite Single Crystal.

Various layered lead halide perovskites based on the Ruddlesden-Popper (RP) structure with the general formula of An+1 Bn X3n+1 (n = 1, 2, …) have been developed. Most of them are comprised of alternating layered perovskite-like stacks and an organic spacer with a long insulating chain especially for the A-site-substituted RP-type perovskites. To date, only a small number of layered X-site-substituted two-dimensional (2D) RP type perovskites are reported. Herein, we report the first inorganic-cation pseudo-halide 2D phase perovskite single crystal, Cs2 Pb(SCN)2 Br2 . It was synthesized by the anti-solvent vapor assisted crystallization (AVC) method, and 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 sheets. Notably, 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). Surprisingly, this 2D single crystal exhibits a reversible first-order phase transformation to 3D CsPbBr3 (Pm3m #221) at 450 K. The Cs2 Pb(SCN)2 Br2 single crystal exhibits a very low exciton binding energy of 160 meV, as measured by temperature-dependent photoluminescence (PL), which is one of the lowest values reported for 2D perovskites (n = 1) and is comparable to the binding energies of quasi-2D RP-phase perovskites. A photodetector based on Cs2 Pb(SCN)2 Br2 single crystal is demonstrated with a responsivity of 8.46 mA W-1 and detectivity of ∼1.2 x 1010 Jones requiring a low bias voltage of only 0.5 V. This article is protected by copyright. All rights reserved.