Liao C.-HChen C.-HBing JBailey CLin Y.-TPandit T.MGranados LZheng JTang SLin B.-HHUNG-WEI YENMcCamey D.RKennedy B.JChueh C.-CCHU-CHEN CHUEH et al.2022-11-162022-11-16202209359648https://www.scopus.com/inward/record.uri?eid=2-s2.0-85121528031&doi=10.1002%2fadma.202104782&partnerID=40&md5=4593c07a72f3383007a80977dc3e90f8https://scholars.lib.ntu.edu.tw/handle/123456789/625357Most of the reported 2D Ruddlesden–Popper (RP) lead halide perovskites with the general formula of An+1BnX3n+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, Cs2Pb(SCN)2Br2, 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)2Br2 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 Cs2Pb(SCN)2Br2-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. © 2021 The Authors. Advanced Materials published by Wiley-VCH GmbHinorganic 2D perovskites; low exciton binding energy; photodetectors; photoresponse; pseudohalide; reversible phase transformation; single crystals[SDGs]SDG7Binding energy; Excitons; Lead compounds; Perovskite; Phase transitions; Photodetectors; Positive ions; Single crystals; Advanced materials; Exciton-binding energy; Inorganic 2d perovskite; Inorganic cations; Inorganics; Low exciton binding energy; Photoresponses; Pseudo Halide; Reversible phase transformations; Ruddlesden-Popper; Photonsjournal article10.1002/adma.202104782348662522-s2.0-85121528031