Inorganic‐Cation Pseudohalide 2D Cs₂Pb(SCN)₂Br₂ Perovskite Single Crystal

Abstract

Pb 2+ ; X = Cl − , Br − , I − ) have demonstrated impressive performance. Capitalizing on their tunable dimensionality, layered perovskites such as 2D and 2D/3D (or quasi-2D) structures have also emerged significantly expanding semiconducting material candidates due to the wide diversity of potential A-site cation and/or X-site anion spacer selections. [9,10] The unique advantages of layered lead halide perovskites are high stability, quantum confinement effect, and tunable photophysical properties. They are increasingly sought after for optoelectronic research in recent years. [11][12][13] The most popular layered lead halide perovskites are based on the Ruddlesden-Popper (RP) structure with the general formula of A n+1 B n X 3n+1 (n = 1, 2, …), where A-site-substituted layered RP-type perovskites are generally hybrid organic-inorganic compounds containing long organic chains. These long organic chains however are insulating. Added to this, the spatial confinements of charge carrier and dielectric contrast between perovskite stack and spacer [11,12] result in a large exciton binding energy, typically ten times larger than the those of 3D perovskites. [14] Most of the reported 2D Ruddlesden-Popper (RP) lead halide perovskites with the general formula of A n+1 B n X 3n+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, Cs 2 Pb(SCN) 2 Br 2 , 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 Br 2 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 CsPbBr 3 (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 Cs 2 Pb(SCN) 2 Br 2 -single-crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W −1 and detectivity of ≈1.2 × 10 10 Jones at a low bias voltage of 0.5 V.

Temperature-induced structural transition in an organic–inorganic hybrid layered perovskite (MA)2PbI2−xBrx(SCN)2

“…MA + and SCN − . Since a temperature-induced transition was not reported in Cs 2 PbBr 2 (SCN) 2 , 41 at least the MA + ion contributes to the transition.…”

Halide perovskite quantum dots for photocatalytic CO2reduction

“…122 Compared to halide perovskite, these pseudo-halide perovskites have better stability at room temperature because of the SCN − or HCOO − anion in the axial positions of the lead halide octahedron which results in an asymmetric electronic structure to maintain the desired framework. 123 The tunable band structure and relatively low exciton binding energy of pseudo-halide perovskites also contribute to obtaining desirable bandgaps and electron transfer. From these perspectives, the pseudo-halide perovskite materials also show promising prospects for photocatalytic CO 2 reduction.…”

Structural Dynamics of a Novel Pseudohalide Perovskite Cs2Pb(SeCN)2Br2 Investigated with Nonlinear Infrared Spectroscopy

“…Most of the SeCN − s are 13 C 15 N. Only 1−2% are 12 C 14 N, which gives rise to the band at 2090 cm −1 , on which we do the experiments. Therefore, the vast majority of the 12 C 14 Ns we study will have 13 C 15 N as the other SeCN − bound to a Pb 2+ . The separation between the 12 C 14 N and 13 C 15 N peaks is ∼74 cm −1 .…”

“…11−13 The first purely inorganic pseudohalide 2D perovskite single-crystal reported was Cs 2 Pb-(SCN) 2 Br 2 , which has excellent performance in photodetector applications. 12 There have been extensive efforts to investigate the dynamical properties of 2D perovskites. Their deformable and highly polar lattices play a central role in the excited-state dynamics by exciton-or charge-lattice interactions.…”