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

While solar cells, light-emitting diodes, photodetectors, and field-effect transistors based on 3D metal halide perovskite with the molecular formula of ABX₃ (A = Cs⁺, CH3NH³⁺, CH(NH2)²⁺; B = Sn²⁺, Pb²⁺; X = Cl⁻, Br⁻, I⁻) have demonstrated outstanding performance in recent years, 2D or quasi-2D perovskites achieved by space insertions have attracted intensive research effort. This is because of the added stability that accompanies these materials while still inheriting the advantages of 3D counterparts The versatility of engineer A-site cations or X-site anions in the spacers to achieve lower dimensionality also expands perovskite material choices.<br/>Most of the reported 2D Ruddlesden–Popper (RP) phase lead halide perovskites with the general formula of A_n+1B_nX_3n+1 (n = 1, 2, …) comprise of layered perovskites separated by A-site-substituted organic spacers. This layered structure is constructed with long carbon chain spacers introducing a large separation between BX₆ octahedron inorganic layers. The organic spacers work as an insulating layer that provides large exciton binding energy and reduces the conductivity in a vertical direction through each inorganic layer. Therefore, the X-site substituted layered perovskite presents a much smaller separation between the constituent perovskite layers than the A-site substituted layered perovskite, moderating the exciton binding energy.<br/>To-date, only three X-site-substituted RP phase perovskites have been reported [1-3]. Herein, we reported the first inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs₂Pb(SCN)₂Br₂. It is synthesized by the antisolvent vapor-assisted crystallization (AVC) method at room temperature. The crystal exhibits a standard single-layer (n = 1) RP phase structure described in the space group of <i>Pmmn</i> (#59) with a slight separation (<i>d</i> = 1.69 Å) between the perovskite sheets. Interestingly, 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 &gt; 1). Above 450K, the 2D single crystal undergoes an unusual but reversible first-order phase transformation to 3D CsPbBr₃ (<i>Pm3m</i> #221). According to the small separation between perovskite sheets, Pb-Br-Pb coordination can be formed to drive the SCN^- anion away and transform into a more stable 3D CsPbBr₃ structure at high temperatures. Once the temperature cools down to 250K, the existed SCN^- free anions within the grains break the weak Pb-Br-Pb coordination to reconstruct the Pb(SCN)₂Br₂octahedral due to the preferable Gibbs energy. Again, due to the small interlayer separation, Cs₂Pb(SCN)₂Br₂ exhibits a minuscule exciton binding energy of 160 meV measured by temperature-dependent PL. It is one of the lowest values reported for 2D perovskites (n = 1) and comparable to the quasi-2D A-site substituted RP phase perovskite values. Finally, a Cs₂Pb(SCN)₂Br₂ single crystal photodetector is demonstrated with a respectable responsivity of 8.46 mA W⁻¹ and a detectivity of ≈1.2 × 10¹⁰ Jones at a low bias voltage of 0.5 V.<br/><br/><b>References</b><br/>[1] M. Daub, H. S. Hillebrecht, <i>Angew. Chem.</i>, <b>2015</b>, 127, 11168.<br/>[2] J. Li, Q. Yu, Y. He, C. C. Stoumpos, G. Niu, G. G. Trimarchi, H. Guo, G. Dong, D. Wang, L. Wang, M. G. Kanatzidis, <i>J. Am. Chem. Soc.</i>, <b>2018</b>, 140, 11085.<br/>[3] C. H. Liao, C. H. Chen, J. Bing, C. Bailey, Y. T. Lin, T. M. Pandit, L., Granados, J. Zheng, S. Tang, B. H. Lin, H. W. Yen, D. R. McCamey, B. J. Kennedy, C. C. Chueh, A. W. Ho-Baillie, <i>Adv. Mater.,</i> <b>2022</b>, 34(7), 2104782.