[Methylhydrazinium]₂PbBr₄, a Ferroelectric Hybrid Organic–Inorganic Perovskite with Multiple Nonlinear Optical Outputs

Abstract

An expansive library of structurally complex two-dimensional (2D) and three-dimensional (3D) lead halide perovskites has emerged over the past decade, finding applications in various aspects of photon management: photovoltaics, photodetection, light emission, and nonlinear optics. Needless to say, the highest degree of structural plasticity enjoys the former group, offering a rich playground for modifications of relevant optoelectronic parameters such as exciton energy. Structural tailorability is reflected in the ease of modification of the chemistry of the organic layers residing between inorganic slabs. In this vein, we show that the introduction of methylhydrazinium cation (MHy+, CH3NH2NH2 +) into 2D perovskite gives a material with a record low separation of the inorganic layers (8.91 Å at 300 K). Optical studies showed that MHy2PbBr4 features the most red-shifted excitonic absorption among all known A2PbBr4 compounds as well as a small exciton binding energy of 99.9 meV. MHy2PbBr4 crystallizes in polar Pmn21 symmetry at room emperature (phase III) and at 351 K undergoes a phase transition to modulated Pmnm phase (II) followed by another phase transition at 371 K to Pmnm phase (I). The ferroelectric property of room-temperature phase III is inferred from switching of the pyrocurrent, dielectric measurements, and optical birefringence results. MHy2PbBr4 exhibits multiple nonlinear optical phenomena such as second-harmonic generation, third-harmonic generation, two-photon excited luminescence, and multiphoton excited luminescence. Analysis of MHy2PbBr4 single-crystal luminescence spectra obtained through linear and nonlinear optical excitation pathways indicates that free exciton emission is readily probed by the ultraviolet excitation, whereas crumpled exciton emission is detected under two- and multiphoton excitation conditions. Overall, our results demonstrate that incorporation of MHy+ into the organic layer is an emergent strategy for obtaining a 2D perovskite with polar character and multifunctional properties.

Temperature-Dependent Raman Studies of FAPbBr3 and MAPbBr3 Perovskites: Effect of Phase Transitions on Molecular Dynamics and Lattice Distortion

“…Hybrid lead halide perovskites crystallizing in various crystal structures, including zero-, one-, two-and three-dimensional (0D, 1D, 2D and 3D), have attracted a lot of interest in recent years due to their various functional properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Regarding 3D perovskites, the most famous ones are MAPbI 3 and FAPbI 3 (MA = methylammonium; FA = formamidinium) since they exhibit excellent photovoltaic properties with a power conversion efficiency exceeding 21% [7,8].…”

Nonlinear Optical Properties of Emerging Nano‐ and Microcrystalline Materials

“…Mączka and coworkers recently presented linear and nonlinear optical studies of exciton emission of 2D layered perovskite MHy 2 PbBr 4 with ferroelectric property (Table 2, entry 16). [89] In their work, they also observed that the sharp CE emission is clearly visible under 2PA excitation conditions at 800 nm and 1000 nm but not under 1PA excitation (367 nm). Temperature-resolved studies (80-370 K) showed that CE emission is present up to room temperature, although it significantly subsides in intensity and shifts towards longer wavelengths, fully merging with self-trapped excitons (STEx) emission at ≈320 K. They have also studied exciton luminescence of the same material using longwavelength 1300 nm femtosecond laser; at that wavelength the luminescence is of mixed three-photon and four-photon origin, as revealed by power-dependent measurements.…”

Structure‐Property Relationships and Idiosyncrasies of Bulk, 2D Hybrid Lead Bromide Perovskites

“…[177] Surprisingly, beside the 3D lead bromide perovskite structure, the MHy cations can simultaneously template the n = 1 structure (MHy) 2 PbBr 4 . [178] Just recently, FA 2 PbBr 4 (n = 1) phases with the corrugated (110)-orientated structure were reported in two polymorphs: triclinic (t-FA 2 PbBr 4 ) and monoclinic (m-FA 2 PbBr 4 ) (Table 2). [139] The FA 2 PbBr 4 phases are synthesized in pure form with the use of excess FABr salt and subsist in a labile equilibrium with the 3D FAPbBr 3 phase.…”

“…As discussed above, structurally and synthetically, the majority of bulk, lead bromide perovskites reported are the thin-layered (n = 1) members. [55,[88][89][91][92][93][95][96][97][102][103][104][105]113,[123][124][125][126][128][129][141][142][143][144][145][146][147] In the early 1990s, Thorn and colleagues report the first evidence of thick-layer lead halide perovskites (n > 1) by producing films of (C 9 H 19 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 and (C 9 H 19 NH 3 ) 2 (CH 3 NH 3 ) nÀ 1 Pb n Br 3n + 1 (n = 1-3) series with the nonylammonium spacer cation, along with films of (PhNH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 with Ph: phenylammonium. [148] Although unable to obtain single crystal structures at the time, Thorn and colleagues noted the excitonic absorption peak of (C 9 H 19 NH 3 ) 2 (CH 3 NH 3 )Pb 2 Br 7 films at 430 nm and (C 9 H 19 NH 3 ) 2 (CH 3 NH 3 ) 2 Pb 3 Br 10 at 450 nm, lower in energy than their n = 1 member.…”