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.
Highlights
Layered perovskites have aroused interest for a long time as a result of their ability to generate versatile molecular structures with improved stabilities and continue to exhibit unique physicochemical properties
Those high hopes stem from the fact that 2D perovskites surpass 3D counterparts in terms of higher resistance to moisture and chemical and irradiation stresses and can be thought of as active and stability-fortifying components of photovoltaic layers.[2−4] There is an enticing prospect for using 2D perovskites in lighting applications owing to the improved photoluminescence quantum yield (PLQY) of the bulk samples.[5]
The large diversity of available organic spacers offers enormous opportunities for tuning the structural, physical, and chemical properties of 2D perovskites.[2−5] The inorganic layers contribute to the semiconductor electronic structure; due to the spatial separation, they form quantum-like structures with 2D electronic confinement.[6−9] Apart from quantum confinement, the separation of the inorganic layers by organic ones provides for the dielectric confinement of charge carriers.[7,8,10]
Summary
Layered perovskites have aroused interest for a long time as a result of their ability to generate versatile molecular structures with improved stabilities and continue to exhibit unique physicochemical properties. The large diversity of available organic spacers offers enormous opportunities for tuning the structural, physical, and chemical properties of 2D perovskites.[2−5] The inorganic layers contribute to the semiconductor electronic structure; due to the spatial separation, they form quantum-like structures with 2D electronic confinement.[6−9] Apart from quantum confinement, the separation of the inorganic layers by organic ones provides for the dielectric confinement of charge carriers.[7,8,10] As a result, stable excitons (electron−hole pairs) may exist at room temperature (RT).[5−11] Typically, the exciton binding energies of 2D perovskites are as high as a few hundreds of meV, which is an order of magnitude higher relative to 3D lead halide perovskites.[5−11] the large exciton binding energies and wide band gaps are both responsible for the Received: November 18, 2020 Revised: March 16, 2021 Published: March 31, 2021. It is worth noting that the band gap narrows and the exciton energy decreases with decreasing octahedral tilting (crumpling of the extended corner-shared PbX6 layers).[8,13,16,17]
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