Colloidal Nanocrystals of Monomethylhydrazinium Lead Bromide—A 2.5D Perovskite

The field of functional inorganic materials for optoelectronics has witnessed in the past decades a rapid development of metal halide perovskites with ABX₃ formula, where B = Pb, Sn, X = Cl, Br, I and A is a small cation, either inorganic (Cs⁺) or organic (methylammonium, MA⁺ or formamidinium, FA⁺). Every constituent of the perovskite lattice has been scrutinized and numerous organic cations came into consideration for A site position. The rule for retaining a three-dimensional (3D) perovskite lattice in good approximation is summarized by the concept of Goldsmith tolerance factor, limiting the size of A cation to not much larger than 260 pm.¹At the edge of the 3D perovskite stability window are a couple of non-trivial organic cations, <i>e.g.</i> monomethylhydrazinium (MMH⁺, 263 pm), azetidinium (AZT⁺, 250 pm) and aziridinium (AZR⁺, 227 pm).^3-5While AZR⁺ still allows for a cubic perovskite lattice, AZT⁺ and MMH⁺ introduce lattice distortions, resulting in the retention of 3D structural connectivity with the adoption of symmetry lower than cubic. Among other organic cations, MMH⁺ especially stands out with its lone electron pair, readily coordinating to metals. Recently discovered bulk MMHPbBr₃ features two MMH⁺cations in coordination environment of one Pb atom together with distorted octahedral environment of Br atoms.^{&lt;span style="font-size:10.8333px"&gt;6&lt;/span&gt;} The other crystallographically inequivalent Pb atom resides in a conventional octahedral perovskite environment. Both PbBr₆ octahedron types retain corner-sharing and assemble into a typical perovskite ABX₃ structure, albeit with lowered monoclinic noncentrosymmetric space group, and alternating layers of distorted and undistorted PbBr₆ octahedrons layers. The unique MMHPbBr₃ structure cannot be immediately categorized as one of two classical structural types, a 3D perovskite or a layered 2D perovskite with n=1. The degree of wavefunction hybridization between the distorted and undistorted layers would define where MMHPbBr₃ falls. Using density functional theory (DFT), we reveal the unique 2.5D structure of MMHPbBr₃ where the electron-wavefunction has three-dimensional confinement and hole-wavefunction is confined within an undistorted perovskite layer. Herein, we for the first time obtain MMHPbBr₃ in the form of colloidal nanocrystals (NCs), revealing the unique excitonic nature of this material. We characterize the synthesized NCs structurally and report the emission properties at the ensemble and single dot levels.<br/><br/>References<br/>1. Jin, S. Can We Find the Perfect A-Cations for Halide Perovskites? <i>ACS Energy Lett.</i> <b>6</b>, 3386-3389, (2021).<br/>2. Pering, S. R.<i> et al.</i> Azetidinium lead iodide for perovskite solar cells. <i>J. Mater. Chem. A</i> <b>5</b>, 20658-20665, (2017).<br/>3. Zheng, C. & Rubel, O. Aziridinium Lead Iodide: A Stable, Low-Band-Gap Hybrid Halide Perovskite for Photovoltaics. <i>J. Phys. Chem. Lett.</i> <b>9</b>, 874-880, (2018).<br/>4. Petrosova, H. R., Kucheriv, O. I., Shova, S. & Gural'skiy, I. A. Aziridinium cation templating 3D lead halide hybrid perovskites. <i>Chem. Commun.</i> <b>58</b>, 5745-5748, (2022).<br/>5. Bodnarchuk, M. I.<i> et al.</i> Colloidal Aziridinium Lead Bromide Quantum Dots. <i>ACS Nano</i> <b>18</b>, 5684-5697, (2024).<br/>6. Maczka, M. a.<i> et al.</i> Methylhydrazinium Lead Bromide: Noncentrosymmetric Three-Dimensional Perovskite with Exceptionally Large Framework Distortion and Green Photoluminescence. <i>Chemistry of Materials</i> <b>32</b>, 1667-1673, (2020).