Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

中文翻译：

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二维 Ruddlesden-Popper 钙钛矿中离散供体-受体对转变的起源

二维（2D）范德华纳米材料因其在纳米尺度光子和光物质应用中的潜在用途而引起了广泛关注。由于其激子特性，二维钙钛矿也是有前途的活性材料，可用于在室温下工作的设备。在这项工作中，我们通过μ光致发光和时间衰减测量研究了二维卤化铅钙钛矿中非常窄且空间局部化的光学跃迁的存在。这些离散光学跃迁的特点是亚毫电子伏线宽 (≃120μeV) 和长衰减时间 (5–8 ns)。 X射线光电子发射和密度泛函理论计算已被用来研究负责这些跃迁的电子态的化学起源。苯乙基铵与甲基铵阳离子结合形成二维 Ruddlesden-Popper 钙钛矿 (PEA)2(MA)n−1PbnI3n+1，特别是在 n≥2 的相中，已被确定为供体-受体对 (DAP) 形成的机制，对应于铅原子的置换及其被甲基铵的取代。电离 DAP 复合被认为是观察到的离散光发射线最可能的物理来源。使用简单模型对实验数据进行分析，评估电离受体和供体之间的库仑相互作用，返回玻尔半径约为 10 nm 的供体。从基础研究的角度来看，对二维钙钛矿中这些单一供体-受体态的光谱和电子特性的分析特别重要，并且能够将这些态的发射与新的光电应用联系起来。需要远距离光学可控相互作用。