Microscopic understanding of the phonon bottleneck in 2D perovskites

钙钛矿构成了一种可再制造的半导体材料体系，在许多方面与众所周知的外延半导体家族有很大的不同。对三维钙钛矿的巨大兴趣引发了对其低维形态的研究，如纳米晶和二维钙钛矿。后者以其优越的发射性能而闻名，表现出比传统半导体高出一到两个数量级的量子发射效率。这种技术上重要特征的微观起源仍然是正在进行的研究的主题，在文献中一直有争议地讨论。软晶格导致了钙钛矿材料中特殊的激子-声子相互作用，特别是已知激子与声学声子的耦合非常弱。结果表明，这极大地减少了声子辅助散射进入暗激子态，从而解释了钙钛矿纳米晶体的强烈发光。然而，2D钙钛矿中亮激子态和暗激子态之间的声子瓶颈仍然缺乏明确的证据，需要一个先进的激子弛豫动力学微观模型。在这个实验-理论联合项目中，我们将研究二维钙钛矿中声子瓶颈的微观来源及其对光学性质的影响。这些技术前景看好的纳米材料为研究亮激子态和暗激子态之间的声子瓶颈提供了一个前所未有的舞台，因为它们为激子精细结构和声子谱的独立修饰提供了可能性。这不仅为深入了解声子介导的激子弛豫提供了机会，而且为确定新型高效发光器件的设计提供了控制和克服声子瓶颈的机会。通过改变二维钙钛矿量子阱的厚度、化学组成和有机间隔分子，我们将能够调节暗-亮激子分裂和光学声子能量之间的光谱差，从而揭示增强或抑制声子瓶颈效应的条件。为了实现这个项目的总体目标，我们将联合实验和理论小组的力量。结合光学光谱学领域的专业知识和先进的激子动力学模型，我们将提供对2D钙钛矿中声子瓶颈背后的基本过程的微观洞察，我们将揭示如何控制这种效应的策略，并优化2D钙钛矿的技术关键量子发射效率。

Perovskites constitute a remakable semiconducting material system, which is in many aspects very different from the well-known and understood family of epitaxial semiconductors. The huge interest in 3D perovskites triggered an investigation of their lower dimensional forms, such as nanocrystals and two-dimensional (2D) perovskites. The latter are known for their superior emissive properties exhibiting a quantum emission efficiency that is one or two orders of magnitude higher than in conventional semiconductors. The microscopic origin of this technologically important characteristics is still the subject of ongoing research and has been controversially discussed in literature. The soft lattice results in a specific exciton-phonon interaction in perovskite materials, in particular, excitons are known to a couple very weakly to acoustic phonons. It was proposed that this leads to extremely reduced phonon assisted scattering into the dark exciton states explaining very intense luminescence from perovskites nanocrystals. However, a clear evidence for this phonon bottleneck between bright and dark exciton states in 2D perovskites is still missing and an advanced microscopic modelling of exciton relaxation dynamics is needed. In this joint experiment-theory project, we will investigate the microscopic origin of the phonon bottleneck in 2D perovskites and its impact on their optical properties. These technologically promising nanomaterials constitute an unprecedented playground to study the phonon bottleneck between bright and dark exciton states as they offer the possibility for independent modification of the excitonic fine structure and the phonon spectrum. This provides an opportunity not only to thoroughly understand phonon-mediated exciton relaxation, but also to control and overcome the phonon bottleneck for a deterministic design of novel highly efficient light emitters. By varying the 2D perovskite quantum well thickness, chemical composition and organic spacer molecule we will be able to tune the spectral difference between the dark-bright exciton splitting and the optical phonon energy revealing the conditions for enhanced or suppressed phonon bottleneck effect. To realize the general objective of this project we will join forces of experimental and theoretical groups. Merging expertise in the field of optical spectroscopy with advanced exciton dynamics modelling we will provide microscopic insights into the elementary processes behind the phonon-bottleneck in 2D perovskites and we will reveal strategies of how to control this effect and optimize the technologically crucial quantum emission efficiency of 2D perovskites.