Microscopic understanding of the phonon bottleneck in 2D perovskites

钙钛矿构成了一种值得注意的半导体材料体系，其在许多方面与众所周知和理解的外延半导体家族非常不同。对3D钙钛矿的巨大兴趣引发了对其低维形式的研究，例如纳米晶体和二维（2D）钙钛矿。后者以其上级发射特性而闻名，其量子发射效率比常规半导体高一个或两个数量级。这一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，并在文献中进行了有争议的讨论。软晶格导致钙钛矿材料中特定的激子-声子相互作用，特别地，已知激子与声学声子的耦合非常弱。有人提出，这导致了极大减少的声子辅助散射到暗激子状态，解释了钙钛矿纳米晶体的非常强烈的发光。然而，在2D钙钛矿中亮激子和暗激子状态之间的声子瓶颈的明确证据仍然缺失，并且需要激子弛豫动力学的高级微观建模。在这个联合实验-理论项目中，我们将研究二维钙钛矿中声子瓶颈的微观起源及其对光学性质的影响。这些技术上有前途的纳米材料构成了一个前所未有的游乐场，研究明亮和黑暗的激子状态之间的声子瓶颈，因为它们提供了独立修改激子精细结构和声子光谱的可能性。这提供了一个机会，不仅要彻底了解声子介导的激子弛豫，而且要控制和克服声子瓶颈的确定性设计的新型高效发光体。通过改变2D钙钛矿量子阱厚度、化学组成和有机间隔分子，我们将能够调节暗-亮激子分裂与光学声子能量之间的光谱差异，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，我们将联合实验组和理论组的力量。将光学光谱学领域的专业知识与先进的激子动力学建模相结合，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见解，我们将揭示如何控制这种效应并优化2D钙钛矿的技术关键量子发射效率的策略。

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.