Ultrafast transient pump-pump-probe spectroscopic studies in colloidal semiconductor heteronanostructures to follow processes involving multiple excitations

多个激发事件在捕光应用中具有最高的相关性，即涉及多电子氧化还原反应的光催化。为了在反应中心实现电荷积累，必须发生几个连续的光致电荷载流子转移步骤，每个步骤都由一个光子的吸收启动，或者从多个激发系统分离电荷并准同时发生多个电荷转移到反应中心。大多数研究报告仅在单一激发条件下研究载流子动力学。为了克服这一局限，本项目旨在研究胶体半导体纳米结构中的激子和电荷载流子动力学。泵浦-泵浦-探测暂态吸收光谱将用于研究多激发态纳米结构中连续的光诱导电子转移过程和过程的动力学。该项目将深入了解原位条件下初始电子转移以外的电子转移级联。关注的焦点将是异构体结构，例如金属尖端的CdSe@CDS纳米棒，它们已被证明对光子到氢的转换具有高效率。探讨了在第一个电子转移步骤之后，由于金属颗粒的带电而在半导体/金属界面形成的电荷分离的额外势垒对电子转移过程的影响。我们将探索结构因素对第二电荷转移步骤的影响，以补充已有的关于第一电荷转移的知识，并支持优化结构的设计。多激子的相互作用及其动力学将在异质结构中进行研究，例如，CdSe@CDS纳米棒。激发波长的选择允许在异质结构中控制产生的激子在定义的子域中的初始局域化。特别是，两个空间上分离良好的激子通过俄歇复合(AR)湮没的时间尺度将提供一个基本的理解，即激子之间的相互作用依赖于纳米结构中的结构和电子因素(粒子的体积和长径比以及能带排列)。此外，将考虑具有不同电子耦合程度的半导体纳米颗粒组装中的多个激发之间的相互作用。相邻粒子之间的耦合强度可以通过修饰粒子的表面配体来调节，这会影响纳米粒子组装中的激子迁移过程。激子在层中的扩散会导致AR产生的多个激发的猝灭。这些研究结果将有助于理解AR时间尺度与结构之间的关系，并将指导具有改进性质的结构的设计，从而使多激子能够在未来的光收集应用中得到应用。

Multiple excitation events are of highest relevance in light-harvesting applications, i.e., photocatalysis involving multielectron redox reactions. To achieve charge accumulation at the reaction center either several consecutive light-induced charge carrier transfer steps, each step initiated by absorption of a photon, or charge separation from multiple excited systems and quasi simultaneous multiple charge transfer to a reaction center have to occur. The majority of research reports on investigations of charge carrier dynamics solely under single excitation conditions. To overcome this limitation, this project aims to study the exciton and charge carrier dynamics in colloidal semiconductor nanostructures involving multiple excitations. Pump-pump-probe transient absorption spectroscopy will be applied to investigate the dynamics of consecutive light-induced electron transfer processes and processes in multiply excited nanostructures.The project will deliver insight into the electron transfer cascade beyond the initial electron transfer under in situ conditions. In the focus will be heteronanostructures, e.g., metal tipped CdSe@CdS nanorods, which have proven high efficiencies for photon-to-hydrogen conversion. The impact of the formation of an additional barrier for the charge separation at the semiconductor/metal interface due to charging of the metal particle after the first electron transfer step on the electron transfer process will be explored. The influence of structural factors on the second charge transfer step will be explored to complement the already available knowledge for the first charge transfer and support the design of optimized structures.The interaction of multiple excitons and their dynamics will be studied in heteronanostructures, e.g, CdSe@CdS nanorods. The choice of the excitation wavelength allows in heterostructures to control the initial localization of a generated exciton in a defined subdomain. Especially, the timescale of annihilation via Auger recombination (AR) of two initially spatially well-separated excitons will provide a fundamental understanding of the interaction of excitons in dependence on structural and electronic factors (volume and aspect ratio of the particle and band alignment) in the nanostructure. Further, interactions between multiple excitations in assemblies of semiconductor nanoparticles with varying degree of electronic coupling will be regarded. The coupling strength between neighboring particles can be tuned via modification of the surface ligands of the particles, which impacts exciton migration processes within the nanoparticle assembly. Exciton diffusion in the layer can lead to quenching of multiple excitations generated via AR. The results of these investigations will help to understand the relations between AR time scale and structure and will guide the design of structures with improved properties to enable future use of multiexcitons in light-harvesting applications.