Electron dynamics at surface-modified photocathodes

本项目旨在了解控制原型光电极表面电子动力学和能量学的基本过程，半导体表面附近的相关内部界面以及光电化学产氢的相关模型系统。界面电子传递过程的详细机理及其动力学尚不清楚。我们建议专门修改III-V型化合物半导体吸收系统的表面电子和化学性质，以促进多电子过程。用于显式表面敏感分析的时间分辨双光子光电发射（tr-2PPE）将与基于密度泛函理论（DFT）的数值模拟相结合，以获得对关键电子转移和重组过程的基本理解。Tr-2PPE是一种独特的技术，可以直接探测光电子的动能和动力学，同时获得电子结构和近表面态的时间占用。III-V化合物半导体作为相关的模型系统来研究相对于选择的表面修饰程序的界面动力学。修饰III-V表面的方法包括薄膜外延生长、原位表面转化和催化剂沉积。这些方法可以产生准二维覆盖层，以减缓腐蚀并提高光催化活性。这种表面层的修饰可以通过选择性的电子结构修饰来调整电子传递动力学。研究不同类型的表面修饰将使我们能够大致了解界面设计如何有利于电子向催化活性表面的传递。关键的科学问题是：(i)在相关的激发密度下，改性表面上热电子的冷却是否比块体材料中的热电子冷却要慢得多；（ii）界面状态是否可以在导带附近特别引入；（iii）如何增强电子收集和抑制非辐射复合。从研究联盟的长远角度来看，这些问题的答案将有利于设计设想的新型多结吸收系统的功能化层，以便为多电子催化过程提供最佳的电荷分离和转移。

This project aims to understand the fundamental processes that govern electron dynamics and energetics of prototypical photoelectrode surfaces, the associated internal interfaces near semiconductor surfaces and related model systems in view of photoelectrochemical hydrogen generation. The detailed mechanisms of interfacial electron transfer processes and their dynamics are still insufficiently understood. We propose to specifically modify the surface electronic and chemical properties of III-V compound semiconductor absorber systems to promote multi-electron processes. Time-resolved two-photon photoemission (tr-2PPE) for explicitly surface-sensitive analysis will be combined with density-functional theory (DFT)-based numerical simulations in order to gain a fundamental understanding of key electron transfer and recombination processes. Tr-2PPE is a unique technique that directly probes the kinetic energy and dynamics of photoemitted electrons accessing at the same time the electronic structure and temporal occupation of surface-near states. III-V compound semiconductors serve as relevant model systems to investigate interfacial dynamics with respect to selected surface modification procedures. Ways to modify III-V surfaces include epitaxial growth of thin films, in-situ surface transformation and catalyst deposition. These methods can produce quasi-two dimensional overlayers to slow down corrosion and enhance photocatalytic activity. The modification with such surface layers will enable tuning of the electron transfer dynamics by selective electronic structure modifications. Studying different types of surface modification will allow us to draw a general picture how interface design benefits electron transport towards the catalytically active surface. Key scientific questions are (i) if cooling of hot electrons at modified surfaces can be substantially slower than in bulk material at relevant excitation densities, (ii) if interface states can be introduced specifically near the conduction band, (iii) how electron collection can be enhanced and non-radiative recombination suppressed. In the long-term perspective of the Research Consortium, answers to these questions will benefit the design of functionalization layers of the envisaged new multi-junction absorber systems in order to provide optimum charge separation and transfer for multi-electron catalytic processes.