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

• Applicant: Krishan Kumar, Ph.D.
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2021 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 468735112

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.

• DFG Programme: WBP Position