Properties and dynamics of excited charge carriers in colloidal nanocrystals studied via optical and terahertz pump-probe spectroscopy

Abstract

Colloidal nanocrystals are extensively studied for their unique, size-dependent optical and electronic properties, enabling possible applications in photocatalysis, solar cells or light-emitting devices as well as providing a platform for studying fundamental physical phenomena such as light-matter coupling or superfluorescence. The optical properties of metal nanoparticles are dictated by the localized surface plasmon resonance, a collective vibration of the conduction band electrons, that falls into the optical frequency region for gold. Research on gold nanoparticles focuses on the utilization of highly excited electrons that are generated during the dephasing of the plasmon oscillation. Semiconducting nanocrystals have recently attracted the attention of a wider public, since the Nobel Prize in Chemistry in 2023 was devoted to research on their size-tunable band gap. Material parameters such as size, shape and composition have a significant effect on the optical and electronic properties of excited charge carriers. Therefore, with smart design, multicomponent nanocrystals can be tailored for specific functions, such as efficient charge separation for photocatalytic applications or efficient charge recombination for light-emitting devices. Ultrafast lasers allow to study the properties and dynamics of optically excited charge carriers via various pump-probe spectroscopic methods and hence help to make smart design choices for optimizing nanocrystals towards the various applications. This thesis aims to contribute to the understanding of how the nanocrystal’s material parameters like size, shape, crystallinity or surface chemistry alter the dynamics of optically excited carriers. To do so, both metal and semiconductor nanocrystals as well as hybrid nanostructures that combine the properties of multiple components were studied via optical-pump-optical-probe (transient absorption) and optical-pump-terahertz-probe spectroscopy.

Employing transient absorption spectroscopy, it could be shown that in gold nanoparticles, the electron-phonon coupling constant is a size-dependent quantity. However, the size dependency can only be observed in monocrystalline particles, as in polycrystalline particles, it is blurred due to efficient electron-surface scattering at internal grain boundaries. In a second study, it has been shown that in order to enable strong optical interaction of gold nanoparticles with dyes, carefully designing the dye and applying it to the nanoparticle’s surface is crucial.

In a project focusing on semiconductor nanocrystals, the charge carrier localization in spherical and elongated CdSe-CdS core-shell nanocrystals was compared. Employing a combination of transient absorption and optical-pump-terahertz-probe spectroscopy, it could be shown that shell-excited carriers are mobile in elongated particles for hundreds of picoseconds, whereas they quickly form bound electron-hole pairs in spherical particles. Furthermore, hybrid semiconductor-metal nanoparticles were studied for their photocatalytic hydrogen generation, whereby it was shown that ionic electron donating agents scavenge holes from the photocatalyst faster than alcoholic ones due to their enhanced surface activity, thereby enabling fast charge carrier dynamics and efficient hydrogen generation.

Finally, in a technical study on optical-pump-terahertz-probe spectroscopy, spectral distortions in the acquired data caused by too small pump beam spot sizes were discussed. A plain model was set up to account for such distortions and different experimental implementations were compared.