Abstract
Understanding the electronic structure and dynamics of semiconducting nanomaterials at the atomic level is crucial for the realization and optimization of devices in solar energy, catalysis, and optoelectronic applications. We report here on the use of ultrafast X-ray linear dichroism spectroscopy to monitor the carrier dynamics in epitaxial ZnO nanorods after band gap photoexcitation. By rigorously subtracting out thermal contributions and conducting ab initio calculations, we reveal an overall depletion of absorption cross sections in the transient X-ray spectra caused by photogenerated charge carriers screening the core-hole potential of the X-ray absorbing atom. At low laser excitation densities, we observe phase-space filling by excited electrons and holes separately. These results pave the way for carrier- and element-specific probing of charge transfer dynamics across heterostructured interfaces with ultrafast table-top and fourth-generation X-ray sources.
Highlights
The past decade has seen the development of photovoltaic cells with great chemical and structural complexity
Our results demonstrate the use of ps-resolved transition-metal K-edge spectroscopy to track the evolution of excited charge carriers in a widely used prototypical Zinc oxide (ZnO) semiconducting material. 68,69 We use transient X-ray linear dichroism (XLD) as a new methodology to isolate non-thermal contributions in the transient XANES
For the first time, the screening of the core-hole potential by delocalized excited electrons in the transient XAS of photoexcited semiconducting materials. These results are in contrast with a large body of previous ultrafast XAS studies on semiconducting materials that report on charge trapping and/or polaron formation upon photoexcitation
Summary
The past decade has seen the development of photovoltaic cells with great chemical and structural complexity. We use transient ps-resolved XLD to reveal non-local electronic effects in the Zn K-edge XAS of photoexcited epitaxial ZnO nanorods as a fingerprint of carrier transport.
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