Charge Carrier Screening in Photoexcited Epitaxial Semiconductor Nanorods Revealed by Transient X-ray Absorption Linear Dichroism



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 unravel 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.


Supplementary material

Supporting Information
The Supporting Information is available free of charge which contains sample synthesis and characterizations, experimental details about the XTA measurements, calculation of the excitation densities, additional kinetics, evolution of XAS with lattice temperature, simulation of the timescale of heat diffusion, effect of the penetration depth mismatch between the laser and the X-rays, chemical shift simulations, thermal and screening effects in previous XTA measurements at the metal K-edge, mathematical derivation of transient XLD, XTA fluence dependence, XAS of ZnO under solar illumination – non-local screening effects, ab initio calculations.