Coexisting Large and Small Polarons in Photoexcited CeO${}_2$

Abstract

Time-resolved terahertz (THz) spectroscopy is used to probe polaron formation in CeO${}_2$. The ultrafast THz photoconductivity is dominated by contributions from large hole and small electron polarons. Delocalized large hole polarons are formed on a $820\pm 50$ fs time scale and are of Fröhlich type, with optical conductivity well described by a simple Drude model and a transient hole mobility of $100$ cm${}^2$/Vs. Small electron polarons form via localized coupling to Ce lattice sites, revealed by reduction in the lattice Born effective charge causing a transient phonon softening. These results establish the dual polaron nature of CeO${}_2$ and provide a basis for understanding charge transfer dynamics in metal oxide photocatalysts.

11 pages, 6 figures (including Supplementary Information)