Probing Polaron Design Principles in Iron Oxides with Transient Extreme Ultraviolet Spectroscopy

Long timescale charge separation and high carrier mobilities are necessary for efficient devices. In highly polar transition metal oxide materials, photoexcited polarons remain a bottleneck to realizing highly mobile carriers, while also offering an exciting route for long timescale charge separation. Controlling photoexcited polaron formation in transition metal oxides is therefore necessary. This thesis uses transient extreme ultraviolet (XUV) reflection-absorption spectroscopy and ab initio density functional theory (DFT) and the Bethe–Salpeter equation (BSE) to elucidate photoexcited polaronic dynamics in a series of iron oxide photocatalysts. By systematically varying structure, on-site repulsions, electron–phonon coupling, and exchange interactions, this work identifies the fundamental materials parameters that govern ultrafast polaron formation. The results demonstrate that photoexcited polaron formation can be tuned and, in some cases, suppressed. In CuFeO2, small polarons form on ~100 fs timescales, followed by dynamic delocalization mediated by coherent lattice expansion and charge sharing with surrounding Fe sites. In ErFeO3, frustrated iron-centered octahedra enable favorable exchange pathways, where exchange-mediated charge hopping thermalizes carriers prior to localization as polarons. In GdFeO3, excitation-wavelength-dependent XUV spectroscopy reveals that ligand-to-metal and metal-to-metal charge transfer pathways modulate polaron formation, demonstrating that strong electronic and spin correlations can either suppress or enable polaron formation. In Fe2SiO4 polaron formation is present following metal-to-metal charge transfer. These experimental observables are then mapped onto the Holstein, Hubbard–Holstein, and t-J–Holstein models to define a parameter space for polaron control. These ground-state Hamiltonians act as a ground-state calculable roadmap for materials design parameters based upon measured excited state polaronic dynamics. Together, these studies provide design principles for controlling polaronic behavior in transition metal oxides for photocatalytic, electronic, and quantum materials applications.