Small polaron formation in transition metal oxides poses a barrier to their use in solar energy devices by limiting the mobility and lifetimes of photoexcited carriers. Methods to mitigate the effect of small polarons, such as altering the composition of ligands, changing the coordination of ligands, and nanostructuring, have not been explored because the femtosecond timescale of polaron formation inhibits measurement. In this work, we employ extreme ultraviolet (XUV) transient absorption spectroscopy of the Fe M2,3 edge to measure small polaron formation kinetics in photoexcited goethite (α-FeOOH) crystalline nanorods. Small polaron formation is observed via the breaking of the Fe 3p level degeneracy, which appears in the XUV spectrum 180 ± 30fs after visible excitation. Excited electrons are measured to have an 80 ± 10% probability of forming small polarons, and the signal associated with the polaron persists for over 300 ps. At short excitation wavelengths, there is a significant increase in polaron formation time to 350 ± 40fs at 2.6 eV. The ultrafast small polaron formation in goethite is compared to similar studies of hematite (α-Fe2O3) thin films. In particular, a longer polaron formation time was measured in goethite (180 ± 30fs) compared to hematite (90 ± 5 fs), suggesting that polaron formation may be tuned by altering the ligand composition and coordination environment. This work suggests that incorporating small changes in the ligands and morphology could allow for the control of photoexcited small polaron formation in transition metal oxides.

Small polaron formation in iron oxide nanoparticles: the effect of ligand field and morphology on formation rates

256th National Meeting (2018)

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