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Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3

Abstract

Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion efficiency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (α-Fe2O3). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M2,3 edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t2g conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite’s photoconversion efficiency.

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Figure 1: Change in core level transitions with photoexcitation.
Figure 2: Transient absorption after 480 nm excitation in Fe2O3.
Figure 3: Energy dependence of polaron formation and decay.
Figure 4: Effect of polaron formation across haematite’s absorption.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05-CH11231 within the Physical Chemistry of Inorganic Nanostructures Program (KC3103). S.K.C. acknowledges support by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Solar Energy Technologies Office.

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L.M.C., S.K.C. and S.R.L. designed the study. L.M.C. and S.K.C. performed the transient XUV measurements and data analysis. S.K.C. modelled the polaron spectral signature and dynamics. C.L. and Y.S. were responsible for sample fabrication and characterization. L.M.C., S.K.C., C.L., Y.S., P.Y., A.P.A. and S.R.L. wrote and revised the manuscript.

Corresponding author

Correspondence to Stephen R. Leone.

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Carneiro, L., Cushing, S., Liu, C. et al. Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3. Nature Mater 16, 819–825 (2017). https://doi.org/10.1038/nmat4936

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