Publication date: 31st March 2013
Often the presence of electronic states within the band gap is claimed to be the origin of fast charge carrier trapping and electron-hole recombination in transition metal oxides. Prominent among these mid-gap states are the intersite and intrasite d-d transitions. We report transient optical spectroscopy of Co3O4 where these d-d transitions can be selectively excited and probed. The data suggests that excited electrons thermalize within a few picoseconds to the mid-gap, near infrared d-d states, while recovery due to hole capture takes place on the ~0.6 ns time scale. The dependence of the transient probe spectrum and kinetics on excitation density, pump wavelength, and across different morphologies of Co3O4 supports the primary conclusion; resonant two photon absorption just below the d-d transitions prevents the data from ruling out the possible contribution of lower energy states. How hole capture by such electron trap states could mediate charge flow during the water oxidation reaction on Co3O4 and materials with similar mid-gap states is discussed. Future experiments will elucidate the extent to which this type of carrier relaxation is generic to transition metal oxides.
M. Waegele, H. Doan, and T. Cuk. Implications of the mid-gap, d-manifold to photoexcited carrier dynamics in cobalt oxide. Submitted to Phys. Rev. B. 2013.
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