Characterization of Photo-Induced Charge Transfer and Hot Carrier Relaxation Pathways in Spinel Cobalt Oxide (Co3O4)

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• 作者：Chang-Ming Jiang ; L. Robert Baker ; J. Matthew Lucas ; Josh Vura-Weis ; A. Paul Alivisatos ; Stephen R. Leone
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：October 2, 2014
• 年：2014
• 卷：118
• 期：39
• 页码：22774-22784
• 全文大小：498K
• ISSN：1932-7455

文摘

The identities of photoexcited states in thin-film Co₃O₄ and the ultrafast carrier relaxation dynamics of Co₃O₄ are investigated with oxidation-state-specific pump鈥損robe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M_2,3-edge of cobalt are probed in transient absorption with femtosecond high-order harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific Op>2鈥?/sup>(2p) 鈫?Cop>3+p>(e_g) charge-transfer transition, out of six possible excitation pathways involving Cop>3+p> and Cop>2+p> in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 卤 10 fs at 13.2 mJ/cmp>2p>) together with a slight redshift in spectral shape (535 卤 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.