Charge, lattice and spin interplay in the ultrafast response of photoexcited spinel Co3O4
Giulia Fulvia Mancini a
a University of Pavia, Corso Strada Nuova, 65, Pavia, Italy
Proceedings of International Conference on Emerging Light Emitting Materials (EMLEM23)
Peyia, Cyprus, 2023 November 13th - 15th
Organizers: Grigorios Itskos, Maksym Kovalenko and Maryna Bodnarchuk
Invited Speaker, Giulia Fulvia Mancini, presentation 031
Publication date: 18th August 2023

Among the transition metal oxides (TMOs), spinel Co3O4 has recently emerged as promising material for photovoltaic and photocatalytic applications. Spinel Co3O4 contains Co[3+] and Co[2+] centers respectively occupying octahedral and tetrahedral sites surrounded by O[2-] anions. The two nonequivalent Cobalt centers are characterized by paired and unpaired d-orbital electronic configurations, determining the presence of Mott-Hubbard and charge transfer gaps close in energy. Because of its complex physics involving the correlated interaction of multiple degrees of freedom, the investigation of its relaxation dynamics calls for experimental approaches able to disentangle electronic, spin and lattice photo-responses with ultrafast time resolution. With ultrafast X-ray emission spectroscopy (XES) at the FXE instrument of the European X-ray free electron laser (EXFEL), we studied the temporal evolution of metal-centered transient electronic configurations, which are degenerate in the optical domain. Specifically, we excited the ligand-to-metal charge transfer (LMCT) and metal-to-metal charge transfer (MMCT) transitions of Co3O4 by pumping a 27 nm thin film at 400 nm and 800 nm, respectively. Our results rule out a stepwise relaxation cascade from the highest LMCT to the lowest d-d gap through the intermediate MMCT state, presenting a radically different picture compared to previous time-resolved optical studies on Co3O4. These results establish correlative time-resolved X-ray emission and optical spectroscopy as a novel strategy for the investigation of condensed matter systems.

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