Oliviero Cannelli
In this thesis, we exploited optical and X-ray pump-probe methods in a synergistic approach to study the interplay of the electronic, spin and structural degrees of freedom in two class of complex systems relevant for solar energy conversion applications: lead-halide perovskites and transition metal oxides.CsPbBr3 perovskites are promising optoelectronic materials characterized by a flexible ionic inorganic network that strongly interacts with the charge carriers. Upon photoexcitation, the lattice response leads to the polaronic distortions affecting the optical properties of the system. In our study, we combined time-resolved X-ray absorption spectroscopy and ab-initio simulations to quantify the structural deformations induced in the lattice through electron-phonon coupling. Complementing our pump-probe results with temperature-dependent X-ray absorption measurements, we disentangled the photodynamics of the system from thermally-induced effects. Additionally, we clarified the thermal response of CsPbBr3 comparing temperature-dependent X-ray investigations with first principles computations. A consistent atomic level picture is provided, in which the role of thermal fluctuation and phonon anharmonicity are rationalized with the experimental evidence.Spinel Co3O4 represents a model system for the investigation of the correlated electronic-spin-nuclear degrees of freedom in transition metal oxides. By combining femtosecond broadband reflectivity and ultrafast time-resolved X-ray emission spectroscopy, we monitored the material's photoresponse upon selective excitations of the ligand-to-metal and metal-to-metal charge transfer optical transitions. In the former case, sub-picosecond spin and electronic dynamics occurs together with a displacive excitation of coherent phonons, in the latter a slower electronic relaxation is measured in presence of impulsively stimulated Raman scattering phonons. We propose a possible explanation in terms of excitation-selective ultrafast intersystem crossing. We also present the preparation of parallel time-resolved X-ray emission and X-ray diffraction experiments, which will harness the structural- and spin-sensitivity of these techniques to disentangle the interactions determining the photodynamics of the system.In the last part of the thesis, we show a preliminary study aiming at extending the temperature-jump pump-probe method to the X-ray domain. In this experiment, a near-infrared pump is used to vibrationally excite the water molecules of the solvent, which undergo ultrafast relaxation causing a sudden increase of the temperature in the bulk of the solution. This process triggers a thermal chemical reaction of the dissolved solutes, which is monitored in a time-resolved fashion. We studied a multistep ligand substitution reaction of a hexacoordinated Cobalt ion complex in chlorinated water solution, showing the great sensitivity of the X-ray absorption technique to subtle structural changes. This work opens new perspectives for the investigation of thermally-driven reactions with element-selective and site-specific X-ray methods.
EPFL2021
