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Hard X-ray helical dichroism of disordered molecular media

Camila Bacellar Cases Da Silveira, Oliviero Cannelli, Majed Chergui, Christian David, Dominik Kinschel, Jérôme Lacour, Giulia Fulvia Mancini, Malte Oppermann

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life sciences to materials science. Chirality-sensitive spectroscopic methods, such as circular dichroism, exhibit weak signal contributions on an achiral background. Helical dichroism, which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an angstrom-size orbital. Here we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the helical dichroism spectra of a disordered powder sample of enantiopure salts of the molecular complex of Fe(4,4 '-diMebpy)(3) at the iron K edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the helical dichroism spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the OAM of light. Linearly polarized orbital angular momentum-carrying hard X-ray beams are induced using spiral Fresnel zone plates. By sending the hard X-ray beams to disordered enantiopure molecular complexes, the helicity-dependent and chiral-sensitive signal is obtained.

NATURE PORTFOLIO2022

Photo- and thermally-induced electronic and structural dynamics in perovskites and transition metal oxides

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

Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals

Andre Al Haddad, James Michael Budarz, Oliviero Cannelli, Majed Chergui, Nicola Colonna, Ludmila Maria Diniz Leroy, Dominik Kinschel, Giulia Fulvia Mancini, Michele Puppin, Thomas Charles Henry Rossi

The development of next-generation perovskite-based optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron-phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of timeresolved and temperature-dependent studies at Br K and Pb L-3 X-ray absorption edges with refined ab initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperature-dependent XAS results rules out thermal effects as the primary source of distortion of the Pb-Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

2021