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

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.

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

The environment is important for the exact course of a chemical reaction. As an example of the strong influence of the environment on the reaction, this work studies the isomerization reaction of the chromophore retinal in the binding pocket of the protein bacteriorhodopsin. Three selected distance scales with reference to the chromophore are addressed: The distant protein surface at a length scale of 3-5 nm, the fuzzy environment of the binding pocket (1-2 nm) and the immediate neighborhood represented by the amino acid tryptophan Trp86 (5 Å). First, static absorption and fluorescence spectroscopies are applied to three dimensionally crystallized bacteriorhodopsin. An especially adapted set-up was developed for each task. The analysis of data is carried out in comparison with the measured spectra of the native protein membrane in solution. While the absorption spectrum shows a broadening on the high-energy side, the fluorescence spectrum deviates in the general shape of that measured in solution. The synoptic analysis of all spectra reveals the existence of three spectroscopically distinct species in the crystal: BR490 (absorption maximum at 490 nm) with a relative contribution of 28%, BR570 (native spectrum of solution, 68%), and BR610 ("blue membrane", 4%). BR490 appears particularly in the additional fluorescence band at 550 nm and is assigned to the dehydrated species of bacteriorhodopsin. The presence of that particular species indicates the suppression of rehydration in a partially dehydrated crystal and shows that the three dimensional arrangement hinders water diffusion. A simple model, close domains are assumed to form, eventually suppressing free water diffusion in a crystal. The lack of water on the protein surface modifies the protonation states of the amino acids in the binding pocket and therefore the optical properties of the retinal. The chromophore actually reacts on the alteration of its distant environment upon incorporation in the protein crystal. The second distance scale - the fuzzy environment - is addressed by a time-integrated fluorescence spectroscopy on native bacteriorhodopsin as function of the excitation wavelength (430 nm to 650 nm). The optimized experimental set-up guaranteed that the fluorescence of only the excited state I460 contributed. The measured spectra show maxima at 740 nm, and exhibit a Stokes shift of 5000 cm-1, independent of the excitation wavelength. The similarity of the fluorescence excitation spectrum with the spectrum of absorbed photons excludes energy-dependent loss channels. However, the spectra show a growing asymmetric broadening on the high-energy side with decreasing excitation wavelength. The additional fluorescence originates from vibrational hot states, also supported by the decomposition by Gaussian functions. A comparison with the fluorescence spectrum of blue membranes shows that the low-energy part also arises from vibrational hot states. Consequently, the first fast intramolecular vibrational energy relaxation does not entirely distribute the access-energy to other modes. The remaining energy is found in a quasi-static occupation of vibrational modes. The subsequent second vibrational relaxation — probably not only involving the retinal but also the protein — takes place on a time scale similar to that of the isomerization. The fast isomerization reaction prevents dissipation of energy relaxation of the excited state of the retinal into the protein environment. On a third distance scale, femtosecond spectroscopy in the UV offers a direct view into the electrostatic interaction of the chromophore with its immediate neighborhood. For the first time, the retinal isomerization was studied within the spectral window of the tryptophan absorption from 265 nm to 310 nm. For this, a set-up adapted to UV wavelengths was realized, basing on non-linear optics. The developed single-shot detection system yields a sensitivity of 10-5 in optical density. In the transient absorption measurements, a time resolution of 90 fs was achieved. At short probe wavelengths (265 nm - 294 nm), the measured transients show first a fast reduction of the absorption which subsequently recovers on three time scales (380 fs, 3 ps, > 16 ps). With increasing probe wavelength an induced transient absorption becomes dominant (282 nm - 294 nm). It has a rise time of 280 fs, and decays with a time constant of 4.3 ps. With further increasing probe wavelength the absorption signal decays faster. In response to the complex signature of the transients, an iterative and model-independent transient-decomposition is introduced. A set of three basis elements were identified: one bleach, and two absorption transients. The two induced absorption transients are assigned to retinal. The dynamics of the excited state (I460) in the first retinal-transient is described by two time constants (280 fs and 1 ps). The fast time constant leads to the photoproduct, while the slower one leads back to the ground state, presumably by internal conversion. The 250 fs delayed onset of the second transient (photo-intermediate J) is connected to a wave packet motion in the excited state. This delayed onset of the transient was observed for the first time. It demands a rise time of the J photo-intermediate of only 280 fs. This contrasts common estimates of 500 fs given in literature. For the first time, the temporal behavior of the permanent dipole moment of retinal is observed experimentally. Its evolution is imprinted in the dominant bleach feature (265 nm - 294 nm), originating from tryptophan Trp86. In the excited state, the permanent dipole moment increases strongly within the first 250 fs, and decays exponentially afterwards, following the formation of the photoproduct. However, the related time constant of 380 fs is slightly greater than that of 280 fs of the photoproduct J. This discrepancy may indicate a remaining long-lived polarization of the binding pocket. In brief, by using the intrinsic tryptophan as a probe, the local evolution of the electric field during the isomerization reaction was investigated. Indeed the isomerization reaction is reflected in the immediate environment. The variety of optical spectroscopy provides methods especially adapted for studying the influences of the environment on a chemical reaction. The results achieved here highlight the importance of these influences.

EPFL2004

Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

Pascal Martelli

The dynamics and bonding of the complex hydrides LiBH4 and LiAlH4 have been investigated by vibrational spectroscopy. The combination of infrared, Raman, and inelastic neutron scattering (INS) spectroscopies on hydrided and deuterided samples reveals a complete picture of the dynamics of the BH-4 and AlH-4 anions respectively as well as the lattice. The straightforward interpretation of isotope effects facilitates tracer diffusion experiments revealing the diffusion coefficients of hydrogen containing species in LiBH4, and LiAlH4. LiBH4 exchanges atomic hydrogen starting at 200 °C. Despite having an iso-electronic structure, the mobility of hydrogen in LiAlH4 is different from that of LiBH4. Upon ball-milling of LiAlH4 and LiAlD4, hydrogen is exchanged with deuterium even at room temperature. However, the exchange reaction competes with the decomposition of the compound. The diffusion coefficients of the alanate and borohydride have been found to be D ≃ 7 × 10-14 m2 s -1 at 473 K and D ≃ 5 × 10-16 m2 s-1 at 348 K, respectively. The BH-4 ion is easily exchanged by other ions such as I- or by NH-2. This opens the possibility of tailoring physical properties such as the temperature of the phase transition linked to the Li-ion conductivity in LiBH4 as measured by nuclear magnetic resonance and Raman spectroscopy. Temperature dependent Raman measurements on diffusion gradient samples Li(BH4)1-cIc demonstrate that increasing temperature has a similar impact to increasing the iodide concentration c: the system is driven towards the high-temperature phase of LiBH4. The influence of anion exchange on the hydrogen sorption properties is limited, though. For example, Li4(BH 4)(NH2)3 does not exchange hydrogen easily even in the melt. © The Royal Society of Chemistry 2011.

2011

Cu-Sn-S system: Vibrational properties and coexistence of the Cu2SnS3, Cu3SnS4 and Cu4SnS4 compounds

Andrew Wayne Fairbrother

In the present study the transformation between Cu2SnS3, Cu3SnS4 and Cu4SnS4 compounds is analyzed by means of X-ray diffraction and Raman scattering spectroscopy. A detailed analysis of the X-ray diffractograms showed the dependence of formation of the indicated compositional phases from the [Cu]/[Sn] ratio, which was also correlated with the evolution of the Raman spectra. The results are complemented with the detailed analysis of microscopic Raman imaging. Finally, fingerprint Raman spectrum for each compositional polymorph is presented, which should exclude any misinterpretations during phase analysis in future works. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

PERGAMON-ELSEVIER SCIENCE LTD2020

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

EPFL2004