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Synchrotron Radiation as a Modern Tool in Materials Science

Gemeinsames Kolloquium des Fachbereichs Physik und des SFB 1083

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Abstract:
What governs the efficiency of functional materials or the rate and selectivity of energy conversion and chemical processes remains experimentally challenging. Soft x-ray spectroscopy from Synchrotron Radiation Sources and Free-Electron Lasers allows to break down complex electronic structure problems into the atomic constituents and molecular moieties. This element specific and chemical state selective projection of soft X-rays - down to the atomic level - I will utilize and illustrate in this talk. The example I focus on deals with the mechanisms that allow MoS2 to act as an efficient material for photo-electrochemical water splitting. In this effort, we have investigated for one the electronic structure properties of the layered Dichalcogenide MoS2 interacting with the relevant molecular units, its photo excitation and charge transfer dynamics. On the other side, we have conducted quantitative investigations on the interacting fluctuating network of liquid water itself, where - up to now - diverging views have existed, dating back to Wilhelm Conrad Röntgen, postulating distinct phases to coexist in liquid water even under ambient conditions, competing with a single-phase liquid in a fluctuating hydrogen bonding network. Over time, X-ray spectroscopic methods have repeatedly been interpreted in support of Röntgens postulate. However, our recent, quantitative X-ray spectroscopic analyses show that this is not the case. As I go through the specific scientific insights, I will also give attention to the more general aspects of how to use soft X-ray spectroscopic methods wisely and how to avoid common mistakes.

 (1)      Inhomogeneity of Cleaved Bulk MoS2 and Compensation of Its Charge Imbalances by Room-Temperature Hydrogen Treatment, E. Giangrisostomi et al. Advanced Materials Interfaces, 10, 32, 2300392 (2023)
(2)      Photodriven transient picosecond top‐layer semiconductor to metal phase‐transition in p‐doped molybdenum disulfide. NLAN Sorgenfrei et al. Advanced Materials 33, 14, 2006957 (2021)
(3)      Cuts through the manifold of molecular H2O potential energy surfaces in liquid water at ambient conditions, A. Pietzsch et al. PNAS 119, 28, e2118101119 (2022)
(4)      Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions, J Niskanen, et al.  PNAS 116, 4058 (2019)
(5)      Probing hydrogen bond strength in liquid water by resonant inelastic X-ray scattering, V. Vaz da Cruz, et al. Nat. Com. 10, 1013 (2019)

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