Coherent X-ray scattering study of polymer dynamics and nanostructures of metal nanoparticles.

In the 90's, the advent of insertion devices in synchrotrons resulted in an improvement of the coherence of X-rays and offered a large range of new experimental possibilities. The interference between coherent X-rays allows retrieving local properties of the sample, which would be otherwise averaged in the case of incoherent X-rays. In this talk, we describe two different experimental techniques making use of coherent X-rays to study the relaxation dynamics of polymer and the structural changes in metal nanoparticles. The first part is dedicated to the demonstration of the feasibility of the X-ray photon correlation spectroscopy (XPCS) technique with XFEL source. The first generation of XFEL being based on the self-amplified spontaneous emission (SASE) process, random by nature, fluctuations in energy, intensity and beam position form the essence of the X-ray beam. We retrieve here the dynamics of gold nanoparticles in a polystyrene matrix above its melting temperature, and deduce in the meantime several beam characteristics such as the number of transverse modes and the average beam size, and estimate the sample heating and damage. In the second part of this seminar we study the internal deformation change in platinum nanocrystal catalyst during the catalytic oxidation of methane, a reaction that takes place in car exhaust systems. Using the coherent X-ray diffraction technique (CXD) in Bragg geometry, we show that the diffraction pattern, centered on a Bragg peak, presents radically different behaviors depending on the gas ambiance and the temperature. Using a phase retrieval algorithm, we are able to deduce both the displacement and the strain distribution in the nanocrystal. Finally, we present an experimental plan to be conducted at ID10 beamline during the length of the postdoctoral position. Chalcogenide elements are interesting from the viewpoint of the dynamics in the super-cooled liquid phase and the evolution of the coherence length close to the glass transition temperature. The reversible amorphous-crystalline transition is promising for the use, e. g., in non-volatile random-access memory. Combining XPCS with a spatial technique such as X-ray cross-correlation analysis (XCCA) would give a full overview of the structural evolution of the chalcogenide glass.