Search

We investigate K-shell ionization of N2 at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the Auger electron, and both N+ fragments. These fully differential data show that the dissociation process of the N2+2 ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately 30∶70 between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.

International audience; The electrochemical activation of oxygen is the cornerstone of energy storage and conversion devices, such as metal-air batteries, electrolysers and fuel cells. In proton-exchange membrane fuel cells, the high cost and scarcity of Pt continues to drive research aimed at enhancing its mass activity (the current produced per gram of Pt) for the oxygen reduction (ORR). Recent studies have shown that the ORR kinetics is maximal on bimetallic alloys and at (111) facets 1 , hence, PtNi/C octahedra are still envisioned as the dream ORR catalyst 2. Strikingly, it was also found recently that structurally-disordered PtNi nanocatalysts feature highly desirable and sustainable ORR activity 3. However, to date, the mechanisms of this unexpected ORR activity enhancement remain unclear. To shed fundamental light onto this, state-of-the art PtNi/C nanocatalysts with distinct atomic composition, size, shape and density of disorder were synthesized. Their disorder was quantified experimentally, using the values of microstrain (a parameter accessible by the Rietveld refinement of wide-angle X-ray scattering patterns). Thanks to experimental measurements and ab initio calculations, the contributions of chemical and structural disorder were disentangled and a new parameter, the surface distortion (SD), was established. The SD descriptor was used to rationalize the ORR activity enhancement of the two classes of materials (structurally-ordered and structurally-disordered), and to probe their stability in simulated PEMFC cathode operating conditions

Physical review letters 123(19), 193001 (2019). doi:10.1103/PhysRevLett.123.193001, We investigate K-shell ionization of N2 at 40 keV photon energy. Using a cold target recoil ionmomentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, theAuger electron, and both Nþ fragments. These fully differential data show that the dissociation process ofthe N2þ2 ion is significantly modified not only by the recoil momentum of the photoelectron but also by thephoton momentum and the momentum of the emitted Auger electron. We find that the recoil energyintroduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately30∶70 between the Auger electron and fragment ion kinetic energies, respectively.We also observe that thephoton momentum induces an additional rotation of the molecular ion., Published by APS, College Park, Md.

The local atomic structure and the magnetic response of Co films intercalated between Graphene and Ir(111) were investigated combining polarized X-ray Absorption Spectroscopy at the Co K edge with Magneto-Optic Kerr Effect. The structural and magnetic evolution upon a 500 ° C annealing was evaluated as a function of the film thickness. After the thermal treatment, our thick film (10 monolayers) presented a lower perpendicular magnetic anisotropy (PMA) as well as a reduced average structural disorder. On the other hand, in our thin film (5 monolayers), the annealing enhanced the perpendicular magnetic response and induced a local anisotropy by stretching the Co-Co bonds in the film plane and compressing those outside the plane. Our finding emphasizes the close relationship between the local structure of Co within the film and its magnetic properties.

Thin Co films intercalated between a Graphene capping layer and the Ir(111) surface are of interest for spintronics applications due to their peculiar magnetic properties and to their chemical stability. The structure, and then the magnetic properties, of the Co films depend on the intercalation process which is strongly influenced by the temperature, total amount of Cobalt and quality of the capping Graphene layer. In order to identify and disentangle the effects of these contributions, we report on the structural characterisation of four Co films as a function of thickness, annealing temperature, and Graphene capping. From the structural point of view, the deposition of Co on a hot Ir substrate mimics quite well the intercalation process proving the validity of the colander model describing the Graphene role during the process.

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) ${\mathrm{SrTiO}}_{3}$ single crystals. We find that the polar instability of the $1\ifmmode\times\else\texttimes\fi{}1$ surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of ${\mathrm{SrTiO}}_{3}$ layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of $3\ifmmode\times\else\texttimes\fi{}3$ (111) ${\mathrm{SrTiO}}_{3}$ surfaces consisting in an incomplete Ti (surface)--${\mathrm{O}}_{2}$ (subsurface) layer covering the $1\ifmmode\times\else\texttimes\fi{}1$ Ti-terminated (111) ${\mathrm{SrTiO}}_{3}$ truncated crystal. In both cases, the ${\mathrm{TiO}}_{6}$ octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) ${\mathrm{SrTiO}}_{3}$ surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

International audience; X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 mu m X 10 mu m, individual flakes are probed by specular X-ray reflectivity. By deploying a recursive Parratt algorithm to model the experimental data, we gain access to characteristic crystallographic parameters of the samples. Notably, it is possible to directly extract the bi/multilayer graphene c-axis lattice parameter. The latter is found to increase upon lithiation, which we control using an on-chip peripheral electrochemical cell layout. These experiments demonstrate the feasibility of in situ X-ray diffraction on individual, micron-sized single crystallites of few- and bilayer two-dimensional materials.

Graphene can be used to avoid the oxidation of metallic films. This work explores the effectiveness of such stabilizing effect on Cobalt (Co) films intercalated between Graphene and Ir(1 1 1). After intercalation at 300 °C, two Co films are exposed to ambient pressure and investigated using Co-K edge X-ray Absorption Near Edge Spectroscopy. The formation of a disordered oxide phase is observed, and associated to the presence of some non-intercalated Co. Further annealing at 500 °C causes the oxide reduction to metallic Co which further intercalates below the Graphene. Once the intercalation is completed, Graphene prevents the Co from oxidation under ambient pressure conditions.