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Atomic resolution imaging

Adsorption/reaction-induced reconstruction

FIM is applied to reveal structural changes of metal crystals due to adsorption of gases. Admitting oxygen to a Rh crystal at ~500 K causes a shape transformation from nearly hemispherical to polyhedral. High-index planes are largely absent in the polyhedral morphology. {011} and {113} planes are reconstructed according to the missing-row type.

Observation of oscillations by FIM

Hydrogen oxidation to water on Rhodium is studied under dynamic reaction conditions. Video-FIM is applied to image moving reaction fronts with nanoscale resolution.

Starting with a mixed layer of hydrogen and oxygen atoms on a polyhedrally reconstructed Rh crystal, the nucleation and growth of an oxide layer begins in the central (001) area of the crystal. This layer extends towards the outskirts of the Rh crystal (preferentially along the < 100 > zone lines). The image brightness decreases over time before the oxide layer is suddenly reacted off as water to reestablish the initial image of a mixed (and reactive) layer of hydrogen and oxygen atoms. Self-sustained oscillatory reaction behavior is observed as the catalytic cycle described above repeats itself in a periodic manner. PFDMS during imaging proves water species to image the catalytic reaction. The mass spectra demonstrate that the formation rate of water is higher on an oxidized Rh surface.

Conditions : 550K, PH2=1.3 10-3Pa, field strenghts 9 V/nm

Nonlinear dynamics studied by FEM

Field Emission Microscopy (FEM) is used to image the catalytic reduction of NO2 by H2 on platinum tips in real time (with a time resolution of 20ms). Several dynamical behaviors have been observed: adsorption hysteresis, bistability, self-sustained periodic oscillations and self-sustained bimodal periodic oscillations. In FEM, the brightness signal is monitored and reflects the work function of the region analyzed. This works function varies with the chemical nature of the adsorbates. Following the brightness signal vs time allows us to qualitatively follow the local composition of the surface.

During the self-sustained periodic oscillations, we can observe a regular increase of the brightness signal which has been assigned to water formation, and then a decrease due to dissociative adsorption of NO2.

Analysis of the data is performed by evaluating the Fourier Transforms, the autocorrelation functions, the histogram, the Poincaré section, the next maximum map as well as reconstructed attractors of the time series.

Chiral adsorption

Despite the high symmetry of metallic crystals, the surface can exhibit enantiomorphous orientations, which could show differences towards adsorption of chiral molecules. Imaging the behaviour of chiral molecules on chiral facets is of great interest as it would provide new knowledge on the enantioselective adsorption on chiral facets and how one chiral surface orientation may dominate over another in the context of a catalytic reaction.