M. Cabié

Dynamic observations of Au catalysts by environmental electron microscopy

Au clusters in the size range (1–8 nm), supported on MgO and TiO2, are studied at the atomic scale by High Resolution Transmission Electron Microscopy (HRTEM), in standard conditions and during cycles of gas treatments by environmental HRTEM. Their morphology, the structure of the interface with the substrate and their adhesion energy are deduced from top and profile views, according to the preparation techniques, by atomic deposition in ultra high vacuum or by deposition precipitation in wet conditions.Under hydrogen atmosphere the gold clusters exhibit a truncated octahedron shape while under oxygen they show a rounding of the shape. These evolution are reversible in hydrogen/oxygen cycles. In the case of the CO oxidation by Au clusters supported on TiO2, the formation of CO2 in the microscope sample holder is evidenced by a differentially pumped mass spectrometer connected to the environmental sample holder, at the same time as the rounding of the clusters. The implication of the shape evolution in CO oxidation is discussed.

TEM and HRTEM Evidence for the Role of Ligands in the Formation of Shape-Controlled Platinum Nanoparticles

The morphology of platinum nanoparticles synthesized using an organometallic approach from PtMe(2) (C(8) H(12) ) is influenced by the nature of the ligands used as stabilizing agents. The use of long alkyl chain amines leads to the formation of multipodal nanoparticles that transform into compact nano-objects, adopting cubic, truncated cubic, or cuboctahedral shapes. In contrast, the use of diamine ligands allows the growth of compact (111) arrowlike faces, forming polycrystalline nanoparticles of an overall desert-rose aspect. Different reaction parameters are studied ([ligand]/[metal] ratio, temperature, solvent identity) in order to optimize the various shapes.

Transmission electron microscopy and Raman measurements of the misfit stress in a Si tensile strained layer

A tensile-strained Si layer grown on a Si0.8Ge0.2 pseudo substrate with a nominal lattice mismatch of −0.76% has been studied by transmission electron microscopy using a curvature method and Raman scattering in order to determine experimentally the in-plane component of the epitaxial stress. The stress is obtained by measuring the curvature and the thickness on different areas of a thinned sample. Experimental values of the stress given by the two methods are in good agreement and are close to the nominal one.

Geometrical criteria required for the determination of the epitaxial stress from the transmission electron microscopy curvature method

The epitaxial stress of a Ga0.8In0.2As thin layer deposited on a GaAs substrate has been measured by the curvature method adapted to transmission electron microscopy. It is shown that even if the geometrical characteristics of the specimens thinned to be observed by transmission electron microscopy are very different from the ones of a thick sample, the conditions of validity of the model can still be verified. Finite element calculations have been performed to determine the geometry of the specimen answering to these conditions. Once these conditions are satisfied, the stress measured on a Ga0.8In0.2As layer is −1.30±0.13GPa.

Adsorbate-induced restructuring on Pt nanoparticles studied by environmental transmission electron microscopy

The reactivity of metal nanoparticles depends on their shape, their structure and their morphology. Furthermore, the adsorption of reactive species on the surface of these catalysts can modify their morphology. These considerations illustrate the need of in situ characterization in order to better understand the elemental mechanisms of catalytic reactions.

Environmental High Resolution Electron Microscopy With a Closed Ecell: Application to Catalysts

Metal nano-clusters have catalytic properties related to their size, their shape and crystalline structure. The most important advances for catalysis in electron microscopy, was the possibility to insert gas in the microscope during observations with the aim to close simultaneously the material and the pressure gap in catalysis [1–3].