Toward quantitative core-loss EFTEM tomography.

Core-loss EFTEM tomography provides three-dimensional structural and chemical information. Multiple inelastic scattering occurring in thick specimens as well as orientation-dependent diffraction contrast due to multiple elastic scattering, however, often limit its applications. After demonstrating the capability of core-loss EFTEM tomography to reconstruct just a few monolayers thin carbon layer covering a Fe catalyst particle we discuss its application to thicker samples. We propose an approximate multiple-scattering correction method based on the use of zero-loss images and apply it successfully to copper whiskers, providing a significant improvement of the reconstructed 3D elemental distribution. We conclude this paper by a general discussion on experimental parameters affecting the accuracy of EFTEM 3D elemental mapping.

Shape changes of supported Rh nanoparticles during oxidation and reduction cycles.

The microscopic insight into how and why catalytically active nanoparticles change their shape during oxidation and reduction reactions is a pivotal challenge in the fundamental understanding of heterogeneous catalysis. We report an oxygen-induced shape transformation of rhodium nanoparticles on magnesium oxide (001) substrates that is lifted upon carbon monoxide exposure at 600 kelvin. A Wulff analysis of high-resolution in situ x-ray diffraction, combined with transmission electron microscopy, shows that this phenomenon is driven by the formation of a oxygen-rhodium-oxygen surface oxide at the rhodium nanofacets. This experimental access into the behavior of such nanoparticles during a catalytic cycle is useful for the development of improved heterogeneous catalysts.

Lateral motion of SiGe islands driven by surface-mediated alloying.

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.

Structures of BaF2-CaF2 heterolayers and their influences on ionic conductivity.

Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF2-CaF2 using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l > 50 nm; 8 < l < 50 nm; l < 8 nm). In accordance with conductivity studies, the samples with spacings of 10 nm or greater (A,B) are epitaxial and continuous, whereas in the case of extremely small spacing (C) the continuity of the layers is destroyed by formation of a column-like structure. Analytical electron microscopy reveals that, instead of forming multilayers, Ca and Ba separate in different columns in sample C. The structure properties of sample A (large l) are quite ideal: Planar interfaces with regular arrays of misfit dislocations with their Burgers vectors on the interface are observed. In the case of sample B (medium l) the lattice misfit is accommodated, in addition, by wavy interfaces associated with dislocations characterized by a Burgers vector that makes a large angle to the interfaces. The (111) lattice spacing very close to the interfaces is markedly changed due to this novel relaxation mechanism in the multilayer. The influences of the crystallographic defects on the ionic conductivity are also discussed.