Transmission electron microscopy, perfected in 1931 by Ernst Ruska and Max Knoll, has evolved enormously over the last ten years. Thanks to the introduction of Cs (and Cc) corrected lenses, spatial resolution has been greatly improved and now reaches values well below the Angstrom. But, perhaps even more remarkably, the development of techniques such as EELS and EDX has been such that it is now possible not only to obtain chemical information on an atomic scale, but also to determine the valence of ions on an atomic scale. The power of modern electron microscopy is that it is possible to simultaneously combine different structural, chemical and electronic information on the same region.
In this presentation, we take cerium oxide as an example. These nanocrystals are found in octahedral form with {111} surfaces or as "truncated octahedra" with {111} and {100} planes. Super-high resolution shows that the {111} planes are oxygen-terminated, but the {100} planes are cerium-terminated. In addition, EELS analysis reveals that Ce4+ is reduced to Ce3+ at the surface. This reduction is more pronounced on {111} surfaces than on {100} planes. EELS imaging enables us to quantify the number of planes affected by this reduction.
By its very nature, transmission electron microscopy is a purely 2D technique, but it is possible to reconstruct 3D images using tomography. Recently, we have shown that it is possible to reconstruct 3D images down to the atomic scale (provided the nanostructure is crystalline) !
