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of Transmission Electron Microscopy Knut W. Urban

Abstract The following article is based on the Von Hippel Award address given by Knut W. Urban, chair of experimental physics at RWTH Aachen University and director of the Institute of Microstructure Research within the Department of Solid-State Research at the Jülich Research Center. Urban presented his award talk on November 29, 2006, during the 2006 Materials Research Society Fall Meeting in Boston. He was recognized with MRS’s highest honor, the Von Hippel Award, for his “sustained contributions to the development and use of electron microscopy, and for major discoveries in the defect physics of quasicrystals and high-temperature superconductors.” The Von Hippel Award honors those qualities most prized by materials scientists and engineers—brilliance and originality of intellect, combined with vision that transcends the boundaries of conventional scientific disciplines, as exemplified by the life of Arthur von Hippel (http://vonhippel.mrs.org). In this article, Urban describes the advances in spherical-aberration-corrected electron optics that have set transmission electron microscopy on an entirely new track in materials science. The new imaging theory for aberration-corrected instruments is based on exploiting both lens focus and spherical aberration as variable parameters to optimize contrast, resolution, and point spread. The novel negative-spherical-aberration imaging technique makes it possible to image low-nuclear-charge atoms, which were previously inaccessible directly by electron microscopy. This technique has proven particularly successful for research on oxides. Due to negligible point spread, quantitative measurements of atomic-site occupancies, which are equivalent to local concentrations, have become feasible. These improvements are dramatically expanding the potential of electron microscopy for quantitative studies in materials science.

Introduction In 1932, Ernst Ruska and Max Knoll, working at the Technical University of Berlin, had just successfully constructed and tested the first two-lens electron microscope when they learned about the wave nature of electrons. Curious about the maximum resolution that electron microscopy would be able to achieve, they applied Ernst Abbe’s equation in which they inserted the de Broglie wavelength of electrons and arrived at a value below 1 Å (10−8 cm), that is, on the order of the interatomic distances in solids. However, the level of the optical aberrations of their electromagnetic lenses was so high that in their day, it was impossible to make atomic resolution a reality. It took more than 60 years to find a way of constructing aberration-corrected electron lenses, and it is only recently that 946

aberration-corrected electron microscopes have come onto the market. Corrected electron optics is, however, only one of the ingredients required to realize atomicresolution transmission electron microscopy. To take advantage of this technical improvement, a new contrast theory and new imaging modes had to be d