Lee Hsun Lecture Series

Topic:  Aberration-Corrected Electron Microscopy
　　　　How the overcoming of a basic law of nature allowed to see atoms in materials

Speaker: Prof. Knut W. Urban
　　　　　Ernst Ruska Center for Microscopy and Spectroscopy with Electrons,
　　　　　Research Center Juelich and RWTH Aachen University

Abstract:

Since the pioneering work of Ernst Abbe in the second half of the nineteenth century we know that in an optical system resolution is limited by lens aberrations. Abbe also showed that the most important geometrical lens aberration, spherical aberration,can be eliminated by the construction of systems of converging and diverging lenses whose aberrations are mutually compensating. Unfortunately in electron-optical systems this technique cannot be directly applied since, due to the fact that magnetic fields have to fulfil Gauss’s law for magnetism, one of the four Maxwell equations, all round magnetic lenses are converging and no diverging lenses can be constructed.

During the nineteen nineties, i.e. more than sixty years after the invention of the electron microscope by Ernst Ruska, against all odds, it became possible to realize aberrationcorrected electron optics [1]. For this we developed a system of two hexapoles and two transfer round lenses which together are acting like a diverging lens. This allowed us to construct the word’s first aberration-corrected electron microscope. In the past twenty years this has revolutionized electron microscopy. Our double-hexapole correction principle is employed in all but a few of the about 600 commercial aberration-corrected instruments installed to date world-wide independent on whether these are TEMs or STEMs. The prominent feature of these is that we can see atoms and study these with the stunning precision of one picometer,i.e. one hundredth of the Bohr diameter of the hydrogen atom. This provides access to the heart of physical properties of materials and allows to compare TEM and STEM results with ab-initio theoretical calculations. On this basis the electron microscope has become a unique high-precision measurement tool for materials science.

In our own work we have employed aberration-corrected electron microscopy to oxide materials to study dislocations, coherent grain boundaries, heterostructure interfaces as well as the structural background of functional properties of superconductors and multiferroics. In oxides we take advantage of the fact that aberration-corrected electron microscopy for the first time allows to image oxygen, the key element in these compounds, directly.
After a general introduction this lecture will illustrate by the discussion of a few examples the way how modern aberration-corrected electron microscopy is helping us to better understand materials and their properties.

[1] M. Haider, S. Uhlemann, E. Schwan, H. Rose, B. Kabius & K. W. Urban,Nature 392, 768 (1998).