Microscopy at the Bottom

About twenty years ago, many experts were still pessimistic about the possibility for making qualitative significant advancements in resolution improvement of the electron microscope. The basis for their scepticism was the Scherzer Theorem [1], which states that axially symmetric electromagnetic lenses have large intrinsic spherical and chromatic aberrations, limiting the resolution of an electron microscope to about 100 times the wavelength of the imaging electrons. This is exactly the situation to which Richard Feynman referred 50 years ago when writing: “It would be very easy to make an analysis of any complicated chemical substance; all one would have to do would be to look at it and see where the atoms are. The only trouble is that the electron microscope is one hundred times too poor” [2]. Feynman further stated that it should, however, not be impossible to improve the electron optic of the microscopes as “it is not against the law of diffraction of the electron. The wave length of the electron in such a microscope is only 1/20 of an angstrom. So it should be possible to see individual atoms. What good would it be to see the individual atoms distantly!” Prior to Rose's designing the first non-axially symmetric lenses that actually could correct the aberrations inherent to round magnetic lenses [3], the instrumental resolution of TEMs was limited primarily by the spherical aberration (CS) of the objective lens and the wavelength of the electrons. Therefore, reduction of the wavelength was the only possibility for improving the resolution. However, because of the occurrence of radiation damage, highvoltage instruments could not be used for many applications as even thick crystalline materials can be observed under MeV beams only for a few minutes [4]. After the practical realization of the CS-corrector for the TEM in 1998 [5] and STEM in 1999 [6] and its commercial premiere in 2005, atomic resolution of material’s heavy and light single atom columns and even single heavy atoms at medium accelerating voltages has become a reality. An enormously huge wealth of materials science questions can now be addressed and answered. Atom column dumbbells, which never were resolved so far can now be imaged distantly (see Fig. 1).