Monochromated STEM with a 30 meV-wide, atom-sized electron probe

The origins and the recent accomplishments of aberration correction in scanning transmission electron microscopy (STEM) are reviewed. It is remembered that the successful correction of imaging aberrations of round lenses owes much to the successful correction of spectrum aberrations achieved in electron energy loss spectrometers 2-3 decades earlier. Two noteworthy examples of the types of STEM investigation that aberration correction has made possible are shown: imaging of single-atom impurities in graphene and analyzing atomic bonding of single atoms by electron energy loss spectroscopy (EELS). Looking towards the future, a new all-magnetic monochromator is described. The monochromator uses several of the principles pioneered in round lens aberration correction, and it employs stabilization schemes that make it immune to variations in the high voltage of the microscope and in the monochromator main prism current. Tests of the monochromator carried out at 60 keV have demonstrated energy resolution as good as 12 meV and monochromated probe size of ∼1.2 Å. These results were obtained in separate experiments, but they indicate that the instrument can perform imaging and EELS with an atom-sized probe <30 meV wide in energy, and that an improvement in energy resolution to 10 meV and beyond should be possible in the future.