Maarten Bischoff

Detection of Single Atoms and Buried Defects in Three Dimensions by Aberration-Corrected Electron Microscope with 0.5-Å Information Limit

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instruments new capabilities were exploited to detect a buried Sigma3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

Using a monochromator to improve the resolution in TEM to below 0.5 Å. Part II: Application to focal series reconstruction

We apply monochromated illumination to improve the information transfer in focal series reconstruction to 0.5 Å at 300 kV. Contrary to single images, which can be taken arbitrarily close to Gaussian focus in a C(S)-corrected microscope, images in a focal series are taken at a certain defocus. This defocus poses limits on the spatial coherence of the illumination, and through this, limits on the brightness of the monochromated illumination. We derive an estimate for the minimum spatial coherence and the minimal brightness needed for a certain resolution at a certain defocus and apply this estimate to our focal series experiments. We find that the 0.5 Å information transfer would have been difficult and probably impossible to obtain without the exceptionally high brightness of the monochromated illumination.

Using a monochromator to improve the resolution in focal-series reconstructed TEM down to 0.5Å

The resolution of present-day spherical-aberration corrected TEMs is limited by the chromatic aberration of the objective lens to about 0.7a at 300kV. The resolution can be improved by Cc correction or by reducing the energy spread in the illumination with a monochromator. We used TEAM 0.5, a special Titan column built within the TEAM project [1], and on this microscope we succeeded to improve the resolution to 0.5a by monochromation, not only in single images but also in images reconstructed from focal series. Such reconstructed images are free of possible contrast reversals due to incorrect focusing and possible artifacts due to non-linear interferences. However, focal series are more demanding to acquire than single images because of the higher demand on the stability of the column, and because they must be taken over some focus interval and this significantly increases the demands on the parallelness or coherence of the beam.